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High-valent oxo-molybdenum and oxo-rhenium complexes as efficient catalysts for X–H (X = Si, B, P and H) bond activation and for organic reductions Downloaded by Universidade Tecnica de Lisboa (UTL) on 20 August 2012 Published on 11 July 2012 on http://pubs.rsc.org | doi:10.1039/C2CS35155B

Sara C. A. Sousa, Ivaˆnia Cabrita and Ana C. Fernandes* Received 23rd April 2012 DOI: 10.1039/c2cs35155b High-valent oxo-complexes have recently emerged as powerful catalysts for the activation of X–H (X = Si, B, P and H) bonds and for the reduction of several functional groups. This new reactivity represents a complete reversal from the traditional role of these complexes as oxidation catalysts and opened a new research area for high-valent oxo-complexes. This tutorial review highlights the work developed using high-valent oxo-molybdenum and oxo-rhenium complexes as excellent catalysts for X–H (X = Si, B, P and H) bond activation and for organic reductions.

1. Introduction During many years, high-valent oxo-molybdenum1,2 and oxorhenium3,4 complexes were employed as excellent catalysts for oxidation reactions such as the oxidation of alkenes, sulfides, and pyridines to the corresponding epoxides, sulfoxides and pyridine N-oxides (Scheme 1). As the active sites of several molybdoenzymes e.g. sulfite oxidase, DMSO reductase and xanthine oxidase are formed by a dioxo- or a monooxo-molybdenum unit, many oxomolybdenum complexes have been studied as chemical models of their active sites.5–7

Centro de Quı´mca Estrutural, Instituto Superior Te´cnico, Av. Rovisco Pais, Lisbon 1049-001, Portugal. E-mail: [email protected]

Sara C. A. Sousa

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Sara Sousa received her BS and MS degrees in Chemistry from the Faculty of Sciences, University of Lisbon, Portugal. Then, she obtained a Euromaster degree in Measurement Science in Chemistry from the University of Tartu, Estonia. In 2010, she started her PhD work on the use of the high-valent oxo-complexes as catalysts for organic reductions at Centro de Quı´mica Estrutural, Instituto Superior Te´cnico, Lisbon.

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In recent years, several new developments into the chemistry of high-valent oxo-complexes were reported. Among the most interesting ones was the successful reduction of a variety of functional groups promoted by oxo-complexes in high oxidation states. In 2003, Toste and co-workers8 described a novel method for the hydrosilylation of carbonyl compounds with silanes catalyzed by the high-valent oxo-rhenium complex ReIO2(PPh3)2 1, affording the corresponding silyl ethers in good to excellent yields, with tolerance of a wide range of functional groups (Scheme 2). This reaction provides an efficient and practical one-step reduction-protection method of carbonyl compounds. The use of high-valent oxo-complexes as catalysts for organic reductions represents a complete reversal from the traditional role of these complexes as oxidation catalysts. Toste and co-workers9 have also proposed a catalytic cycle (Scheme 3) for the hydrosilylation of carbonyl compounds,

Ivaˆnia Cabrita

Ivaˆnia Cabrita completed her MSc postgraduate degree in Chemistry at the University of Lisbon, Portugal. She also obtained a Euromaster postgraduate degree in Measurement Science in Chemistry from the University of Tartu, Estonia. Recently, she joined the organic chemistry research group as a PhD student at Centro de Quı´mica Estrutural, Instituto Superior Te´cnico, Lisbon. Her research interests include the synthesis and use of carbohydrates as scaffolds for asymmetric catalysis.

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Scheme 1 Oxidation reactions catalyzed by oxo-complexes.

Scheme 3 Proposed catalytic cycle for hydrosilylation of carbonyl compounds.

2. Si–H bond activation 2.1 Organic reductions using the catalytic system silane/ high-valent oxo-molybdenum complexes Scheme 2 Hydrosilylation of carbonyl compounds catalyzed by 1.

which involves the addition of a silane Si–H bond across one of the rhenium-oxo bonds to form the siloxyrhenium hydride intermediate 4, that reacts with a carbonyl substrate to generate siloxyrhenium alkoxide 5, which in turn, affords the silyl ether product. This proposed mechanism was supported by DFT calculations performed by Wu and co-workers.10 The formation of the hydride 4 demonstrated for the first time that the oxorhenium complex ReIO2(PPh3)2 1 activates the Si–H bond of the silane. The results reported by Toste8,9 opened a new research area for high-valent oxo-complexes as catalysts for Si–H bond activation and for organic reductions. High-valent oxo-molybdenum complexes have also been used as efficient catalysts for C–X bond forming reactions, including carbon–carbon and carbon–heteroatom bonds.11

