Catalytic Hydroboration and Reductive Amination of Carbonyl Compounds by HBpin using a Zinc Promoter.

Abstract

The chemoselective hydroboration of aldehydes and ketones, catalyzed by Zinc(II) complexes [κ2 -(PyCH=NR)ZnX2 ] [R=CPh3 , X=Cl (1) and R=Dipp (2,6-diisoropylphenyl) and X=I (2)], in the presence of pinacolborane (HBpin) at ambient temperature and under solvent-free conditions, which produced the corresponding boronate esters in high yield, is reported. Zinc metal complexes 1 and 2 were derived in 80-90% yield from the reaction of iminopyridine [PyCH=NR] with anhydrous zinc dichloride in dichloromethane at room temperature. The solid-state structures of both zinc complexes were confirmed using X-ray crystallography. Zinc complex 1 was also used as a competent pre-catalyst in the reductive amination of carbonyl compounds with HBpin under mild and solvent-free conditions to afford a high yield (up to 97%) of the corresponding secondary amines. The wider substrate scope of both reactions was explored. Catalytic protocols using zinc as a pre-catalyst demonstrated an atom-economic and green method with diverse substrates bearing excellent functional group tolerance. Computational studies established a plausible mechanism for catalytic hydroboration.