Mechanistic Study of the L2Pd-Catalyzed Reduction of Nitrobenzene with CO in Methanol: Comparative Study between Diphosphane and 1,10-Phenanthroline Complexes

The catalytic activity of PdII compounds supported by 1,10-phenanthroline (phen) or the bidentate diarylphosphane ligand L4X has been studied in the reaction of nitrobenzene with CO in methanol. Both systems are ∼70% selective for azoxybenzene and azobenzene but also produce carbonylation products (methyl phenyl carbamate (MPC) and N,N’-diphenylurea (DPU)) and hydrogenation products (aniline and DPU). The PdII(L4X) system also produces methanol oxidation products (dimethyl carbonate, dimethyl oxalate, water). Upon the addition of a catalytic amount of acid, the coupling reaction is suppressed in favor of either the carbonylation reaction (for PdII(phen)) or of both the carbonylation and hydrogenation reaction (for PdII(L4X)). The palladacycle L2PdC­(O)­N­(Ph)­OC­(O) (C7) and palladium–imido species L2PdNPh (C3) were considered as possible carbonylation product-releasing species, where L2 is phen or the diphosphane ligands L4X and L3X. A ligand exchange experiment of phen-C7 with L4X and L3X, ESI-MS analysis of L3X-C7 and phen-C7, and a DFT study of nitrobenzene deoxygenation intermediates to L2PdNPh all suggest that C7 is not the major product-releasing intermediate; all data suggest that the barrier for C7 decarbonylation (−CO) is lower than that for decarboxylation (−CO2). C7 is thus thought to be part of an “NPh reservoir” consisting of palladacycles that are mutually accessible through carbonylation/decarbonylation. Under acidic conditions the decarboxylation barrier is lowered; for phen-C7 apparently to the point where decarboxylative alcoholysis is favored relative to decarbonylation, but for L4X-C7 the decarbonylation barrier still seems lowest due to the destabilizing effect that this bulkier ligand has on such palladacycles. It is thus concluded that the L2PdNPh complex C3 is the prime “NPh” product-releasing intermediate and only under acidic conditions and in an alcoholic environment may C7for phenanthrolinebecome the predominant carbamate product releasing intermediate.