Selective hydrogenation of benzonitrile and its homologues to primary amines over platinum

Various supported precious metal (Pt, Rh, Ru, Ir) catalysts have been screened in the reduction of benzonitrile (BN), benzyl cyanide (BC) and 3-phenylpropionitrile (PPN) to benzylamine (BA), 2-phenylethylamine (PEA) and 3-phenylpropylamine (PPA), using our method for selective, heterogeneous, palladium-catalysed hydrogenation of nitriles to primary amines developed previously. A readily available platinum on carbon catalyst (10% Pt/C) afforded the best results obtaining complete conversion of nitriles, as well as comparatively high isolated yields (58–70%) and selectivities to primary amines (57–68%) under mild and optimized conditions [6 bar, 30 °C, two immiscible solvents (dichloromethane/water or toluene/water), acidic additives (NaH2PO4, (NH4)H2PO4 or H2SO4)]. Contrary to the typical high secondary amine selectivity of platinum, surprisingly, this Pt/C catalyst proved to be much more effective and selective in the formation of PEA and PPA than a Pd/C one applied earlier. To clear the divergences in the primary amine selectivity observed, quantum chemical calculations (DFT) were accomplished related to the adsorption interactions between the imine intermediates and platinum.