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Asymmetric synthesis in solid state photochemistry

University of British Columbia

The Norrish/Yang type II photochemistry of sixteen ketones has been studied in the crystalline state as well as in solution. In both media, the ketones, for the most part, undergo stereoselective cyclobutanol formation in which the cis-stereochernistry is produced exclusively. The reactive γ-hydrogen atoms are identified and the distance and angular parameters associated with their abstractions are derived from crystallographic data. Four of the sixteen ketones were found to be photochemically inert as a result of their 1,4-biradical intermediates being poorly aligned for cleavage and also as a result of an excessive distance between the radical centres. The chiral ionic auxiliary concept was shown to be useful for inducing asymmetric induction in solid state photochemistry. Chiral crystals from prochiral carboxyJic acids and optically pure amines were made (seventeen in total), and solid state photolyses of these salts afforded cyclobutanol photoproducts in optically active form (10% to 100% ee). Solid state structure-reactivity relationships are discussed based on X-ray crystal structures of the starting chiral salts. Due to the ionic character of the salts described above, they tend to have strong lattice forces and relatively high melting points. These "natural" two-component crystalline materials proved to be more robust than purely molecular crystals and survived photolysis to higher conversions without loss of topochemical control. In this thesis, we report the first example of a topotactic, enantioselective solid state photorearrangement that involves an ionic chiral auxiliary of known absolute configuration, so that X-ray diffraction studies allowed mapping of the absolute steric course of the photochemical reaction. This was accomplished by obtaining an X-ray crystal structure of the same single crystal at the beginning, midpoint and final stages of reaction. The results of asymmetric induction studies of the Norrish type II photoreaction conducted within zeolites are also presented in this thesis. The results show that zeolites, when modified with chiral inductors, yield photoproducts with low to moderate enantiomeric excess.

Thesis/Dissertation

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2009-04-01

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http://hdl.handle.net/2429/6725

Chemistry

University of British Columbia

UBCV

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