Hydroxylation of mesitylene by a nonheme manganese(IV)-oxo complex, [(N4Py)Mn(IV) (O)](2+) (1), proceeds via one-step hydrogen-atom transfer (HAT) with a large deuterium kinetic isotope effect (KIE) of 3.2(3) at 293 K. In contrast, the same reaction with a triflic 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 K in the reaction of 2, however, the mechanism changes again from ET to HAT with a large KIE of 2.9(3). Such a switchover of the reaction mechanism from ET to HAT is shown to occur by changing only temperature in the boundary region between ET and HAT pathways when the driving force of ET from toluene derivatives to 2 is around -0.5 eV. The present results provide a valuable and general guide to predict a switchover of the reaction mechanism from ET to the others, including HAT.

We report the oxidation of cyclic olefins, such as cyclohexene, cyclohexene-d10, and cyclooctene, by mononuclear nonheme manganese(IV)-oxo (Mn(IV)O) and triflic acid (HOTf)-bound Mn(IV)O complexes. In the oxidation of cyclohexene, the Mn(IV)O complexes prefer the C-H bond activation to the C═C double bond epoxidation, whereas the C═C double bond epoxidation becomes a preferred reaction pathway in the cyclohexene oxidation by HOTf-bound Mn(IV)O complexes. In contrast, the oxidation of cyclohexene-d10 and cyclooctene by the Mn(IV)O complexes occurs predominantly via the C═C double bond epoxidation. This conclusion is drawn from the product analysis and kinetic studies of the olefin oxidation reactions, such as the epoxide versus allylic oxidation products, the formation of Mn(II) versus Mn(III) products, and the kinetic analyses. Overall, the experimental results suggest that the energy barrier of the C═C double bond epoxidation is very close to that of the allylic C-H bond activation in the oxidation of cyclic olefins by high-valent metal-oxo complexes. Thus, the preference of the reaction pathways is subject to changes upon small manipulation of the reaction environments, such as the supporting ligands and metal ions in metal-oxo species, the presence of HOTf (i.e., HOTf-bound Mn(IV)O species), and the allylic C-H(D) bond dissociation energies of olefins. This is confirmed by DFT calculations in the oxidation of cyclohexene and cyclooctene, which show multiple pathways with similar rate-limiting energy barriers and depending on the allylic C-H bond dissociation energies. In addition, the possibility of excited state reactivity in the current system is confirmed for epoxidation reactions.

Outer-sphere electron transfer from styrene, thioanisole, and toluene derivatives to a triflic acid (HOTf)-bound nonheme Mn­(IV)–oxo complex, [(N4Py)­MnIV(O)]2+–(HOTf)2 (N4Py = N,N-bis­(2-pyridylmethyl)-N-bis­(2-pyridyl)­methylamine), has been shown to be the rate-determining step of different types of redox reactions such as epoxidation, sulfoxidation, and hydroxylation of styrene, thioanisole, and toluene derivatives, respectively, by [(N4Py)­MnIV(O)]2+–(HOTf)2. The rate constants of HOTf-promoted epoxidation of all styrene derivatives with [(N4Py)­MnIV(O)]2+ and electron transfer from electron donors to [(N4Py)­MnV(O)]2+ exhibit a remarkably unified correlation with the driving force of outer-sphere electron transfer in light of the Marcus theory of electron transfer. The same electron-transfer driving force dependence is observed in the oxygen atom transfer from [(N4Py)­MnIV(O)]2+–(HOTf)2 to thioanisole derivatives as well as in the hydrogen atom transfer from toluene derivatives to [(N4Py)­MnIV(O)]2+–(HOTf)2. Thus, mechanisms of oxygen atom transfer (epoxidation and sulfoxidation) reactions of styrene and thioanisole derivatives and hydrogen atom transfer (hydroxylation) reactions of toluene derivatives by [(N4Py)­MnIV(O)]2+–(HOTf)2 have been unified for the first time as the same reaction pathway via outer-sphere electron transfer, followed by the fast bond-forming step, which exhibits the singly unified electron-transfer driving force dependence of the rate constants as outer-sphere electron-transfer reactions. In the case of the epoxidation of cis-stilbene by [(N4Py)­MnIV(O)]2+–(HOTf)2, the isomerization of cis-stilbene radical cation to trans-stilbene radical cation occurs after outer-sphere electron transfer from cis-stilbene to [(N4Py)­MnIV(O)]2+–(HOTf)2 to yield trans-stilbene oxide selectively, which is also taken as evidence for the occurrence of electron transfer in the acid-catalyzed epoxidation.

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.