SEMIEMPIRICAL AND AB-INITIO STUDY OF 1,3-DIPOLAR ADDITION OF AZIDE ANION TO ORGANIC CYANIDES

The 1,3-polar azide anion cyloaddition to nitriles was studied by semi empirical and ab initio methods. Two mechanisms were considered: a con certed and a two-step mechanism. AM1 strongly supports the two-step me chanism that involves a nucleophilic attack of the azide ion on the ca rbon of the nitrile group (formation of iminoazide intermediate) follo wed by tetrazole ring closure. The calculated activation energies are in good agreement with the experimentally obtained reactivity of the n itriles. The calculations agree with the fact that electron-withdrawin g substituents on the nitrile group decrease the activation barrier an d facilitate the reaction. Although RHF/3-21G was unable to locate the first transition state in all cases of the two-step mechanism, its co ncerted transition state structures are very unsymmetrical. The distor tion depends much more on the polarity of the substituents than on the ir size.