Switchover of the Mechanism between Electron Transfer and Hydrogen‐Atom Transfer for a Protonated Manganese(IV)–Oxo Complex by Changing Only the Reaction Temperature

Abstract

Hydroxylation of mesitylene by an onheme manganese(IV)-oxo complex, [(N4Py)Mn IV (O)] 2+ (1), proceeds via one-step hydrogen-atom transfer (HAT) with alarge deuterium kinetic isotope effect (KIE) of 3.2(3) at 293 K. In contrast, the same reaction with at riflic acid-bound manganese(IV)-oxo complex, [(N4Py)Mn IV (O)] 2+ -(HOTf) 2 (2), proceeds via electron transfer (ET) with no KIE at 293 K. Interestingly,when the reaction temperature is lowered to less than 263 Kinthe reaction of 2,however,the mechanism changes again from ET to HATwith al arge KIE of 2.9(3). Such aswitchover of the reaction mechanism from ET to HAT is shown to occur by changing only temperature in the boundary region between ET and HATp athwaysw hen the driving force of ET from toluene derivatives to 2 is around À0.5 eV.T he present results provideavaluable and general guide to predict as witchover of the reaction mechanism from ET to the others,including HAT.

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

“…18–20,50,51 Rate enhancements are observed if the driving force of electron transfer between the metal−oxo/Lewis acid adduct and the C−H substrate is reasonably favorable, as in the case of toluene derivatives as substrates. 18,51 For these substrates, the mechanism of C−H cleavage is described as proton-coupled electron transfer (PCET), where rate-limiting ET controls the kinetics and no KIE (i.e., KIE ≈ 1) for C−H/C−D is observed.…”

Oxidative Kinetic Resolution of Cyclic Benzylic Ethers

“…Communications might proceed through a pathway involving a sequential ratedetermining SET followed by a proton transfer. [19] Accordingly, electron transfer probe experiment was next conducted (Scheme 5 E). [20] Under standard conditions with flask open to air, cyclopropane 16 was efficiently transformed to endoperoxide 17 in 88 % yield.…”

Oxidative Kinetic Resolution of Cyclic Benzylic Ethers

“…Zuschriften might proceed through a pathway involving a sequential ratedetermining SET followed by a proton transfer. [19] Accordingly, electron transfer probe experiment was next conducted (Scheme 5 E). [20] Under standard conditions with flask open to air, cyclopropane 16 was efficiently transformed to endoperoxide 17 in 88 % yield.…”