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Asymmetric induction and triplet sensitization in the solid state photochemistry of Anti-9-carboxybenzonorbornadiene

University of British Columbia

[Formula] Acid 19 and its methyl ester 45 were prepared, and studies of their photochemical reactivity were made in both the solution phase and solid state. Unexpectedly, both compounds underwent di-7r.-meth.ane rearrangement to the corresponding undecatrienes 54 and 53 upon direct irradiation in solution. Ester 45 reacted more rapidly in the solid state than in solution, although acid 19 did not react at all in the solid state. Although di-7t-methane rearrangement usually occurs only from the excited triplet state in constrained cyclic systems such as these, benzonorbornadiene derivatives 19 and 45 appear capable of undergoing the reaction from either the singlet or triplet excited state. Although the di-Tt-methane rearrangement would occur upon direct irradiation, the reaction was much faster when a triplet sensitizer was used. The solution phase reaction could be sensitized with acetone, acetophenone, or benzophenone. Sensitization was also possible in the solid state. Crystals of the complex formed between acid 19 and p-acetylpyridine, upon irradiation with long-wave ultraviolet light that was not absorbed by the acid (A, > 330 nm), gave rearranged product. Asymmetric induction could also be carried out. Although acid 19 is achiral, its undecatriene rearrangement product is chiral. Irradiation of crystals of salts formed between acid 19 and commercially available, optically pure, chiral amines gave optically active undecatriene products at low conversions. Enantiomeric excesses as high as 41 % were observed. Rearrangement was slow. The most rapid rearrangement occurred for the (-)-ephedrine salt of acid 19, which reached 3 1 % conversion to product after 6 hours. Crystals of acid 19 with chiral triplet sensitizer amines were also irradiated. These chiral triplet sensitizer auxiliaries were prepared by coupling natural amino acids to benzyl alcohols and phenols which contained aryl ketone groups. The auxiliaries were isolated as their hydrobromide salts and converted to the free base form immediately before use. Rearrangement of the salts formed with these chiral triplet sensitizer amines was very rapid. In the solid state, complete conversion to product occurred in less than 1 hour. Enantiomeric excesses (22% - 91%) were generally higher than those obtained upon irradiation of salts prepared with commercially available amines (5.9% - 41%).

Thesis/Dissertation

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2009-05-25

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http://hdl.handle.net/2429/8159

Chemistry

University of British Columbia

UBCV

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