Investigation of electronic effects in the pyridine and pyridine N-oxide rings. Part 3. Reactions of substituted 2-carboxypyridine N-oxides with diazodiphenylmethane

Abstract

Rate constants for the reactions of diazodiphenylmethane (DDM) with a series of substituted 2-carboxypyridine N-oxides (3-, 4-, 5-, and 6-CO₂Me, 4-NO₂, 4-Cl, 4-Br, 6-Me, H, 4-OMe, 4-NH₂) have been determined in absolute ethanol over the range 20–50 °C. The existence of a strong intramolecular hydrogen bond has previously been established for this particular arrangement of the N-oxide and carboxy group, and it was previously believed that this type of interaction did not conform to the Hammett relationship. It was shown, however, that log(rate constants) for the reactions of 4-substituted derivatives are well correlated with ordinary σ values of substituents with ρ 1.340, and with a sliding scale of σ values with ρ 0.677. A satisfactory correlation was obtained between log(rate constants) for the reaction of a number of the above acids with DDM and the pK_a values of the conjugate acids of correspondingly substituted pyridine N-oxides. On the basis of the results, it was concluded that an equilibrium between the hydrogen-bonded and the free acid precedes proton transfer in the rate-determining step in the reaction with DDM.