Optically active silanes reduction

Reduction Reactions of Some Optically Active Silanes and Representative Enones... 

Biotransformation as a Preparative Method for the Synthesis of Optically Active Silanes, Germanes, and Digermanes Studies on the (l )-Selective Microbial Reduction of MePh(Me3C)ElC(0)Me (El = Si, Ge), MePh(Me3Ge)GeC(0)Me, and MePh(Me3Si)GeC(0)Me Using Resting Cells of Saccharomyces cerevisiae (DHW S-3)... 

In conclusion, enantioselective microbial reductions of silicon and germanium compounds containing an El-C(0)Me (El = Si, Ge) moiety El-CH(OH)Me] proved to be an efficient preparative method for the synthesis of optically active silanes, germanes, and digermanes. Furthermore, the commercially available yeast Saccharomyces cerevisiae (DHW S-3) is considered to be an efficient biocatalyst for this particular type of bioconversion. 

Growing cells of Trigonopsis variabilis (DSM 70714) were found to reduce the acyclic chiral acetylsilane vac-111 enantioselectively to give the diastereomeric optically active (l-hydroxyethyl)silanes (R,R)-228 and (S,R)-228 (yield of reduction product 50%, enantiomeric purity 90 [(R,R)-228] and 94% ee [(S,R)-228], respectively)285,286. Both the yield and the enantiomeric purity of the products could be significantly improved by using resting free cells (yield 74%, enantiomeric purity of both diastereomers 96% ee substrate concentration 0.35 g/1, 37 °C)282. The optically active silanes (R,R)-228 and (SyR)-118 can be separated by chromatography on silica gel without a noticeable decrease in yield. 

Recently, Schaumann et al. 153,154 an(j Bienz et tf/.155,156 have developed dependable routes for the resolution of racemic functionalized organosilanes with Si-centered chirality using chiral auxiliaries, such as binaphthol (BINOL), 2-aminobutanol, and phenylethane-l,2-diol (Scheme 2). For instance, the successive reaction of BINOL with butyllithium and the chiral triorganochlorosilanes RPhMeSiCl (R = /-Pr, -Bu, /-Bu) affords the BINOL monosilyl ethers 9-11, which can be resolved into the pure enantiomers (A)-9-ll and (7 )-9-11, respectively. Reduction with LiAlFF produces the enantiomerically pure triorgano-H-silanes (A)- and (R)-RPhMeSiH (12, R = /-Pr 13, -Bu 14, /-Bu), respectively (Scheme 2). Tamao et al. have used chiral amines to prepare optically active organosilanes.157... 

Stereoselective enzymatic hydrolyses of esters represent a further type of biotransformation that has been used for the synthesis of optically active organosilicon compounds. The first example of this particular type of bioconversion is illustrated in Scheme 15. Starting from the racemic (l-acetoxyethyl)silane rac-11, the optically active (l-hydroxyethyl)silane (5)-41 was obtained by a kinetic racemate resolution using porcine liver esterase (PLE E.C. 3.1.1.1) as the biocatalyst7. The silane (5)-41 (isolated with an enantiomeric purity of 60% ee bioconversion not optimized) is the antipode of compound (R)-41 which was obtained by an enantioselective microbial reduction of the acetylsilane 40 (see Scheme 8). 

This mechanism consists of several steps (1) oxidative addition of hydrosilane to the rhodium(I) complex (2) and (3) coordination and insertion of the ketone into the rhodium-silicon bond to form a diastereomeric a-silyloxyalkylrhodium intermediate (4) reductive elimination of alkoxysilane as a primary product and (5) hydrolysis of the alkoxysilane yielding an optically active alcohol. Hydrosilylation of prochiral ketones by prochirally disubstituted silanes leads to asymmetry on the silicon atom as well as on the carbon atom and, in the presence of chiral rhodium complexes, results in optically active monohydrosilanes (eq. (5)) [2] ... 

