Rearrangement aryl migration

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. 

Addition of bromine to 1 in chloroform solution at 10°C led in high yield to the formation of the exo-5-a/ih -7-dibromide 2. No other products were isolated. The formation of this rearranged product can be explained in terms of Wagner-Meerwein rearrangement where migration of the aryl group is involved (eqn. 1). 

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 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 ... 

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]. 

Rearrangement of benzil to benzylic acid via aryl migration. 

From a study of his and van Alphen s results, Huttel identified three classes of rearrangement (Scheme 40).137 Type A involved migration of a group from C-3 to unsubstituted C-4 (e.g., 114 - 11564), Type B an acyl migration from C-4 or C-5 to N (e.g., 116 -> 11764or 118 - 117137), and Type C an aryl migration from C-3 to N (e.g., 118 - 119137) the latter two modes were observed when all carbon atoms were fully substituted. 

Chalcones under acidic conditions are known to undergo 1,2-aryl migrations with concomitant reductive elimination of iodobenzene yielding rearranged acetals [Eq. (61)] [106]. 

RB3LYP calculations indicate that the s-cis conformer of peroxy acids is more stable than the s-trans conformer. Calculations on the reaction of prop-2-enol with some peroxy acids showed that trans-transition states collapse to the epoxide via a 1,2-shift, whereas a 1,4-shift is operable for cis-transition states.195 Quantum mechanical calculations have been performed on the migration step of the Baeyer-Villiger rearrangements of some substituted acetophenones with m-chloroperbenzoic acid (m-CPBA). The energy barriers, charge distributions and frontier molecular orbitals, determined for the aryl migration step, have been used to explain the effects of substituents on the reactivity of the ketones.196... 

ArCH(CH3)COOH. These acids (or the esters) have been obtained by a 1,2-aryl migration of propiophenones (cf. 12,452). This rearrangement is effected most easily with iodine or IC13 in trimethyl orthoformate.23... 

A parallel biogenetic scheme has been suggested by Theuns et al. (24) for the formation of neodihydrothebaine (7) in Papaver bracteatum. In this case the route proceeds through the isomeric salutaridinol (103), which is derived by para-ortho coupling of reticuline (Scheme 29). It has been suggested that salutaridinol (103) (or thebaine) is the precursor of bractazonine (8), but in this case via a proerythrinadienone formed by aryl migration in the dienol-benzene rearrangement (24) (Scheme 30). 

---