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Ruthenium-Catalyzed Isomerizations of Allylic and Propargylic Alcohols in Aqueous and Organic Media: Applications in Synthesis Ruthenium-Cat lyzedIsomerizationsofAlylicandPropargylicAlcohols Cadierno, Pascale Crochet, José Gimeno* Victorio Departamento de Química Orgánica e Inorgánica, Instituto Universitario de Química Organometálica ‘Enrique Moles’ (Unidad Asociada al CSIC), Facultad de Química, Universidad de Oviedo, Julián Claveria 8, 33006 Oviedo, Spain Fax +34(985)103446; E-mail:
[email protected] Received 7 November 2007
Abstract: Recent research work on ruthenium-catalyzed isomerizations of allylic and propargylic alcohols into carbonyl compounds is reviewed. Tandem processes based on these isomerization reactions are also included. 1 2 2.1 2.2 2.3 3 3.1 3.2 3.3 4
redox isomerization R R1
R4 5
R R2
Introduction Isomerizations of Allylic Alcohols Redox Isomerization The 1,3-Rearrangement Tandem Processes Isomerizations of Propargylic Alcohols Meyer–Schuster and Rupe Rearrangements Redox Isomerization Tandem Processes Conclusions and Outlook
Key words: ruthenium, isomerizations, alcohols, homogeneous catalysis, tandem reactions
(R5
3
Introduction
= H)
SYNLETT 2008, No. 8, pp 1105–1124xx. 208 Advanced online publication: 16.04.2008 DOI: 10.1055/s-2008-1072593; Art ID: A47607ST © Georg Thieme Verlag Stuttgart · New York
O R3
1,3-rearrangement
R1
R4
R2 OH
Meyer–Schuster rearrangement
OH R1
R3
[Ru]cat
(R3 = CHR4R5)
R5
R1 R3
O
Rupe rearrangement
H
R2
R2
R5 R4 R1
O
redox isomerization (R3 = H)
The search for organic reactions that proceed with efficiency, selectivity, and atom economy (i.e., all of the atoms in the reactants end up in the final products)1 has emerged as a prime goal in synthetic chemistry. A large variety of catalytic processes have been devised mainly to achieve these important objectives. Among them, ruthenium-catalyzed transformations constitute one of the most powerful strategies, disclosing a large number of new approaches, and therefore have been widely used to address these fundamental synthetic issues.1d Furthermore, since the 1990s there has been growing attention paid to the use of water as solvent in organic chemistry, in both stoichiometric and catalytic processes.2 It is nowadays well documented that even when reactants are sparingly soluble in water, due to hydrophobic effects, an enhancement on the reactivity and selectivity of the reaction is often observed.2 The ability of ruthenium complexes to develop new catalytic processes, not only in organic solvents,3 but also in water or biphasic organic–aqueous media,4 has made them a preferred tool in modern organic synthesis that also fulfils some of the fundamental principles of green chemistry.5
R4 R2
[Ru]cat
OH
R2
1
R3 R1
H R1
O R2
Scheme 1 Ruthenium-catalyzed isomerizations of allylic and propargylic alcohols
Among the organic processes that proceed with atom economy, isomerization reactions are typical examples because no byproducts are generated. To this regard, the isomerizations of readily accessible allylic and propargylic alcohols, mainly giving carbonyl compounds, provide a simple synthetic route to these very valuable raw materials in organic chemistry (Scheme 1). Although pioneering transformations include treatment with acids and heterogeneous catalysts, new synthetic approaches based on homogeneous catalysis involving transition-metal complexes are preferred because they generally proceed under milder conditions with much more efficiency and selectivity. Herein, we summarize our efforts to develop new ruthenium-catalyzed isomerization processes disclosing powerful tools for the synthesis of carbonyl compounds, including a synthetic approach for the isomerization of allylic alcohols in aqueous media. Our catalytic methodologies have also been used for performing one-pot tandem processes in which the resulting carbonyl derivatives are transformed into (i) saturated alcohols through transfer hydrogenation reactions and (ii) conjugated dienones and dienediones through aldol-type condensations. Important contribu-
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tions by other research groups on related catalytic isomerizations and sequential transformations are also included.
