Journal of the American Chemical Society
Page 4 of 6
mechanism is supported by the present and previous stud-
ies.17-25 The sequential addition of two protons to the oxo lig-
and of II is consistent to the titration experiment of AcOH
(Figure S1). The binding of two protons has been reported for
nonheme oxomanganese(IV) complexes.18 The conversion
between II and iron(III) porphyrin π-cation radical with acid
and base has been reported in many studies.7, 19-21 The ex-
change of the axial ligand to X is also reasonable because the
aqua ligand has been known as a very weak axial ligand.22-23
The oxidation of II to I with an iron(III) porphyrin π-cation
radical has been reported by Balch et al.24 and also supported
by the redox potentials: E1/2(II) < E1/2(I).25 In addition, the
rate law based on this reaction mechanism predicts the first-
order for the concentration of II and the second-order for the
concentration of HX when the concentration of AcOH is low
(see S.I.). This is also consistent to the present kinetic study.
Finally, we studied the effect of the disproportionation reac-
tion of II on the oxidation of substrate. We examined the reac-
tion of TMP-II with p-methoxystyrene and cyclooctene in the
absence and presence of AcOH. II has been reported to be a
less reactive compound than the corresponding I.6-7 Therefore,
the reactions of TMP-II with p-methoxystyrene and cy-
clooctenewere are very slow and afforded corresponding
epoxides in 9 and 2 %, respectively. On the other hand, the re-
actions of TMP-II with p-methoxystyrene and cyclooctene in
the presence of AcOH (2.0 equiv) produced corresponding
epoxides in 24 % and 12 %, respectively. The reactions of au-
thentic TMP-I afford the epoxides in 61 % and 39 % yields.
These results indicate that the yields of the epoxides from the
disproportionation reaction of TMP-II are higher than those
from TMP-II and about half for those from TMP-I. These re-
sults clearly indicate that the disproportionation reaction of II
increases the yield of the product by the formation of I and
may be a significant reaction pathway when II is generated in
the presence of an acid under catalytic conditions.
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Funding Sources
No competing financial interests have been declared.
ACKNOWLEDGMENT
This work was supported by grants from JSPS (Grant 17H03032
and 19H04581) and CREST. The authors thank IMS for accom-
modation of NMR and EPR measurements.
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In conclusion, we show direct evidence on the dispropor-
tionation reaction of oxoiron(IV) porphyrin complex with ab-
2
sorption, H NMR, and EPR spectroscopy. Kinetic study of
the disproportionation reaction indicates the second-order
for the concentration of proton and the first-order for the con-
centration of II. Based on these results, we propose the dispro-
portionation mechanism, involving the sequential addition of
two protons to the oxo ligand of II, followed by the oxidation
of another II to afford I and III.
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is avail-
able free of charge on the ACS Publications website. Experi-
mental and computational details, Figures S1 – S13, Table S1,
and derivation of reaction rate based on Scheme 1.
16. Kujime, M.; Fujii, H., Spectroscopic Characterization of Reaction
Intermediates in a Model for Copper Nitrite Reductase. Angew. Chem. Int.
Ed. 2006, 45, 1089-92.
17. Ehudin, M. A.; Gee, L. B.; Sabuncu, S.; Braun, A.; Moenne-Loccoz, P.;
Hedman, B.; Hodgson, K. O.; Solomon, E. I.; Karlin, K. D., Tuning the
Geometric and Electronic Structure of Synthetic High-Valent Heme
Iron(IV)-Oxo Models in the Presence of a Lewis Acid and Various Axial
Ligands. J. Am. Chem. Soc. 2019, 141, 5942-5960.
AUTHOR INFORMATION
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