Dramatically accelerated selective oxygen-atom transfer by a nonheme iron(IV)-oxo complex: Tuning of the first and second coordination spheres

Article abstract of DOI:10.1021/ja410240c

The new ligand N3Py^amideSR and its Fe^II complex [Fe^II(N3Py^amideSR)](BF₄)₂ (1) are described. Reaction of 1 with PhIO at -40 C gives metastable [Fe ^IV(O)(N3Py^amideSR)]²⁺ (2), containing a sulfide ligand and a single amide H-bond donor in proximity to the terminal oxo group. Direct evidence for H-bonding is seen in a structural analogue, [Fe ^II(Cl)(N3Py^amideSR)](BF₄)₂ (3). Complex 2 exhibits rapid O-atom transfer (OAT) toward external sulfide substrates, but no intramolecular OAT. However, direct S-oxygenation does occur in the reaction of 1 with mCPBA, yielding sulfoxide-ligated [Fe ^II(N3Py^amideS(O)R)](BF₄)₂ (4). Catalytic OAT with 1 was also observed.

Full text of DOI:10.1021/ja410240c

Communication
pubs.acs.org/JACS
Dramatically Accelerated Selective Oxygen-Atom Transfer by a
Nonheme Iron(IV)-Oxo Complex: Tuning of the First and Second
Coordination Spheres
Leland R. Widger,† Casey G. Davies,∥ Tzuhsiung Yang,† Maxime A. Siegler,† Oliver Troeppner,§
Guy N. L. Jameson,*,∥ Ivana Ivanovic-Burmazovic,§ and David P. Goldberg*,†
́
́
†Department of Chemistry, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States
∥Department of Chemistry & MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Otago, P.O. Box 56,
Dunedin 9054, New Zealand
§Department of Chemistry and Pharmacy, University of Erlangen-Nurnberg, 91058 Erlangen, Germany
̈
S
* Supporting Information
intermediate was detected. In contrast, the FeIV(O) complex
ABSTRACT: The new ligand N3PyamideSR and its FeII
complex [FeII(N3PyamideSR)](BF4)2 (1) are described.
Reaction of 1 with PhIO at −40 °C gives metastable
[FeIV(O)(N3PyamideSR)]2+ (2), containing a sulfide ligand
and a single amide H-bond donor in proximity to the
terminal oxo group. Direct evidence for H-bonding is seen
in a structural analogue, [FeII(Cl)(N3PyamideSR)](BF4)2
(3). Complex 2 exhibits rapid O-atom transfer (OAT)
toward external sulfide substrates, but no intramolecular
OAT. However, direct S-oxygenation does occur in the
reaction of 1 with mCPBA, yielding sulfoxide-ligated
[FeII(N3PyamideS(O)R)](BF4)2 (4). Catalytic OAT with 1
was also observed.
of the all-nitrogen analogue N4Py is well-characterized.3
Encouraged by our success in ligand design to incorporate an
S-bound substrate mimic, we have made further efforts to
control the reactivity of non-heme Fe centers by tuning both
the first and second coordination spheres through ligand
modification.4 In the present work, a new thioether, amide-
appended ligand N3PyamideSR (Figure 1) has been prepared
from the N3PyS scaffold. The thiolato donor was converted to
a thioether group to discourage S oxidation in the first
coordination sphere and potentially allow for the stabilization
of an FeIV(O) complex, similar to the proposed protective
function of N−H---S hydrogen bonds in thiolate-ligated
metalloenzymes.5 In addition, a pendant amide group was
installed in the second coordination sphere to examine the
influence of a single H-bond donor on the reactivity of Fe/Ox
intermediates, including FeIV(O) species. The influence of
multiple H-bond donors on sterically encumbered metal−
oxygen species has been examined, but there remains relatively
little known about the influence of a single H-bond donor on
sterically accessible MOx sites.6
etermining the influence of both first- and second-
D
coordination-sphere elements on non-heme iron-Ox
species has been the focus of much recent effort.1 Well
characterized, high-valent FeIV(O) complexes are known, but
most of these systems contain structurally similar polyamino/
pyridyl ligands.1a Very few FeIV(O) complexes have been
reported with heteroatoms other than nitrogen in the first
coordination sphere. The inclusion of sulfur donors is
important for modeling key metal−sulfur interactions in non-
heme enzymes, but is challenging because of the inherent
problems associated with the facile oxidation of sulfur.2 In
earlier work we described an FeII complex with a thiolato donor
built into the first coordination sphere, N3PyS− (Figure 1),
which undergoes facile O2-mediated S-oxygenation.2b This
complex provided a functional model of the non-heme iron
enzyme cysteine dioxygenase (CDO), but no FeIV(O)
The new ligand N3PyamideSR was used to prepare the FeII
complex [FeII(N3PyamideSR)](BF4)2 (1). Complex 1 reacts
with PhIO to give a metastable ferryl complex at low
temperature, [FeIV(O)(N3PyamideSR)]2+ (2). This species is a
rare example of a well characterized FeIV(O) complex with
sulfur ligation.7 Remarkably, complex 2 shows no propensity
for intramolecular sulfoxidation, but instead exhibits rapid
intermolecular O-atom transfer (OAT) to external thioether
substrates. In addition, the rates of OAT are dramatically
accelerated when compared to the same reaction exhibited by
other FeIV(O) complexes, and this increase in reactivity may be
due, at least in part, to the presence of the designed NH---O
Fe hydrogen bond.