Ana Fernandes graduated in Chemistry at the Faculdade de Cieˆncias of the Universidade de Lisboa, Portugal (1991), where she received her PhD in Organic Chemistry (1996). Then, she worked as a Researcher at Herbex, Produtos Quı´micos for two years in drug synthesis. From 1999–2008 she held an Assistant Professor position at Universidade Luso´fona de Humanidades e Tecnologias and in 2008 she became an Ana C. Fernandes Auxiliary Researcher at Instituto Superior Te´cnico, Lisbon. Her research interests include development of novel methodologies for organic chemistry catalyzed by high-valent oxo-complexes and use of carbohydrates as scaffolds for asymmetric catalysis and for the synthesis of biologically important compounds. 5642

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2.1.1 Hydrosilylation of carbonyl compounds. After the discovery of Toste8,9 that demonstrated the ability of the high-valent oxo-rhenium complex ReIO2(PPh3)2 to activate the Si–H bond of silanes and catalyze the hydrosilylation of carbonyl compounds, Royo and co-workers12 investigated the use of the dioxo-molybdenum complex MoO2Cl2 713 as catalyst for the same reaction. The catalytic system silane/ MoO2Cl2 was investigated in the hydrosilylation of aldehydes and ketones, affording the corresponding silyl ethers in moderate to good yields (Scheme 4). The reaction is chemoselective, tolerating several functional groups such as –CF3, –NO2, –Br, –CN and ester. This is the first example of the reduction of carbonyl compounds catalyzed by a high-valent oxo-molybdenum complex, showing a complete reversal of the traditional role of this catalyst. Royo and co-workers14,15 have explored the catalytic activity of other dioxo-molybdenum complexes such as MoO2(S2CNEt2)2 10, MoO2(acac)2 11, CpMoO2Cl 12, MoO2(mes)2 13, and the polymeric organotin-oxomolybdates (R3Sn)2MoO4 14 and 15 in the hydrosilylation of aldehydes and ketones (Scheme 5). The results obtained show that complexes 11–15 catalyze these reactions but required heating at 80 1C and longer reaction times compared to MoO2Cl2. Compound 10 is inactive. A mechanism for the hydrosilylation of aldehydes and ketones catalyzed by MoO2Cl2 was proposed based on DFT calculations.16

Scheme 4 Hydrosilylation of carbonyl compounds catalyzed by MoO2Cl2.

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Scheme 5

Hydrosilylation catalyzed by oxo-molybdenum complexes.

Scheme 7 Free energy profile [kcal mol1] for the concerted reduction of aldehyde by [MoCl2(H)(O)(OSiH3)] (top), and classical reduction followed by silyl migration (bottom).

Scheme 8 Reduction of imines catalyzed by MoO2Cl2.

Scheme 6 Two pathways for Si–H addition to MoO2Cl2: [2+2] addition to MoQO (above), [3+2] addition to MoQO (bottom). Free energies are given in kcal mol1.

These studies demonstrated that the most favourable pathway to the first step, the Si–H activation, is a [2+2] addition to the MoQO bond, forming the hydride species MoCl2H(O)(OSiR3) B (Scheme 6). However, the participation and concurrent formation of B1 in the reaction, which corresponds to the [3+2] addition of Si–H to MoO2Cl2, cannot be entirely ruled out because its interconversion to B has lower activation energy (33.2 kcal mol1). In the following step, the aldehyde approaches the hydride species and coordinates weakly through the oxygen atom. Two alternative pathways can be envisaged (Scheme 7): the classical reduction, in which a hydrogen atom migrates to the carbon atom to form an alkoxide, which then proceeds to generate the final silyl ether, or a concerted mechanism involving the migration of a hydrogen atom to a carbon and of a silyl ether to an oxygen atom to generate the silyl ether weakly bounded to the molybdenum atom. However, the authors did not rule out the possibility of a radical mechanism for the hydrosilylation of aldehydes and ketones catalyzed by MoO2Cl2. 2.1.2 Reduction of imines. Fernandes and Roma˜o investigated the reduction of imines to the corresponding amines with the catalytic system silane/MoO2Cl2 (10 mol%) (Scheme 8).17 These reductions were explored with imines bearing different functional groups e.g. –NO2, –CF3, –F, –Cl, –CO2CH3, affording the amines in moderate to excellent yields and good chemoselectivity. This journal is

