Rearrangements donor radical cations

The fate cS the contact ion pair [RH A 1 is critical to electrm-transfer oxidation. Oxidative efficimey is the highest with those organic donors diat yield unstable radical cations, such as hexamethyl(Dewar benzene), which und goes spontaneous rearrangement (equation 7). > ... 

Bauld and coworkers, especially, developed the analogous Diels-Alder (4 + 2) cycloaddition reactions. These reactions are conveniently catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate (78) or by photosensitization with aromatic nitriles. The radical cation-catalyzed Diels-Alder reaction is far faster than the uncatalyzed one, and leads to some selectivity for attack at the least substituted double bond for the monoene component (Scheme 18, 79 —> 80), but only modest endo selectivity (e- and x-80) [105]. Cross reactions with two dienes proved to be notably less sensitive to inhibition by steric hindrance of alkyl groups substituted on the double bonds than the uncatalyzed reactions, as cyclohexadiene adds detectably even to the trisubstituted double bond of 2-methylhexadiene (82), producing both 83 and 84. Dienes such as 85 react with donor-substituted olefins (86) to principally give the vinylcyclobutene products 87, but they may be thermally rearranged to the cyclohexene product 88 in good yield [105]. Schmittel and coworkers have studied the cation radical catalyzed Diels-Alder addition of both... 

When donor-substituted and acceptor-substituted alkenes complex and suffer a one-electron transfer, the cation radical of the radical cation-radical anion pair may suffer rearrangement at rates competitive with back electron transfer or with radical ion pair recombination. Some vinylcyclopropanes show this phenomenon (127 —> 128 —> 129 —> 130) 2 TCNE and the diphenylbenzocyclobutene (131) react similarly to form (134) by way of (132) and (133).2 ... 

Radical cations exhibit a wide variety of reactions, including unimolecular reactions such as rearrangement, fragmentation, and intramolecular bond formation as well as bimolecular reactions with ionic, radical, or ground state species. Notable processes include reaction with nucleophiles to produce radicals, reaction with radicals to produce cations, reaction with electron donors to produce biradicals, and reaction with ground state molecules to give addition products. Often the products of reactions of radical cations with neutral species are different from those observed by reaction of the corresponding carbocation with the same reactant... 

Newcomb et suggested that reaction of the Fe—OOH species as an OH donor might be the source of the cationic rearrangement products. Most recently, Ortiz de MonteIiano and co-workers, with still different probes and enzymes, concluded that carbon hydroxylation proceeds exclusively by the radical pathway shown in Figures 8.5 and 8.6, and that the minor cationic pathway may be due to electron transfer from a radical intermediate to some reducible component of the peptide. It also was concluded, based on the similar products in DjO and HjO, that Fe OjH is not the hydroxylating agent. 

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