Electron Transfer Properties of Alkoxyl Radicals. A Time-Resolved Kinetic Study of the Reactions of thetert-Butoxyl, Cumyloxyl, and Benzyloxyl Radicals with Alkyl Ferrocenes

Abstract

A time-resolved kinetic study on the reactions of the tert-butoxyl (t-BuO*), cumyloxyl (CumO*), and benzyloxyl (BnO*) radicals with alkylferrocenes has been carried out in MeCN solution. With all radicals, clear evidence for an electron transfer (ET) process has been obtained, and with the same ferrocene donor, the reactivity has been observed to increase in the order t-BuO* < CumO* < BnO*, with the difference in reactivity approaching 3 orders of magnitude on going from t-BuO* to BnO*. With BnO*, an excellent fit to the Marcus equation has been obtained, from which a value of the reduction potential of BnO* (E degrees(BnO*/BnO(-)) = 0.54 V/SCE) has been derived. The latter value appears, however, to be significantly higher than the previously determined reduction potential values for alkoxyl radicals and in contrast with the differences in the computed solution-phase electron affinities determined for t-BuO*, CumO*, and BnO*, indicating that the reaction of BnO* with ferrocene donors may not be described in terms of a straightforward outer sphere ET mechanism. From these data, and taking into account the available value of the reduction potential for CumO*, a value of E degrees (BnO*/BnO(-)) = -0.10 V/SCE has been estimated. On the basis of computational evidence for the formation of a pi-stacked prereaction complex in the reaction between BnO* and DcMFc, an alternative ET mechanism is proposed for the reactions of both CumO* and BnO*. In these cases, the delocalized nature of the unpaired electron allows for the aromatic ring to act as an electron relay by mediating the ET from the ferrocene donor to the formal oxygen radical center. This hypothesis is also in line with the observation that both BnO* and CumO* react with the ferrocene donors with rate constants that are in all cases at least 2 orders of magnitude higher than those measured for t-BuO*, wherein the radical is well-localized.