Radical-initiated P,P-metathesis reactions of diphosphanes: evidence from experimental and computational studies

Abstract

By combining the diphosphanes Ar₂P–PAr₂, where Ar = C₆H₅, 4-C₆H₄Me, 4-C₆H₄OMe, 3,5-C₆H₃(CF₃)₂, it has been shown that P,P-metathesis generally occurs rapidly under ambient conditions. DFT calculations have shown that the stability of unsymmetrical diphosphanes Z₂P–PZ′₂ is a function of the difference between the Z and Z′ substituents in terms of size and electronegativity. Of the mechanisms that were calculated for the P,P-metathesis, the most likely was considered to be one involving Ar₂P˙ radicals. The observations that photolysis increases the rate of the P,P-metatheses and TEMPO inhibits it, are consistent with a radical chain process. The P,P-metathesis reactions that involve (o-Tol)₂P–P(o-Tol)₂ are anomalously slow and, in the absence of photolysis, were only observed to take place in CHCl₃ and CH₂Cl₂. The role of the chlorinated solvent is ascribed to the formation of Ar₂PCl which catalyses the P,P-metathesis. The slow kinetics observed with (o-Tol)₂P–P(o-Tol)₂ is tentatively attributed to the o-CH₃ groups quenching the (o-Tol)₂P˙ radicals or inhibiting the metathesis reaction sterically.