Optical active silanes

In the case of the optically active silane, only cymantrene undergoes oxidative addition -... 

Dimanganese decacarbonyl reacts with optically active silanes, but the complex cannot be crystallized ... 

The stability of five-coordinate intermediates also makes possible the ready racemization of optically active silanes by catalytic amounts of base. The base can add readily to form a five-coordinate intermediate. The latter can undergo Berry pseudorotation with complete scrambling of substituents followed by loss of the base to yield the racemized silane. 

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

Enantioselective enzymatic transesterifications have been successfully used for the synthesis of optically active silanes with the silicon atom as the center of chirality. As shown in Scheme 20, the prochiral bis(hydroxymethyl)silanes 86 and 88 were transformed into the corresponding chiral dextrorotatory isobutyrates (+)-87 and (+)-89, respectively, using Candida cylindracea lipase (CCL, E.C. 3.1.1.3) as the biocatalyst73. For these bioconversions, methyl isobutyrate was used as solvent and acylation agent. When using acetoxime isobutyrate as the acylation agent and Chromobacterium viscosum lipase (CVL ... 

Optically active silanes of the type R3SiH react with lithium aluminum deuteride, halogens, oxygen bases, and perbenzoic acid stereoselectively, with retention. Generally, cyclic transition states have been proposed... 

NMR time scale for all counterions studied, i.e., Br, I-, C104, CF3S03, including also Cl- at low temperature. The rate constant was estimated to be at least of the order of 103 second-1 at 30°C. In light of this result it is reasonable to postulate a ligand exchange process for the nucleophile-induced racemization of optically active silanes (discussed in Section V,F). 

It should be stressed that Eq. (66) is not the sole representation of kinetics of nucleophile-activated racemization of optically active silanes. Silylonium complexes often appear to be thermodynamically more stable than the substrate in the reaction systems, i.e.,, [Nu] > -l. The steady-state conditions are not met, which usually leads to complex kinetics. On the other hand, the different kinetic laws observed in these processes (287,288) are related to other types of mechanisms, which are discussed later. 

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 previous publications we have demonstrated that biotransformations with whole microbial cells or free enzymes can be used for the preparation of optically active silicon and germanium compoimds [1]. In continuation of these studies, we synthesized a series of optically active silanes, germanes, and digermanes... 

Synthesis of Optically Active Silanes, Germanes, and Digermanes 239... 

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. 

The geometry of most organosUanes is tetrahedral using the sp hybridized orbitals on sihcon. Like carbon, optically active silanes are possible when central chirality exists. A series of monofunctional optically active silanes can be prepared and used for stereochemical studies. These are the methyl(naphthyl)phenylsilyl derivatives, which are commoifly abbreviated to R3 Si X. Together with the similar germanium compound, the isoconfigurational series of compounds are shown in Figure 1. 

Before reviewing the different routes to optically active silanes and discussing some relevant points, namely, the determination of enantiomeric purity and configuration, we shall give a brief overview of the stereochemical stability of chiral silicon species. 

Optically active silanes were obtained from symmetrical ketone, RCOR, in up to 46% enantiomeric excess, the catalyst being the only source of chirality. Asymmetric induction was also observed at the prochiral carbon of constitutionally unsymmetrical ketones, RCOR The optical yield at the carbon atom is different from that at silicon. This is well understood on the basis of kinetic Scheme 13. The diastereomeric complexes 56 and 57 interconvert rapidly in solution. Each complex reacts with different rates at the two faces (a and 0) of the ketone. The optical purity at the silicon center depends on the relative rates of... 

Owing to the interest of optically active C-centered organosilanes, Paquette and co-workers (refs. 116-118) have applied the Haller-Bauer reaction to optically active non-enolizable a-silyl phenyl ketones. An optically active silane (Fig. 23) was obtained with retention of configuration (96 to 98 %). These results are interpreted on the basis of an initial a-silyl carbanion formation within a solvent shell that also encases benzamide. 

Optically active silanes by use of asymmetric synthesis methods 312... 

Optically active silanes were obtained from symmetrical ketones RCOR, in up to 46% enantiomeric excess, the catalyst being the only source of chirality. Asymmetric induction was also observed at the prochiral carbon of constitutionally unsymmetrical ketones ... 

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. 

By analogy to the biotransformation of rac-221, the structurally related acetyldisilane rac-229 and the acetylsilane rac-231 were also reduced enantioselectively using resting free cells of Trigonopsis variabilis (DSM 70714)282,287. The conversion of rac-229 leads to a mixture of the optically active disilanes (R,R)-230 and (S,R)-230 (yield 75%, enantiomeric purity of both diastereomers >98% ee) and the transformation of rac-231 leads to a mixture of the optically active silanes (R,R)-232 and (S,R)-232 (yield 72%, enantiomeric purity of both diastereomers 94% ee). These reactions were carried out at 37 (rac-229) and 44 °C (rac-231), respectively the substrate concentrations used were 0.53 g/1. Especially remarkable is the biotransformation of the disilane rac-229 which could be realized without a noticeable degree of hydrolytic cleavage of the Si-Si bond. 

Stereoselective hydrolyses of (l-acetoxyalkyl)silanes have also been performed with intact microbial cells. An example of this is the highly enantioselective transformation of rac-(S,R/R,S)-235 into the optically active silanes (R,S)-226 using growing cells of the yeast Pichia pijperi (ATCC 20127)19 285 288. The product (R,S)-226 could be isolated with a yield of 80% [related to (R,S)-235] and an enantiomeric purity of >96%ee. 

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