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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 (i)Pr(2)NEt and HCO(2)H 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 tertiary 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.
Narayanam et al. (Fri,) studied this question.