Novel photochemical behaviour of the oximes and hydrazones of β,γ-unsaturated carbonyl compounds

Abstract

A study of the photochemical reactivity of a series of β,γ-unsaturated oximes and hydrazone derivatives under triplet sensitized conditions has been carried out. The oximes 3c, 4a and 4c cyclize to the corresponding dihydroisoxazoles 5c, 6a and 6c while the tosyl hydrazone 8a affords the dihydropyrazole 9a. An intramolecular single electron-transfer mechanism from the alkene moiety to the oximino group, in the case of the oximes 3c, 4a and 4c, and to the tosyl group for the tosyl hydrazone 8a, is proposed to account for these results. Oximes and hydrazine derivatives from aldehydes behave differently. Thus, the oxime 3d yields the cyclopropyl oxime 10 by an aza di-π-methane (ADPM) rearrangement while the aldoxime 3e gives a mixture of the corresponding dihydroisoxazole 5d and cyclopropane 11a resulting from an ADPM process. Irradiation of the hydrazine derivatives 8b, 8c and 8d gives a mixture of the corresponding dihydropyrazoles 9b, 9c and 9d and the cyclopropanes 11b, 11c and 11d, respectively. However, under the same experimental conditions, dihydronaphthalene derivatives such as the oxime 12a and the tosyl hydrazone 12b undergo ADPM rearrangements exclusively, affording the cyclopropanes 13a and 13b, respectively. Sensitized irradiation of the tosyl hydrazone 12c yields the cyclopropane 13c, as the major product. In this instance a small amount of the hexahydrophenanthroline 14, resulting from an endo cyclization is also formed. The influence of substitution on the outcome of the reaction is discussed.