Recent Advances in Sonogashira Reactions

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Mar 18, 2011 - copper cross-coupling reactions.13. The copper-free Sonogashira reaction has been explained suggesting the mechanism depicted in Fig.
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Recent Advances in Sonogashira Reactions Article  in  Chemical Society Reviews · June 2011 DOI: 10.1039/c1cs15071e · Source: PubMed

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Carmen Najera

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Cross coupling reactions in organic synthesis themed issue Guest editor: Matthias Beller All authors contributed to this issue in honour of the 2010 Nobel Prize in Chemistry winners, Professors Richard F. Heck, Ei-ichi Negishi and Akira Suzuki

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Rafael Chinchilla* and Carmen Na´jera* Received 18th March 2011 DOI: 10.1039/c1cs15071e The coupling of aryl or vinyl halides with terminal acetylenes catalysed by palladium and other transition metals, commonly termed as Sonogashira cross-coupling reaction, is one of the most important and widely used sp2–sp carbon–carbon bond formation reactions in organic synthesis, frequently employed in the synthesis of natural products, biologically active molecules, heterocycles, molecular electronics, dendrimers and conjugated polymers or nanostructures. This critical review focuses on developments in the Sonogashira reaction achieved in recent years concerning catalysts, reaction conditions and substrates (352 references).

1. Introduction The palladium catalysed C–C bond formation process which is able to couple a terminal sp hybridized carbon from an alkyne with a sp2 carbon of an aryl or vinyl halide (or triflate) is commonly termed as a Sonogashira coupling. The reaction name arises from the discovery in 1975 by Sonogashira, Tohda, and Hagihara that this process could be performed

Departamento de Quı´mica Orga´nica, Facultad de Ciencias and Instituto de Sı´ntesis Orga´nica (ISO), Universidad de Alicante, Apartado 99, 03080 Alicante, Spain. E-mail: [email protected], [email protected]; Fax: +34 965903549; Tel: +34 965903728 w Part of a themed issue on the topic of palladium-catalysed cross couplings in organic synthesis in honour of the 2010 Nobel Prize winners Professors Richard F. Heck, Ei-ichi Negishi and Akira Suzuki.

Scheme 1

easily at room temperature using a palladium source such as PdCl2(PPh3)2 as catalyst, combined with a co-catalytic amount of CuI in an amine as solvent (Scheme 1).1 This finding came several months after Cassar2 and Dieck and Heck3 disclosures that it was possible to perform this coupling only under palladium catalysis but working at high temperatures. Sonogashira’s insight was to combine the copper-mediated transmetalation

Rafael Chinchilla (left) was born in Alicante and studied chemistry at the University of Alicante from which he was graduated (1985) and doctorated (1990). After a postdoctoral stay at the University of Uppsala (1991–1992) with J.-E. Ba¨ckvall, he moved back to the University of Alicante where he was appointed Associate Professor in 1997. He is co-author of more than 80 papers and four patents and his research interests are mainly focused on peptide coupling reagents, asymmetric synthesis of a-amino acids and enantioselective syntheses by using organocatalysis. Carmen Na´jera (right) obtained her BSc at the University of Saragossa in 1973 and her PhD at the University of Oviedo under the supervision of J. Barluenga and M. Yus in 1979. She performed postdoctoral work with D. Seebach, J. E. Baldwin, E. J. Corey, and J.-E. Ba¨ckvall and was appointed Associate Rafael Chinchilla and Carmen Na´jera Professor in 1985 at the University of Oviedo and Full Professor in 1993 at the University of Alicante. She was awarded ‘‘2006 Organic Chemistry Prize’’ from the Spanish Royal Chemical Society of Chemistry, ‘‘2006 Rosalind Franklin International Lectureship’’ from the English Royal Society and the SCF 2010 French–Spanish Prize from the Socie´te´ Chimique de France. In October 2010 has been named Correspondent Member of the Royal Spanish Academy of Sciences. Her contributions are focused on sulfone chemistry, peptide coupling reagents, asymmetric synthesis of a-amino acids, palladium carbon–carbon coupling reagents, asymmetric metal catalysis and organocatalysis. 5084

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of alkynes that was studied, with a metal more versatile and powerful in catalytic terms such as palladium, thus providing an extension of Cassar and Heck’s results that yielded a robust C–C bond formation procedure. However, the addition of copper, although beneficial in terms of increasing the reactivity of the system, added some shortcomings, the principal being the necessity of avoiding the presence of oxygen in order to block the undesirable formation of alkyne homocoupling through a copper-mediated Hay/Glaser reaction.4 To address such a problem, a solution was to eliminate the copper in the so-called ‘‘copper-free’’ Sonogashira reaction, although perhaps in this case the process could easily be termed as Heck–Cassar reaction, Heck alkynylation or perhaps Sonogashira–Heck–Cassar coupling. The term ‘‘Sonogashira reaction’’ however, is nowadays a blanket description applied to the palladium(0)-catalysed coupling of a sp2 (or even sp3) halide or triflate with a terminal alkyne, regardless of whether copper(I) salts are present. The obtained products from the Sonogashira reaction have found applicability in the most diverse areas of chemistry, such as dyes, sensors, electronics, polymers, guest–host constructs, natural products and heterocycle synthesis. The Sonogashira reaction, including differently employed catalysts, reaction conditions and applications, was extensively reviewed in 2007.5,6 Despite the few years after these revisions, the number of reports showing different modifications and applications of this process has reached impressive records. Thus, a simple search for the topic ‘‘Sonogashira’’ in the SciFinder database showed more than 1500 references published in journals during the period 2007–2010. Most of these publications deal with applications of the typical coppercocatalysed Sonogashira protocol to the synthesis of the most diverse systems and cannot be considered as real advances in the Sonogashira reaction. Thus, in the presented revision we will focus mainly on contributions published from 2007 to the very beginning of 2011 that represent a novelty in the Sonogashira cross-coupling methodology when dealing with the use of catalysts, reaction conditions, procedures, substrates or mechanism insights. The review has been ordered, after some mechanistic considerations, by the different types of catalysts, supported or not, that have been recently employed in this reaction.

2. Mechanism The exact mechanism of the palladium/copper-catalysed Sonogashira reaction is still at this moment not well understood, mainly due to the difficulties of analysing the combined action of the two present metal catalysts, although it is generally supposed to take place through two independent catalytic cycles (Fig. 1).5 The first ‘‘palladium-cycle’’ (cycle A) is classical from C–C cross-coupling formations7 and starts in the catalytically active species Pd(0)L2, which can be of colloidal nature and/or a low-ligated Pd(0)-species stabilised by the ligands present, including the base and/or solvent molecules. In the case of using phosphanes as ligands, the corresponding bis(phosphane)palladium, as well as other involved species, has been observed in the gas phase by This journal is

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Fig. 1 Supposed mechanism for the copper-cocatalysed Sonogashira reaction.

negative-ion electrospray ionization mass spectrometry.8 This [Pd(0)L2] complex can be formed from Pd(0) complexes such as Pd(PPh3)4, or can be created from Pd(II) complexes such as PdCl2(PPh3)2 through formation of a [Pd(II)L2(CRCR2)2] species, which gives [Pd(0)L2] after reductive elimination by forming a diyne, a route that can be significant in the presence of an oxidant such as molecular oxygen.9 In addition, amines may also reduce Pd(II) to Pd(0) through formation of iminium cations, structural evidence of the reduction mechanism of Pd(II) to Pd(0) by inorganic bases being also reported.10 Once complex [Pd(0)L2] has been formed, the first step in the catalytic cycle is initiated by oxidative addition of the aryl or vinyl halide, which is considered to be the rate-limiting step of the Sonogashira reaction, the barriers of oxidative addition of ArX (X = Cl, Br, I) increasing in the order of ArI o ArBr o ArCl.11 High-throughput kinetics and descriptor modeling suggested that the initial aryl halides participate in the turnover-determining step of the Sonogashira reaction.12 Presumably, this step is preceded by an end-on ligation of the halogen atom to palladium and therefore could be regarded as an electron-donating step. Thus, the higher the EHOMO of the substrate (electron-donor groups), the more stable the complex should be and, the higher the rate determining activation barrier for subsequent steps. On the contrary, a low EHOMO (electron-withdrawing groups) would facilitate the oxidative addition reaction. The formed [Pd(II)R1L2X] adduct in Fig. 1 is then transformed into a [Pd(II)L2R1(CRCR2)] species after transmetalation with a copper acetylide formed in the ‘‘copper-cycle’’ (cycle B). This adduct suffers reductive elimination, after cis/trans-isomerization, to the final alkyne, regenerating the catalyst [Pd(0)L2]. While the ‘‘palladium-cycle’’ is classical from C–C couplings, the ‘‘copper-cycle’’ (cycle B) is poorly known. The base (organic or inorganic) is believed to assist the copper acetylide formation with the help of a p-alkyne copper complex, which would make the alkyne terminal proton more acidic. However, things can even be more complicated, as it has been shown that Chem. Soc. Rev., 2011, 40, 5084–5121

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Fig. 4 Proposed mechanistic changeover in the mechanism of the Sonogashira reaction when going from electron-rich to electron-poor alkynes.

Fig. 2

Supposed mechanism for the copper-free Sonogashira reaction.

CuI-polyphosphane adducts can be formed and ligands can be transferred from one metal to other, showing that copper– ligand interactions might also be likely in tandem palladium/ copper cross-coupling reactions.13 The copper-free Sonogashira reaction has been explained suggesting the mechanism depicted in Fig. 2.5 This proposed catalytic cycle is initiated, as usual, by the oxidative addition of the aryl or vinyl halide to the catalytic species [Pd(0)L2]. The following step is a reversible p-coordination of the alkyne, producing an alkyne–Pd(II) complex where the acetylenic proton is acidified facilitating its removal by the base with coordination of the acetylene ligand to the metal. This [Pd(II)R1(CRCR2)L2] complex releases the cross-coupled product by reductive elimination re-forming the catalytic species [Pd(0)L2]. A recent study has revealed the multiple role of amines in the copper-free Sonogashira reaction.14 Besides their expected function as deprotonating agents, amines may be involved in different steps preceding the deprotonation. Amines can interfere in the oxidative addition by an accelerating effect due to the formation of more reactive [Pd(0)L(amine)] complexes, and they can also substitute one ligand in the complex formed after the oxidative addition. Depending on the rate of the competition between amine and alkyne in the substitution of one ligand in this complex, other mechanism could also operate (Fig. 3) together with the one presented in Fig. 2. Thus, the mechanism in Fig. 2 would be preferential if the amine is a less good ligand than the alkyne for the palladium(II) center in [Pd(II)XR1L2] (i.e. L = PPh3, amine = piperidine or morpholine). However, the mechanism in Fig. 3 would be

Fig. 3 Proposed mechanism involving amines for the copper-free Sonogashira reaction.

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preferred if the amine is a better ligand than the alkyne for the palladium(II) center in [Pd(II)XR1L2] (i.e. L = AsPPh3, amine = piperidine). Hammett correlation studies using a model reaction with differently p-substituted phenyl acetylenes and 4-iodobenzotrifluoride as coupling partners and a Pd2(dba)3CHCl3/AsPh3 catalyst system in methanol suggested a possible mechanistic changeover when going from electron-rich to electron-poor alkynes.15 The changeover point was found to be dependent on the nature of the amine base and its concentration. It is hypothesised that the reaction mechanism depicted in Fig. 2 changes from a pathway involving a fast proton transfer from a slowly forming cationic palladium complex (Fig. 4, cationic pathway) to a pathway involving a slow proton transfer from a neutral Pd complex (Fig. 4, anionic pathway) on going from electron-rich to electron-poor alkynes. The amine base is proposed to act as a base in both pathways and also as a nucleophile promoting the formation of the cationic complex in the reactions involving electron-rich alkynes, the optimum choice of base therefore differing substantially with the substrate. Therefore, although recent studies have paid attention to the mechanism of the Sonogashira reaction, particularly to the simplest copper-free process, this reaction is still far of being clearly understood. It seems that different catalytic cycles can operate depending on substrates or bases, and even that palladium–copper synergistic exchange of ligands could be present when possible.

3. Palladium–phosphorous complexes 3.1 Unsupported palladium–phosphorous complexes 3.1.1 Copper-cocatalysed reactions. As previously mentioned, the typical Sonogashira reaction has been performed using a phosphane-including palladium complex as catalyst in the presence of a catalytic amount of a copper(I) salt and an excess of an amine (or inorganic base) under homogeneous conditions, although recently any solvent but a ball mill has been used.16 Although catalysts with bidentate ligands such as PdCl2(dppe),17 PdCl2(dppp),18 or PdCl2(dppf)19 [dppe = 1,2bis(diphenylphosphino)ethane; dppp = 1,3-bis(diphenylphosphino)propane; dppf = 1,10 -bis(diphenylphosphino)ferrocene] have been employed, the most commonly used palladium complexes have been Pd(PPh3)4 [sometimes formed in situ by mixing a palladium(II) salt and triphenylphosphane] and PdCl2(PPh3)2, the latter being more stable and soluble. These last two triphenylphosphane-containing complexes continue to be nowadays by far the most frequently employed catalysts when a copper-cocatalysed This journal is

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Sonogashira cross-coupling reaction is going to be used in a practical application.20 Just an example, the use of PdCl2(PPh3)2 (1 mol%) combined with CuBr, using diisopropylethylamine as organic base in DMF, has been found optimal for a Sonogashira coupling reaction leading to 10- and 100-gram order production of a metalloproteinase inhibitor performed using an automated-flow microreactor system.21 Triphenylphosphane has been the most frequent phosphane employed in the Sonogashira cross-coupling reaction, even though the positive effect of increasing steric bulk of the phosphane in C–C cross-coupling reactions was recognised in 1983 when using P(o-tolyl)3 as a ligand in a palladium complex.22 Thus, electron-rich, bulky phosphanes are highly useful ligands for palladium catalysis in many types of crosscoupling reactions,23 and the Sonogashira reaction is no exception. The electron-richness favors the oxidative addition step, whereas the steric bulkyness favors the formation of lowcoordinate and highly active palladium complexes, this low coordination being more easily obtained with monodentate phosphanes.24 Particularly concerning the effect of the ligand bulkyness, a study based on high throughput techniques has demonstrated that the rate of a given Sonogashira reaction depends on a combination of the steric bulk of the reactants and the steric bulk of the phosphanes coordinated to the catalytically active palladium.25 Then, normal substrates are more efficiently coupled with palladium complexes of large phosphanes with cone angles of 1821 (PtBu3) to 1901 (PAd2tBu, Ad = adamantyl). However, with very bulky substrates the best results can be obtained with palladium–phosphane complexes of slightly smaller steric demand such as tBu2PCy (1781) or tBuPCy2 (1741), the catalytical activity dropping drastically below a critical cone angle of ca. 1701. Thus, in spite of the still tremendous popularity of triphenylphosphane as palladium ligand, the use of bulky phosphanes such as PtBu326 or PCy327 as palladium ligands in Sonogashira reactions has been more common in the last few years and synthetic examples can be found. Recent cases include the use of tBu3PHBF4 (6 mol%) combined with PdCl(PhCN)2 (3 mol%) in the CuI (3 mol%) co-catalysed coupling of terminal alkynes and inactivated aryl bromides performed using a weak base such as diluted aqueous ammonia (1 M) in THF as solvent, the reaction being carried out at room temperature.28 This catalytic combination has also been used in the alkynylation of 5-bromobenzofurans with prop-2-yn-1-yl acetate, as part of a synthesis of the natural benzofurans Ailanthoidol and Egonol.29 Considering the mentioned higher reactivity of bulky phosphanes, the search for new of these types of ligands amenable to improve the efficiency of palladium complexes used in the Sonogashira process, particularly with less reactive aryl bromides or chlorides, less catalyst loading and in milder and environmentally friendlier conditions, has been frequent. Thus, neopentylphosphanes such as 1 and 2 have been used as ligands in the cross coupling of aryl bromides with phenylacetylene and aliphatic alkynes.30 The reaction between aryl bromides and phenylacetylene was performed using Pd2(dba)3 (1.5 mol%) as palladium source and the ligand (4.75 mol%) in the presence of CuI (2 mol%) and triethylamine, in toluene at room temperature, whereas the coupling of aryl bromides with This journal is

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aliphatic alkynes required to change the palladium source to PdCl2(PhCN)2 (3 mol%), to increase the amount of ligand (8 mol%) and the use of diisopropylamine as base in dioxane at room temperature.