Reaction of N3PyamideSR with FeII(BF4)2 in MeCN led to the
isolation of crystalline 1 (88%). The X-ray structure of 1 at 110
K (Figure 2) reveals an octahedral FeII complex with the amide
group coordinated in the open site. Bond distances (Table S1)
Received: October 5, 2013
Figure 1. Ligands discussed in the present study.
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
(δ = 0.04 mm/s, ΔEQ = 0.80 mm/s, Figure 3). These
parameters, especially the low isomer shift, are definitive for an
FeIV(O) complex.1 The lack of peaks for 1 indicates that the
FeII complex has been quantitatively converted to the FeIV(O)
complex (2).
A structural analogue of metastable 2 was synthesized from
FeCl2, N3PyamideSR, and anion metathesis with NaBF4,
affording yellow, crystalline [FeII(Cl)(N3PyamideSR)](BF4)
(3). The X-ray structure of 3 (Figure 2) shows that the
carbonyl group has been displaced by Cl−. Importantly, the free
amide N−H group is clearly oriented for H-bonding to the
terminal Cl− (Cl---H−N = 2.497 Å; Cl−H−N = 167.53°).8
The structure of 3 fully supports the proposed structure for 2,
in which the pendant N−H amide forms a stable, 6-membered
H-bond ring with the terminal oxo ligand.
Figure 2. Displacement ellipsoid plots (50% probability level) of the
dication of 1 (left) and the cation of 3 (right). H-atoms omitted for
clarity, except for the amide N−H.
The structures and Mossbauer parameters for 1 and 2 were
̈
examined by density functional theory (DFT) methods.
Assuming a ground state singlet for 1, the optimized geometry
(B3LYP) (Figure S14, Table S1) and calculated Mossbauer
̈
parameters (δ = 0.39 mm/s, ΔEQ = 0.82 mm/s) are in good
agreement with the experimental data, providing a calibration of
our theoretical methods. Geometry optimization for 2 confirms
the proposed structure (Figure S15), including the amide
NH---O H-bond, and a triplet ground state is found for 2 (32)
with a close-lying quintet (52) excited state 2.3 kcal/mol higher
in energy (Table S2). The experimental isomer shift for 2 falls
3
within the range for [FeIV(O)] complexes as well as the low
5
end of the range for [FeIV(O)] complexes (Table S4), and
thus the Mossbauer data alone do not conclusively show a
̈
triplet ground state. However, the DFT-derived Mossbauer
̈
parameters for 32 are a good match for the experimental values
(calcd: δ = 0.03 mm/s, ΔEQ = 0.55 mm/s, Table S3), whereas
the calculated parameters for 52 deviate significantly from
experiment. Moreover almost all well-defined 5[FeIV(O)]
species are 5-coordinate, whereas the optimized geometry for
2 is 6-coordinate. The experimental data, together with DFT
calculations and reactivity studies (vide infra), provide strong
evidence that the ground state for 2 is most likely a triplet state.
