Keto–Enol tautomerism and ionisation of 1-phenacylpyridinium ions: a model for carbanion-stabilisation of azomethine ylides

Abstract

Measurement of equilibrium constants for keto–enol tautomerism (K_T) and ionisation (K_a) of 1-phenacylpyridinium and 1-phenacyl(4-dimethylamino) pyridinium ions gives pK_T(–log K_T)= 6.10 and 5.55 and pK_a= 10.90 and 13.2 respectively. The enol content and acidity of the 1-phenacylpyridinium ion is lower than that of its 2-, 3- and 4-isomers. and the possibility that this reflects impaired –M resonance by a 1-pyridinium substituent is discussed. Notional (proton) activating factors reflecting the influence of the positive charge of the 1-pyridinium substituent upon equilibrium ionisation and rates of deprotonation by lutidine and hydroxide bases are estimated from free energy correlations as 10³, 17 and 5 × 10³ respectively. These compare with a (methyl) activating factor of 10⁹ derived from equilibrium ionisations of 4-chlorobenzaldehyde oxime and nitrone and a (notional) value of 10⁶ for pyridine-N-oxide. The implications of these values for the activating effect of N-protonation of an azomethine group in models for pyridoxal-catalysed azomethine rearrangements are discussed.