A broad substrate-scope method for fast, efficient and selective hydrogen peroxide-epoxidation

Article abstract of DOI:10.1002/adsc.200800650

The efficient epoxidation of a broad range of olefins using hydrogen peroxide (H₂O₂) as the oxidant has been accomplished by a manganese catalyst that exhibits an uncommon chemoselectivity.

Full text of DOI:10.1002/adsc.200800650

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DOI: 10.1002/adsc.200800650
A Broad Substrate-Scope Method for Fast, Efficient and Selective
Hydrogen Peroxide-Epoxidation
a
a,
a,
Isaac Garcia-Bosch, Xavi Ribas, * and Miquel Costas *
a
Departament de Quꢀmica, Facultat de Ciꢁncies, Campus de Montilivi, 17071 Girona, Spain
Fax : (+34)-97-241-8150; phone: (+34)-97-241-8333; e-mail: miquel.costas@udg.edu or xavi.ribas@udg.edu
Received: October 23, 2008; Published online: February 6, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800650.
Abstract: The efficient epoxidation of a broad
range of olefins using hydrogen peroxide (H O ) as
2
2
the oxidant has been accomplished by a manganese
catalyst that exhibits an uncommon chemoselectivi-
ty.
Keywords: catalysis; chemoselectivity; epoxidation;
hydrogen peroxide; manganese
Olefin epoxidation is an important reaction because
epoxides are valuable compounds amenable for a
[
1]
number of subsequent transformations. In addition,
chemoselective epoxidations constitute an ongoing
[
2]
challenge in synthetic organic chemistry. Because of
environmental, atom-economy, cost and safety consid-
erations, activations of H O by first-row transition
2
2
metal complexes constitute very attractive epoxida-
[
3]
tion technologies. Furthermore, careful design of
transition metal catalysts can also open the door to
distinct chemo- and stereoselectivity patterns. Howev-
er, the combination of hydrogen peroxide with transi-
tion metal complexes is often unpredictable as dispro-
Scheme 1.
[
6]
portionation and free-radical types of processes can oxidant. We considered that peracetic acid bears im-
compete with and/or dominate the metal-based epoxi- portant drawbacks such as poorer atom economy than
dation. Selected examples of H O -epoxidations cata- H O , is more expensive, and commercial solutions
2
2
2
2
[4]
[5]
lyzed by Mn and Fe complexes have attracted are very acidic, limiting their use in the epoxidation
much interest, but the search for environmentally of acid-sensitive substrates. Because of that, we
benign epoxidation methods exhibiting broad sub- sought to extend this chemistry to the use of H O2
2
strate scope remains a challenge. In this work we (Table 1). Initial attempts to epoxidize 1-octene with
report our advances towards this goal and describe a H O (1.2 equiv.) in the presence of 1 (0.1 mol%) re-
2
2
novel method for the fast, efficient and selective ep- sulted in the immediate evolution of O , arising from
2
oxidation of a broad range of olefins with H O using peroxide disproportionation, and minimum (<1%)
2
2
small loadings of a manganese catalyst.
We recently reported that
epoxide product was obtained. The use of 1.1 equiv.
[Mn AHCTUNGTERNNG(UN OTf) - of H O in the presence of 1.4 equiv. of acetic acid
2
2
2
H,Me
(
PyTACN)] (OTf=CF SO ) (1) (Scheme 1) is a (with respect to substrate) was also tested because it
3 3
robust, efficient and selective catalyst for the epoxida- has been proposed that peracetic acid may be formed
[
7]
tion of a broad range of olefins using peracetic acid as in situ under these conditions. However, low epoxi-
348
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Adv. Synth. Catal. 2009, 351, 348 – 352

Method for Fast, Efficient and Selective Hydrogen Peroxide-Epoxidation
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Table 1. Effect of the addition of acetic acid in the epoxida- these conditions, epoxidation of styrene with H O2
2
[a]
tion of 1-octene by 1/H O .
