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

Abstract

Outer-sphere electron transfer from styrene, thioanisole, and toluene derivatives to a triflic acid (HOTf)-bound nonheme Mn(IV)-oxo complex, [(N4Py)-mn Mn-IV(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)-Mn-IV(O)](2+)-(HOTf)(2). The rate constants of HOTf-promoted epoxidation of all styrene derivatives with [(N4Py)-Mn-IV(O)](2+) and electron transfer from electron donors to [(N4Py)Mn-V(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)Mn-IV(O)](2+)-(HOTf)(2) to thioanisole derivatives as well as in the hydrogen atom transfer from toluene derivatives to [(N4Py)Mn-IV(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)Mn-IV(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)Mn-IV(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)Mn-IV(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.