Catalytic hydroboration of carbonyl derivatives by using phosphinimino amide ligated magnesium complexes

Reduction of carbonyl derivatives by using Earth-abundant, cheap, and environmentally benign metal-based catalysts through an atom-efficient method is a challenging task. Herein, we report the synthesis and characterization of dinuclear magnesium complexes 1-3 chelated by a phosphinimino amide skeleton. In combination with pinacolborane (HBpin) as a reducing agent, complex 1 bearing an ortho-methyl substituent on the phenyl ring of the ligand showed excellent reduction capability for a broad range of carbonyl derivatives under mild reaction conditions. Aldehydes, ketones, and acrolein substrates were efficiently reduced to the corresponding alkoxy-borane products with a record high TOF. Besides, acrolein derivatives were exclusively reduced to 1,2-regioselective products. Using two equiv. of HBpin, ester substrates were reduced to two kinds of alkoxy-borane products. Carbonate reduction accomplished by using complex 1 and three equiv. of HBpin afforded diols and a methanol precursor, respectively. When chiral substrates such as (S)-1,2-propanediol carbonate and L-lactide or polymeric P(L-LA) were employed, the chirality was almost retained in their reductive products.