Three phenols with pendant, hydrogen-bonded bases (HOAr-B) have been oxidized in MeCN with various one-electron oxidants. The bases are a primary amine (-CPh(2)NH(2)), an imidazole, and a pyridine. The product of chemical and quasi-reversible electrochemical oxidations in each case is the phenoxyl radical in which the phenolic proton has transferred to the base, (*)OAr-BH(+), a proton-coupled electron transfer (PCET) process. The redox potentials for these oxidations are lower than for other phenols, predominately from the driving force for proton movement. One-electron oxidation of the phenols occurs by a concerted proton-electron transfer (CPET) mechanism, based on thermochemical arguments, isotope effects, and DeltaDeltaG(++)/DeltaDeltaG degrees . The data rule out stepwise paths involving initial electron transfer to form the phenol radical cations [(*)(+)HOAr-B] or initial proton transfer to give the zwitterions [(-)OAr-BH(+)]. The rate constant for heterogeneous electron transfer from HOAr-NH(2) to a platinum electrode has been derived from electrochemical measurements. For oxidations of HOAr-NH(2), the dependence of the solution rate constants on driving force, on temperature, and on the nature of the oxidant, and the correspondence between the homogeneous and heterogeneous rate constants, are all consistent with the application of adiabatic Marcus theory. The CPET reorganization energies, lambda = 23-56 kcal mol(-)(1), are large in comparison with those for electron transfer reactions of aromatic compounds. The reactions are not highly non-adiabatic, based on minimum values of H(rp) derived from the temperature dependence of the rate constants. These are among the first detailed analyses of CPET reactions where the proton and electron move to different sites.

A series of seven substituted 4,6-di-tert-butyl-2-(4,5-diarylimidazolyl)-phenols have been prepared and characterized, along with two related benzimidazole compounds. X-ray crystal structures of all of the compounds show that the phenol and imidazole rings are close to coplanar and are connected by an intramolecular ArOH⅐ ⅐ ⅐N hydrogen bond. One-electron oxidation of these compounds occurs with movement of the phenolic proton to the imidazole base by concerted proton-electron transfer (CPET) to yield fairly stable distonic radical cations. These phenol-base compounds are a valuable system in which to examine the key features of CPET. Kinetic measurements of bimolecular CPET oxidations, with E rxn between ؉0.04 and ؊0.33 V, give rate constants from (6.3 ؎ 0.6) ؋ 10 2 to (3.0 ؎ 0.6) ؋ 10 6 M ؊1 s ؊1 . There is a good correlation of log(k) with ⌬G°, with only one of the 15 rate constants falling more than a factor of 5.2 from the correlation line. Substituents on the imidazole affect the (O-H⅐ ⅐ ⅐N) hydrogen bond, as marked by variations in the 1 H NMR and calculated vibrational spectra and geometries. Crystallographic dO⅐ ⅐ ⅐N values appear to be more strongly affected by crystal packing forces. However, there is almost no correlation of rate constants with any of these measured or computed parameters. Over this range of compounds from the same structural family, the dominant contributor to the differences in rate constant is the driving force ⌬G°.Marcus Theory ͉ oxyl radicals ͉ proton-coupled ͉ ROS ͉ tyrosyl radicals R eactive oxygen species (ROS) exhibit a wide range of reactivity, from the extraordinarily potent hydroxyl radical to much more inert aryloxyl and ascorbyl radicals. These species are sometimes categorized by their redox potentials at pH 7, but most reactions of ROS do not proceed by simple electron transfer (ET). Typically, ROS react by proton-coupled electron transfer (PCET), as in the disproportionation of hydrogen peroxide to dioxygen and water and the interconversions of tyrosyl radicals and tyrosine residues. Understanding the detailed mechanisms of these and other PCET

Proton-coupled electron transfer (PCET) reactions are of much current interest because of their role in many chemical and biological processes.[1] The oxidation of tyrosine residues to tyrosyl radicals, for example, is important in the function of a number of proteins.[2] The Kok S-state cycle of photosystem II involves oxidations of tyrosine-Z through longrange electron transfer to P 680 + whereby the phenolic proton is likely transferred to a hydrogen-bonded imidazole (H 190 ) in a single kinetic step, [3] called separated CPET (concerted proton-electron transfer).[4] This conclusion has been generally supported by studies of phenol model systems in which electron transfer (ET) is coupled to proton transfer (PT) to a different species. [4][5][6][7][8][9] Concerted transfer of e À and H + is often preferred because DG CPET is more favorable than DG ET for initial electron transfer. [4][5][6][7][8][9] When the driving forces are equal, however, separated-CPET reactions typically occur with lower rate constants than related ET reactions. The origin of the slower rates has been discussed in theoretical treatments [1,10] and the importance of hydrogen bonding has been noted by Hammarström and co-workers.[6b] Herein, we report that the nature of the hydrogen bond has a large influence on the facility of CPET.We recently described chemical and electrochemical oxidations of phenol base compounds HOAr-B (B = base) that proceed by intermolecular ET concerted with intramolecular PT to the hydrogen-bonded base (Scheme 1). [9] CPET reactions of pyridylphenol 1 are around 10 2 times faster than those of amine analogue 2 with the same type of oxidant and for the same value of DG CPET .[9] This result was unexpected because free pyridine is a weaker base than a primary amine (pK BH + = 12 and 18, respectively, in MeCN). [11] The difference between 1 and 2 is unlikely to be due to different outer-sphere (solvent) reorganization because these molecules are of comparable size and should have similar changes in solvation upon CPET. The average crystallographic proton donor-acceptor distance d(O···N) is slightly shorter in 1 than in 2 (by 0.01 ).[9] However, this distance does not appear to be the dominant factor as a phenolimidazole analogue of 1 has a much longer d(O···N) than 2 (by 0.06 ) yet is oxidized substantially faster.[9] The key feature seems to be that the faster phenol-pyridine and phenolimidazole compounds contain resonance-assisted hydrogen bonds (RAHBs) owing to conjugation between the proton donor and acceptor. This conjugation leads to stronger hydrogen bonds and influences the NMR and IR spectra (see below). [9, 12] To test the apparent correlation between RAHBs and increased CPET rates, the nonconjugated phenol-pyridine 4,6-di-tert-butyl-2-(2'-pyridinylmethyl)phenol (3) was prepared. 2-Pyridylmagnesium chloride (2-pyMgCl) was added to benzyl ether protected 3,5-di-tert-butylsalicylaldehyde, followed by acylation of the resulting alcohol and catalytic hydrogenation. Compound 3 has been fully characterized, in...

Crystals of the 2,4,6-tri-tert-butylphenoxyl radical have been isolated and characterized by X-ray diffraction, and calculations have been performed that give the distribution of spin density in the radical.

A new reaction for carbon: Separating the electron and proton allows C–H bonds to be cleaved and formed with mild reagents.