Homolytic Cleavage Energies of R−H Bonds Centered on Carbon Atoms of High Electronegativity: First General Observations of O-type Variation on C−H BDEs and the Implication for the Governing Factors Leading to the Distinct O/S Patterns of Radical Substituent Effects

A systematic investigation of the methylene C−H bond dissociation energies (BDEs) of the onium-substituted toluene series (1−6) and of the adjacent electron-pulling group-substituted acetophenone and fluorene series (7 and 8) (Chart 1) was carried out using a modified eletrochemical method (eq 1) incorporating the pKa's measured in DMSO solution. This provided the first opportunity to examine the stabilization effect of substituents on carbon radicals (or C−H bonds) of varying degrees of electron deficiency. The relative BDE (ΔBDE) values estimated for these substrates within a family showed that for most systems, the adjacent or remote electron-withdrawing groups (EWGs) were found to strengthen the benzylic (or methylene) C−H bonds (i.e., the O-type), which is in sharp contrast to the universally observed C−H bond-weakening effect of EWGs (i.e., the S-type) in the literature. This general phenomenon reveals that it is the apparent electronegativity of the methylene carbon, rather than the nonbonded elect...