We have previously shown that the thiolato donor in
[FeII(N3PyS)]+ is S-oxygenated by O2, as is the coordinated S-
Cys donor in CDO.2b,9 Thus it is notable that protection of the
thiolato donor to a thioether allows for a metastable FeIV(O)
complex 2 to be trapped at low temperature, exhibiting no
intramolecular S-oxygenation. However, when 2 is exposed to
external thioether substrates, rapid OAT occurs. Reaction of 2
with PhSMe at −40 °C leads to the fast decay of the 750 nm
band and isosbestic return of the starting FeII complex 1 (82%
recovery in 30 min) (Figure S7). Analysis by 1H NMR showed
PhS(O)Me was produced in 80% yield (eq 1).
Figure 3. Synthesis of 2 (top); UV−vis spectral changes for the
conversion of 1 to 2 (bottom, left); Mossbauer spectra (47 mT) for 1
̈
and 2 (bottom, right) at 5 K in CH3CN.
for 1 are consistent with low-spin (ls) FeII (S = 0), and
Mossbauer spectroscopy at 5 K reveals a sharp, well-resolved
̈
doublet (δ = 0.47 mm/s; ΔEQ = 0.77 mm/s, Figure 3) typical
for ls-FeII. However, the room-temperature 1H NMR spectrum
for 1 reveals paramagnetically shifted peaks, but this is likely
due to a small paramagnetic impurity and further work is
ongoing to definitively assign the spin state at higher
temperatures for this complex.
Reaction of 1 with PhIO (1.2 equiv, dissolved in minimal
MeOH) in MeCN at −40 °C leads to the rapid decay of the
peaks for 1 at 350 and 450 nm (ε = 5 × 103 and 6 × 103 M−1
cm−1, respectively), and the isosbestic production of a new peak
at 750 nm (ε = 400 M−1 cm−1, Figure 3). This broad, weak
band in the near-IR region is characteristic of non-heme
FeIV(O) complexes.1 The new species with λmax = 750 nm is
stable at −40 °C for hours but immediately decays upon
warming above −30 °C. This behavior contrasts that of
[FeIV(O)(N4Py)]2+, which is stable at room temperature (t1/2
=
60 h).3a Analysis of the reaction mixture by low-temperature,
high-resolution ESI-MS (+ mode) (−40 °C) shows a parent
ion at m/z 311.0914 and isotope pattern corresponding to the
dicationic [Fe(O)(N3PyamideSR)]2+ (Figure S3). The 750 nm
species is EPR silent (X-band, 15 K), consistent with an
integer-spin FeIV(O) complex. Confirmation of the nature of
the new species from 1 + PhIO was obtained through
The kinetics of intermolecular sulfoxidation were monitored
by UV−vis under pseudo-first-order conditions, and could be
well fit to a single exponential model (Figure S8). A linear
dependence was found for kobs versus [PhSMe], yielding a
second-order rate constant of k = 4.3 M−1 s−1 (Figure S9). The
rate constants for other non-heme FeIV(O) complexes are given
in Table 1. Complex 2 shows dramatically enhanced OAT
reactivity, exhibiting a nearly 70-fold rate enhancement at −40
Mossbauer spectroscopy, which shows a well-resolved doublet
̈
B
dx.doi.org/10.1021/ja410240c | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
with mCPBA further emphasizes the remarkable stability of 2
toward intramolecular OAT.
