Quantum chemical study of the nature of regioselectivity in reactions of 2,4,6-triazidopyridines with tert-butylphosphaacetylene

We have performed PM3 calculations for 2,4,6-triazido-3,5-dicyanopyridine and 2,4,6-triazido-3-chloro-5-cyanopyridine, and also the cycloadducts of these compounds with one and two tert-butylphosphaacetylene molecules. We have established that the selectivity of addition of a phosphaacetylene molecule at the γ-azido group of triazidopyridines is due to the specifics of the electronic properties and geometry of these groups, characterized by enhanced electrophilicity of the terminal nitrogen atoms, a greater contribution from atomic orbitals to the LUMO (lowest unoccupied molecular orbital), and greater bending of the chain of N-N-N atoms. The cycloaddition itself corresponds to the dipole-LUMO-controlled reaction type, and sequential addition of phosphaacetylene molecules at the azido groups of pyridines leads to formation of cycloadducts having lower LUMO energies. Nevertheless, the γ-azido groups of the triazidopyridines can remain stronger dipoles than the azide groups of the cycloadducts when the difference between the LUMO energies in the triazidopyridines and cycloadducts is no greater than 10 kcal/mole.