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2.1.3 Reduction of sulfoxides. The reduction (or deoxygenation) of sulfoxides to the corresponding sulfides is an important organic and biological reaction. The catalytic system PhSiH3/MoO2Cl2 was also tested in the reduction of sulfoxides by Fernandes and Roma˜o using 5 mol% of MoO2Cl2 and one equivalent of phenylsilane in THF at reflux temperature under an inert atmosphere (Scheme 9).18 This novel method is suitable for the deoxygenation of aromatic and aliphatic sulfoxides in excellent yields, with tolerance of other functional groups such as halo, carboxyl, and vinyl. 2.1.4 Reduction of pyridine N-oxides. These authors18 have also performed a brief investigation on the deoxygenation of pyridine N-oxides by MoO2Cl2 7 and MoO2Cl2(H2O)2 2019 (Scheme 10). The substrates 3-picoline N-oxide and 4-picoline N-oxide were reduced with the catalytic systems PhSiH3/ MoO2Cl2 and PhSiH3/MoO2Cl2(H2O)2 in good yields (83–85%), demonstrating that these systems are also very efficient for the deoxygenation of pyridine N-oxides. 2.1.5 Reduction of esters. The conversion of esters to the corresponding alcohols is a fundamental process in organic synthesis.

Scheme 9 Reduction of sulfoxides catalyzed by MoO2Cl2.

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Scheme 13 Reductive amination catalyzed by MoO2Cl2.

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Scheme 10 Reduction of pyridine N-oxides catalyzed by oxomolybdenum complexes.

Scheme 11 Reduction of esters catalyzed by MoO2Cl2.

Fernandes and Roma˜o have demonstrated that the catalytic system PhSiH3/MoO2Cl2 (5 mol%) is very efficient for the reduction of a variety of aliphatic and aromatic esters to the corresponding alcohols in good yields (Scheme 11).20 The mechanism proposed for this reduction should involve the formation of an alkyl silyl acetal, resulting from the reaction between the ester and the hydride species (Mo–H), which is easily converted into the corresponding aldehyde. Then, the aldehyde reacts with a second equivalent of hydride species (Mo–H) producing a silyl ether, followed by a rapid hydrolysis to the alcohol.

both electron-deficient and electron-rich aldehydes, using MoO2Cl2 as catalyst and phenylsilane as the reducing agent (Scheme 13). This method is environmentally friendly, using ethanol or methanol as solvent in place of the standard chlorinated solvents, and it is also chemoselective, tolerating a number of reducible functional groups (e.g. –F, –Cl, –I, –OMe, –NO2, –CO2Me, –CN) and heterocyclic ring systems. 2.1.8 Reduction of azides. Prabhu and co-workers23 have developed a neutral and efficient strategy for the reduction of azides to the corresponding amines catalyzed by MoO2(S2CNEt2)2 10 in the presence of phenylsilane (Scheme 14). This method tolerates a wide variety of reducible functional groups e.g. chloro, nitro, cyano, aldehyde, ketone, ester, amide, epoxide, alcohol, and double bonds. 2.2 Organic reductions using the catalytic system silane/ high-valent oxo-rhenium complexes 2.2.1 Hydrosilylation of carbonyl compounds. After the initial studies developed by Toste8,9 about the hydrosilylation of carbonyl compounds catalyzed by the oxo-rhenium complex ReIO2(PPh3)2, Royo and co-workers24,25 explored the catalytic activity of the oxo-rhenium complexes Re2O7 32, CH3ReO3 (MTO) 33, ReOCl3(PPh3)2 34, and HReO4 35 in the same reaction (Scheme 15). Among all the oxo-rhenium

2.1.6 Reduction of amides. The catalytic efficiency of the system PhSiH3/MoO2Cl2 was also explored in the reduction of amides to the corresponding amines (Scheme 12).21 Several amides were reduced in moderate to good yields. This novel methodology proved to be especially suitable for the reduction of tertiary amides with bulky N-substituents. 2.1.7 Reductive amination of aldehydes. Direct reductive amination is a powerful method for C–N bond formation and for the synthesis of higher order amines from a carbonyl compound and an amine. Smith and co-workers22 have developed an efficient method for direct reductive amination of

Scheme 12 Reduction of amides catalyzed by MoO2Cl2.

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Scheme 14 Reduction of azides catalyzed by MoO2(S2CNEt2)2.

Scheme 15 Hydrosilylation catalyzed by oxo-rhenium complexes.

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Scheme 18 Reduction of styrene derivatives catalyzed by ReIO2(PPh3)2.

