Harnessing Light for Novel Radical Chemistry from Intermediates with Established Polar Reactivity

Identifier:  TDX:4410

Handle: https://hdl.handle.net/20.500.11797/TDX4410

Authors:  Hin-Fung Wong, Thomas

Abstract:
This thesis aimed at repurposing intermediates with established ionic reactivity for a radical paradigm. By semiempirical choices of photocatalyst and radical precursors, this enables distinct reactivity patterns to overcome defined challenges in organic chemistry. This provides new opportunities for selective C-C and C-S bond formations, affording different scaffolds with potential bioactivity. Iminium ion is a privileged motif in the polar domain. It was shown that iminium ion catalysis could render activate enals towards intercepting nucleophilic primary radicals for conjugate addition. The chiral iminium intemediates were effective in surpassing background processes and trapping short-lived open-shell species, securing good enantioselecitivity in 1,7-dicarbonyl scaffold formation. A radical umpolung strategy was also developed for chiral iminium ions. SET reduction of electrophilic, chiral iminium ions switched the electrophilic -position of parent enal to nucleophilic within the resultant chiral 5 -electron radical. This permitted cross-electrophile to afford chiral 1,6- dicarbonyl compounds as a single regioisomer with excellent enantioselectivity. The first example of general asymmetric conjugate cynation of enals was also disclosed with good enantioselectivity and exclusive chemoselectivity.