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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 C(9)H(19)C(CH(3))(2)SH. Rate constants for reaction of the phenylthiyl (PhS•) radical with two NHC-boranes have been measured to be ~10(8) M(-1) s(-1) by laser flash photolysis experiments. An analysis of the available evidence suggests the operation of polarity reversal catalysis.
Pan et al. (Tue,) studied this question.