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Scheme 16 Hydrosilylation of carbonyl compounds catalyzed by catalysts 38 and 39.

complexes tested in the hydrosilylation of aldehydes, Re2O7 and HReO4 proved to be the most active catalysts, giving the silyl ethers in good yields in few minutes. These oxo-rhenium complexes were also tested in the hydrosilylation of aromatic and aliphatic ketones (Scheme 15). In contrast to the high reactivity of the complex Re2O7 in the hydrosilylation of aldehydes, with ketones no product was formed. MTO and ReOCl3(PPh3)2 were the most effective catalysts for the hydrosilylation of ketones, producing the corresponding silyl ethers in excellent yields. At the same time, Abu-Omar and co-workers26–28 examined the catalytic activity of other oxo-rhenium complexes such as complexes 3826 and 3928 in the hydrosilylation of carbonyl compounds (Scheme 16). Both the complexes proved to be very efficient catalysts for the hydrosilylation of aromatic and aliphatic aldehydes or ketones under mild, and open-flask conditions. 2.2.2 Reduction of aromatic nitro compounds. The reduction of aromatic nitro compounds to the corresponding amines was also studied with silanes catalyzed by high-valent oxo-rhenium complexes.29 The catalytic system PhMe2SiH/ ReIO2(PPh3)2 (5 mol%) reduced efficiently a series of aromatic nitro compounds in the presence of a wide range of functional groups e.g. ester, halo, amide, sulfone, lactone, and benzyl (Scheme 17). The mechanism proposed for the reduction of aromatic nitro compounds with the system silane/oxo-rhenium complexes initiates with the coordination of the nitro group to the rhenium with liberation of the phosphines. Then, occurs the addition of the silane to the complex containing the nitro compound, forming a hydride species, followed by the reduction

Scheme 17 Reduction of aromatic nitro compounds catalyzed by ReIO2(PPh3)2.

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of the nitro compound to the corresponding nitroso derivative with liberation of R3SiOH. In the next two steps, the nitroso intermediate is rapidly deoxygenated to the hydroxylamine, which is then reduced to the amine by addition of two equivalents of silane. 2.2.3 Reduction of alkenes. A novel method for the reduction of alkenes to the corresponding alkanes with silanes catalyzed by several oxo-rhenium complexes was developed by Fernandes and co-workers.30 The catalytic system PhMe2SiH/ ReIO2(PPh3)2 (5 mol%) was tested in the reduction of a series of styrene derivatives in excellent yields under solvent-free conditions (Scheme 18). This catalytic system proved to be also very efficient for the reduction of mono- and disubstituted alkenes. 2.2.4 Reduction of sulfoxides. Fernandes and co-workers31,32 also tested several oxo-rhenium complexes in the reduction of sulfoxides and found that the catalytic system PhSiH3/ ReIO2(PPh3)2 (1 mol%) was the most efficient for the reduction of a wide range of aromatic and aliphatic sulfoxides in excellent yields under mild conditions (Scheme 19). This novel methodology is also highly chemoselective, tolerating several functional groups such as –CHO, –CO2R, –Cl, –NO2, and double or triple bonds. 2.2.5 Reduction of nitriles. The reduction of nitriles is a powerful tool to synthesize primary amines and it is also a fundamental process in organic chemistry. Cabrita and Fernandes studied the reduction of nitriles to the corresponding primary amines with silanes catalyzed by oxo-rhenium complexes.33 The catalytic system PhSiH3/ReIO2(PPh3)2 (10 mol%) reduced efficiently a variety of nitriles in the presence of a wide range of functional groups (Scheme 20). Furthermore, this novel methodology avoids the formation of secondary amines, by unwanted side-reactions, which is a general problem observed in the reduction of nitriles by catalytic hydrogenation.

Scheme 19 Reduction of sulfoxides with the system PhSiH3/ ReIO2(PPh3)2.

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Scheme 20 Reduction of nitriles catalyzed by ReIO2(PPh3)2

Scheme 22 Synthesis of secondary amines catalyzed by ReIO2(PPh3)2.

Scheme 23 Synthesis of tertiary amines catalyzed by ReIO2(PPh3)2.

Scheme 21 Proposed mechanism for the reduction of nitriles.

The mechanism proposed for the reduction of nitriles with the system silane/oxo-rhenium complexes involves the following steps: coordination of two nitriles to the rhenium with liberation of two phosphines, affording the complex ReIO2(nitrile)2 46; formation of the hydride species ðnitrileÞ2 ðOÞIReðHÞOSiR03 47 as a result of the addition of the Si–H bond of the silane to one of the oxo-rhenium bonds; dihydrosilylation of the nitrile to the corresponding N-disilylamine 52; and formation of amine 53 by hydrolysis of N-disilylamine, probably due to the presence of a trace of water in the reaction mixture (Scheme 21). 2.2.6 Reductive amination of aldehydes. The synthesis of secondary and tertiary amines was explored by Sousa and Fernandes through direct reductive amination of aldehydes with silanes in the presence of a catalytic amount of oxorhenium(V) and (VII) complexes (Scheme 22).34 The system PhSiH3/ReIO2(PPh3)2 (2.5 mol%) proved to be the most efficient and chemoselective for the synthesis of secondary amines, tolerating a wide range of functional groups e.g. –NO2, –CF3, –SO2R, –CO2R, –Cl, –Br, –CN, –OH, –OMe, –NCOR, and double bonds. 5646

