Iodine atom-catalysed isomerisation of symmetrically substituted cis-stilbenes

Abstract

Rate constants have been measured for the iodine-catalysed cis–trans isomerisation of 19 symmetrically substituted cis-stilbenes between 70 and 120°. The reaction rate is of the first order in cis-stilbene derivative and of half order with respect to iodine, which indicates that iodine atoms are the catalytic species. For most compounds value of log k fit a Hammett relation with a negative ρ-value, which can be explained on the basis of the electrophilic character on the iodine atom. Some para-substituted cis-stilbenes deviate from the Hammett line, apparently because in these cases the benzyl-type radical which is formed as an intermediate has additional resonance stabilisation and the energy of the transition state is consequently reduced. In this respect a parallel is found between the isomerisation and homolytic phenylations.

From measurements at different temperatures Arrhenius activation parameters have been determined. It appears that for almost all compounds studied the frequency factor is constant whereas the activation energies vary, depending on the nature of the substituents, from 21·3 to 23·6 kcal./mole. For meta- and para-substituted compounds a general rate equation has been derived. For ortho derivatives in which steric effects interfere, k-values have been correlated with Taft's steric parameters and van der Waals radii.

From a comparison of log A factors found with values for the formation of the rotation of the intermediate an argument has been derived for the supposition that the rotation step is rate-determining.