Highly active and selective gold catalysts for the aerobic oxidative condensation of benzylamines to imines and one-pot, two-step synthesis of secondary benzylamines

Abstract

Supported gold nanoparticles catalyze the oxidation of benzylamines to N-benzylidene benzylamines by molecular oxygen in toluene. The process is general, and para-substituted benzylamines as well as heterocyclic methanamines undergo oxidative condensation. 1-Phenylethanamine and diphenylmethanamine form imines with much less selectivity than benzylamines due to the unfavourable steric hindrance introduced by the substituents at the α-carbon. Secondary and tertiary dibenzyl and tribenzylamines form N-benzylidene benzylamines accompanied with aromatic ketones and oximes arising from C–N bond rupture. The efficiency of gold catalyst increases exponentially as the average particle size is reduced, and the TOF increases when the gold crystallite size is decreased. The solid support plays an important role as it has been found that gold supported on active carbon is more efficient than gold supported on anatase. Although palladium nanoparticles are also active in promoting benzylamine oxidation, only the catalyst based on platinum was found to exhibit an activity comparable to that of gold. Cross condensation of benzylamines in the presence of aromatic or aliphatic primary amines forms the mixed N-aryl or N-alkyl benzylimines with high yields. A one-pot, two-step procedure using a single gold catalyst has been devised to produce secondary symmetric and asymmetric amines derived from benzylamines with high atom efficiency.