74454-40-3Relevant academic research and scientific papers
Diastereomeric Transition States. Relative Energies of the Chair and Boat Reaction Pathways in the Cope Rearrangement
Shea, Kenneth J.,Phillips, Richard B.
, p. 3156 - 3162 (1980)
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 C2v (boat) for the meso isomer and C2h (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 sigmatropic rearrangements.The "characteristic" activation parameters of the Cope rearrangement, in particular the large negative entropy of activation, are consistent with the chair-like (C2h) transition state.The boat-like transition state (C2v), 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 C2h (twist) and C2v (plane) are at substantially higher energy than the chair and do not, therefore, play an important role in sigmatropic rearrangement of conformationally flexible 1,5-dienes.
