Efficient and selective peracetic acid epoxidation catalyzed by a robust manganese catalyst

Article abstract of DOI:10.1021/ol800329m

(Chemical Equation Presented) A manganese catalyst containing a tetradentate ligand derived from triazacyclononane exhibits high catalytic activity in epoxidation reactions using peracetic acid as oxidant. The system exhibits broad substrate scope and requires small (0.1-0.15 mol %) catalyst loading. The catalyst is remarkably selective toward aliphatic cis-olefins. Mechanistic studies point toward an electrophilic oxidant delivering the oxygen atom in a concerted step.

Full text of DOI:10.1021/ol800329m

ORGANIC
LETTERS
2
008
Vol. 10, No. 11
095-2098
Efficient and Selective Peracetic Acid
Epoxidation Catalyzed by a Robust
Manganese Catalyst
2
†
†
‡
Isaac Garcia-Bosch, Anna Company, Xavier Fontrodona, Xavi Ribas,* and
Miquel Costas*
,†
,†
Departament de Qu ´ı mica, Campus de MontiliVi, E-17071 Girona, Catalonia, Spain,
and SerVeis T e` cnics de Recerca, UniVersitat de Girona, E-17071 Girona,
Catalonia, Spain
xaVi.ribas@udg.edu; miquel.costas@udg.edu
Received February 13, 2008
ABSTRACT
A manganese catalyst containing a tetradentate ligand derived from triazacyclononane exhibits high catalytic activity in epoxidation reactions
using peracetic acid as oxidant. The system exhibits broad substrate scope and requires small (0.1-0.15 mol %) catalyst loading. The catalyst
is remarkably selective toward aliphatic cis-olefins. Mechanistic studies point toward an electrophilic oxidant delivering the oxygen atom in
a concerted step.
Development of environmentally benign and selective ep-
oxidation methods with broad substrate scope under mild
conditions is an important research topic because epoxy
compounds are widely used as intermediates to obtain value-
added chemical products. This has fueled the development
of oxidation technologies based on the activation of peroxides
that only generates acetic acid as byproduct. Although this
represents smaller atom efficiency compared to O and H O ,
2
2
2
and it is comparatively more expensive than H O , most
2
2
interestingly, it bypasses the common peroxide dispropor-
tionation reactions associated with the latter. Via wide
screening of a number of catalysts, Stack and co-workers
also demonstrated that the efficiency and selectivity of the
reactions, as well as the catalyst stability, are dramatically
1
by transition-metal catalysts. Very recently, Stack and co-
II
workers described [Mn (CF
3
SO
3
)
2
(MCP)] (MCP ) N,N′-
dimethyl-N,N′-bis(2-pyridylmethyl)cyclohexane-trans-1,2-
2
modulated by their structure, thus opening the door for the
2
+
diamine) and [Mn(bpy)
2
]
(bpy ) 4,4′-bipyridine) as very
3
development of selective catalysts via careful ligand design.
active epoxidation catalysts in combination with peracetic
Inspired by this idea, in this work we describe a novel Mn
complex as a very efficient epoxidation catalyst capable of
selectively epoxidizing a wide range of olefins reaching
turnover numbers of 1000 at >98% conversion and >99%
2
acid. Peracetic acid is an environmentally friendly oxidant
†
Departament de Qu ´ı mica.
Serveis T e` cnics de Recerca.
‡
(
1) (a) Lane, B. S.; Burgess, K. Chem. ReV. 2003, 103, 2457. (b)
Adolfsson, H., Transition Metal-catalyzed Epoxidation of Alkenes. In
Modern Oxidation Methods, B a¨ ckvall, J.-E., Ed.; Wiley-VCH: Weinheim,
(2) (a) Murphy, A.; Dubois, G.; Stack, T. D. P. J. Am. Chem. Soc. 2003,
125, 5250. (b) Murphy, A.; Pace, A.; Stack, T. D. P. Org. Lett. 2004, 6,
3119.
