An Acidity Scale of Triazolium-Based NHC Precursors in DMSO.

An acidity scale of triazolium-based N-heterocyclic carbene precursors was established by measuring 25 compounds' (10a-15b) equilibrium acidities in dimethyl sulfoxide (DMSO) solution using overlapping indicator method. The pKa values ranged from 12.08 to 15.5, responding not only to the N-aryl motif, but also to the core structure. Excellent correlation was observed between the pKa values and the Hammett substituent constants (σp).