Lower amounts of the palladium source sodium tetrachloropalladate (0.5 mol%) and the indole-containing ligand 3 (1 mol%) in the presence of CuI (1 mol%) are required for the cross-coupling of aryl and heteroaryl bromides with trimethylsilylacetylene (TMSA) using tetramethylethylenediamine (TMEDA) as solvent at 80 1C, the amount of palladium being amenable to be reduced down to 0.005 mol% when coupling the activated 4-bromoacetophenone.31 When applied to the coupling of heteroaryl bromides and aryl alkynes, these catalytic systems and reaction conditions afforded diarylacetylenes amenable to be transformed into maleimides after a double carbonylation.32 Moreover, this catalytic combination, also under similar reaction conditions, has been applied to the synthesis of Free Fatty Acid 1 (FFA1) receptor agonists, such as TUG-424 (6), obtained from aryl bromide 4 by a first Sonogashira coupling with TMSA affording alkyne 5 after silyl deprotection, the addition of water as cosolvent improves the yield of the coupling step (Scheme 2).33 Further cross-coupling with 2-bromotoluene and hydrolysis gave the final product 6. The indenyldialkylphosphane 7 has been found to be a suitable palladium ligand for a Sonogashira reaction with aryl chlorides. Thus, 1 mol% of the combination Na2PdCl4/7/CuI (4 : 8 : 3) has been employed as catalytic mixture in the presence of sodium carbonate in DMSO as solvent at 100–120 1C in the cross-coupling of activated and inactivated

Scheme 2

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aryl chlorides with phenylacetylene or 1-hexyne in high yields.34 Furthermore, the 9-fluorenylphosphane 8 has been developed as a trialkylphosphane with good electron-donating properties and large steric bulk. This ligand has been used (0.04 mol%), combined with a low loading of a palladium source such as Na2PdCl4 (0.02 mol%) and CuI (0.015 mol%), for creating a catalytic system able to perform the Sonogashira coupling of aryl bromides with phenylacetylene, using triethylamine as solvent at 50 1C.35 This catalyst is also able to perform the coupling of activated and inactivated aryl chlorides with phenylacetylene, although the loading of the components of the catalytic mixture should be increased [Na2PdCl4 (1 mol%), 8 (2 mol%), CuI (0.75 mol%)] and the solvent changed to DMSO and heating at 100 or 120 1C. Scheme 4

The chiral ferrocenyl-containing phosphane 9 [(R,R)-Taniaphos] has been employed in a reported rare asymmetric double Sonogashira coupling of diiodoparacyclophanes (up to 80% ee), although high loadings of palladium, ligand and coppercocatalyst were necessary.36 Thus, the combination of PdCl2(MeCN)2 (10 mol%), the ligand 9 (10 mol%) and CuI (10 mol%) in the presence of caesium carbonate as base in THF at room temperature has allowed the enantioselective synthesis of planarly chiral dialkynylparacyclophanes such as 11, formed by double Sonogashira reaction of racemic diiodoparacyclophane 10 and 3-methoxyprop-1-yne (Scheme 3). Water-soluble palladium catalysts have received significant interest due to the potential environmental and economic benefits of performing catalysis in aqueous–organic biphasic systems.37 Thus, although the typical Sonogashira conditions employing the usual Pd(PPh3)4 as catalyst have been used in water as solvent,38 water-soluble phosphanes such as the known trisodium tri(3-sulfonatophenyl)phosphane (12, TPPTS) have

Scheme 3

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been employed, combined with a palladium salt, as catalyst for copper-cocatalysed Sonogashira reactions,39 such as the alkynylation of 5-iodouridine,40 7-iodo-7-deazapurine nucleosides and nucleotides41 and 8-bromo-2 0 -deoxyadenosine.42 The analogous catalytic system formed using phosphane TXPTS (13) as ligand (30 mol%), palladium(II) diacetate (10 mol%) and CuI (10 mol%) has shown to be more reactive and has been employed in the aqueous-phase Sonogashira coupling of unprotected halonucleosides,43 an example being the coupling of 8-bromoadenosine 14 with phenylacetylene, which was performed in the presence of triethylamine in an mixture acetonitrile/water (1 : 1) as solvent at 80 1C giving the alkynylated system 15 (Scheme 4). Refluxing water has been the only solvent employed in the coupling of aryl iodides, bromides and even chlorides with phenylacetylene when using as catalyst a combination of palladium(II) chloride (2.5 mol%), the phosphazane ligand 16 (0.3 mol%) and CuI (1 mol%), the presence of an additional base being not required.44

In spite of all these developments concerning phosphane ligands used in Sonogashira couplings, still the standard methodology involving the catalytic combination formed by Pd(PPh3)4 or PdCl2(PPh3)2 in the presence of a copper(I) salt is the most frequently employed. We will continue this section illustrating very briefly some recent uses of the standard Sonogashira methodology in the most important areas where this synthetic procedure is common. The synthesis of the enyne moiety by reaction of a vinyl halide with a terminal alkyne is a synthetic application of the Sonogashira reaction which has been carried out frequently under the standard copper-cocatalysed cross-coupling conditions. Reported examples include the use of vinyl halides such as (E)-1-iodovinyl sulfones,45 (E)-1-iodo-2-arylselenoethylenes,46 2-brominated allyl alcohol derivatives,47 furanose- and pyranosederived mono- and dihalo-exo-glycals,48 or 3-iodocyclopent-2enones,49 as well as the gem-dibromoolefins in the preparation of This journal is

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Scheme 5

highly conjugated diethynylethenes with optical properties.50 This standard enyne synthesis has also been applied frequently to the preparation of products of natural origin. Thus, examples of the typical Sonogashira methodology applied to the coupling of terminal alkynes to vinyl iodides,51 bromides52 and also triflates53 for the synthesis of natural products can be found, as shown in Scheme 5 in the coupling of alkyne 17 with (E)-1-bromoprop1-ene to give enyne 18, which is an intermediate in the total synthesis of the (+)-Sapinofuranone B (19), a furanone from Streptomyces griseus.54 In addition, enynes have been obtained by coupling of (Z)-3-iodoalk-2-en-1-ols with terminal propargylic alcohols, and have been used for a subsequent Au(I) or Pd(II)catalysed cyclisation–aromatisation to give polysubstituted furans.55 However, the use of an aryl or heteroaryl halide in the coupling reaction with a terminal alkyne is the most frequently performed reaction under standard Sonogashira conditions and the one that has found most applications. Particularly interesting is the case of the alkynylation of haloarenes possessing a nucleophilic substituent, such as an oxygen of nitrogen, at the ortho position, which can be used for a subsequent 5-exo-trig or a 6-endo-dig cyclisation to produce an array of different heterocycles.56 An example of the application of this methodology is shown in Scheme 6, where vanillin-derived iodoarene 20 was cross-coupled with alkyne 21 under the standard Sonogashira conditions to give diarylalkyne 22 which, after cyclisation with iodine, gave benzofuran 23, an intermediate in the total synthesis of the natural product XH-14, a lignan isolated from Salvia miltiorrhiza.57 The synthesis of Moracin O58 or Erypoegin H59 are other examples of the use of this alkynylation–cyclisation concept to the total synthesis of natural benzofurans, as well as the preparation of other of these heterocycles60 or related starting from iodohetarenes,61 as well as pyran-1-ones from a 6-endo-dig cyclisation from haloheteroaromatic carboxylic acid derivatives.62 In addition, examples of cross-couplings devoted to the preparation of indoles63 or related systems,64 including some natural products such as ()-Mersicarpine65 can be found. Moreover, a domino reaction via Sonogashira coupling of benzimidoyl chlorides with 1,6-enynes and cyclisation to synthesise quinoline derivatives has been reported.66 Acid chlorides are compounds amenable to be used under the standard Sonogashira conditions for the synthesis of many heterocyclic systems.67 As an example, a one-pot three-component synthesis of N-Boc-4-iodopyrroles has been This journal is

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Scheme 6

developed by coupling aryl, heteroaryl, alkenyl and some alkyl-substituted acid chlorides such as 24, with N-Bocprotected propargylamine, to produce ynones such as 25 which are cyclised in the presence of sodium iodide and p-toluenesulfonic acid (PTSA) to the final iodopyrrole 26 (Scheme 7).68 This final pyrrole 26 can be 4-alkynylated after another one-pot Sonogashira coupling. Indolyl-containing ynones have been prepared by a decarbonylative Sonogashira reaction methodology involving glyoxylyl chlorides, an example being shown in Scheme 8. Thus, indole-3glyoxylyl chloride 27 is decarbonylatively coupled with 1-hexyne in the presence of PdCl2(PPh3)2 (1 mol%) as catalyst and CuI (1 mol%) as co-catalyst, using triethylamine as base in THF at room temperature, to afford indolyl-ynone 28.69 This process can be performed following an in situ protocol starting from indoles, 7-aza-indoles or pyrroles, consisting of glyoxylation with oxalyl chloride followed by the palladium/copper-catalysed decarbonylative alkynylation.

Scheme 7

Scheme 8

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The alkynylation of halogenated heterocycles under the typical Sonogashira reaction conditions has been profusely used for the preparation of many systems of interests. Examples are the alkynylation of halogenated pyrroles,70 phthalimides,71 isoindigo derivatives,72 benzofurans,73 dihydroselenophenes,74 pyridines,75 quinolines,76 coumarins,77 pyrimidines,78 2,3-dihydropyrimidin-4-ones,79 uracil80 and adenosine81 nucleosides, triazines82 and also 2-trifloxyoxazoles in the total synthesis of Leucascandrolide A83 and Neopentolide.84 The traditional halides or triflates present in the alkynylation of heterocycles can be substituted by other leaving groups in some recent cases. Thus, nonaflates (nonafluorobutanesulfonates), which are the C4 homologues of triflates,85 have also been used as leaving groups. As an example, a typical catalyst combination for Sonogashira couplings [Pd(OAc)2/PPh3/CuI] using diisopropylamine as base in DMF as solvent has been used for the alkynylation of 8-oxa- and 8-thiabicyclo[3.2.1]octanonederived alkenyl nonaflates86 and 6-(trifluoromethyl)pyridyl nonaflates.87 This alkynylation of heterocycles has been also performed in a much less common way by using phenyl- or alkylsulfanyl substituents as leaving groups in the standard Sonogashira reaction.88,89 Thus, 5-chloro-3-(phenylsulfanyl)pyrazin-2-(1H)ones such as 29 has been alkynylated at the 3-position with terminal acetylenes such as phenylacetylene, using PdCl2(PPh3)2/CuI as catalytic combination in the presence of caesium carbonate in DMF at 95 1C under microwave irradiation, to afford the corresponding heteroaromatic alkyne 30 (Scheme 9).88 When starting from 5-methyl-3-(methylthio)-1,2,4-triazine, the alkynylation takes place at the 3-position using PdCl2(dppf)2 (10 mol%)/CuI (20 mol%) as catalytic mixture and using triethylamine as base in THF at 120 1C, working also under microwave conditions.89 However, the alkynylation of aryl systems leading to monoaryl or diarylacetylenes is probably the most traditional use of the typical Sonogashira reaction. Thus, examples of application of this standard methodology to the preparation of compounds of interests, such as pharmacologically active substances,90 natural products,91 as well as other compounds such as arylalkynylphosphonates,92 alkynylaryltrifluoroborates,93 a-arylethynyl-a,b-unsaturated esters,94 or even rotaxanes95 can be found. Particularly important is the use of the Sonogashira methodology to the preparation of highly conjugated structures. These highly p-conjugated systems with a well-defined structure are considered useful molecular materials for electronics and photonics.96 In particular, oligo-p-aryleneethynylenes (OAEs)

have attracted special interest for their applications, including molecular electronics. These kinds of systems are obtained using profusely the Sonogashira reaction following iterative synthetic approaches based on the use of acetylene surrogates. The most frequent of these protected acetylenes are trialkylsilylacetylenes such as TMSA,97 which can be silyl-deprotected using a fluoride source or an inorganic base. An example of the use of this iterative procedure for the preparation of a fragment of an OAE is illustrated in Scheme 10, which shows the coupling of alkyne 31 with diiodoarene 32 under the standard coupling conditions to give 33. Further coupling with TMSA afforded 34 and subsequent deprotection with tetra-n-butylammonium fluoride (TBAF) yielded 35, this system being cross-coupled with another diiodoarene for the creation of an OAE.98 Other examples of these one-dimensional ‘‘molecular wires’’ have been prepared using the standard Sonogashira coupling,99 as well as other diarylalkyne-including one-,100 two-101 and three-dimensional102 architectures supporting a high degree of electronic conjugation. The synthesis of diarylacetylenes is sometimes limited by the availability of the appropriate monoarylacetylenes due to their often moderate stability. This process can be more efficiently carried out in one-pot without the necessity of isolating the monoarylated intermediate. An example of this methodology employing TMSA is shown in Scheme 11, where aryl bromide 36 is cross-coupled with TMSA under the typical Sonogashira conditions to give silylated alkyne intermediate 37. In situ treatment with potassium hydroxide and further addition of 4-iodotoluene produced silyl-deprotection giving intermediate 38, and a subsequent second Sonogashira coupling afforded the final diarylacetylene 39.103 The former sequential process has also been reported using 1-ethynylcyclohexanol as

Scheme 9

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Scheme 13 Scheme 11

acetylene surrogate,104 its behavior being similar to the known 2-methyl-3-butyn-2-ol,105 although liberation of the acetylene under basic conditions now releases the chemically more inert cyclohexanone instead of acetone. This alcohol has also been used as masked acetylene in the coupling with 2-iodophenols for the synthesis of benzofurans.106 In addition, if no final aryl halide is added and air is present, a copper-promoted Hay/Glaser coupling of the monoarylated intermediate can also occur, which has been used recently in the case of using TMSA for the direct preparation of symmetrical diynes by a consecutive one-pot Sonogashira–Glaser process achieved by a sequential palladium/copper catalysis.107 It has been shown previously that it is possible to perform the direct palladium–copper catalysed coupling of alkynylsilanes without the above commented necessity of a preliminary silyl-deprotection, in what is called a ‘‘sila’’-Sonogashira reaction. This reaction has been performed under the base-free original conditions, which used aryl or vinyl triflates,108 using Pd(PPh3)4 (5 mol%) as catalyst now in the coupling of aryl iodides, although the presence of a higher amount of CuCl (50 mol%) was required, the reaction being performed in DMF as solvent at 80 1C.109 In addition, the diphosphane ()-DIOP (40) has been shown as a suitable ligand combined with a palladium(II) source such as palladium(II) acetate for generating a catalyst able to crosscouple alkynyl silanes, such as 41, with activated aryl chlorides, such as 42, in the presence of substoichiometric amounts of CuI also under nonbasic conditions and in DMF as solvent at 120 1C, giving the corresponding unsymmetrical diarylethyne 43 (Scheme 12).110

Scheme 12

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The use of TMSA as acetylene surrogate in Sonogashira couplings has allowed us to develop a route to 4-aryl-1,2,3triazoles by using a sequential Sonogashira–click reaction.111 Thus, by using typical Sonogashira reaction conditions, aryl iodides such as 44 have been cross-coupled with TMSA to afford the corresponding alkyne intermediate 45, which was treated with benzyl azide and TBAF, as silyl-deprotecting agent, allowing to obtain the final click-triazole 46 (Scheme 13). A similar process has also been carried out with aryl iodides and bromides, this last case in moderate yields,112 obtaining the Sonogashira adduct under microwave irradiation as a way of accelerating the cross-coupling reaction,113 and performing the click cycloaddition in the presence of CuF2. Arenediazonium salts have been found recently to be a suitable alternative to aryl halides for the Sonogashira coupling reaction. Thus arenediazonium salts such as 47 have been cross-coupled at room temperature with terminal alkynes such as prop-2-yn-1-ol using a typical Sonogashira catalytic combination [PdCl2(PPh3)2 (1 mol%), CuI (4 mol%)], in the presence of diethylamine and tetra-n-butylammonium iodide (TBAI) in acetonitrile as solvent, to give diarylalkyne 48 (Scheme 14), the reaction proceeding through a domino iododediazoniation/Sonogashira cross-coupling sequence.114 In addition, tautomerizable heterocycles have been alkynylated using a process involving in situ C–OH activation with bromotripyrrolidinophosphonium hexafluorophosphate (PyBrOP) followed by Sonogashira coupling using a catalytic system formed by PdCl2(PPh3)2 (5 mol%) and CuI (10 mol%), using triethylamine or diisopropylamine as bases in dioxane at 25–80 1C.115 The phosphanes 49 and 50 bearing an ammonium salt have been explored as triphenylphosphane-like ligands (16 mol%) combined with palladium(II) chloride (8 mol%) and CuI (10 mol%) in the cross-coupling of 4-iodo- and 4-bromotoluene

Scheme 14

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with phenylacetylene. The reaction was performed in the presence of triethylamine at 70 1C in the ionic liquid [bmim]PF6, affording high yield in the first case but only moderate in the second.116 These phosphanes resulted poorly soluble in ether and remain in the ionic liquid, thus allowing the easy extraction of the coupled products from the reaction mixture.

