Unified View of Oxidative C–H Bond Cleavage and Sulfoxidation by a Nonheme Iron(IV)–Oxo Complex via Lewis Acid-Promoted Electron Transfer

Abstract

Oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by a nonheme iron(IV)-oxo complex, [(N4Py)Fe(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), were remarkably enhanced by the presence of triflic acid (HOTf) and Sc(OTf)3 in acetonitrile at 298 K. All the logarithms of the observed second-order rate constants of both the oxidative C-H bond cleavage and sulfoxidation reactions exhibit remarkably unified correlations with the driving forces of proton-coupled electron transfer (PCET) and metal ion-coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes between PCET and MCET were taken into account, respectively. Thus, the mechanisms of both the oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by [(N4Py)Fe(IV)(O)](2+) in the presence of HOTf and Sc(OTf)3 have been unified as the rate-determining electron transfer, which is coupled with binding of [(N4Py)Fe(IV)(O)](2+) by proton (PCET) and Sc(OTf)3 (MCET). There was no deuterium kinetic isotope effect (KIE) on the oxidative C-H bond cleavage of toluene via the PCET pathway, whereas a large KIE value was observed with Sc(OTf)3, which exhibited no acceleration of the oxidative C-H bond cleavage of toluene. When HOTf was replaced by DOTf, an inverse KIE (0.4) was observed for PCET from both toluene and [Ru(II)(bpy)3](2+) (bpy =2,2'-bipyridine) to [(N4Py)Fe(IV)(O)](2+). The PCET and MCET reactivities of [(N4Py)Fe(IV)(O)](2+) with Brønsted acids and various metal triflates have also been unified as a single correlation with a quantitative measure of the Lewis acidity.

Synthetic High‐Valent M–O–X Oxidants

“…A protonated intermediate was assumed to be the key ET intermediate, although the hydroxide compound [Fe IV (OH)(N4Py)] 3+ ( 29H + ) was not spectroscopically observed. In later work, the binding of up to two molecules of HOTf [ 29–(HOTf) 2 or 29H 2 2+ ] was postulated . Analogous behaviour was observed in sulfoxidation reactions , .…”

Moving Protons and Electrons in Biomimetic Systems

“…121 Modeling such reactions is challenging because of the fleeting nature of the intermediates, and the lack of a strong chromophore. In terms of PCET reactivity, among the most comprehensively investigated systems are those of Borovik, 45 Collins, 122 Nam 123 and Que 124 (examples given in Figure 11). In addition, the design criteria and reaction chemistry from Collins and co-workers provide some insight into important enzymatic features that protect proteins from oxidation by their own cofactors.…”

High-Valent Manganese–Oxo Valence Tautomers and the Influence of Lewis/Brönsted Acids on C–H Bond Cleavage

“…16 The addition of Lewis/Brönsted acids (e.g., Sc 3+ , HOTf) has been found to enhance electron transfer (ET), OAT, and PCET (PCET = proton-coupled electron transfer) rates of nonheme Fe IV (O) complexes. 18 Addition of triflic acid and Sc(OTf) 3 to nonheme Mn IV (O) complexes was found to accelerate ET and OAT rates but inhibit rates of H-atom abstraction. 19,20 Nonredox active metal ions have also been found to accelerate the rates and improve efficiencies of Mn-mediated catalytic oxidation reactions.…”