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This methodology was also employed in the synthesis of tertiary amines, in moderate yields, reacting N-methylaniline 58 with several aldehydes using the system PhSiH3/ ReIO2(PPh3)2 (Scheme 23). A plausible mechanism (Scheme 24) for the direct reductive amination of aldehydes with the catalytic system PhSiH3/ ReIO2(PPh3)2 should initiate with the formation of the imine, and coordination of this molecule to the catalyst by substitution of the two phosphines, affording the complex ReIO2(imine)2 60. In the second step, the hydride species (imine)2(O)IRe(H)OSiR3 61 is formed as a result of the addition of the Si–H bond of the silane to one of the oxorhenium bonds. Then, the hydrosilylation of the imine occurs, followed by hydrolysis to the corresponding amine 65. Finally, the ReIO2(imine) 63 species formed will be stabilized by the entry of another molecule of imine, regenerating the dioxorhenium complex ReIO2(imine)2 60. 2.2.7 Asymmetric reductions catalyzed by oxo-rhenium complexes. Toste and co-workers35,36 reported the synthesis of a series of new chiral, non-racemic (CN-box)Re(V)-oxo complexes 68, reacting ReOCl3(OPPh3)(SMe2) 66 with different cyanobis(oxazoline) ligands 67 in dichloromethane at room temperature (Scheme 25). These catalysts were tested in the asymmetric hydrosilylation of ketones and imines (Scheme 26). The catalysts 71 and 72 reduced several aromatic, heteroaromatic and five-, six-, and seven-membered cyclic ketones with good to excellent enantioselectivity. In contrast, the reduction of non-aryl ketones proceeded in good to excellent yields, but with modest enantioenrichment of the resultant alcohols (6–15% ee). This journal is

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Scheme 27 Asymmetric reduction of enones catalyzed by complexes 71 and 72.

Scheme 24 Proposed mechanism for the reductive amination of aldehydes.

Scheme 25 Synthesis of (CN-box)Re(V)-oxo complexes.

Scheme 28 Tandem Meyer–Schuster rearrangement–hydrosilylation.

alcohols (Scheme 27). Modest enantioselectivities, 45–55% ee, were obtained in the reaction with a,b-unsaturated conjugated ketones and higher enantiomeric excesses, 55–63% ee, were observed with a-substituted enone. The 1,4-reduction of the enones was not observed under these conditions. These authors also found that the in situ generation of chiral catalyst 77, from complex 66 and ligand 76, provided the synthesis of several allyl alcohols in moderate to good enantiomeric excess by one-pot Meyer–Schuster rearrangement– reduction of racemic propargyl alcohols 75 (Scheme 28). Several allenyl alcohols were also subjected to the tandem reaction conditions using the catalyst 71, generated in situ from complex 66 and ligand 81, affording the corresponding allylic alcohols in good yields (63–71%) and moderate to good enantiomeric excess (56–77%) (Scheme 29). The catalytic activity of the complex 72 was also examined in the asymmetric reduction of phosphinyl imines (Scheme 30). The results obtained demonstrated that acyclic and cyclic ketimines were reduced in good yields with excellent enantioselectivity.

Scheme 26 Asymmetric hydrosilylation of ketones catalyzed by complexes 71 and 72.

The Re(V)-oxo catalyzed enantioselective hydrosilylation of ketones was extended to the synthesis of chiral allylic This journal is

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Scheme 29 Asymmetric synthesis of allylic alcohols catalyzed by 71.

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3.1 Reduction of sulfoxides using the catalytic system borane/ high-valent oxo-molybdenum complexes

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Scheme 30 Asymmetric reduction of phosphinyl imines catalyzed by 72.

Scheme 31 Asymmetric reduction of a-imino esters catalyzed by 72.

The study of the activation of the B–H bond of catecholborane (HBcat) and borane (BH3THF) by oxo-molybdenum complexes was carried out in the reduction of sulfoxides.37 The systems HBcat/MoO2Cl2(H2O)2 (5 mol%) and BH3THF/ MoO2Cl2 (5 mol%) proved to be very efficient for the deoxygenation of sulfoxides to the corresponding sulfides (Scheme 33). The reusability of MoO2Cl2(H2O)2 was evaluated and it was observed that this catalyst can be reused in five catalytic cycles with the same catalytic activity. This study represents the first example of the B–H bond activation by high-valent oxo-complexes and extends the role of MoO2Cl2 and MoO2Cl2(H2O)2 as excellent catalysts for B–H bond activation. In order to understand the activation of the B–H bond of boranes by the oxo-molybdenum complex MoO2Cl2(H2O)2, Calhorda and Costa39 carried out a preliminary DFT study showing that the [2+2] addition, with formation of a hydride, is a feasible process (Scheme 34). 3.2 Reduction of sulfoxides using the catalytic system borane/ high-valent oxo-rhenium complexes

Scheme 32 Asymmetric synthesis of allylic amines catalyzed by oxocomplex 72.

