Photosensitized ring opening in phenyl oiranes

Abstract

Under energy transfer sensitization by pulse-radiolytically or laser-flash-photolytically generated benzene, acetone, or chromone triplet, the ring opening in cis or trans-stilbene oxide leads to a mixture of exo,endo and exo,exo forms of carbonyl ylides that are spectrally and kinetically distinguishable from each other. The fact that the same molar ratio of the two ylide forms is produced from the two isomeric oxiranes suggests the involvement of a common ylide triplet. On the other hand, under reversible electron transfer sensitization by 1,4-dicyanonaphthalene singlet excited state, the two ylide forms are produced in significantly different ratios when cis and trans-stilbene oxides are used as donors; this indicates the intermediacy of distinct, non-equilibrating, ring-opened radical cations in the two cases. Spectrally observable carbonyl ylide transients with short lifetimes (85-550 ns) are also produced from tri- and tetraphenyl oxiranes under both energy and electron transfer sensitization. Unlike the ylides from di- and trisubstituted phenyl oxiranes, that from tetraphenyl oxirane is practically non-quenchable by electron-poor dipolarophiles (e.g., methyl acrylate), suggesting a steric hindrance in attaining the "two-plane orientation complex" in the course of 1,3-dipolar cycloaddition.