Oxidation Potentials of α-Hydroxyalkyl Radicals in Acetonitrile Obtained by Photomodulated Voltammetry

Abstract

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.

Tandem Intramolecular Hydroalkoxylation–Hydroarylation Reactions: Synthesis of Enantiopure Benzofused Cyclic Ethers from the Chiral Pool

“…Indeed, at a low starting propan‐2‐ol concentration (= 0.1 m ), Φ red (SS)≅2× Φ red (FD), indicating that virtually every ketyl radical is oxidized. This is in accord with the easy oxidation of such radicals— E (Me 2 C ⋅ OH/Me 2 C + OH) = −0.61 V versus SCE27—and, coupled with the rapid deprotonation of the cation to give acetone, supports electron transfer according to Equation (7) as a viable pathway 19. The rate constant for radical reaction ( k ox ) was again determined by competition with addition to electrophilic alkenes (see below).…”

A comprehensive picture of the one-electron oxidation chemistry of enols, enolates and α-carbonyl radicals: oxidation potentials and characterization of radical intermediates

“…The oxidation potential of 46d is similar to that of an a-hydroxyalkyl radical, 54 as reported by Wayner et al, 55 all of them placed in the range of À0.61 V SCE to þ0.38 V SCE depending on the substituents. It was also noted that the oxidation potentials of a-hydroxyalkyl radicals are strongly dependent on the solvent used and are significantly lower in water than those obtained in acetonitrile.…”

Photochemical Formation of Au/Cu Bimetallic Nanoparticles with Different Shapes and Sizes in a Poly(vinyl alcohol) Film

“…and the PVA radical in acetonitrile are -0.5 and -0.86 V versus NHE (normal hydrogen electrode), [11] respectively (the oxidation potential of (CH 3 ) 2 C . OH is quoted as the oxidation potential of the PVA radical).…”