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

Dicyclooctatetraenylmethane (1) and dicyclooctatetraenyldimethylsilane (2) in THF-d delta at 272 R exist as mixtures of diastereomers in ratios of 1:0 .8 and 1:1, respectively. Nine energy minima (four meso and five racemic co nformers) were located for each compound by geometry optimization at the HF /6-31G* level of theory. The effects of torsional strain, steric interactio ns and dynamic electron correlation were analyzed. The diastereomeric ratio s for 1 and 2 were reproduced reasonably well from the total energy calcula ted for each conformer corrected for its conformational enthalpy and entrop y 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 inve rsion. This suggests that additional substituent effects, such as pi intera ctions, are operative in the transition state for BS.