Optically active silanes synthesis

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 ... 

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)... 

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. 

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

Asymmetric hydrosilylation of alkenes, catalysed by ferrocenylphosphine-palladium complexes, can be utilized in a synthesis of optically active alcohols from alkenes (Scheme 1). The initial optically active silane adducts are converted to alkyl pentafluorosilicates (1) which are then cleaved oxidatively, with retention of configuration, using peracid (c/. 3,132). Complex (2) was the best of several hydrosilylation catalysts examined using this, alcohols with enantiomeric purities of approx. 50% were obtained from norbornene and styrene as the alkenes. 

In this section, we comprehensively focused on the controlled synthesis, chiroptical characterization, and manipulation of optically active poly(dialkyl-silane)s. Although many artificial polymers adopting preferential screw sense... 

The axially chiral (allenylmethyl) silanes 110 were also prepared in optically active form using chiral Pd catalysts [98]. For the asymmetric synthesis of 110, a Pd/(R)-segphos system was much better in terms of enantioselectivity than the Pd/(R)-binap catalyst. Under the optimized conditions, 110m and llOt were obtained in 79% ee (57% yield) and 87% ee (63% yield), respectively (Scheme 3.56). The enantio-merically enriched (allenylmethyl) silanes 110 served for Lewis acid-promoted SE reaction with tBuCH(OMe)2 to give conjugated dienes 111 with a newly formed chiral carbon center (Scheme 3.56). During the SE reaction, the allenic axial chirality was transferred to the carbon central chirality with up to 88% transfer efficiency. 

Phosphine-containing compounds are prepared by reaction of [PPh3(CO) Mn]2 with silanes and this reaction allows the synthesis of an optically active complex ... 

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). 

Enantioselective enzymatic amide hydrolyses can also be applied for the preparation of optically active organosilicon compounds. The first example of this is the kinetic resolution of the racemic [l-(phenylacetamido)ethyl] silane rac-84 using immobilized penicillin G acylase (PGA E.C. 3.5.1.11) from Escherichia coli as the biocatalyst (Scheme 18)69. (R)-selective hydrolysis of rac-84 yielded the corresponding (l-aminoethyl)silane (R)-85 which was obtained on a preparative scale in 40% yield (relative to rac-84). The enantiomeric purity of the biotransformation product was 92% ee. This method has not yet been used for the synthesis of optically active silicon compounds with the silicon atom as the center of chirality. 

Enantioenriched ( )-allyl silane 376 is a highly useful building block for the synthesis of optically active tetra-hydropyrans 377, reacting with aldehydes and simple ketones with very effective chirality transfer in the presence of a Lewis acid (Equation 155, Table 18) <1998JOC6096>. 

The preparation of the optically active title compounds is based on the synthesis of the racemic silane roc-la and the racemic germane roc-lb, followed by their resolution into the respective Ry- and (5)-enantiomers and subsequent transformation of the latter compounds into the antipodes of the quaternary ammonium derivatives 2a and 2b. 

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. 

Palladium-catalyzed hydrosilylation of 1,3-dienes is one of the important synthetic methods for allylic silanes, and considerable attention has been directed to the asymmetric synthesis of the latter by catalytic methods [9]. Optically active allyhc silanes have been used as chiral allylating reagents in S reactions with electrophiles, typically aldehydes [38,39]. In the presence of Pd catalysts the reaction with hydrosilanes containing electron-withdrawing atoms or substituents on sihcon usually proceeds in a 1,4-fashion giving allyHc silanes [40,41]. Asymmetric hydrosilylation of cyclopentadiene (29) forming optically active 3-silylcyclopentene (30) has been most extensively studied (Scheme 13). In the first report, hydrosilylation of cyclopentadiene (29) with methyldichlorosilane in the presence of 0.01 mol % of palladium-(l )-(S)-PPFA (15a) as a catalyst gave... 

The synthesis of optically active syn- and tf/ / -diastereomers of ( )-crotylsilanes is achieved by the Ireland protocol starting with the vinyl silanes 14 and 16480 481. 

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... 

The selectivity shown by allylsilanes towards electrophiles makes asymmetric synthesis via chiral silanes an attractive prospect. A series of optically active allylsilanes has been prepared... 

Tris(trimethylsilyl)silane in organic synthesis step feature in a concise synthesis of optically active (—)-zearalenone °. 

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