Photochemistry of imidazolium cations. Water addition to methylimidazolium ring induced by UV radiation in aqueous solution

• Photochemical reactions of methylimidazolium cations in water have been investigated.
• UV-C radiation induces water addition to methylimidazolium ring.
• Hydroxyl adds preferably at the side of the ring having the longer alkyl substituent.
• Diastereoisomers are formed at equal proportions.

ABSTRACTThe UV-C induced photoaddition of water to N-alkyl-N′-methylimidazolium cations was studied. The main photoreaction products exhibit chemical additions of a proton and a hydroxyl group to either positions 4 or 5 of the imidazolium ring. For unsymmetrical imidazolium cations, two positional isomers were obtained as products. In these cases, the most abundant isomer is the one in which the hydroxyl group adds at the side of the ring having the longer alkyl substituent. Experiments performed in D2O solutions reveal that the additions of proton and hydroxyl group never take place at the same carbon atom, in a reaction that produces equal amounts of diastereoisomers. Moreover, the formation of diastereoisomers at equal proportions suggests that the reaction proceeds in an unconcerted fashion.

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