1,5-Aryl migration

The intramolecular addition of silyl radicals to aromatic rings has also attracted some attention. Early work on the silyl radical obtained by the reaction of silanes 41 with thermally generated t-BuO radicals at 135 °C showed the formation of rearranged products only for = 3 or 4, whereas for = 1, 2, 5, and 6 no rearrangement took place [20], 

From the reaction of photogenerated t-BuO radicals with silanes 48, in the cavity of the EPR spectrometer, the corresponding spiro cyclohexadienyl radicals are the only detectable species [22]. For n = 3, the intermediate radical 49 showed a spectral feature that suggests a chair-like arrangement and a chair-chair interconversion barrier of 18.4kJ/mol was obtained from a detailed line-shape analysis. 

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Diaryloxaziridines are even less stable. With oxaziridines (66 Ar = Ph or 4-Me2NC6H4) acid amide formation at 25 °C proceeded in the course of 66 and 10 h respectively in the latter case there were equal amounts of H and aryl migration. Ethanol as solvent again accelerated the reaction, in the latter case by a factor of lO. ... 

The more facile migration of aryl and other unsaturated groups involves bridged intermediates formed by an addition process. In the case of aryl migration, the intermediate is a cyclohexadienyl radical ... 

Aryl migrations are promoted by steric crowding in the initial radical site. This trend is illustrated by data from the thermal decomposition of a series of diacyl peroxides. The amount of product derived from rearrangement increases with the size and number of substituents ... 

This rearrangement can be considered to occur via a transition state in which C(2)—C(4) bridging is accompanied by a 4 3 aryl migration ... 

In 1968, the aforementioned reaction was repeated and found to produce a rearranged product (11). This was the first report of aryl migration with the Pictet-Gams conditions. The expected product was l-methyl-3-phenyl isoquinoline, but only 1-methyl-4-isoquinoline (11) was observed. Interestingly, the authors did not suggest a mechanism for the formation of the isolated product. 

An intermolecular 1,3-dipolar cycloaddition of diazocarbonyl compounds with alkynes was developed by using an InCl3-catalyzed cycloaddition in water. The reaction was found to proceed by a domino 1,3-dipolar cycloaddition-hydrogen (alkyl or aryl) migration (Eq. 12.68).146 The reaction is applicable to various a-diazocarbonyl compounds and alkynes with a carbonyl group at the neighboring position, and the success of the reaction was rationalized by decreasing the HOMO-LUMO of the reaction. 

When we allowed pentafluorophenyl-lithium to decompose in ether in the presence of an excess of N, ZV-dimethy laniline we obtained the compounds (92) 70, X = F), (94), the latter as the major compound, and a product which was shown to be (97). That this latter compound did not arise by metallation of 2V,lV-dimethylaniline followed by addition to tetrafluorobenzyne was shown by quenching the reaction mixture with deuterium oxide. No deuterium incorporation was detected. The compound (97) provides a rare example of a product derived by a Stevens rearrangement in which aryl migration has occurred b>. 

The aryl migration undoubtedly occurs as a result of the strong electron with drawing effect of the four fluorine atoms. 

The rearrangements that we have considered to date all have one feature in common the migration of an alkyl or aryl group, with its electron pair, to a carbon atom which, whether it be a carbocation or not, is electron-deficient. Another atom that can similarly become electron-deficient is nitrogen in, for example, R2N or RN (a nitrene, cf. carbenes above), and it might be expected that alkyl or aryl migration to such centres would take place, just as it did to R3C and R2C this is indeed found to be the case. 

Aryl migrations are not confined to carbon/carbon rearrangements, as is seen in the behaviour of (Ph3CO)2 (120, cf. p. 300) on heating ... 

Aryl Migrations in Organometallic Compounds of the Alkali Metals, 16, 167... 

Diarylboron chelates C-(l-hydroxyalkyl)nitrones (238) on boiling in toluene tend to undergo 1,4-aryl migration and form heterocyclic arylboronates (239) (Scheme 2.82) (446). 

STEELE, C.L., GLJZEN, M., QUTOB, D., DIXON, R.A., Molecular characterization of the enzyme catalyzing the aryl migration reaction of isoflavonoid biosynthesis in soybean, Arch. of Biochem. and Biophys., 1999, 367, 146-150. 

The most widely used route to l-benzazepin-2-ones involves the Beckmann or Schmidt reaction of the easily accessible 1-tetralones. Many biologically active compounds described in this review have been prepared on the basis of these reactions they have been fully reviewed [2], In the Beckmann reaction of 1-tetralone oximes, polyphosphoric acid is used as a catalyst-solvent in most instances. Aryl migration generally takes precedence over alkyl migration under these reaction conditions, and various 1-tetralone oximes substituted on the aromatic and/or aliphatic rings can be converted to the appropriate 2,3,4,5-tetrahydro-l//-l-benzazepin-2-ones (51) [5, 20-23, 36, 59, 65, 80, 107-112]. Both courses of the rearrangement occur in some instances, yielding l-benzazepin-2-ones (51) and the isomeric 2-benzazepine-l-ones, probably due to electronic effects of the substituents [90, 113, 114]. 

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