Theoretical Determination of the Electronic Mechanisms of 1,3-Dipolar Cycloaddition Reactions of Fulminic Acid and Diazomethane

The electronic mechanisms of 1,3-dipolar cycloaddition reactions of fulminic acid (ONCH) and diazomethane (NNCH 2 ) with HC⋮CH, N⋮CH, and P⋮CH are studied using ab initio molecular orbital methods. The reaction mechanisms can be classified into two classes: ionic electrocyclic and diradical coupling mechanisms. Ionic electrocyclic mechanism occurs in reaction systems with a large difference in electronegativities between the two atoms of the dipolarophile and their atoms react with the atoms with each opposite charge of a 1,3-dipole. Diradical coupling mechanism occurs through two processes: the first process is a one-electron movement from the edge atom to the center atom of a 1,3-dipole, which leads to a biradical state (for ONCH, the one-electron movement occurs from the oxygen to the nitrogen atom; for NNCH 2, the movement occurs from the carbon to the center nitrogen atom). The second process is the breaking of the π-bond of the dipolarophile, which is strongly associated with the activation energy, followed by the formation of the new σ-bond (cyclic reaction) through concerted or stepwise modes.