The effect of high pressure on the Heck reaction - A contribution to a deeper understanding of the mechanism

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 (DeltaH(double dagger), DeltaS(double dagger), DeltaV(double dagger)) were determined at temperatures between 50 and 120 degreesC 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 (DeltaV(double dagger) = -12cm(3) mol(-1)) is more negative than that of the reaction of iodobenzene with ethyl acrylate (DeltaV(double dagger) = -7 cm(3) mol(-1)) or methyl acrylate (DeltaV(double dagger) = -5 cm(3) 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 cm(3) 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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).