Cobalt-catalyzed transfer hydrogenation of nitroarenes to arylhydroxylamines with NH3BH3: Scope and mechanism

Abstract

A novel cobalt-catalyzed transfer hydrogenation of nitroarenes to arylhydroxylamines using ammonia borane (NH₃BH₃) in methanol has been developed. The cobalt catalyst exhibits good selectivity without producing over-reduced by-products. Density functional theory (DFT) calculations and control experiments provide valuable insights into the reaction mechanism, revealing the N–H moiety of NH₃BH₃ as a key participant in nitroarene reduction. Moreover, DFT calculations reveal that selective control is achieved via the unique square-pyramidal coordination of the cobalt complex with NH₃BH₃ and hydroxylamine. This coordination ensures that the reduction of nitrobenzene halts at the hydroxylamine stage, enabling the selective reduction of nitrobenzene to hydroxylamine. Moreover, methanol is indirectly involved in the reaction and does not provide hydrogen atoms directly for nitroarene reduction, which differs from the classical solvent-mediated protonolysis mechanism. This study presents a novel and straightforward method for synthesizing arylhydroxylamines.