A study of the tendency of organic bases towards cationic heteroconjugation in polar non-aqueous solvents

Abstract

By using the potentiometric method, cationic heteroconjugation constants, ssK_BHB1⁺, were determined in systems involving a cation of protonated 4-N′,N′-dimethylaminopyridine N-oxide and N-oxides of 4-methylpyridine, 3-methylpyridine, 2-methylpyridine, unsubstituted pyridine, as well as protonated trimethylamine N-oxide and 4-N′,N′-dimethylaminopyridine N-oxide in polar non-aqueous solvents, protophobic aprotic acetone and amphiprotic methanol. In order to estimate the influence of the basicity of the proton donors and proton acceptors on the cationic heteroconjugation constants, their values determined in the N-oxide systems studied were compared with the previously determined ones for other N-oxide systems in the two above-mentioned solvents. Furthermore, to evaluate the influence of the medium on the tendency towards cationic heteroconjugation, all the determined heteroconjugation constant values in acetone and methanol were compared with those determined in other polar solvents, such as nitromethane, nitrobenzene, acetonitrile, propylene carbonate, N,N-dimethylformamide and dimethyl sulfoxide. Moreover, a comparison of the constant values obtained in the N-oxide systems (with [OHO]⁺ bridges) with those determined in the heterocyclic N-base systems (with [NHN]⁺ bridges) and in systems involving both kinds of bases (with mixed [NHO]⁺–[OHN]⁺ bridges) provided a basis for discussion of the influence of the types of H-bonded bridges on their stability.