Green Asymmetric Oxidation Using Air as Oxidant

We found that (NO)- and (aqua)ruthenium-salen complexes are efficient catalysts for the asymmetric aerobic oxygen atom transfer reactions such as epoxidation and sulfoxidation at ambient temperature. (NO)ruthenium-salen complexes 2 and 3 could catalyze oxidation of sulfides and the epoxidation of conjugated olefins with good to high enantioselectivity using dioxygen as oxidant, albeit under visible light-irradiation, respectively. On the other hand, (aqua)ruthenium-salen complex 5 was found to catalyze highly enantio-selective epoxidation in air even without irradiation. Although the mechanism of this ruthenium-catalyzed aerobic oxidation has not been completely elucidated, water that is bound to the ruthenium ion has been considered to play a critical role in proton coupled electron transfer, a key step for oxygen activation, and to be regenerated via oxo-hydroxo tautomerization. We also found that (di-mu-hydroxo)iron-salan complexes catalyzes asymmetric dehydrogenative oxidation reactions such as 2-naphthol coupling, alcohol oxidation, and dearomatization, using air as oxidant.