3.1.2 Copper-free reactions. The required addition of the copper(I) halide in the standard Sonogashira reaction presents some drawbacks concerning not only economical and environmental reasons, but mainly the necessity of performing the coupling reaction in the absence of oxygen to minimise the occurrence of the Hay/Glaser copper-catalysed homocoupling of the employed terminal acetylenes. Reagent-grade argon or nitrogen is typically used to provide the inert atmospheric conditions required to minimise this undesired reaction, although special equipment destinated to avoid the presence of oxygen, such as two-chamber reaction systems, has been developed.117 However, the most direct way of avoiding formation of Hay/Glaser is logically to perform the reaction in the absence of any copper(I) salt, in the so-called copper-free Sonogashira reaction. It is necessary to point out that the possible presence of copper contaminants in these reactions usually is not carefully studied and, therefore, some doubts about the reality of a ‘‘copper-free’’ reaction could arouse. This has been the case of metal contaminants detected in other catalytic systems used in Sonogashira couplings (see Section 10.1). Some substrates have been found to be appropriate for performing the copper-free process efficiently using the common palladium catalysts after the addition of some additives. Thus unactivated aryl iodides have been coupled with terminal acetylenes following a copper-free Sonogashira protocol involving the use of a mixture of palladium(II) acetate (5 mol%) and triphenylphosphane (10 mol%) as catalyst, adding tetra-n-butylammonium bromide (TBAB) as additive and working in wet THF as solvent at room temperature.118 The same catalytic combination, but in the presence of triethylamine as base in acetonitrile at 80 1C, has been employed for the 3-alkynylation of 2-chloro-3-iodoquinoline with TMSA in a synthesis of an indole-containing KDR kinase inhibitor.119 In addition, 3-iodoselenophenes have been alkynylated using a high loading (10 mol%) of PdCl2(PPh3)2 as catalyst and triethylamine as base in DMF as solvent at room temperature.120 This palladium(II) catalyst (6 mol%) has also been used in the presence of TBAF for the alkynylation of o-dihaloarenes employed in the synthesis of pharmacologically interesting indoles.121 Increasing the reactivity of the catalytic system by using bulkier phosphane ligands has been shown often as an appropriate way of performing the copper-free Sonogashira reaction. Thus, aryl and heteroaryl bromides can be cross-coupled with 5092

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Scheme 15

terminal alkynes employing as catalyst a rather high load of the combination of Pd(PPh3)4 (6 mol%) and the known 2-(di-tert-butylphosphane)-1,1 0 -biphenyl (51) (12 mol%) in the absence of copper(I) co-catalysis and in the presence of caesium hydroxide as base in acetonitrile at 90 1C.122 This procedure has been applied to the C-6-alkynylation of protected 2 0 -deoxyadenosines such as 52 to give the corresponding alkynylated system 53 when coupled with phenylacetylene (Scheme 15). In addition, iodo- and bromoaryl carboxylic acids have been cross-coupled with phenylacetylene or 2-methyl-3-butyn-2-ol under copper-free conditions using as catalyst PdCl2(PPh3)2 (4 mol%) in piperidine at 85 1C.123 The always hard-to-couple aryl chlorides have been found as suitable coupling partners of phenylacetylene or 1-octyne when using as catalyst a combination of PdCl2(PPh3)2 (2 mol%) and PtBu3 (4 mol%), in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and caesium carbonate in DMF as solvent under microwave irradiation, the reaction affording high yields of the final coupled products in only 10 min reaction time.124 Aryl chlorides and 3-chlorothiophene have been coupled to terminal alkynes using as catalyst a low loading mixture of PdCl2(MeCN)2 (1 mol%) and the N-substituted bulky heteroaryl phosphane ligand 54 (3 mol%), the reaction being performed in the presence of sodium carbonate as base in toluene at 90 1C.125 Several functional groups, including amino, silyl and vinyl, are tolerated under these conditions, an example being shown in Scheme 16, where 2-chlorostyrene (55) is coupled with 1-octyne to give the corresponding alkyne 56 in rather low yield, although formation of stilbene or stilbene oligomers was not observed, proving the chemoselectivity of the system for the coupling of the alkyne. Diaryl- and dihetarylacetylenes have been obtained under copper-free conditions by a one-pot palladium-catalysed, double Sonogashira coupling of inactivated aryl chlorides with This journal is

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Scheme 18

Scheme 16

Scheme 19

Scheme 17

2-methyl-3-butyn-2-ol as acetylene equivalent, the reaction being carried out using a catalyst bearing a bulky ligand such as PdCl2(PCy3)2 (5 mol%) in the presence of piperidine and caesium carbonate, in DMSO at 120 1C.126 An example of this methodology is shown in Scheme 17, where 2-chlorothiophene (57) is double-coupled to 2-methyl-3-butyn-2-ol to give the symmetrical disubstituted acetylene 58. This procedure has also been applied to the preparation of unsymmetrical diarylated acetylenes, although final yields were rather low. In addition, 3-chloro-3-formylquinolines have been cross-coupled with phenylacetylene using palladium(II) acetate (10 mol%)/triphenylphosphane (20 mol%) as catalyst in the presence of triethylamine, using acetonitrile as solvent at 80 1C. One-pot addition of methanol to the Sonogashira intermediate allowed intramolecular cyclisation to pyrano[4,3-b]quinolines.127 A decarboxylative palladium-catalysed coupling of alkynylcarboxylic acids and aryl and heteroaryl bromides has allowed using these types of acids as terminal alkyne surrogates.128 As an example, phenylpropionic acid (59) reacted with an heteroaryl bromide such as 3-bromopyridine in a reaction catalysed by a combination of Pd2(dba)3 (2.5 mol%) and the bulky phosphane PtBu3 (10 mol%) in the presence of TBAF in NMP as solvent at 90 1C, to give the corresponding diarylated acetylene 60 (Scheme 18). 2-Octynoic acid can also be used for this process, but the reaction is then performed using PdCl2(PPh3)2 (1 mol%)/ 1,4-bis(diphenylphosphino)butane (dppb) (2 mol%) and TBAF in DMSO as solvent at 110 1C. Related to this decarboxylative methodology is the synthesis of symmetrical diarylalkynes from propiolic acid or 2-butynedioic acid and aryl iodides or bromides using PdCl2(PPh3)2 (5 mol%)/dppb (10 mol%), and DBU as base in DMSO as solvent at 110 1C. Unsymmetrical diarylalkynes were obtained when all reagents were added at the beginning of the reaction at 50 1C and then heated at 80 1C.129 In addition, unsymmetrical diarylalkynes have also been obtained by a Pd2(dba)3 (5 mol%)/dppf (10 mol%)-catalysed coupling reaction of aryl iodides with propiolic acid in the presence of TBAF in NMP as solvent at room temperature, This journal is

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followed by in situ decarboxylation and further addition of aryl bromides, raising the reaction temperature up to 90 1C.130 The copper-free Sonogashira cross-coupling of ynamides with aryl iodides has been accomplished using as catalyst the combination palladium(II) acetate (5 mol%)/triphenylphosphane (10 mol%) in the presence of sodium acetate in DMF as solvent at 80 1C,131 the reaction giving good yields for activated aryl iodides and moderate for hindered or deactivated. This methodology has allowed the synthesis of acetylenes such as 62, formed by coupling of alkynylated urethane ynamide 61 with phenylacetylene (Scheme 19). Although aryl triflates can be an alternative as coupling partners to the use of the typical aryl halides in the Sonogashira cross-coupling reaction, its use has been very limited due to high cost of the triflating agent and their low hydrolytic stability. Therefore, as the use of phenolic compounds as electrophiles in the Sonogashira reaction is an interesting possibility, several copper-free alternative methods to the use of aryl triflates have been reported in the last few years. Thus, aryl and heteroaryl mesylates and tosylates have been crosscoupled with terminal alkynes in the presence of a mixture of palladium(II) acetate (2 mol%) and the indolyl phosphane 63 (6 mol%) as catalyst and potassium phosphate as base in tert-butanol at 100 1C.132 A similar coupling of aryl and heteroaryl tosylates with terminal acetylenes has been reported using a combination of palladium(II) trifluoroacetate (3 mol%) and the ferrocene-derived phosphane 64 (7 mol%) as catalyst, the reaction also being carried out in the presence of potassium phosphate and in tert-butanol as solvent at 85 1C.133 In addition, nucleoside C-6 arylsulfonates have been crosscoupled with terminal alkynes following a copper-free procedure involving the combination palladium(II) chloride (5 mol%)/Xphos (65) (15 mol%) as catalyst, and working in the presence of triethylamine in THF as solvent at 90 1C.134

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Scheme 20

achieved by performing the process in the presence of tosyl chloride, thus generating in situ the 4-tosylcoumarin which suffers the coupling reaction. This transformation has been carried out using PdCl2(PPh3)2 as catalyst (5 mol%) in the presence of diisopropylethylamine and using acetonitrile as solvent at 60 1C, an example being the cross coupling of 4-hydroxycoumarin 66 with ethynylcyclopropane to give the 4-alkynylated coumarin 67 (Scheme 20).135 In addition, tautomerizable heterocycles have been alkynylated using a process involving in situ C–OH activation with PyBrOP followed by Sonogashira coupling using a catalytic copperfree system formed by PdCl2(MeCN)2 (5 mol%) and 2-(dicyclohexylphosphino)biphenyl (15 mol%).136 Moreover, there is a report on the use of imidazol-1-ylsulfonates as electrophiles in a copper-free Sonogashira reaction.137 Thus, aryl imidazylates such as 68 have been coupled with phenylacetylene using as catalyst a combination of palladium(II) acetate (10 mol%) and 65 (Xphos) (20 mol%), working in the presence of potassium phosphate in DMSO as solvent at 65 1C, to afford the corresponding alkynylated product 69 (Scheme 20). Aryltrimethoxysilanes have been employed as suitable coupling partners with terminal alkynes when reacting in the presence of a fluoride source such as silver(I) fluoride, the reaction being catalysed by PdCl2(dppf) (3 mol%) using sodium bicarbonate as base in acetonitrile as solvent at room temperature.138 Moreover, triarylantimony diacetates, such as 70, have been found as a new class of electrophiles for copper and base-free Sonogashira-type cross-coupling reactions with terminal alkynes, such as 4-(dimethylamino)phenylacetylene (Scheme 21). Thus, under aerobic conditions and PdCl2(PPh3)2 catalysis (1 mol%) in 1,4-dioxane as solvent at 80 1C, two of the three aryl groups could be involved in the reaction and the corresponding acetylenes, such as 71, are obtained.139

The palladium-catalysed carbonylation reaction of aryl halides with nucleophiles is an interesting synthetic transformation140 that, when performed using terminal alkynes as nucleophilic counterparts, allows the synthesis of alkynones in the so-called Sonogashira carbonylation reaction. Although there are examples involving the use of an atmospheric pressure of carbon monoxide, as the coupling of iodoferrocene with terminal alkynes catalysed by PdCl2(PPh3)2 (5–10 mol%) in THF at 60 1C, the presence of CuI (2–4 mol%) is necessary.141 This process has been carried out in the absence of coppercocatalysis using [PdCl(cinnamyl)]2 as catalyst (2 mol%) and the bulky BuPAd2 (Ad = adamantyl) as ligand (6 mol%) using potassium carbonate as base in DMF as solvent under a carbon monoxide atmosphere (10 bar) at 100 1C, allowing the carbonylative coupling of aryl bromides with aryl alkynes, to give the corresponding alkynones.142 In addition, the same palladium source (1 mol%) combined with the Xantphos (72) (2 mol%) phosphane ligand has been employed as catalyst in a carbonylative Sonogashira alkynone formation starting from aryl triflates and arylacetylenes, the reaction being performed in the presence of triethylamine as base in toluene at 110 1C under a carbon monoxide atmosphere (10 bar).143 The methodology has also been applied to the carbonylative coupling of vinyl triflates such as 73, which gives ynone 74 after reaction with phenylacetylene (Scheme 22).143 Flavones have been obtained from 2-iodophenols by using a one-pot copper-free carbonylative Sonogashira-annulation protocol catalysed by a combination of Pd2(dba)3 (1.5 mol%) and 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phenyl-6-phosphaadamantane (75) (3 mol%).144 An example of this methodology is shown in Scheme 23, where 2-iodophenol (76) is carbonylatively crosscoupled with 1-hexyne in the presence of the former catalytic mixture and DBU under a carbon monoxide atmosphere in DMF at room temperature, to give the alkynylated intermediate 77 which suffers cyclisation yielding the final flavone 78. A related procedure, starting from 2-iodoanilines, and using PdCl2(dppp) (1 mol%) as catalyst in the presence of triethylamine as base in toluene at 80 1C under a carbon monoxide atmosphere (5 bar), has allowed the synthesis of quinolin-4(1H)-ones.145 In addition, 1-bromo-2-iodobenzene has been used as starting material in a PdCl2(PPh3)2-catalysed (5 mol%) copper-free Sonogashira coupling–carbonylation– hydroamination sequence leading to 3-methyleneisoindolin-1-ones carried out in phosphonium salt-based ionic liquids.146

Scheme 21

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Scheme 23

The three-component carbonylative Sonogashira process has also been carried out using iodoalkanes, carbon monoxide (45 atm) and terminal alkynes to give alkyl alkynyl ketones, the reaction being performed under photoirradiation conditions using xenon light in the presence of PdCl2(PPh3)2 as catalyst (5 mol%), triethylamine as base and benzene as solvent.147 The carbonylative process can also be carried out avoiding the use of gaseous carbon monoxide. Thus, Mo(CO)6 has been employed as carbon monoxide source in the palladiumcatalysed alkynylcarbonylation of aryl iodides performed using Pd(PtBu3)2 as catalyst and triethylamine as base in acetonitrile as solvent at room temperature, the use of the bulky tritert-butylphosphane ligand proving crucial as the use of PdCl2(PPh3)2 as catalyst required heating at 80 1C.148 A formal inverse copper-free Sonogashira reaction for the direct alkynylation of arenes and heterocycles with alkynyl halides has been developed.149 An example of using this methodology is the alkynylation of indolizine, pyrroloquinoline, pyrroloisoquinoline and pyrrolo-oxazole cores with bromoalkynes catalysed by PdCl2(PPh3)2 (3–5 mol%) using potassium acetate as base in toluene at 30–80 1C.150 This chemistry has also been applied to the regioselective C3-alkynylation of indole (79), which reacted with (bromoethynyl)benzene in the presence of PdCl2(PPh3)2 (10 mol%) as catalyst and sodium acetate as base in THF as solvent at 50 1C, to give the corresponding 3-alkynylated indole 80 (Scheme 24).151 Examples on the use of economically and environmentally convenient aqueous solvent systems in copper-free Sonogashira reactions have been reported. Thus, a sec-butyl amine/water (1 : 1) mixture at room temperature has been found to be an

Scheme 24

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appropriate solvent for the copper-free coupling of aryl and heteroaryl iodides and terminal acetylenes, the reaction being catalysed by PdCl2(PPh3)2 (2 mol%).152 In addition, an acetone/water (1 : 1) mixture at 60 1C has been used for the coupling of the former substrates using palladium(II) acetate (1 mol%) as catalyst in the presence of sodium hydroxide, the coupling of aryl bromides requiring the use of the combination palladium(II) chloride (5 mol%)/triphenylphosphane (10 mol%) as catalyst as well as the presence of piperidine.153 Aqueous solvents have also been found suitable for performing the copper-free Sonogashira transformations when using sterically demanding hydrophilic sulfonates as ligands. Thus, the sulfonated triarylphosphanes TXTPS (47) and the more electron-rich TMAPTS (81) have been employed, combined with palladium(II) acetate (3 mol%) as water-soluble ligands (8 mol%) for the generation of a catalyst suitable for the copper-free cross-coupling of aryl bromides with phenylacetylene, the reaction being carried out in the presence of sodium carbonate as base in a mixture of acetonitrile/water (1 : 1) at 50 1C as solvent.154 Aliphatic alkynes proved unsuccessful using this catalytic system, even adding CuI. In addition, the zwitterionic trialkylphosphonium sulfonates 82 (2 mol%) have been mixed to palladium(II) acetate (2 mol%), and the in situ created catalyst has been employed in the cross-coupling reaction of aryl bromides and terminal alkynes performed in the presence of caesium hydroxide and in acetonitrile/water (1 : 1) as solvent at room temperature, the use of the unreactive 4-chloroanisole in the coupling requires to raise the temperature up to 80 1C.155 Moreover, the water soluble fluorenyl-derived phosphane 83 has been used as ligand (2 mol%) combined with Na2PdCl4 (1 mol%) in the coupling of aryl and heteroaryl bromides, as well as chloropyridines and 2-chloropyrimidine, with terminal acetylenes, the reaction being carried out using potassium carbonate as base and an isopropanol/water (1 : 1) mixture at 90 1C as solvent.156

Examples of copper-free Sonogashira reactions performed using neat water as solvent instead of aqueous organic solvents can also be found. Thus, a mixture of palladium(II) chloride (2 mol%) and triphenylphosphane (4 mol%) has been used for the in situ generation of the active catalyst able to perform the cross-coupling reaction of aryl and heteroaryl bromides with terminal acetylenes in water at 120 1C as solvent, the reaction requiring the addition of pyrrolidine.157 Water has also been used as solvent, although at room temperature, in the coupling Chem. Soc. Rev., 2011, 40, 5084–5121

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Scheme 25

reaction of aryl bromides with terminal alkynes, the employed catalyst being the generated in situ by mixing PdCl2(MeCN)2 (1 mol%) and Xphos (65, 2.5 mol%).158 The reaction was performed in the presence of the lipophilic vitamin E coreincluding nanomicelle-forming amphiphile 84 (3 mol%), triethylamine being the most appropriate of the bases explored. The reaction was very slow (days) when electron-rich aryl bromides were attempted, even though this catalytic combination has been particularly developed for less reactive substrates such as aryl chlorides or tosylates.159 This was improved by using cyclopropylphosphane 85 as ligand instead of Xphos (65), an example of the application of this methodology being the coupling of aryl bromide 86 with undec-10-yn-1-ol to yield alkyne 87 (Scheme 25). A recyclable catalytic system for the coupling of aryl halides with phenylacetylene in water has been developed, using as catalyst the complex generated by mixing palladium(II) acetate (2 mol%) and a 2-aminophenyl diphenylphosphinite ligand 88 (6 mol%). The reaction was performed in the presence of sodium hydroxide, in water at 80–95 1C as solvent, and allowed the copper-free Sonogashira coupling of aryl iodides, bromides and chlorides with phenylacetylene in good to excellent yields.160 The coupling of aryl iodides could be achieved at room temperature by addition of substoichiometric amount of TBAB. The suspended catalyst was recovered after the reaction by centrifugation and reused up to six times without loss of activity. In addition, symmetrical diarylalkynes have been obtained in water as solvent by a palladiumcatalysed decarboxylative coupling of propiolic acid and aryl bromides, using as catalytic system both PdCl2(PPh3)2 (5 mol%)/dppb (10 mol%) or the water-soluble phosphaneincluding mixture PdCl2(TPPMS)2 (2.5 mol%)/TPPMS

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(89, 5 mol%), using DABCO as base and in the presence of octadecyltrimethylammonium chloride as a surfactant.161 Ionic liquids have also been employed as solvents in copper-free Sonogashira reactions in order to achieve a recyclability of the catalytic system once the final products have been extracted. Thus, the cross-coupling of aryl iodides with terminal alkynes have been found to proceed in the ionic liquid [bmim][PF6] at 60 1C, using as catalyst a combination of palladium(II) chloride (2 mol%) and 1,1-bis(diphenylphosphino)cobaltocenium hexafluorophosphate (90, dppc+PF6) (2 mol%).162 No apparent leaching of palladium to the extractive organic phase was observed, and the ionic liquid with the catalysts was reused up to eight times with minimal loss of activity. In addition, aryl iodides and activated aryl bromides have been coupled with terminal alkynes using palladium(II) acetate (4 mol%) and triphenylphosphane (8 mol%) dissolved in N-butylpyridinium tetrafluoroborate at 75 1C in the presence of triethylamine, deactivated aryl bromides such as 4-bromoanisole affording low yields.163 The catalytic mixture in the ionic liquid was reused two times observing a certain loss of activity.