2
004. (c) Joergensen, K. A. Chem. ReV. 1989, 89, 431. (d) Noyori, R.;
Aoki, M.; Sato, K. Chem. Commun. 2003, 1977.
10.1021/ol800329m CCC: $40.75 2008 American Chemical Society
Published on Web 04/25/2008


selectivity. The system also exhibits a remarkable selectivity
for cis-aliphatic olefins.
restricted to 1. An elliposoid diagram of the crystal structure
of 1 is showed in Figure 1, which consists in a mononuclear
We targeted the preparation and study of a novel family
of Mn catalysts based on a triazacyclononane macrocyle,
derivatized with a pyridylmethyl group because we envi-
sioned that this structure would form highly stable and robust
complexes under oxidative and/or acidic conditions. Indeed,
3
Me TACN-based Mn complexes are well-known as quite
4
robust epoxidation catalysts. To this end, tetradentate ligands
R,R′
PyTACN were prepared following standard procedures
for the alkylation of dialkyl-substituted triazacyclononane
5
backbones. The corresponding complexes of general formula
R,R′
[
Mn(CF
3
SO
3
)
2
(
PyTACN)] 1-4 were subsequently ob-
tained by straightforward reaction of each ligand with
Figure 1. Ellipsoid diagram (50% probability) of 1.
Mn(CF SO )
3 3 2
in THF (Scheme 1, top). The complexes were
II
Mn complex with a distorted octahedral geometry. Four
Scheme 1
coordination sites are occupied by the N atoms of the
H,Me
tetradentate
PyTACN ligand, and the two remaining
positions are occupied by the two oxygen atoms of triflate
groups. The structure also shows that this type of ligand
enforces the cis coordination of the CF SO ligands, where
3 3
the peracid molecule will most likely interact with the Mn
ion.
We developed reaction procedures to optimize the activity
of 1 under lower catalyst loading conditions (cat. 0.1-0.15
mol %) using peracetic acid diluted in CH
added at 0 °C over 30 min. Under these conditions (Table
), catalyst 1 exhibits an excellent performance in the
3
CN (1:1 v/v) and
1
epoxidation of a wide range of olefins. Oxidations proceed
smoothly between 1 and 6 h to reach completion revealing
a remarkable stability of the catalyst. Thus, styrene and its
derivatives (Table 1, entries 1-5) were epoxidized with
excellent yields (91 to >99%). More challenging targets such
as terminal aliphatic olefins were also efficiently and
selectively oxidized to the corresponding epoxides (96 and
isolated as colorless crystalline compounds in 57-85%
yields. We initially tested their ability to epoxidize 1-octene
in a 1 mol % catalyst scale and delivering commercially
available peracetic acid via syringe pump over 3 min
(
Scheme 1, bottom). A remarkable dependence of the
catalytic activity on the catalyst structure was observed. Both
and 4 are virtually inactive (1% epoxide yield), 2 is
9
1% epoxide yields, 100% conversion, entries 13,14).
Aliphatic cis-olefins proved particularly reactive substrates
and they were epoxidized in quantitative yields and selectivi-
ties (entries 8-10). Remarkably, the oxidation of cyclohex-
ene was chemoselective toward the epoxide, and only trace
amounts (<1%) of allylic oxidation products were obtained.
On the other hand, trisubstituted and trans-olefins were
efficiently oxidized, yet lower yields for the epoxide were
obtained (72-86%, entries 11, 12, and 18). The system also
epoxidizes electron-deficient olefins affording excellent yields
3
moderately active (27%), but most remarkably, the activity
of 1 exhibits a rewarding 97% epoxide yield. On the basis
of these preliminary experiments, further studies were
(
3) (a) Guillemot, G.; Neuburger, M.; Pfaltz, A. Chem. Eur. J. 2007,
1
3, 8960. (b) G o´ mez, L.; Garcia-Bosch, I.; Company, A.; Sala, X.;
Fontrodona, X.; Ribas, X.; Costas, M. Dalton Trans. 2007, 5539. (c) Kang,
B.; Kim, M.; Lee, J.; Do, Y.; Chang, S. J. Org. Chem. 2006, 71, 6721. (d)
Nehru, K.; Kim, S. J.; Kim, I. Y.; Seo, M. S.; Kim, Y.; Kim, S.-J.; Kim, J.;
Nam, W. Chem. Commun. 2007, 4623.
