Phosphoranyl

Product branching. The reactions of phosphoranyl radical with disulfides leads to the product shown 41... 

This chapter surveys the literature published from 1995 to 2003, concerning the reactivity and the chemical applications of the four main families of phosphorus centered radicals, i.e., phosphinyl (L2P )> phosphonyl (L2P =0), phospho-niumyl (L3P ) and related charged species, and phosphoranyl (L4P ) radicals. Due to their specificity, a section is devoted to the generation and properties of persistent and stable phosphorus centered radicals. 

Although their chemistry is less developed than that of phosphonyl, phospho-niumyl or phosphoranyl radicals, many structural studies have been devoted to phosphinyl radicals [1]. Like their nitrogen analogs,phosphinyl radicals are 7i-type radicals (Fig. 1) and because of the very small s character of their SOMO, the magnitude of their phosphorus hyperfine coupling constants flp is below 15 mT [1]. 

Fluorinated phosphonates exhibit interesting properties as enzyme inhibitors, chelating agents or as fuel cell electrolytes [29] however, only few methods of preparation for these compounds are available. Burton et al. [30] developed several methods to prepare fluorinated phosphates which involve phosphonyl, and likely phosphoranyl radicals as chain carriers (Scheme 11). 

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. 

The various structures [59], EPR parameters [1] and reactivity [59] of phosphoranyl radicals have been extensively reviewed over the last three decades. This section presents only the main trends, and the last developments concerning these species. 

The displacement of a ligand L from the phosphorus atom of a phosphoranyl radical can easily occur via a-scission of the L-P bond (Scheme 23). The fragmentation is a regiospecific process, i.e., the leaving group must be apical (Scheme 23) and it occurs via an intermediate o" structure (Fig. 5). 

Three- and pentacoordinate organic phosphorus compounds can be oxidized through a free radical Arbuzov reaction, i.e., formation and p-scission of a phosphoranyl radical (Scheme 24). The P-scission is regioselective homolysis occurs on a ligand located in an equatorial site. Both a- and P-scissions are strongly dependent on the strength (bond dissociation energy) of the cleaved... 

The trapping of alkyl, alkoxyl and alkylthiyl radicals by trivalent phosphorus compounds, followed by either a-scission or p-scission of the ensuing phosphoranyl radical, is a powerful tool for preparation of new trivalent or pen-tavalent phosphorus compounds [59]. However, the products of these reactions strongly depend on the BDE of the bonds, which are either formed or cleaved. For example, the addition of phenyl radicals on a three-coordinate phosphorus molecule occurs irreversibly, while that of dimethylaminyl (Me2N ) or methyl radicals is reversible, the amount of subsequent P-scission (formation of compound C) depending on the nature of Z and R (Scheme 25). For tertiary alkyl radicals and stabilized alkyl radicals no addition is observed (Scheme 25) [63]. 

The strained hydrocarbon [1,1,1] propellane is of special interest because of the thermodynamic and kinetic ease of addition of free radicals (R ) to it. The resulting R-substituted [ 1.1.1]pent-1-yl radicals (Eq. 3, Scheme 26) have attracted attention because of their highly pyramidal structure and consequent potentially increased reactivity. R-substituted [1.1.1]pent-1-yl radicals have a propensity to bond to three-coordinate phosphorus that is greater than that of a primary alkyl radical and similar to that of phenyl radicals. They can add irreversibly to phosphines or alkylphosphinites to afford new alkylphosphonites or alkylphosphonates via radical chain processes (Scheme 26) [63]. The high propensity of a R-substituted [1.1.1] pent-1-yl radical to react with three-coordinate phosphorus molecules reflects its highly pyramidal structure, which is accompanied by the increased s-character of its SOMO orbital and the strength of the P-C bond in the intermediate phosphoranyl radical. 

Scheme 25 Influence of the nature of ligands on the a- and p-scission of phosphoranyl radicals. Reprinted with permission from [63]. Copyright 1997 American Chemical Society...

Alkoxy (R0 ) radicals react at near diffusion controlled rates with trialkyl phosphites to give phosphoranyl radicals [ROP(OR )3] that typically undergo very fast -scission to generate alkyl radicals (R ) and phosphates [OP(OR )3]. In a mechanistic study, trimethyl phosphite, P(OMe)3, has been used as an efficient and selective trap in oxiranylcarbinyl radical systems formed from haloepoxides under thermal AIBN/n-Bu3SnH conditions at about 80 °C (Scheme 27) [64]. The formation of alkenes resulting from the capture of allyloxy radicals by P(OMe)3 fulfils a prior prediction that, under conditions close to kinetic control, products of C-0 cleavage (path a. Scheme 27), not just those of C-C cleavage (path b. Scheme 27) may result. 

