Study of influence of anion nature on crystal packing features and feasibility of [2 + 2] photocycloaddition reaction in protonated forms of dimethoxystyrylheterocycles

Abstract

A series of protonated dimethoxystyrylheterocycles containing 4-pyridine, 4-quinoline, and 2-benzothiazole residues, as well as various anions, were synthesized. The ability of these compounds to undergo solid-phase [2 + 2] photocycloaddition (PCA) reaction was studied using 1 H NMR. It was established that only 4-styrylpyridine and 2-styrylbenzothiazole hydrotosylates undergo the PCA with the formation of rctt isomers of cyclobutane derivatives. X-ray diffraction studies have shown that the most protonated styrylheterocycles form crystal packing motifs that are not suitable for the implementation of the PCA.Only 4-styrylpyridine and 2-styrylbenzothiazole hydrotosylates yielded packings with a syn-head-to-tail arrangement of their cations. In the packings, the ethylene fragments of adjacent cations have antiparallel mutual arrangement and been brought closer in space. This allows the formation of centrosymmetric rctt isomers of cyclobutane derivatives upon irradiation of these substances with visible light. The PCA was observed as a single crystal-to-single crystal process for dimethoxystyrylbenzothiazole hydrotosylate. The cyclobutanes were isolated in a free state and their structures were confirmed by single crystal X-ray diffraction.