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 Pd^II 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 <i>N,N’</i>-diphenylurea (DPU)) and hydrogenation products (aniline and DPU). The Pd^II(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 Pd^II(phen)) or of both the carbonylation and hydrogenation reaction (for Pd^II(L4X)). The palladacycle L₂PdC­(O)­N­(Ph)­OC­(O) (<b>C7</b>) and palladium–imido species L₂PdNPh (<b>C3</b>) 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-<b>C7</b> with L4X and L3X, ESI-MS analysis of L3X-<b>C7</b> and phen-<b>C7</b>, and a DFT study of nitrobenzene deoxygenation intermediates to L₂PdNPh all suggest that <b>C7</b> is not the major product-releasing intermediate; all data suggest that the barrier for <b>C7</b> decarbonylation (−CO) is lower than that for decarboxylation (−CO₂). <b>C7</b> is thus thought to be part of an “NPh reservoir” consisting of palladacycles that are mutually accessible through carbonylation/decarbonylation. Under <i>acidic conditions</i> the decarboxylation barrier is lowered; for phen-<b>C7</b> apparently to the point where decarboxylative alcoholysis is favored relative to decarbonylation, but for L4X-<b>C7</b> 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 <b>C3</b> is the prime “NPh” product-releasing intermediate and only under acidic conditions and in an alcoholic environment may <b>C7</b>for phenanthrolinebecome the predominant carbamate product releasing intermediate