Dependence of transition-state structure on nucleophile in the reaction of aryl oxide anions with aryl diphenylphosphate esters

Abstract

Second-order rate constants have been measured for reaction of aryl oxide anions with substituted phenyl diphenylphosphate esters in aqueous media at 25 °C. The rate constants obey good Brønsted type correlations with the pK_a of either the leaving group or nucleophile. Both β_nuc and β_Ig values vary substantially with basicity of leaving group and nucleophile, respectively, providing evidence for coupling between bond fission and formation.

Attack of phenolate ions on 2,4-dinitrophenyl diphenylphosphate has a β_nuc value of 0.12 giving a Kreevoy's tightness parameter value, τ, of 0.18 which is consistent with an ‘exploded’ transition state for the identity reaction involving 2,4-dinitrophenolate ion as the nucleophile. Transfer of the diphenylphosphinoyl group between 2,4-dinitrophenolate ion nucleophiles has a τ value of 0.28, consistent with an ‘exploded’ transition state for this reaction also.

The identity reaction involving transfer of the diphenylphosphoryl group between phenolate ions has a τ value of 1.20, indicating an almost synchronous process. The data for transfer of the diphenylphosphinoyl and diethylphosphoryl groups between phenolate ion nucleophiles indicates that the identity reaction for the parent phenol has an almost synchronous mechanism for the former phosphinoyl species (τ= 0.92) whereas the latter (τ= 1.45) is almost associative.