Formation and Stability of N-Heterocyclic Carbenes in Water: The Carbon Acid pK_a of Imidazolium Cations in Aqueous Solution

We report second-order rate constants k_DO (M^-1 s ^-1) for exchange for deuterium of the C(2)-proton of a series of simple imidazolium cations to give the corresponding singlet imidazol-2-yl carbenes in D₂O at 25 °C and l = 1.0 (KCl). Evidence is presented that the reverse protonation of imidazol-2-yl carbenes by solvent water is limited by solvent reorganization and occurs with a rate constant of k_HOH = k_reorg = 10¹¹ s^-1. The data were used to calculate reliable carbon acid pK_as for ionization of imidazolium cations at C(2) to give the corresponding singlet imidazol-2-yl carbenes in water: pK_a = 23.8 for the imidazolium cation, pK _a = 23.0 for the 1,3-dimethylimidazolium cation, pK_a = 21.6 for the 1,3-dimethylbenzimidazolium cation, and pK_a = 21.2 for the 1,3-bis-((S)-1-phenylethyl)benzimidazolium cation. The data also provide the thermodynamic driving force for a 1,2-hydrogen shift at a singlet carbene: K₁₂ = 5 × 10¹⁶ for rearrangement of the parent imidazol-2-yl carbene to give neutral imidazole in water at 298 K, which corresponds to a favorable Gibbs free energy change of 23 kcal/mol. We present a simple rationale for the observed substituent effects on the thermodynamic stability of N-heterocyclic carbenes relative to a variety of neutral and cationic derivatives that emphasizes the importance of the choice of reference reaction when assessing the stability of N-heterocyclic carbenes.