Electron-Transfer Photoredox Catalysis: Development of a Tin-Free Reductive Dehalogenatioin Reaction

We report an operationally simple, tin-free reductive dehalogenation system utilizing the well-known visible-light-activated photoredox catalyst Ru(bpy)(3)Cl-2 in combination with 'Pr2NEt and HCO2H or Hantzsch ester as the hydrogen atom donor. Activated C-X bonds may be reduced in good yields with excellent functional-group tolerance and chemoselectivity over aryl and vinyl C-X bonds. The proposed mechanism involves visible-light excitation of the catalyst, which is reduced by the 3 degrees amine to produce the single-electron reducing agent Ru(bpy)(3)(+). A subsequent single-electron transfer generates the alkyl radical, which is quenched by abstraction of a hydrogen atom. Reductions can be accomplished on a preparative scale with as little as 0.05 mol % Ru catalyst.