The kinetics and mechanism of the reaction of triarylphosphines with 7,7,8,8-tetracyanoquinodimethane and tetracyanoethylene

Abstract

Triarylphosphines react with 7,7,8,8-tetracyanoquinodimethane (TCNQ) in chloroform, acetonitrile or acetonitrile–benzene in the presence of water and catalytic quantities of hydrochloric acid to give quantitative yields of 1,4-bis-(dicyanomethyl)benzene and triarylphosphine oxide. Similarly, in acetonitrile in the presence of aqueous acid, tetracyanoethylene (TCNE) and triarylphosphine give tetracyanoethane and the phosphine oxide. Using an optically active phosphine, the oxide recovered from both reactions is completely racemic. Kinetic data on the reaction of TCNQ with triarylphosphines are reported and by consideration of the order, deuterium isotope effect and activation parameters coupled with the stereochemical evidence it is proposed that the reaction proceeds through a phosphinium radical cation intermediate. A Hammett correlation of rate with σ-substituent parameters of the substituted phosphines gives a ρ value of –3·2. The reactions of triarylphosphines with TCNQ and TCNE in absolute aprotic media are also discussed.