| Abstract
| - The use of chiral (salen)TiCl2 complexes to induce the asymmetric addition of trimethylsilyl cyanideto aldehydes has been investigated. The complexes are catalytically active at substrate-to-catalyst ratios ashigh 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% enantiomericexcess at ambient temperature. Water plays a key role in these reactions since under strictly anhydrous conditionsmuch lower enantiomeric excesses are produced. The role of water has been shown to be to generate dimericcomplexes of the form [(salen)Ti(μ-O)]2 (4) which are the real catalyst precursors. A structure for one of thesecomplexes (4a derived from (R,R)-1,2-diaminocyclohexane and 2-hydroxybenzaldehyde) has been determinedby 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% enantiomericexcess in less than 1 h at ambient temperature.
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