SCHEMES 12.5. Free-radical rearrangements

Scheme 12.6 gives some examples of reactions in which free-radical rearrangements have been observed. 

Scheme 3.4. Walton s and McCarroll s classification of unimolecular free-radical rearrangements.

An excellent review by Walton and McCarroll has recapitulated the various processes which can be featured during a radical cascade [2]. Moreover, these authors have elaborated a compilation of classes of unimolecular free-radical rearrangements, as illustrated in Scheme 3.4. 

Phenylpropanoic, 4-phenylbutanoic and 3,3-diphenylpropanoic acids were transformed with xenon difluoride to the corresponding fluorinated products without indication of free-radical rearrangement or cyclization, while phenylacetic acid in the presence of benzene produced diphenylmethane, indicating the involvement of a trivalent species (Scheme 44). Trifluoromethylbenzene was formed in 42% yield by reaction of trifluo-roacetic acid with benzene35-37. 

However, the evidence increasingly points to the substitution reactions as simply being an adjunct of the general scheme for radical rearrangement and as taking place via nucleophilic capture of one or other of the radical cation/anion pairs or of the free-radical cation. 

Scheme 11.6 gives some examples of reactions in which free radical rearrangements have been observed. Entries 1 and 2 are phenyl group migrations in primary alkyl radicals generated by decarbonylation. The migration is competitive with the... 

The situation is not as clearly solved in a positive or negative sense for arenediazo phenyl ethers. Here three alternatives have to be considered, namely an intramolecular rearrangement of the arenediazo phenyl ether (Scheme 12-11, A), and two types of intermolecular rearrangement, either by heterolytic dissociation into a diazonium ion and a phenoxide ion (B) or by homolytic dissociation into a radical pair or two free radicals (C). 

Scheme 2 summarizes the mechanism for the formation of thiosulfonate from sulfinyl radicals it is shown that the sulfinyl radicals combine to give both yic-disulfoxides and O, S-sulfenyl sulfinates, although they may rearrange to thiosulfonates either via a free radical route or via a concerted mechanism. The reader is referred to the recent review of Freeman22, who has collected and discussed the vast amount of information published on yic-disulfoxides and O, S-sulfenyl sulfinates. 

The trimethylsilyl ethers 212 of four-membered 1-alkenyl-1-cyclobutanols rearrange to the ring-expanded 0-mercuriocyclopentanones 213. These can be converted into the a-methylenecyclopentanones 214 through elimination or further expanded by one-carbon atom into cyclohexanones 215 via the Bu3SnH-mediated free radical chain reactions [116]. A similar radical intermediate is suggested to be involved in the ring expansion of a-bromomethyl-fi-keto esters [117]. (Scheme 84)... 

The rearrangement of compounds 131 to 132 in a number of solvents at 155°C has been shown to proceed via a free radical process (Scheme 31). 00.101 jjjg reaction is first order, little affected by solvent, and there is... 

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