Similar selectivity was observed with heteroaromatic compounds. In contrast, the reduction of aliphatic phosphinyl imines gave low enantiomeric excess. The synthesis of phenyl glycine derivatives was also achieved through the reduction of the corresponding a-imino esters catalyzed by oxo-rhenium complex 72 in moderate to good yields with excellent enantioselectivity (Scheme 31). Finally, Toste and co-workers have also investigated the synthesis of chiral allylic amines (Scheme 32) through the chemo- and enantioselective reduction of the corresponding imines. Conjugated aromatic imines were reduced in good yields with excellent selectivity and unconjugated vinyl imines were reduced with good enantioselectivity in moderate yields.

Fernandes and co-workers have also explored the reduction of sulfoxides with the boranes HBcat, HBpin, and BH3THF catalyzed by the oxo-rhenium complexes ReIO2(PPh3)2, ReOCl3(PPh3)2, ReOCl3(dppm), Re2O7, MTO, and HReO4.32,38 The catalytic systems HBcat/ReIO2(PPh3)2 (1 mol%), HBcat/ Re2O7 (1 mol%) and HBcat/MTO (1 mol%) were highly efficient for the reduction of several sulfoxides at room temperature under air atmosphere in few minutes (Scheme 35).

Scheme 33 Reduction of sulfoxides with the system borane/oxomolybdenum complexes.

3. B–H bond activation The activation of the B–H bond of boranes with high-valent oxo-molybdenum and oxo-rhenium complexes and the catalytic activity of the system borane/high-valent oxo-complexes in the reduction of sulfoxides were also investigated by Fernandes and co-workers.32,37,38 5648

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Scheme 34 Activation of the B–H bond by MoO2Cl2(H2O)2.

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Scheme 35 Reduction of sulfoxides with the system boranes/oxorhenium complexes.

Scheme 37 Proposed mechanism for the reduction of sulfoxides.

with release of H2 and BcatOBcat. Finally, occurs the coordination of a molecule of sulfoxide, regenerating the catalyst 90.

4. P–H bond activation Scheme 36 Synthesis of rhenium hydrides 88 and 89.

In order to study the activation of the B–H bond of boranes by high-valent oxo-rhenium complexes Fernandes and co-workers performed the reaction between the catecholborane and the oxo-rhenium complex ReIO2(PPh3)2 in THF at room temperature, obtaining the rhenium hydride (PPh3)2(O)(I)Re(H)OBcat 88 in 50% yield (Scheme 36).40 A similar reaction between pinacolborane (HBpin) and the oxorhenium complex ReIO2(PPh3)2 gave a similar rhenium hydride, (PPh3)2(O)(I)Re(H)OBpin 89, in 60% yield (Scheme 36).40 The structural characterization by X-ray diffraction of 88 and 89 showed that these hydrides are formed by addition of the B–H bond across the Re-oxo bond without dissociation of any phosphine or substitution of the iodide ligand. The synthesis of the novel hydrides 88 and 89 is a clear demonstration that the high-valent oxo-rhenium complex ReIO2(PPh3)2 activates the B–H bond of boranes. Usually, the activation of boranes reported in the literature involves transition metal complexes in a low oxidation state.41–43 A mechanism for the reduction of sulfoxides with the catalytic system HBcat/ReIO2(PPh3)2 was also proposed based on DFT calculations (Scheme 37). These studies suggested that the reaction starts with the formation of ReIO2(R2SO)2 90 by coordination of two molecules of sulfoxide to the rhenium with substitution of the two phosphines. In the second step, the addition of the first molecule of HBcat to yield the hydride ReHIO(R2SO)2(OBcat) 91 occurs, followed by the loss of the sulfide, and oxidation of the metal to the oxidation state VII. Then, a second HBcat molecule attacks the Re(VII) intermediate, reducing the metal back to Re(V), This journal is

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After the study of the activation of Si–H and B–H bonds by oxo-molybdenum and oxo-rhenium complexes, the activation of P–H bond was also investigated. 4.1 Hydrophosphonylation of aldehydes using the catalytic system HP(O)(OEt)2/high-valent oxo-molybdenum complex Fernandes and co-workers have reported the use of MoO2Cl2 as a novel catalyst for P–H bond activation, exemplified by the synthesis of a-hydroxyphosphonates and a-aminophosphonates obtained in the hydrophosphonylation of aldehydes44 and imines.45 The synthesis of a-hydroxy- and a-aminophosphonates has attracted much attention due to their important biological activities as antibiotics,46 anti-tumor agents,47 and enzyme inhibitors.48 A series of a-hydroxyphosphonates was prepared, in excellent yields, using the catalytic system HP(O)(OEt)2/MoO2Cl2 (5 mol%) under solvent-free conditions or at refluxing THF (Scheme 38).

