Why Cyclooctatetraene Is Highly Stabilized: The Importance of “Two-Way” (Double) Hyperconjugation

Despite its highly nonplanar geometry, the tub-shaped D 2d cyclooctatetraene (COT) minimum is far from being an unconjugated polyene model devoid of important π interactions. The warped skeleton of D 2d COT results in the large stabilization (41.1 kcal/mol) revealed by its isodesmic bond separation energy (BSE). This originates largely from the “two-way” hyperconjugation, back and forth across the C–C single bonds, between the CC/CH σ(σ*) and the CC (π*)π orbitals. These hyperconjugative effects compensate for the substantial, but not complete, loss of π conjugation upon ring puckering. C–C single bond rotation of 1,3-butadiene involves a similar interplay between π conjugation and “two-way” double hyperconjugation and serves as a simple model for the inversion of D 2d to D 4h COT. The perpendicular rotational transition states of many other systems, e.g., the allyl cation, styrene, biphenyl, and ethene, are stabilized similarly by “two-way” hyperconjugation.