Journal of Organic Chemistry p. 6542 - 6546 (1992)
Update date:2022-08-05
Topics:
Bordwell, F. G.
Cheng, Jin-Pei
Satish, A. V.
Twyman, Cary L.
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 pKHA 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 occured 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.
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