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 (Ea) for concerted DielsAlder 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 Ea and that for diradicals is called the energy of concert (Econ). If experiment represents concert, Econ must be large, and if it represents diradicals Econ will be very small. In this paper, 42 examples are adduced in which firm experimental data show that Econ = 0. These cycloadditions cannot be concerted. While concert remains possible for all cycioadditions not proven stepwise, there is none with compelling evidence for concert.