2
2
(1.2 equiv.) affords the corresponding epoxide in 90%
conversion/90% yield, reaching 4500 turnovers
[b]
[c]
Entry
Equiv. Acetic Acid
Conv.
Yield
(
Table 2, entry 2). By comparison, the same experi-
1
2
3
4
0.05
1.4
7
20
27
63
90
17
27
63
90
ment was performed using peracetic acid (1.2 equiv.)
as oxidant, affording substrate conversion of 40% and
epoxide yield below <10%. That suggests that the
less acidic conditions of the hydrogen peroxide/acetic
acid method allow for the survival of both the catalyst
14
[
a]
Reaction conditions: 1-octene (1.66 mmol) in CH CN
15 mL), 1 (1.66 mmol), acetic acid (0.083–23.3 mmol, and the products formed.
3
(
0
.05–14 equiv.), and H O 32% dissolved in CH CN 1:1
An excellent regioselectivity of the 1/H O /
3 2
2
2
3
2 2
(
v:v) (1.83 mmol, 1.1 equivalent with respect to sub- CH CO H system was also demonstrated by perform-
strate) added at 08C in 30 min. Cat:H O :olefin ratio=
2
2
ing the monoepoxidation of the terminal olefin site of
R(À)-carvone (99% yield, entry 16) and the monoe-
poxidation of the cis-olefin site of 4-vinylcyclohexene
to afford the corresponding 4-vinylcyclohexane 1-ep-
oxide (83% yield, entry 15). Furthermore, the cis-ole-
finic site in the trans-2-cis-6-nonadienyl ester
1
:1100:1000.
[
[
b]
c]
Substrate conversion into products.
Epoxide yield determined by GC.
(
entry 17) was selectively epoxidized to furnish the 2-
trans-olefin 6-monoepoxide product in 95% yield. Ad-
ditional chemoselectivity tests were performed by
doing competition experiments between pairs
Scheme 2.
of olefins ([cat.]/
ACHTUNGTRENNUNG[ H O ]/ ACHTNUTGRENNUGN[ olefin A]/ ACHTNGURTEUNNG[ olefin B]=
2 2
1
:100:1000:1000). These experiments show that cis-2-
dation activity was observed (27% epoxide yield). Im- heptene is epoxidized 5 times faster than trans-2-hep-
portantly, the increase of the acetic acid load promot- tene (lower discrimination compared to peracetic acid
[
6]
ed the epoxidation activity, and upon addition of system, cis/trans=9). On the other hand cis- and
4 equiv. of CH CO H, high substrate conversion was trans-b-methylstyrene are epoxidized at roughly equal
1
3
2
achieved. The corresponding epoxide was now ob- rates (cis/trans=0.9). Finally, much to our surprise,
tained in a rewarding 90% yield (Table 1). trans-stilbene is epoxidized 6 times faster than cis-stil-
To test the versatility of the method (Scheme 2), it bene (cis/trans=0.17), indicating a higher preference
was applied to a series of substrates (Table 2). A for the trans-olefin than that observed in the peracetic
broad spectrum of olefins was rapidly epoxidized acid case (cis/trans=0.5). This selectivity for aromatic
À1
(
<90 min, TOF > 1700 turnovers·h ) in good to excel- trans-olefins is remarkable, since most of the systems
lent conversions and yields. Examples include aromat- described so far showed a major reactivity for aromat-
[
3a]
ic (entries 1–6), aliphatic (entries 7–9) and terminal ic cis-olefins,
and there is no precedent to our
aliphatic olefins (entries 10 and 11). The epoxidations knowledge of the reversal of cis/trans selectivity upon
of electron-deficient (entry 12) and electron-rich tri- epoxidizing aliphatic or aromatic olefins.
