Carbon acidity of the α-pyridinium carbon of a pyridoxamine analog

Abstract

We report second-order rate constants of k_DO = 120 dm³ mol⁻¹ s⁻¹ and k_B = 6.4 × 10⁻⁴ dm³ mol⁻¹ s⁻¹ for exchange for deuterium of the first α-methylene proton of the 4-(aminomethyl)pyridine dication in D₂O at 25 °C and I = 1.0 (KCl). These data are consistent with a carbon acid pK_a between 17 and 19 for ionization of this simple carbon acid and they show that the effect of an α-pyridinium substituent on carbon acidity is similar to that of an α-ester substituent.