Homogeneous Catalytic Carbonylation of Nitroaromatics. 5. Kinetics and Mechanism of Aniline and Carbamate Formation Using Ru(Ph2PCH2CH2PPh2)(CO)3

Jerry D. Gargulak, Amanda J. Berry, Mark D. Noirot, Wayne L Gladfelter

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Abstract

Kinetic and mechanistic studies of the catalytic conversion of nitroaromatics and methanol to methyl N-arylcarbamates utilizing elevated pressures of CO, elevated temperatures, and the presence of a homogeneous catalyst, Ru(dppe)(CO)3 where dppe = 1,2-bis(diphenylphosphino)ethane, are described. The kinetics were studied in o-xylene over a range of CO pressures (22-103 atm), temperatures (125-155 °C), catalyst concentrations (1.3-11.2 mM), methanol concentrations (1.2-24.7 M), and initial aniline concentrations (2.2-52 mM). Aniline was confirmed to be a byproduct of the reaction as well as an intermediate in the formation of carbamate. Aniline formation was first order in catalyst concentration and inverse first order with respect to the pressure of CO. The rate of carbamate formation was first order in both catalyst concentration and aniline concentration. The integrated kinetic model gave good fits to the data over the above range of parameters. Stoichiometric reactions established that the catalytic cycle can be separated into three distinct segments each occurring with an increasing kinetic barrier. Phase 1, which occurs at room temperature, involves the reaction of Ru(dppe)(CO)3 with ArNO2 and CO to give the unusual complex Ru(dppe)(CO)2[C(O)N(Ar)O] which was characterized using solution spectroscopic and, for Ar = p-chlorophenyl, X-ray crystallographic methods [triclinic crystal system, PI space group, a = 9.372 (5) Å, b = 10.59 (2) Å, c = 17.24 (1) Å, α = 102.0 (1)°, β = 104.40 (4)°, γ = 100.46 (9)°, V = 1572 (6), Z = 2], Phase 2 involves the reaction of Ru(dppe)(CO)2[C- (O)N(Ar)O] with methanol and CO. Aniline and CO2 are released, and the new bis(methoxycarbonyl) complex, Ru- (dppe)(CO)2[C(O)OMe]2, was isolated in high yield. This reaction proceeds through an intermediate identified as the bis(methanolato) complex, Ru(dppe)(CO)2(OMe)2. Carbamate formation occurs during Phase 3 of the catalysis and involves the reaction of ArNH2 with Ru(dppe)(CO)2[C(O)OMe]2. Both in situ infrared spectroscopy and 31P NMR spectroscopy of rapidly quenched samples withdrawn from the catalytic solutions establish that Ru(dppe)(CO)2 [C(O)OMe] 2 is the major species present during the catalysis. Details of the proposed mechanism of the catalysis are discussed along with comments relating these results to other catalytic systems.

Original languageEnglish (US)
Pages (from-to)8933-8945
Number of pages13
JournalJournal of the American Chemical Society
Volume114
Issue number23
DOIs
StatePublished - Nov 1 1992

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