Oxidation Potential Gap (ΔEox): The Hidden Parameter in Redox Chemistry

Oxidative biaryl coupling of aryls with different electronic features generally fails. However, this has not been systematically studied via theoretical analysis, and thus, the crucial factor governing coupling efficiency remains unclear. Herein, we propose that the “oxidation potential gap (ΔEox)” is a key parameter in predicting the efficiency of an intramolecular oxidative coupling reaction, with ΔEox defined as a difference in the oxidation potentials of the relevant aromatic rings. Our experimental and computational analyses revealed that the efficiency of an aromatic intramolecular coupling reaction correlates with the activation energy (ΔE≠) of C−C bond formation of the radical cation intermediates. Furthermore, ΔE≠ correlates with ΔEox. Therefore, we demonstrate the tuning of ΔEox by attaching cleavable extra electron‐donating/‐withdrawing groups, enabling the rational synthesis of a phenanthridone skeleton using aromatic rings with an electronic gap. Experimental and computational studies have revealed that the efficiency of intramolecular oxidative cross‐coupling reactions is governed by the SOMO–HOMO gap of the coupling partners, and that the reaction efficiency can be predicted by the oxidation potential gap (ΔEox). Furthermore, tuning of the ΔEox value by the addition of cleavable electron‐donating or ‐withdrawing groups enabled the electrochemical synthesis of phenanthridone skeletons.