Homogeneous catalytic carbonylation of nitroaromatics. 8. Kinetic and mechanistic studies of the carbon-nitrogen bond and product forming steps from Ru(Ph2PCH2CH2PPh2)(CO) 2[C(O)OCH3]2: The turnover limiting reactions in the catalytic cycle

Article abstract of DOI:10.1021/ja00088a015

Mechanistic studies of the reaction of (OC-6-32)-dicarbonylbis(methoxycarbonyl)[1,2-bis(diphenylphosphino)-ethane] ruthenium(II) with p-toluidine to form N,N'-di-p-tolylurea are presented. The overall reaction was studied from 22 to 103°C and was found to be first order with respect to each reactant. Spectroscopic and kinetic studies between 22 and 52°C showed that the reaction proceeds through a species, Ru(dppe)(CO)₂[C(O)OCH₃] [C(O)-NH(p-tolyl)], which is in equilibrium with Ru(dppe)(CO)₂[C(O)OCH₃]₂. The mechanism of the C-N bond forming step is proposed to involve nucleophilic attack on a coordinated Ru-CO moiety with subsequent cleavage of the C(O)-OMe bond. The methoxycarbonyl-carbamoyl complex decomposes in a unimolecular fashion to liberate CH₃OH, the starting catalyst Ru(dppe)(CO)₃, and p-tolyl isocyanate, which is immediately scavenged by excess amine to form N,N'-di-p-tolylurea. Studies of the analogous bis(isopropylcarbamoyl) complex provided supporting evidence for isocyanate elimination. Thermolysis of (OC-6-32)-dicarbonylbis(isopropylcarbamoyl)[1,2-bis(diphenylphosphino)-ethane] ruthenium(II) yields isopropylamine, the starting catalyst Ru(dppe)(CO)₃, and isopropyl isocyanate, which reacts with isopropylamine over time to form diisopropylurea. The kinetics obtained from all of the stoichiometric reactions were combined into a suitable expression and found to lie on the same Arrhenius activation energy plot as the overall rate of the catalytic reaction.