Mechanistic insight into concerted proton-electron transfer of a Ru(IV)-oxo complex: A possible oxidative asynchronicity

Article abstract of DOI:10.1021/jacs.0c05738

We have thoroughly investigated the oxidation of benzyl alcohol (BA) derivatives by a RuIV(O) complex (RuIV(O)) in the absence or presence of Br?nsted acids in order to elucidate the proton-coupled electron-transfer (PCET) mechanisms in C-H oxidation on the basis of a kinetic analysis. Oxidation of BA derivatives by RuIV(O) without acids proceeded through concerted proton-electron transfer (CPET) with a large kinetic isotope effect (KIE). In contrast, the oxidation of 3,4,5-trimethoxy- BA ((MeO)3-BA) by RuIV(O) was accelerated by the addition of acids, in which the KIE value reached 1.1 with TFA (550 mM), indicating an alteration of the PCET mechanism from CPET to stepwise electron transfer (ET) followed by proton transfer (PT). Although the oxidized products of BA derivatives were confirmed to be the corresponding benzaldehydes in the range of acid concentrations (0-550 mM), a one-electron-reduction potential of RuIV(O) was positively shifted with increases in the concentrations of acids. The elevated reduction potential of RuIV(O) strongly influenced the PCET mechanisms in the oxidation of (MeO)3-BA, changing the mechanism from CPET to ET/PT, as evidenced by the driving-force dependence of logarithms of reaction rate constants in light of the Marcus theory of ET. In addition, dependence of activation parameters on acid concentrations suggested that an oxidative asynchronous CPET, which is not an admixture of the CPET and ET/PT mechanisms, is probably operative in the boundary region (0 mM < [TFA] < 50 mM) involving a one-protoninteracted RuIV(O) H+ as a dominant reactive species.

Full text of DOI:10.1021/jacs.0c05738

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Article
Mechanistic Insight into Concerted Proton-Electron Transfer
of a Ru(IV)-Oxo Complex: A Possible Oxidative Asynchronicity
Hiroaki Kotani, hinatsu shimomura, Kei Ikeda, Tomoya Ishizuka,
Yoshihito Shiota, Kazunari Yoshizawa, and Takahiko Kojima
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c05738 • Publication Date (Web): 14 Sep 2020
Downloaded from pubs.acs.org on September 14, 2020
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