Radical, aryl, rearrangement intermediate

This general mechanistic scheme readily explains a number of experimental observations. For instance it is very clear why such ester shifts only ever take place between vicinal carbons [1], as it is only this arrangement that permits the formation of an alkene radical cation as intermediate. Intermolecular ester shifts are excluded for the same reason. Rearrangements of o-(acyloxy)aryl radicals (Scheme 7) [13, 14] and their vinyl counterparts would require the intermediacy of very high energy benzyne radical cations, as such no examples are known. Failed migrations between two secondary radicals (Scheme 8) may now be seen as being due not so... 

In recent years, radical aryl migrations have received increased attention within the synthetic organic chemistry community. Yet, these reactions are also found as key steps in complex natural product synthesis [78]. For example, the neophyl rearrangement-which is the 1,2-phenyl migration of the neophyl radical 42 to form the tertiary radical 44 (probably via spirocyclohexadienyl radical 43)-was discovered by Urry and Kharasch more than 60 years ago (Scheme 13.9) [79], since which time numerous reports on neophyl-type rearrangements have been presented [80]. However, despite these efforts the postulated intermediate 43 has not yet been identified [81], The slow neophyl rearrangement (k = 762 s at 25 °C, [82]) can be used as a radical clock [83], The 1,2-aryl migration can also occur from C- to... 

IV-Nitrosqanilides are an alternative source of aryl radicals. There is a close mechanistie relationship to the decomposition of azo compounds. The JV-nitrosoanilides rearrange to intermediates that have a nitrogen-nitrogen double bond. The intermediate then decomposes to generate aryl radieals. ... 

The aryl radical thus formed attacks the substrate to give the intermediate 1 (p. 898), from which the radical 26 abstracts hydrogen to give the product. N-Nitroso amides probably rearrange to N-acyloxy compounds, which cleave to give aryl radicals ... 

The enthalpy changes associated with proton transfer in the various 4, -substituted benzophenone contact radical ion pairs as a function of solvent have been estimated by employing a variety of thermochemical data [20]. The effect of substituents upon the stability of the radical IP were derived from the study of Arnold and co-workers [55] of the reduction potentials for a variety of 4,4 -substituted benzophenones. The effect of substituents upon the stability of the ketyl radical were estimated from the kinetic data obtained by Creary for the thermal rearrangement of 2-aryl-3,3-dimethylmethylenecyclopropanes, where the mechanism for the isomerization assumes a biradical intermediate [56]. The solvent dependence for the energetics of proton transfer were based upon the studies of Gould et al. [38]. The details of the analysis can be found in the original literature [20] and only the results are herein given in Table 2.2. 

Acetoxylation proceeds mostly via the radical cation of the olefin. Aliphatic alkenes, however, undergo allylic substitution and rearrangement predominantly rather than addition [224, 225]. Aryl-substituted alkenes react by addition to vic-disubstituted acetates, in which the dia-stereoselectivity of the product formation indicates a cyclic acetoxonium ion as intermediate [226, 227]. In acenaphthenes, the cis portion of the diacetoxy product is significantly larger in the anodic process than in the chemical ones indicating that some steric shielding through the electrode is involved [228]. 

A more detailed evaluation of the diverse structures proposed for the secondary species goes beyond the scope of this review. We mwely emphasize that the ESR results provide detailed evidence for the nature of the radical center, but fail to elucidate the cationic site. The identity of this center is left to secondary considerations or speculation. We also note that any alternative structure has the virtue of not contradicting the ab irutio calculations the potential c ture of chloride ion has precedent in the nucleophilic substitution at a cyclopropane carbon (see Section 7). Another type of ring-opened structure has been postulated as an intermediate in the aminium radical cation catalyzed rearrangement of l-aryl-2-vinylcyclopropanes (see Section 5). 

The point at which one can expect SN2 and E2 reactions to go faster than radical formation as the structures of the halides and the nature of the metal are changed is not yet clearly defined. However, it is becoming increasingly evident that there are substitution reactions of unactivated aryl halides that proceed without rearrangement by way of radical intermediates. The key step in these reactions is donation of an electron to one of the unfilled tt orbitals of the ring and subsequent ejection of a halide ion ... 

When a radical is formed in a chain four or five carbon atoms removed from an aryl group, rearrangements that amount to intramolecular aromatic substitutions can occur. The initial radical attack will yield a cyclohexadienyl radical intermediate with an attached five- or six-membered ring, as in Equation 9.109. 

Scheme 22 presents some selected aryl radical cyclizations. The first example shows that isomerization of the intermediate radicals is again a concern. This isomerization, which is usually called the neo-phyl rearrangement,101 is promoted by the activating aldehyde group in the case at hand.102-103 The other examples illustrate that aryl radical cyclizations provide practical routes to benzo-fused heterocycles.104-105... 

Although the two competing intermediates, the hypothetical ketyl-aryl radical pair (4) and the oxaspirooctadienyllithium (3), are not the rate-determining transition states, they should lie at almost the same energetic level. The rearrangement is in accord with the intramolecular nucleophilic addition/elimination mechanism rather than with homolytic cleavage/recombination. 

The reaction mechanism of photocyclization of aryl vinyl ethers was derived from results obtained by means of flash photolysis. The ground state intermediate rearranges by mono-or bi-molecular 1,4-hydrogen shifts to yield the products (Scheme 62) (81JOC978). The photocyclization of 2-aryloxyenones was used in the total synthesis of ( )-lycoramine (77JA8065). The formation of dihydrobenzo[6 ]furans by radical cyclization from o-allenyl-oxyarenediazonium salts with tri-n-butyltin(IV) hydride was successful (81CC136). 

---