Diazaphosphinyl-Radical-Catalyzed Halogen Atom Transfer: Inverting Reactivity Trends for Chloride Activation

Catalytic radical halogen atom transfer (XAT) typically follows the bond dissociation free energy (BDFE)-dependent reactivity order R-I > R-Br > R-Cl, leaving abundant alkyl chlorides largely underutilized. We report diazaphosphinyl radicals (NHP^•) derived from N-heterocyclic phosphines as efficient organic radical catalysts that invert this trend, enabling catalytic hydrodehalogenation of alkyl chlorides over bromides and iodides. Comprehensive thermodynamic, kinetic, and mechanistic studies reveal dual roles of NHP derivatives: XAT abstractors and hydrogen donors. The overall catalytic efficiency is governed not by the radical XAT step, but by a polar bond-metathesis process that regenerates the active P-H reductant (NHP-H). This step is thermodynamically much favorable for chlorides, due to the strong Si-Cl bond formed, thereby establishing a closed catalytic cycle uniquely effective for R-Cl substrates. The catalyst exhibits good functional-group tolerance and enables mono-dechlorination of dichlorides with high selectivity, as well as hydroalkylation of activated olefins. This work highlights the important roles of polar steps in radical catalysis that can dictate substrate selectivity, and provides a sustainable, metal-free strategy for valorizing alkyl chlorides.