The alkaline hydrolysis of some tri-(2-thienyl)phosphonium salts. Inductive effects on the rate of nucleophilic attack at phosphorus

Abstract

The kinetics of alkaline hydrolysis of the heteroarylphosphonium salts, methyltri-(2-thienyl)phosphonium iodide (IV) and benzyltri-(2-thienyl)phosphonium bromide (V), which proceed with cleavage of a phosphorus–thiophen bond, have been studied at different temperatures in 50%(v/v) aqueous ethanol, and the rates of hydrolysis have been compared with those for the hydrolysis of analogous triphenylphosphonium salts. The hydrolysis of both the above thienylphosphonium salts follows a third-order rate law; under the same conditions at 40·2°, methyltri(2-thienyl)phosphonum iodide (IV) hydrolyses 1·45 × 10⁸ times more rapidly than methyltriphenylphosphonium iodide. Similarly, benzyltri-(2-thienyl)phosphonium bromide hydrolyses 2·5 × 10⁵ times faster than benzyltriphenylphosphonium bromide.

It is suggested that in the alkaline hydrolysis of the above salts, the inductive effect of the 2-thienyl group and the stability of the 2-thienyl carbanion are of the greatest importance in controlling the rate of the reaction; d_π–p_π bonding between the ‘π-excessive’ heterocycle and the phosphorus 3d orbitals does not appear to play a significant role in controlling the rate of nucleophilic attack at phosphorus.