This presentation will describe N-heterocyclic carbenes (NHCs) that can be modulated with light. Emphasis will concentrate on 4,5-dithienylimidazolylidene-based scaffolds, which were shown to undergo photo-induced cyclization upon exposure to UV irradiation; subsequent irradiation with visible light effectively reversed the process. We will show how photomodulation of the NHCs changes key spectroscopic signatures, intrinsic electronic properties, and chemical reactivity. The latter feature was used to control a series of NHC-catalyzed condensation and polymerization transformations wherein the rates of the corresponding reactions were increased or decreased through exposure to UV or visible light. The NHCs were also successfully ligated to a variety of transition metals, including ruthenium and rhodium, and used to control C-C bond forming chemistry.  In some cases, the intrinsic substrate selectivities displayed by the switchable catalysts were found to be dependent on the state of the photochromic ligand. Details of the key contributors were deduced using computational methods, which revealed how steric and electronic effects separately influence the reaction pathways and provided activation energies that were in good agreement with the relative reaction rate constants that were measured by experiment. The presentation will conclude with some perspectives on using photoswitchable catalysts to control other types of useful transformations.

Photoswitchable carbenes: Using light to control organo- and metal-mediated transformations

256th National Meeting (2018)

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