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Asymmetric induction in solid state photochemistry Gudmundsdottir, Anna D.

Abstract

The enantioselectivity of the di-rr-methane photo rearrangement in chiral crystals was studied. Three achiral dibenzobarrelene carboxylic acid derivatives and one achiral amino dibenzobarrelene derivative were synthesized for this work. These compounds were induced to crystallize as chiral crystals by forming salts with chiral counterions. Photolysis of the chiral crystals resulted in asymmetric induction in the products. The regioselectivity of the photorearrangements of the starting materials prior to salt formation was studied in solution as well as in the solid state. In addition, the photochemistry of the methyl and ethyl esters of the carboxylic acids was also investigated. The photochemistry of some of these dibenzobarrelene derivatives was found to be medium-dependent. Possible structure-reactivity correlations for these compounds, based on X-ray crystallographic data, are discussed. The chiral salts of the dibenzobarrelene derivatives were photolyzed in solution and in the solid state. The extent of asymmetric induction in the crystalline phase was studied by measuring the enantiomeric excess of the photoproducts. No optical activity was observed for the solution photoproducts, presumably because the salts had dissociated. The enantioselectivity of the di-rc-methane photorearrangement in the solid state varied from poor to good, depending on the chiral salt in each instance. The chiralcounterion ensures chiral crystals, but the crystal lattice alone is accountable for asymmetric induction, and different salts crystallized in different crystal packing arrangements leading to varying degrees of asymmetric induction. The absolute steric course of the di-n-methane rearrangement in the solid state was studied. Chiral salts for which the photorearrangements were both regio- and enantioselective were selected for this study. The reaction pathway was mapped by comparing the absolute configuration of the starting material and the photoproduct. After the reaction pathways had been determined, the crystal structures of the starting materials were analyzed in order to identify the crystal forces that control the enantioselectivity of the di-n-methane rearrangement. A general explanation of the factors that control the asymmetric induction of the di-m-methane rearrangement in dibenzobarrelene derivatives was proposed. It was concluded that forming chiral salts of achiral molecules is a convenient and an effective way to study reactions in chiral crystals. Finally, solid state photochromism was discovered for some of the bridgehead-substituted dibenzobarrelene derivatives. Possible intermediates responsible for the photochromic phenomena are suggested.

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