A superior, readily available enantiopure ligand for the catalytic enantioselective addition of diethylzinc to a-substituted aldehydes

The lithium perchlorate-induced ring opening of (S)-triphenylethylene oxide (3) with secondary amines (piperidine (a), N-methylpiperazine (b), AT-phenylpiperazine (c) and morpholine (d)) takes place in a stereospecific and completely regioselective manner to afford CR)-2-(dialkylamino)-l, 1, 2-triphenylethanols (4a-d). These amino alcohols catalytically induce the addition of diethylzinc to benzaldehyde with high enantioselectivity at 0 °C and at room temperature. Ligand 4a, which provides the highest enantioselectivity at 0 °C, has been studied in the addition of Et2Zn to a family of 20 representative aliphatic and aromatic aldehydes 5a-t. For a 17-membered set of a-substituted substrates (5a-m, q-t), including ortho-, meta-, and para-substituted benzaldehydes, the naphthaldehydes, ct./î-unsaturated and aliphatic (cyclic and acyclic) aldehydes, the mean enantiomeric excess of the resulting alcohols 6a-m, q-t is 97%, whereas for three a-unsubstituted specimens (5n-p) the addition takes place with an enantioselectivity of 92-93%. © 1998 American Chemical Society.