Activated and inactivated aryl bromides have been crosscoupled with phenylacetylene or 1-decyne in good yields in the ionic liquid [bmim][PF6] at 130 1C, using a copper-free Sonogashira protocol that includes the use of the combination Pd(allyl)Cl2 (0.5 mol%)/triphenylphosphane (3 mol%) as catalyst in the presence of pyrrolidine.164 The recycling of the ionic liquid containing the catalyst was carried out four times observing a certain loss of activity, the addition of additional amount of triphenylphosphane being necessary. The activity of this system suggested a molecular nature for the catalyst. Thus, upon recycling, the ionic liquid phase would be a reservoir of palladium atoms in a colloidal form. Moreover, the former palladium source (0.5 mol%), combined with the donor-stabilised phosphenium adduct 91 (3 mol%), has been employed in the coupling of electron-rich and deficient aryl bromides with phenylacetylene in the presence of pyrrolidine, the reaction being performed in [bmim][PF6] at 130 1C.165 In this case, the recyclability of the system also proved difficult, as formation of bis(5-methyl-2furyl)bromophosphane and its corresponding phosphane oxide, product of the C–P bond cleavage through protonation of the carbenic moiety and further oxidation, was detected in the organic phase after the recycling experiments.

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3.2

Supported palladium–phosphorous complexes

The significant costs associated with precious metal catalysts and their undesired tendency to remain in the organic products have generated an increasing interest for developing ways to recover and reuse these metals. Supporting the phosphane ligands to a resin or other solid support would allow the possible recovery of the catalyst by simple means, usually just filtration.166 Difficulties associated with the study of processes involving supported palladium catalysts are being nowadays overcome by the use of techniques such the HRMAS (HighResolution Magic Angle Spinning) NMR.167 3.2.1 Copper-cocatalysed reactions. The Merrifield resin has been used for anchoring a diarylphosphane and to furtherly prepare the supported palladium complex 92, which has been used in low loading (0.05 mol% Pd) in the copper(I)cocatalysed coupling of aryl iodides with phenylacetylene or prop-2-yn-1-ol, in the presence of piperidine in acetonitrile at 80 1C, recycling experiments showing almost no change in catalyst performance after four cycles.168 No structural modifications of the catalyst after use were observed, suggesting that the ‘‘soluble’’ catalytic entities of the reaction are palladium complexes and not nanoparticles.

Silica frameworks show characteristics related to stability and morphology that makes them interesting for supporting a palladium catalyst amenable of performing the different C–C couplings.169 They are usually easy to handle (free flowing, no static charge) and their high density makes them suitable for small volume work, not requiring extensive washing for high recoveries and without the possible sticking to glassware. In addition, they are robust (mechanically and thermally stable), and work well with overhead stirring and stand high temperatures, having low swelling properties, which frequently makes them solvent independent. Thus, the mesoporous silicate MCM-41 has been employed for anchoring a bidentate phosphane palladium(II) creating complex 93 and obtaining the corresponding palladium(0) complex after reduction with hydrazine.170 This supported palladium(0) catalyst has been employed (0.05 mol%) in the coupling reaction of terminal alkynes with aryl iodides in high yields in the presence of CuI, in piperidine at room temperature. The catalyst was reused up to ten times without any decrease in its catalytic activity. This stability was probably due to the chelating action of the bidentate ligand and also because the catalytic palladium was anchored on the inner surface of the mesopore of MCM-41, therefore no strong complexing and solvolytic activity of solvent being present. This catalyst has also been used (5 mol%) in the carbonylative Sonogashira coupling of terminal alkynes with aryl iodides under atmospheric pressure This journal is

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of carbon monoxide, using as solvent triethylamine or aqueous ammonia in THF, the catalysts being reused up to ten times without loss of activity.171

SiliaBondsdiphenylphosphane is a commercial silicaanchored ligand that can be used for the immobilization of palladium(0) by reaction with the appropriate complex such as Pd(PPh3)4, thus producing complexes of the type 94. This supported complex has been used in the one-pot/four step/ palladium catalysed synthesis of indole derivatives, an example being shown in Scheme 26, taking advantage of its high stability together with high reactivity.172 Thus, 2-iodoaniline reacted with TMSA in a copper-cocatalysed Sonogashira coupling to give silylated aryl alkyne intermediate 95, which was silyl-deprotected in situ with TBAF affording ethynylaniline 96. Further Sonogashira reaction with iodobenzene gave intermediate 97 which was cyclised by the addition of palladium(II) chloride to give indole 98. The average yield of each step in the one-pot sequence was estimated in 88%. Pd EnCatt TPP30 is a commercially available palladium(II) acetate/triphenylphosphane mixture microencapsulated in a low-leaching highly crosslinked polyurea matrix, which has been employed as a catalyst in Sonogashira couplings. For instance, Pd EnCatt TPP30 (3.5 mol% Pd) in the presence of CuI and DBU in DMF under microwave irradiation at 100 1C has allowed the formation of enynes from 1,2-dichloroethylenes such as 100, product of the reaction of cis-1,2-dichloroethylene with acetal 99 (Scheme 27).173 Concerning catalyst recycling, no significant loss of activity up to the fifth use was observed, although small quantities of inactive palladium black were observed in the solution, addition of the palladium scavenger resin Quadrapure-TU to the reaction being able to fully eliminate the contamination. In addition, a phosphonated polymer-incarcerated palladium(0) catalyst has been obtained

Scheme 26

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Scheme 27

by microencapsulation of Pd(PPh3)4, followed by crosslinking, and has been used (3 mol% Pd) in the crosscoupling of aryl iodides with terminal alkynes, in the presence of caesium carbonate as base in THF as solvent at 80 1C.174 Palladium leaching was diminished by addition of CuI, and no recycling attempts for this reaction were reported. 3.2.2 Copper-free reactions. The Merrifield resin has been employed for the preparation of the polymer-supported palladium(0) diphenylphosphinoethane complex 101, which has been used as catalyst (1 mol%) for the copper-free coupling reaction of aryl iodides with terminal alkynes in piperidine as solvent at room temperature,175 the polymer being recycled and reused up to five times with almost no loss of activity. Moreover, an amphiphilic polystyrene– poly(ethylene glycol) resin-supported palladium–phosphane complex 102 has been developed, acting as a catalyst (5 mol% Pd) in the copper-free Sonogashira cross-coupling reaction of aryl iodides, and activated aryl bromides with terminal alkynes, inactivated aryl bromides and activated aryl chlorides affording low final yields.176 An environmental advantage of this transformation, added to supporting the catalyst, is that it was performed in water as solvent,177 with temperatures ranging 30–100 1C (depending on the reactivity of the aryl halide) in the presence of triethylamine, and the supported catalyst was recovered by filtration and reused up to four times keeping its activity.

3-Aminopropyl-functionalised silica has been used for creating the appropriate support of the bidentate phosphane palladium(II) complex 93, which has been used as recyclable catalyst (2 mol%) in the copper-free Sonogashira coupling of aryl and heteroaryl iodides and activated aryl bromides with arylacetylenes, the reaction being performed in piperidine at 70 1C.179 Indoles have also been obtained using this catalyst (5 mol%) by a Sonogashira coupling involving 2-iodoanilines, followed by in situ-heterocyclisation in DMF at 100 1C. In both cases, the supported catalyst could be recycled, although a significative drop in reactivity after the fourth run was observed. In addition, a bidentate palladium(II) complex closely related to 93 has been anchored to mesocellular foam-type mesoporous silica powder and has been able to catalyse (0.05 mol%) a copper-free Sonogashira coupling of aryl iodides and activated aryl bromides with phenylacetylene in good yields, the process being performed in the presence of tetra-n-butylammonium acetate (TBAA) in DMF at room temperature.180 Furthermore, other copper-free Sonogashira reaction of aryl iodides and activated aryl bromides with terminal alkynes has been reported using as catalyst the palladium(II) complex supported on commercial silica gel (100–200 mesh) 104 (0.01 mol% Pd), as in the reaction of trifluoromethylated iodoarene 105 with phenylacetylene to give alkyne 106 (Scheme 28).181 The reaction was carried out in the presence of triethylamine as base in ethylene glycol as solvent at 100 1C, and the catalyst retained totally its activity after being reused six times. SiliaCats DPP-Pd is commercially sold as a diphenylphosphane palladium(II) heterogeneous catalyst made from a leach-resistant organically modified silica matrix obtained by

The dendritic palladium(II) complex 103 has been prepared by a covalent grafting via a reaction between carboxylic acid groups of core–shell g-Fe2O3/polymer superparamagnetic nanoparticles and amino groups at the focal point of an amino-terminated dendron.178 Its catalytic activity was investigated in the copper-free Sonogashira coupling (2.4 mol% Pd) of aryl iodides and bromides in methanol or water at 70 1C, using Tritons X-405 as a surfactant and lithium hydroxide as base. The amount of palladium leached to the solution was 2.3% per cycle, the catalysis reactivity reported being not associated with leached palladium (corresponding to 0.05 mol% of Pd of the starting palladium 5098

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Scheme 29

as catalyst in the coupling of terminal alkynes with aryl iodides, bromides and chlorides (the latter in low yields) in a mixture of DMF/water (2 : 1) as solvent at 60–120 1C and using piperidine as base and TBAB as additive.186 However, neat water at 100 1C has been used as the only solvent when employing as catalyst (0.01 mol%) the saccharin-derived palladium complex 113. High yields of the corresponding acetylenic compounds were obtained using this complex when coupling aryl iodides with monoarylacetylenes, but moderate or low results were observed using aryl bromides or chlorides, working in the presence of potassium hydroxide as base and TBAB as additive.187 In addition, a water soluble palladium(II)–salen complex 114 has been used in the copper-free Sonogashira reaction of aryl iodides and terminal alkynes, using caesium carbonate as base and sodium lauryl sulfate as a surfactant, in neat water at 60 1C.188

an encapsulation process, which has been used to perform different C–C-bond forming reactions. Concerning the Sonogashira process, it has been assayed as catalyst (down to 0.002 mol%) in a copper-free almost quantitative coupling of the very reactive 4-iodonitrobenzene with phenylacetylene carried out in the presence of potassium carbonate as base in refluxing ethanol.182

4. Palladium–nitrogen complexes 4.1

Unsupported palladium–nitrogen complexes

4.1.1 Copper-cocatalysed reactions. Palladium(II) complexes with nitrogenated ligands such as PdCl2(PhCN)2 have been used for the creation of enynes from low-reactive and not frequently employed vinyl chlorides. Thus, chloroenyne 107 has been cross-coupled with glycine-derived acetylene 108 using PdCl2(PhCN)2 (2 mol%) and CuI (2 mol%) as catalytic combination, in the presence of piperidine as base in THF at room temperature, to give enediyne 109 (Scheme 29).183 An aqueous mixture (DMF/water 1 : 1) has been used as reaction solvent at 50–90 1C when using as catalyst a palladium(II) complex 110 from an azetidine-derived ligand in the presence of sodium carbonate as base. Aryl iodides, bromides and chlorides were coupled with terminal alkynes using this catalyst in good to excellent yields, although the reaction requires the presence of an stoichiometric amount of CuI.184

4.1.2 Copper-free reactions. Unsymmetrical palladium(II) complexes with benzimidazolium-pyrazole ligands 111 have been prepared and used as catalysts (0.2 mol%), in the coupling of aryl bromides and phenylacetylene, potassium carbonate being employed as base and N,N-dimethylacetamide (DMA) as solvent at 100 1C.185 In addition, a pyridinebis(ferrocene-isoxazole) ligand has been employed for the preparation of palladium(II) complex 112, which has been used This journal is

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Soluble but recyclable palladium–nitrogen complexes have been prepared and used in fluorous biphasic catalysis, allowing the easy separation of the fluorous phase containing the catalyst. Thus, palladium perfluorooctanesulfonate [Pd(OPf)2] catalyses the Sonogashira reaction of aryl bromides or chlorides such as 116 with terminal alkynes to give the corresponding adduct 117 (Scheme 30) in the presence of a perfluoroalkylated pyridine 115 as a ligand in a fluorous system composed of toluene and perfluorodecalin at 80 1C.189 The catalytic system in the fluorous phase has been reused up to five times without a significant yield loss, more of the 99.7% of the palladium and more of the 99.8% of ligand being retained in the fluorous phase.

Scheme 30

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Palladium complex 118 bearing fluorous-ponytails in the structure of a 2,2 0 -bipyridine ligand was employed (1 mol%) in the Sonogashira reaction of aryl iodides or aryl bromides bearing electron-withdrawing groups with phenylacetylene or 1-hexyne, using potassium carbonate or DABCO as bases and performing the process in perfluorinated solvents at 130–140 1C.190 Alternatively, the reaction can be carried out in DMF as solvent at 135 1C, the catalyst precipitating when reaching room temperature. These catalysts have also been used under carbon dioxide pressure-induced conditions due to the CO2-philic nature of the fluorous ponytails.191 Thus, the catalyst, in the presence of triethylamine as base, dissolves under carbon dioxide pressure at 80 1C and catalyses the coupling of iodobenzene and phenylacetylene. Depressurization precipitates the catalyst that is separated from the reaction mixture by centrifugation. However, the recyclability of the system is not very high, the conversion decreasing to half after the second recycling run.

4.2

Supported palladium–nitrogen complexes

4.2.1 Copper-cocatalysed reactions. Examples of the use of supported palladium–nitrogen ligand complexes suitable for performing the Sonogashira reaction have been not rare in recent years. Thus, the polystyrene-supported palladium(II) ethylenediamine complex 119 has been used for the synthesis of heterocyclic systems through heteroannulation of copper-cocatalysed Sonogashira intermediates.192,193 For instance, this polymeric system has been used in the preparation of 2-benzylimidazol[2,1-b]pyridines such as 122 after Sonogashira coupling of activated aryl iodides such as 4-iodobenzonitrile with alkynylated pyridinium salt 120 and

Scheme 31

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further in situ heterocyclisation of the intermediate 121 followed by double bond isomerization (Scheme 31).193 The catalyst could be recovered by filtration and reused up to five times although observing a certain loss of activity, a 0.5 mol% of the palladium being leached to the solution during the course of the reaction. The polystyrene-anchored palladium(II) Schiff base complex 123 has been obtained and employed (3.7 mol% Pd) in the coupling of aryl iodides, bromides and chlorides and phenylacetylene in the presence of CuI and triethylamine in water as solvent at 70 1C, the catalyst being reused up to five times without observing loss of activity or palladium leaching to the solution.194

Superparamagnetic cobalt–iron nanoparticles with a CoFe2O4 structure have been functionalised with Schiff-base groups on the surface to form immobilised bidentate palladium(II) complex 124, which has been used as catalysts for a copper-cocatalysed Sonogashira coupling of iodobenzene with phenylacetylene using potassium phosphate as base in DMF at 70 1C.195 The supported catalyst was recovered by magnetic separation and reused five times without loss of activity, no contribution from homogeneous catalysis of active palladium species leaching into the solution being detected.

