The Effect of High Pressure on the Heck Reaction − A Contribution to a Deeper Understanding of the Mechanism

Abstract

Abstract 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 (Δ H ‡ , Δ S ‡ , Δ 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 (Δ V ‡ = −12 cm 3 mol −1 ) is more negative than that of the reaction of iodobenzene with ethyl acrylate (Δ V ‡ = −7 cm 3 mol −1 ) or methyl acrylate (Δ V ‡ = −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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)