kcal/mol) for the nucleophilic substitution reaction between carbanion V' and 4-cyanopyridine (Fig. , for the full free energy profile, see Supplementary Fig. and). These computational findings are consistent with the observed difference in reactivity between performing the reaction in the presence of 1.0 and 2.0 equivalents of t BuOK. Additionally, the radical pathway for the C-C coupling can be ruled out because of the higher energy required for the corresponding SET process (see Supplementary Fig. for detailed calculations).