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Reaction selectivity and asymmetric synthesis in solid state organic photochemistry

University of British Columbia

The Norrish/Yang type II photochemistry of four bicyclo[3.3.1]nonyl phenyl ketones was studied in solution and in the crystalline state. In both media, the bicyclic ketones without a-methyl substituents were found to be photochemically inert, while the cc-methyl substituted ones underwent Yang photocyclization to give endo-aryl cyclobutanols as predominant products. An unusual 1,4-hydroxybiradical disproportionation reaction was also observed during the photolysis of the α-methylated bicyclo[3.3.1]nonyl phenyl ketones, although it was a minor process (< 5%). By utilizing the ionic chiral auxiliary method, chiral ammonium carboxylate salts (7 in total) were formed between optically pure amines and the achiral bicyclo[3.3.1]nonyl phenyl ketone derivative containing a carboxylic acid substituent. Photolysis of these salts in the solid state afforded the cyclobutanol product with fair to excellent enantiomeric excesses (45% - 96%). A homologous series of spirobicyclo[3.3.1]nonyl ketones (4 in total) was synthesized and their Norrish/Yang type II photochemistry was investigated both in solution and in the solid state. These compounds were chosen for investigation because the conformation of the 1,4-biradicals formed by γ-hydrogen atom abstraction could be varied in a regular and incremental fashion by changing the size of the ketone-containing spiro ring. X-ray crystallographic analysis of these compounds provided detailed information about the geometric factors responsible for the partitioning of the intermediate 1,4-hydroxybiradicals among cleavage, cyclization, and reverse hydrogen transfer. Parameters governing the efficiency of the type II cleavage and Yang cyclization were established, respectively. The results indicate that while geometry does have a strong influence on the 1,4-hydroxybiradical partitioning among cyclization, cleavage and reverse hydrogen transfer, a full understanding of the results requires that the strain involved in forming the cyclization products be taken into account. The ionic chiral auxiliary approach for asymmetric induction was also investigated in a c/s-bicyclo[4.3.0]non-8-ylacetophenone system, which undergoes the Norrish type II photocleavage reaction to yield a simple chiral alkene — cis-3a, 4, 5, 6, 7, 7a-hexahydro-1H-indene. Irradiation of chiral salts (5 in total) in the crystalline state gave enantioselectivities ranging from poor to moderate (ee = 11% - 67%). Single crystal X-ray diffraction studies were performed on a number of these compounds, and on this basis, the structure-reactivity relationships involved were discussed.

Text

2009-12-16

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http://hdl.handle.net/2429/16882

Chemistry

University of British Columbia

UBCV

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