Heterogeneously catalyzed self-condensation of primary amines to secondary amines by supported copper catalysts

Abstract

Self-condensation of primary amines to symmetrically substituted secondary amines could efficiently be promoted by an inexpensive supported copper catalyst, Cu/Al₂O₃, easily prepared by the reduction of the hydroxide precursor, Cu(OH)_x/Al₂O₃. Various kinds of structurally diverse primary amines including benzylamine, picolylamine, and aliphatic amine derivatives could selectively be converted into the corresponding secondary amines in moderate to excellent yields without any cocatalysts such as bases and stabilizing ligands in 1 atm of Ar or H₂. The reactions in H₂ showed higher selectivities to desired secondary amines than those in Ar. The roles of H₂ are the promotion of hydrogenation of N-alkylimines and the stabilization of active Cu(0) species. In addition, in the presence of Cu/Al₂O₃, unsymmetrically substituted secondary amines could efficiently be synthesized by N-alkylation of primary amines with alcohols and reductive amination of aldehydes. The observed catalysis was truly heterogeneous, and the retrieved Cu/Al₂O₃ catalyst could be reused for self-condensation without a significant loss of its catalytic performance. The reaction mechanism involving dehydrogenation of primary amines and condensation to N-alkylimines followed by hydrogenation, the so-called “borrowing hydrogen pathway”, has been proposed.