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UBC Theses and Dissertations

Solution and crystalline state photochemistry of 2,3-diacyl-substituted benzobarrelenes Yap, Melvin Peng-Kwun


A series of 2,3-diacyl-substituted benzobarrelenes was synthesized and their photochemistry investigated in the crystalline state and in solution media. The differences in photoreactivity and product selectivity between these compounds in the two phases were determined, and structure- reactivity correlations were established based on X-ray crystallographic data. The photochemistry of the title series is multiplicity-dependent; benzocyclooctatetraene is formed through the singlet excited state and benzosemibullvalenes are products of the triplet excited state. Regioisomeric benzosemibullvalenes are derived from the different "di-TT-me thane" rearrangement pathways taken. Solution phase, triplet-state photoisomerizations of a number of symmetrically disubstituted benzobarrelenes gave two benzosemibullvalenes1,2- disubstituted and 2a,6c-disubstituted. Selectivity is controlled predominantly by electronic effects. The use of a benzoyl substituent as one of the acyl groups enhances intersystem crossing, thereby enabling the substrate to achieve the triplet excited state through direct irradiation. Photolyses of several unsymmetrical benzoyl/ester benzobarrelenes in the solid state gave strikingly unusual regioselectivities between the 1,2-disubstituted and the 2a,5c-disubstituted products. The presence of specific lattice interactions was determined to be the major factor responsible; electronic effects were shown to play a much less significant role. Photochemical investigation of the benzoyl/methyl ester system was also conducted in polymer matrix media to show variation in product selectivity. Ammonium and metal ion salts of benzobarrelene-2,3-dicarboxylic acid were photolyzed in solution and the crystalline phase; their triplet product selectivities were recorded to further support the dominance of specific lattice interaction effects over electronic effects. Discovery of a rare "tri-TT-methane" rearrangement in some dibenzobarrelenes prompted the design and synthesis of dimethyl 1,4-dihydro1,4,5,8- tetramethyl-1,4-ethenonaphthalene-2,3-dicarboxylate. This successfully underwent the tri-TT-methane rearrangement in both solution and solid states. The novel selectivity was rationalized in terms of intramolecular steric effects rather than on the basis of electronic stabilizing effects. Many benzosemibullvalene photoproducts were found to undergo [1,3] shifts from a secondary photolysis; coincidentally, these rearrangements lead to regioisomeric benzosemibullvalenes. As a result, many exist in unusual photostationary states. It was found that benzosemibullvalenes require a carbonyl substituent adjacent to the bond cleaved in order for the [1,3] shift reaction to take place.

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