Enantioselective α-Fluorination and α-Chlorination of N-Acyl Pyrazoles Catalyzed by a Non-C2-Symmetric Chiral-at-Rhodium Catalyst

A non-C2-symmetric and sterically demanding chiral-at-rhodium catalyst is demonstrated to efficiently catalyze the highly enantioselective α-fluorination [12 examples, up to >99% enantiomeric excess (ee)] and α-chlorination (12 examples, up to 98% ee) of N-acyl pyrazoles in high yields. Based on two sterically distinct cyclometalating ligands, the nonracemic rhodium(III) catalyst can conveniently be accessed in an enantiomerically pure fashion (>99% ee) via an established auxiliary-mediated approach. Comparison of the catalytic performance with the related C2-symmetric rhodium catalysts revealed the explicit superiority of the non-C2-symmetric design for the presented α-halogenation reactions, which are generally featured by a very simple synthetic protocol.