Solvent-caged radical pair

The authors advocated a redox reaction mechanism (Scheme 1.82), whereby 304 reacts with ArCH2Y to produce the solvent cage radical pair 306, which collapses to 307. Elimination of methanol from 307 yielded 308. Here, the loss of HY was believed to occur homolytically to yield 305. ... 

However, solvent viscosity, rather than polarity, has been a useful tool for mechanistic purposes. Although the quantum yield of the ortho-rearranged product of 4-methylphenyl acetate (20) does not change with viscosity of the medium, the formation of 4-methylphenol (22) is highly sensitive to this factor. Thus, its quantum yield is 0.45 in ethanol (1.00 cP) but only 0.02 in Carbowax 600 (109 cP) (Scheme 8 Table 3) [13], This clearly supports the mechanism involving caged radical pairs. A related aspect is the intramolecular nature of the process confirmed by the lack of cross-coupling products in crossover experiments with mixtures of different esters [10]. 

The SET mechanism was suggested by Meyers and coworkers610, who proposed a solvent caged radical-anion radical pair (RARP) intermediate (equation 81). 

Photolysis of 1,4-diiodocubane (494) in nucleophilic solvents, such as methanol, was found to provide effectively the substitution product 495 (Scheme 6.241).1348 It was argued that the reaction proceeds via the formation of a caged radical pair and electron transfer to give iodide ion and a carbocation, which is trapped by the solvent (SN1 pathway). Hyperconjugative stabilization in cubyl cation cannot assist much from an extremely strained olefin cubene. However, the authors expected that photochemical fission of the C—I bond would still provide sufficient energy for production of the cation intermediate. 

Cage escape to the solvent-separated radical pair opens further reaction channels such as secondary recombination and other reactions. 

In certain cases, e.g. with Z = tert-butyl, the experimental findings may better be rationalized by an ion-pair mechanism rather than a radical-pair mechanism. A heterolytic cleavage of the N-R bond will lead to the ion-pair 4b, held together in a solvent cage ... 

The UV spectra suggest that the equilibrium between the diazonium ion and the solvent, on the one hand, and an electron donor-acceptor complex (8.58) on the other, lies on the side of the complex. The latter may possibly exist also as a radical pair (8.60) or a covalent compound (8.59). Dissociation of this complex within a cage to form an aryl radical, a nitrogen molecule, and the radical cation of DMSO is slow and rate-determining. Fast subsequent steps lead to the products observed. 

The other mechanism which has been advocated58 is that known as the radical-pair mechanism , in which two cation radicals are thought of as intermediates held in a solvent cage so preserving the intramolecularity of the reaction, viz. 

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