Scheme 38 Hydrophosphonylation of aldehydes catalyzed by MoO2Cl2.

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Scheme 41 Hydrophosphonylation of imines catalyzed by MoO2Cl2.

Scheme 39 Two pathways for P–H addition to MoO2Cl2: [3+2] addition (above), [2+2] addition (bottom) to MoQO. (DE0 top and DG bottom in kcal mol1).

This novel method proved to be very efficient and chemoselective, tolerating several functional groups e.g. –NO2, –CF3, –F, –CN, –CO2Me, –OMe and double bonds. To understand the activation of the P–H bond of HP(O)(OMe)2 by MoO2Cl2 several computational studies were carried out, which indicated that the activation occurs with a [3+2] addition, starting with coordination of PQO to molybdenum and hydrogen transfer from P–H to the oxo in MoQO, forming Mo–OH (Scheme 39). In this case, [2+2] addition with hydride formation is more unfavorable than the [3+2] addition (Scheme 39). Further computational studies were also performed for the hydrophosphonylation of aldehydes catalyzed by MoO2Cl2 (Scheme 40). The new intermediate O reacts with the aldehyde substrate in two steps. In the first, the aldehyde binds to the metal through the carbonyl, forming the C–P bond, and in the second, the hydrogen is transferred from Mo–OH to the oxygen of the final product. 4.2 Hydrophosphonylation of imines using the catalytic system HP(O)(OEt)2/high-valent oxo-molybdenum complex Fernandes and co-workers have also extended the use of MoO2Cl2 as catalyst for the synthesis of a-aminophosphonates by addition of HP(O)(OEt)2 to imines (Scheme 41).45

Scheme 40 Reaction pathway for the reaction between the intermediate O and the aldehyde substrate (DE0 top and DG bottom in kcal mol1).

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This novel methodology was carried out under mild and solvent-free conditions, with high yields, fast reaction times and tolerance of several functional groups such as –CF3, –F, –CN, –NO2, –OMe or a double bond conjugated to the imino group. This is the first example of the synthesis of a-hydroxyand a-aminophosphonates catalyzed by a high-valent oxomolybdenum complex. This work opens a new research area for high-valent oxo-molybdenum complexes as excellent catalysts for C–P bond forming reactions.

5. H–H bond activation In the last few years, the activation of H–H bond by highvalent oxo-molybdenum and oxo-rhenium complexes was also studied and the reactivity of the catalytic system H2/oxo-complexes was explored in different organic reductions. 5.1 Organic reduction using the catalytic system H2/highvalent oxo-molybdenum and oxo-rhenium complexes 5.1.1 Reduction of alkynes. Royo and co-workers49 have studied the addition of hydrogen to alkynes, affording the corresponding alkenes, using the catalytic system H2/MoO2Cl2 (Scheme 42). The reduction of 1-hexyne gave 1-hexene in 100% yield. However, low or moderate yields were obtained in the reduction of other alkynes. This reaction was also studied with the oxo-rhenium complexes MTO and ReIO2(PPh3)2 (Scheme 42).

Scheme 42 Reduction of alkynes catalyzed by oxo-complexes.

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Scheme 45 Reduction of aromatic nitro compounds with the system H2/MoO2Cl2.

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Scheme 43 Activation of the H–H bond by MoO2Cl2.

Both complexes are catalytically active in the hydrogenation of 1-hexyne, phenylacetylene and 3-hexyne, giving conversions of 100, 70 and 47%, respectively, for MTO and 100% yield of 1-hexene for ReIO2(PPh3)2. DFT computational studies showed that the mechanism for dihydrogen activation by the Mo(VI) complexes starts with a [2+2] addition of the H–H bond to the MoQO bond (Scheme 43). The barrier is relatively high (57.8 kcal mol1), but the range of pressure and temperature required for the catalytic conditions is consistent with it (typically 8 atm and 80 1C). The hydride complex formed may then catalyze the reduction of approaching substrates or it may undergo a reductive elimination reaction, which leads to formation and later elimination of water. 5.1.2 Reduction of sulfoxides. The deoxygenation of methyl phenyl sulfoxide and dibutyl sulfoxide with hydrogen in the presence of a catalytic amount of MoO2Cl2 or MoO2(S2CNEt2)2 was also investigated by Royo and co-workers.49 The reduction catalyzed by MoO2Cl2 gave the sulfides in quantitative yields, but the reactions in the presence of MoO2(S2CNEt2)2 afforded only low yields of the corresponding sulfides (31–55%) (Scheme 44). In these reactions, the oxygen of sulfoxides is removed as H2O. In the presence of the oxo-rhenium complexes ReIO2(PPh3)2 and ReOCl3(PPh3)2, this reduction afforded the corresponding sulfides in excellent yields (Scheme 44).49

