Part I Photochemical Deconjugation Reactions Of 3-alkyl-2-cyclohexenones Part Ii Biradicals In 2+2 Photocycloaddition Reactions Of 2-cyclopentenone

Part I. Ultraviolet irradiation of 3-methyl-2-cyclohexenones leads to the formation of the deconjugated exocyclic isomers in low quantum yields. The reaction proceeds via the triplet excited state of the enone but the mechanism has not been determined satisfactorily. Part I of this thesis describes evidence for acid catalysis of the reaction. The presence of small quantities of carboxylic acid greatly enhances the quantum yield of the deconjugation reaction. The proposal of the intermediacy of a carbocation was supported by the detection of products derived from addition to the conjugate base of the acid to the carbocation. In addition, the presence of 10% methanol in the irradiation mixture led to the formation of products derived from addition of the alcohol to the carbocation. From the kinetic data obtained the lifetime of the trans-enone was determined to be at least 1 {dollar}\mu{dollar}s.;The presence of alkenes in the reaction mixture led to a decrease in the quantum yield of the deconjugation reaction; instead products derived from 2+2 cycloaddition between the enone and the alkene were observed.;Contrary to literature reports it was found that the deconjugation reaction proceeds not only for 3-methyl substituted 2-cyclohexenones but also for 3-ethyl-5,5-dimethyl-2-cyclohexenone. Irradiation of the latter compound led to the formation of two geometrical isomers of the exocyclic deconjugated isomer. No deconjugation products were found in the irradiation of 3-isopropyl substituted 2-cyclohexenones which was believed to be due to steric hindrance to formation of the trans-enone. Acid catalysis was also found in the deconjugation reaction of R-10-methyl-{dollar}\Delta\sp{lcub}1,9{rcub}{dollar}-2-octalone, a bicyclic cyclohexenone.;Part II. Irradiation of 2-cyclopentenone in the presence of alkenes leads to the formation of cyclobutane ring containing 2+2 cycloadducts. The reaction proceeds via the triplet excited state of the enone which is quenched by the alkene to give an intermediate triplet 1,4-biradical.;Irradiation of the enone with vinylcyclopropane led to the detection of products derived from radical rearrangement of the intermediate biradical in addition to 2+2 cycloadducts. From the product distribution the lifetime of the triplet 1,4-biradical intermediate in this reaction was estimated to be 20-50 ns.;A number of differently substituted diketones based on the bicyclo (3.3) octane system were synthesized. It was hoped that these would undergo a photochemical decarbonylation reaction generating the desired biradicals; however, only the cyano substituted diketones underwent the expected reaction. The results suggest that the cycloaddition reaction regiochemistry is indeed determined by the fates of the biradical intermediates rather than their relative rates of formation. (Abstract shortened by UMI.