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).