Highly Enantioselective Cyanosilylation of Aldehydes Catalyzed by a Chiral Oxazaborolidinium Ion

The chiral oxazaborolidinium salts 1 and 2 are excellent catalysts for the enantioselective cyanosilylation of a wide variety of aldehydes (see Table 1) using trimethylsilyl (TMS) cyanide and triphenylphosphine oxide as the source of a new reactive cyanide donor. This donor appears to be the isocyanophosphorane Ph3P(OTMS)(N=C:) (4). The novel process described herein has several advantages: predictability of absolute configuration of cyanohydrin products from a mechanistic model (3), high yields and very good enantiomeric purity of products (>/=90%), and, finally, easy and efficient recovery of the catalytic ligand.