Carbon–Hydrogen Bond Activation of Arenes by a [Bis(oxazolinyl)phenyl]rhodium(III) Acetate Complex
✍ Scribed by Jun-ichi Ito; Hisao Nishiyama
- Publisher
- John Wiley and Sons
- Year
- 2007
- Tongue
- English
- Weight
- 144 KB
- Volume
- 2007
- Category
- Article
- ISSN
- 1434-1948
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✦ Synopsis
Abstract
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)