Table 1. Second-Order Rate Constants for OAT of Various
FeIV(O) Complexes to Thioanisole
A preliminary examination of 2 for its ability to abstract
hydrogen atoms reveals strongly muted reactivity. Addition of
the H-atom donor 2,4-di-tert-butylphenol to 2 at −40 °C leads
to the slow decay of the 750 nm band and return of the peaks
for 1 (Figure S11). The reaction rate with a large excess of
phenol (500 equiv) leads to a kobs of only 4.0 × 10−3 s−1
(Figure S12), which is 2.5 times slower than H-atom transfer
(HAT) to [FeIV(O)(TMC)(NCCH3)]2+ at 0 °C for the same
phenol.11 This observation is in line with calculations that
predict that H-bonds in non-heme FeIV(O) and MnIV(O)
complexes should lower H-atom abstraction reactivity.12 Given
that only one tBu group on the substrate is in proximity to the
O−H bond, it is also unlikely that sterics play a significant role
in lowering the reaction rate. These results also argue against
complex
k (M−1 s−1
4.3
8.7 × 10−1
(1.5 × 103)
6.5 × 10−2
2.9 × 10−2
4.4 × 10−1
3.5 × 10−1
)
T (°C)
ref
[FeIV(O)(N3PyamideSR)]2+
[FeIV(O)(N4Py)]2+
(+HClO4)
[(FeIV(O)(N4Py)]2+
[FeIV(O)(TMC)]2+
[FeIV(O)(TPA)]2+
[FeIV(O)(TBC)]2+
−40
25
this work
10a
0
35
10b
10a
10b
10c
−45
−10
°C vs the parent [FeIV(O)(N4Py)]2+ at 0 °C.10b The reaction
between [FeIV(O)(N4Py)]2+ and PhSMe at −40 °C was run as
a direct comparison, but we observed no spectroscopic change
under these conditions over 40 min. The only example in Table
1 that also exhibits significantly increased reactivity is found in
row 3, in which the strong proton donor HClO4 is postulated
to activate the FeIV(O) complex through monoprotonation of
the oxo ligand ([(N4Py)FeIV(OH)]3+).10a We suggest that the
unique amide H-bond donor in 2 may activate the terminal oxo
ligand in an analogous fashion, increasing the electrophilicity of
the oxo ligand.
The choice of oxidant for 1 is critical to the reaction pathway.
If PhIO is replaced with the more powerful oxidant mCPBA,
the UV−vis signature for the ferryl complex is not obtained, but
instead an intense band at 700 nm (1000 M−1 cm−1) is
produced. This new species is thermally stable, unlike 2, and an
X-ray structure of this product (Figure 4) reveals that the
5
the involvement of a high-spin [FeIV(O)] ground state (or
accessible quintet excited state) in the reactivity of 2, because a
5[FeIV(O)] species should exhibit accelerated HAT.4,13
Finally, the efficient intermolecular OAT facilitated by 2 to
PhSMe can be exploited for catalytic sulfoxidation. Complex 1
was combined with excess PhSMe (100 equiv) at room
temperature and then treated with PhIO (50 equiv). A transient
color change signaling the formation of 2 was observed,
followed by an immediate return of the orange color for 1.
1
Product analysis by H NMR gave 40 turnovers of PhS(O)Me
in 2 h (Figure S13), before catalyst decomposition. These
results demonstrate that complex 1 is able to function as a
potent catalyst for the oxidation of thioethers.
The new findings reported here are summarized schemati-
cally in Figure 5. We have shown that a rare, metastable non-
Figure 4. Synthesis of sulfoxide complex 4 (top); UV−vis spectral
changes for the formation of 4 (bottom left); and displacement
ellipsoids (50% probability level) for the dication of 4 showing the
major S-bound isomer, with H-atoms omitted for clarity, except for the
amide N−H (bottom right).
Figure 5. Summary of reactivity for 1 and 2.
heme FeIV(O) complex can be generated with a pendant
thioether donor in the first coordination sphere. Despite the
fact that 2 does not undergo intramolecular S-oxygenation, it
does facilitate the rapid sulfoxidation of external thioether
substrates. Although more work is needed to determine the
origins of this high selectivity, coordination of the thioether
ligand to iron may constrain the orientation of the S donor,
and/or raise its redox potential, such that oxidation is
discouraged. The dramatic rate enhancement in OAT facilitated
by 2 is similar to that seen for [FeIV(O)(N4Py)]2+ upon
addition of strong H+ donors, suggesting that the inclusion of
the amide H-bond donor in the second coordination sphere
may enhance OAT reactivity through a similar increase in
ligated sulfide has been S-oxygenated to sulfoxide, affording
[FeII(N3PyamideS(O)R)]2+(BF4)2 (4). The major isomer of 4 is
S-bound, while a minor isomer is O-bound to iron (Figure S6).
Complex 4 also provides an authentic iron(II)-sulfoxide
complex with a characteristic UV−vis band at λmax = 700 nm.
The complete absence of this peak during the formation of 2 or
its subsequent reactions with external substrates confirms that
intramolecular S-oxygenation does not occur for 2. The
susceptibility of the coordinated thioether to sulfoxidation
C
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Journal of the American Chemical Society
Communication
Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 2005,
127, 12046. (c) Chiou, S. J.; Innocent, J.; Riordan, C. G.; Lam, K. C.;
Liable-Sands, L.; Rheingold, A. L. Inorg. Chem. 2000, 39, 4347.
