The photochemical Beckmann rearrangement

Abstract

The direct irradiation of aryl aldoximes gives the amide (the photochemical Beckmann rearrangement) by intramolecular oxygen migration. This has been shown by the rearrangement of 18O-labelled benzaldoxime in the presence of p-tolualdoxime: no exchange of label was observed.The rearrangement usually gives the amide, but in the case of the anisyl derivative the anilide was also obtained. The highest quantum yield for benzamide formation noted (in acetic acid) was 0.034, but the figure is dependent on irradiation time. The results of low-temperature irradiation suggest that an oxazirane is an intermediate. Phenyl-N-methyl oxazirane is converted into the amide on irradiation (though not thermally) and it seems likely that the reaction is induced, under the conditions of the reaction, by benzylic hydrogen abstraction by thermally produced small amounts of benzaldehyde; a fact which may explain the variation in quantum yield of amide formation with extent of irradiation. An additional route from oxazirane to amide may also be available.The activation of the oxime by high energy sensitizers under the most favorable conditions gives Φamide = 0.002, and so a singlet pathway for oxazirane formation is preferred. The main route for energy degradation appears to be by syn–anti isomerism. This can be induced by a variety of sensitizers and so can be a triplet process. The direct irradiation of benzaldoxime leads to isomerism with Φαβ = 0.40 and Φβα = 0.38. From these figures, together with the composition of the stationary state found, the composition of the sensitized stationary state can be calculated in fair agreement with the experimentally found value. This suggests that the isomerization following direct irradiation is also a triplet process.In a single example of an aralkyl ketoxime (p-anisylmethyl ketoxime) irradiation products of both aryl and alkyl migration, were found.