R3 1
Isomerizations of Allylic Alcohols
R3
R3 R1
2.1
Redox Isomerization
O
R2
b
2
R4
R
R4 R2
R1
a
R4 R2
OH
O
The redox isomerization of allylic alcohols catalyzed by transition-metal complexes is a useful and straightforward synthetic route to carbonyl compounds (Scheme 2, pathway a) which conveniently replaces the classical two-step sequential oxidation/reduction reactions (Scheme 2, pathway b or c).
Scheme 2
Besides accomplishing total atom economy, the redox isomerization avoids the use of toxic and/or expensive oxidation and reduction reagents. Hence, during the last two decades considerable efforts have been devoted to devel-
oping efficient catalytic systems for this process. In this context, a great variety of group 6, 8, 9, and 10 metal complexes have been found to be active in this transformation.
c
R3 R1
R4 R2
OH
The redox isomerization of allylic alcohols
Biographical Sketches
Victorio Cadierno (left) studied chemistry at the University of Oviedo, Spain, and obtained his Ph.D. degree in 1996 working under the supervision of Professor J. Gimeno. He then joined the group of Professor J. P. Majoral at the Labora-
toire de Chimie de Coordination (LCC-CNRS) in Toulouse, France, for a two-year postdoctoral stay. Thereafter, he returned to the University of Oviedo where he is currently a ‘Ramón y Cajal’ researcher. In 2002, he was awarded the Spanish Royal
Society of Chemistry (RSEQ) Young Investigator Award. His research interests cover the chemistry of ruthenium complexes and their catalytic applications, and he is a co-author of more than 80 publications in the field of organometallic chemistry.
Pascale Crochet (centre) studied chemistry at the University of Rennes I, France, and obtained her Ph.D. in 1996 under the supervision of Professor P. H. Dixneuf and B. Demerseman. After a two-year postdoctoral stay in the research group of Professor M. A. Esteruelas at the University
of Zaragoza, Spain, and working one year as an assistant professor at the National High School of Physics and Chemistry of Bordeaux, France, she moved to the University of Oviedo, Spain, to collaborate with the Professor J. Gimeno in 1999. She is currently an associate professor in the
Department of Organic and Inorganic Chemistry. Her research interests are the design and synthetic applications of organometallic complexes, with a particular focus on water-soluble ruthenium catalysts.
José Gimeno (right) received his B.Sc. (1969) and Ph.D. (1973) degrees in chemistry from the University of Zaragoza, Spain. He has spent postdoctoral periods at the University of Athens in Georgia, USA (1975– 1977), and at the University of Regensburg, Germany (1981), where he worked under the supervision of Pro-
fessors R. B. King and W. A. Herrmann, respectively. After becoming an assistant professor at the University of Zaragoza, he moved to the University of Oviedo, Spain, in 1982, where he is now Professor of Inorganic Chemistry. In 2004, he was awarded the Prize in Inorganic Chemistry of the Spanish Royal Soci-
ety of Chemistry. He is co-author of more than 150 publications in the field of organometallic chemistry. His current research interests are mainly devoted to ruthenium-catalyzed reactions, particularly those involving atom-economical processes both in organic solvents and aqueous medium.
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In particular, the best performances in terms of selectivity, turnover frequencies (TOF), and turnover numbers (TON) have been obtained using iron, ruthenium, and rhodium compounds.6 Nevertheless, only a few catalysts have proven to be efficient under mild conditions and then, in most cases, temperatures ranging from 60 to 180 °C are usually required to reach reasonable conversions.6 In particular, allylic alcohols bearing substituents on the carbon–carbon double bond, i.e. substrates of the type R1CH=CHCH(OH)R2, CH2=C(R1)CH(OH)R2, 1 2 3 1 R CH=C(R )CH(OH)R , or (R )(R2)C=CHCH(OH)R3 (R1, R2, R3 ≠ H), are rarely isomerized under smooth temperature conditions, and the isomerizations require high catalyst loadings and long reaction times. Additionally, the catalytic isomerization of allylic alcohols into carbonyl compounds in aqueous media has been scarcely studied despite the great interest in this atom-economical transformation in synthesis.6,7 With these precedents in mind, and because of our previous experience in ruthenium chemistry,8 we became interested in initiating the search for new ruthenium catalysts for the redox isomerization of allylic alcohols in aqueous media. We chose this particular transformation as an ideal example to develop new, synthetically useful procedures compatible with ‘cleaner’ chemistry since the reactions can usually be performed under biphasic conditions (water–organic solvent), allowing the recycling of the catalyst by phase separation. Firstly, almost simultaneously, we checked the ruthenium-catalyzed reactions in aqueous media with precatalysts of two types: (i) water-soluble (h6-arene)ruthenium(II) complexes 1 and 2, and (ii) the bis(allyl)ruthenium(IV) derivative 3. We note that no ruthenium(IV) complex had been used as catalyst before.
