Stereo- and regioselective cycloaddition of norbornene to 2,4,9-triazidopyridine

4-(3-Azatricyclo[3.2.1.0]oct-3-yl)-2,6-diazido-3,5-dicyanopyridine has been obtained by the reaction of 2,4,6-triazido-3,5-dicyanopyridine with an equimolar quantity of norbornene. The product reacted readily at room temperature with an excess of norbornene giving the corresponding trisazatricyclooctane cycloadduct. An analogous trisadduct was obtained in the reaction of 4-(3-azatricyclo[3.2.1.0]oct-3-yl)-1,6-diazido-3-chloro-5-cyanopyridine with norbornene on boiling in CCl₄, and also in ether at room temperature in the presence of the complexes Rh₂(OAc)₄ and Cu(AcAc)₂. The cycloaddition proceeds stereoselectively in all cases with the exclusive formation of exo-conformers. Calculations have been carried out using the PM3 and RHF/3-21G* methods on 2,4,6-triazido-3-chloro-5-cyanopyridine and on 2,4,6-triazido-3,5-dicyanopyridine and also on the cycloadducts of these compounds with one or two molecules of norbornene. It was established that the addition of norbornene at the azide groups of pyridine is a dipole-LUMO controlled type of reaction and leads to the formation of cycloadducts having higher LUMO energy than the initial azides. The energy of the LUMO is increased to a lesser extent as a result of the addition of norbornene to a triazide containing identical substituents in the β positions of the pyridine ring, and is due to the special features of the symmetry of the LUMO of the cycloadducts formed.