Long-range proton transfer catalysed by 1,4-dioxane proton craning in a non-polar medium

Abstract

A study is presented of the effect of solvent polarity on the photo-induced long distance intramolecular overall transfer of a proton from the OH group in 7-hydroxyquinoline to the N atom in the ring by a rotationally mobile amine attached at position 8 of the latter. An intramolecular hydrogen bond is formed when the OH group is deprotonated by the side group. The formation of an intramolecular hydrogen bond causes the process to stop at the zwitterionic form, when the solvent is an alkane. However, the overall proton transfer is shown to be feasible in alkanes, provided that the solution also contains some 1,4-dioxane. It is shown that 1,4-dioxane acts as a catalyst by making a 1:1 complex with a hydrogen bond to the protonated amine of the side group. The sacrifice of the intramolecular hydrogen bond introduces rotational mobility of the side group to the zwitterionic form. It is argued that similar complex formation in the case of solutions of the solute in polar solvents, between polar solvent molecules and the zwitterionic form, may be of much greater importance in the overall process than bulk solvent polarity.