Radical paths homolytic cleavage

Serves as a Flag to Alert You to Shift Into Using One-Electron Paths Homolytic Cleavage by Heat or Light Common Radical Initiators... 

The photoinduced -elimination of 1,2,3-triazole from 1-(A,A-bisacyl)amino-l,2,3-triazoles (142), itself formed from the photochemical isomerization of triazoles (141), proceeds either via an intra-or intermolecular hydrogen abstraction or electron-transfer mechanism followed by homolytic cleavage of the A,A-bond (path a) or via t -assisted )8-cleavage of the same weak bond (path b). The composition of the products suggests that in all cases a c-type 1,2,3-triazolyl radical (143) is eliminated which is further quenched by hydrogen abstraction as shown in Scheme 24 <93JHC1301>. 

The ferric peroxide intermediate could undergo either heterolytic or homolytic cleavage to give either a ferryl or diferryl oxene. In the path shown in Fig. 14, the organic radical is generated by reaction with the Fe center after heterolytic 0—0 bond cleavage. For RNRB2 this is the... 

Electrochemically generated cation radical 252 derived from the cyclic alkylphenyl selenide 251 can react via two parallel paths deprotonization with the formation of compounds containing double bonds in the selenium-bearing rings and homolytic cleavage of the C(sp3)-selenium bond, followed by dimerization and formation of a diselenide 253 with an extended ring (Scheme 38) <1987ZOB609>. 

Since almost all stable species are even electron and spin paired (molecular oxygen is the obvious exception), radical processes must start either by single electron transfer or by homolytic cleavage. It is important to recognize these initiation steps in order to know when to use the one-electron paths from this chapter rather than the two-electron paths described previously. Half-headed arrows are used to symbolize the movement of one electron. Homolytic cleavage is the simple extension of a bond-stretching vibration (Fig. 11.4). The process is always endothermic, for the barrier must be at least equal to the strength of the bond cleaved. 

Free radicals are important in atmospheric systems, flames, explosions, polymerizations, and biological systems. They have one or more free electrons that result from homolytic cleavage of normal chemical bonds. Free radicals are very active and typically appear as intermediates on the reaction path from one stable species to another, hence their experimental study is difficult. Like A Hin of positive and negative ions, A H29S of free radicals can be computed ab initio. 

Given the well documented role of homolytic M-M bond cleavage in photoreactions of dinuclear carbonyls, a logical hypothesis for the photofragmentations of trinuclear complexes would be to follow a similar path to give a diradical. Indeed such an intermediate is attractive in the context that the lower quantum yields could be attributed to rapid recombination of the radicals held together by the rest of the cluster. However, experiments with added chlorocarbons in Ru3(CO)i2 photolysis solutions have shown that a trappable diradical is not an intermediate in the photoreaction chemistry, but instead the intermediate formed is susceptible to trapping by two electron donors (i.e., Lewis base nucleophiles) (eq. 13) [17,20,48-50]. 

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