Showcase of Undergraduate Research and Creative Endeavors (SOURCE)

Photoredox Mediated Alkylation of Imines with Potassium Organotrifluoroborates in the Presence of an Organic Photocatalyst

Our group is investigating the application of an organic photocatalyst, 9-mesityl-10-methyl acridinium tetrafluoroborate (Mes-Acr-Me), to the alkylation of aryl imines with potassium organotrifluoroborates. In the past, our group has employed a potassium isopropyltrifluoroborate for model reactions, but we desired to expand our BF₃K scope to see the effects of variation in the alkyl group of the organotrifluoroborate. In addition, we wanted to study the substituent electronic effects in the imine, which was done using a series of competition experiments. The protocol appears to be applicable to broad range of substituted aldimines, although extended reaction time was necessary to complete the reaction with highly electron withdrawing substituents (e.g. CF₃). Secondary, and tertiary potassium alkyltrifluoroborates were also shown to be competent reaction partners. Potassium cyclobutyltrifluoroborate gave a low yield compared to other secondary alkyltrifluoroborates, presumably because it required formation of the less stable cyclobutyl radical. To assess substituent effects, competition experiments were carried out, and the data were plotted using several substituent constant scales (including σ· values developed for free radical reactions; see for example Dust and Arnold JACS 1983, 105, 1221); the best fit was obtained when σ⁺ values were used, suggesting that polar effects are more important than radical stabilization effects. The small negative value of ρ (-0.61) seems to indicate that the t-butyl radical exhibits weakly electrophilic behavior in this reaction.