(88-89%, entries 16 and 17). Finally, cis- and trans-stilbene
(
4) (a) Hage, R.; Iburg, J. E.; Kerschner, J.; Koek, J. H.; Lempers,
E. L. M.; Martens, R. J.; Racherla, U. S.; Russell, S. W.; Swarthoff, T.;
van Vliet, M. R. P.; Warnaar, J. B.; van der Wolf, L.; Krijnen, B Nature
(entries 6 and 7) proved difficult substrates for the system,
and modest epoxide yields (24 and 58% respectively) were
obtained at 0.1 mol % catalyst loading. Increase of the
catalyst loading led to full substrate conversion but epoxide
yields decreased substantially.
1
994, 369, 637. (b) de Vos, D. E.; de Wildeman, S.; Sels, B. F.; Grobet,
P. J.; Jacobs, P. A. Angew. Chem., Int. Ed. 1999, 38, 980. (c) Sibbons,
K. F.; Shastri, K.; Watkinson, M. Dalton Trans. 2006, 645. (d) Vos, D. D.;
Bein, T Chem. Commun. 1996, 917. (e) de Vos, D. E.; Bein, T. J.
Organomet. Chem. 1996, 520, 195. (f) de Vos, D. E.; Sels, B. F.; Reynaers,
M.; Subba Rao, Y. V.; Jacobs, P. A. Tetrahedron Lett. 1998, 39, 3221. (g)
Shul’pin, G. B.; S u¨ ss-Fink, G.; Shul’pina, L. S. J. Mol. Catal. A: Chem.
Besides being a very active catalyst, 1 also allows selective
monoepoxidation of substrates containing two olefinic sites.
2
001, 170, 17–34.
5) (a) Flassbeck, C.; Wieghardt, K., Z. Anorg. Allg. Chem. 1992, 608,
0. (b) Halfen, J. A.; Tolman, W. B. Inorg. Synth. 1998, 34, 75. (c) Berreau,
(
R)-(-)-carvone and 4-vinylcyclohexene (entries 15 and 19)
were selectively epoxidized to 5,6-monoepoxide carvone
89% yield) and 4-vinylcyclohexane-1-epoxide (78% yield),
(
6
L. M.; Halfen, J. A.; Young, V. G., Jr.; Tolman, W. B. Inorg. Chim. Acta
000, 297, 115. (d) Company, A.; G o´ mez, L; G u¨ ell, M.; Ribas, X.; Luis,
J.-M.; Que, L, Jr; Costas, M. J. Am. Chem. Soc. 2007, 129, 15766.
(
2
respectively. Furthermore, the cis-olefin site in trans-2-cis-
2096
Org. Lett., Vol. 10, No. 11, 2008

These experiments indicate that styrene is 14 times more
reactive than cis-2-heptene and that the latter is 9 times more
reactive than trans-2-heptene. This selectivity toward ali-
phatic cis-olefins is more pronounced than in stoichiometric
a
Table 1. Epoxidation of Different Olefins Using Catalyst
6
7
oxidations using m-CPBA (1.2), dimethyldioxirane (8.3),
catalytic systems such as Venturello’s tungstate-H
2
O
2
6
catalyst (3.7-7.3), Mn-Me
3
TACN-H
2
O
2
and related
3
CO H
4
e
II
catalysts (2.0-4.7), [Mn (CF SO (MCP)]-CH
3
3
)
2
3
2
a
8
catalysts (3), iron-PhIO (5.8 for cis/trans-2-butene), and
manganese-NaOCl porphyrin catalysts (5.0 for cis/trans-
9
2
-hexene) and approaches the high selectivity observed for
10
2 2
Mizuno’s silicotungstate-H O system (11.5). On the other
hand, cis/trans selectivity is dramatically altered upon
inclusion of aromatic rings in the olefin. Thus epoxidation
of cis- and trans-ꢀ-methylstyrene occurs at identical rates,
and trans-stilbene is two times more reactive than cis-
stilbene. The inversion in the cis/trans selectivity upon
changing from aliphatic to aromatic olefins suggests that its
origin is not a pure topological discrimination.
