KETO-ENOL-TAUTOMERISM AND IONIZATION OF 1-PHENACYLPYRIDINIUM IONS - AMODEL FOR CARBANION-STABILIZATION OF AZOMETHINE YLIDES

Measurement of equilibrium constants for keto-enol tautomerism (K(T)) and ionisation (K(a)) of 1-phenacylpyridinium and 1-phenacyl(4-dimethy lamino) pyridinium ions gives pK(T) (-log K(T)) = 6.10 and 5.55 and pK (a) = 10.90 and 1 3.2 respectively. The enol content and acidity of th e 1-phenacylpyridinium ion is lower than that of its 2-, 3- and 4-isom ers, and the possibility that this reflects impaired -M resonance by a 1-pyridinium substituent is discussed. Notional (proton) activating f actors reflecting the. influence Qf the positive charge of the 1-pyrid inium substituent upon equilibrium ionisation and rates of deprotonati on by lutidine and hydroxide bases are estimated from free energy corr elations as 10(3), 17 and 5 x 10(3) respectively. These compare with a (methyl) activating factor of 10(9) derived from equilibrium ionisati ons of 4-chlorobenzaldehyde oxime and nitrone and a (notional) value o f 10(6) for pyridine-N-oxide. The implications of these values for the activating effect of N-protonation of an azomethine group in models f or pyridoxal-catalysed azomethine rearrangements are discussed.