Acidity-Oxidation-Potential (AOP) Values as Estimates of Relative Bond Dissociation Energies and Radical Stabilities in Dimethyl Sulfoxide Solution

Article abstract of DOI:10.1021/ja00268a041

Oxidation potentials (E_ox) for the fluorenide ion and 21 substituted fluorenide ions have been measured in Me2SO solution.The E_ox values for 2-substituted fluorenide ions were found to plot linearly with the corresponding pK_HA values in Me2SO for the conjugate acids of these anions.Points for 3-methoxy-, 3-methyl-, 3-(methylthio)-, 3-(phenylthio)-, 1,2-benzo- and 2,3-benzofluorenide ions deviated from this plot.A combination of pK_HA and E_ox values gave acidity-oxidation-potential (AOP) values, which, when related to the AOP for fluorene itself, indicate that these remote substituents stabilize the fluorenyl radical by 0.4-1.5 kcal/mol.The 3-F and 3-PhSO2 substituents do not stabilize the radical appreciably.A similar analysis for GC6H4CHCN(1-) anions revealed radical stabilization by 4-Me2N and 4-Ph, as well as by 4-MeO, 4-Me, and 4-PhS substituents, but not by 4-F or 4-CF3.A similar analysis for 9-substituted fluorenes revealed much larger Δ(AOP) values, indicating radical-stabilizing effects for Me2N, MeO, Me, MeS, and Ph donor substituents and also for the acceptor substituents, H2NCO, MeOCO, and CN.These effects for substituents attached to a carbon atom bearing a relatively high spin density were in the range of about 2-11 kcal/mol.The effects on anion and radical stabilizations deduced from ΔpK_a and Δ(AOP) values, respectively, are compared.