Radical Aryl Migration Reactions

O-centered radicals, and subsequently is referred to as the O-neophyl rearrangement [84]. 

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Radical aryl migration reactions leading to formation of heterocycles 01T9649. 

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... 

Scheme 13.9 Radical 1,2- and 1,3-aryl migration reactions (DLP = dilauroyl peroxide).

Neopentyl and Neophyl Iodides. The halides mentioned above formed the corresponding organocobalt complexes on reaction with pentacyanocobaltate(II). While neopentyl radical is stable to rearrangement, the neophyl radical undergoes aryl migration (56) as follows ... 

In the 1,2-aryl migration (homobenzylic rearrangement) the delocalized spiro[2.5]octadienyl radical 26 is too short-lived for EPR detection, but has been identified by absorption spectroscopy. Many examples of this reaction are known,but products containing the cyclopropane ring are not obtained. 

Later, Motherwell developed a new biaryl synthesis using 1,4- as well as 1,5-aryl migrations from sulfur to aryl radicals [101], Easily available arenesulfonates as well as arenesulfonamides were used as starting materials in these reactions 72). As in the Speckamp studies with primary alkyl radicals, the aryl radicals 71 were shown to also react by ortho attack to afford the side product 73. However, by judicious choice of the migrating aryl group, it is possible to completely suppress the ortho attack [102]. 

Several benzo-fused tetrahydropyrans have been prepared either from aryl-propanhydroperoxides in the presence of iron(II) and copper(II) salts, from the corresponding arylpropanol either by oxidation with FeS20g, or in a photochemical reaction with iodine and mercury(II) oxide (type III radical precursors). These reactions are considered to proceed via alkoxyl radical intermediates which add to the aromatic part of the molecule. Subsequent C- or O-migration in spirocyclized intermediates affords substituted chromanes. However, ionic cyclizations may significantly interfere with the key step of the C-O bond formation [57-59]. 

Radical reactions. In the presence of air (or oxygen), EtsB promotes radical formation. A tin-free cyclization of w-iodo carbonyl compounds is effected. Regioselective arylation can take advantage of 1,4-aryl migration from tin to a carbon radical center. a-Hydroxyalkylation at C-2 of THF is conveniently accomplished with RCHO. ... 

Scheme 13.10 A complex cascade reaction comprising a radical 1,4-aryl migration.

With 4,4-diarylcyclohexenones, the reaction takes a slightly different course involving aryl migration. In compounds in which the two aryl groups are substituted differently, it is found that substituents that stabilize radical character favor migration. Thus the p-cyanophenyl substituent migrates in preference to the phenyl... 

In some cases, aryl radical migration appeared as a side reaction [16a, 17], with the most extensively investigated one being the 1,2-aryl migration in p-aryl carbon-centered radicals, called the neophyl-type rearrangement (Eq. 9.5), where the neophyl radical 30 is converted to tertiary radical... 

Aryl migrations in free radicals chemistry are not restricted to 1,2-migrations 1,4- and 1,5-migrations of aryl groups are also an efficient method for C—C bond formation. Again, these migrations are usually considered to be side reactions, although some synthetically useful examples have been reported in the literature [17]. 

Zimmerman, H.E. and Hoffacker, K.D. Novel radical cation reactions of bichromophoric systems. Transannular aryl migrations mechanistic and exploratory organic photochemistry, /. Org. Chem., 61, 6526-6534, 1996. 

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