(6) (a) Lacy, D. C.; Mukherjee, J.; Lucas, R. L.; Day, V. W.; Borovik,
A. S. Polyhedron 2013, 52, 261. (b) Gupta, R.; Lacy, D. C.; Bominaar,
E. L.; Borovik, A. S.; Hendrich, M. P. J. Am. Chem. Soc. 2012, 134,
9775. (c) Lacy, D. C.; Gupta, R.; Stone, K. L.; Greaves, J.; Ziller, J. W.;
Hendrich, M. P.; Borovik, A. S. J. Am. Chem. Soc. 2010, 132, 12188.
(d) Sen Soo, H.; Komor, A. C.; Iavarone, A. T.; Chang, C. J. Inorg.
Chem. 2009, 48, 10024. (e) Wada, A.; Harata, M.; Hasegawa, K.;
Jitsukawa, K.; Masuda, H.; Mukai, M.; Kitagawa, T.; Einaga, H. Angew.
Chem., Int. Ed. 1998, 37, 798. (f) Kim, S.; Saracini, C.; Siegler, M. A.;
Drichko, N.; Karlin, K. D. Inorg. Chem. 2012, 51, 12603.
electrophilicity of the terminal oxo ligand. However other
factors may contribute to the strong increase in reactivity seen
for 2, including the accessibility of a high-spin quintet state,
which cannot be completely ruled out by the available data. The
metal-oxo group may also be more exposed in 2 as compared to
[FeIV(O)(N4Py)]2+ because of replacement of an equatorial py
donor with a less sterically encumbered sulfide donor, allowing
for easier approach of substrate. The novel reactivity seen for 2
provides motivation for further examination of first- and
second-coordination-sphere effects in this, and other non-heme
iron model complexes.
(7) (a) McDonald, A. R.; Bukowski, M. R.; Farquhar, E. R.; Jackson,
T. A.; Koehntop, K. D.; Seo, M. S.; De Hont, R. F.; Stubna, A.; Halfen,
ASSOCIATED CONTENT
* Supporting Information
Detailed experimental procedures, ESI-MS data, crystallo-
■
S
J. A.; Munck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc. 2010, 132,
̈
17118. (b) Annaraj, J.; Kim, S.; Seo, M. S.; Lee, Y. M.; Kim, Y.; Kim, S.
J.; Choi, Y. S.; Jang, H. G.; Nam, W. Inorg. Chim. Acta 2009, 362, 1031.
(c) Bukowski, M. R.; Koehntop, K. D.; Stubna, A.; Bominaar, E. L.;
graphic information for 1, 3, and 4, Mossbauer spectroscopy,
̈
details for kinetic studies, and computational investigations.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Halfen, J. A.; Munck, E.; Nam, W.; Que, L., Jr. Science 2005, 310, 1000.
̈
(8) (a) Sickerman, N. S.; Park, Y. J.; Ng, G. K. Y.; Bates, J. E.; Hilkert,
M.; Ziller, J. W.; Furche, F.; Borovik, A. S. Dalton Trans. 2012, 41,
4358. (b) Aullon, G.; Bellamy, D.; Brammer, L.; Bruton, E. A.; Orpen,
A. G. Chem. Commun. 1998, 653.
AUTHOR INFORMATION
■
Corresponding Authors
gjameson@chemistry.otago.ac.nz
dpg@jhu.edu
(9) (a) Kumar, D.; Sastry, G. N.; Goldberg, D. P.; de Visser, S. P. J.
Phys. Chem. A. 2012, 116, 582. (b) Kumar, D.; Thiel, W.; de Visser, S.
P. J. Am. Chem. Soc. 2011, 133, 3869.
Notes
(10) (a) Park, J.; Morimoto, Y.; Lee, Y. M.; Nam, W.; Fukuzumi, S. J.
Am. Chem. Soc. 2012, 134, 3903. (b) Park, M. J.; Lee, J.; Suh, Y.; Kim,
J.; Nam, W. J. Am. Chem. Soc. 2006, 128, 2630. (c) Seo, M. S.; Jang, H.
G.; Kim, J.; Nam, W. Bull. Korean Chem. Soc. 2005, 26, 971.
