Aryl group migration

The Hammett p value for migrating aryl groups in the acid-catalysed Beckmann rearrangement is -2.0. What does this tell us... 

Initially, one of the geometrical isomers, presumably Z, forms faster. The competing thermal E-Z interconversion complicates the situation sufficiently that it is not yet clear whether the transposition reaction forms this geometrical isomer exclusively. If so, the mechanism is likely to be of the dyoiropic kind, involving a shift of both migrating aryl groups and a biradicaloid transition state with a bent or planar bicyclobutane-like structure 24. An alternative that cannot yet be ruled out with confidence is a disilene-to-silylsilylene shift followed by a silylsilylene-to-disilene shift (cf. Sections II.A.3 and II.B.l.b). Both possibilities are displayed in equation 19. 

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]. 

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