ELSEVIER Theoretical THEO CHEM Journal of Molecular Structure (Theochem) 343 (1995) 149-155 study of azide anion addition to nonpolar and polar double and triple bonds Branko S. Jursic*, Zoran Zdravkovski Department of Chemistry, University of New Orleans, New Orleans, LA 70148, USA Received 12 January 1995; accepted 22 January 1995 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 MP2/6-3 1 +G* and MP3/6- 3 1 +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. Keywords: Ab initio calculation; Azide anion addition; Double bond; Transition structure; Triple bond 1. Introduction One of the best synthetic procedures for the synthesis of triazole, tetrazole and pentazole and their saturated derivatives is the polar addition of the corresponding azide to the unsaturated com- pound [ 11. By varying the substituents, for example on the dienophile and/or the azide, a wide variety of nitrogen-containing heterocyclic five membered compounds can be synthesized. 1,2,3-Triazole has been obtained by several direct and indirect * Corresponding author. synthetic approaches. The direct addition of azide anion in acidic media (hydrazoic acid) is a straight- forward procedure but is somewhat dangerous [2]. Although the direct preparation of 1,2,3-triazole from acetylene and hydrazoic acid is experi- mentally feasible, the compound can be obtained from acetylenecarboxylic acid and sodium azide in acid media with cycloaddition and elimination in one synthetic step [3]. One of the adducts, pentazole, has very interesting properties. It should be an aromatic heterocycle with high acidity [4]. The chemistry of pentazole has developed in the last couple of decades because the compound is a 0166-1280/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0166-1280(95)04147-8