The photochemical Beckmann rearrangement

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 a...