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 d escribed. The products of the reaction included 1 equiv of carbon diox ide and an eta(2)-nitrosoarene ruthenium complex (Ru(dppe)(CO)(2)[ON(A r)] for Ar = 4-chloro-2-trifluoromethylphenyl), which was isolated and fully characterized by solution spectroscopic methods and by single c rystal X-ray diffraction [monoclinic crystal system, space group P2(1) /c (#14), a = 14.556 (8) Angstrom, b = 12.903 (6) Angstrom, c = 20.10 (1) Angstrom, beta = 105.60 (6)degrees, V = 3636 (8) Angstrom(3), Z = 4]. The deoxygenation reaction was determined to be first-order with r espect to both Ru(dppe)(CO)(3) and nitroarene. Electron withdrawing su bstituents on the nitroarene and polar solvents accelerated the rate, and a substituent study provided a rho of +3.45 indicating negative ch arge buildup on the nitroarene in the rate determining step of the rea ction. Activation enthalpies for 2-CF3, 4-Cl, 4-H, and 4-CH3 substitut ed nitroarenes were 9.3, 9.9, 10.5, and 10.7 kcal mol(-1), and the ent ropies of activation were -35, -33, -36, and -37 eu, respectively. Cor relation between the reduction potentials of the nitroarenes (E(ArNO2) degrees) and log k(2) was also observed for substituted nitroarenes yi elding a slope of 10 V-1. Monosubstituted nitroarenes bearing a single methyl, phenyl, or chloro group in the ortho position and disubstitut ed 2,6-dimethyl- and 2,3-dichloronitrobenzene showed no attenuation in the rate, from what would be expected based on the E(ArNO2)degrees - log k(2) correlation. Large rate attenuation was observed for nitroare nes bearing both ortho and meta groups. Analysis of the kinetic and th ermodynamic data using Marcus theory indicated that the rates were too high for an outer-sphere electron-transfer mechanism. The data were i nterpreted in terms of an inner-sphere electron-transfer mechanism whe re the unfavorable energetics are mitigated by bonding interactions be tween the donor and acceptor.