Epoxidations catalyzed by manganese(v) oxo and lmido complexes: Role of the oxidant mn oxo (lmido) intermediate

Article abstract of DOI:10.1021/ic902517j

The manganese(V) oxo complex (TBP₈Cz)Mn^v(O) (1) is shown to catalyze the epoxidation of alkenes with a series of iodosylarenes (ArIO) as oxidants. Competition experiments reveal that the identity of ArIO influences the product ratios, implicating an unusual coordinated oxo-metal-ArIO intermediate (1-OIAr) as the active catalytic species. The isoelectronic manganese(V) imido complex (TBP₈Cz)Mn^v(NMes) (2) does not participate in NR transfer but does catalyze epoxidations with ArIO as the O-atom source, suggesting a mechanism similar to that seen for 1. Direct evidence (ESIMS) is obtained for 1-OIMes.

Full text of DOI:10.1021/ic902517j

Inorg. Chem. 2010, 49, 3083–3085 3083
DOI: 10.1021/ic902517j
Epoxidations Catalyzed by Manganese(V) Oxo and Imido Complexes:
Role of the Oxidant-Mn-Oxo (Imido) Intermediate
Pannee Leeladee and David P. Goldberg*
Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218
Received December 17, 2009
V
The manganese(V) oxo complex (TBP Cz)Mn (O) (1) is shown to
catalyze the epoxidation of alkenes with a series of iodosylarenes
species are thought to play key roles in metal-catalyzed N- or
NR-group transfer.
8
(ArIO) as oxidants. Competition experiments reveal that the
identity of ArIO influences the product ratios, implicating an unusual
In recent years, however, alternative mechanisms have
been described for both porphyrin- and non-heme-type
1
2
coordinated oxo-metal-ArIO intermediate (1-OIAr) as the active
catalytic species. The isoelectronic manganese(V) imido complex
catalytic OAT reactions. These studies have implicated a
metal-oxidant complex as the reactive intermediate (Scheme 1,
pathway A) in lieu of a metal-oxo complex (pathway B).
However, an alternative mechanistic hypothesis that had
previously not been considered involves the coordination of
the oxidant after the formation of the high-valent metal-oxo
species (pathway C). We first proposed this mechanism in the
course of studying catalytic OAT reactions with iodosylben-
zene (PhIO) as the oxidant and the manganese(V) oxo
V
TBP Cz)Mn (NMes) (2) does not participate in NR transfer but
(
does catalyze epoxidations with ArIO as the O-atom source,
8
suggesting a mechanism similar to that seen for 1. Direct evidence
(
ESIMS) is obtained for 1-OIMes.
V
High-valent manganese and iron complexes have been
complex (TBP Cz)Mn (O) [1; TBP Cz = octakis(4-tert-
8
8
3
implicated as key intermediates in catalytic atom- or group-
transfer reactions for a number of synthetic and biological
systems. For example, the metal-catalyzed transfer of an
O atom is fundamental to a wide range of transformations,
from asymmetric epoxidations, which rely on Mn-salen
catalysts, to the selective hydroxylations carried out by the
heme enzyme cytochrome P450. In these systems, a high-
valent metal-oxo species is normally invoked as the inter-
mediate that carries out the critical O-atom-transfer (OAT)
step. Similarly, high-valent metal-nitrido and metal-imido
butylphenyl)corrolazinato; Chart 1] as the catalyst. Metal-
catalyzed oxidations involving hypervalent iodine reagents
such as PhIO are widely used in organic synthesis and thus
4
are of particular importance.
Zdilla and Abu-Omar subsequently found that the same
mechanism is operative in analogous catalytic aziridinations
V
with manganese(V) imido complex (tpfc)Mn (NTs) [tpfc =
5
tris(pentafluorophenyl)corrolato] as the catalyst. Thus, we
speculated that this mechanism may have broad implications
for group-transfer reactions. Herein we show that pathway
C is a general mechanism for catalytic epoxidations carried
out by the manganese(V) oxo complex 1 and provide the first
direct evidence for the ArIO-metal-oxo adduct. In addition,
*
To whom correspondence should be addressed. E-mail: dpg@jhu.edu.
1) (a) Collman, J. P.; Zeng, L.; Decr ꢀe au, R. A. Chem. Commun. 2003,
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(
2
J. Am. Chem. Soc. 2000, 122, 11098–11100. (c) Song, W. J.; Sun, Y. J.; Choi,
S. K.; Nam, W. Chem.;Eur. J. 2006, 12, 130–137. (d) Nam, W.; Choi, S. K.;
Lim, M. H.; Rohde, J. U.; Kim, I.; Kim, J.; Kim, C.; Que, L., Jr. Angew. Chem.,
Int. Ed. 2003, 42, 109–111. (e) Franke, A.; Fertinger, C.; van Eldik, R. Angew.
