Acidities and Hemolytic Bond Dissociation Energies (BDEs) of Benzyl-Type C-H Bonds in Sterically Congested Substrates

Equilibrium acidities in DMSO and BDEs for the benzylic C-H bonds are reported for 19 triphenylmethanes, three 9,10-dihydroanthracenes, and nine xanthenes. The phenyl groups in triphenylmethane, 9-phenyl-9,10-dihydroanthracene, and 9-phenylxanthene are shown to be constrained in their ability to delocalize either the negative charges in the anions formed by loss of a proton or the odd electrons released by loss of hydrogen atom. Analysis of the pK_HA values showed, however, that strong solvation of para electron-withdrawing substituents in a phenyl ring of each of these substrates caused the corresponding anion to adopt a conformation where effective conjugative overlap occurred between the substituent and the carbanion via the phenyl ring. In other words, a conformational change was induced via a substituent solvation assisted resonance (SSAR) effect. In sharp contrast to these large solvation-induced substituent effects on anion stabilities, remote substituent effects on radical stabilities in these congested species were found to be negligible. Substitution of either a p-toluenesulfonyl or CN group into the 9-position of xanthene caused a large increase in acidity, but the p-CH₃C₆H₄SO₂ group caused an increase in the BDE of the acidic C-H bond, whereas the CN group caused a small decrease.