Rearrangement of radicals

Although rearrangements are less prevalent in radical chemistry than in the chemistry of cations, they do occur under various circumstances.180 

Rearrangement of phenyl groups and halogen to an adjacent center are more favorable than those of alkyl and hydrogen. 1,2-Shifts of phenyl occur most readily when the rearrangement will yield a more highly stabilized radical, for example, in Equations 9.105 and 9.106. The rearrangement does nevertheless 

180 (a) A comprehensive review is J. W. Wilt, in Free Radicals, J. K. Kochi, Ed., Vol. I, p. 333 see also (b) R. Kh. Friedlina, in Advances in Free Radical Chemistry, Vol. 1, G. H. Williams, Ed., Logos Press and Academic Press, London, 1965, p. 211 (c) C. Walling, in Molecular Rearrangements, P. DeMayo, Ed., Wiley-Interscience, New York, 1963, Part I, p. 407. 

Because raising the concentrations of substances from which the radicals can abstract hydrogen decreases the amount of rearrangement,186 it is clear that rearrangement follows the formation of the radical rather than being concerted with it the half-migrated structure 44, if an intermediate at all, must be a very 

188 See, for example M. L. Poutsma and P. A. Ibarbia, Tetrahedron Lett., 3309 (1972). Other studies are cited by Wilt, in Free Radicals, J. K. Kochi, Ed., pp. 346-347. 

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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. 

Something worth noting about the photoreactivity of 2-aza-1,4-dienes under DCA sensitization is that some of the products obtained result from the rearrangement of radical-cations centered on the C—C double bond, whereas in other... 

The quartet spectrum observed for the amorphous samples at temperatures higher than — 70°C. was thought to be caused by rearrangement of radical XVI, giving the radical structure XIX and a chain scission. 

The rearrangement of radical cations often leads to products not formed from the parent neutral compounds, and this fact is increasingly exploited in reactions initiated by single electron transfer. 1,8-Naphthoquinodimethane and its alkane-derivatives have been characterized spectroscopically as persistent species in... 

The third mechanism of isomerization, photoinduced rearrangements of radical cations, has been pursued in a variety of systems. Matrix isolated radical cations have been noted to undergo some rigorous reorganizations as well as subtle ones. For example, the ring opening of cyclohexadiene to hexatriene radical cation and the interconversion of its different rotamers have been achieved by irradiation with UV or visible light [173-174]. 

While solid matrices have been employed successfully, they may be less than ideal for controlled mechanistic studies. A more appropriate technique for controlled doublet photochemistry appears to be two-photon excitation in solution. In this experiment, the first photon is used to initiate radical ion formation, whereas the second photon, appropriately delayed to coincide with the maximum concentration of the radical cation so generated and tuned to its absorption maximum, serves to excite these intermediates. However, we hasten to add that the benefits of this technique have yet to be demonstrated. The photoinduced rearrangement of radical cations very likely will benefit substantially from a mismatch between (quartet vs. doublet) potential surfaces, much as triplet sensitized isomer-izations can be ascribed to mismatches between triplet and ground state surfaces. 

Rearrangements of radicals are much less common than rearrangements of carbocations. 

Rearrangement of Radicals The S[subscript RN] 1 Reaction The Birch Reduction... 

Organic and inorganic radical formation in zeolites can occur spontaneously, on adsorption of molecules into a suitably activated zeolite, or as the result of radiolysis of adsorbed species. Once a radical is formed, EPR spectroscopy can be used to follow its subsequent reactions. For example, Trifunac et al have recently described the use of variable temperature EPR to investigate reactions of olefin radical cations generated in ZSM-5 zeolites. [4]. This work shows clearly the facile rearrangement of radical cations produced by irradiation of... 

Evidence for a surface-bound radical was also provided by work of Ruchardt et al. [59]. Reaction of neophyl chloride 55 with magnesium led, after carbonylation, to acid 61 in 65% yield, without any contamination by acid 60, which could have arisen by rearrangement of radical 56 to radical 57 (Scheme 24). 

Vysotskii, Y. B., Bryantsev, V. S. Quantum-Chemical Treatment of Cyclization and Recyclization Reactions XXV. Skeletal Rearrangements of Radicals, and Lowest Triplet-State Systems. Russ. J. Org. Chem. 2002, 38,1244-1251. 

In summary, the rearrangement of radicals and anions of the 5-hexenyl type is a useful mechanistic probe if the two reaction types can be clearly separated. 

T6429>. Pyrolysis of trispyrazolylmethane (94) alfords bispyrazolylmethane (88 R = H) and, probably, AT,iV -bipyrazole (14) while pyrolysis of bispyrazolylmethane (88 R = H) affords NH-pyrazole (7), trispyrazolylmethane (94), 1-methylpyrazole and, at higher temperatures, pyrimidine. To explain the formation of pyrimidine a radical mechanism involving the rearrangement of radical (70) was proposed. Trichloromethyl radicals react with 3(5)-methyl-pyrazole to afford 4-methylpyrimidine and its 2-chloro derivative <89PJS377>. 

Butyryl-CoA mutase interconverts isobutyryl-CoA and n-butyryl-CoA (Table 1, entry 4) in an adenosyl-cobamide-dependent way and is presumed to involve the rearrangement of radicals in a mechanism similar to that of methylmalonyl-CoA mutase (10,81). 

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