Photoisomerization of ammonium α,β-unsaturated carboxylates in the solid state: effect of the hydrogen-bond network on the reactivity

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Abstract
EZ-Photoisomerization has been attained for several kinds of salts of α,β-unsaturated carboxylic acids with amines. This photoreaction has been proven to be an effective method for preparing (Z)-isomers from ammonium (E)-α,β-unsaturated carboxylates. The isomerizability is strikingly altered upon changing the ammonium group, which implies that the crystal structure affected the reactivity to a considerable extent. However, the cavity in the crystal has been found to have less influence than expected on the isomerizability. A further detailed study concerning the crystal structures suggested that the characteristics of the hydrogen-bond network had a close correlation with the isomerizability. Moreover, both the bulkiness of the carboxylate and ammonium groups as well as their difference in molecular length are considered to be the most important factors influencing the isomerizability.