Reversibly Photoswitchable Catalysts for Olefin Metathesis Reactions

Azobenzene-bearing photoswitchable ruthenium catalysts were developed for olefin metathesis reactions. These catalysts exhibited an on–off switching ability in ring-closing metathesis depending on UV irradiation (>99% conversion without UV vs <1% with UV). Photoswitching phenomena have been observed in various types of olefin metathesis reactions, such as ring-opening metathesis, cross-metathesis, and ethenolysis. The photoswitching feature is also reversible, as the ring-closing metathesis reaction can repeatedly be turned on and off by switching the UV light source. Photoisomerization of azobenzene-bearing cyclic­(alkyl)­(amino)­carbene (azo-CAAC) ligands and the corresponding ruthenium catalysts was observed by UV–vis spectroscopy.