Aryl groups migrate intramolecularly

Photolysis of alkyl azides bearing pendant aryl groups does not lead to intramolecular trapping of a nitrene by the aryl group. However, intramolecular capture of the putative nitrene is observed upon pyrolysis of the azide precursor. These observations convinced Kyba and Abramovitch" ° that 1,2-migration is concerted with loss of nitrogen from the excited state of the azide, but that a free singlet nitrene is formed upon thermal decomposition. The chemistry of acyl azides will be shown later to exhibit the opposite pattern. 

It is important to note the similarities and differences between the benzylic acid rearrangement and the Cannizzaro reaction (Scheme 9.4). The aryl group migrates in the benzylic acid rearrangement. The hydride migrates in the Cannizzaro reaction. Further, the Cannizzaro reaction is intermolecular (second-order in aldehyde and first-order in base), while the benzylic acid rearrangement is intramolecular and first-order both in base and in diketone. 

Carbenes can also be stabilized by migration of alkyl or aryl groups. 2-Methyl-2-phenyl-1-diazopropane provides a case in which products of both phenyl and methyl migration, as well as intramolecular insertion, are observed. 

The molecular structure of diazocine 31 (R = R = H) (see Section II,A,2) was determined by X-ray diffraction to be a tub conformation with no extensive 7r-conjugation and a very short C-H—O intramolecular distance [76AX(B)2314]. The mass spectra of a number of diaryl-1,2-diazocines 28 (X = H2) indicated no migration of aryl groups (79MI1). 

Many other cases of free-radical migration of aryl groups have been found. Intramolecular radical rearrangements are known. The C-4 radicals of a- and p-thujone undergo two distinct rearrangement reactions, and it has been proposed that these could serve as simultaneous, but independent radical clocks. ... 

The migration can be either intramolecular or intermolecular, and these can be distinguished by cross-over experiments. Aryl groups tend to migrate more easily than alkyl groups. 

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