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(μ-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.
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