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 (H 2 O)] [ 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)(κ 2 ‐OAc)] [Ar = C 6 H 5 ( 2 ), 3,5‐Me 2 C 6 H 3 ( 3 ), 3,4‐Me 2 C 6 H 3 ( 4 ), C 6 H 4 Me ( 5 ), C 6 H 4 CF 3 ( 6 ), C 6 H 4 OMe ( 7 ), C 6 H 4 COMe ( 8 ), C 6 H 4 Cl ( 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 C 6 H 5 X was determined to be: X = OMe (1.8) > COMe (1.6) > CF 3 (1.2), Cl (1.2) > CH 3 (1). The acetato ligand of 1 appears to be essential for C–H bond activation of arenes as no reaction of the Rh III complex [( dm ‐Phebox‐ dm )RhCl 2 (H 2 O)] is observed in chlorobenzene at 120 °C. The first‐order rate constants obtained by thermolysis of 1 in [D 8 ]toluene at 97.1–116.5 °C yielded the following activation parameters: Δ H ‡ = 22(2) kcal mol –1 , Δ S ‡ = –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 k H / k D = 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)