Homogeneous Pd-Catalyzed Heck Coupling in gamma-Valerolactone as a Green Reaction Medium: A Catalytic, Kinetic, and Computational Study

gamma-Valerolactone (GVL) was proposed as an environmentally benign reaction medium for phosphine-free Pd-catalyzed homogeneous Heck coupling reaction of iodobenzene and styrene derivatives. Detailed catalytic, kinetic, and computational studies were performed for this industrially important transformation in GVL, which shows remarkable tolerance for the moisture content of the reaction mixture and gives good efficiency for Pd(II) catalyst precursors such as PdCl2 and Pd(OAc)(2). Excellent functional group tolerance for both iodoaromatic substances and styrene derivatives was shown, and the reaction efficiency was correlated by the Hammet-constant of corresponding substrates. A simplified kinetic model was subsequently found to model the transformation, which can be represented by an excellent fitting of the calculated concentration of corresponding species to the experimentally determined ones. Delta H-app(double dagger) = +103.0 kJ.mol(-1) for the apparent overall activation enthalpy of the reaction in GVL, and Delta H-app(double dagger) = +103.7 kJ.mol(-1) in the case of DMF have been obtained. Delta S-app(double dagger) = +139.7 J. mol(-1).K-1 was calculated for the apparent overall activation entropy of the reaction in the presence of GVL, and Delta S-app(double dagger) = +138.1 J. mol(-1).K-1 in the presence of DMF, very similar values suggesting that no medium effect on the mechanism can be proposed. The computational study confirmed the experimentally observed trends regarding the effect of electron-donating and -withdrawing substituents of iodobenzene and styrene as follows: the electron-donating substituents accelerate the reaction rate for iodobenzene but decrease the reactivity for styrene derivatives.