U.V. and 15N N.M.R. integrated study of the protonation of aminoazoles

Abstract

The behaviour on protonation of a series of 3-, 4-, and 5-aminoisoxazoles has been studied by ¹⁵N n.m.r. spectroscopy. The results confirm that the first protonation site in the 3- and 5-amino derivatives is located at the endocyclic nitrogen atom, while in 4-amino derivatives it is at the exocyclic nitrogen. In addition, the protonation of 3-, 4-, and 5-amino substituted 1-methyl- and 1-phenyl-pyrazoles has been investigated by an integrated u.v. and ¹⁵N n.m.r. approach. The first protonation site in the 5-amino derivatives is the pyridine-like endocyclic nitrogen, while in the 4-amino derivatives it is the exocyclic nitrogen. The 3-amino series behaves exceptionally because a tautomeric equilibrium is possible between the endocyclic and exocyclic monocations. In sulphuric acid the position of this equilibrium is dependent on H₂SO₄ concentration. Diprotonation of these derivatives in concentrated sulphuric acid solutions has also been studied.