Kinetic Effects of Increased Proton Transfer Distance on Proton-Coupled Oxidations of Phenol-Amines

Abstract

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.

Semiclassical evaluation of kinetic isotope effects in 13-atomic system

“…7,9,10 Nuclear zero-point energy effects can result in the values of KIE up to k H /k D = 7 at room temperatures, 11,12 while higher values of KIE suggest involvement of quantum mechanical tunneling in the mechanism of hydrogen transfer. [13][14][15][16] A KIE close to 100 has been experimentally observed in reactions of enzyme catalysis even at physiological temperatures, 17 and KIE of more than 400 has been reported for a non-enzymatic system. 18 Development of an adequate theoretical approach for quantitative description of KIEs is of much interest for understanding the physical origin of the observed KIEs and mechanisms of the underlying chemical reactions.…”

Influence of the Protonatable Site in the Photo-Induced Proton-Coupled Electron Transfer between Rhenium(I) Polypyridyl Complexes and Hydroquinone

“…As the ET-PT is likely the major pathway in the quenching reaction, it is not expected significant isotope effects, as previous discussed by Mayer and co-workers. 44 Nevertheless, the slightly superior KIE value found for 2 may indicate a higher contribution of concerted electron-proton transfer (EPT) pathway for this complex in relation to 3 and 4, since the PT-ET pathway is thermodynamically improbable. It is also important to highlight that the absence of significant KIEs is not an argument against EPT, since this pathway can involve vibrationally excited species.…”

“…It is also important to highlight that the absence of significant KIEs is not an argument against EPT, since this pathway can involve vibrationally excited species. 10,44 An additional reaction pathway that could be considered is the initial proton transfer after MLCT excitation of 2 (*PT-ET), followed by reduction of the protonated complex, but in this case, a much higher KIE is expected. Moreover, the determined KIE values do not justify by themselves the differences of the k q values between 2 and 3 in bare acetonitrile.…”

Experimental and theoretical insights into photochemical transformation kinetics and mechanisms of aqueous propylparaben and risk assessment of its degradation products

“…The hydrogen transfer reaction was further evaluated by assessing the deuterium kinetic isotope effect, because the exchangeable O-H protons in PPB could be replaced by deuterons from the large molar excess of D 2 O in the solution [33,34]. Experiments were performed in N 2 -saturated H 2 O and D 2 O solutions with lower irradiation energy, to produce the PPB degradation kinetics curve and some formation products ( Figure 7).…”