P-Fragmentation

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). 

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]. 

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). 

Maslak, P. Fragmentations by Photoinduced Electron Transfer. Fundamentals and Practical Aspects. 168, 1-46 (1993). 

Krauss, D. Mainx, H.G. Tauscher, B. Bischof, P. Fragmentation of Trimethyl-silyl Derivatives of 2-Alkoxyphenols a Further Violation of the Even-Electron Rule. Org. Mass Spectrom. 1985, 20, 614-618. 

Barros M, De Souza Silva MA, Huston JP, Tomaz C (2002) Anxiolytic-like effects of substance P fragment (SP(l-7)) in non-human primates (Callithrix penicillata). Peptides 23 967-973... 

The parent peak, usually having the highest m/e value, can easily be identified in all cases. Triphenyl-X -phosphorin also has an intense peak at m/e = 120, which Markl attributes to the C Hs-C = P fragment. Fig. 11 shows the mass spectra... 

Quantum yields, free energy changes, — AG°, for the electron transfer reaction from the P fragment to the Q fragment of P-L Q molecules in methylene chloride solution [55]... 

Cyclobutane derivatives can also be opened under reductive conditions. The reductive a,P-fragmentation of y-halocyclobutylketones (Eq. (19)) has been studied... 

The first complexes to contain the P=P fragment were synthesized in the metal coordination sphere. The parent diphosphene, HP=PH, is unknown in the free state, but is quite stable in (41).285... 

ADDITIONS TO CARBON-NITROGEN AND CARBON-OXYGEN MULTIPLE BONDS, AND p-FRAGMENTATIONS... 

Water Elimination, Heterolytic P-Fragmentation and Formation of Radical Cations 118... 

Carbon-centered radicals may undergo homolytic P-fragmentation reactions, whereby an olefin and a new radical is formed. This reaction is, in fact, the reverse of the polymerization reaction. With neighboring C-C bonds, these P-frag-mentation reactions are usually slow, and only observable, at least on the pulse radiolysis time-scale with negatively-charged polymeric radicals whose lifetime is prolonged by electrostatic repulsion. Then, even the situation of equilibrium polymerization maybe approached (Ulanski et al. 2000 Chap. 9.4). 

Many O-centered radicals undergo facile P-fragmentation. For example, acyl-oxyl radicals which are intermediates in the electrolytic oxidation of acids (Kolbe electrolysis), rapidly decompose into alkyl radicals and C02 [reaction (1)]. The rate of these reactions is in the order of 109 s 1 and increases with increasing branching of the alkyl substituent, i.e., decreasing C-C02 bond energy (Table 7.1). 

Similarly, the p-fragmentation of tertiary alkoxyl radicals [reaction (2)] is a well-known process. Interestingly, this unimolecular decay is speeded up in a polar environment. For example, the decay of the ferf-butoxyl radical into acetone and a methyl radical proceeds in the gas phase at a rate of 103 s 1 (for kinetic details and quantum-mechanical calculations see Fittschen et al. 2000), increases with increasing solvent polarity (Walling and Wagner 1964), and in water it is faster than 106 s 1 (Gilbert et al. 1981 Table 7.2). 

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