A B3LYP Study of the Effects of Phenyl Substituents on 1,5-Hydrogen Shifts in 3-(Z)-1,3-Pentadiene Provides Evidence Against a Chameleonic Transition Structure

The effects of one or two phenyl substituents on the activation enthalpy for a 1,5-hydrogen shift in 3-(Z)-1,3-pentadiene (1) and on the geometry of the transition structure (TS) have been investigated by B3LYP/6-31G calculations. The phenyl-substituent effects on the experimentally measured activation enthalpies are predicted to be sizable, spanning a range of nearly 10 kcal/mol. However, if differences between steric effects in the transoid isomers of the reactants are factored out by comparing the activation enthalpies in the cisoid conformers, the electronic components of the phenyl-substituent effects on both the barrier heights and the TS geometries are found to be quite modest in size. Unlike the TS in the Cope rearrangement, the TS for a 1,5-hydrogen shift in 1 is not highly variable in nature, and the reason the 1,5-hydrogen shift TS is not chameleonic is discussed.