Radicals rearrangements, 1,2 phenyl migration

With 4,4-diarylcyclohexenones, the di-Tc-methane rearrangement occcurs. In compounds in which the two aryl groups are substituted differently, it is found that substituents which stabilize radical character favor migration. Thus, the p-cyanophenyl substituent migrates in preference to the phenyl substituent in 4 ... 

With regard to the mechanism of the solid state photoreactions, there are two possible pathways from the starting amides 53 to phthalides 54 either phenyl migration, which is the same as in the mechanism of thioester derivatives 51, or the mechanism initiated by homolytic cleavage of the C( = 0)-N bond to generate a radical pair intermediate. The radical mechanism has been confirmed in the photo-Fries rearrangement of aromatic amides. To answer the question of the... 

The scrambling of 12% deuterium to the 2-position is consistent with a relative reactivity of hydrogen at the 1-position to hydrogen at the 2-position of (very roughly) 3 1 toward hydrogen abstraction. Further evidence for the formation of 2-tetralyl radical comes from the observation of 1-methylindan from the thermal rearrangement of tetralin. Work in our laboratory has shown that 1-methylindan results from the 1,2-phenyl migration of 2-tetralyl radical and does not occur via 1-tetralyl... 

It is not surprising that the concerted Wolff-type expansion is particularly favorable in 69. It is in effect a migration of a phenyl-group, as made clear in the limiting resonance form 75. Phenyl-migrations are wellknown in Wolff-and Curtius rearrangements >, in radicals 1, and in cations b... 

Scheme 11.6 gives some examples of reactions in which free radical rearrangements have been observed. Entries 1 and 2 are phenyl group migrations in primary alkyl radicals generated by decarbonylation. The migration is competitive with the... 

The reaction in Entry 5 involves a phenyl migration. The rearranged radical undergoes hydrogen atom elimination rather than the usual abstraction of bromine. 

Another reaction of radicals is rearrangement. Radicals are generally less susceptible to rearrangement than are carbocations, and the 1,2 hydrogen or carbon shifts seen with carbocations are not observed with radicals. However, apparent phenyl migration has been observed (Scheme 4.37). Treatment of neophyl chloride with phenyhnagnesium bromide and cobaltous chloride produced isobutylbenzene... 

Even in the most favorable cases, such as rearrangement of the primary radical A to the tertiary B by phenyl migration, there is a modest activation energy required ... 

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... 

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