The asymmetric addition of trimethylsilyl cyanide to aldehydes catalyzed by chiral (salen)titanium complexes

The use of chiral (salen)TiCl2 complexes to induce the asymmetric addition of trimethylsilyl cyanide to aldehydes has:been investigated. The complexes are catalytically active at substrate-to-catalyst ratios as high as 1000:1, and the optimal catalyst (2e) which is derived from (R,R)-1,2-diaminocyclohexane and 3,5-di-tert-butyl-2-hydroxybenzaldehyde produces trimethylsilyl ethers of cyanohydrins with up to 90% enantiomeric excess at ambient temperature. Water plays a key role in these reactions since under strictly anhydrous conditions much lower enantiomeric excesses are produced. The role of water has been shown to be to generate dimeric complexes of the form [(salen)Ti(mu-O)](2) (4) which are the real catalyst precursors. A structure for one of these complexes (4a derived from (R,R)-1,2-diaminocyclohexane and 2-hydroxybenzaldehyde) has been determined by X-ray crystallography. The dimeric complexes are more active than the dichloride precursors, and at substrate-to-catalyst ratios between 100 and 1000:1 give cyanohydrin trimethylsilyl ethers with up to 92% enantiomeric excess in less than 1 h at ambient temperature.