Radical Reactions of Phosphines

Radical reactions may play an important role in the biological chemistry of phosphines. As discussed in Sects. 4.1 and 4.2, under certain conditions phosphines can react with dialkyl peroxides, disulphides and thiols by radical pathways rather than ionic mechanism. The autoxidation of phosphines also appears to involve a radical mechanism. For all of these examples the intermediate species is a phosphoranyl radical R4P which contains 

9 electrons in the P valence shell. However there are three additional types of phosphorus radical species that may be relevant to the biological chemistry of phosphines. These are phosphonium radical anions (R3P ) which also have 9 valence electrons, and the two 7-electron species, phosphinium radical cations (RsP ) and phosphino radicals (RzP). 

The chemistry of phosphorus radicals has been studied extensively and the subject has been reviewed However, there appear to have been no investigations of the radical chemistry of phosphines under conditions relevant to biology. In Table 14 we give examples of the types of radical reactions that phosphines could potentially undergo in a biological system. 

The ability of phosphines to act as either one-electron acceptors or one-electron donors may be crucial in the mechanism of their cytotoxicity and allow them to interfere with electron transport processes. Phosphinium radical cations have been generated by oxidation of phosphines at a mercury anode or by y-irradiation of tertiary phosphines on silica and in sulphuric acid The oxidation potentials appear to be within the range accessible to biological systems Aryl phosphines are more easily oxidised than alkyl phosphines Ejq (polarography in CH3CN, anodic oxidation) PPha -H 50 mV, PEt3 - 415 Phosphine dimer cation radicals (R3PPR3) have been identified by 

ESR spectroscopy as products generated during electrochemical oxidation of PR3 compounds in solution the initially-formed phosphinium radical cation reacts rapidly with a further molecule of phosphine Phosphonium radical anions are the least well characterised of all phosphorus radicals. Electrolytic reduction of PPh3 in CH3CN, at a dropping mercury cathode, produced biphenyl and diphenylphosphonic add. The radical anion was presumed to be an intermediate  

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