(11) Sastri, C. V.; Lee, J.; Oh, K.; Lee, Y. J.; Lee, J.; Jackson, T. A.;
Ray, K.; Hirao, H.; Shin, W.; Halfen, J. A.; Kim, J.; Que, L., Jr.; Shaik,
S.; Nam, W. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 19181.
(12) Latifi, R.; Sainna, M. A.; Rybak-Akimova, E. V.; de Visser, S. P.
Chem.Eur. J. 2013, 19, 4058.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The NIH (D.P.G., GM62309 and GM101153) is gratefully
acknowledged for financial support. G.N.L.J. thanks the
Marsden Fund and The International Mobility Fund
administered by Royal Society of New Zealand. I. I.-B. and
O. T. gratefully acknowledge support through the “Solar
Technologies Go Hybrid” initiative of the German Federate
State of Bavaria.
́
(13) (a) Wilson, S. A.; Chen, J.; Hong, S.; Lee, Y. M.; Clemancey,
M.; Garcia-Serres, R.; Nomura, T.; Ogura, T.; Latour, J. M.; Hedman,
B.; Hodgson, K. O.; Nam, W.; Solomon, E. I. J. Am. Chem. Soc. 2012,
134, 11791. (b) McDonald, A. R.; Guo, Y. S.; Vu, V.; Bominaar, E. L.;
Munck, E.; Que, L., Jr. Chem. Sci. 2012, 3, 1680.
̈
REFERENCES
■
(1) (a) McDonald, A. R.; Que, L., Jr. Coord. Chem. Rev. 2013, 257,
414. (b) Shook, R. L.; Borovik, A. S. Inorg. Chem. 2010, 49, 3646.
(c) de Visser, S. P.; Rohde, J. U.; Lee, Y. M.; Cho, J.; Nam, W. Coord.
Chem. Rev. 2013, 257, 381.
(2) (a) McQuilken, A. C.; Goldberg, D. P. Dalton Trans. 2012, 41,
10883. (b) McQuilken, A. C.; Jiang, Y.; Siegler, M. A.; Goldberg, D. P.
J. Am. Chem. Soc. 2012, 134, 8758. (c) Jiang, Y.; Widger, L. R.; Kasper,
G. D.; Siegler, M. A.; Goldberg, D. P. J. Am. Chem. Soc. 2010, 132,
12214. (d) Sallmann, M.; Siewert, I.; Fohlmeister, L.; Limberg, C.;
Knispel, C. Angew. Chem., Int. Ed. 2012, 51, 2234. (e) Badiei, Y. M.;
Siegler, M. A.; Goldberg, D. P. J. Am. Chem. Soc. 2011, 133, 1274.
(f) Liu, T. B.; Li, B.; Singleton, M. L.; Hall, M. B.; Darensbourg, M. Y.
J. Am. Chem. Soc. 2009, 131, 8296. (g) O’Toole, M. G.; Kreso, M.;
Kozlowski, P. M.; Mashuta, M. S.; Grapperhaus, C. A. J. Biol. Inorg.
Chem. 2008, 13, 1219. (h) Lugo-Mas, P.; Taylor, W.; Schweitzer, D.;
Theisen, R. M.; Xu, L.; Shearer, J.; Swartz, R. D.; Gleaves, M. C.;
DiPasquale, A.; Kaminsky, W.; Kovacs, J. A. Inorg. Chem. 2008, 47,
11228.
(3) (a) Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde, J. U.; Song, W. J.;
Stubna, A.; Kim, J.; Munck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc.
̈
2004, 126, 472. (b) Klinker, E. J.; Kaizer, J.; Brennessel, W. W.;
Woodrum, N. L.; Cramer, C. J.; Que, L., Jr. Angew. Chem., Int. Ed.
2005, 44, 3690.
(4) Sahu, S.; Widger, L. R.; Quesne, M. G.; de Visser, S. P.;
Matsumura, H.; Moenne-Loccoz, P.; Siegler, M. A.; Goldberg, D. P. J.
̈
Am. Chem. Soc. 2013, 135, 10590.
(5) (a) Zheng, P.; Takayama, S. I. J.; Mauk, A. G.; Li, H. B. J. Am.
Chem. Soc. 2012, 134, 4124. (b) Dey, A.; Okamura, T.; Ueyama, N.;
D
dx.doi.org/10.1021/ja410240c | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX