The Low Energy of Concert in Many Symmetry‐Allowed Cycloadditions Supports a Stepwise‐Diradical Mechanism

The thesis of this paper is that orbital symmetry allowedness does not guarantee concert. The activation energy (E_a) for concerted Diels–Alder and 1,3‐dipolar cycloadditions must be substantially lower than that for a stepwise‐diradical pathway because the diradical has two fewer bonding electrons than anything on the concerted reaction coordinate, including the transition state. Two bonding electrons provide tens of kcal/mol of stabilization. The difference between the experimental E_a and that for diradicals is called the energy of concert (E_con). If experiment represents concert, E_con must be large, and if it represents diradicals E_con will be very small. In this paper, 42 examples are adduced in which firm experimental data show that E_con = 0. These cycloadditions cannot be concerted. While concert remains possible for all cycioadditions not proven stepwise, there is none with compelling evidence for concert.