The electron transfer photochemistry of tertiary butyl vinyl ether : yet another type of photo-NOCAS reaction

Abstract

The normal photochemical nucleophile-olefin combination aromatic substitution (photo-NOCAS) reaction is a well-established type of photochemical reaction involving the regioselective combination of a cyano-substituted arene, an alkene, and a nucleophile in "one pot". A recently discovered novel photo-NOCAS reaction involves the combination of 2,3 dimethylbut-2-ene, 1,4-dicyanobenzene, methanol, and ethyl vinyl ether. In this work, the generality of this rather unique reaction, which combines four molecules in "one pot", was investigated by substituting tertiary-butyl vinyl ether for ethyl vinyl ether. t-Butyl vinyl ether was intentionally used as products incorporating it would possess simpler NMR spectra than products incorporating ethyl vinyl ether. Furthermore, any steric effects would also be revealed. Irradiation of all four components in the presence of phenanthrene as the redox photosensitiser resulted in the formation of several new compounds, three of which were purified and characterised. The major product from this reaction was the expected novel photo-NOCAS adduct which was obtained in high yield. This fit the NMR data perfectly and acetal hydrolysis served as confirmatory proof of structure. This thereby adds to the generality of this novel photo-NOCAS reaction and supports the originally proposed mechanism for this reaction. The normal photo-NOCAS product with t-butyl vinyl ether was obtained as a white solid-the first ever recorded normal photo-NOCAS adduct in solid form. This diether was produced by the Markovnikov addition of methanol to the radical cation oft-butyl vinyl ether. The acetal regioisomer, which would have been produced by the relatively less favoured anti-Markovnikov addition, was not detected in this reaction. The study, however, also provided a completely unexpected result. The third major product from this reaction was found to incorporate 1,4-dicyanobenzene, methanol, and two molecules oft-butyl vinyl ether (but no 2,3-dimethylbut-2-ene). Such a product must have formed via yet another different type of photo-NOCAS reaction that still leads to the eventual combination of four molecules in "one pot". This product possesses an 0,0 mixed acetal and a separate alkoxy functional group and as a result, the reaction possesses synthetic potential.