Scope and Mechanistic Study of the Coupling Reaction of α β-Unsaturated Carbonyl Compounds with Alkenes: Uncovering Electronic Effects on Alkene Insertion vs Oxidative Coupling Pathways

摘要

The cationic ruthenium-hydride complex [(C6H6)(PCy3)(CO)RuH]⁺BF4⁻ (>1) was found to be a highly effective catalyst for the intermolecular conjugate addition of simple alkenes to α,β-unsaturated carbonyl compounds to give (Z)-selective tetrasubstituted olefin products. The analogous coupling reaction of cinnamides with electron-deficient olefins led to the oxidative coupling of two olefinic C–H bonds in forming (E)-selective diene products. The intramolecular version of the coupling reaction efficiently produced indene and bicyclic fulvene derivatives. The empirical rate law for the coupling reaction of ethyl cinnamate with propene was determined as: rate = k[>1]¹[propene]⁰[cinnamate]⁻¹. A negligible deuterium kinetic isotope effect (kH/kD = 1.1±0.1) was measured from both (E)-C6H5CH=C(CH3)CONHCH3 and (E)-C6H5CD=C(CH3)CONHCH3 with styrene. In contrast, a significant normal isotope effect (kH/kD = 1.7±0.1) was observed from the reaction of (E)-C₆H₅CH=C(CH₃)CONHCH₃ with styrene and styrene-d₁₀. A pronounced carbon isotope effect was measured from the coupling reaction of (E)-C₆H₅CH=CHCO₂Et with propene (¹³C(recovered)/¹³C(virgin) at C_β = 1.019(6)), while a negligible carbon isotope effect (¹³C(recovered)/¹³C(virgin) at C_β = 0.999(4)) was obtained from the reaction of (E)-C₆H₅CH=C(CH₃)CONHCH₃ with styrene. Hammett plots from the correlation of para-substituted p-X-C₆H₄CH=CHCO₂Et (X = OCH₃, CH₃, H, F, Cl, CO₂Me, CF₃) with propene and from the treatment of (E)-C₆H₅CH=CHCO₂Et with a series of para-substituted styrenes p-Y-C₆H₄CH=CH₂ (Y = OCH₃, CH₃, H, F, Cl, CF₃) gave the positive slopes for both cases (ρ = +1.1±0.1 and +1.5±0.1, respectively). Eyring analysis of the coupling reaction led to the thermodynamic parameters, Δ H^‡ = 20±2 kcal mol⁻¹ and S^‡ = −42±5 e.u. Two separate mechanistic pathways for the coupling reaction have been proposed on the basis of these kinetic and spectroscopic studies.