An investigation of excited state interactions between carbon-chlorine bonds and the enol ether chromophore

John Joseph Nash, Purdue University

Abstract

The solution phase photochemistry and photophysics of the epimeric 7-chloro-2-trimethylsiloxynorbornenes were studied in order to elucidate the mechanism of photodehalogenation in $\beta$-chloro enol ethers. In addition, the solution phase photochemistry and photophysics of the epimeric 9-chloro-3-methoxyexotricyclo (5.2.1.0$\sp{2,6}$) dec-3-enes were studied to examine the distance dependance upon $\pi$*/$\sigma$* LUMO mixing. The first stage of these studies involved molecular orbital (MO) calculations to determine the minimal basis set necessary to adequately describe $\pi$*/$\sigma$* LUMO mixing and to determine the degree of C-Cl $\sigma$* character mixed into the LUMO of the enol ethers. The minimal STO-3G basis set was found to be the best choice since larger basis sets suffer from collapse of the virtual orbitals onto continuum solutions and are not physically meaningful. The 7-chloronorbornyl trimethylsilyl enol ethers show minimal mixing (ca. 3.5%) compared to the relatively photoactive exo-6-chloro-2-trimethylsiloxynorbornene, ExoCl (23.6%). MO calculations on the $\gamma$-chlorides show 11.2% C-Cl $\sigma$* character mixed into the LUMO of the exo isomer and 7.0% in the endo isomer. The second stage involved the preparation of anti and syn-7-chloro-2-trimethylsiloxynorbornenes (AntiCl and SynCl) and the exo and endo-9-chloro-3-methoxyexotricyclo (5.2.1.0$\sp{2,6}$) dec-3-enes, GExoCl and GEndoCl. AntiCl and SynCl were prepared by the silylation of the respective chloro-ketones. GExoCl and GEndoCl were prepared in several steps starting from dicyclopentadiene initially utilizing an acid-catalyzed Wagner-Meerwein rearrangement. These derivatives were chosen because of their rigid structures thereby allowing an examination of the stereoelectronic effects in these systems. The third stage was the photochemistry and photophysics of AntiCl and SynCl. The predicted negation of orbital overlap in AntiCl and SynCl was experimentally verified by the hypsochromicity in the UV absorption spectra and the minimal photodehalogenation observed upon photolysis in hexane compared to ExoCl ($\Phi\sb{\rm dis}$(AntiCl) = 0.0072, $\Phi\sb{\rm dis}$(SynCl) = 0.0027). These observations are completely consistent with the $\pi$*/$\sigma$* LUMO mixing hypothesis and the possibility of an electron-transfer mechanism in these molecules is eliminated. The fourth stage was the photochemistry and photophysics of GExoCl and GEndoCl. There is no observable UV spectral perturbation in either compound and electron transmission spectroscopy (ETS) shows no observable splitting of the virtual orbitals in GEndoCl. These compounds are virtually photoinert under direct irradiation at 254 nm. Triplet sensitization studies also gave no observable photochemistry. These results are consistent with the UV and ETS spectra of these molecules but not with the MO calculations. It is likely that through-bond coupling in these molecules does not lead to observable spectroscopic or photochemical characteristics.

Degree

Ph.D.

Advisors

Morrison, Purdue University.

Subject Area

Organic chemistry

COinS