Scheme 46 Reduction of pyridine N-oxides with the system H2/MoO2Cl2.

5.1.4 Reduction of pyridine N-oxides. The same group further extended the catalytic system H2/MoO2Cl2 (10 mol%) to the reduction of several pyridine N-oxides, obtaining the corresponding pyridines in quantitative yields, with tolerance of chloro, cyano, aldehyde, and methoxy substituents (Scheme 46).50 5.1.5 Deoxygenation of epoxides and vicinal diols. AbuOmar and co-workers51 have reported a novel method for the deoxygenation of epoxides and vicinal diols to the corresponding alkenes using hydrogen as the reductant, at 150 1C and 80–500 psi, catalyzed by MTO. This deoxygenation method was found applicable to several aliphatic, aromatic, and cyclic epoxides (Scheme 47). For example, 1-hexene oxide afforded 1-hexene in 95% yield, styrene oxide gave styrene in 80% yield and cyclohexene oxide produced cyclohexene in 73% yield. The only byproduct formed in this methodology is water. This reaction was also tested in the deoxygenation of diols (Scheme 48) and proved to be selective for the deoxygenation of cis cyclic diols to the corresponding alkenes, signaling a mechanism of extrusion from a coordinated epoxide via a metallaoxetane intermediate (Scheme 49).

5.1.3 Reduction of aromatic nitro compounds. Royo and Reis have also explored the reduction of aromatic nitro compounds with H2 in the presence of a catalytic amount of MoO2Cl2 in quantitative yields (Scheme 45).50 This method is chemoselective, and it was successfully applied to the reduction of several aromatic nitro compounds containing carbonyl, cyano and halo groups or double bonds. The reusability of the oxo-molybdenum complex was studied and it was observed that MoO2Cl2 can be reused in three catalytic cycles with the same catalytic activity. Scheme 47 Deoxygenation of epoxides catalyzed by MTO.

Scheme 44 Reduction of sulfoxides with the system H2/oxo-complexes.

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Scheme 48 Deoxygenation of diols catalyzed by MTO.

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References

Scheme 49 Proposed mechanism for the deoxygenation of epoxides and vicinal diols.

6. Conclusion This review gives an overview of the investigation in the X–H (X = Si, B, P, and H) bond activation catalyzed by high-valent oxo-molybdenum and oxo-rhenium complexes. The examples compiled in this tutorial review clearly demonstrate the new role of high-valent oxo-molybdenum and oxo-rhenium complexes as excellent catalysts for X–H (X = Si, B, P, and H) bond activation. Using these oxo-complexes as catalysts several classes of functional groups such as aldehydes, ketones, esters, amides, imines, sulfoxides, pyridine N-oxides, azides, alkenes, and aromatic nitro compounds were easily reduced in an efficient manner. These methodologies were also successfully applied to the synthesis of secondary and tertiary amines by reductive amination of aldehydes and for the preparation of a-hydroxyphosphonates and a-aminophosphonates in excellent yields. In many cases, these new methods provide milder conditions and simpler procedures, tolerating several functional groups, than previously reported methodologies. The successful results obtained by Toste and co-workers in asymmetric reductions of ketones and imines catalyzed by oxo-rhenium complexes suggest that future use of these catalysts will bring great benefits to both academia and industry for the production of fine chemicals such as bioactive and pharmaceutical compounds. The activation of the X–H (X = Si, B, P, and H) bond is a new and active area of research for high-valent oxo-complexes that will certainly grow exponentially in the future providing advantageous alternatives to existing catalysts. Future applications of these catalysts can include the activation of other X–H bonds, hydrosilylation, hydroboration or hydrophosphonylation of alkenes, alkynes and imines, and new C–C and C–heteroatom forming reactions.

Acknowledgements This research was supported by FCT through projects PTDC/ QUI-QUI/110080/2009 and PTDC/QUI-QUI/110532/2009. Sara C. A. Sousa and I. Cabrita thank FCT for grants (SFRH/BD/63471/2009 and SFRH/BD/74280/2010). 5652

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