Rhodium-Catalyzed Imine-Directed C-H Bond Functionalization

Abstract

A stereocenter can be set with high enantioselectivity (90%) using a catalytic C-H bond activation. The aromatic aldimines are more difficult substrates than their corresponding ketimines in the Rh-chiral phosphoramidite-mediated enantioselective cyclization reactions. Thus, it was necessary to modify the N-benzyl imine moiety to increase the efficiency of the cyclization. Going from the unsubstituted benzylaldimine 1b to the electron-poor modified 1c, the yield significantly increased from 31% to 57%. Decreasing the reaction temperature to 90 °C also afforded better results (61% yield, 90% ee). Intriguingly, the methylene moiety of the benzyl group appears to be critical in the transfer of chirality from catalyst to product; although imine 1a, derived from aniline, gives a higher yield than N-benzyl imine 1b, the ee value drops precipitously.