Phosphoranyl radical intermediates

Phosphoranyl radical intermediates (Z4P e.g., Z = alkyl, aryl, dialkylamino, aryloxy and combinations thereof) have been well-studied over the past two decades. The subject has been reviewed several times (1-8). The formation of these radical intermediates by oxidative addition processes and their subsequent scission processes to yield the products of oxidation and substitution are shown in Scheme I (R = alkyl, benzyl). Somewhat less well characterized are bimolecular processes in which intact phosphoranyl radicals (Z4P = a bicyclic or spiro species) are trapped, as illustrated in Scheme I by the reaction with a disulfide (9). 

The rate of a or 3 scission of a phosphoranyl radical intermediate also can greatly effect the overall reactivity of a given radical towards a particular trivalent phosphorus derivative (26). Alkoxy radicals react with trialkyl phosphites with ki (reaction 4) about 10 sec" mol" and E 2 kcal/mol (24). For R equal jt-... 

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). 

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. 

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. 

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

Several very stable radicals (e.g. those derived from AIBN) react with tetraphenyl-biphosphine in a homolytic displacement reaction.39 Phosphoranyl radicals, e.g. (36), were postulated as being intermediates. The reaction of the triplet state of olefin (37) is thought to occur via a similar mechanism. 

Some peroxyphosphorus radical intermediates such as tertraalkoxy phosphoranyl peroxy radical, tetrachlorophosphoranyl peroxy radical , phenylphosphorus peroxy radicaP and diphenyl phosphinic peroxy radicaP have been reported. 

The structures in brackets are phosphoranyl radicals with nine electrons on phosphorus and are considered to be transient intermediates. The radical, R-, presumably represents some initiator fragment, but since the phosphoranyl radical in Reaction 11 is symmetric, R exchanges can (and do) take place. In this regard, triaryl phosphites autoxidize much more slowly, and it has been suggested (3) that here phenoxy radicals are generated via another exchange (Reaction 13) and then terminate chains. 

The autoxidation of trialkyl phosphines occurs similarly, except that here the second intermediate phosphoranyl radical can cleave by two paths ... 

The phosphorus homologues of R As" radicals, the phosphoranyl radicals, have been considerably studied during the past 15 years and several review articles have been dedicated to this species - ". This great interest for phosphoranyl radicals is due to the fact that the R P radical often appears as an important intermediate and that its structure frequently governs the reaction mechanism. The structure and reaction properties of arsoranyl and phosphoranyl radicals are very similar nevertheless, some notable differences exist, due for example to the greater electron affinity of arsenic than of phosphorus . 

The radical nature of this reaction has been confirmed by Griffin (118, 119), Crofts and Downie (87), and Cadogan and co-workers (71,77). These latter workers favor an alternative mechanism involving chain propagation by reaction of the phosphoranyl radical with carbon tetrachloride to give a phosphonium intermediate capable of valency expansion and a free trichloromethyl radical. 

That a typical free radical initiator, azobisisobutyronitrile, would also catalyze this process was demonstrated by Walling and Rabinowitz (299,300), who proposed direct reaction of thiyl radicals on phosphite to yield an intermediate phosphoranyl radical identical to that produced in the homolytic reaction of disulfides with phosphites. For thiophenol and triethyl phosphite, an ionic reaction to produce phenyl ethyl sulfide and diethyl phosphite is favored even in the presence of di- cr butyl peroxide (298). 

The PH2 phosphinyl radical is formed as an intermediate in the photolysis of phosphine below, but the parent phosphinyl and phosphoranyl radicals are generally not easily obtained. They can, however, be produced by y-irradiation of phosphine trapped in a krypton matrix at 4.2 K (13.156), (2.1) and (2.2). 

Under conditions where alkoxy radicals can be generated, phosphines will also react with dialkylperoxides via a radical mechanism. For instance Buckler observed that BU3P reacted with di-t-butyl peroxide at 130 C to give an 80% yield of Bu2PO Bu and only 20% "BU3PO. This indicates that a-scission of the intermediate phosphoranyl radical [ Bu3PO Bu] predominates over P-scission despite formation of the strong P=0 bond. 

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... 

Phosphoranyl radicals (R4P ) are the most widely studied type of phosphorus radi-cali 200). These may be important intermediates of the reactions of phosphines with radical spedes that are potentially available in biology, e.g. thiyl (RS ), alkoxy (RO ) or alkylperoxy (RO2) radicals. The reaction of alkoxy or alkylthiyF radicals with PR3 compounds take place at close to diffusion controlled rates (ca. 10 - 10 M s" ), although BuOO reacts very much slower than BuO with R3P . An important feature of the chemistry of phosphoranyl radicals X3P-A-B is that they can undergo two types of fragmentation involving cleavage of the P-A bond (a-scission) or A-B bond (P-scission). For reactions of phosphines with oxy or thiyl radicals the two pathways lead to either substitution or oxidation products ... 

Phosphoranyl radicals (5) are postulated as probable intermediates in several types of radical processes involving trivalent phosphorus compounds (] ) as demonstrated in the scheme below. 

However, the evidence is very strong (30) that in such reaction systems those phosphoranyl radicals observed by ESR have five-membered dioxa ring systems attached to phosphorus apical-equatorial as in 15 (15) and its mirror form 25. This applies to six-membered ring 1,3-diaza species too (39) The apparent failure of the isomerization of radical 15 to 16 (24) (a thermoneutral process which keeps the odd electron equatorial) to compete with loss of R (R = t-Bu), indicates that the cyclic phosphoranyl radicals are not simple analogs of pentacovalent phosphorus intermediates, at least where permutational properties are concerned. Thus by contrast to the apparent high AG of >11 kcal/mol assigned to process Mi of J.5 (15 24), that for the... 

Most recently we have examined (43) the reaction of 23 (R = jt-Bu or Me) with R 0-to give phosphite 24. Our earlier substitution stereochemical studies are open to some ambiguity of interpretation in that the intermediate phosphoranyl radical is too unstable to be detected by ESR, so no circumstantial evidence is available supporting its presence or suggesting its geometry. Inversion is most easily explained then by intermediate 25 or its corresponding transition state. In fact, since Me2N additions are reversible, initial intermediate 22 could conceivably not lead to product at all but break down instead to reform MezN and... 

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