Asymmetric addition of dialkylzincs to benzaldehyde derivatives catalyzed by chiral β-amino alcohols. Evidence for the monomeric alkylzinc aminoalkoxide as catalyst

Abstract

3- exo-(Dimethylamino)isoborneol efficiently promotes the asymmetric addition of dialkylzincs to aldehydes. In order to characterize the catalytic organozinc species, relative reactivities of various substituted benzaldehydes and organozincs have been examined under enantiopure and racemic conditions. The relative rates are not affected by the enantiomeric purity of the amino alcohol. The Hammett plots of methylation of p-substituted benzaldehydes gave a ϱ value of +1.2. Methylation of o-methylbenzaldehyde is slower than that of benzaldehyde by a factor of 2.3. Relative reactivities of (CH 3) 2Zn, (C 2H 5) 2Zn, and ( n-C 4H 9) 2Zn with benzaldehyde were 1:21:8. The results strongly support the mechanism involving a monomeric alkylzinc aminoalkoxide as catalyst.