C O MMU N I C A T I O N S
Table 1. Rate Constants for Reactions of Mn -Oxo Intermediatesa
V
than relative rate constants that are obtained under catalytic turnover
conditions, and comparisons of the reactivity of various metal-oxo
species with one substrate are possible. Given that Mn -oxo
V
ox (M-1 s-1
)
Mn -oxo
substrate
k
V
5
5
5
4
4
3
4
3
2
cis-stilbene
(6.1 ( 0.3) × 10
(1.3 ( 0.3) × 10
(1.28 ( 0.03) × 10
(5.5 ( 0.5) × 10
(1.1 ( 0.1) × 10
<2 × 10
intermediates are more reactive than analogous iron-oxo species,
the potential is good for extending the methods for studies of iron-
oxo species and possibly for generation of iron-oxo intermediates
diphenylmethane
ethylbenzene
ethylbenzene-d10
cis-stilbene
diphenylmethane
cis-stilbene
4
6
15
in enzymes.
b
(4.3 ( 0.3) × 10
(5.8 ( 0.1) × 10
Acknowledgment. This work was supported by a grant from
the National Institutes of Health (GM-48722). We thank Dr. John
H. Horner for assistance with the LFP experiments.
diphenylmethane
a
In acetonitrile at (22 ( 2) °C. The results for 2 are from 2 to 4
determinations, and those for 4 and 6 are from 1 to 2 determinations. b The
reaction was too slow to determine a rate constant accurately; the value is
the lower limit of the method.
Supporting Information Available: Experimental details, results
of LFP experiments, and results of competition experiments (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(
1) Watanabe, Y.; Fujii, H. In Metal-Oxo and Metal-Peroxo Species in
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9. Meunier, B. Chem. ReV. 1992, 92, 1411-1456. Feichtinger, D.;
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2) Yachandra, V. K.; Sauer, K.; Klein, M. P. Chem. ReV. 1996, 96, 2927-
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(
3) Groves, J. T.; Lee, J.; Marla, S. S. J. Am. Chem. Soc. 1997, 119, 6269-
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Figure 3. Left: Time-resolved spectrum of 6, produced by LFP of complex
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5
in acetonitrile, reacting with diphenylmethane over 11 ms. Right: Time-
III
resolved spectrum from reaction of Mn (TMPyP)(Cl) with m-CPBA in
acetonitrile over 40 s. The growing absorbances are from Mn species.
III
(5) MacDonnell, F. M.; Fackler, N. L. P.; Stern, C.; O’Halloran, T. V. J.
Am. Chem. Soc. 1994, 116, 7431-7432. Miller, C. G.; Gordon-Wylie, S.
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Note that the formation of 6 in acetonitrile was “instant” in the stopped-
3
flow unit under our reaction conditions.
1
1541. Gross, Z.; Golubkov, G.; Simkhovich, L. Angew. Chem., Int. Ed.
2
000, 39, 4045-4047. Mandimutsira, B. S.; Ramdhanie, B.; Todd, R.
highly stereoselective formation of cis-stilbene oxide (>95:5, cis:
C.; Wang, H.; Zareba, A. A.; Czemuszewicz, R. S.; Goldberg, D. P. J.
Am. Chem. Soc. 2002, 124, 15170-15171. Liu, H.-Y.; Lai, T.-S.; Yeung,
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III
trans) from oxidations of cis-stilbene by catalytic Mn (TPFPP)-
(
Cl) with m-CPBA and by photoactivated (300-400 nm irradiation)
(
6) Suslick, K. S.; Watson, R. A. New J. Chem. 1992, 16, 633-642. Hennig,
III
Mn (TPFPP)(ClO
4 3 3
) in CH CN and in PhCF solutions.
H. Coord. Chem. ReV. 1999, 182, 101-123.
III
(7) Abbreviations: TPP ) tetraphenylporphyrinato, TPFPP ) tetrakis-
Manganese(V)-oxo species also were generated by LFP of Mn -
(
pentafluorophenyl)porphyrinato; TMPyP ) tetra(4-(N-methylpyridini-
7
III
7
(TPP)(ClO
4
) (3), giving oxo species 4, and of Mn (TMPyP)(ClO
4
)
umyl))porphyinato; m-CPBA ) m-chloroperoxybenzoic acid.