ble ruthenium(II) complexes 1 and 2 by the reaction of the dimers [{RuCl(m-Cl)(h6-arene)}2] (arene = benzene, pcymene, C6Me6) with two or four equivalents of P(CH2OH)3, respectively.12 These compounds, when associated with cesium carbonate (Cs2CO3), are efficient catalysts for the redox isomerization of oct-1-en-3-ol to give octan-3-one under water–n-heptane (1:1) biphasic conditions (Table 1). Although quantitative and selective transformations were observed in all cases, the neutral complexes 1a–c (entries 1–3) were found to be much more active than their cationic counterparts 2a–c (entries 3–6) (TOF = 29–67 vs 4–6 h–1). In addition, the catalytic efficiency of these complexes was found to be strongly dependent on the nature of the arene ligand. Thus, the rate order observed in both series with the different arene ligands, i.e. benzene > p-cymene > hexamethylbenzene, indicates that the less sterically demanding and electronrich arenes show higher performance. Interestingly, the catalytically active ruthenium species could be recycled in all cases, leading to conversions 86% in a second catalytic run; nevertheless, only the neutral derivative 1a showed good performance in a third and fourth cycle. Other allylic alcohols, mono- and disubstituted at the olefin, could also be efficiently isomerized into the corresponding carbonyl compounds using 1a and 1b as catalysts under the same reaction conditions. Excellent results in terms of both activity and catalyst recycling (up to seven consecutive runs) were obtained when but-3-en-2-ol was used as the substrate, leading to TOF values up to 600 h–1 and TON values up to 782.12 Table 1 Isomerization of Oct-1-en-3-ol Catalyzed by Complexes 1 and 2 under Biphasic Conditions [Ru] (1 mol%) Cs2CO3 (2 mol%)
OH Rn
Rn
OH
OH
[Cl]
Ru Cl OH P HO HO OH 1 OH 2 arene = C6H6 (a), p-cymene (b), C6Me6 (c)
HO
Ru Cl
P
HO
P
O
H2O–n-heptane 75 °C
Cl Ru Cl
Cl
Figure 1
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Ruthenium-Catalyzed Isomerizations of Allylic and Propargylic Alcohols
3
Structure of the ruthenium complexes 1–3
The introduction of hydrophilic ligands in the coordination sphere of a transition metal is probably the most popular method for the preparation of water-soluble catalysts.9 Thus, a wide variety of functionalized phosphine ligands containing highly polar sulfonate, hydroxyalkyl, ammonium, phosphonium, carboxylate, carbohydrate, or phosphonate groups are presently known and their effectiveness in biphasic catalysis has been largely demonstrated.9,10 In this context, tris(hydroxymethyl)phosphine [P(CH2OH)3] is a simple, commercially available, and moderately air-sensitive phosphine ligand that has been successfully used for the preparation of a large number of water-soluble transition-metal complexes.11 On this basis, we prepared the novel water-solu-
Entry
Catalyst
Yielda (%)
1
1a
100
1.5
67
2
1b
100
2.25
44
3
1c
100
3.5
29
4
2a
100
18
6
5
2b
100
21
5
6
2c
100
24
4
Time (h)
TOFb (h–1)
a
Determined by GC. Turnover frequencies [(moles of product/moles of Ru)/time] were calculated using the reaction time indicated in each case.
