Conformational Analysis and Kinetics of Ring Inversion for Methylene- and Dimethylsilyl-Bridged Dicyclooctatetraene

Abstract

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.

Bond shift and charge transfer dynamics in methylene- and dimethylsilyl-bridged dicyclooctatetraene dianions

“…Since the p orbitals on C-1, C-2, C-5 and C-6 are directed towards the endo K + , an interaction between this cation and both the C-1-C-2 and C-5-C-6 π-bonds is a reasonable explanation for the syn preference. The C-1-C-1Ј distances in the CIPs of 1 2Ϫ -2K + and 2 2Ϫ -2K + are the same, within 0.04 Å, as those in the corresponding neutral compounds optimised at HF/6-31G*, 29 i.e., 2.55, 2.52, 3.08 and 3.04 Å for 1, 1 2Ϫ -2K + , 2 and 2 2Ϫ -2K + , respectively.…”

“…However, similar natural LWs of 1 and 2 and the neutral rings of their dianions were assumed, and values of k BS for 1 and 2 were derived after correction for RI broadenings. 29 For 1 2Ϫ and 2 2Ϫ it was found from spectra acquired at low temperature, where BS exchange broadening of the 13 C peaks was absent, that the natural LW of C-5 (for which the magnetic environment is not changed by BS) was very similar to those of the other proton-bearing carbons in the neutral ring of 1 2Ϫ and 2 2Ϫ . Therefore, the LW of C-5 at higher temperatures was subtracted from the LWs of C-2, C-3, C-4, C-6, C-7 and C-8 to give the broadenings due to BS exchange.…”

“…This is not central to the mechanism of CT and is discussed elsewhere. 29 Cation transfer and low energy conformational changes, such as bond length changes, bond rotations and solvent reorganisation, apart from those involved in the BS process, should all be considered in a refined analysis. However, it is likely that these effects are almost identical in 1 2Ϫ and 2 2Ϫ since the 13 C NMR chemical shifts indicate that the tightness of the CIP is very…”

“…Interestingly, other investigations have led to different or opposite conclusions. In a 29 Si and 13 C NMR study of the same series of compounds (along with the corresponding CH 2bridged series), van Walree et al concluded that Si(CH 3 ) 2 is a better transmitter of substituent effects (by a σ-bond polarization mechanism) than the carbon bridges. However, as these authors point out, 68 this is an effect primarily associated with the σ-bonds and not with the π-bonds as in the previous study.…”