Photoswitchable N-heterocyclic carbenes : modulating electronic structure and catalytic properties with light
MetadataShow full item record
The ubiquity of N-heterocyclic carbenes (NHCs) as ligands in transition metalmediated and as organocatalysts have led us to focus on the development of photochromic NHCs in order to influence the reactivity of various chemical transformations. A novel NHC based on the photochromic diarylethene scaffold has been synthesized and isolated. The free NHC was found to undergo electrocyclic isomerization upon exposure to UV radiation (λIrr = 313 nm); subsequent exposure to visible light (λIrr > 500 nm) reversed the aforementioned cyclization. This photoinduced cyclization resulted in remarkable changes in the electronic properties of the NHC. Furthermore, the ringclosed NHC was found to undergo facile N-H bond activation, facilitating the ondemand capture and release of ammonia. Collectively, these results demonstrated the first known example of an isolable NHC whose electronic structure and, by extension, donicity could be photochemically tuned. Moreover, the ability to use light as a means to achieve the selective capture of ammonia represents the first example of its kind and provided an unprecedented degree of control over a difficult transformation. We sought to expand the utility of this NHC by preparing a photoswitchable analogue of second-generation Hoveyda-Grubbs' catalyst. Similar to the free NHC, this complex was found to undergo reversible ring-closure upon successive exposure to UV (λIrr = 313 nm) and visible light (λIrr > 500 nm) radiation. Changes in the corresponding ¹H NMR spectra indicated > 80% conversion to the ring-closed isomer. This complex was found to facilitate ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) reactions, with both cases proceeding to high degrees of conversion for a variety of substrates. The activity of this catalyst was successfully modulated upon alternating UV (λIrr = 313) and visible light (λIrr > 500 nm) irradiation. Interestingly, comparatively faster RCM reaction rates (k [subscript open] /k [subscript closed] ≥ 0.50) were observed when the ring-closed isomer was employed, while utilization of the ring-opened isomer resulted in comparatively faster ROMP rates (k [subscript open] /k [subscript closed] ≤ 1.67). Furthermore, these examples represent an extremely rare instance of photoswitching the activity of a transition metal catalyst through modular electronics and highlight the potential for broadly applicable photoswitchable catalysis.