Investigations in light controlled reactivity using dithienylethenes

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(Thesis) Ph.D.
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Author: Sud, David
Compounds that undergo reversible photochemical transformations have been investigated for use in optoelectronic technologies, molecular devices and to a lesser extent, in influencing chemical reactivity. Photoresponsive 1,2-dithienylethenes (DTEs) represent a significant improvement over azobenzenes, used in previous research, primarily because they undergo thermally irreversible photochemical ring-closing and ring-opening reactions. The light-induced isomerization between ring-open and ring-closed isomers results in steric, electronic and localized p-bond arrangement changes. This makes DTEs appealing in the design of systems controlling the chemical reactivity of photoresponsive catalysts and reagents. The research presented in this thesis demonstrates control of reactivity using the DTE architecture. The initial approach of modulating reactivity used the flexible-to-rigid changes of the DTE backbone to control the stereochemical outcome of a catalytic reaction. The results showed that only the flexible ring-open form of a bis(oxazoline) DTE ligand, where the metal-binding groups could converge towards each other and chelate copper(I). With this binding geometry, the cyclopropanation reaction of styrene with ethyldiazoacetate afforded stereoselectivity in the product distribution. Irradiation with UV light generated the rigid ring-closed isomer, rendering it ineffective towards metal-chelation by forcing the metal-binding groups to diverge away from one another. In a second study, a DTE bearing bis(phosphine) groups was prepared, representing a new class of photoresponsive ligands with steric and electronic differences between the two photogenerated isomers. It was also shown that DTE-metal complexes remained photochromic, albeit with decreased photoconversion. Results also indicated that the extended conjugation in the ring-closed isomer resulted in greater electron-withdrawing effects on the phosphine compared to the ring-open isomer. In a third study, the localized p-bond rearrangement accompanying the ring-opening/ring-closing isomerization reactions of a DTE were used to activate/deactivate an enediyne towards Bergman cyclization. This was done by installing/removing a localized p-bond shared between an enediyne and the DTE backbone. Only the ring-open isomer contained the enediyne structure required to produce a diradical, which is responsible for the potent antitumor activity of enediyne derivatives. The ring-open isomer was created by irradiating the thermally stable ring-closed isomer with visible light, thus unmasking an enediyne, which could subsequently undergo a Bergman cyclization.
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