Highly Chemoselective Catalytic Photooxidations by Using Solvent as a Sacrificial Electron Acceptor

Catalyst recovery is an integral part of photoredox catalysis. It is often solved by adding another component‐a sacrificial agent‐whose role is to convert the catalyst back into its original oxidation state. However, an additive may cause a side reaction thus decreasing the selectivity and overall efficiency. Herein, we present a novel approach towards chemoselective photooxidation reactions based on suitable solvent‐acetonitrile acting simultaneously as an electron acceptor for catalyst recovery, and on anaerobic conditions. This is allowed by the unique properties of the catalyst, 7,8‐dimethoxy‐3‐methyl‐5‐phenyl‐5‐deazaflavinium chloride existing in both strongly oxidizing and reducing forms, whose strength is increased by excitation with visible light. Usefulness of this system is demonstrated in chemoselective dehydrogenations of 4‐methoxy‐ and 4‐chlorobenzyl alcohols to aldehydes without over‐oxidation to benzoic acids achieving yields up to 70 %. 4‐Substituted 1‐phenylethanols were oxidized to ketones with yields 80–100 % and, moreover, with yields 31‐98 % in the presence of benzylic methyl group, diphenylmethane or thioanisole which are readily oxidized in the presence of oxygen but these were untouched with our system. Mechanistic studies based on UV‐Vis spectro‐electrochemistry, EPR and time‐resolved spectroscopy measurements showed that the process involving an electron release from an excited deazaflavin radical to acetonitrile under formation of solvated electron is crucial for the catalyst recovery. New way to drive photoredox catalysis: Highly chemoselective photooxidations of benzylic alcohols to carbonyl compounds in the presence of various easily‐oxidizable groups are possible in a simple oxygen‐free system consisting of a substrate, unique deazaflavinium catalyst and acetonitrile which acts simultaneously as a sacrificial electron acceptor and solvent.