Cooperative and competitive substituent effects on the Cope rearrangementsof phenyl-substituted 1,5-hexadienes elucidated by Becke3LYP/6-31G*calculations

B3LYP/6-31G* calculations have been performed on the Cope rearrangements of 1,5-hexadienes that are substituted with up to four phenyl groups. Experim ental activation enthalpies are available for all of the compounds on which the calculations were performed, and the excellent agreement between the c omputed and the experimental values provides evidence that the calculated g eometries of the transition structures and intermediates are reliable. The calculations confirm that, as suggested by the experimental data, phenyl su bstituent effects on the Cope rearrangement can be either cooperative or co mpetitive. Based on the computed geometries of the transition structures, i t is possible to explain why these two different types of substituent effec ts are observed.