Cofactor-Inspired Quinone Catalysis Following a One-Electron Hydrogen Atom Transfer Pathway

Alcohol dehydrogenation catalysts in an aerobic atmosphere are often inspired by biological cofactors, which play a major role in controlling the redox chemistry. A large body of work mimicking quinone-containing cofactors established two major mechanistic routes, addition–elimination or transamination. Both of these pathways are completely regulated by two-electron processes, despite the possibility of the cofactor motif being reduced by one electron. In stark contrast to the established mechanisms, we demonstrate a one-electron pathway in quinone catalysis toward dehydrogenating alcohols. The described pyrene dione molecule is efficient in catalytic dehydrogenation of primary, secondary, and aliphatic alcohols. The process starts with the photoexcitation of the dione motif which becomes photoreduced by KOtBu to generate a persistent semiquinonate radical. The substrate alcohol interacts with the quinone backbone to forge a hydrogen-bonded intermediate, which leads to a crucial hydrogen atom transfer (HAT) step, to accomplish the dehydrogenation reaction. A series of kinetic experiments including Bell–Evans–Polanyi correlation with the bond dissociation enthalpy firmly establish HAT to be rate-determining during dehydrogenation reactions. A kinetic isotope effect measured for the dehydrogenation process at 30 °C is 7.7 ± 0.9. Interception of a series of intermediates by a radical quencher in conjunction with a radical-probe substrate further affirms the radical-mediated, one-electron pathway to be operative that is in striking contrast to two-electron-driven quinone catalysis established so far.