Reduction of Quinones by NADH Catalyzed by Organoiridium Complexes

摘要

Quinones (Q) are a class of fully conjugated cyclic dione compounds that are widely distributed in nature.They can function as electron carriers in electron-transport chains (e.g. aerobic respiration) and undergo one- or two-electron reduction, coupled with protonation, leading to the corresponding semiquinones (QH·) or hydroquinones (QH2), respectively (Scheme 1). In biological systems, quinones are often reduced by coenzyme NAD(P)H (Scheme 2) in the presence of cellular reductases, such as NADH ubiquinone oxidoreductase, NADH cytochrome b5, and NADPH cytochrome P-450 reductase. Apart from enzymatic methods, the reduction of quinones by chemical, photochemical, and electrochemical methods has been investigated because of its biological and industrial importance. However, there are only a few reports of metal involvement in the reduction of quinones. For example, Sc~(3+) can form metal-quinone radical adducts using the cyclometalated iridium complex [Ir(ppy)3] (PPy = 2-phenylpyridine) as an electron donor. Organome-tallic iridium complexes have potential for application in catalysis and biology. Coenzyme NADH can transfer hydride ions to cyclopentadienyl-Ir~(III) complexes, generating iridium-hydride complexes. Here we investigate whether cyclopentadienyl-Ir~(III) complexes can use NADH as a hydride source for the reduction of quinones, thus mimicking the action of reductases. We chose quinones vitamin K3 (menadione) and the related duroquinone as substrates (Scheme 2), because vitamin K is required for the synthesis of certain proteins that are involved in blood coagulation and metabolic pathways in bones and other tissues, and may play an important role in the treatment of cancer, Alzheimer's and other diseases. This appears to be the first report of the catalytic reduction of quinones by an organometallic complex, and intriguingly, appears to involve the unusual Ir~(II) oxidation state in a novel mechanism.