Mechanism of cis/trans equilibration of alkenes via iodine catalysis

Iodine is commonly used to speed the equilibration of Wittig cis/trans alkene products. This study uses computational chemistry to study the catalyzed isomerization mechanism in detail for seven different examples of 1,2-disubstituted alkenes. We find that the iodo intermediates of the conventional three-step reaction path are weakly stable, bound by less than 7 kJ mol(-1) in five cases and nonexistent in the other two. These variations in relative stability appear to be closely related to the degree of conjugation interruption in the alkene upon attachment of iodine. The rate-determining reaction barrier always occurs in the middle step, the internal rotation of the iodo intermediate, and the variations in the barrier heights are dictated by varying levels of steric hindrance in the seven cases. Regiospecificity of I-atom addition and noticeable hyperconjugative effects are discussed. Comparisons between various theoretical approximations are performed to demonstrate the great difficulty in obtaining accurate results for iodine-atom bond-forming and bond-breaking energies.