Chiral organosilicon compounds

A chiral cationic rhodium complex has been shown to catalyse the enantioselective conjugate addition of silyl anion equivalents to cyclic a,fl-unsaturated ketones and esters, thus providing a facile access to chiral organosilicon compounds.247... 

Various metalated chiral organosilicon compounds bearing the SMP moiety have been alkylated to synthesize chiral alcohols. Excellent regio- and stereoselectivities have been observed in the alkylation of chiral silylpropargyl anions (eq 8). ... 

Kaye, P. T., Learmonth, R. A. Chiral organosilicon compounds. Parts. Peracid oxidation of chiral silyl enol ethers. Synth. Commun. 1990, 20, 1333-1338. 

A. Transition Metal Catalyzed Reactions of Chiral Organosilicon Compounds... 

Whereas the preparation of optically active materials using the principles of asymmetric synthesis has been widely used in carbon chemistry, the examples of asymmetric induction at heteroatoms are limited36. However, synthetic methods based on asymmetric synthesis have been applied successfuly to organosilicon chemistry and provide interesting routes to chiral organosilicon compounds. 

E. Asymmetric Synthesis of Chiral Organosilicon Compounds via Hydrosilylation. 1515... 

Our discussion to this point has been limited to molecules m which the chirality center IS carbon Atoms other than carbon may also be chirality centers Silicon like carbon has a tetrahedral arrangement of bonds when it bears four substituents A large number of organosilicon compounds m which silicon bears four different groups have been resolved into their enantiomers... 

This short review deals with the topical subject chirality in bioorganosilicon chemistry . Two different aspects will be discussed (i) biological recognition of enantiomeric organosilicon drugs and (ii) biocatalysis as a method for the preparation of optically active organosilicon compounds . Most of the experimental material described in this article (which does not lay claim to completeness) comes from our own laboratory. 

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. 

As discussed above, the cross-coupling reaction of organosilicon compounds proceeds stereospecifically, depending on the reaction conditions. Thus, the transformation C—Si C —C is demonstrated to be accompanied by chirality transfer. Now, the question arises of how to prepare organosilicon compounds whose chiral allylic carbon is substituted by a silyl group. The most accessible solution is asymmetric hydrosilylation of olefins [35]. We studied asymmetric hydrosilylation of 1-substituted 1,3-butadienes using... 

A number of organosilicon compounds with chiral silicon atoms have been obtained as single enantiomers by routes involving ... 

Transition metal catalyzed carbene insertion into the Si—H bond provides a direct and efficient method for the synthesis of organosilicon compounds. When chiral spiro diimine ligand (/ )-24a was applied in Cu-catalyzed asymmetric insertion of a-diazo-a-arylacetates with silanes, the Si—H insertion products were obtained in high yields (85-97%) and excellent enantioselectivities (90-99% ee) (Scheme 51) [26a],... 

Chiral organosilanes have been shown to undergo stereospecific catalytic reactions leading to the preparation of optically active silyl-transition metal complexes. We first discuss the stereochemistry and mechanism of transition metal catalyzed reactions of organosilicon compounds. Then the stereochemistry of chiral organosilyl-transition metal complexes are described. The chemistry of optically active silyl- and germyl-transition metals has been the subject of a recent review (12), and we concentrate here on mechanistic implications, especially in the field of homogeneous catalytic reactions. 

Organosilicon compounds containing C-Sn or C-B bonds undergo the reactions with various electrophiles preferentially at these bonds rather than at the C-Si bonds. A typical example is 2-(silylmethyl)allylstannmie The C-Sn bond exclusively reacts with aldehydes in the presence of a chiral titanium catalyst to give allylsilanes having a chiral center (Scheme 3-57). ... 

Kohra S, Hayashida H, Tominaga Y, Hosomi A. Pentaco-ordinate organosilicon compounds in synthesis asymmetric reduction of carbonyl compounds with hydrosilanes catalyzed by chiral bases. Tetrahedron Lett. 1988 29 89-92. 

Scheme 15)62. After terminating the reaction at a conversion of 38% (relative to total amount of substrate rac-78), the product (S)-43 was separated from the nonreacted substrate by column chromatography on silica gel and isolated on a preparative scale in 71% yield (relative to total amount of converted rac-78) with an enantiomeric purity of 95% ee. Recrystallization led to an improvement of the enantiomeric purity by up to >98% ee. The biotransformation product (S)-43 is the antipode of compound (/ )-43 which was obtained by enantioselective microbial reduction of the acylsilane 42 (see Scheme 8)53. The nonreacted substrate (/ )-78 was isolated in 81% yield (relative to total amount of nonconverted rac-78) with an enantiomeric purity of 57% ee. For further enantioselective enzymatic hydrolyses of racemic organosilicon esters, with the carbon atom as the center of chirality, see References 63 and 64. 

---