The photoinduced electron transfer chemistry of ethyl vinyl ether

Abstract

The electron transfer photochemistry of ethyl vinyl ether was investigated. Irradiation in acetonitrile in the presence of 1,4-dicyanobenzene and the redox sensitizer phenanthrene results in [2 + 2] cyclodimerisation of the alkene. The cis- and trans 1,2-diethoxycyclobutanes thus formed have been reported previously in the literature, but were characterised fully here for the first time. The reaction is highly regiospecific; only the head-to-head cyclodimers were formed. This is the first time that alkyl vinyl ether [2 + 2] cyclodimers have been synthesised via the photoinduced electron transfer route. Irradiation under identical conditions, but in acetonitrile/methanol results in the formation of the photo-NOCAS (nucleophile-olefin combination, aromatic substitution) product, 1-( 4-cyanophenyl)-1-ethoxy-2-methoxyethane as a major product. The regioisomeric acetal photo-NOCAS compound is thought to have been produced but could not be isolated successfully. Unexpectedly, the redox photosensitiser phenanthrene was also consumed, resulting in an apparently complex mixture of products as indicated by gas chromatography (GC). Attempts to isolate and purify these products were unsuccessful, but also revealed that the compounds formed were possibly only a mixture of two isomers that was decomposing under GC conditions. Preliminary spectroscopic analysis of the (impure) isolated products indicated that they might be acetal-type structures that result from addition of ethyl vinyl ether at the 9, 10-postion of the phenanthrene ring, with the concomitant involvement of methanol. This is the first time that phenanthrene has been reported to react under photo-NOCAS conditions. Theoretical calculations, as well as photophysical studies, support the electron transfer mechanistic pathways proposed for the above reactions.