Tertiary phosphines, in the absence of special effects 2 ), have relatively high barriers 8) ca. 30-35 kcal/mol) to pyramidal inversion, and may therefore be prepared in otically stable form. Methods for synthesis of optically active phosphines include cathodic reduction or base-catalyzed hydrolysis 3° 31) of optically active phosphonium salts, reduction of optically active phosphine oxides with silane hydrides 32), and kinetic 3 0 or direct 33) resolution. The ready availability of optically pure phosphine oxides of known absolute configuration by the Grignard method (see Sect. 2.1) led to a study 3 ) of a convenient, general, and stereospecific method for their reduction, thus providing a combined methodology for preparation of phosphines of known chirality and of high enantiomeric purity. 

Pybox L8 (Fig. 4) was synthesized from pyridine-2,6-dicarboxylic acid and optically active amino alcohols via an amido chloride intermediate [ 16,23 ] or via BFj-catalyzed cychzation of intermediate amino alcohols [24]. The combination of Pybox-i-Pr (L8a) and [Rh(COD)Cl]2 (Rl) exhibited catalytic activity as an in-situ catalyst for the reduction of acetophenone (Kl) to give 76% ee (S) [16].However, the complex RhCl3(Pybox-z-Pr) R4a under assistance with AgBF4 accelerated the reduction in THF to give 94-95% ees [23]. Diphenylsilane (SI) was also the best silane in this system. Most aromatic methyl ketones were reduced in 90-99% ees, and reactions of levurinate KIO and 2-octanone Kl 1 resulted in 95% ee and in 63% ee, respectively. The Pybox-Rh catalyst R4a reduced selectively 2-phenylcyclohexanone K12 to give the S-alcohols for both trans- and cis-isomers PI and P2 in 96-99% ees [25]. The catalyst R4a can differentiate only the enan-... 

The use of (i7-C5Hs)2TiCl2 as catalyst provides a more powerful reduction method similar to the use of LiAlH4 (230). In this case, reduction of optically active functional silanes occurs with a high degree of retention of configuration (eq. [65]). The intervention of a transition-metal hydride catalyst is probably involved in these reductions. 

Isotactic polycarbosilane was synthesized for the first time by polyaddition via the hydrosilylation reaction. The starting optically active aHylsUane was synthesized from methylphenyldi[(—)-bomyloxy]silane, another optically pure starting material, and allyllithium, followed by the reduction by lithium aluminum hydride to give a colorless oil. = — 16.0(c 0.50, pentane). The reaction scheme of the synthesis... 

Optically active cinerolone 361 was obtainable by asymmetric catalytic reduction with diphenyl silane and an optically active rhodium complex [750] (Reaction scheme 243). 

Other important molecules that are useful intermediates in the synthesis of natural products are chiral diols. anli-l,2-Diols of type 30 were obtained in good yields (75-85%) and moderate to good diastereoselectivity (76-96% de) by a nickel-catalyzed three-component addition of a-silyloxy aldehydes 27, alkynyl silanes 28a, and reduction with triisopropyl silane (29a) (Scheme 11.11) [31]. The diastereoselectivity of this process could be explained by the Felkin model. Alternatively, a chiral alkynyl derivative can control the outcome of the reaction. Thus, the coupling of optically active, oxazolidinone-derived ynamides, aldehydes, and silane as reducing agent led to the formation of y-siloxyenamide derivatives with diastereoselectivities up to 99% [32]. 

A proposed mechanism for the rhodium-catalyzed alcoholysis is represented in Scheme 49 (77). In the first step, activation of the hydrosilane occurs through oxidative addition. Formation of the alkoxysilane then takes place by nucleophilic attack of a noncoordinated alcohol molecule. The dihydro-rhodium complex 143 thus obtained liberates a hydrogen molecule upon reductive elimination. Nucleophilic cleavage of the silicon-rhodium bond, without prior coordination of the alcohol at the rhodium is supported by results obtained in asymmetric alcoholysis (cf. Sect. II-D). Optical yields were shown to be little dependent on the catalyst ligands (in marked contrast with the asymmetric hydro-silylation), indicating but weak interaction between alcohol and catalyst during the reaction. Moreover, inversion of configuration at silicon, which occurs in the particular case of methanol as solvent, is not likely to occur in a reaction between coordinated silane and alcohol. 

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