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Unified Mechanism of Oxygen Atom Transfer and Hydrogen Atom Transfer Reactions with a Triflic Acid-Bound Nonheme Manganese(IV)–Oxo Complex via Outer-Sphere Electron Transfer
journal contribution
posted on 2019-01-15, 00:00 authored by Yong-Min Lee, Surin Kim, Kei Ohkubo, Kyung-Ha Kim, Wonwoo Nam, Shunichi FukuzumiOuter-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.
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4Pyouter-sphere electron-transfer reactionsrate constantsoxygen atom transferHydrogen Atom Transfer Reactionsforce dependenceouter-sphere electron transferthioanisole derivativesOuter-Sphere Electron Transfer Outer-sphere electron transferOxygen Atom Transferelectron transfertoluene derivativesstyrenestilbeneepoxidationIVHOTfhydrogen atom transfer
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