Equilibrium between 2-oxomorpholin-3-yl radicals and viologen radicals. Determination of reduction potentials

Abstract

via a bond. For H202 and hydroquinone as reducing agents there are acidic protons in bonds and redox orbitals which are either ow or ?r in character.In a simultaneous electron-proton transfer, intramolecular electronic interactions between and the redox orbitals lead to electronic communication between sites and the correlated transfer of both groups to the oxidant. The critical mode is a Y (~~) stretch and the motions of the electron and proton are coupled.The utilization of a simultaneous electron/proton-transfer pathway places special demands on the oxidant as well as the reductant. The oxidant must be capable of accepting both an electron and a proton simultaneously. This requirement is met by either [ (bpy)2(py)Ru1v(0)] 2+ or [(bpy)2(py)Ru11'(OH)] 2+ as oxidants. These contain both electron and proton acceptor capabilities, which are also based on separate sites. The electron is accepted into a vacancy in the d n orbitals. The proton is transferred to the 0-atom of, what is initially, the oxo group of Ru(IV) or the hydroxo group of Ru(II1). The reactions in Scheme V stand in contrast to the allylic oxidation of olefins3la or the oxidations of HC02or of PhCH20H by [(bpy)2(py)Ru1v(0)]2+. In these reactions H 2 0 / D 2 0 kinetic isotope effects are negligible, but large C-H/C-D kinetic isotope effects are ~b s e r v e d .~*~' It has been suggested in these cases that the mechanisms involve direct attack on a C-H bond. The C-H bond serves as the source of both electrons and protons. Both are transferred in the redox step.We propose that there are at least two distinct mechanisms for simultaneous electron/proton transfer. We also propose that one (31) (a) Curry, M.; Dobson, J. C.; Stultz, L.; Meyer, T. J., manuscript in preparation. (b) Dobson, J. C.; Seok, W. K.; Meyer, T. J. Inorg. Chem. 1986, 25, 1514.be called proton-coupled electron transfer. In this mechanism (Scheme V) the electron and proton donor functions are located at separate sites in the reductant but are transferred simultaneously. It can be anticipated that other examples will be found based on proton transfer from dissociable -NH or -SH bonds.Although the examples studied here involve one-electron transfer, there is also the possibility that proton-coupled two-electron transfer may occur in some reactions. In a second mechanism involving simultaneous electron-proton transfer, the transfer occurs from the same bond. We propose that the term hydrogen atom transfer be reserved for this direct transfer of an e-/H+ pair from the same bond of the reductant to the oxidant.

Free-Radical Reactions in the Synthesis of α-Amino Acids and Derivatives

“…The radicals formed by bond homolysis of the dimers act as one electron reducing agents, reflecting the ease of electron transfer from α-carbon-centered amino acid radicals, referred to above. Reductions by the dimers of compounds such as adriamycin and daunomycin have been studied in detail, as models for the in vivo manipulation of quinone antitumor drugs. ,,,−

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“…Reductions by the dimers of compounds such as adriamycin and daunomycin have been studied in detail, as models for the in vivo manipulation of quinone antitumor drugs. 240,245,246,[248][249][250][251][252][253][254][255][256][257][258] As a final example of the production of amino acid radicals through functional group transformations, photolysis of the pyrazolines 232a-c gave the corresponding cyclopropylamino acid derivatives 234a-…”

Remote Control of Electron Transfer Reaction by Microwave Irradiation: Kinetic Demonstration of Reduction of Bipyridine Derivatives on Surface of Nickel Particle

“…of Diquat (DQ •+ ), 14 Methylviologen (MV •+ ), 15 and N,N′ethylene-2,2′-bipyridinium (3DQ •+ ). 16 In these experiments, we used a microwave cavity with ellipsoidal shape to generate the TM 110 mode of 2.45 GHz microwaves, where the sample vessels are positioned at the antinode of the alternating electric field (see Supporting Information). Here, the temperature was maintained at 373 K under both microwave and oil bath heating.…”

“…Figure a–c shows the numbers of reduced Bpys 2+ radical cations generated by a one-electron transfer reaction from Ni particles dispersed in water, which were estimated from UV–vis absorption spectra (Figures S1 and S2) of reductant cations of Diquat (DQ ·+ ), Methylviologen (MV ·+ ), and N , N ′-ethylene-2,2′-bipyridinium (3DQ •+ ) . In these experiments, we used a microwave cavity with ellipsoidal shape to generate the TM 110 mode of 2.45 GHz microwaves, where the sample vessels are positioned at the antinode of the alternating electric field (see Supporting Information).…”