Carbonylation of nitrobenzene catalysed by palladium and heteropolyanions; a mechanistic approach

The previously reported catalytic system PdCl2-Keggin heteropolyanion for the carbonylation of nitrobenzene to methyl phenylcarbamate has been analysed in more detail. PdCl2 has been confirmed to be the best palladium precursor and the addition of pyridine hinders the reaction. The addition of aniline promotes the reaction at high nitrobenzene concentrations, but depresses it at low nitrobenzene concentrations. Performing the reaction in the presence of aniline, but in the absence of methanol, and using deuterated nitrobenzene, data are collected indicating that the reaction, which affords the corresponding urea, follows at least three different pathways, one of which consumes more aniline than nitrobenzene. Aniline is an intermediate in this as well as in one of the other two processes, but not in the third. The fate and possible reactivation of the spent catalyst have been examined. (C) 1997 Elsevier Science B.V.