b
As an alternative, taking advantage of the known stability of bis(allyl)ruthenium(IV) complexes toward water and their ability to form aquo complexes,13 we also studied the isomerization of oct-1-en-3-ol in aqueous media using the mononuclear complex 3 (Figure 1) as the catalyst in the presence of Cs2CO3. Complex 3 is insoluble in water, but it readily becomes soluble in the presence of the allylic alSynlett 2008, No. 8, 1105–1124
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cohol, probably through the dissociation of the chloride ligands in the resulting mixture. We found that the reaction performed at 75 °C using the reagents oct-1-en-3-ol/ Ru/Cs2CO3 in a ratio of 500:1:2 proceeded not only at a higher rate than those reactions observed using the watersoluble ruthenium(II) complexes 1a–c and 2a–c, but also than that achieved when tetrahydrofuran (THF) is used as the solvent (100% yield, 15 min, TOF = 2000 h–1 vs 100% yield, 70 min, TOF = 429 h–1).14 The catalytic activity of complex 3 was also tested in the reactions of a number of other allylic alcohols and, as previously observed with other catalytic systems,6 the results had a strong dependence on the substitution of the carbon–carbon double bond.14 Thus, monosubstituted secondary alcohols, such as hex-1-en-3-ol, pent-1-en-3-ol, and but-3-en-2-ol, were readily isomerized in an aqueous medium into the corresponding ketones using 0.2 mol% of 3 (TOF = 300–2000 h–1 without Cs2CO3 or 1500–2000 h–1 with Cs2CO3). In contrast, when 1,1- and 1,2-disubstituted allylic alcohols were used, longer reaction times and higher catalyst loadings (5–10 mol%) were required to achieve total conversion. The high catalytic activity of 3 at a lower loading (10–4 mol%) was also confirmed for the isomerization of but-3-en-2-ol (0.2 M in H2O) to give butan-2-one (100% yield, 18 h, TON = 106, TOF = 55556 h–1 without Cs2CO3). It is also interesting to note that under the same reaction conditions (e.g., isomerization of oct-1-en-3-ol in THF, Table 2), complex 3 is more active than classical ruthenium(II) catalysts, such as [{RuCl(m-Cl)(h6-arene)}2] (4b, arene = p-cymene; 4c, arene = C6Me6),12,15 [RuCl2(PPh3)3] (5),15,16 [RuCl(h5-C9H7)(PPh3)2] (6),17 and [RuCl(h5Cp)(PPh3)2] (7)17,18 (TOF = 429 h–1 vs 200000 h–1 has been claimed): (b) See ref. 18b TON values up to 17200 (TOF up to 18400 h–1) have been reported for the isomerization of oct1-en-3-ol into octan-3-one using the heterogeneous catalytic system Ru(OH)x/Al2O3, see: (c) Yamaguchi, K.; Koike, T.; Kotani, M.; Matsushita, M.; Shinachi, S.; Mizuno, N. Chem. Eur. J. 2005, 11, 6574.
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(56) (57) (58) (59) (60) (61)
2797. (c) Touchard, D.; Dixneuf, P. H. Coord. Chem. Rev. 1998, 178-180, 409. (d) Puerta, M. C.; Valerga, P. Coord. Chem. Rev. 1999, 193-195, 977. For a special issue devoted to the chemistry of vinylidenes, allenylidenes, and related metallacumulenes, see: (e) Coord. Chem. Rev. 2004, 248, 1531–1715. (a) Cadierno, V.; Díez, J.; García-Garrido, S. E.; Gimeno, J. Chem. Commun. 2004, 2716. (b) Cadierno, V.; GarcíaGarrido, S. E.; Gimeno, J. Adv. Synth. Catal. 2006, 348, 101. Bustelo, E.; Dixneuf, P. H. Adv. Synth. Catal. 2007, 349, 933. Ma, D.; Lu, X. J. Chem. Soc., Chem. Commun. 1989, 890. Trost, B. M.; Livingston, R. C. J. Am. Chem. Soc. 1995, 117, 9586. A related redox isomerization has been evoked in the rearrangement of but-2-yne-1,4-diol to give butyrolactone catalyzed by [RuH2(PPh3)4] via cyclization of the initially formed enal, see: Shvo, Y.; Blum, Y.; Reshef, D. J. Organomet. Chem. 1982, 238, C79. Nishibayashi, Y.; Uemura, S. Curr. Org. Chem. 2006, 10, 135; and references cited therein. Onodera, G.; Matsumoto, H.; Nishibayashi, Y.; Uemura, S. Organometallics 2005, 24, 5799. Cadierno, V.; Díez, J.; García-Garrido, S. E.; Gimeno, J.; Nebra, N. Adv. Synth. Catal. 2006, 348, 2125. Tsuji, Y.; Yokoyama, Y.; Huh, K.-T.; Watanabe, Y. Bull. Chem. Soc. Jpn. 1987, 60, 3456. Pridmore, S. J.; Slatford, P. A.; Daniel, A.; Whittlesey, M. K.; Williams, J. M. J. Tetrahedron Lett. 2007, 48, 5115. Pridmore, S. J.; Slatford, P. A.; Williams, J. M. J. Tetrahedron Lett. 2007, 48, 5111.