4.2.2 Copper-free reactions. A polystyrene-supported palladium(II) N,N-bis(naphthylideneimino)diethylenetriamine complex 125 has been found to catalyse (1 mol%) the copperfree Sonogashira cross-coupling of aryl iodides with terminal alkynes, working in the presence of triethylamine in DMF as solvent at room temperature, the catalyst being reused up to ten times with a slightly low activity.196 In addition, a 2-pyridinealdoxime-based palladium(II) complex 126, anchored via the oxime moiety to a polystyrene phase which is part of a highly megaporous glass/polymer composite matrix shaped as Raschig rings, has been used as catalyst (0.7 mol%) under thermal (100 1C) and microwave irradiating conditions in water in the copper-free cross-coupling reaction of aryl and heteroaryl iodides and bromides with phenylacetylene, using diisopropylamine or sodium hydroxide as base and TBAB as additive.197 Similarly, the 2-pyridinealdoxime motif has been incorporated into the polysaccharide chitosan to generate palladium(II) complex 127 after reaction with palladium(II) acetate, which has been assayed as supported catalyst in the cross-coupling of iodobenzene and phenylacetylene, in the presence of sodium acetate in DMF at 150 1C under microwave irradiation, although resulting poorly selective.198 This journal is

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Scheme 32

A polystyrene-supported palladium(II) 1-phenyl-1,2propanedione-2-oxime thiosemicarbazone complex 128 (0.01 mol%) has been found active for the copper-free coupling of aryl iodides and bromides (much less for aryl chlorides) with terminal acetylenes. The reaction was performed in triethylamine (for iodides) or pyridine (for bromides) at room temperature and the supported catalyst was recycled up to four times with a slight loss of activity.199 This supported palladium(II) complex has been reduced using hydrazine to give a polystyrene resin-supported Pd(0) complex which has been employed as catalyst (1 mol% Pd) in the coupling of acyl chlorides with terminal alkynes in triethylamine at room temperature to give ynones.200 After centrifugation, the supported catalyst was recycled up to four times with a slight decrease in activity. In addition, a 1,10-phenanthroline moiety has been anchored to a porous crosslinked chloromethylated polystyrene monolith filling a capillary column, and used as ligand for the preparation of a supported palladium(II) complex 129, which has been employed in a Sonogashira coupling of iodobenzene and phenylacetylene carried out in a microfluidic device.201 Triethylamine was used as base in this process and DMF/water (4 : 1) as mobile solvent at 80 1C, a 4% palladium leaching being observed after 8 days of continual operation.

in the presence of triethylamine as base and in water at 80 1C, the observed leaching after the fifth run being only 11.6 ppm.202 Highly porous micelle templated structures from spherical silica particles have been employed for the preparation of anchored 3-(aminopropyl)triethoxysilane-derived palladium(II) complex 132, which has been assayed as catalyst (1 mol%) in the copper-free coupling of aryl iodides and phenylacetylene using piperidine as base and solvent at 70–90 1C, few details being given about recycling and loss of activity.203 A similar recoverable catalyst, although 1,2-diaminocyclohexane-derived, supported on amorphous silica gel has been reported and used (1 mol%) in the cross-coupling of aryl iodides and bromides with phenylacetylene, using piperidine as base and solvent at 70 1C, with palladium leaching lower than 0.3 ppm and with a 10% reduction of the catalyst activity after 6 reaction cycles.204

Nanostructured silica has been functionalised with pyridine sites, being transformed, after reaction with palladium acetate, into the supported palladium complex 133 (X = I, Br, OAc).205 This complex has been used as recoverable catalyst (10 mol% of Pd) in the Sonogashira reaction of iodobenzene, 2-iodothiophene and 1-iodonaphthalene with phenylacetylene and p-tolylacetylene, in the presence of triethylamine in acetonitrile at reflux. Low leaching of 0.9 ppm was observed although with a certain catalyst deterioration upon recycling. Moreover, a quinoline-2-carboimine palladium complex 134 immobilised on the mesoporous silicate MCM-41 has been used in the heterogeneous copper-free Sonogashira coupling of aryl iodides and bromobenzene with aryl alkynes by adding piperidine as base in NMP at 80 1C. Moderate results were

\ The basic amine-containing resin Amberlyst A-21 has been used as ligand when reacting with palladium perfluorooctanesulfonate to give a recyclable complex 130, which has been used as catalyst (1 mol%) in the copper-free Sonogashira coupling of aryl iodides, bromides and activated chlorides, with terminal alkynes, as shown in Scheme 32 in the coupling of 4-chlorobenzaldehyde with phenylacetylene to give the corresponding alkyne 131. These reactions were performed This journal is

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observed when 1-hexyne was used as coupling partner.206 The catalyst was reused without apparent loss of activity up to four times, and the addition of water enhanced its reactivity but observing significant palladium leaching. Hybrid silica materials such as 135, containing the di(2-pyridyl)methylamine–palladium dichloride complex, have been used to catalyse (0.2 mol% Pd) the heterogeneous Sonogashira reaction of 4-iodoanisole and 4-bromoacetophenone with phenylacetylene, in the presence of TBAA in DMF as solvent at 110 1C, the formation of palladium nanoparticles not being observed.207 Leaching experiments showed that an homogeneous pathway is, in part, responsible for the reaction, although with some heterogeneous participation.

Scheme 33

6. N-Heterocyclic carbene (NHC) palladium complexes

5. Palladium–P, N, O complexes Palladium(II) b-oxoiminatophosphane complexes such as 136 have been prepared and used in the copper-free Sonogashira cross-coupling reaction of aryl iodides, bromides and even chlorides with terminal alkynes, usually in high yields.208 The reaction was carried out with low catalyst loadings (0.01 mol%) using piperidine as base at 40–50 1C, the amount of catalyst being reduced to 0.0002 mol% in the coupling of iodobenzene and phenylacetylene and still affording 91% yield of the final diphenylacetylene.

The use of N-heterocyclic carbenes (NHCs) as alternative ligands in palladium-catalysed cross-coupling reactions is rapidly gaining in popularity.210 These two electron donor ligands combine strong s-donating properties with a shielding steric pattern that allows for both stabilisation of the metal center and enhancement of its catalytic activity. They generally exhibit high stability, allowing for indefinite storage and easy handling. As a result, the number of well-defined NHC containing palladium(II) complexes is growing, and their use in coupling reactions is witnessing increasing interest. 6.1 Unsupported NHC–palladium complexes 6.1.1 Copper-cocatalysed reactions. The previously prepared N-carbamoyl-1-substituted heterocyclic carbene complex of palladium(II) 140 has been used as precatalyst, in a copper co-catalysed Sonogashira coupling of aryl iodides or bromides and terminal alkynes in DMF as solvent at 25 1C (for aryl iodides) or 80 1C (for aryl bromides), using propylene oxide as an acid scavenger instead of a base.211 The absence of base allows performing the coupling reaction with basesensitive substrates such as the ketone 141, which can be

A supported version of these catalysts has been reported.209 Thus immobilisation of the palladium(II) b-oxoiminatophosphane complex to a magnetic nanoparticle formed by coating commercial iron(III) oxide with silica gave anchored complex 137, which have been used as catalyst (0.5 mol%) in different cross-coupling reactions. Concerning its use in the Sonogashira coupling, when the reaction was performed in the presence of piperidine and TBAB as additive in water at 60 1C, the usually unreactive aryl chlorides were able to cross-couple with terminal alkynes in high yields. An example is shown in Scheme 33, where hindered dimethylated chlorobenzene 138 is coupled with phenylacetylene under these reaction conditions to give the corresponding alkyne 139. The catalyst was recovered from the reaction mixture by magnetic separation and reused without loss of activity after ten reaction cycles, detecting less than 0.06% palladium leaching. No formation of visible nanoparticles was observed on the support surface. 5102

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cross-coupled with iodobenzene giving rise to the corresponding acetylene 142 together with a small amount of the isomerized product 143 (Scheme 34), this last product being obtained exclusively when using triethylamine as base. The use of the trans-(NHC)2PdBr2 complexes 144 as precatalysts (3 mol%) in the presence of CuBr (10 mol%) has allowed the coupling of aryl iodides with terminal alkynes at 100 1C in a polar mixed aqueous media formed by DMF/water (3 : 1) using caesium hydroxide as base.212 In addition, water combined with the nonionic surfactant Brij 30 at 90 1C has been the solvent employed when coupling 4-nitrobromobenzene with phenylacetylene using as catalyst the bis-NHC–palladium complex 145 derived from caffeine (2 mol%) in the presence of potassium hydroxide as base, CuI co-catalysis being necessary.213 Scheme 35

6.1.2 Copper-free reactions. A series of Pyridine Enhanced Precatalyst, Preparation, Stabilisation and Initiation (PEPPSI)214 N-heterocyclic carbene palladium(II) precatalysts 146–149 have been prepared and employed in copper-free Sonogashira coupling reaction.215 Thus, NHC complexes based on the C4–C5 unsaturated imidazole 146, imidazole 147 and triazole 148 proved highly stable and have been used (3 mol%) in the cross-coupling reaction of aryl iodides and activated aryl bromides with phenylacetylene in the absence of copper co-catalysis, using caesium carbonate as base in a

mixed aqueous medium of DMF/water (1 : 1) as solvent at 100 1C.216,217 In addition, a series of PEPPSI themed precatalysts of abnormal NHCs 149 have been prepared and used in a similar copper-free Sonogashira reaction of iodides and activated aryl bromides and arylacetylenes in mixed aqueous medium of DMF/water (3 : 1) at 100 1C.218 Moreover, couplings under similar conditions have been carried out using the trans-(NHC)2PdCl2 complexes 150.214,217 The NHC–palladium complex 151 has been employed as precatalyst (1 mol%) in a domino approach to dihydroisobenzofurans by a sequential Sonogashira/hydroalkoxylation process.219 Thus, iodinated benzyl alcohol 152 was crosscoupled with phenylacetylene to give alkyne intermediate 153 that suffered internal cyclisation to give dihydroisobenzofuran 154 (Scheme 35). When the reaction was performed with a brominated alcohol, the obtained yields were moderate, whereas aryl chlorides gave low yields. The sulfonated imidazolinium salt 155 has been used for the in situ generation of a water-soluble NHC–palladium complex formed after being combined (0.5–1.0 mol%) with Na2PdCl4 (0.25–0.5 mol%) and potassium hydroxide, allowing the crosscoupling of electron-rich aryl bromides and heteroaryl bromides and chlorides with terminal alkynes in high conversions using isopropyl alcohol/water (1 : 1) as solvent at 90–95 1C.220

Pincer complexes are formed by the binding of a chemical structure to a metal atom with at least one carbon–metal bond. Usually the metal atom has three bonds to a chemical

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7. Palladacycles Palladium compounds containing at least one metal–carbon bond intramolecularly stabilised by at least one donor atom, termed cyclopalladated compounds or palladacycles are one of the most popular class of organopalladium derivatives.225 They show interesting mixed characteristics such as high catalytic activity and, at the same time, high stability and are usually encountered as precatalyst precursors of active palladium nanoparticles in many C–C coupling reactions,225,226 such as copper-free Sonogashira couplings. 7.1 Unsupported palladacycles Scheme 36

backbone, enclosing the atom like a pincer. These complexes have become a very promising family of complexes due to an excellent balance between stability and reactivity that enables them to catalyse a good number of organic reactions.221 An example is palladium(II) complex 156 bearing a 3-butyl1-(1,10-phenanthrolin-2-yl)imidazolydinene ligand. This complex has been used as precatalyst (1 mol%), although combined with triphenylphosphane (1 mol%), in a copper-free Sonogashira cross-coupling reaction of aryl iodides and bromides with phenylacetylene or 1-hexyne using caesium carbonate as base and neat water as solvent at 80 1C.222 6.2

Supported NHC–palladium complexes

A couple of polymer-supported NHC–palladium complexes applicable to the Sonogashira reaction have been reported recently. Thus, the dipeptide-containing supported monocarbene palladium complex 157 has been applied (2.5 mol%) to the cross-coupling of activated aryl iodides, such as 1-iodo2-nitrobenzene, with terminal alkynes, such as TMSA, to give the corresponding alkyne 158 (Scheme 36). The reaction was performed in the presence of CuI as cocatalyst using an excess of triethylamine as base in DMF as solvent at 50 1C.223 Once recovered by filtration, this catalyst showed no loss of activity after eight runs. The crosslinked polystyrene-supported NHC–palladium(II) complex 159 has been employed in the copper-free coupling of aryl iodides with terminal alkynes, using as base piperidine or caesium carbonate and as solvent a polar aqueous mixture formed by DMF/water (1 : 1) 60–100 1C.224 This polymer-supported catalyst has consistent activity after four runs, and analysis by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) showed no significant decrease in palladium content on polymer beads after recovery.

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Examples of palladacycles exploited in the copper-free Sonogashira reaction under homogeneous conditions can be seen in the use of cyclopalladated ferrocenylimine 160 (1 mol%) which, in the presence of TBAB as additive and potassium acetate as base in DMA as solvent at 80 1C, has allowed the cross coupling of aryl iodides, aryl bromides and some activated aryl chlorides with terminal alkynes.227 Interestingly, this catalyst is also able to catalyse (1 mol%) the cross-coupling reaction of arylboronic acids with terminal alkynes, using potassium acetate as base in dichloromethane at room temperature, although the presence of stoichiometric amounts of silver(I) oxide is necessary.228 In addition, the orthopalladated palladium(II) complex 161 has been used as catalyst in the coupling of iodobenzene and phenylacetylene using triethylamine as base and using the ionic liquid [bmim][PF6] as solvent at 80 1C, the use of ionic liquid facilitating the recycling of the catalyst with a small decrease in its activity.229

Different pincer complexes have been employed as catalysts in the copper-free Sonogashira reaction. Thus, the PCP pincer complex 162 has been used as catalyst for the cross-coupling of aryl bromides and phenylacetylene in triethylamine as solvent at 90 1C, the TON of the process being just in the range 400–900.230 More efficiency has been shown by the PCP complex 163, which has been used as catalyst in low loading (0.005 mol%) for the cross coupling of aryl or heteroaryl iodides or bromides with aryl or heteroaryl alkynes (TONs up to 20 000) using potassium phosphate as base in ethylene glycol as solvent at 140 1C.231 An example of use of this catalyst is shown in Scheme 37, where 2-bromopyridine is cross-coupled with ethynyltrimethylbenzene 164 to give the corresponding alkyne 165. The PCN pincer palladium(II) complex 166 has been employed as catalysts (2 mol%) in the copper-free coupling reaction of aryl iodides and bromides with phenylacetylene or 1-hexyne performed in the presence of caesium carbonate as base in methanol at room temperature or 70 1C, aryl bromides affording moderate yields.232 Moreover, the coupling of aryl iodides with phenylacetylene in water as solvent at 50 1C has been achieved using the unsymmetrical PCN complex 167 as This journal is

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Scheme 37

complex 171 as reusable catalyst.235 The use of this supported palladacycle (1 mol% Pd) was able to perform the coupling of aryl iodides and bromides with phenylacetylene in high yields, aryl chlorides behaving much poorly. The reaction took place under copper-free conditions, using triethylamine as base and TBAB as additive, and the recovered supported catalyst was reused up to five times showing almost unchanged activity. Moreover, an example of the preparation of an ammonium tagged oxime palladacycle 172 has been reported.236 This species has been used for preparing a clay-nanocomposite by ion exchange of 172 into Na-MMT clay interlayers, creating an organic–inorganic hybrid catalytic system which can act as a precatalyst (0.047 mol% Pd) for the high-yielding crosscoupling of arylacetylenes with aryl iodides and activated aryl bromides, the reaction being carried out in an ionic liquid in the presence of triethylamine at 80 1C. Recycling experiments of this supported palladacycle in the ionic liquid showed just a slight lowering of the reactivity after seven runs, negligible amounts of palladium leaching being detected.

catalyst (2 mol%), although the addition of an stoichiometric amount of pyrrolidine as base was necessary.233

7.2

Supported palladacycles

The polystyrene-anchored palladium(II) azo complex 168 has shown to behave as a heterogeneous catalyst in the copper-free Sonogashira reaction performed in water medium. Thus, aryl iodides and bromides, such as 169, have been cross-coupled with phenylacetylene in the presence of this polymeric catalyst (0.5 mol% Pd), triethylamine as base, TBAB additive as a probable nanoparticle stabiliser and in water at 70–80 1C, to give the corresponding alkyne such as 170 (Scheme 38).234 However, the coupling of chlorobenzene gave rise to a very low yield (6%). The catalyst was recovered after filtration and reused more than six times without loss of activity. Water has also been the solvent suitable for performing the Sonogashira cross-coupling reaction when using the poly(3,6-dibenzaldimino-N-vinylcarbazole)-anchored palladium(II)

Scheme 38

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8. Ligand-free palladium species 8.1 Unsupported ligand-free palladium catalysts The use of simple commercial palladium salts as catalysts in the Sonogashira reaction has advantages related to price avoiding possibly sensitive ligands. Some examples on the use of these catalysts have been reported, exploring their efficiency under very different reaction conditions, although the results are usually limited to the use of aryl iodides. Thus, palladium(II) acetate in a rather high loading (10 mol%) has been employed as catalyst in the presence of diisopropylethylamine for the preparation of a series of derivatives of the non-steroidal anti-inflammatory drug nimesulide using a copperfree Sonogashira reaction, as is the case of the synthesis of alkyne 174, which has been prepared by coupling of iodosulfonamide 173 and tert-butylacetylene (Scheme 39), the reaction being performed in acetonitrile at room temperature.237 Palladium(II) acetate (3 mol%) has also been used as catalyst in the copper-free coupling reaction of aryl iodides with trimethyl(prop-2-yn-1-yloxy)silane, in the presence of potassium carbonate as base in DMF under microwave conditions.238 In addition, aryl iodides have been crosscoupled with phenylacetylene using palladium(II) chloride as catalyst (2 mol%) and triethylamine as base, using as solvent Chem. Soc. Rev., 2011, 40, 5084–5121

