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

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.