A study of the photochemistry of the syn-7 and anti-7 benzonorbornene chlorides and the intermolecular sensitization of three-membered ring heterocycles

Alan Jay Post, Purdue University

Abstract

The solution phase photochemistry and photophysics of the epimeric 7-chlorobenzonorbornenes were studied in order to determine if ab initio calculations were correct in predicting the increased photoreactivity of the anti isomer. In additions, the intermolecular photochemical sensitization of three-membered ring heterocycles were studied in order to elucidate the mechanism by which this occurs. The first part of this study involved the preparation and subsequent study of the photochemistry and photophysics of syn and anti-7-chlorobenzonorbornene (BSynCl and BAntiCl). The BSynCl isomer was predicted by ab initio calculations to have little C-Cl $\sigma\sp*$ character in its LUMO, and this is reflected in its relative photostability upon photolysis in methanol $(\phi\sb{\rm dis}=0.0014).$ In contrast, BAntiCl is calculated to have a significant amount of C-Cl $\sigma\sp*$ in its LUMO, consistent with the greater reactivity of this isomer $(\phi\sb{\rm dis}=0.23).$ The second part was a study of the intermolecular photosensitization of 2-isobutyl-1,2-oxazaspiro(2.5) octane (IBOX) by aryl sensitizers to give the corresponding ring expanded lactam. It was discovered that sensitization occurs via a singlet mechanism involving "nonvertical" energy transfer. Photolysis of a hexane solution containing toluene and IBOX gives a quantum efficiency for loss of IBOX (ca. 0.6) greater than that observed with direct irradiation of IBOX $(\phi\sb{\rm dis}=0.29).$ Simple singlet energy transfer cannot account for this difference. Attempts to expand this observation to intramolecular systems with steroidal frameworks proved unsuccessful. The third part was the study of the triplet sensitization of IBOX to give the ring opened N-isobutylcaproamide as a product. This reaction was shown to proceed only with ketone sensitizers that have a lowest lying n,$\pi\sp*$ triplet excited state. A mechilnism involving ground state ketyl radicals is proposed. The fourth part was the expansion of the above sensitization to other three-membered ring heterocycles. Ketone sensitization resulted in the formation of carbenoid intermediates. A mechanism involving electron transfer from the diaziridine to the ketone is proposed.

Degree

Ph.D.

Advisors

Morrison, Purdue University.

Subject Area

Organic chemistry|Chemistry

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