First insight into the active species responsible for this
chemistry was obtained by competitive oxidation of styrene
versus p-substituted styrenes. A Hammett plot analysis of
2
the data provides a F ) -0.67 (R ) 0.98) indicating that
an electrophilic type of active oxidant is active in these
reactions. This value is in agreement with that determined
for Mn-Me
3
2 2
TACN-H O catalyzed epoxidation of cin-
1
1
namic acids (F ) -0.63). On the other hand, epoxidation
of cis- and trans-olefins exhibit a large degree of stereospeci-
ficity. Only trace amounts of epimerized products were
detected in the epoxidation of aliphatic olefins and ꢀ-methyl-
styrenes, and only 10% of epimerized epoxide was obtained
in the epoxidation of the very sensitive cis-stilbene (Scheme
2
, top). These observations rule out the implication of long-
Scheme 2
a
Reaction conditions: olefin 0.12 M, catalyst 1 (0.1-0.15 mol %) 0.12/
lived carbocationic and/or radical type of intermediates,
which are commonly involved in epoxidations by
Mn-salen, Mn-Me TACN, and Mn salts in bicarbonate
3
0
.17 mM and 1.4 equiv of CH3CO3H 32% dissolved in CH3CN (1:1) added
over 30 min at 0 °C. 1/CH3CO3H/olefin 1-1.5:1400:1000. See the
Supporting Information for blank experiments. Substrate conversion into
products. Unless stated, epoxide yield determined by GC. 97% cis. 90%
cis. 99% cis. 1 equiv of CH3CO3H; yield refers to monoepoxide. Yield
b
12
4d
c
d
e
f
g
h
1
i
determined by H NMR. 8% diepoxide.
(6) Sato, K.; Aoki, M.; Ogawa, M.; Hashimoto, T.; Panyella, D.; Noyori,
R. Bull. Chem. Soc. Jpn. 1997, 70, 905.
(
(
7) Baumstark, A. L.; Vasquez, P. C. J. Org. Chem. 1988, 53, 3437.
8) Groves, J. T.; Nemo, T. E. J. Am. Chem. Soc. 1983, 115, 5786.
6
-nonadienyl acetate was selectively epoxidized to furnish
the 6-monoepoxide 2-trans-olefin product in 96% yield (entry
0).
The chemoselectivity of 1 was further explored by
performing competition experiments between pairs of olefins
[cat.]/[CH CO H]/[olefin A]/[olefin B] ) 1:100:1000:1000).
(9) (a) Meunier, B.; Guilmet, E.; Carvalho, M.-E. D.; Poilblanc, R. J. Am.
Chem. Soc. 1984, 106, 6668. (b) Battioni, P.; Renaud, J. P.; Bartoli, J. F.;
Reina-Artiles, M.; Fort, M.; Mansuy, D. J. Am. Chem. Soc. 1988, 110, 8462.
2
(
10) Kamata, K.; Yonehara, K.; Sumida, Y.; Yamaguchi, K.; Hikichi,
S.; Mizuno, N. Science 2003, 300, 964.
(11) Lindsay Smith, J. R.; Gilbert, B. C.; Mairata i Payeras, A.; Murray,
J.; Lowdon, T. R.; Oakes, J.; Pons i Prats, R.; Walton, P. H. J. Mol. Catal.
A: Chem. 2006, 251, 114.
