The influence of high pressure on the Heck reactions of iodobenzene with methyl, ethyl and tert-butyl acrylate, and of both 4-nitrophenyl iodide and 4-nitrophenyl triflate with methyl acrylate, has been studied for the first time by quantitative on-line FT-IR spectroscopy. Reaction rates and activation parameters (∆H‡, ∆S‡, ∆V‡) were determined at temperatures between 50 and 120 °C and pressures up to 3000 bar. A clear influence on the kinetics of the reactions was observed by the nature of the leaving group and by the steric and electronic features of the acrylate. tert-Butyl acrylate, which is more electron-rich than methyl acrylate according to PES measurements, was found to react 1.4-times faster on average than methyl acrylate and, at elevated pressures, triflate turned out to be a better leaving group than iodide.
Thus, the observed activation volume for the reaction of iodobenzene with tert-butyl acrylate (∆V‡ = −12 cm3 mol−1) is more negative than that of the reaction of iodobenzene with ethyl acrylate (∆V‡ = −7 cm3 mol−1) or methyl acrylate (∆V‡ = −5 cm3 mol−1). For the variation of the leaving group in 4-nitrophenyl iodide and 4-nitrophenyl triflate, the activation volumes for the reaction of each with methyl acrylate are −9 and −37 cm3 mol−1, respectively. The results suggest that the rate-determining step of the overall reaction is not the oxidative addition, but is either the alkene coordination or the subsequent carbopalladation.