Iodine-catalysed isomerisation of p-methoxy-cis-stilbene at room temperature, an ionic reaction

Abstract

4-Methoxy- and 4,4′-dimethoxy-cis-stilbene isomerise under the influence of molecular iodine at low temperatures and without illumination. The reactions are of the first order in cis-compound but of high order (3–4) in iodine. The rate constants do not depend on the presence of oxygen and vary only slightly with temperature, but are very sensitive to changes in the solvent. These facts suggest an ionic mechanism.

Apparently the isomerisations proceed via a molecular complex between iodine and the cis-compound, in which the iodine–iodine bond is heterolytically broken under the influence of an aggregate of several iodine molecules. The presence of a strongly electron-donating substituent in the cis-stilbene stablising the transition state seems essential for this type of reaction.