(
3
3
Org. Lett., Vol. 10, No. 11, 2008
2097

1
3
buffer. Finally, an epoxidation reaction of cis-2-heptene
(Scheme 3, right) as responsible for the oxygen atom transfer
event. More recently, it has also been proposed that the
1
2
(
100 equiv substrate/25 equiv CH
large excess of H
of O into the epoxide (Scheme 2, bottom) indicating that
the oxidant does not exchange with water. Although high-
valent manganese-oxo species usually undergo exchange of
3
CO
3
H) in the presence of
18
IV
2
O (1000 equiv) showed no incorporation
oxidant may be Lewis acid activated by a high valent (Mn
1
8
V
17
or Mn ) species (Scheme 3, left). We favor the latter
explanation because oxygen atom transfer from peracid type
of species is a concerted reaction, while significant loss of
1
8
1
4
the oxo oxygen atom with water, the present observation
stereochemistry in cis-stilbene epoxidation is commonly
1
8
12,15
should be taken with caution, as failure to see O-exchange
is not a definitive proof to rule out the implication of
manganese oxo species. It is possible that water exchange
may be simply slower than attack on the substrate (Scheme
observed in oxidations by high valent metal oxo species.
This mechanistic scenario would also account for the lack
of water exchange in these reactions. However, none of the
two arguments is indeed conclusive.
3
, kexc , kox), precluding tautomerization.
In conclusion, the present work describes a novel catalyst
which epoxidizes a wide range of olefins with excellent
efficiency and remarkable selectivity under mild experimental
conditions. The method is environmentally non aggressive,
and the reactions only require 1.0-1.4 oxidant/substrate
molar ratio. Substrate conversions are quantitative (or nearly),
and selectivities for the epoxide usually surpass 90%. Most
remarkably, the catalyst is very robust and supports turnover
numbers that range from 500 to 1000, which results in
minimum catalyst loadings. The present system compares
Scheme 3
4
f
well with state of the art Mn-Me
3
2 2
TACN-oxalate-H O ,
II
2
and [Mn (CF
3
SO
3
)
2
3 3
(MCP)]-CH CO H-based systems in
terms of epoxide yields, stereochemical retention, catalyst
activity and broad substrate scope. Despite the fact that the
H,Me
laborious preparation of the tetradentate
PyTACN ligand
still constitutes a drawback when compared with com-
mercially available Me TACN and the relatively simple
At present, the absence of both intermediate accumulation
and observation of water incorporation into products preclude
the identification of the active species responsible for this
chemistry. Indeed, the exact nature of the epoxidation species
3
2
a
three-step synthesis of MCP, 1 exhibits a much more
pronounced discrimination between structurally different
aliphatic olefin sites, which allows the site selective epoxi-
dation of polyolefins. Finally, preliminary mechanistic studies
indicate the implication of an electrophilic oxidant capable
of delivering the oxygen atom to the olefin in a concerted
step.
1
5
12
in other manganese systems such as porphyrins, salen,
4
d,16
Me
3
TACN,
and also in recently described nonporphyrin
2
,3,17
type of complexes
remains a matter of debate. This
chemistry is found to be highly sensitive to the catalyst
nature, oxidant and reaction conditions, which suggests the
presence of a diverse mechanistic landscape. In general, two
basic mechanistic scenarios are proposed. The first one
Acknowledgment. Financial support from MEC Spain
(CTQ2006-05367 to M.C.). I.G.-B. and A.C. thank MEC-
Spain for PhD grants. We thank Dr. R. Hage (Unilever) for
helpful discussions and a reviewer for helpful suggestions
to improve the clarity of the paper.
V
implicates high-valent (usually Mn ) Mn-oxo species
(
(
12) McGarrigle, E. M.; Gilheany, D. G. Chem. ReV. 2005, 105, 1563.
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(
(
(
14) Bernadou, J.; Meunier, B. Chem. Commun. 1998, 2167.
15) Meunier, B. Chem. ReV. 1992, 92, 1411.
Supporting Information Available: Experimental pro-
cedures and a CIF file for 1. This material is available free
of charge via the Internet at http://pubs.acs.org.
16) de Boer, J. W.; Browne, W. R.; Brinksma, J.; Alsters, P. L.; Hage,
R.; Feringa, B. L Inorg. Chem. 2007, 46, 6353
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(
OL800329M
1
.
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Org. Lett., Vol. 10, No. 11, 2008