The photo-EOCAS reaction : scope and limitations

Abstract

The photo-EOCAS (electrophile olefin combination with aromatic substitution) reaction between methoxyaromatics and electron-poor alkenes in methanol was re-investigated to further define the scope and limitations of this potentially-useful reaction. In the presence of acrylonitrile, the dimethoxybenzenes yielded the same substitution products previously reported by others, and these were fully characterised spectroscopically for the first time. In the case of 1-4-dimethoxybenzene, however, a new cyclohexadiene ketal product that had not been reported earlier was also identified. Furthermore, 1,2-dimethoxybenxene additionally yielded a previously-missed photo- EOCAS di-adduct. New chromophores such as trimethoxybenzenes and 3,4- dimethoxytoluene were also investigated, and, in some cases, new photosubstitution (EOCAS) products were isolated and characterised. The effect of varying the olefin from acrylonitrile, to various other electron-poor alkenes, including methyl acrylate and methyl methacrylate, was studied. A large majority of these olefins (including the latter two esters) showed no reactivity when 1,4- dimethoxybenzene was irradiated in their presence. Stern-Volmer-type studies indicated that the fluorescence of 1,4-dimethoxybenzene is quenched at the diffusion-controlled limit for methanol solution by acrylonitrile, as well as by methyl acrylate and methyl methacrylate; it therefore appears likely that photoinduced electron transfer from the first excited singlet of 1,4-dimethoxybenzene (which is feasible according to Weller equation calculations) is also occurring in the latter two cases, despite their inertness. Mechanistic studies using deuterated methanol were carried out to try and gain a better understanding of the reason for the lack of reactivity. Even though these results showed that reaction utility is limited to methoxybenzenes and cyanoethene, preliminary studies indicate that 1-methoxynaphthalene and methyl methacrylate do undergo a photo-EOCAS reaction in methanol. This would constitute the first known example of an ester-substituted alkene undergoing this type of reaction, and merits further study. Attempts to intercept the α-cyanoethyl radical intermediate proposed in these reactions using 2,3-dimethylbut-2-ene as a trapping agent were unsuccessful.