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Photoswitchable Organocatalysis: Using Light To Modulate the Catalytic Activities of N‑Heterocyclic Carbenes

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journal contribution
posted on 20.02.2016 by Bethany M. Neilson, Christopher W. Bielawski
A 4,5-dithienylimidazolium salt was found to undergo electrocyclic isomerization upon exposure to UV radiation (λirr = 313 nm) under neutral and basic conditions; subsequent exposure to visible light reversed the reaction. Under ambient light and in the presence of base, the imidazolium species catalyzed transesterifications as well as amidations in a manner similar to those of previously reported N-heterocyclic carbene precatalysts. However, upon UV irradiation to effect the aforementioned photocyclization, the rate of the transesterification reaction between vinyl acetate and allyl alcohol was significantly attenuated (kvis/UV = 12.5), as was the rate of the condensation of ethyl acetate with aminoethanol (kvis/UV = 100). The rates of these reactions were successfully toggled between fast and slow states by alternating exposure to visible and UV light, respectively, thus demonstrating a rare example of a photoswitchable catalyst that operates via photomodulation of its electronic structure.