Conformational analysis and kinetics of ring inversion for methylene- and dimethylsilyl-bridged dicyclooctatetraene

Dicyclooctatetraenylmethane (1) and dicyclooctatetraenyldimethylsilane (2) in THF-d(8) at 272 K exist as mixtures of diastereomers in ratios of 1:0.8 and 1:1, respectively. Nine energy minima (four meso and five racemic conformers) were located for each compound by geometry optimization at the HF/6-31G level of theory. The effects of torsional strain, steric interactions and dynamic electron correlation were analyzed. The diastereomeric ratios for 1 and 2 were reproduced reasonably well from the total energy calculated for each conformer corrected for its conformational enthalpy and entropy contributions. The ratio of rate constants for bond shift (BS) (k(BS)(1)/k(BS)(2)) is three times greater than the corresponding ratio for ring inversion. This suggests that additional substituent effects, such as pi interactions, are operative in the transition state for BS.