Investigation of electronic effects in the pyridine and pyridine N-oxide rings. Part 4. Kinetics and mechanism of the reaction of diazodiphenylmethane with substituted carboxypyridine and 2-carboxypyridine N-oxides in dimethylformamide

Abstract

Rate constants for the reactions of 15 substituted carboxypyridines and four substituted 2-carboxy-pyridine N-oxides with diazodiphenylmethane (DDM) in dimethylformamide (DMF) were determined at 30 °C, by the known spectrophotometric method. For comparison, rate constants for the reaction of 12 substituted benzoic acids were determined under the same conditions. Kinetic and thermodynamic parameters obtained, together with products analyses, essentially verify the previously proposed reaction mechanism in this solvent. The empirical Hammett treatment has been applied to the results, and it was shown that a scattered overall plot (ρ= 1.019, logk_o=–2.98, r= 0.91, s= 0.22, n= 31) was obtained with additive σ constants from the corresponding σ values for the pyridine aza-group and N-oxide group and Hammett σ constants for substituents. Fairly good separate correlations were obtained from the data for substituted benzoic acids (ρ= 1.213, logk_o=–2.91, r= 0.985, s= 0.12, n= 12), halogen- and nitro-substituted acids (ρ= 1.07, logk_o=–2.99, r= 0.998, s= 0.02, n= 5) and for the carboxypyridine N-oxides (ρ= 1.18, logk_o=–3.35, r= 0.995, s= 0.04, n= 4). A far less satisfactory correlation was obtained when it was applied to the substituted carboxypyridines, including the N-oxides (ρ= 1.15, logk_o=–3.23, r= 0.91, s= 0.18, n= 19). It was concluded that powerful solvent nucleophilic stabilisation of the reactant carboxylic acid in the initial state is greatly influenced by the type of substitution, substituent solvation, and probably also by the kind of electronic interaction of the particular substituent with the heteroaromatic nuclei.