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

By combining the diphosphanes Ar 2 P-PAr 2 , where Ar = C 6 H 5 , 4-C 6 H 4 Me, 4-C 6 H 4 OMe, 3,5-C 6 H 3 (CF 3 ) 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 Z 2 P-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 Ar 2 P&z.rad; 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) 2 P-P( o -Tol) 2 are anomalously slow and, in the absence of photolysis, were only observed to take place in CHCl 3 and CH 2 Cl 2 . The role of the chlorinated solvent is ascribed to the formation of Ar 2 PCl which catalyses the P,P-metathesis. The slow kinetics observed with ( o -Tol) 2 P-P( o -Tol) 2 is tentatively attributed to the o -CH 3 groups quenching the ( o -Tol) 2 P&z.rad; radicals or inhibiting the metathesis reaction sterically. The mechanism by which P,P-exchange reactions can occur rapidly under ambient conditions has been investigated and the experimental evidence, combined with computational studies, support a radical process.