Further investigation into the trapping of the photo-NOCAS intermediate : a novel photo-NOCAS reaction

Abstract

The previously reported photo-NOCAS (nucleophile olefin combination aromatic substitution) reaction is a photosensitised electron transfer reaction between alkyl olefins and dicyanobenzenes in acetonitrile solution. Nucleophilic addition to the photochemically generated radical cation of the olefin occurs on photolysis of the mixture, to yield a β-alkoxy radical, which then couples with the radical cation of the dicyanobenzene. Cyanide loss leads to the photo-NOCAS product. The P-alkoxy radical intermediate was successfully trapped recently. The trapping was achieved by adding an electron-poor olefin, such as methyl acrylate and acrylonitrile, to the reaction mixture. The P-alkoxy radical intermediate is trapped prior to the aromatic coupling stage, resulting in a product that is a formal combination of a nucleophile, an electron-rich alkene and an electron-poor one. In this study, the generality of trapping mechanism was investigated. Trapping was attempted with five other electron-poor olefins. These were crotononitrile, methyl crotonate, methyl-2-furoate, 1-cyanovinyl acetate and ethyl vinyl ether. Crotononitrile, methyl crotonate and methyl-2-furoate did not successfully trap the P-alkoxy radical intermediate. This was attributed to steric factors in the former two cases, and aromatic stability in the latter. Only the investigation involving 1-cyanovinyl acetate gave the sought product i.e. 1-cyano-4- methoxy-3 ,3 ,4-trimethylpentyl-1-ethanoate. Use of ethyl vinyl ether afforded an unexpected result. A novel photo-NOCAS type of reaction was observed, and led to a new product resulting from the formal addition of methanol, 2,3-dimethyl-2-butene, ethyl vinyl ether, and 1,4- dicyanobenzene. This new reaction possesses considerable synthetic potential.