The chemistry of heteroarylphosphorus compounds. Part V. The alkaline hydrolysis of 2-furyldiphenyl-(2-thienyl)phosphonium bromide, and some mixed heteroarylmethylphosphonium salts. Relative stabilities of the forming carbanions

Abstract

Alkaline hydrolysis of 2-furyl(diphenyl)-2-thienyl-phosphonium bromide (V) gives both furan and thiophen in a 1 : 3 molar ratio, indicating that the forming 2-thienyl carbanion is more stable than the 2-furyl analogue, and implying stabilisation of the forming negative charge in the former by sulphur 3d orbital participation. A kinetic isotope effect, k_H/k_D≈ 1·2 is observed for the formation of both furan and thiophen in this reaction. The previously reported greater rate of alkaline hydrolysis of alkyltri-(2-furyl)phosphonium salts compared to the 2-thienyl analogues is now attributable to the greater electron-withdrawing effect of the 2-furyl substituent on the magnitude of the equilibrium constants of the pre-equilibria involved in the reactions. The products of alkaline hydrolysis of the mixed arylmethyldiphenylphosphonium salts (X)–(XII) indicate the relative stabilities of the forming carboanions to be in the order 2-furylmethyl > 2-thenyl > benzyl. The implications of this order are discussed in relation to earlier kinetic studies.