Iodine atom-catalysed isomerisation of substituted cis,trans-1,4-diphenylbutadienes

Abstract

Rate constants have been measured for the iodine-catalysed cis-trans-isomerisation of several substituted cis,-trans-diphenylbutadienes in hexane between –40 and 65°. To obtain reproducible results the measurements were carried out at relatively high iodine atom concentrations, generated by short, intense illumination of the reaction mixtures. The non-linearity of the Arrhenius plots in the temperature range investigated revealed that the isomerisation is a multiple rate-determining process: the rate constants for dissociation and internal rotation of the intermediate radical are about equal. By the appliance of a curve-fit program the independent parameters of the Arrhenius equation could, however, be determined. The isomerisations proceed via cinnamyl, not via benzyl radicals. The activation energy is ca. 6 kcal mol^–1 lower than in the isomerisation of cis-stilbenes. This reduction can be ascribed to the larger gain of stabilisation energy in the formation of a cinnamyl radical in comparison with a benzyl radical, and to the lower activation energy of the rotation step. The influence of substituents on the isomerisation is discussed briefly.