Photo-Wagner-Meerwein rearrangement

Extensive information concerning the operation of stereoelectronic effects on the photoreactivity of homobenzylic halides has also been obtained for dibenzo[2.2.2]-systems223-228 such as 51-54. The photo-Wagner-Meerwein rearrangements and photo-solvolyses of these compounds also occur via exclusive223 or preferential224 reaction of the p-C—Cl bond anti to the excited aryl group. Electron transfer between these two moieties is considered to be the key step here too. Indeed, the thermodynamics of such a step seem... 

Further studies on the isomeric dichlorides of (47) with different Y and Y substitution support the proposal that an electron transfer occurs from the photoexcited n orbital of the aromatic ring to the a orbital of the carbon-chlorine bond [72], When Y = H and Y = CN, COCH3 or N02 no photosolvolysis or photo-Wagner-Meerwein rearrangements were observed. Using the Weller equation it was determined that electron transfer from the presumed triplet states of the COCH3 or N02 substituted compounds or the singlet state of... 

Cristol, S. J., Opitz, R. J. Photochemical transformations. 40. syn and anti Migration in photo-Wagner-Meerwein rearrangements. J. Org. Chem. 1985, 50, 4558-4563. 

Reaction with unsaturated bicyclic hydrocarbons. From a study of the chlorination of unsaturated bicyclic compounds, Masson and Thuillier1 conclude that the reaction follows, a radical addition mechanism when initiated thermally or photo-chemically. The stereochemistry of the addition is markedly influenced by steric effects. No Wagner-Meerwein rearrangements are observed under these conditions. An ionic mechanism is involved without initiation or in the presence of trifluoroacetic acid in this case the usual carbonium ion rearrangements are observed. 

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