Catalytic asymmetric synthesis of O-acetylcyanohydrins from potassium cyanide, acetic anhydride, and aldehydes, promoted by chiral salen complexes of titanium(IV) and vanadium(V)

The utility of the chiral [Ti(mu-O)(salen)](2) complexes (R)- and (S)-1 (H(2)salen was prepared from (R,R)- or (S,S)-cyclohexane-1,2-diamine and 3,5-di(tert-butyl)-2-hydroxybenzaldehyde) as catalysts for the asymmetric addition of KCN and Ac2O to aldehydes to produce O-acetylcyanohydrins was investigated. It was shown that the complexes were active at a substrate/catalyst ratio of 100: 1 and produced the O-protected cyanohydrins with ee in the range of 60-92% at -40degrees. Other complexes, [Ti-2(AcO)(2)(mu-O)(salen)(2)] ((R)-4) and [Ti(CF3COO)(2)(salen)] ((R)-5). were prepared from (R)-1 by treatment with different amounts of Ac2O and (CF3CO)(2)O, and their catalytic activities were tested under the same conditions. The efficiency of (R)-4 was found to be even greater than that of (R)-1, whereas (R)-5 was inactive. The synthesis of the corresponding salen complexes of V-IV and V-V, [V(O)(salen)] ((R)-2) and [V(O)(salen)(H2O)] [S(O)(3)OEt] ((R)-3), was elaborated, and their X-ray crystal structures were determined. The efficiency of (R)-3 was sufficient to produce O-acetyl derivatives of aromatic cyanohydrins with ee in the range of 80-91% at -40degrees.