Thermal stability of phosphinoacetic acids

Abstract

Phosphinoacetic acids decarboxylate smoothly in toluene solution at 99 °C and the corresponding alkylphosphine is formed in quantitative yields. Electron-withdrawing substituents at the α position of the carboxylic acid lead to a large increase in the reaction rate. In contrast, electron-withdrawing substituents at the phosphorus atom lead to a small decrease in the rate. We have concluded from the substituent effects, solvent effects, and the influence of bases and acids that both the lone pair of the phosphorus atom and the carboxylate hydrogen atom play a crucial role in the reaction. A mechanism is proposed that proceeds via an ylide. Sodium phosphinocarboxylates do not decarboxylate in an aqueous solution at 95 °C. Instead a carbon–phosphorus bond cleavage occurs probably by an intramolecular nucleophilic substitution.