V
(8) Suslick, K. S.; Acholla, F. V.; Cook, B. R. J. Am. Chem. Soc. 1987, 109,
(5), giving oxo species 6. Groves et al. produced the Mn -oxo
2
818-2819.
species 6 via stopped-flow methods in water, where λmax was at
(
9) The efficiencies of the LFP reactions are functions of the solvent, the
3
13
porphyrin ligand, and the added substrates.
4
43 nm, but 6 has λmax at 451 nm in acetonitrile. Figure 3 shows
(
10) For the photochemical generation of MnIV-oxo porphyrin intermediates,
time-resolved UV-vis spectra of 6 produced by photolysis of the
see: Suslick, K. S.; Watson, R. A. Inorg. Chem. 1991, 30, 912-919.
III
IV
perchlorate salt 5 and from the reaction of Mn (TMPyP)(Cl) with
The Mn -oxo species did not react rapidly with substrates (Supporting
Information).
m-CPBA. In the absence of substrates, decay of 6 was faster than
(
11) Cagnina, A.; Campestrini, S.; Di Furia, F.; Ghiotti, P. J. Mol. Catal. A
1998, 130, 221-231.
III
8
formation of Mn species. In the reaction with m-CPBA, we used
3
equiv of oxidant to drive the reaction to formation of the MnV-
(12) Variations in ratios as a function of sacrificial oxidant and solvent might
reflect multiple active oxidant forms. See: Collman, J. P.; Chien, A. S.;
Eberspacher, T. A.; Brauman, J. I. J. Am. Chem. Soc. 2000, 122, 11098-
11100. Newcomb, M.; Shen, R.; Choi, S. Y.; Toy, P. H.; Hollenberg, P.
F.; Vaz, A. D. N.; Coon, M. J. J. Am. Chem. Soc. 2000, 122, 2677-
2686.
oxo species 6, and the rate of decay of 6 is a measure of the rate
V
of consumption of m-CPBA. We note that Mn -oxo species 2 was
not detected from reaction of Mn(TPFPP)(Cl) with m-CPBA.
Second-order rate constants for reaction of 4 with cis-stilbene
and for reaction of 6 with cis-stilbene and diphenylmethane are
(
13) The absorbance change is a solvent effect. For example, λmax of the Soret
III
band of Mn (TMPyP)(Cl) changes from 462 nm in water to 470 nm in
3
CH CN.
V
(14) Dolphin, D.; Traylor, T. G.; Xie, L. Y. Acc. Chem. Res. 1997, 30, 251-
listed in Table 1. The reactivities of the Mn -oxo intermediates
259 and references therein. Palucki, M.; Finney, N. S.; Pospisil, P. J.;
with cis-stilbene and with diphenylmethane are in the order 2 > 6
Guler, M. L.; Ishida, T.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120,
948-954.
>
4, which is in agreement with the general observation that more
(
15) Porphyrin-iron(IV)-oxo radical cations, putative intermediates in cyto-
chrome P450-catalyzed oxidations, have not been observed in catalytic
processes, even at low temperatures. See: Davydov, R.; Macdonald, I.
D. G.; Makris, T. M.; Sligar, S. G.; Hoffman, B. M. J. Am. Chem. Soc.
highly electrophilic metal-oxo complexes, by virtue of the electron-
1
4
withdrawing aryl groups, are more reactive oxidants.
V
LFP formation of Mn -oxo species demonstrates that highly
reactive transition metal-oxo intermediates can be produced pho-
tochemically and studied directly. The method permits measure-
ments of absolute rate constants for oxidations of substrates rather
1
999, 121, 10654-10655. Davydov, R.; Makris, T. M.; Kofman, V.;
Werst, D. E.; Sligar, S. G.; Hoffman, B. M. J. Am. Chem. Soc. 2001,
23, 1403-1415.
1
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J. AM. CHEM. SOC.
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