Phosphoniumyl radicals

The electrochemical generation and reactivity of phosphoniumyl and related charged radicals have been recently reviewed by Kargin and Bunikova [8]. In 1995, Yasui reviewed the reactivity of trivalent phosphorus compounds in single electron transfer (SET) processes [41] and, in 1990, the EPR features and reactivity of phosphoniumyl radicals were reviewed by Tordo [42]. 

The structures of phosphoniumyl and related charged radicals have been extensively listed over the last three decades [1]. Phosphoniumyl radicals are a... 

The p-scission of a phosphoniumyl radical yields a cation and a phosphonyl radical, while its reaction with a nucleophile generates a phosphoranyl radical which can undergo SET reactions and a- or p-fragmentations (Scheme 14). 

Recent results of Bentrude et al. [44] suggest that a vinyl radical cation is first generated, and that the 1,3-distonic radical cation is reduced to a diradical involving a phosphoranyl radical moiety. However, because the phosphite and styryl moieties of phenylallylphosphites exhibit very close oxidation potentials [45], the presence of a phosphoniumyl radical cannot be totally ruled out. 

Romakhin et al. [49] showed that anodically generated phosphoniumyl radicals can add onto alkenes to yield phosphonylated alkenes through an anodic oxi-dation/addition/anodic oxidation/elimination/nucleophilic attack sequence (Scheme 17). 

Scheme 17 Oxidative addition of phosphoniumyl radicals onto alkenes...

SET reactions involving phosphoniumyl radicals have been developed by Yasui et al. [41,55], particularly for the dediazoniation of arenediazonium salts [56] (Scheme 20) and for the oxidation of trivalent phosphorus compounds [41,57] (Scheme 21). 

Recently, Lee [58] showed that phosphoniumyl radicals generated via SET can be used to generate carbocations (Scheme 22). 

Scheme 22 Carbocations resulting from the SET generation of phosphoniumyl radicals. Reprinted with permission from [58]. Copyright 2000 Korean Chemical Society...

A different approach must be used for the photochemical hydrophosphination of electron-poor olefins, and this involves a PET reaction. Silyl phosphites (e.g., 30) were used as electron donors, whereas conjugated ketones have the double role of electron acceptors and absorbing species. Thus, the irradiation of a mixture containing 2-cydohexenone and 30 generated an ion pair. The phosphoniumyl radical cations decomposed to give trimethylsilyl cations (which in turn were trapped by the enone radical anion) and phosphonyl radicals. A radical-radical combination afforded the 4-phosphonylated ketones in yields ranging from 78% to 92% (Scheme 3.20) [49]. This reaction was exploited for the preparation of substituted phosphonates, which serve as key intermediates in the synthesis of a class of biologically active compounds. 

The phosphoniumyl radicals are very well known radicals which have already been reviewed in the present series. The possibility of forming arsoniumyl radicals by photolysis was mentioned twenty years ago hy Preer and coworkers. Symons and coworkers showed that y-irradiation of solutions of Ph3 As in sulphuric add at 77 K produces the radical cation. In this sample the irradiated molecule is in fact the arsonium cation, PhjAsH", which, after ejection of an electron, loses a proton and forms R3AS " ... 

Phosphoniumyl cation radicals XsP, phosphine dimer cation radicals X3P P X3, phosphate anion radicals, and related species... 

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