Electron-Rich Phenoxyl Mediators Improve Thermodynamic Performance of Electrocatalytic Alcohol Oxidation with an Iridium Pincer Complex

• Published in: Journal of the American Chemical Society 2020-11, Vol.142 (45), p.19368-19378
• Main Authors: Galvin, Conor M, Waymouth, Robert M
• Format: Article
• Language: English
• Subjects: Chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.0c09605

Electron-rich phenols, including α-rac-tocopherol Ar 1 OH, 2,4,6,-tri-tert-butylphenol Ar 3 OH, and butylated hydroxy-toluene Ar 4 OH, are effective electrochemical mediators for the electrocatalytic oxidation of alcohols by an iridium amido dihyride complex (PNP)­Ir­(H)2 (IrN 1, PNP = bis­[2-diisopropylphosphino)­ethyl]­amide). Addition of phenol mediators leads to a decrease in the onset potential of catalysis from −0.65 V vs Fc+/0 under unmediated conditions to −1.07 V vs Fc+/0 in the presence of phenols. Mechanistic analysis suggests that oxidative turnover of the iridium amino trihydride (PNHP)­Ir­(H)3 (IrH 2, PNHP = bis­[2-diisopropylphosphino)­ethyl]­amine) to IrN 1 proceeds through two successive hydrogen atom transfers (HAT) to 2 equiv of phenoxyl that are generated transiently at the anode. Isotope studies and comparison to known systems are consistent with initial homolysis of an Ir–H bond being rate-determining. Turnover frequencies up to 14.6 s–1 and an average Faradaic efficiency of 93% are observed. The mediated system shows excellent chemoselectivity in bulk oxidations of 2-propanol and 1,2-benzenedimethanol in THF and is also viable in neat 2-propanol.