Ni–O Cooperation versus Nickel(II) Hydride in Catalytic Hydroboration of N-Heteroarenes

Abstract

An air-stable half-sandwich nickel(II) complex bearing a phosphinophenolato ligand, Cp*Ni(1,2-Ph2PC6H4O) (1), has been designed and synthesized for activation of HBpin and catalytic hydroboration of N-heteroarenes such as pyridine. Through addition of the H–B bond across the Ni–O bond, 1 reacts with HBpin to afford an 18-electron Ni(II)–H intermediate [H1(Bpin)] featuring an oxygen-stabilized boron moiety, which readily reduces pyridine analogues to give the 1,2-hydroborated product, thus accomplishing the catalytic cycle under mild conditions. The necessity of the phosphinophenolato ligand to deliver the boryl group was manifested by the clear difference in the activity of 1 and Cp*NiH(PPh3) (3H) in catalytic hydroborations. The latter lacks a functional oxygen atom and is inert to process the catalysis.