In the course of the 1990s, Yasui et al. [41b, 68] showed that, depending on the ligands attached to the phosphorus atom, phosphoranyl radicals may decay via three main processes a-scission, -scission and SET (Scheme 31). For example, in the presence of 10-methylacridinium iodide, phosphoranyl radicals generated from phenyl diphenylphosphinite decayed mainly via a-scission (Scheme 32) whereas phosphoranyl radicals generated from /so-propyl diphenylphosphinite decayed only via a SET process (Scheme 33). The reactivity of the phosphoni-umyl/phosphoranyl radical tandem has already been discussed in Sect. 3. 

Scheme 31 Decay of phosphoranyl radicals via a-scission, p-scission and SET. Reprinted with permission from [68]. Copyright 1994 Wiley Interscience...

During the last two decades, Bentrude et al. [70] has shown that phosphoranyl radicals exhibiting very slow a- and P-fragmentations react with alkyl disulfides via Sh2 homolytic substitution (Scheme 35) [70b]. The reactivity of phosphoranyl radicals in these Sh2 reactions depends strongly on the substituents attached to the phosphorus atom and on the structure of the disulfides [70c]. 

Scheme 35 Sh2 reactions of phosphoranyl radicals with alkyl disulfides...

Roberts et al. [74] took advantage of the rapid and selective p-scissions of phosphoranyl radicals, to develop a radical chain desulfurization affording new substituted a-alkyl acrylates in good to moderate yields (Scheme 37). 

Scheme 37 Radical chain desulfurization involving phosphoranyl radicals as chain carriers. Reprinted with permission from [74]. Copyright 1998 Pergamon Press...

As already mentioned, the reduction of di-p-tolyliodonium salt intop-tolylio-dide in the presence of triphenylphosphine (Scheme 39) [69] involves a SET, the intermediate phosphoranyl radical behaving as one electron-reductant. 

Phosphoranyl radicals can be involved [77] in RAFT processes [78] (reversible addition fragmentation transfer) used to control free radical polymerizations [79]. We have shown [77] that tetrathiophosphoric acid esters are able to afford controlled/living polymerizations when they are used as RAFT agents. This result can be explained by addition of polymer radicals to the P=S bond followed by the selective p-fragmentation of the ensuing phosphoranyl radicals to release the polymer chain and to regenerate the RAFT agent (Scheme 41). 

Scheme 41 Phosphoranyl radicals as intermediates in RAFT polymerizations...

Phosphoranyl radicals were observed by FPR at the end of the sixties [91]. For a long time, phosphoranyl radicals, particularly the alicyclic ones [59], were considered as elusive species. However, recently. Marque et al. [92] observed the first strongly persistent (ti/2=45 min at RT) alicyclic phosphoranyl radicals (Fig. 10) when they irradiated bis(trialkylsilyl)peroxides in the presence of tris(trialkylsilyl)phosphites. The increased lifetime of the ensuing phosphoranyl radicals is a consequence of the presence of four bulky R3SiO groups around the phosphorus. The bulkiness of the substituents hampers the dimerization and the Sh2 reaction of phosphoranyl radicals with the peroxide initiator. Furthermore, the high strength of the P-0 and 0-Si bonds results in slow a- and p-scissions [93]. 

Further evidence has been adduced for the configurational stability of phosphoranyl radicals. Thus photolysis of iodobenzene in the presence of (11) gave a 95% yield of (12). Reaction of the phosphonium salt (13) with lithium alkyls produces the phosphoranyl radical (14). ... 

Further investigations for the radical isomerisation of hydro-spirophosphoranes have shown that, on heating with di-t-butyl peroxide in benzene, phosphoranes (66) and (67) are transformed into (68) and (69) respectively93. The proposed mechanism, as written for (67), proceeds through the phosphoranyl radical (70) which rearranges to (71) which in turn reacts with (67) to form (69) and (70) in the propagation step. 

E.s.r. showed that, X. ray irradiation of tetraalkyldiphosphine diphosphides gave phosphoranyl radicals with t.b.p. structures (39).114 A structure has been assigned to phosphiny1hydrazy1s (40). The dimethy1 ami no radical was particularly persistent.115 The e.s.r. parameters of the electrogenerated pyrazine radical cations (41) have been recorded.116 The spectra of a stable furanyl phosphate radical adduct117 and a phenalene radical anion which involves injection of spin density into half an attached cyclophosphazene ring,11 are reported. 

The 10-57-5-hydridosiliconate ion 62 is known in association with lithium,323 tetrabutylammonium,101 and bis(phosphoranyl)iminium93 cations. It is synthesized by hydride addition to the 8-.S7-4-silane 63, which is derived from hexafluoroacetone.101 Benzaldehyde and related aryl aldehydes are reduced by solutions of 62 in dichloromethane at room temperature101 or in tetrahydrofuran at 0°96 within two hours. The alkyl aldehyde, 1-nonanal, is also reduced by 62 in tetrahydrofuran at O0.96 Good to excellent yields of the respective alcohols are obtained following hydrolytic workup. The reactions are not accelerated by addition of excess lithium chloride,96 but neutral 63 catalyzes the reaction, apparently through complexation of its silicon center with the carbonyl oxygen prior to delivery of hydride from 62.101... 

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