Catalytic Asymmetric Synthesis of O-Acetylcyanohydrins from Potassium Cyanide, Acetic Anhydride, and Aldehydes, Promoted by Chiral Salen Complexes of Titanium (IV) and Vanadium (V)

摘要

The utility of the chiral [Ti(mu-O)(salen)]_2 complexes (R)-and (S)-1 (H_2salen 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 Ac_2O 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 -40 deg. Other complexes, [Ti_2(AcO)_2(mu-O)(salen)_2] ((R)-4) and [Ti(CF_3COO)_2(salen)] ((R)-5), were prepared from (R)-1 by treatment with different amounts of Ac_2O and (CF_3CO)_2O, 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) (H_2O)] [S(O)_3OEt] ((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 -40 deg.