Mechanistic Overview
There were two original proposals for the mechanism of the 1,3-dipolar cycloaddition: the concerted pericyclic cycloaddition mechanism, proposed by Rolf Huisgen, and the stepwise mechanism involving a diradical intermediate, proposed by Firestone. The former proposal is now generally accepted. The 1,3-dipole reacts with the dipolarophile in a concerted, often asynchronous, and symmetry-allowed π4s + π2s fashion through a six-electron Huckel aromatic transition state.
Read more about this topic: 1,3-Dipolar Cycloaddition