Chem., Int. Ed. 2008, 47, 5238–5242. (f) Machii, K.; Watanabe, Y.; Morishima, I.
J. Am. Chem. Soc. 1995, 117, 6691–6697. (g) Nam, W.; Lim, M. H.; Lee, H. J.;
Kim, C. J. Am. Chem. Soc. 2000, 122, 6641–6647. (h) Toy, P. H.; Newcomb, M.;
Coon, M. J.; Vaz, A. D. N. J. Am. Chem. Soc. 1998, 120, 9718–9719. (i) Suzuki,
N.; Higuchi, T.; Nagano, T. J. Am. Chem. Soc. 2002, 124, 9622–9628. (j) Jin, S.;
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(
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(4) (a) Smegal, J. A.; Schardt, B. C.; Hill, C. L. J. Am. Chem. Soc. 1983,
105, 3510–3515. (b) Yang, Y. H.; Diederich, F.; Valentine, J. S. J. Am. Chem.
Soc. 1991, 113, 7195–7205. (c) Franklin, C. C.; VanAtta, R. B.; Tai, A. F.;
Valentine, J. S. J. Am. Chem. Soc. 1984, 106, 814–816. (d) Zhdankin, V. V.;
Stang, P. J. Chem. Rev. 2002, 102, 2523–2584. (e) Kim, S. J.; Latifi, R.; Kang,
H. Y.; Nam, W.; de Visser, S. P. Chem. Commun. 2009, 1562–1564. (f) Ochiai,
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2003, 125, 13006–13007. (g) Zhdankin, V. V.; Stang, P. J. Chem. Rev. 2008, 108,
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003, 125, 3406–3407. (k) Wadhwani, P.; Mukherjee, M.; Bandyopadhyay, D.
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(5) (a) Zdilla, M. J.; Abu-Omar, M. M. J. Am. Chem. Soc. 2006, 128,
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enesulfonyl] and ArINTs* [Ar = 2-(tert-butylsulfonyl)benzene and Ts* = p-(tert-
butylbenzene)sulfonyl].
(6) For examples of mixed oxo/imido complexes that mediate OAT, see:
(a) Ison, E. A.; Cessarich, J. E.; Travia, N. E.; Fanwick, P. E.; Abu-Omar,
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(
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r 2010 American Chemical Society
Published on Web 03/04/2010
pubs.acs.org/IC

3
084 Inorganic Chemistry, Vol. 49, No. 7, 2010
Leeladee and Goldberg
Scheme 1
Scheme 2
Table 1. Competitive Epoxidations of Styrene and cis-Stilbene with Different
a
Oxidants
Chart 1
b
b
c
ArIO
3 (%)
cis-4 þ trans-4 (%)
[3]/[cis-4 þ trans-4]
PhIO
PFIB
MesIO
26
17
24
19
12
22
1.39 ( 0.05
1.41 ( 0.02
1.05 ( 0.05
we report the first example, to our knowledge, of a manga-
nese terminal imide complex that catalyzes not NR group
a
III
Reaction conditions: (TBP Cz)Mn (0.78 μmol), ArIO (48 μmol),
8
b
6
styrene (280 μmol), and cis-stilbene (280 μmol). Percent yields are
determined by gas chromatography and based upon the amount of ArIO
transfer but OAT.
18
Previously, it was shown conclusively through O labeling
that the terminal oxo ligand in 1 does not transfer to the
substrate, remaining as a “spectator” oxo group, while the
c
added. Calculated from the ratio of the moles of the products deter-
mined by gas chromatography. Average of at least three runs.
observed for MesIO as compared to PhIO or PFIB. Similar
findings have been obtained for both porphyrin and salen
complexes, where product distributions are influenced by the
nature of the oxidant but do not correlate with an obvious
steric or electronic property.
Zdilla and Abu-Omar have shown that the manganese(V)
imido complex (tpfc)Mn (NTs) catalyzes the aziridination
of styrene through a coordinated oxidant intermediate
(tpfc)Mn (NTs)(ArINTs*). We speculated that a suitable
manganese(V) imido complex might be able to promote the
epoxidation of an alkene via a coordinated ArIO intermedi-
coordinated O atom of PhIO in the proposed intermediate
V
(
TBP Cz)Mn (O)(OIPh) (1-OIPh) becomes incorporated in
8
3
the oxidized product. However, direct spectroscopic evi-
dence for 1-OIPh was lacking, and only one oxidant (PhIO)
and one alkene substrate had been examined.
