Oxidative dehydrogenation of alcohols and amines is one of the most fundamental and important reactions in organic chemistry and industrial processes. To avoid the environmental pollution, the development of aerobic oxidative dehydrogenation without the use of large amount of heavy-metal oxidants has been demanded. In this account, the aerobic oxidative dehydrogenation of amines using ruthenium complexes bearing a cyclometalated ligand has been described. A pincer ruthenium complex exhibited the oxidative dehydrogenation of the coordinated imidazoline units with aerial oxygen to give an imidazole-ligated pincer complex; the σ-donor character of the cyclometalated ligand assisted the aerobic oxidation of the coordinated imidazoline moieties. Based on this observation, the aerobic oxidative dehydrogenation of 2-substituted imidazolines to their corresponding imidazoles has been achieved using a cyclometalated ruthenium complex, (PF_(6)) (ppy = 2-phenylpyridine, bpy = 2,2'-bipyridyl), as a catalyst under mild conditions. The ruthenium-catalyzed aerobic oxidation of benzylamines was also demonstrated.
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