Oxidation potentials of alpha-hydroxyalkyl radicals in acetonitrile obtained by photomodulated voltammetry.

Oxidation potentials, E1/2(ox) of alpha-hydroxyalkyl radicals of the type R(1)R(2)C(*)OH (denoted K(1)H(*)) have been obtained in acetonitrile by photomodulated voltammetry. The values of E1/2(ox) increase as the R(1) and R(2) groups are changed from alkyl to aryl and, in particular, strong electron-withdrawing functionalities such as CN and CF3. Using rate data available in the literature for the pinacol photoexchange reaction K + K(1)H(*) --> KH(*) + K(1), it is found that as the difference in the standard potential of the ketone K, EK degrees and the oxidation potential of K(1)H(*), E1/2(ox), increases there is a modest increase in the exchange rate constant, k(ex). This indicates that even if some charge transfer may occur between the hydroxyalkyl radical and the ketone in the transition state, it is certainly not to the extent of a complete electron transfer. If the exchange reaction is treated as a simple hydrogen atom transfer process within the Marcus model, the intrinsic barrier is found to be 8-13 kcal mol(-1) due to the changes occurring in bonds, hybridizations, and bond angles. Finally, acid dissociation constants for K(1)H(*) are provided by means of a thermochemical cycle.

Substituent effects on the oxidation and reduction potentials of phenylthiyl radicals in acetonitrile.

Oxidation (E(1/2)(ox)) and reduction potentials (E(1/2)(red)) of a series of para-substituted phenylthiyl radicals XC(6)H(4)S* generated from the pertinent disulfides or thiophenols have been measured by means of photomodulated voltammetry in acetonitrile. The values of E(1/2)(ox) are of particular interest as they give access to the hitherto unknown thermochemistry of short-lived phenylsulfenium cations in solution. Both E(1/2)(OX) and E(1/2)(red) decrease as the electron-donating power of the substituent raises, resulting in linear correlations with the Hammett substituent coefficient sigma(+) with slopes rho(+) of 4.7 and 6.4, respectively. The finding of a larger substituent effect on than is a consequence of a corresponding development in the electron affinities and ionization potentials of XC(6)H(4)S* as revealed by quantum-chemical calculations. Solvation energies extracted for XC(6)H(4)S(+) and XC(6)H(4)S(-) from thermochemical cycles show the expected substituent dependency; i.e., the absolute values of the solvation energies decrease as the charge becomes more delocalized in the ions. Acetonitrile is better in solvating XC(6)H(4)S(+) than XC(6)H(4)S(-) for most substituents, even if there is a substantial delocalization of the charge in the series of phenylsulfenium cations. The substituent effect on is smaller in aqueous solution than acetonitrile, which is attributed to the ability of water to stabilize in particular localized anions through hydrogen bonding.

Evidence for ionic samarium(II) species in THF/HMPA solution and investigation of their electron-donating properties

The fundamental nature of samarium(II) complexes in THF/HMPA (HMPA = hexamethylphosphoramide) solutions containing SmI2 has been clarified by means of cyclic voltammetry, conductivity measurements, UV spectroscopy, and kinetic measurements. The principal species is not [SmI2(hmpa)4] as previously suggested, but either the ionic cluster [Sm(hmpa)4(thf)2+2I- if four equivalents of HMPA is present in the THF solution or [Sm(hmpa)6]2+ 2I- in the presence of at least 10 equivalents of HMPA. The formal potential of the [Sm(hmpa)4(thf)2]3+ 2I-/[Sm(hmpa)4(thf)2]2- 2I- redox couple determined by cyclic voltammetry was -1.79 +/- 0.08 V versus SCE. The order of reactivity of the samarium(II) complexes was found to be [Sm(hmpa)6]2+2I- > [Sm(hmpa)4(thf)2]2+2I- > SmI2 in their respective reactions with 1-iodobutane and with benzyl chloride. Very high rate enhancements, of the order of 1,000-15,000-fold, were observed upon addition of HMPA to the THF solution containing SmI2, Comparison of these rate constants with the corresponding rate constants for electron transfer (ET) reactions involving aromatic radical anions revealed that none of the reactions studied can be classified as outer-sphere ET processes and that the inner-sphere electron-donating abilities of the [Sm(hmpa)4(thf)2]2+ 2I- and SmI2 complexes are comparable. The inner-sphere ET character of the transition state increases on going from 1-iodobutane and benzyl bromide to benzyl chloride and acetophenone.