6,6'-Substituent effect of BINOL in bis-titanium chiral Lewis acid catalyzed 1,3-dipolar cycloaddition of nitrones

摘要

By the accommodation of modified BINOLs as chiral lig-ands, enantioselectivities in the bis-titanium chiral Lewis acid catalyzed 1,3-dipolar cycloaddition of N-diphenylmethyl nitrones and methacrolein could be improved. In the field of asymmetric Lewis acid catalysis, 1,1'-binaphthyl-2,2'-diol (BINOL) is one of the best known privileged ligands having axial chirality, and has been successfully applied in a great number of enantioselective reactions. It is also known that the introduction of an electron-withdrawing group at 6,6'-position of BINOL affects the reactivity and enantioselectivity by changing the Lewis acidity and the chiral environment of the catalyst. In our recent study, the oxygen-bridged bis-titanium chiral Lewis acid (S,S)-la (X = H, Fig. 1) containing BINOL ligands has been revealed to catalyze some asymmetric reactions, such as asymmetric allylation and 1,3-dipolar cycloaddition of nitrones. We report herein a detailed study of the 6,6'-substituent effect of BINOL in our catalytic system, which led to the identification of a new highly efficient catalyst.