Enantioselective cyanosilylation of aldehydes catalyzed by a diastereomeric mixture of atropisomeric thioureas

Atropisomeric thioxothiazolidinylphenyl dimethylaminocyclohexyl thiourea I and its (S,S)-(dimethylamino)cyclohexyl diastereomer are prepd.; in the presence of I (as a 1:1 mixt. of atropisomers about the aryl-thiazolinethione bond), trimethylsilyl cyanide adds to aldehydes to give (trimethylsilyloxy)nitriles which are hydrolyzed and acetylated to give nonracemic a-acetyloxy nitriles (S)-RCH(OAc)CN [R = Ph, cyclohexyl, 4-ClC6H4, 2-naphthyl, 2-furyl, (E)-PhCH:CH, PhCH2CH2, Me2CH] in 47-69% ee. The atropisomers of aminophenylthiazolidinethione II are sepd. by prep. HPLC and characterized; the barrier to rotation about the aryl-thiazolidinethione bond is > 145 kJ/mol. A phenyl-substituted thiourea catalyzes the addn. of trimethylsilyl cyanide to benzaldehyde to yield (S)-O-acetylmandelonitrile in 32% ee, while I acts as a catalyst for the same reaction to yield (S)-O-acetylmandelonitrile in 66% ee; sepd. diastereomers of I with (R)-biaryl stereochem. and with (R,R) and (S,S)-aminocyclohexyl stereochem. give (S)- and (R)-O-acetylmandelonitriles in 47% ee and 55% ee, resp.