Diastereomeric Transition States. Relative Energies of the Chair and Boat Reaction Pathways in the Cope Rearrangement

Article abstract of DOI:10.1021/ja00529a045

Thermal rearrangement of dl- and meso-2,2'-bismethylenecyclopentane and 2,2'-bismethylenecyclohexane is reported.The rates of rearrangement permit independent measurement of the high- and low-energy conformations of the Cope rearrangement.The transition-state geometry for each diastereomer is uniquely defined as C_2v (boat) for the meso isomer and C_2h (chair) for the dl diastereomer.Correlation of these findings with available data from thermal rearrangement of 1,5-hexadiene establishes that bismethylenecycloalkanes are reasonable models for the isomeric transition states of <3.3> sigmatropic rearrangements.The "characteristic" activation parameters of the Cope rearrangement, in particular the large negative entropy of activation, are consistent with the chair-like (C_2h) transition state.The boat-like transition state (C_2v), on the other hand, exhibits a slightly positive entropy of activation.Both reactions satisfy mechanistic criteria for concertedness.These results indicate that the entropy of activation does not provide a valid criteria of concertedness.Consideration of stereochemical results from a number of highly substituted derivatives of 1,5-dienes reveals that alternative transition-state geometries C_2h (twist) and C_2v (plane) are at substantially higher energy than the chair and do not, therefore, play an important role in <3.3> sigmatropic rearrangement of conformationally flexible 1,5-dienes.