Applications of the Linnett electronic theory to organic chemistry. Part III. Linnett structures for 1,3-dipoles and for the diradical intermediates in 1,3-dipolar cycloadditions

When the diradical intermediates in 1,3-dipolar cycloadditions are written as Lewis structures with paired-electron bonds, the losses in bond energy which accompany their formation are much greater than the experimental activation energies. However, the use of Linnett structures for the 1,3-dipoles and diradicals produces good agreement between bond-energy losses and activation energies. Although the results do not uniquely confirm the diradical mechanism, they do show that it cannot be ruled out on the basis of energy arguments alone.