A mechanistic investigation of alkene epoxidation by sterically encumbered trans-dioxoruthenium(VI) porphyrins

The highly substituted dioxoruthenium(VI) porphyrins [Ru-VI(DPP)O-2] (1a; H2DPP = 2,3,5,7,8;10,- 12,13,15,17, 18,20-dodecaphenylporphyrin), [Ru-VI(TDCPP)O-2] (Ib; H2TDCPP = meso-tetrakis(2,6- dichlorophenyl)porphyrin), and [Ru-VI(TMOPP)O-2] (1c; H2TMOPP = meso-tetrakis(2,4,6-trimethoxyphenyl)porphyrin) are competent oxidants for alkene epoxidation. The oxidations were carried out in a CH2Cl2/Hpz solution, and a paramagnetic bis(pyrazolato)ruthenium(IV) porphyrin, [Ru-IV(Por)-(pz)(2)] (2; H(2)Por = H2DPP, H2TDCPP, H2TMOPP), was isolated and characterized. For the oxidation of cis-alkenes, stereoselectivity is dependent upon both the alkenes and the ruthenium oxidants, and it decreases in the order: cis-stilbene > cis-beta-methylstyrene > cis-beta-deuteriostyrene. The observation of inverse secondary KIE for the oxidation of beta-d(2)-styrene [k(H)/k(D) = 0.87 (1a); 0.86 (1b)] but not for the alpha-deuteriostyrene oxidations suggests that the C-O bond formation is more advanced at the C(beta) atom than at the C(alpha) atom of styrene, consistent with a nonconcerted mechanism. By consideration of spin delocalization and polar effects, the second-order rate constants for the oxidation of para-substituted styrenes by complexes la-c can linearly correlate with the carboradical substituent constants sigma(mb) and sigma(JJ)(.) (Jiang, X.-K. Acc. Chem. Res. 1997, 30, 283). This implies that the styrene oxidation by the dioxoruthenium(VI) porphyrins should involve rate-limiting generation of a benzylic radical intermediate, and the magnitude of \rho(JJ)(.)\rho(mb)\ > 1 suggests that the spin delocalization effect is more important than the polar effect in the epoxidation reactions. The spontaneous epoxidation of trans-beta-methylstyrene by the sterically encumbered [Ru-VI(TDCPP)O-2] and [Ru-VI(TMOPP)O-2] complexes and the comparable Delta S-double dagger values for their reactions with trans-beta-methylstyrene and styrene are incompatible with the "side-on approach" model; a "head-on approach" model is implicated.