Asymmetric Cyanosilylation of Aldehydes

Optically pure cyanohydrins are versatile synthetic intermediates in the synthesis of a wide range of homochiral products such as a-hydroxy acids and P-hydroxy 

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Mori et al.149 also reported the asymmetric cyanosilylation of aldehyde with TMSCN using 132 (X = CN) as the precatalyst. The chiral dicyano complex was generated in situ, and the asymmetric cyanosilylation gave ee values of up to 75%. Scheme 2-58 depicts the proposed reaction process. 

Another example is the asymmetric cyanosilylation of aldehydes catalyzed by bifunctional catalyst 131.100 Compound 131 contains aluminum, the central metal, acting as a Lewis acid, and group X, acting as a Lewis base. The asymmetric cyanosilylation, as shown in Scheme 8-50, proceeds under the outlined... 

For other catalytic asymmetric cyanosilylation of aldehydes, see C.-D. Hwang, D.-R, Hwang, B.-J. Uang, Enantioselective Addition of Trimethylsilyl Cyanide to Aldehydes Induced by a New Chiral TiflV) Complex, J. Org Chem 1998,63,6762-6763, and references tited therein. 

Y. Hamashima, D. Sawada, M. Kanai, M Shibasaki A New Bifunctional Asymmetric Catalysis An Effident Catalytic Asymmetric Cyanosilylation of Aldehydes, J. Am Chem Soc 1999,121, 2641-2642. 

Although HCN also adds to these unsaturated bonds to afford cyanohydrins and a-amino nitriles, use of TMSCN instead of HCN provides an efficient and safer route to these compounds. Cyanosilylation with TMSCN is accelerated by acid and base catalysts. In recent years a variety of organic and inorganic compounds have been found to work as effective catalysts, and much attention has been devoted for the development of chiral catalysts for asymmetric cyanosilylation of aldehydes, ketones, and imines. 

Inoue et al. reported that a complex prepared from AlMes and peptide Schiff base 166 is available for asymmetric cyanosilylation of aldehydes (Scheme 10.239) [630]. The enantioselectivity observed is not as high, even with a stoichiometric amount of the complex (up to 71% ee). A more recent study by Snapper and Hoveyda has, however, revealed that a similar catalyst system using Al(()t-Pr) ), and peptide Schiff base 167 is quite effective in catalytic asymmetric cyanosilylation of both aromatic and aliphatic ketones (66->98%, 80-95% ee wifh 10-20 mol% of fhe catalyst) [631]. 

Al-BINOL complex 168a, developed by Shibasaki et al., is one of the most efficient catalysts for asymmetric cyanosilylation of aldehydes (Scheme 10.239) [632]. 

Scheme 19.2 Asymmetric cyanosilylation of aldehydes catalysed by Al(salen) complexes.

Scheme 19.3 Possible mechanism for asymmetric cyanosilylation of aldehydes catalysed by a bimetallic Al(salen) complex.

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