Theoretical study of azide anion addition to nonpolar and polar double and triple bonds

Abstract

Transition structures for the 1,3-polar addition of azide anion to hydrogen cyanide, formaldimine, nitrogen, cis- and trans -diazene, ethylene and acetylene were obtained at the MP2/6–31 + G ∗ theoretical model. The additions can be divided into two groups: addition to a triple bond, giving rise to an aromatic heterocyclic product, and addition to a double bond, forming a non-aromatic product. All transition structures correspond to a concerted mechanism for the polar cycloaddition. Symmetrical dienophiles, apart from cis -diazene, give rise to synchronous transition structures. The anomaly is explained in terms of strong n-n repulsion of the reactants in the transition structure. The reactivity of the compounds can be rationalized in terms of the bond orders of the newly forming bonds, from the frontier orbital energy differences and from the charge transfer from the azide anion to the dienophile. The quantitative correlation of the reactivity has been judged on the basis of the activation energies of the reactions calculated at MP2/6–31 + G ∗ and MP3/6–31 + G ∗ . It is predicted that the addition of azide to nitrogen is the slowest and that the additions to hydrogen cyanide and acetylene have the lowest barriers, in agreement with literature data.