Reductive Amination by Photoredox Catalysis and Polarity-Matched Hydrogen Atom Transfer

The excitation of a Ru-II photosensitizer in the presence of ascorbic acid leads to the reduction of iminium ions to electron-rich alpha-aminoalkyl radical intermediates, which are rapidly converted into reductive amination products by thiol-mediated hydrogen atom transfer (HAT). As a result, the reductive amination of carbonyl compounds with amines by photoredox catalysis proceeds in good to excellent yields and with broad substrate scope and good functional group tolerance. The three key features of this work are 1)the rapid interception of electron-rich alpha-aminoalkyl radical intermediates by polarity-matched HAT in a photoredox reaction, 2)the method of reductive amination by photoredox catalysis itself, and 3)the application of this new method for temporally and spatially controlled reactions on a solid support, as demonstrated by the attachment of a fluorescent dye on an activated cellulose support by photoredox-catalyzed reductive amination.