A Superior, Readily Available Enantiopure Ligand for the Catalytic Enantioselective Addition of Diethylzinc to alpha-Substituted Aldehydes.

The lithium perchlorate-induced ring opening of (S)-triphenylethylene oxide (3) with secondary amines (piperidine (a), N-methylpiperazine (b), N-phenylpiperazine (c) and morpholine (d)) takes place in a stereospecific and completely regioselective manner to afford (R)-2-(dialkylamino)-1,1,2-triphenylethanols (4a-d). These amino alcohols catalytically induce the addition of diethylzinc to benzaldehyde with high enantioselectivity at 0 degrees C and at room temperature. Ligand 4a, which provides the highest enantioselectivity at 0 degrees C, has been studied in the addition of Et(2)Zn to a family of 20 representative aliphatic and aromatic aldehydes 5a-t. For a 17-membered set of alpha-substituted substrates (5a-m,q-t), including ortho-, meta-, and para-substituted benzaldehydes, the naphthaldehydes, alpha,beta-unsaturated and aliphatic (cyclic and acyclic) aldehydes, the mean enantiomeric excess of the resulting alcohols 6a-m,q-t is 97%, whereas for three alpha-unsubstituted specimens (5n-p) the addition takes place with an enantioselectivity of 92-93%.