Soluble and Polymer-Anchored Rhodium Catalyst for Carbonylation Reaction: Kinetics and Mechanism of Diphenylurea Formation

The high catalytic activity of rhodium(I) complexes toward the reductive carbonylation of nitroaromatics under high carbon monoxide pressure prompted investigation of the polymer-bound complex of rhodium(I). The polymer-supported complex [RhA(Ph2P–CH2–polystyrene)CO] having the same coordination environment as [RhA(CO)PPh3] (HA; 2-aminobenzoic acid) was prepared and the catalytic behavior studied under various reaction conditions and methanol concentrations. The carbonylation of nitrobenzene at low temperature and atmospheric or elevated pressure using both soluble and polymer-supported rhodium species is reported. N,N′-Diphenylureais the main product under moderate carbon monoxide pressure while methyl-N-phenyl carbamateis formed predominantly as Pco and methanol concentration is increased. Spectroscopic and kinetic studies showed that the reaction proceeds through the species [Rh(A)(PPh3)(COOCH3)(μ-OCH3)]2 and the isocyanate formed at an intermediate stage is immediately scavenged by excess amine to form N,N′-diphenylurea. A tentative reaction mechanism based on the identification of reactive intermediates has been proposed for the carbonylation process.