As seen previously for cis-stilbene, the catalytic epoxida-
tion of styrene with (TBP Cz)Mn as the catalyst and PhIO
1a,2d
3
V
III
8
proceeds smoothly, with a high selectivity for epoxide (3)
V
5
III
over aldehyde (Scheme 2). Under these conditions, the Mn
complex is rapidly (<1 min) converted to 1. The oxidant can
be changed to the more reactive pentafluoroiodosylbenzene
V
ate. The corrolazine complex (TBP Cz)Mn (NMes) (2;
Chart 1) is a rare example of a stable, isolable Mn terminal
imide and shows no direct reactivity toward alkenes. Thus,
was an ideal candidate for this study. We were pleased to
8
(PFIB), which yields 3 (42%) with a trace amount of alde-
V
hyde (4%). The isolated manganese(V) oxo complex 1 can
also be added directly to the reaction as the catalyst. Utilizing
8
2
1
as the catalyst with PFIB as the oxidant and expanding the
find that the stoichiometric reaction of 2-PFIB-cyclooctene
substrate scope to include cyclooctene results in 34% cyclo-
octene oxide (Scheme 2). These results point to the general
scope of the catalytic epoxidations mediated by 1/ArIO.
Competition experiments were next performed in which a
(
1:1:1 molar ratio) in CD Cl at 23 °C over 24 h produced
2
2
1
cyclooctene oxide (23%). Analysis by H NMR spectroscopy
showed that diamagnetic 2 remained intact at the end of the
reaction, and quantitation by UV-vis [λ
(ε) 419 (3.60 ꢀ
max
-1
1
:1 mixture of alkenes was reacted with different ArIO oxi-
4
-1
-1
3
-1
1
8
0 M cm ), 659 (9.29 ꢀ 10 M cm )/nm] revealed an
dants in CH Cl at 23 °C. With limiting oxidant and excess
2
2
6% recovery for 2. The majority of cyclooctene oxide (19%)
substrate, the molar ratio of the products should reflect the
inherent selectivity of the active catalytic species. If the active
species is 1-OIAr, as predicted for pathway C (Scheme 1),
then the product ratio should depend on the identity of ArIO.
Product analysis of the competition reaction between styrene
and cis-stilbene (Table 1) shows that the ratio of epoxides is
is formed within 1 h, at which point the recovery of 2 is nearly
quantitative (g95%). If excess substrate (20-40 equiv) is
employed, the yield of epoxide (50-68%) increases drama-
tically, and there is no loss of 2 (98-103% by UV-vis). We
conclude that 2 mediates OAT through a reactive interme-
diate that is intercepted by substrate until the substrate
becomes diminished, at which point 2 begins to degrade.
With the manganese(V) imido complex employed as the
catalyst (2:PFIB:cyclooctene = 1:25:250 molar ratio), the
oxide is formed in 34% (9 turnovers), similar to catalysis with
7
similar for PhIO and PFIB but different for MesIO. This
result is consistent with the steric encumbrance of ArIO
having a dominant effect, as might be expected based on
pathway C, providing good support for the proposed mecha-
nism. However, it is not clear why an increase in stilbene
oxide, the seemingly more sterically hindered product, is
1
. The proposed catalytic mechanism is shown in Scheme 3,
in which a coordinated oxidant-Mn-imido intermediate is
(
7) The [cis-4]/[trans-4] ratio also varies with the identity of the oxidant
(8) Lansky, D. E.; Kosack, J. R.; Sarjeant, A. A. N.; Goldberg, D. P.
(
see the Supporting Information).
Inorg. Chem. 2006, 45, 8477–8479.

Communication
Inorganic Chemistry, Vol. 49, No. 7, 2010 3085
Scheme 3
the active epoxidizing agent. We considered the possibility
that 2 was being hydrolyzed to 1 to a small extent by exo-
genous H O, allowing 1 to serve as the catalyst. However,
2
þ
stirring 2 in a mixture of CH Cl -H O (1:1, v/v) for 24 h did
Figure 1. ESIMS of 1-OIMes Na . The inset shows observed (black)
3
2
2
2
and theoretical (gray) isotope distribution patterns.
not produce any 1 as monitored by UV-vis. Even the addi-
þ
tion of the strong H donor trifluoroacetic acid to this
mixture failed to convert 2 to 1. Examining the stability of
þ
12
corresponding to 1-OIMes Na (Figure 1). The isotope
3
2
under catalytic conditions [2-PFIB (1:25) or 2-PFIB-
þ
pattern firmly establishes the composition of 1-OIMes Na
inset, Figure 1). A cluster centered at m/z 1449.8 is assigned
to 1 Na (calcd m/z 1449.8). Another cluster near m/z 1427.0
corresponds to 1 (calcd m/z 1426.8). Structural character-
3
H O (1:25:excess) in CH Cl ] revealed only the decay of 2
2
2
2
(
(Figure S3 in the Supporting Information), which likely
þ
3
occurs in the absence of substrate from overoxidation by
PFIB. These results rule out 1 as the active catalyst during the
manganese(V) imido-mediated epoxidations.
