Ruthenium-based photocatalyst

Photocatalytic enantioselective oxidative arylic coupling reactions have been investigated by two different groups. Both studies involved the use of ruthenium-based photocatalysts [142, 143]. In 1993, Hamada and co-workers introduced a photostable chiral ruthenium tris(bipyridine)-type complex (A-[Ru(menbpy)3]2+) 210 possessing high redox ability [143]. The catalytic cycle also employed Co(acac)3 211 to assist in the generation of the active (A-[Ru(menbpy)3]3+) species 212. The authors suggested that the enantioselection observed upon binaphthol formation was the result of a faster formation of the (R)-enantiomer from the intermediate 213 (second oxidation and/or proton loss), albeit only to a rather low extent (ee 16 %) (Scheme 54). 

Yoon and co-workers showed that pyrroles 2 are formed when vinyl and aryl azides 1 are irradiated with visible light in the presence of a ruthenium-based photocatalyst. The reaction mechanism proceeds through a nitrene intermediate which can undergo several different reactions with pyrroles as one of the classes of products that are formed (14AG(I)793). 

Diaryliodonium salts are frequently used as photoinitiators in cadmiic polymerization, and aryl radicals are generated with ruthenium- or itidium-based photocatalysts [10]. hi 2012, Sanford exploited this feature in a combined Pd/Ir-catalyzed system for photoredox arylations of arenes with N-directing groups. The generation of aryl radicals allowed ort/io-arylations at room temperature in MeOH, i.e., much milder conditions than previously described [216]. 

There are several photocatalysts mimicking hydrogenase activity that are not based on metalloporphyrin systems. Among them there are mixed-valence complexes of rhodium or iridium, [41] as well as complex systems encompassing photosensitizers (eg ruthenium complexes) attached to a catalytic bimetallic centre [43], The design of more sophisticated systems approaches that of photosynthetic processes [44],... 

Since a few years, ruthenium- and iridium-based polypyridyl complexes are privileged photocatalysts in visible light photocatalysis. However, organic dyes should constitute a valuable alternative not only due to their relatively lower cost and wider availability but also by giving access to new transformations. The direct arylation of heteroarenes (e.g., furan, thiophene, and pyrrole)... 

While the majority of transition metal photocatalysts are based upon ruthenium and iridium complexes, similar intermolecular cyanation reactions can also be accomplished using gold-based transition metal photocatalysts. 

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