Ni(II)-Catalyzed Transfer Hydrogenation of Azoarenes with NH₃BH₃

Abstract

A nickel-catalyzed semihydrogenation of azoarenes to hydrazoarenes with NH₃BH₃ is developed. The catalytic system exhibits good functional group tolerance and a high turnover frequency at room temperature. Results of control and deuterium-labeling experiments indicate that the ethanol hydroxyl and BH₃ groups each donated one hydrogen to this transfer hydrogenation, and the main byproducts were B(OEt)₃ and H₂. Moreover, density functional theory calculations indicated that the reaction proceeded via a ligand-to-ligand hydrogen transfer mechanism. This study presents a novel nickel catalytic system for the semihydrogenation of azoarenes.