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Scheme 39

Scheme 40

biodegradable ionic liquids derived from nicotinic acid under ultrasonic irradiation conditions.239 Moreover, palladium(II) acetate has been used as catalyst (0.01 mol%) in the presence of silver(I) oxide (1.5 eq.) and potassium carbonate as base in acetonitrile at 70 1C for the cross-coupling reaction of boronic acids, such as o-tolylboronic acid, with electron-poor terminal alkynes such as the vinyl amide 175, to give the corresponding alkynylated product 176 (Scheme 40).240 A direct oxidative Sonogashira 2-alkynylation of indoles in the absence of any leaving group has been reported by using potassium tetrachloropalladate as precatalyst (10 mol%) in the presence of caesium carbonate as base and an excess of pivalic acid in DMSO as solvent at 80 1C, under an oxygen atmosphere (1 atm).241 An example of using this methodology is shown in Scheme 41, where N-methyl indole 177 is alkynylated with phenylacetylene to give the 2-alkynylated heterocycle 178. Water at high pressure (16 MPa) and high temperature (250 1C) in a microfluidic system has been used as solvent for the coupling of aryl iodides and phenylacetylene, using palladium(II) chloride as catalyst (2 mol%) and sodium hydroxide as base, quantitative yields being obtained in up to 4 seconds and with turnover frequencies of up to 4.3  106 h1. 242 Recently, no solvent at all has been used in the coupling of aryl iodides with phenylacetylene promoted by palladium(II) acetate (5 mol%) using DABCO as base when performing the process in a planetary ball mill, the coupling of aryl bromides requiring the use of Pd(PPh3)4 as catalyst or a change in the material of the milling beaker or in the filling.243

8.2.1 Copper-cocatalysed reactions. Palladium on charcoal is considered to be a very suitable heterogeneous form of palladium in C–C coupling reactions, particularly for industrial purposes, because of its low cost, availability and stability.244 Although it is in fact an immobilised form of ligand-free palladium, it is frequently used in the presence of soluble phosphane ligands which interact with leached palladium to form palladium(0) complexes that actually catalyse the coupling process. An example of the use of palladium on charcoal in Sonogashira coupling reactions is the coupling of b-chloroacroleins such as 179 with terminal alkynes such as 2-methylbut-3-yn-2-ol, using 10% palladium on charcoal (2.6 mol%) in the presence of CuI and triphenylphosphane in triethylamine as solvent at 80 1C, to give alkynylated chromene-3-carbaldehyde 180 (Scheme 42), the palladium catalyst not being reused.245 The above mentioned combination, Pd/C–CuI–PPh3 in the presence of triethylamine, has been employed in the synthesis of 2-alkynylquinolines from 2-chloro- and 2,4-dichloroquinoline in water as solvent at 80 1C,246 as well as in the preparation of 8-alkynyl-1,2,3,4-tetrahydroisoquinolines from the corresponding 8-iodinated systems in the presence of 2-aminoethanol in water at 80 1C,247 and also in the synthesis of isoquinolones by intramolecular acetylenic Schmidt reaction of alkynylated 2-iodobenzyl azides in ethanol at 80 1C.248 It also has been used in the preparation of 2-substituted 6-oxopyrrolo[3,2,1-ij]quinoline derivatives by alkynylation of 8-iodo-4-oxo-1,4-dihydro quinoline carboxylic acids and intramolecular cyclisation in ethanol at 80 1C,249 or in the synthesis of liquid crystals of trans-cyclohexyltolans by reaction of 1-iodo-4-(trans-4-alkylcyclohexyl)benzene with terminal alkynes in a mixture of acetone/water (5 : 2).250 None of these cases reported recycling of the catalyst. The mesoporous silicate MCM-41 has been used for the immobilisation of palladium(0), formed by soaking in a palladium(II) salt and further reduction with hydrazine, this material being used as supported ligand-free catalyst in several C–C bond forming reactions.251 Thus, the Sonogashira reaction has been carried out using this catalyst in the presence of CuI and TBAA in DMF at 80–100 1C, allowing the coupling of iodobenzene and activated aryl bromides with phenylacetylene in high yields, p-bromoanisole and chlorobenzene affording low yields. The catalytic system proved recyclable up to the fourth reuse in a Heck reaction, no recycling experiments for the Sonogashira coupling being performed. 8.2.2 Copper-free reactions. The use of the bulky ligand Xphos (65, 1 mol%) has allowed us to use 10% palladium on

Scheme 41

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Scheme 43

charcoal (1 mol% Pd) as precatalyst in an efficient copper-free Sonogashira cross-coupling reaction of activated and inactivated aryl chlorides with terminal alkynes, the reaction being performed in the presence of potassium carbonate as base in N,N-dimethylacetamide (DMA) as solvent at 110 1C.252 An example of application of this methodology is shown in Scheme 43, where aryl chloride 181 is cross-coupled with phenylacetylene to give the corresponding alkyne 182. Interestingly, addition of CuI to this reaction mixture, even in a minimal amount, caused disruption to the catalytic activity. The filtered homogeneous reaction mixture showed that the reaction took place after the separation of the supported palladium, proving the presence of leached palladium species. Attempted recycling of the catalyst showed decrease of the activity after each cycle, addition of ligand being necessary in every repeated run. A new form of palladium on charcoal has been prepared by dissolving palladium(II) nitrate in water, addition of activated charcoal, ultrasonication and drying.253 This palladium on charcoal, named UC Pd, has shown to be most active and consistent that many sold it commercially for Sonogashira reactions, allowing the copper-free coupling of aryl bromides with terminal alkynes (2 mol% Pd), in the presence of Xphos (65) as ligand (2 mol%) and potassium carbonate as base in ethanol as solvent at 50 1C. This UC Pd was recycled up to four times with only a slight lowering of the activity, other commercially available Pd/C forms being unreactive after four runs. After filtering the supported palladium, the filtrate showed only traces of leached palladium (2–4 ppm), whereas as above mentioned, addition of CuI drove to losing half of the reactivity of the catalyst. Not many examples amenable to be really called ‘‘ligandfree’’ can be found when using palladium on charcoal as precatalyst. Thus, aryl iodides have been coupled with aromatic and aliphatic terminal alkynes by using 0.4 mol% of the heterogeneous 10% palladium on charcoal in the absence of ligands, using sodium phosphate as base and aqueous 50% isopropanol as solvent at 80 1C.254 However, the recyclability of the catalyst resulted rather low, yield dropping dramatically after the second run. This methodology has also been applied using a low pyrophoric wet-type 10% palladium on charcoal. Phthalides have been prepared by a ligand-free tandem palladium immobilised on carbon nanotubes (C/CNTs)catalysed coupling-cyclisation process, although no mention about recyclability of the supported catalyst was made.255 Thus, o-iodobenzoic acids such as 183 have been coupled with terminal alkynes such as methyl propargylate, using C/CNTs as catalyst (0.1 mol%) in the presence of DABCO and sodium acetate in wet DMF at 100 1C, to give the corresponding phthalide 185 after cyclisation of the Sonogashira intermediate 184 (Scheme 44). In addition, acid chlorides has been coupled This journal is

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Scheme 44

with terminal alkynes in the presence of 10% palladium on charcoal (1 mol%), using triethylamine as base and toluene as solvent at 110 1C. Recycling experiments showed that the catalyst diminished its reactivity ca. 30% after six consecutive runs with a 10% decrease in the palladium content by leaching.256 Moreover, 5% palladium on charcoal in triethylamine as solvent under a carbon monoxide atmosphere (0.5 MPa) at 130 1C has allowed us to achieve the carbonylative Sonogashira coupling of aryl iodides and terminal alkynes, the catalyst being recycled up to three times with no significant loss of activity.257 There are nowadays two commercially available forms of Pd EnCatt deprived of any phosphane ligand (Pd EnCatt 30 and 40, the former being more porous) and both have been used in Sonogashira couplings. Thus, Pd EnCatt 30 (1 mol%) have been employed as catalyst in the coupling of aryl iodides with terminal alkynes, piperidine being used as base in aqueous acetonitrile (1 : 1) at 40 1C.258 After filtration, the catalyst was recovered and reused up to three times with slight decrease in the final yield of the cross-couples product, although the reaction time increased very noticeably. The less porous Pd EnCatt 40 has been reported to perform (0.1–0.01 mol% Pd) the coupling of aryl iodides and a few aryl bromides using pyrrolidine as solvent at 85 1C, microwave irradiation accelerating the process.259 The catalyst was reused up to three times without loss of activity. Palladium(II)-exchanged mesoporous sodalite and NaA zeolite have been assayed as heterogeneous catalysts (1 mol% Pd) in some cross-coupling reactions such as the Sonogashira coupling of bromobenzene with phenylacetylene or triisopropylsilylacetylene in the presence of sodium carbonate as base in DMF/water (4 : 1) as solvent at 80 1C.260 The recovered catalyst was maintained during five recycling experiments and showed no palladium leaching. The presence of air proved crucial, and it was proposed that palladium(0) species, which catalyse the reaction, are generated in situ and immediately oxidised back to palladium(II) by oxygen, thus preventing the formation of palladium aggregation. Polyionic Amberlite resin formate derived from commercially available Amberlite resin chloride by rinsing with aqueous formic acid has been soaked with palladium(0) from palladium salts, the formate counteranion being the reducing source.261 This resin supported with palladium has been attempted as ligand-free catalyst in several C–C coupling Chem. Soc. Rev., 2011, 40, 5084–5121

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processes such as the Sonogashira reaction. Thus, several deactivated aryl iodides have been cross-coupled with phenylacetylene or 1-ethynyl-4-methoxybenzene using this supported catalyst (0.01 mol% Pd) in the presence of triethylamine in acetonitrile as solvent at room temperature. Recycling experiments were not carried out on these Sonogashira processes, but in a Suzuki–Miyaura reaction in DMF as solvent, results not being easily extendable as discharge of palladium from the polymer surface generally depends on the nature of the solvent.

9. Palladium nanoparticles as catalysts Metal nanoparticles have attracted much attention in the last few years owing to their relatively high chemical activity and specificity of interaction. One of the reasons for the rapidly developing field of nanoparticle research is the distinctly differing physicochemical properties presented by metal nanoparticles compared to their bulk counterparts due to their large surface-to-volume ratio, providing many highly active metal uncoordinated sites. Among these metallic species, palladium nanoparticles are particularly interesting due to their applicability to all the array of palladium-catalysed reactions, particularly the C–C couplings.262 Although palladium(0) nanoparticles can be the real catalyst in many of the processes described in former sections as a consequence of decomposition of the original palladium salt or complex, perhaps just acting as a reservoir for molecular palladium species, this section will deal with the use of specifically created and identified palladium(0) nanoparticles in the Sonogashira cross-coupling reaction. As mentioned, the question if palladium-catalysed crosscoupling reactions are achieved by the palladium nanoparticles themselves or by molecular palladium species in solution is always present and has been addressed by using a simple approach based on using a special membrane reactor, this membrane allowing the passage of palladium atoms and ions, but not of species larger than 5 nm.263 Using this procedure, it has been shown that palladium atoms and ions do leach from palladium clusters under Heck and Suzuki coupling conditions. It has been proposed that two leaching mechanisms can occur, depending on the reaction conditions used. In the absence of any oxidising agent, palladium(0) atoms leach from the clusters. Conversely, in the presence of an aryl halide such as iodobenzene, palladium(II) complexes can form by oxidative addition, either on the cluster surface or through reaction with palladium(II) atoms that have already been leached into solution. Both palladium(0) atoms and palladium(II) complexes can then enter the cross-coupling catalytic cycle and the remaining palladium clusters would not be catalytically active species. 9.1

palladium(0) nanoparticles. Thus, star-shaped heavily fluorinated aromatic sulfurs have been prepared and used as stabilisers of palladium nanoparticles active in the coupling of 4-iodoanisole and phenylacetylene, the process being carried out in the presence of CuI and triphenylphosphane, using potassium carbonate as base in ethanol at 80 1C.264 Hollow palladium–iron nanospheres have been obtained through a vesicle-assisted chemical reduction method and employed as catalyst in several palladium-catalysed coupling reactions.265 Thus, the Sonogashira coupling of aryl iodides and terminal alkynes has been carried out using this catalyst (0.02 mol% Pd) in the presence of CuI, triphenylphosphane and potassium carbonate as base in water at 80 1C. The catalyst could be separated from the reaction mixture by a magnet and reused up to five times without loss of activity. In addition, bimetallic hollow palladium–cobalt nanospheres have also been obtained and have been used as catalysts (0.02 mol%) in the cross-coupling of aryl iodides, as well as bromo- and chlorobenzene, with terminal acetylenes.266 The reaction was also performed using copper(I) co-catalysis in the presence of triphenylphosphane as ligand, using potassium carbonate as base in water at 80 1C, and the catalyst was separated by centrifugation and reused up to seven times. 9.1.2 Copper-free reactions. Homogeneous copper- and ligand-free Sonogashira cross-couplings have been carried out using different procedures in order to stabilise generated palladium(0) nanoparticles. Thus, oil-in-water microemulsions (ME) formed by the mixture Tritons X100/n-heptane/ n-butanol/water/PEG at 80 1C have been employed in the fast cross coupling of aryl iodides, such as 186, with phenylacetylene (5 min) catalysed by palladium(0) nanoparticles generated by combining palladium(II) chloride (0.5 mol%) and sodium hydroxide, yielding the corresponding alkynes such as 187 (Scheme 45).267 However, aryl bromides performed poorly under these conditions. Catalytically active palladium(0) nanoparticles have been synthesised in water by reduction of potassium tetrachloropalladate(II) with a Fisher carbene complex in the presence of PEG as nanoparticles stabiliser. This nanoparticle solution has been used as catalyst (0.01 eq. with PEG6000 1.0 eq.) in the coupling of activated aryl iodides with terminal alkynes, using potassium carbonate as base in water at 55 or 65 1C, no final product being detected when starting from aryl bromides.268 Palladium(0) nanoparticles have also been created by irradiating palladium(II) acetate (2 mol%) with ultrasounds in the presence of TBAA, and have been used as catalyst in a one-pot synthesis of substituted benzofurans by a Sonogashira coupling-5-endo-dig cyclisation of 2-iodophenols, the reaction being carried out in acetonitrile at room temperature.269 This one-pot coupling-cyclisation process for the preparation of benzofurans has been performed using as catalyst

Unimmobilised palladium nanoparticles as catalysts

9.1.1 Copper-cocatalysed reactions. Although one of the main interests in using palladium(0) nanoparticles in the Sonogashira process lies in their high reactivity, and therefore the suitability of performing copper- and ligand-free couplings, there are examples where copper(I) and a phosphane ligand have been added to a homogeneous reaction catalysed by 5108

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Scheme 46

palladium(0) nanoparticles generated in water by reduction of Na2PdCl4 with sodium dodecyl sulfate (SDS), which also acted as a stabiliser.270 Thus, benzofurans such as 190 have been obtained after coupling of 2-iodophenols such as 188 with terminal alkynes such as phenylacetylene in the presence of the palladium nanoparticles and triethylamine in refluxing water at 100 1C, the Sonogashira intermediate 189 being cyclised in situ to give the final heterocycle (Scheme 46). The aqueous suspension containing the catalyst after the reaction was recycled for three runs with gradual loss of efficiency due to agglomeration of the nanoparticles after each cycle. Water has also been the solvent for a Sonogashira reaction catalysed by b-cyclodextrin-capped palladium nanoparticles obtained from Na2PdCl4 and perthiolated b-cyclodextrin.271 Using this catalytic species (10 mol%), the coupling of iodobenzene and 1-iodonaphthalene with aryl alkynes was achieved in high yields, bromobenzene giving moderate results. The process was carried out in the presence of a big excess of diisopropylamine as base in water at room temperature. The role of the cyclodextrin is suggested to be facilitating the inclusion of hydrophobic species inside the cavity bringing the molecules to the vicinity of the palladium surface. Moreover, another example of the use of cyclodextrin (CD)-capped palladium nanoparticles has been reported. Thus, hydroxypropyl-a-cyclodextrin-capped palladium nanoparticles have been obtained by addition of a-cyclodextrin partially O-hydroxypropylated to an aqueous solution of palladium(II) chloride.272 The active palladium(0) generated [Pd(0)/CD] was used as catalyst (0.5 mol%) in the coupling of aryl iodides, such as 4-iodoanisole, or bromobenzene with phenylacetylene or 1-octyne, in the presence of triethylamine in water at 60–70 1C to give the corresponding coupled alkynes such as 191 (Scheme 47). It is known that the use of ionic liquids as solvents allows stabilisation of metal nanoparticles,273 although examples of their use in Sonogashira couplings are rather limited. Thus, a basic ionic liquid at room temperature has been explored as medium for the generation of palladium(0) nanoparticles from PdCl2(PPh3)2 with a small amount of piperidine or methanol, attempting the coupling of iodobenzene or 4-iodoanisole with phenylacetylene, although the homocoupling of the alkyne as side-reaction was observed in 10–40%.274

