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

Area of Science:

• Organic Chemistry
• Catalysis
• Radical Chemistry

Background:

• Catalytic radical halogen atom transfer (XAT) usually favors C-I and C-Br bonds over C-Cl bonds due to bond dissociation free energy (BDFE).
• This selectivity leaves abundant and inexpensive alkyl chlorides underutilized in synthetic chemistry.

Purpose of the Study:

• To develop novel catalysts that can efficiently perform hydrodehalogenation of alkyl chlorides.
• To invert the typical reactivity order of XAT, enabling selective C-Cl bond activation.
• To establish a sustainable, metal-free catalytic system for alkyl chloride valorization.

Main Methods:

• Synthesis and characterization of N-heterocyclic phosphine-derived diazaphosphinyl radicals (NHP•) as catalysts.
• Comprehensive thermodynamic, kinetic, and mechanistic studies to elucidate the catalytic cycle.
• Evaluation of catalyst performance in hydrodehalogenation of various alkyl chlorides, bromides, and iodides.

Main Results:

• NHP• radicals efficiently catalyze the hydrodehalogenation of alkyl chlorides, outperforming bromides and iodides.
• The catalytic cycle involves dual roles of NHP derivatives as XAT abstractors and hydrogen donors.
• The rate-determining step is a polar bond-metathesis process forming a strong Si-Cl bond, favoring chlorides.

Conclusions:

• NHP• catalysts enable a unique, metal-free hydrodehalogenation strategy with high selectivity for alkyl chlorides.
• Polar steps in radical catalysis can dictate substrate selectivity, challenging traditional BDFE-dependent reactivity.
• This approach offers a sustainable method for utilizing alkyl chlorides in organic synthesis, including hydroalkylation reactions.