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

Mechanistic studies of the reaction of (OC-6-3 2)-dicarbonylbis(methoxycarbonyl)[1, 2-bis(diphenylphosphino)- ethane]ruthenium(II) with p-toluidine to form N, Nʹ-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, Nʹ-di-p-tolylurea. Studies of the analogous bis(isopropylcarbamoyl) complex provided supporting evidence for isocyanate elimination. Thermolysis of (OC-6-32)-dicarbonylbis(isopropylcarbamoyl)[l, 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.