•þ
+
ization of 1-OIMes Na was obtained by collision-induced
3
dissociation tandem ESIMS/MS (Figure S4 in the Support-
ing Information). The ion at m/z 1711.2 dissociates cleanly to
give a single fragment ion at m/z 1449.8, corresponding to a
loss of MesIO. This fragmentation is consistent with the
OIMes group being axially ligated to 1.
As a further test of the mechanism in Scheme 3, we added
Lewis bases as potential donors that could compete for the
binding site trans to the imido group. The addition of MeOH
as the cosolvent [CH Cl -CH OH (3:1, v/v)] to 2-PFIB-
2
2
3
cyclooctene lowers the catalytic yield of cyclooctene oxide to
7%, consistent with MeOH inhibiting the reaction through
In conclusion, we have demonstrated that 1 functions as a
general catalyst for the selective epoxidation of alkenes with a
range of ArIO oxidants. It was also shown that the isoelec-
tronic manganese(V) imido complex 2 is capable of mediat-
ing alkene epoxidation, but not aziridination, under con-
ditions similar to those of 1. These reactions most likely
proceed through an unusual oxidant-metal-oxo/imido
intermediate as the active catalytic species. The first direct
evidence for one of these intermediates has been obtained by
ESIMS. The mechanism in pathway C may have broad
significance for both porphyrin- and non-heme-type oxida-
tions. Although heme enzymes are less likely to proceed
through pathway C because of the proximal ligand, this
work strengthens the notion that coordinated metal-oxidant
adducts may be viable alternatives to the generally accepted
metal-oxo intermediates.
1
binding to the open site on the metal. Similarly, the addition
of either excess Bu NCl or 1-methylimidazole (1-MeIm;
4
2
2
5 equiv) causes a decrease in the epoxide yield (23% and
9
18
4%, respectively). Finally, the addition of H2 O to 2-
18
PFIB-cyclooctene under catalytic conditions gives O-cyclo-
octene oxide (79% isotopic enrichment). The incorporation
of O into metal-bound PhIO adducts from H O is well-
precedented.
18
18
2
1c,d,3,10
Taken together, all of the data indicate
that 2 functions as a catalyst for the epoxidation of alkenes
V
and provide strong evidence for (TBP Cz)Mn (NMes)(OI-
8
C F ) as the active epoxidizing agent.
6
5
No UV-vis spectral change is seen upon the addition of
ArIO to 1 or 2. However, negligible changes to the optical
V
IV
•þ
spectra of both Mn (O)(corrole) and Fe (O)(porph )
complexes have been observed upon the coordination of
11
axial ligands. Direct evidence for the intermediate in path-
way C was obtained by electrospray ionization mass spectro-
metry (ESIMS). Rapid injection of freshly prepared mixtures
of 1 and MesIO (2.5 equiv) gives rise to a prominent peak at a
mass-to-charge ratio (m/z) of 1711.2 (calcd m/z 1711.8),
Acknowledgment. This work was supported by the NSF
Grants CHE0909587 and CHE0606614). P.L. is grateful
(
for the Queen Sirikit Scholarship (Thailand).
Supporting Information Available: Experimental details,
Table S1, and UV-vis and ESIMS/MS spectra (Figures S1-
S4). This material is available free of charge via the Internet at
http://pubs.acs.org.
(
9) No UV-vis change is seen for 2 upon the addition of Cl- or 1-MeIm,
and thus binding affinities could not be obtained. However, laser-desorption
mass spectrometry data provide direct evidence that isoelectronic 1 does bind
-
-
Lewis bases (e.g., F , CN ). Prokop, K.; Goldberg, D. P. Unpublished
results.
(12) Alkali-metal cation adducts are commonly observed in ESIMS. See:
(a) Wang, H. Y.; Yim, W. L.; Kl u€ ner, T.; Metzger, J. O. Chem.;Eur. J. 2009,
15, 10948–10959. (b) O'Toole, M. G.; Kreso, M.; Kozlowski, P. M.; Mashuta, M.
S.; Grapperhaus, C. A. J. Biol. Inorg. Chem. 2008, 13, 1219–1230. (c) Colton, R.;
D'Agostino, A.; Traeger, J. C. Mass Spectrom. Rev. 1995, 14, 79–106.
(10) Nam, W.; Valentine, J. S. J. Am. Chem. Soc. 1993, 115, 1772–1778.
(11) (a) Liu, H. Y.; Zhou, H.; Liu, L. Y.; Ying, X.; Jiang, H. F.; Chang, C.
K. Chem. Lett. 2007, 36, 274–275. (b) Takahashi, A.; Kurahashi, T.; Fujii, H.
Inorg. Chem. 2009, 48, 2614–2625.