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

Abstract

The highly substituted dioxoruthenium(VI) porphyrins [Ru(VI)(DPP)O2] (1a; H2DPP = 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin), [Ru(VI)(TDCPP)O2] (1b; H2TDCPP = meso-tetrakis(2,6- dichlorophenyl)porphyrin), and [Ru(VI)(TMOPP)O2] (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; H2Por = 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-β-methylstyrene > cis-β-deuteriostyrene. The observation of inverse secondary KIE for the oxidation of β-d2-styrene [k(H)/k(D) = 0.87 (1a); 0.86 (1b)] but not for the α-deuteriostyrene oxidations suggests that the C-O bond formation is more advanced at the C(β) atom than at the C(α) 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 1a-c can linearly correlate with the carboradical substituent constants σ(mb) and σ(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 |ρ·(JJ)/ρ(mb)| > 1 suggests that the spin delocalization effect is more important than the polar effect in the epoxidation reactions. The spontaneous epoxidation of trans-β-methylstyrene by the sterically encumbered [Ru(VI)(TDCPP)O2] and [Ru(VI)(TMOPP)O2] complexes and the comparable ΔS((+)) values for their reactions with trans-β-methylstyrene and styrene are incompatible with the 'side-on approach' model; a 'head-on approach' model is implicated.