Rearrangements Involving Radicals

Intramolecular rearrangements of free radicals are not nearly so common as those of carbo-cations. In fact, the most important rearrangements of free radicals are those associated with free radical clocks , as discussed in Section 8.8.8 and listed in Table 8.7. Here we describe a few other rearrangements of radical. systems. 

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As depicted in Scheme 53 the photo-Smiles rearrangement involves radical ion pairs. Intermediately the spiro-type Meisenheimer complex 277 is formed... 

The fragmentation of alkoxyl radicals is especially favorable because the formation of a carbonyl bond makes such reactions exothermic. Rearrangements of radicals frequently occur by a series of addition-fragmentation steps. The following two reactions involve radical rearrangements that proceed through addition-elimination sequences. 

Rearrangements that involve radicals are found to be much less common than otherwise similar rearrangements that involve carbo-cations. In this they resemble carbanions (cf. p. 292), and the reason for the resemblance becomes apparent when we compare the T.S.s for a 1,2-alkyl shift in the three series ... 

Although cyclic azoalkanes are well known as biradical precursors [159] they have been used as 1,2- and 1,3-radical cation precursors only recently [160-164]. Apart from the rearrangement products bicyclopentane 161 and cyclopentene 163, the PET-oxidation of bicyclic azoalkane 158 yields mostly unsaturated spirocyclic products [165]. Common sensitizers are triphenyl-pyrylium tetrafluoroborate and 9,10-dicyanoanthracene with biphenyl as a cosensitizer. The ethers 164 and 165 represent trapping products of the proposed 1,2-radical cation 162. Comparison of the PET chemistry of the azoalkane 158 and the corresponding bicyclopentane 161 additionally supports the notion that the non-rearranged diazenyl radical cation 159 is involved (Scheme 31). 

A-Alkyl groups in neutral azoles can rearrange thermally to carbon. For example, 2-alkylimidazoles can be prepared in this way in a reaction which is irreversible, uncatalyzed, intramolecular and does not involve radicals (80AHC(27)24l). 

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