Chemoselective heterogeneous iridium catalyzed hydrogenation of cinnamalaniline

Abstract

Selective hydrogenation of unsaturated imines over heterogeneous catalysts is an ecologically feasible and effective way to produce commercially valuable saturated imines and unsaturated amines under mild conditions, avoiding the utilization of toxic halides. The liquid-phase hydrogenation of a model imine, cinnamalaniline, over Ir, Ru, Pd and Au catalysts was studied in polar protic (methanol, 2-propanol), polar aprotic (methyl tert-butyl ether) and non-polar aprotic (toluene) solvents at 40–80 °C under atmospheric hydrogen pressure. Different metal oxides (Al₂O₃, ZrO₂, SiO₂) and carbon composites based on carbon nitrides synthesized by pyrolysis of ethylenediamine or melamine modified by KOH, HNO₃, NH₄Cl or TEOS (including template KIT-6), porous carbon (samples prepared by pyrolysis of sucrose, including template KIT-6) and mesoporous carbon Sibunit were applied to study the effect of the support. Among the tested metals, iridium exhibited the most promising catalytic performance in terms of hydrogenation activity and chemoselectivity towards unsaturated amine. The use of Ir on carbon nitrides (C₃N₄–NH₄Cl, C_xN_y-KIT-6) and amphoteric metal oxides (ZrO₂, Al₂O₃) in nonpolar aprotic toluene solvent provided the best selectivity towards unsaturated amine by minimizing oligomerization. Computational studies indicate that the selective hydrogenation of the CN group on Ir results from a favorable cinnamalaniline adsorption geometry at high surface coverage. Comparable heterogeneously catalyzed highly chemoselective hydrogenation of unsaturated imine into unsaturated amine under atmospheric hydrogen pressure and low temperatures has not been reported previously.