Interconversion and rearrangement of radical cations. Part 2.1 Photoinduced electron transfer and electrochemical oxidation of 1,4-bis(methylene)cyclohexane

Abstract

The photoinduced electron transfer and electrochemical oxidation of 1,4-bis(methylene)cyclohexane (2) in acetonitrile have been studied in the presence and absence of a nucleophile (methanol). The photoinduced electron transfer reactions of 2 in acetonitrile–methanol solution with 1,4-dicyanobenzene (8) as the electron acceptor gives two products: 4-(methoxymethyl)-1-methylenecyclohexane (16) and 4-(4-cyanophenyl)-4-(methoxymethyl)-1-methylenecyclohexane (17). These products arise from nucleophilic attack on the radical cation followed by either reduction and protonation or combination with the radical anion of the electron acceptor, 1,4-dicyanobenzene (8^˙-). These results are in accord with the proposed mechanism of the photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. In the absence of a nucleophile, the photoinduced electron transfer reaction of 2 gives rise to several interesting and unexpected products 18–25 which result from complex reaction mechanisms involving radical, ionic and radical ion intermediates.The electrooxidation of 2 in acetonitrile in the presence of methanol leads to products 27–35. Under these conditions the radical cation 2^˙+ reacts with the nucleophile followed by a second oxidation and subsequent reactions leading to products (electrochemical–chemical–electrochemical, ECE). One of the products (28) is the result of protonation of 2 followed by nucleophilic attack. The electrochemical oxidation of 2 in acetonitrile (no methanol) yields 2^˙+ which is deprotonated and then further oxidized to give 39–43. These products arise from ionic intermediates (ECE); oxidation of 2 all the way to aromatic compounds was observed in 39–41.In none of these experiments was there any evidence for the formation of cyclized products, nor was there any indication of carbon–carbon bond cleavage in 2^˙+. The products are consistent with the initial formation of the intermediate radical cation. The products as well as the possible mechanisms of formation of these species are discussed.