Polarity Reversal Catalysis in Radical Reductions of Halides by N-Heterocyclic Carbene Boranes

Otherwise sluggish or completely ineffective radical reductions of alkyl and aryl halides by N-heterocyclic carbene boranes (NHC-boranes) are catalyzed by thiols. Reductions and reductive cyclizations with readily available 1,3-dimethylimidazol-2-ylidene borane and a water-soluble triazole relative are catalyzed by thiophenol and tert-dodecanethiol [C₉H₁₉C­(CH₃)₂SH]. Rate constants for reaction of the phenylthiyl (PhS•) radical with two NHC-boranes have been measured to be ∼10⁸ M^–1 s^–1 by laser flash photolysis experiments. An analysis of the available evidence suggests the operation of polarity reversal catalysis.