substituted olefins (entries 13 and 14) were also ac-
complished with good to excellent yields and selectivi- tivity of manganese complexes [Mn
ty. Overall, the efficiency and selectivity of these reac- [Mn(OTf) (BPMCN)] 2, and [Mn(OTf) ACHTUNGERTNUNNG( BPMEN)] 3
2
For comparative purposes, we also studied the ac-
A
H
U
G
R
N
N
ACHTUNGTRENNUNG
2
2
A
H
U
G
R
N
N
A
H
U
G
R
N
U
G
A
H
U
T
E
N
N
2
tions resemble those where peracetic acid is employed under the novel reaction conditions (0.1 mol% cata-
as oxidant, and are significantly faster. Particularly re- lyst, 1.2 equiv. H O , 14 equiv. CH CO H). These
2
2
3
2
markable aspects are also the small excess of H O re- complexes have showed a very high epoxidation activ-
2
2
[
11]
quired, and the high turnover numbers attained by ity in combination with peracetic acid,
and could
the catalyst (810–4500 TN). Noteworthy, very acid- eventually represent a synthetically less elaborate cat-
sensitive substrates such as stilbenes and 1-phenyl-1- alyst than 1. Preliminary tests showed no epoxidation
[
12]
cyclohexene (entries 5, 6, and 14), which are not suita- of 1-octene when [Mn
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
was tested as
2
2
[
6]
ble for the peracid system, can also be epoxidized in catalyst and it was not further explored. On the other
good to excellent yields in the present reaction condi- hand 2 afforded good epoxide yields in the oxidation
tions. In addition, epoxidation of aromatic and ali- of aliphatic olefins, and good stereoretention was ob-
phatic cis-olefins was also highly stereospecific even served (Table 2). However, serious difficulties were
[
9]
in the case of highly sensitive cis-stilbene (entry 6).
evidenced when aromatic and electron-deficient sub-
No cis-dihydroxylation products were found in any of strates were employed. Preliminary experiments also
[
10]
the reactions tested.
showed that the activity of 3 is significantly reduced
[13]
We have also tested the activity of the catalyst with respect to that of 2. Therefore, while the versa-
when its loading was reduced to 0.02 mol%. Under tility exhibited by 1 is rather unique, the new method-
Adv. Synth. Catal. 2009, 351, 348 – 352
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
349

Isaac Garcia-Bosch et al.
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[a]
Table 2. Epoxidations of different substrates using catalysts 1–2, H O and CH CO H.
2
2
3
2
[b]
Entry
Substrate
Cat. (mol%)/H O (equiv)/t (min)
Conv. (yield)
2
2
Catalyst 1
Catalyst 2
1
2
0.1/1.2/60
0.02/1.2/60
100 (94)
90 (90)
89 (77)
–
3
0.1/1.2/5
91 (91)
31 (31)
[c]
4
0.1/1.2/5
100 (94 )
83 (78)
[
d]
[e]
5
6
0.1/1.2/5
0.1/1.2/5
100 (95 ) (92 )
26 (26)
36 (28)
[d,f]
[e]
100 (88 ) (86 )
7
0.1/1.1/5
100 (95)
100 (97)
[g]
8
9
0.1/1.2/60
0.1/1.4/5
0.1/1.1/60
100 (83 )
96 (81)
95 (75)
95 (70)
89 (89)
10
90 (90)
11
0.1/1.4/5
84 (84)
–
[
d]
[e]
1
2
3
0.1/1.4/5
0.1/1.4/5
87 (87 ) (87 )
61 (53)
1
100 (89)
100 (87)
[e]
1
4
5
0.1/1.4/5
100 (96) (90 )
59 (51)
–
[h]
1
0.1/1.2/60
97 (83 )
[
d]
[e]
1
6
7
0.1/1.2/5
0.1/1.2/5
100 (>99 ) (92 )
–
-
[d]
[e]
1
95 (95 ) (86 )
[
a]
Reaction conditions: olefin (1.66 mmol), catalyst 1 or 2 (1.66 mmol, 0.1 mol%), AcOH (23.3 mmol, 14 equiv.) in acetoni-
trile (15 mL), then H O 32% dissolved in CH CN (1:1) (1.83–2.33 mmol, 1.1–1.4 equiv.) added via syringe pump over
2
2
3
3
0 min at 08C; GC yields.
