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

Reactions that involve transfer of an electron and a proton can proceed by stepwise pathways involving initial electron transfer (ET) or initial proton transfer (PT), or by a concerted pathway without an intermediate. The concerted mechanism is termed proton-coupled electron transfer (PCET). Understanding such reactions requires knowledge of the thermodynamics of the possible ET, PT, and PCET steps. Many reactions have a large thermochemical bias favoring the PCET pathway. This bias is often sufficient to rule out stepwise mechanisms. The DeltaG degrees for ET, PT, or PCET has a strong influence on the rate of that step. Using the terminology of Marcus theory, PT and PCET reactions at C-H bonds have higher intrinsic barriers than such reactions at O-H or N-?H bonds. The intrinsic barriers to ET and PCET are often similar when there is a small intrinsic barrier to PT. Reactions with a thermochemical bias toward PCET and with similar intrinsic barriers for all the pathways are most likely to occur by concerted PCET.

To test the effect of varying the proton donor-acceptor distance in proton-coupled electron transfer (PCET) reactions, the oxidation of a bicyclic amino-indanol (2) is compared with that of a closely related phenol with an ortho CPh2NH2 substituent (1). Spectroscopic, structural, thermochemical and computational studies show that the two amino-phenols are very similar, except that the O⋯N distance (dON) is >0.1 Å longer in 2 than in 1. The difference in dON is 0.13 ± 0.03 Å from X-ray crystallography and 0.165 Å from DFT calculations. Oxidations of these phenols by outer-sphere oxidants yield distonic radical cations •OAr–NH3+ by concerted proton-electron transfer (CPET). Simple tunneling and classical kinetic models both predict that the longer donor-acceptor distance in 2 should lead to slower reactions, by ca. two orders of magnitude, as well as larger H/D kinetic isotope effects (KIEs). However, kinetic studies show that the compound with the longer proton-transfer distance, 2, exhibits smaller KIEs and has rate constants that are quite close to those of 1. For example, the oxidation of 2 by the triarylamminium radical cation N(C6H4OMe)3•+ (3a+) occurs at (1.4 ± 0.1) × 104 M-1 s-1, only a factor of two slower than the closely related reaction of 1 with N(C6H4OMe)2(C6H4Br)•+ (3b+). This difference in rate constants is well accounted for by the slightly different free energies of reaction: ΔG°(2 + 3a+) = +0.078 V vs. ΔG°(1 + 3b+) = +0.04 V. The two phenol-amines do display some subtle kinetic differences: for instance, compound 2 has a shallower dependence of CPET rate constants on driving force (Brønsted α, Δln(k)/Δln(Keq)). These results show that the simple tunneling model is not a good predictor of the effect of proton donor-acceptor distance on concerted-electron transfer reactions involving strongly hydrogen-bonded systems. Computational analysis of the observed similarity of the two phenols emphasizes the importance of the highly anharmonic O⋯H⋯N potential energy surface and the influence of proton vibrational excited states.

The hydrogen-bonded phenol 2-(aminodiphenylmethyl)-4,6-di-tert-butylphenol (HOAr-NH2) was prepared and oxidized in MeCN by a series of one-electron oxidants. The product is the phenoxyl radical in which the phenolic proton has transferred to the amine, *OAr-NH3+. The reaction of HOAr-NH2 and tris(p-tolyl)aminium ([N(tol)3]*+) to give *OAr-NH3+ + N(tol)3 has Keq = 2.0 +/- 0.5, follows second-order kinetics with k = (1.1 +/- 0.2) x 105 M-1 s-1 (DeltaG = 11 kcal mol-1), and has a primary isotope effect kH/kD = 2.4 +/- 0.4. Oxidation of HOAr-NH2 with [N(C6H4Br)3]*+ is faster, with k congruent with 4 x 107 M-1 s-1. The isotope effect, thermochemical arguments, and the dependence of the rate on driving force (DeltaDeltaG/DeltaDeltaG degrees = 0.53) all indicate that electron transfer from HOAr-NH2 must occur concerted with intramolecular proton transfer from the phenol to the amine (proton-coupled electron transfer, PCET). The data rule out stepwise paths that involve initial electron transfer to form the phenol radical cation *+HOAr-NH2 or that involve initial proton transfer to give the zwitterion -OAr-NH3+. The dependence of the electron-transfer rate constants on driving force can be fit with the adiabatic Marcus equation, yielding a large intrinsic barrier: lambda = 34 kcal mol-1 for reactions of HOAr-NH2 with NAr3*+.