Study of the 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 the solid-phase [2 + 2] photocycloaddition (PCA) reaction was studied using ¹H NMR. It was established that only 4-styrylpyridine and 2-styrylbenzothiazole hydrotosylates undergo PCA with the formation of rctt isomers of cyclobutane derivatives. X-ray diffraction studies have shown that most of the protonated styrylheterocycles form crystal packing motifs that are not suitable for the implementation of 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. 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.