Time after total H O addition.
[
[
[
[
[
[
[
b]
2
2
c]
d]
e]
f]
98% cis-epoxide.
NMR yield.
Isolated yields.
9
9
4
0% cis-epoxide.
8% cis-epoxide.
% diepoxide.
g]
h]
ology applied to 2 opens up a very simple procedure
First approximation to the possible active species
for fast, environmentally benign and efficient stereo- responsible for this chemistry was obtained by com-
specific epoxidation of aliphatic olefins.
petitive epoxidation of styrene vs. p-substituted styr-
enes catalyzed by 1. A Hammett plot analysis pro-
350
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Adv. Synth. Catal. 2009, 351, 348 – 352

Method for Fast, Efficient and Selective Hydrogen Peroxide-Epoxidation
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2
vides a 1=À1.20 (R =0.99) which indicates that the hibits quite unusual chemoselectivity properties that
active oxidant is an electrophilic species. Further- allow regioselective monoepoxidation of diolefins,
more, the difference between the 1/CH CO H system also including a reversal of the cis/trans selectivity de-
3
3
(
1=À0.67) and the 1/H O /acetic acid system strongly pending on whether aliphatic or aromatic olefins are
2
2
suggests that the active oxidant in the two processes epoxidized, and finally it exhibits an unusual prefer-
is not the same. Further mechanistic considerations ence for trans-stilbenes. Preliminary mechanistic stud-
arise from isotopic labelling experiments. The epoxi- ies suggest a concerted oxygen atom delivery to the
dation reaction of styrene (100 equiv. substrate: olefin mediated by electrophilic yet distinct active
2
5 equiv. CH CO H) in the presence of a large excess species with respect to the oxidations performed with
3 3
18 18
of H O (1000 equiv.) shows no incorporation of
O
peracetic acid. Mechanistic studies on these highly
2
into the epoxide indicating that the oxidant does not active systems and evaluation of the role of different
exchange with water. Moreover, the epoxidation of carboxylic acid additives are warranted.
1
8
styrene using H O shows a high incorporation of
2
2
18
O into the epoxide (92%), indicating that the perox-
ide is the main origin of the oxygen atoms incorporat- Experimental Section
ed into the oxidized products and excluding the for-
mation of free-diffusing carbon centered radicals. On General Procedures for an Epoxidation Reaction
the other hand, the stereoretention observed in the
epoxidation of very epimerization-prone substrates
such as cis-stilbene is a strong indication that oxygen
atom transfer is a concerted process, or alternatively,
An acetonitrile solution (15 mL) of the specific olefin (1.66
mmol, final reaction concentration 0.11M) and 1 (0.11 mM)
was prepared in a 25-mL round-bottom flask equipped with
a stir bar and cooled in an ice bath. 1.4 mL of acetic acid
that it occurs via very short time-living intermediates. (23.3 mmol, 14 equiv.) were added directly to the solution.
The sum of all these observations indicate close simi- Then, 0.35–0.44 mL of 1:1 v:v acetonitrile:hydrogen perox-
larities between the oxidations performed with 1/ ide solution 32% (1.83 to 2.33 mmol, 1.1–1.4 equiv.) was
added by syringe pump over a period of 30 min. The solu-
CH CO H and these carried out with 1/H O /
3
3
2
2
tion was further stirred at 08C for 5–60 min. At this point,
the internal standard (biphenyl) was added and the solution
was filtered through a basic alumina plug, which was subse-
quently rinsed with 2ꢄ1 mL AcOEt. GC analysis of the so-
lution provided substrate conversions and product yields rel-
ative to the internal standard integration.
