Phosphoranyl radicals as reducing agents: SRN1 chains with onium salts and neutral nucleophiles

Abstract

Redox radical chain reactions of trivalent organophosphorus compounds (PZ3) with diaryliodonium (Ar2l+) and triarylsulfonium (AraS-1") salts to give arylphosphonium (ArP+Zg) salts and iodoarenes (Arl) or diaryl sulfides (At2S) are reported. The key propagation step in these SrnI reactions is a single-electron reduction of the onium salts by intermediate phosphoranyl radicals (ArÉZa). 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 » 3 X 108 M"1 s-1) and to trimethyl phosphite (k « 2 x 10® M-1 s-1). The intermediate phosphoranyl radicals can also partition between competitive reaction pathways; the aryltributylphosphoranyl radical, ArPBua, for example, partitions between unimolecular a-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, Aset = 4 X 109 M-1 s'1.Phosphoranyl radicals (Z4P*) are well-known species, often characterizable by ESR spectroscopy.2 Most of the

Persilylated Phosphoranyl Radicals: The First Persistent Noncyclic Phosphoranyl Radicals

“…Inner‐sphere SET would be expected to be very fast and insensitive to steric hindrance, whereas k SH2 increases by a factor of six on going from 6 to 5 44. 53…”

Photoredox Catalysis in Organophosphorus Chemistry

“…The syntheses of quaternary phosphonium salts were first reported by Reutov and co‐workers in 1965 . This chemistry was revisited by Kampmeier approximately thirty years later . This work suggested the occurrence of a charge transfer between the diaryliodonium salt and the tertiary phosphine.…”

Homolytic Reduction of Onium Salts

“…The latter is oxidized by the sulfonium to generate a phosphonium generating a sulfuranyl radical which falls apart. [59] The homolytic reduction of sulfonium is a rather complex molecular process. A main issue which remains to be addressed is the control of the regioselectivity.…”

“…[67] Interestingly, Kampmeier and Nalli reported that phosphoranyl radical intermediates generated by arylation of triphenylphosphine can act as reducing agents with iodonium and sulfonium salts. [59] Along with these electrochemical studies, photolysis of phosphonium salts was investigated by Griffin and Kaufman (Scheme 17). UV Irradiation of benzyltriphenylphosphonium chloride in benzeneethanol led to a mixture of diphenyl, diphenylmethane, dibenzyl, diphenylphosphine, triphenylphosphine and oxidation products.…”