Scheme 47

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Palladium nanoclusters have been generated dissolving palladium(II) chloride in a mixed solvent (acetonitrile/ methanol 1 : 1) in the absence of any stabiliser, and have been employed (5 mol% Pd), in the presence of potassium carbonate at room temperature, for the cross-coupling reaction of aryl iodides with terminal acetylenes, leaching of palladium being observed by TEM images during the catalysis.275 In addition, palladium(0) nanoparticles with nanobelt, nanoplate and nanotree morphologies have been prepared from palladium(II) chloride using vitamin B1 as a reducing agent in water at room temperature without any capping agent to prevent agglomeration. This palladium species were used in the Sonogashira coupling of aryl iodides and bromides in good yields, working in the presence of pyridine as base in acetonitrile as solvent under microwave irradiation.276 Trimetallic Au/Ag/Pd nanoparticle-based catalyst have been obtained and used (0.5 mol%) in the Sonogashira reaction, the coupling of aryl iodides, bromides and chlorobenzene with phenylacetylene being performed in the presence of potassium carbonate in aqueous DMF as solvent at 120 1C.277 The nanoparticle catalyst were collected by ultracentrifugation and reused up to three times without losing the catalytic activity. 9.2 Immobilised palladium nanoparticles as catalysts Immobilisation of the metal nanoparticles on heterogeneous supports allows exploitation of the special properties that occur at this size regime. The fusion between porous materials and nanoparticles technology gives the potential for increased efficiency from nanoparticle catalysis, combined to the advantages of heterogeneous supports concerning possible catalyst recovery.278 Therefore, research on the preparation and use of supported palladium nanoparticles as catalysts in processes such as the Sonogashira reaction has been frequent in the very recent years, although in most cases only aryl iodides have been reported as coupling partners. 9.2.1 Copper-cocatalysed reactions. A carbon aerogel has been doped with palladium(0) nanoparticles and has been used as catalyst (6 mol%) in the Sonogashira cross-coupling of aryl iodides with terminal alkynes, the reaction being performed in the presence of CuI and triphenylphosphane and using diisopropylamine as base in DMF at 100 1C.279 No appreciable leaching of palladium was observed after fifteen cycles and no decrease in the catalytic activity was detected. Nitrogen-doped magnetic carbon nanoparticles have been prepared using iron-doped polypyrrole nanoparticles as the carbon precursor, after carbonisation at 800 1C. To the resulting nitrogen-including carbon nanoparticles was added palladium(II) nitrate followed by calcination in a hydrogen atmosphere. This material was used as catalyst (1 mol% Pd) in several C–C bond forming processes such as the Sonogashira reaction, which was carried out by coupling 4-bromoacetophenone with phenylacetylene under copper-cocatalysis, using sodium carbonate as base in DMSO as solvent at 100 1C in the presence of CuI.280 The recovery of the supported catalyst was performed by magnetic separation, being reused up to three times without loss of activity. Chem. Soc. Rev., 2011, 40, 5084–5121

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9.2.2 Copper-free reactions. Poly(styrene), poly(styrene-co4-vinylphenylboronic acid) and poly(styrene-co-4-vinylbenzoic acid) have been loaded with palladium(0) nanoparticles generated by reduction of palladium nitrate using g-irradiation. The catalytic efficiency of these palladium species was tested in the Sonogashira coupling reaction of iodobenzene or 2-iodothiophene with phenylacetylene, being comparable to when using commercial 10% palladium on charcoal, although no recycling experiments were reported for this reaction.281 In addition, palladium nanoparticles have been entangled in resin plugs following a procedure consisting of loading an aminomethylated styrene resin with palladium(II) acetate, reduction to palladium(0) with hydrazine and cross-linking and entanglement by reaction of the amino groups with succinyl chloride.282 The resulting plugs were used as supported catalysts (9.8 mol% Pd) in several palladium-catalysed C–C bond forming reactions such as the Sonogashira coupling of aryl iodides and phenylacetylene in the presence of potassium carbonate in DMF as solvent at 80 1C. Recycling experiments showed that yields remained unaltered after four runs, leaching determinations after a Heck reaction in DMF at 115 1C showing 0.37 ppm of palladium(0) content in the solution at the fifth run. Recently, palladium nanoparticles were entrapped in gelatin after reduction of palladium(II) salts with this natural material.283 These immobilised nanoparticles were used in the coupling reaction of various aryl iodides, bromides and chlorides as well as heteroaryl halides and also b-bromo styrene with phenylacetylene. The reactions were carried out at 100 1C in molten TBAB or PEG400 in the presence of potassium acetate as a base. Recycling experiments showed a considerable increment in reaction time in the third run which was observed when using molten TBAB as solvent (31% Pd leaching), much less being observed when using PEG400 (12% Pd leaching). Mesoporous cubic carbon containing palladium(0) nanoparticles formed by reduction of palladium(II) with sodium borohydride was studied as supported catalyst in the coupling of iodobenzene and phenylacetylene using different reaction conditions.284 Considerable palladium leaching was observed, being largely dependent on the base nature. Thus, the use of triethylamine revealed very poor recyclability, whereas when sodium acetate was used as base, the reusability was much better. This was not due to the extent of palladium leaching, as it was demonstrated that leaching is more important when using sodium acetate as base, probably because acetate ions bind strongly to soluble palladium(II) created by leaching of the palladium nanoparticles to the solution. Presumably the thus formed palladium(II) complexes are less prone to aggregate to form inactive palladium black than when low coordinating bases such as triethylamine are used. Interestingly, the obtained yields were much higher when using the supported palladium(0) nanoparticles than when using suspended nanoparticles. This would suggest that the palladium(0) loaded support would be a reservoir for the active catalyst, as its interfacial area between the palladium and the reaction mixture results much larger than in suspended nanoparticles which have tendency to aggregate.285 Palladium(0) nanoparticles formed from palladium(II) acetate in electrospun polyacrylonitrile nanofibers by a carbonisation 5110

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process were explored (0.5 mol% Pd) in the coupling of iodobenzene and phenylacetylene showing high catalytic reactivity and high leaching resistance.286 The reaction was carried out in the presence of sodium phosphate as base in refluxing isopropanol, and recycling experiments showed unaltered yield (85%) of the final coupled product after ten runs, together with 100% retrieval of the nanofibers. Porous materials derived from starch have been used for supporting palladium(0) nanoparticles, being used as catalyst in C–C-coupling reactions.287 In the case of the Sonogashira reaction, this was performed by coupling iodobenzene and phenylacetylene in the presence of DABCO under microwave irradiation, no recycling experiments for this reaction being reported. Furthermore, palladium nanoparticles generated by reduction of palladium(II) chloride with sodium borohydride have been deposited on mesoporous carbon formed by pyrolysis of a polymer-coated silica template, this palladium-containing material being used as catalyst in the cross-coupling of iodobenzene and phenylacetylene.288 The coupling was performed in the presence of sodium acetate in aqueous DMF as solvent, but the yield of diphenylacetylene obtained was just a 48% with deactivation of the catalyst by formation of large polydisperse crystalline palladium particles. Perfluoro-tagged palladium nanoparticles immobilised on silica gel through fluorous–fluorous interactions or linked to silica gel by covalent bonds have been used as catalysts (0.1–0.5 mol% of Pd) in the alkynylation of terminal alkynes with a variety of aryl iodides and some aryl bromides, in the presence of pyrrolidine in refluxing water as solvent.289 In the case of the nanoparticles supported to the silica by direct fluorous–fluorous interactions, high levels of palladium (39–240 ppm) were found in the crude products due to the relative weakness of the fluorous–fluorous interactions, and reusing was limited to four runs. However, when the fluorinated system linked to the silica gel 192 was used as support for the nanoparticles, the catalytic system allowed for a number of recover cycles largely higher (eleven runs) than in the previous case. An example of using this supported system is shown in Scheme 48, with the coupling of 4-iodoanisole with terminal alkyne 193 to afford the disubstituted alkyne 194. A metal–organic framework (MOF) of Zn4O-clusters and benzene-1,4-dicarboxylate linkers forming an open cubic

Scheme 48

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Scheme 49

network has been used for supporting palladium(0) nanoparticles after soaking the MOF with palladium(II) chloride and further reduction with hydrazine.290 This nanoparticlecontaining material has been used as catalyst in the Sonogashira reaction of aryl iodides and terminal alkynes, performing the coupling in the presence of potassium phosphate as base in methanol as solvent at 80 1C. The activity of the recycled catalyst decreased noticeably after the third run, deactivation of the catalyst being attributed to palladium oxidation to palladium(II), as determined by XPS, and not to palladium leaching. Dendritic nanoferrites with a micro-pine morphology have been prepared. Functionalisation with dopamine followed by reaction with Na2PdCl4 and reduction with hydrazine has allowed obtaining a material with palladium(0) nanoparticles at the tips of amino-functionalised micro-pine ferrites,291 which is suitable for palladium-catalysed coupling reactions. Thus, the Sonogashira cross-coupling has been carried out using this material, allowing the coupling of aryl iodides and bromides in good yields in the presence of potassium carbonate and pyridine, in DMF as solvent at 100 1C under microwave irradiation. Leaching and recyclability were analysed performing Heck couplings under similar reaction conditions, showing negligible palladium leaching probably due to the well defined amine binding sites located on the surface of the micro-pine ferrites. Impregnation of superparamagnetic Fe3O4 nanoparticles with palladium(0) nanoparticles, generated from a palladium(II) salt and reduction with potassium borohydride, has allowed the preparation of a magnetically separable palladium catalyst that has allowed us to perform the carbonylative Sonogashira coupling reaction for the synthesis of a,b-alkynyl ketones.292 Thus, aryl iodides such as 195 reacted with terminal alkynes such as phenylacetylene in the presence of the palladium catalyst (1 mol% Pd) and triethylamine under an atmosphere of carbon monoxide (2.0 MPa) in toluene at 130 1C, to afford the corresponding alkynylated ketone 196 (Scheme 49). The catalyst was magnetically separated and reused seven times with a slight loss of activity, the operating mechanism being quasi-homogenous and catalysed by small amounts of palladium species in solution.

10.

Other metal-based catalysts and cocatalysts

10.1

Non-palladium-based catalysts

Based mainly on economic considerations, different attempts have been made to achieve the Sonogashira reaction in the absence of the expensive noble metal palladium and using instead a cheaper metal.293 Thus, although there are examples of the use of metals such as samarium powder as catalyst in poly(ethyleneglycol) (PEG),294 other transition metal-based This journal is

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catalysts have experienced a much higher interest. Among them, research on the use of different copper-based catalysts has been rather frequent in recent years.4 Thus, the combination of CuI (5 mol%) and triphenylphosphane (15 mol%) using potassium hydroxide as base in water as solvent at 140 1C has allowed the cross-coupling of aryl iodides and terminal alkynes.295 The same combination of copper salt and ligand but using potassium carbonate as base and TBAB as additive, in water as solvent at 120 1C, has been used for the coupling of aryl iodides and terminal alkynes.296 In addition, a solventfree mixture of octahedral CuO nanoparticles (10 mol%), triphenylphosphane (20 mol%) and TBAB using potassium carbonate as base at 135–140 1C has been employed as reusable system for the cross-coupling of aryl and heteroaryl iodides, bromides and chlorides and terminal alkynes.297 Moreover, octahedral and rod-like g-CuI nanocrystals have been used as catalysts (10 mol%), combined with triphenylphosphane in the presence of potassium carbonate, in the cross-coupling reaction of 4-iodoanisole and phenylacetylene performed in PEG400 as solvent at 130 1C.298 The phosphane ligands for the copper have been substituted more often by other compounds such as amines. Thus, the combination CuBr (20 mol%)–DBU (20 mol%) with potassium carbonate as base in DMF at 140–145 1C has been used for the coupling of terminal alkynes with aryl iodides.299 In addition, the mixture CuI (10 mol%)–DABCO (20 mol%) and caesium carbonate as base in DMF as solvent at 135–140 1C has allowed us to perform the Sonogashira coupling of electron-rich and electron-deficient aryl iodides or bromides to aryl and alkyl acetylenes in high yields, aryl chlorides giving rather low results.300 Also the use of CuI (10 mol%) but in the presence of N,N-dimethylglycine hydrochloride (30 mol%) as ligand, caesium carbonate as base and dioxane as solvent at 80 1C has allowed us to use vinyl iodides to prepare conjugated enynes such as 198 that results from the coupling of phenylacetylene and ethyl (Z)-3-iodoacrylate (197) (Scheme 50).301 However, the combination of CuI (10 mol%) and L-proline (30 mol%) in the presence of potassium fluoride in alumina in DMF at 110 1C has resulted effective in the coupling of aryl iodides with terminal alkynes.302 L-Proline has also been used as ligand in a silica-anchored proline–copper(I) catalyst, which has been used for the coupling of terminal alkynes and aryl iodides and bromides, the catalyst being recovered by filtration and reused up to six times.303 Moreover, o-iodoacetanilide

Scheme 50

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derivatives such as 199 have been coupled with alkynes such as 1-octine using a CuI (30 mol%)–N-methylpyrrolidine-2carboxamide hydrochloride (100 mol%) system in the presence of caesium carbonate as base in DMF to give the corresponding product 200 at room temperature (Scheme 50), this reaction temperature being the lowest reported for copper-catalysed Sonogashira reactions.304 Furthermore, the use of a catalytic amount of the N,N 0 -dibenzyl BINAM–CuI complex (20 mol%) in the presence of potassium carbonate as base in DMF as solvent at 140–145 1C has allowed us to perform efficiently the Sonogashira coupling of aryl iodides and terminal alkynes, although the few examples assayed with aryl bromides afforded moderate yields.305 Simple copper(II) acetate has been reported to catalyse the Sonogashira reaction of aryl iodides or electron-deficient aryl bromides and terminal alkynes, although the use of high copper loadings (50 mol%) and triethylamine as solvent at 140–150 1C is necessary.306 Also, copper(II) acetate (15 mol%) is able to act as catalyst in the cross-coupling of 4-iodoanisole and phenylacetylene when combined with ()-sparteine (30 mol%), using potassium carbonate as base in DMF or DMSO as solvent at 130 1C.307 Some copper complexes have been used in Sonogashira reactions. Thus, the copper(II) complex [Cu(DMEDA)2]Cl2H2O (0.5 or 0.05 mol%) (DMEDA = N,N 0 -dimethylethylenediamine) has been employed to catalyse the coupling of aryl iodides with aryl alkynes, the reaction being carried out in the presence of a large excess of the ligand DMEDA (30 mol%), caesium carbonate as base in dioxane at 135 1C.308 This process is an interesting example of ligand-accelerated catalysis, in which the addition of DMEDA converts the resting state of the catalyst into a very active monomeric copper(I) acetylide species. In addition, the system CuBr (20 mol%)/rac-BINOL (20 mol%), in the presence of caesium carbonate and in DMF as solvent at 130 1C has been employed in the cross-coupling of aryl iodides and bromides and terminal alkynes, usually achieving moderate yields.309 In addition, a bidentate complex formed by combining salicylic acid (20 mol%) and CuCl2 (20 mol%), in the presence of caesium carbonate as base and in DMF as solvent at 130 1C, has been used for the coupling of aryl and heteroaryl iodides with terminal alkynes in good yields.310 Moreover, the combination of copper(II) acetylacetonate (10 mol%) and 1,3-diphenylpropane-1,3-dione (30 mol%) has been used, in the presence of potassium carbonate as base and in DMF as solvent at 90–120 1C, for the cross-coupling reaction of aryl iodides and aryl- and alkyl-alkynes.311 The use of a N,O-bifunctional copper catalyst in the crosscoupling reaction of aryl and heteroaryl halides with terminal alkynes has been reported to furnish moderate to high yields of the corresponding Sonogashira adducts, except in the case of aryl chlorides.312 This catalyst 201 has been generated by combining CuI (10 mol%) and 8-hydroxyquinoline (20 mol%), the Sonogashira coupling being carried out using caesium carbonate as base and DMF as solvent at 110–130 1C. The use of this catalyst has allowed obtaining directly the 2-phenylbenzofuran derivative 203 when coupling 2-iodophenol (76) and phenylacetylene, the reaction proceeding through intermediate 202 (Scheme 51). Other N,O-bifunctional copper 5112

