Addition of triphenylphosphine to electron-deficient alkenes in mixed binary solvents: Overcoming the problem of preferential solvation to determine the reaction order with respect to protic solvent

Abstract

© 2020 Elsevier B.V. Kinetics of addition of PPh3 to electron-deficient alkenes has been studied in aprotic solvent+carboxylic acid binary mixtures as a function of the alkene and carboxylic acid concentrations. For ethyl acetate+acetic acid, ethyl acetate+propionic acid, butyl acetate+acetic acid, butyl acetate+propionic acid mixtures, the reactions of PPh3 with acrylic acid and methyl acrylate show no preferential solvation and follow third-order kinetics, first order in the phosphine, the activated alkene, and acetic (propionic) acid. Other alkene/solvent combinations tested also displayed acceleration in the presence of carboxylic acid, but the total effect of the acidic solvent concentration on the reaction rate was nonlinear due to occurrence of preferential solvation. The acid dependence gives the direct evidence for the rate-determining protonation of intermediate phosphonium enolate zwitterion under acidic solvent condition. Since proton transfers from protic solvents are typically fast, the kinetic data obtained give a new mechanistic insight into chemistry of phosphonium enolates: their decomposition to the reactants proceeds much faster than proton transfer step in acidic media. This unexpected finding emphasizes very short lifetime of phosphonium enolates, key intermediates of synthetically useful phosphine-catalyzed reactions.