Solvent-dependent effect by carbon dioxide on the photoreactions of (9-anthryl)alkylamines

The effect of CO2 on a photoreaction was first studied systematically by using (9-anthryl)alkylamines (APA, AEA, and AMA) as the starting compound. From close scrutiny of the results, the CO2 effect was clearly observed and was well rationalized by the previously reported novel solvent dependence of the amine–CO2 reversible reactions. For instance, the yield of the dimer (h–t from APA or AEA, h–t+h–h from AMA) obtained in MeOH or DMSO was higher under CO2 than under argon and this was ascribed to formation of either ammonium bicarbonate/carbonate in MeOH or carbamic acid in DMSO, which will prevent the nitrogen lone pair from being involved in electron-transfer reactions. In fact, the electron-transfer side reactions producing 1–3 in DMSO were strongly inhibited under CO2. Also, due to formation of noncovalent linkage between the ammonium cation and the carbamate anion in 2-PrOH, the proportion of h–h relative to h–t produced from AMA in 2-PrOH was increased by carrying out the reaction under CO2.