Carbon–Hydrogen Bond Activation of Arenes by a [Bis(oxazolinyl)phenyl]rhodium(III) Acetate Complex



Thermolysis of the rhodium(III) complex [(dm-Phebox-dm)Rh(OAc)2(H2O)] [1; dm-Phebox-dm = 2,6-bis(4,4-dimethyloxazolinyl)phenyl] in various arenes results in the formation of the corresponding aryl complexes [(dm-Phebox-dm)Rh(Ar)(κ2-OAc)] [Ar = C6H5 (2), 3,5-Me2C6H3 (3), 3,4-Me2C6H3 (4), C6H4Me (5), C6H4CF3 (6), C6H4OMe (7), C6H4COMe (8), C6H4Cl (9)]. The reaction of 1 with monosubstituted benzenes such as toluene, anisole, acetophenone, trifluorotoluene, and chlorobenzene produces a mixture of meta- and para-activated complexes, the ratio of which depends on the substituents on the benzene ring. The relative rate of the reaction of 1 with monosubstituted benzenes C6H5X was determined to be: X = OMe (1.8) > COMe (1.6) > CF3 (1.2), Cl (1.2) > CH3 (1). The acetato ligand of 1 appears to be essential for C–H bond activation of arenes as no reaction of the RhIII complex [(dm-Phebox-dm)RhCl2(H2O)] is observed in chlorobenzene at 120 °C. The first-order rate constants obtained by thermolysis of 1 in [D8]toluene at 97.1–116.5 °C yielded the following activation parameters: ΔH = 22(2) kcal mol–1, ΔS = –24(5) cal mol–1 K–1. The kinetic isotope effect for the C–H bond activation of toluene by 1 was determined to be kH/kD = 5.4 at 100 °C. These data suggest that the rate-determining step involves the C–H bond cleavage with a rigid, cyclic transition structure. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)