Understanding why [4+2+1] is favored over [2+2+1] and discovering that strain-release is the key to the success. Now let's answer why [2+2+1] reaction does not occur (Figure ). cycloadditions, but they are disfavored and are only presented in the Supporting Information. TS5 is higher than TS4 by 5.9 kcal/mol, suggesting that only [4+2+1] reaction can happen, agreeing with experiments.