CH CO H, although selectivity considerations clearly
3
2
demonstrate that different reactive species are gener-
ated in both cases. These observations are in good
agreement with our original proposal that reactions
proceed via Lewis acid activation of the peroxide via
[
6,14]
an oxidized Mn center.
Nevertheless, the involve-
ment of high valent manganese oxo species cannot be
ruled out. The decisive role of acetic acid in these re-
actions is intriguing and beyond the scope of this
work. However, possible scenarios consider that it en-
hances the electrophilicity of the peroxide moiety, or
alternatively, it assists heterolytic OÀO bond cleavage
General Procedure for Epoxide Isolation.
Epoxidation of cis-Stilbene
An acetonitrile solution (15 mL) of cis-stilbene (312 mg,
9
7% pure, 1.66 mmol, final reaction concentration 0.11M)
and 1 (1 mg, 1.66 mmol, final reaction concentration
.11 mM) was prepared in a 25-mL round-bottom flask
0
of a manganese peroxide intermediate to generate a
high-valent manganese oxo species, which would be
equipped with a stir bar and cooled in an ice bath. 1.4 mL of
acetic acid were directly added (23.3 mmol, 14 equiv). Then,
the active epoxidation agent. Precedent for the 0.38 mL of an 1:1 v:v acetonitrile:H O solution 32% (1.99
2
2
former includes Sharplessꢃ proposal for the Mo- and mmol, 1.2 equiv.) was added by syringe pump over a period
V-catalyzed epoxidation of olefins by alkyl perox- of 30 min. The solution was further stirred at 08C for 5 min.
[
15]
ides. On the other hand, acetic acid-assisted OÀO
At this point, 50 mL of an aqueous Na CO saturated solu-
tion were added to the mixture. The resultant solution was
extracted with diethyl ether (3ꢄ50 mL). Organic fractions
2
3
[
5f]
bond lysis has been proposed for iron systems. Nev-
ertheless, its exact role may not be limited to a single
aspect.
In conclusion, the present work describes a novel
method for the fast epoxidation of a wide range of
olefins using hydrogen peroxide as terminal oxidant
were combined, dried over MgSO , filtered, and the solvent
4
was removed under reduced pressure to afford of the prod-
uct; yield: 280 mg (1.43 mmol 86%). A product sample was
1
dissolved in CDCl and analyzed by H NMR analysis as
3
pure epoxide (90% cis). For details on the isolation of other
and using low loadings of a manganese catalyst. The epoxide products see the Supporting Information.
present H O /CH CO H oxidant system imposes
2
2
3
2
milder experimental conditions than commercial per-
[6]
acetic acid offering a broader substrate scope, and
lower catalyst loadings. The present system compares
well with state-of-the-art epoxidation technologies in
Acknowledgements
terms of product yields, substrate scope, and environ- We thank MICINN for research grant CTQ2006–05367/
mental considerations. Furthermore, the system ex- BQU. I. G.-B. thanks MICINN for a predoctoral FPI grant.
Adv. Synth. Catal. 2009, 351, 348 – 352
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
351

Isaac Garcia-Bosch et al.
COMMUNICATIONS
We thank Ronlad Hage (Rahucat) for valuable comments on
the manuscript, and Rahucat for a generous gift of tritosyl-
TACN. We also thank STR-Udg as facilities users.
Am. Chem. Soc. 2007, 129, 15964; g) S. Taktak, W. Ye,
A. M. Herrera, E. V. Rybak-Akimova, Inorg. Chem.
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[
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[
[
7] M. Fujita, L. Que, Jr., Adv. Synth. Catal. 2004, 346, 190.
8] Blank experiments with cis-cyclooctene, 1-octene and
styrene confirmed that no epoxidation occurs in the ab-
sence of the manganese catalyst.
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Adv. Synth. Catal. 2009, 351, 348 – 352