Two-step conversion of pyridines into their ylides. Compensatory effects of substituents on reactivity

Abstract

Rates of hydrogen–deuterium exchange at the 2- and 6-positions of pyridine and 3-substituted pyridines were obtained in neutral D₂O generally at 217·9 ∓ 0·5°. The rate constant for pyridine under these conditions is 1·84 × 10^–4 S^–1. Rate constant ratios for the substituted molecules relative to pyridine are as follows: 2-position, Cl (8·6), CO₂^–(0·39), SO₃^–(1·8), CH₃O (13), and ND₂(7·3); 6-position, Cl (0·13), CO₂^–(1·2), SO₃^–(0·42), CH₃O (ca. 0·4), and ND₂(0·31). Evidence is presented to show that the compounds react by a two-step conversion of the pyridines into pyridinium ions and then into pyridinium ylides. The small effect of substituents on overall reactivity is rationalized in terms of compensatory effects on the two steps.