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

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posted on 28.03.2012, 00:00 by Xiangcheng Pan, Emmanuel Lacôte, Jacques Lalevée, Dennis P. Curran
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.