Phosphoranyl Radicals as Reducing Agents: SRN1 Chains with Onium Salts and Neutral Nucleophiles

Article abstract of DOI:10.1021/jo00056a030

Redox radical chain reactions of trivalent organophosphorus compounds (PZ3) with diaryliodonium (Ar2I⁺) and triarylsulfonium (Ar3S⁺) salts to give arylphosphonium (ArP⁺Z3) salts and iodoarenes (ArI) or diaryl sulfides (Ar2S) are reported.The key propagation step in these S_RN1 reactions is a single-electron reduction of the onium salts by intermediate phosphoranyl radicals (ArP^.Z3).The observation of competitions between solvent molecules and phosphine establishes the intermediacy of free aryl radicals and allows estimates of rate constants for addition of p-tolyl radicals to triphenylphosphine (k ca. 3 * 1E8 M^-1 s^-1) and to trimethyl phosphite (k ca. 2 * 1E8 M^-1 s^-1).The intermediate phosphoranyl radicals can also partition between competitive reaction pathways; the aryltributylphosphoranyl radical, ArP^.Bu3, for example, partitions between unimolecular α-cleavage of butyl radical and chain-propagating electron transfer to diaryliodonium salt.The relative amounts of these two pathways allows an estimate of the rate constant for electron transfer, k_SET ca. 4 * 1E9 M^-1 s^-1.