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Asymmetric induction and reaction selectivity in solid state organic photochemistry Netherton, Matthew Russell

Abstract

A number of aromatic ketones were synthesized, and their type II photochemistry studied in solution and in the solid state. The purpose of these investigations was to elucidate the geometric parameters responsible for partitioning of the intermediate 1,4- hydroxybiradicals among the possible cleavage, cyclization, and reverse hydrogen transfer pathways. The four ketones studied proved to be very sensitive to changes in the disposition of the carbonyl chromophore, the position of which dictates the anticipated geometry of the biradical intermediate. In particular, it was found that cleavage was only efficient when reasonable overlap between the radical centres and the a-bond undergoing scission was present. Parameters governing the efficiency of the Yang photocyclization were also established. In addition, γ-hydrogen/deuterium exchange was observed on photolysis of an unreactive aromatic ketone using tert-butanol as solvent. This reaction is unprecedented in ketones that undergo γ-hydrogen abstraction, and is indicative of the high efficiency of reverse hydrogen transfer relative to other reaction pathways in this system. The ionic chiral auxiliary method of asymmetric induction was investigated in the solid state photochemistry of a series of macrocyclic aminoketones. Salts were formed between the achiral aminoketones (ring sizes of twelve, fourteen, and sixteen) and optically pure acids. The resulting chiral crystals were irradiated in the solid state, and the photoproducts analyzed for enantiomeric purity. Enantioselectivities ranging from poor to excellent were obtained, and X-ray crystallographic data from a number of these compounds provide insight on the origin of the observed stereoselectivities. The same technique for asymmetric induction was also studied in a benzocyclohexadienone system which undergoes photochemical rearrangement to form a chiral benzobicyclo[3.1.0]hexanone derivative. The planarity of this system was expected to limit conformational bias in product enantiodetermination, and thus allow the anisotropic directing effects of the chiral cavity to be studied. The enantioselectivities observed for these salts was generally low, although one derivative displayed possible synthetic utility (ee ca. 80%). Two reactant crystal structures were obtained, one of which contained a unique arrangement in which two photoreactive benzocyclohexadienone moieties were present in the optically active crystal as conformational enantiomers. The net result of the analysis is that the enantioselectivity observed on photolysis of this substrate is independent of conformational bias. Further computational studies are required to garner a complete understanding of this reaction

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