TRIARYLMETHANES AND 9-ARYLXANTHENES AS PROTOTYE AMPHIHYDRIC COMPOUNDSFOR RELATING THE STABILITIES OF CATIONS, ANIONS AND RADICALS BY C-H BOND-CLEAVAGE AND ELECTRON-TRANSFER

Thermodynamic stability properties of II p-substituted trityl and seve n 9-phenylxanthyl carbocations are reported in sulfolane and of their conjugate carbanions in DMSO. The cations are compared by calorimetric heats of hydride transfer from cyanoborohydride ion, their first and second reduction potentials, their PK(R)(+)s in aqueous sulfuric acid, C-13 chemical shifts and free energies of methoxy exchange. Carbanlon s are compared by their heats and free energies (pK(HA)) of deprotonat ion and their first and second oxidation potentials. Radicals are comp ared by their oxidation and reduction potentials. Their bond dissociat ion energies are derived by alternative routes: from the carbocation a nd its reduction potential and from the carbanion and its oxidation po tential. The various properties are correlated against each other and against appropriate Hammett-type substituent parameters. Correlations between the different measured properties reported here range from fai r to excellent, Despite their importance as historic prototypes for th e three trivalent oxidation states of carbon, trityl and xanthyl syste ms are atypical models for comparing transmission of electron demand I n other series of carbocations, radicals or carbanions with significan tly different structures. The 9-arylxanthyl series is especially poor because of its insensitivity to substituent effects. The effects of su bstituents on various properties which represent the stabilities of R( +)s correlate surprisingly well against those for corresponding R(-)s. Accordingly, compensating effects on the oxidation and reduction of a series of related RS may lead to a nearly constant electron transfer energy and absolute hardness for the series. In contrast, the free ene rgies for interconversion of the carbocations and carbanions which det ermine the gap between pK(R+) and pK(HA) are very sensitive to structu ral change. (C) 1997 by John Wiley & Sons, Ltd.