Robust Photocatalytic Method Using Ethylene‐Bridged Flavinium Salts for the Aerobic Oxidation of Unactivated Benzylic Substrates
Adam Pokluda, Zubair Anwar, Veronika Boguschová, Iwona Anusiewicz, Piotr Skurski, Marek Sikorski, Radek Cibulka
Abstract
Abstract 7,8‐Dimethoxy‐3‐methyl‐1,10‐ethylenealloxazinium chloride (1a) was found to be a superior photooxidation catalyst among substituted ethylene‐bridged flavinium salts (R=7,8‐diMeO, 7,8‐OCH 2 O‐, 7,8‐diMe, H, 7,8‐diCl, 7‐CF 3 and 8‐CF 3 ). Selection was carried out based on structure vs catalytic activity and properties relationship investigations. Flavinium salt 1a proved to be robust enough for practical applications in benzylic oxidations/oxygenations, which was demonstrated using a series of substrates with high oxidation potential, i. e., 1‐phenylethanol, ethylbenzene, diphenylmethane and diphenylmethanol derivatives substituted with electron‐withdrawing groups (Cl or CF 3 ). The unique capabilities of 1a can be attributed to its high photostability and participation via a relatively long‐lived singlet excited state, which was confirmed using spectroscopic studies, electrochemical measurements and TD‐DFT calculations. This allows the maximum use of the oxidation power of 1a, which is given by its singlet excited state reduction potential of +2.4 V. 7,8‐Dichloro‐3‐methyl‐1,10‐ethylenealloxazinium chloride (1 h) can be used as an alternative photocatalyst for even more difficult substrates. magnified image