The photochemical activation of a remote carbon-chlorine bond by the trimethylsilyl enol ether chromophore

Brad Douglas Maxwell, Purdue University

Abstract

The solution phase photochemistry and photophysics of 6-chloro-2-trimethylsiloxynorbornenes were studied in order to study the chemical consequences of $\sigma$*/$\pi$* LUMO mixing. Promotion of an electron from the HOMO to the LUMO should cause cleavage of a remote C-Cl bond since the LUMO contains $\sigma$* character. Molecular orbital calculations using MNDO ad Gaussian 86 techniques were used to determine the degree of C-Cl $\sigma$*/$\pi$* mixing in the LUMO of trimethylsilyl enol ethers. Gaussian 86 calculations using a 90$\sp\circ$ dihedral angle composed of the carbon carbon double bond and the $\alpha$ and $\beta$ carbons and a 180$\sp\circ$ dihedral angle composed of the last carbons of the first dihedral angle and the chlorine show 31.4% C-Cl character mixed into the LUMO of 4-chloro-2-trihydrosiloxybutene. Rotation of the trihydrosilyl group by 180$\sp\circ$ improves the mixing to 38.1%. Systems which are not in the 90/180 geometry such as exo- and endo-6-chloro-2-trihydrosiloxy-norbornenes show 24.1% and 3.7% mixing respectively. ExoCl (exo-6-chloro-2-trimethylsiloxynorbornene) was prepared by the silylation of exo-6-chloro-2-norbornanone. EndoCl (endo-6-chloro-2-trimethylsiloxynorbornene) was prepared by the silylation of GC purified endo-6-chloro-2-norbornanone. The C-Cl bond and the trimethylsilyl enol ether groups were locked into geometries close to 90/180 and 90/60 in order to test the stereoelectronic consequences of orbital mixing. The predicted lowering of the LUMO orbital by mixing was experimentally verified by hyperchromicity and the bathochromic shift in the absorption spectrum of ExoCl relative to 2-trimethylsilylnorbornene (TMSNB). The UV spectrum of EndoCl shows hyperchromicity and a hypsochromic shift relative to TMSNB. Electron Transmission Spectroscopy (ETS) studies agree with molecular orbital calculations which show more $\sigma$*/$\pi$* LUMO mixing in ExoCl than EndoCl. Upon irradiation of ExoCl with 0.50 equivalents of ($\pm$) sec-butylamine (SBA) in hexane, there is facile cleavage of the C-Cl bond. The quantum efficiency of disappearance for ExoCl is 0.066 and for EndoCl is 0.0079. The quantum efficiency for the appearance of TMSNB from ExoCl is 0.069 indicating excellent mass balance. Multiplicity studies are consistent with the photocleavage resulting from the excited singlet state.

Degree

Ph.D.

Advisors

Morrison, Purdue University.

Subject Area

Organic chemistry

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