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Scheme 51

catalysts have been generated by mixing 10 mol% of CuI/rac-BINOL/1,10-phenanthroline, and used in the crosscoupling of 4-iodoanisole and phenylacetylene.312 In addition, copper bis(2,2,6,6-tetramethyl-3,5-heptanedioate) has been reported to catalyse (5 mol%) the carbonylative Sonogashira coupling reaction of terminal alkynes with iodoaryls, the reaction being performed under 20 atm pressure of carbon monoxide in the presence of triethylamine in toluene as solvent at 90 1C.313 Ligand-free procedures involving copper salts as catalysts have also been explored. Thus, a catalytic system composed of CuI (10 mol%), potassium carbonate and poly(ethyleneglycol) (PEG) has been developed to perform the Sonogashira coupling of aryl iodides and phenylacetylene under microwave irradiation.314 CuI (0.5 mol%) has also been used for preparing furo[2,3-h]quinolines by a one-pot procedure involving sequential Sonogashira coupling of arylacetylenes with 7-iodo-8-hydroxyquinolines and further cyclisation, the reaction being carried out when supported on basic alumina under microwave irradiation.315 In addition, CuI (20 or 100 mol%) has been employed in the heterocyclisation reaction of b-iodopropenoic acid derivatives, such as 204, with terminal alkynes such as phenylacetylene to afford g-alkylidenebutenolides such as 206, which are obtained via 5-exo-dig cyclisation of the corresponding Sonogashira enyne intermediate 205 (Scheme 52).316 Moreover, a Sonogashira-type coupling of arylboronic acids with terminal alkynes has been carried out using ligand-free CuI in the presence of Ag2O and using caesium carbonate as base.317 Furthermore, CuO and copper metal highly dispersed on inert oxides (silica, alumina) have

Scheme 52

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been employed as precatalyst in the ligand-free Sonogashira coupling of aryl iodides and electron-rich alkynes.318 Nonetheless, behind all these copper-catalysed methodologies lingers the question of whether the Sonogashira crosscoupling reaction has resulted as a consequence of minute amounts of palladium contaminants,293 as it has been demonstrated that ppb levels of palladium impurities have a dramatic effect in a reported so-called copper-catalysed reaction.319 Thus, the use of Cu(PPh3)2NO3 [prepared from Cu(NO3)2 99.999% purity] as catalyst, using potassium carbonate as base in toluene at 110 1C, gave negligible results in the coupling of iodobenzene and phenylacetylene, a complete coupling being observed adding only 10 ppb of PdCl2(PPh3)2. The authors also observed that palladium contamination of CuI, caesium carbonate, phenylacetylene and even in the stirring bar caused high conversions, supposing that commercially available copper, carbonate, amine, acetylene materials and equipment can supply enough palladium for these ‘‘copper-catalysed’’ couplings. However, the presence of copper resulted important, playing a role in the formation of the copper acetylide species incorporated by transmetalation into the palladium catalytic cycle. In addition, amines will support the formation of monomeric copper acetylides from polymeric reservoirs by coordination, increasing their solubility, as well as in the deprotonation of acetylenes. Anyway, although extreme caution should be taken when naming a process as ‘‘copper-catalysed’’ (or ‘‘palladiumfree’’), exploiting the Sonogashira process under these conditions using only ppb of palladium would offer an applicable and economic alternative to traditional ‘‘palladium-catalysed’’ procedures. Iron would be another of the most interesting cheap metal candidates to be used as catalyst in the Sonogashira crosscoupling reaction and research on this area has been carried out. Thus, a 1 : 1 combination of iron(II) acetylacetonate and CuI (20 mol%), in the presence of potassium phosphate as base in DMSO as solvent at 140 1C, has been used for the coupling of aryl iodides and terminal alkynes in good yields, moderate results being obtained when using aryl bromides.320 The same iron/copper combination (10 mol%), but employing caesium carbonate as base and N-methylpyrrolidone (NMP) as solvent at 140 1C, has been used in the cross-coupling of aryl iodides and terminal alkynes, no reaction being observed using an aryl bromide or chloride.321 Moreover, iron(III) oxide (10 mol%) combined with copper(II) acetylacetonate (10 mol%) in the presence of TMEDA (20 mol%) performed the Sonogashira coupling of aryl and heteroaryl iodides and terminal alkynes when using caesium carbonate as base in DMF at 135 1C.322 Of course, behind these iron/copper catalytic combinations remains the question commented above in the case of copper-catalysis about the real influence of palladium impurities in the coupling process. Other iron-based procedures in the absence of copper have also been developed. Thus, iron(III) acetylacetonate (10 mol%) combined with 2,2 0 -bipyridine (20 mol%) as ligand has been found as a suitable catalytic mixture for the cross-coupling of terminal alkynes with aryl iodides, working in the presence of caesium carbonate as base in toluene at 135 1C.323 In addition, iron trichloride (15 mol%) in conjunction with DMDA (30 mol%), in the presence of caesium carbonate as base in This journal is

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Scheme 53

toluene at 135 1C, allows the coupling of aryl iodides with alkyl- and aryl-acetylenes, as well as TMSA, this last case illustrated in the coupling of 1-chloro-2-iodobenzene with TMSA to give the silylated aryl acetylene 207 (Scheme 53).324 Moreover, triphenylphosphane has been employed as ligand (30 mol%) combined with iron trichloride (15 mol%) in the coupling of aryl and heteroaryl iodides with terminal alkynes, using potassium phosphate as base and toluene as solvent at 135 1C.325 However, caution is necessary, as these iron trichloride-catalysed Sonogashira couplings could be influenced by the known presence of metal impurities in the iron salt (copper for instance), as shown in iron trichloride-promoted N-, O- and S-arylation reactions,326 or even by the presence of very small quantities of palladium, as it has been suggested in related iron-catalysed Suzuki couplings.327 Nanoparticles of paramagnetic magnetite (Fe3O4) have been reported to act as efficient catalysts (5 mol%) for carbon– carbon bond formation via the Sonogashira reaction under heterogeneous ligand-free conditions in ethylene glycol at 125 1C. By using this catalyst, arylalkynes have been produced from the reaction of aryl iodides and activated heteroaryl bromides with terminal alkynes.328 The catalyst was separated after the reaction by an external magnetic field and reused for five consecutive runs without loss of catalytic activity. Adding different amounts of palladium and copper impurities did not affect significantly the final yield, but the addition of different amounts of the iron nanoparticles affected noticeably, suggesting that the iron nanoparticles are really responsible for the catalytic activity. Other catalysts based on cobalt and nickel have also been developed for the use in Sonogashira reactions. Thus, cobalt hollow nanospheres (3 mol%) in the presence of triphenylphosphane (10 mol%), CuI (2 mol%) and potassium carbonate in NMP at 120 1C have been used in the coupling of aryl iodides or bromobenzene and terminal alkynes, the cobalt nanoparticles being recovered and reused up to three times.329 In addition, the pincer nickel(II) complex 208 (5 mol%) has been reported to behave as an efficient catalyst, when combined with CuI (3 mol%), in the coupling of nonactivated alkyl halides to terminal alkynes, using caesium carbonate as base in dioxane at 100 1C.330 Using this procedure, alkyl iodides can be coupled with terminal alkynes, alkyl bromides requiring the presence of sodium iodide for in situ halogen exchange, and alkyl chlorides the addition of TBAB and working at 140 1C. Scheme 54 shows an example of the use of this catalytic system in the coupling of pyrazole-containing chloride 209 and alkyne 210, giving rise to the corresponding coupled product 211. Obviously, the influence of trace amounts of other metals in all these cobalt and nickel-catalysed procedures remains an open question. Chem. Soc. Rev., 2011, 40, 5084–5121

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Scheme 54

Gold (and its complexes)-catalysed organic transformations have been the focus of attention in recent years,331 and particularly research on gold-promoted cross-coupling reactions has been very active.332 In the particular case of the Sonogashira reaction, gold supported on cerium(IV) oxide has been prepared and showed effective (5 mol%) in the coupling of iodobenzene and phenylacetylene, when using sodium carbonate as base in DMF as solvent at 150 1C.333 As this solid form of supported gold showed the mixed presence of Au(0), Au(I) and Au(III) species, and Au(0) showed lower activity for this reaction, Au(I) and Au(III) soluble complexes with Schiff base ligands derived from 1,1-binaphthyl-2,2 0 diamine, 212 and 213, respectively, were prepared and employed in the former Sonogashira coupling. The obtained results showed that the Au(I) complex gave rise to the corresponding cross-coupled product, whereas the Au(III) complex only catalysed the homocoupling of phenylacetylene, the Au(I) behavior in the Sonogashira reaction being explained by having the same d10 electronic configuration as Pd(0) and Cu(I).333 In addition, versions of Au(I) complexes supported on the silicate MCM-41334,335 have been prepared, such as 214 which had been used (20 mol%) in the above Sonogashira coupling employing potassium phosphate as base in xylene as solvent at 130 1C, the catalyst being recovered and reused.334

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Moreover, it has been reported that a combination of gold(I) iodide (1 mol%) and the ligand dppf (1 mol%) in the presence of potassium carbonate as base in toluene at 130 1C, is very effective for the cross-coupling of aryl iodides or electrondeficient aryl bromides and arylacetylenes.336 However, some controversy has aroused from these results, as it has been reported that Au(I) having less than 3.1 ppm of palladium in combination with the ligand dppe was practically inactive to promote the Sonogashira coupling since it was unable to perform the oxidative addition of iodobenzene in the first step of a Sonogashira-like catalytic cycle.337 The authors of this study claimed that the former Au(I)-promoted Sonogashira reactions were in fact produced by contaminating traces of palladium. This assertion has been addressed after kinetic and theoretical studies, considering that although DFT calculations show that certainly the iodobenzene oxidative cleavage on a Au(I) complex is a difficult process, Au(I) complexes are converted under the reaction conditions into nascent gold nanoparticles, and those are in fact responsible for the catalytic activity, making the presence of traces of palladium to explain the cross-coupling process unnecessary.338 In fact, gold nanoparticles supported on cerium(III) oxide, niobium(V) oxide and silica catalyse the Sonogashira reaction of aryl iodides and bromides and terminal alkynes, when reacting in the presence of potassium carbonate as base in DMF under microwave irradiation.339 In addition, gold nanoparticles have been also supported on the porous linear polysaccharide chitosan, being used as catalyst in the coupling of iodobenzene and phenylacetylene, working in the presence of potassium phosphate as base in DMF at 80 1C.340 Concerning the nature of the gold(0) catalyst and the process, studies based on temperature-programmed reaction measurements supported by scanning tunneling microscopy showed that phenylacetylene and iodobenzene react on Au(111) (high purity 99.999% Au) under vacuum conditions to yield biphenyl and diphenyldiacetylene together with the corresponding Sonogashira coupling product.341 These studies indicate that heterogeneous cross-coupling chemistry is an intrinsic property of extended, metallic pure gold surfaces. The reagents would be initially adsorbed intact on the surface of the gold crystallites until the temperature needed to achieve the C–I bond cleavage of the aryl iodide is achieved, which coincides with that required for the Sonogashira coupling. Additional studies on the behavior of gold species deposited on lanthana or ceria showed that Au(I) and Au(III) species were catalytically inert, but gold nanoparticles were highly selective towards the Sonogashira product diphenylacetylene, although proving active but unselective when supported on silica, alumina or barium oxide, suggesting an effect of metalsupport spillover.342 Finally, an interesting study indicates that the gold-nanoparticle-catalysed Sonogashira coupling of iodobenzene and phenylacetylene is predominantly an heterogeneous process.343 Thus, large gold particles resulted much more selective towards diphenylacetylene than small ones, which would be consistent with the hypothesis that steric limitations adversely affect the efficiency of the process. Moreover, gold leached into the solution phase exhibited immeasurably low catalytic activity, also pointing to the primacy of heterogeneous chemistry. These studies have also This journal is

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been extended to the case of metallic rhodium nanoparticle catalysts, revealing similar preference for a heterogeneous Sonogashira coupling.344

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10.2

Non-copper-based cocatalysts

Apart from the obvious elimination of the copper salt cocatalysis in the Sonogashira reaction in order to avoid the possible formation of Hay/Glaser-type alkyne homocoupling byproducts, the use of other cocatalysts less prone to oxidation and also able to act as transmetalation agents to palladium has also been explored. This is the case of silver,345 which has been used in the form of silver(I) iodide (5.1 mol%), combined with palladium(II) chloride (5 mol%), as a catalytic mixture to use hypervalent iodonium salts such as 215 as electrophiles in the cross-coupling reaction with terminal acetylenes, to give the corresponding Sonogashira products, such as 216, obtained by reaction with 1-hexyne in acetonitrile as solvent at room temperature (Scheme 55).346 In addition, the decarboxylative Sonogashira cross-coupling of aryl iodides and bromides and vinyl bromides and triflates has been performed using Pd2(dba)3 (2 mol%)/PPh3 (16 mol%) or Xantphos (72, 8 mol%) as catalytic combination, in the presence of an equimolecular amount of silver(I) oxide and lithium iodide in DMF at 70–100 1C.347 Moreover, an electro-oxidative method for generating silver acetylides from acetylenes with a silver anode has been developed and integrated in a Sonogashira-type coupling with arylboronic acids catalysed by palladium(II) acetate (5 mol%) in the presence of 4-benzoyl-TEMPO (15 mol%) and DBU as base, the process being performed using sodium perchlorate as electrolyte in acetonitrile/water (7:1) as solvent.348 Gold(I) chloride has been used as co-catalyst (1 mol%) combined with palladium(II) chloride (2 mol%) in the coupling of arenediazonium salts with terminal alkynes, a process carried out in the presence of bis-2,6-diisopropylphenyl dihydroimidazolium chloride (IPr NHC) (5 mol%) to in situ generate a NHC–palladium complex, and 2,6-di-tert-butyl-4-methylpyridine (DBMP) as base in acetonitrile as solvent at room temperature.349 This coupling can be carried out starting from anilines by formation of the diazonium salt followed by in situ Sonogashira coupling, as is shown in Scheme 56, where aniline 217 is transformed into diazonium salt 218 and furtherly converted into alkyne 219 by coupling with phenylacetylene. A series of inorganic and organometallic compounds of gold [AuCl(tht), Au(C6F5)(tht) and NaAuCl4] (tht = tetrahydrothiophene), combined with PdCl2(PPh3)2 (1 mol%) have shown to efficiently co-catalyse (1 mol%) the Sonogashira cross-coupling reaction of aryl iodides and activated aryl bromides with phenylacetylene in the presence of diisopropylamine in THF at room temperature, no traces of Hay/Glasertype compounds being found.350 When the water soluble

Scheme 55

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Scheme 56

complex AuCl(TPPTS) was used as co-catalyst, the reaction could be carried out in an aqueous biphasic water/dichloromethane system, although only in the case of the coupling or aryl iodides. In this work, it was reported that the combination of PdCl2(PPh2)2 (1 mol%)/AuCl(PPh3) (1 mol%) resulted inactive towards the coupling reaction of 4-iodoacetophenone with phenylacetylene (o1% conversion). However, this observation was denied in a recent work which reported 96% conversion in this reaction using this catalytic dual combination, the use of only PdCl2(PPh2)2 as catalyst affording 54% conversion (7% of Hay/Glaser homocoupling).351 In fact, in this report this combination (2 mol% each) was employed as an efficient catalytic system in the cross-coupling of aryl iodides and bromides with terminal alkynes, working in the presence of triethylamine as base in DMF at room temperature. Substoichiometric amounts of zinc(II) chloride (10 mol%) have been found to promote a room temperature, palladium/ tri-tert-butylphosphane-catalysed cross-coupling of aryl bromides with terminal alkynes.352 The best results were achieved using the palladium(I) dimer 220 as catalyst (1 mol%) and diisopropylamine as base in THF as solvent. The role of the zinc is probably formation of a zinc halide– alkyne complex, present traces of copper not appearing to be the active catalyst.

11.

Conclusions

The Sonogashira cross-coupling reaction continues providing a large amount of published work because of its enormous synthetic utility in the preparation of compounds of interest. Most of this work is still carried out following the traditional procedure based on the use as catalyst of a combination of PdCl2(PPh3)2 and CuI in the presence of an amine, although many efforts in the development of more reactive palladium catalysts working also in the absence of copper(I) cocatalysis continue. Among these catalytic systems, those suitable to be recycled and reused have been particularly noticeable in the last few years. Thus, many different ways of anchoring or immobilising palladium species on polymers or inorganic Chem. Soc. Rev., 2011, 40, 5084–5121

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materials have been reported. Palladium nanoparticles occupy a remarkable position in this area, although their activity and recyclability results sometimes limited. In addition, the development of procedures based on the use of aqueous systems or neat water as solvents has also been frequent. However, many of these catalytic systems or procedures still lack a broad applicability and are effective only for the coupling of ‘‘easy’’ aryl iodides and terminal alkynes, some of them only being assayed in a model coupling between iodobenzene and phenylacetylene. The general use of aryl chlorides, activated or not, as coupling partners still remains a not sufficiently resolved matter that surely will be the subject of interest in next future, as well as the development of procedures really effective working under very low catalyst loadings. In addition, particularly interesting in recent years has been the use of other non-palladium metals as catalysts in the Sonogashira reaction. Thus, copper, iron, nickel or gold species have been reported to act as catalysts in this process, sometimes arousing controversies about the role of traces of metal contaminants in reagents or glassware. It seems that some of other transition metals have an effective catalytic activity in the Sonogashira reaction and therefore this is going to be an area where research will focus in next future.

Acknowledgements Financial support from the Spanish Ministerio de Ciencia e Innovacio´n (projects CTQ2010-20387 and Consolider Ingenio 2010, CSD2007-00006), the Generalitat Valenciana (Prometeo/2009/039), FEDER and the University of Alicante is acknowledged.

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The Royal Society of Chemistry 2011

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The Royal Society of Chemistry 2011

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Chem. Soc. Rev., 2011, 40, 5084–5121

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