Activation of nitroarenes in the homogenous catalytic carbonylation of nitroaromatics via an oxygen-atom-transfer mechanism induced by inner-sphere electron transfer

Kinetic and mechanistic studies on the deoxygenation of nitroarenes by Ru(dppe)(CO)₃, where dppe = 1,2-bis(diphenylphosphino)ethane, are described. The products of the reaction included 1 equiv of carbon dioxide and an η²-nitrosoarene ruthenium complex (Ru(dppe)(CO)₂ [ON(Ar)] for Ar = 4-chloro-2-trifluoromethylphenyl), which was isolated and fully characterized by solution spectroscopic methods and by single crystal X-ray diffraction [monoclinic crystal system, space group P2₁/c (#14), a = 14.556 (8) Å, b = 12.903 (6) Å, c = 20.10 (1) Å, β = 105.60 (6)°, V = 3636 (8) ų, Z = 4]. The deoxygenation reaction was determined to be first-order with respect to both Ru(dppe)(CO)₃ and nitroarene. Electron withdrawing substituents on the nitroarene and polar solvents accelerated the rate, and a substituent study provided a p of +3.45 indicating negative charge buildup on the nitroarene in the rate determining step of the reaction. Activation enthalpies for 2-CF₃, 4-Cl, 4-H, and 4-CH₃ substituted nitroarenes were 9.3, 9.9, 10.5, and 10.7 kcal mol^-1, and the entropies of activation were -35, -33, -36, and -37 eu, respectively. Correlation between the reduction potentials of the nitroarenes (E°(ArNO2)) and log k₂ was also observed for substituted nitroarenes yielding a slope of 10 V^-1. Monosubstituted nitroarenes bearing a single methyl, phenyl, or chloro group in the ortho position and disubstituted 2,6-dimethyl- and 2,3-dichloronitrobenzene showed no attenuation in the rate, from what would be expected based on the E°(ArNO2) - log k₂ correlation. Large rate attenuation was observed for nitroarenes bearing both ortho and meta groups. Analysis of the kinetic and thermodynamic data using Marcus theory indicated that the rates were too high for an outer-sphere electron-transfer mechanism. The data were interpreted in terms of an inner-sphere electron-transfer mechanism where the unfavorable energetics are mitigated by bonding interactions between the donor and acceptor.