The activation energies of the rate-determining steps of 1 -6 correlate moderately with the reaction energies (R 2 = 0.80, Fig. ), i.e., the more exergonic the addition step the lower the activation barrier, in line with the Bell-Evans-Polanyi principle. Furthermore, the computed spin densities in the transition state structures show a clear distinction between the reacting diene in its T1 state and the reacting diene in its S0 state (Fig. , ESI), as well as the spin polarization effect which the former has on the latter. These results reveal a reactant-like transition state, in line with the exergonic reaction profiles (Fig. ).