Investigation of electronic effects in the reaction of diazodiphenylmethane with pyridine and pyridine N-oxide carboxylic acids

The empirical Hammett treatment has been applied to the investigation of structure–reactivity relationships for pyridine and pyridine N-oxide carboxylic acids in the reaction with diazodiphenylmethane. The values obtained for the pyridine aza-group (σ2 0·878, σ3 0·740, σ41·105) are in good agreement with the literature values for the alkaline hydrolysis of the corresponding esters. For the N-oxide group, the σ values for the 3- and 4-positions (σ3 1·312, σ41·140) indicate that the overall effect is electron withdrawing. The negative value for the 2-position (σ2–0·395) can be explained on the basis of intramolecular hydrogen bonding. It is also shown that the Hammett relationship holds for pyridinecarboxylic acids with a fixed ortho-substituent as well.