Abstract
The asymmetric aldol reaction of isatin derivatives have been realized by using Pd(II)-BINAP complexes as catalysts from Mikami and co-workers. A series of chiral vanadyl complexes derived from N-salicylidene amino acids and tetrazoles were developed. Their structural idendities were proven by X-ray crystallographic analysis. It was found that they can achieve complementary asymmetric catalytic aldol processes in the addition of silyl ketene acetals (SKA) to isatins at -40oC in CH2Cl2 in 66-69h. The results indicate that complementary stereoelectronic bias imposed by the C3 substituent of the salicylidene template or the tetrazole moiety are responsible for the observed complementary enantiofacial preferences in the aldol addition. On the basis of the intrinsic coordination attributes around the vanadyl center of these chiral vanadyl(V) methoxide complexes, we would expect that the initially generated vanadyl-bound enolate should occupy the basal position trans to the Schiff base and the isatin substrate should take on the apical position trans to the V=O unit. Under such circumstance, si face of the carbonyl group of isatin would be blocked by the C-3 substituent (e.g., tert-butyl) of the salicylidene template. The best enantioselectivity (93% ee) can be achieved by using type-I catalyst bearing 3-tert-butyl and 5-bromo substituents. Complementary re-face blockage with high eneatioselectivity (up to -83% ee) can be effected by the tetrazole moiety by using type-II catalyst bearing only 5-nitro substituent.