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.