Kinetic studies of thermal electrocyclization of a series of 1-and 3-alkyl-1, 3, 5-hexatrienes (R= Me, Et, t-Bu) yielded the following relative rates: 3-t-Bu> 3-Et1 3-Me> I-Et> 1-Me, H. The activation enthalpies of the 3-alkyl series were, in general, 3 kcal/mol less than either the 1- alkyl counterparts or the parent hydrocarbon. These results can be interpreted in terms of the donative ability of alkyl groups, steric retardation at the reaction sites, and differences in ...
