Off-campus UNL users: To download campus access dissertations, please use the following link to log into our proxy server with your NU ID and password. When you are done browsing please remember to return to this page and log out.
Non-UNL users: Please talk to your librarian about requesting this dissertation through interlibrary loan.
Ion pairing of pi-delocalized carbanions
Abstract
The average methylenecyclooctatrienyllithium $\sp{13}$C chemical shift decreases on changing the conditions from NH$\sb3$/$-$35$\sp\circ$C to 2(HMPA):THF/$-$35$\sp\circ$C or to NH$\sb3$/$-$65$\sp\circ$C. However, electron density in the exocyclic double bond is polarized away from the ring in the former case, but toward the ring in the latter case. STO-3G molecular orbital calculations indicate two different solvated methylenecyclooctatrienyllithium contact ion pair potential minima: a symmetrical $\eta\sp3$ (Li$\sp+$/C$\sb3$-C$\sb5$ coordinated) and an unsymmetrical $\eta\sp5$ (Li$\sp+$/C$\sb1$-C$\sb5$ coordinated) structures. Considering these two species along with a solvent-separated ion pair the NMR observations are explained by equilibria between these ion pairs. Conformational changes for a series of 9- (1-(p-X-phenyl)) methylenecyclooctatrienyllithiums as X becomes a better $\pi$-electron donor may be detected by $\sp{13}$C NMR spectroscopy. As X changes from chloro to methoxy, the eight-membered ring appears to flatten. As X changes from methyl to methoxy or as temperature decreases fro methoxy, a C$\sb8$C$\sb9$ twist occurs resulting from a dianion resonance form becoming more important than a monoanion resonance form in the determination of the conformation. 9-(2-Propenyl)-methylenecyclooctatrienyllithium at temperatures greater than $-$45$\sp\circ$C rearranges to 2-methyl-bicyclo (6.3.0) undecapentaenyllithium. This rearrangement may proceed through a 12 $\pi$-electron Mobius transition state representing a novel pericyclic ring closure. Indenyllithium and indenylpotassium $\pi$-electron polarization, indicating contact ion pair tightness, was investigated by $\sp{13}$C NMR spectroscopy. Solvent change is about ${2\over3}$ as great for indenylpotassium as for indenyllithium. The indenyl anion is polarized stronger with Li$\sp+$/DMSO than with Li$\sp+$/NH$\sb3$, whereas the reverse is observed for indenylpotassium. Cation-anion interactions are observed for indenylpotassium (HMPA) and for indenyltetramethylammonium (THF). The $\sp1$H and $\sp{13}$C chemical shifts of lithium and potassium salts of various cyclooctatetrene dianions and methylenecyclooctatrienyl anions were measured in HMPA or NH$\sb3$. Data correlation with the corresponding $\pi$-charge densities (HMO-$\omega$ method) indicate that the chemical shifts can be related to charge. An analysis attributes differences at adjacent positions to changes in $\sigma$ to $\pi\sp\*$ orbital mixing resulting from a decrease in energy for this series' LUMOs.
Subject Area
Organic chemistry|Chemistry
Recommended Citation
Mueller, Ronald Lee, "Ion pairing of pi-delocalized carbanions" (1990). ETD collection for University of Nebraska-Lincoln. AAI9022996.
https://digitalcommons.unl.edu/dissertations/AAI9022996