Homolytic cleavage energies of R-H bonds centered on carbon atoms of high electronegativity:: First general observations of O-type variation on C-HBDEs 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 pK(a)'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 (Delta BDE) 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 Cor methylene) C-H bonds (i.e., the O-type), which is in sharp contrast tb 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 electron pair as suggested in the literature, that governs the: direction of radical substituent effects.