The asymmetric addition of trimethylsilyl cyanide to aldehydes can be catalysed by Lewis acids and/or Lewis bases, which activate the aldehyde and trimethylsilyl cyanide, respectively. It is not always apparent from the structure of the catalyst whether Lewis acid or Lewis base catalysis predominates. To investigate this in the context of using salen complexes of titanium, vanadium and aluminium as catalysts, a Hammett analysis of asymmetric cyanohydrin synthesis was undertaken. When Lewis acid catalysis is dominant, a significantly positive reaction constant is observed, whereas reactions dominated by Lewis base catalysis give much smaller reaction constants. [{Ti(salen)O}(2)] was found to show the highest degree of Lewis acid catalysis, whereas two [VO(salen)X] (X=EtOSO(3) or NCS) complexes both displayed lower degrees of Lewis acid catalysis. In the case of reactions catalysed by [{Al(salen)}(2)O] and triphenylphosphine oxide, a non-linear Hammett plot was observed, which is indicative of a change in mechanism with increasing Lewis base catalysis as the carbonyl compound becomes more electron-deficient. These results suggested that the aluminium complex/triphenylphosphine oxide catalyst system should also catalyse the asymmetric addition of trimethylsilyl cyanide to ketones and this was found to be the case.
Cyanohydrins Q 0521A Bimetallic Aluminum(salen) Complex for Asymmetric Cyanohydrin Synthesis.-Optimized reaction conditions using a chiral aluminum complex and phosphine oxide as catalysts are developed for the addition of trimethylsilyl cyanide to aldehydes. -(NORTH*, M.; WILLIAMSON, C.; Tetrahedron Lett. 50 (2009) 26, 3249-3252; Sch. Chem., Univ. Newcastle, Newcastle upon Tyne NE1 7RU, UK; Eng.) -Mais 41-076
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