Convenient and mild epoxidation of alkenes using heterogeneous cobalt oxide catalysts

Article abstract of DOI:10.1002/anie.201310420

A general epoxidation of aromatic and aliphatic olefins has been developed under mild conditions using heterogeneous Co_xO_y-N/C (x=1,3; y=1,4) catalysts and tert-butyl hydroperoxide as the terminal oxidant. Various stilbenes and aliph

Full text of DOI:10.1002/anie.201310420

Angewandte
Chemie
DOI: 10.1002/anie.201310420
Heterogeneous Catalysis
Convenient and Mild Epoxidation of Alkenes Using Heterogeneous
Cobalt Oxide Catalysts**
Debasis Banerjee, Rajenahally V. Jagadeesh, Kathrin Junge, Marga-Martina Pohl, Jçrg Radnik,
Angelika Brꢀckner, and Matthias Beller*
Abstract: A general epoxidation of aromatic and aliphatic
olefins has been developed under mild conditions using
heterogeneous CoxOy–N/C (x = 1,3; y = 1,4) catalysts and
tert-butyl hydroperoxide as the terminal oxidant. Various
stilbenes and aliphatic alkenes, including renewable olefins,
and vitamin and cholesterol derivatives, were successfully
transformed into the corresponding epoxides with high selec-
tivity and often good yields. The cobalt oxide catalyst can be
recycled up to five times without significant loss of activity or
change in structure. Characterization of the catalyst by XRD,
TEM, XPS, and EPR analysis revealed the formation of cobalt
oxide nanoparticles with varying size (Co3O4 with some CoO)
and very few large particles with a metallic Co core and an
oxidic shell. During the pyrolysis process the nitrogen ligand
forms graphene-type layers, in which selected carbon atoms are
substituted by nitrogen.
peroxide in the presence of TS-1 is a state-of-the-art
epoxidation process.[6]
In addition to molecular oxygen and hydrogen peroxide,
alkyl peroxides such as tert-BuOOH (TBHP) offer interesting
possibilities for oxidation catalysis. For example, TBHP has
been shown to significantly improve the performance of
several transition-metal catalysts in their higher oxidation
states in epoxidations. More specifically, transition metal–
alkyl hydroperoxides of titanium(IV), vanadium(V), molyb-
denum(VI), and tungsten(VI) showed better activity and
solubility compared to the corresponding transition metal–
hydrogen peroxide complexes in nonpolar solvents. From
a synthetic point of view, it is interesting that transition metal
alkyl hydroperoxides allow for highly selective epoxidations
towards allylic double bonds in comparison to other oxi-
dants.[7,8]
Apart from the well-known epoxidation catalysts based
on Ti, Mo, and W, in the last decade several catalysts based on
noble metals, for example, Pt, Pd, Ru, Ir, and Au were
introduced for highly selective epoxidations of olefins.[9] On
the other hand, the use of novel cost-effective non-noble
metal catalysts is less known.[10] Therefore, we started some
time ago a program to develop novel catalysts for the
epoxidation of olefins and related reactions based on bio-
relevant transition-metal complexes.[10]
In 2013, we successfully designed a series of novel Co- and
Fe-oxide supported catalysts for the selective reduction of
functionalized nitroarenes and oxidative esterifications.[11]
Based on this work, we became interested in applying these
materials in the epoxidation of olefins. Although cobalt-based
heterogeneous catalysts are well-known to be active in
various oxidation processes,[12] only a few examples are
known for selective epoxidation of alkenes.[13] Herein, we
report an efficient and general heterogeneous Co-catalyzed
epoxidation of alkenes with tert-butyl hydroperoxide under
mild conditions.
E
poxides constitute important intermediates for the pro-
duction of fine and bulk chemicals, especially for industrial
synthesis of polymers. Furthermore, they are valuable build-
ing blocks for the synthesis of a variety of bioactive
molecules.[1,2] The most efficient synthesis of oxiranes makes
À
use of the in situ formation of two C O bonds from an olefin
and a respective oxidant. Regarding atom efficiency and
waste generation, in general such oxidation reactions should
be performed using molecular oxygen or hydrogen peroxide
in combination with a suitable transition metal catalyst.[3]
Although molecular oxygen is the most abundant oxidant,
the vast majority of epoxidations with air still proceed with
50% atom efficiency.[4] For the synthesis of advanced building
blocks, catalytic epoxidations with O2 often require in situ
activation by hydrides, H2/Pt, ascorbic acid, or aldehydes.[5]
Hence, in recent years there exists an increasing interest in the
utilization of hydrogen peroxide as oxidant, both on labo-
ratory and industrial scales because of availability, price, and
safety issues. In fact, reaction of propylene with hydrogen
The catalyst materials were prepared by wet impregnation
of a combination of cobalt(II) acetate and different nitrogen-
containing ligands on Vulcan XC72R.[11] Subsequent pyrolysis
at 8008C led mainly to oxidic nanoparticles with varying size
(2–10 nm, predominantly Co3O4 with some CoO) and
agglomerates in a range of 20–80 nm, in addition to very
few large particles with a metallic Co core and an oxidic shell.
During the pyrolysis process the nitrogen ligand forms
graphene-type layers, in which selected carbon atoms are
substituted by nitrogen. X-ray photoelectron spectroscopy
(XPS) revealed three distinct nitrogen species (pyridine-type
nitrogen, pyrrole-type nitrogen, and a small amount of
quaternary amine species). Deconvolution showed that
[*] Dr. D. Banerjee, Dr. R. V. Jagadeesh, Dr. K. Junge, Dr. M.-M. Pohl,
Dr. J. Radnik, Prof. Dr. A. Brꢀckner, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
Homepage: http://www.catalysis.de
[**] The research has been funded by the State of Mecklenburg-Western
Pomerania, the BMBF, and the DFG (Leibniz Prize). We thank Dr. W.
Baumann, Dr. C. Fischer, S. Buchholz, S. Schareina, A. Koch, and S.
Rossmeisl (all at LIKAT) for their excellent technical and analytical
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310420.
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Table 1: CoxOy–N/C (x=1,3; y=1,4)-catalyzed epoxidation of trans-
stilbene with tert-butyl hydroperoxide.
nanopowder gave only 7% yield of trans-stilbene oxide
(Table 1, entry 14). It is worth noting that in cases with lower
product yield, 5–10% of benzaldehyde was observed as
a side-product in GC-MS analysis of the crude reaction
mixture. The catalytic epoxidation also took place in iso-
propyl alcohol and tert-amyl alcohol, albeit with lower
activity.
For the development of advanced, cost-effective, and
benign industrial processes, catalyst stability and recycling are
important. In metal-containing heterogeneous catalysts, the
leaching of the active metal from the supported materials is
often a problem. Such leaching processes result in diminished
reactivity and selectivity in catalytic processes. When studying
the recycling of our catalyst system, we did not observe any
leaching of the metal by atomic absorption spectroscopy
(AAS), even after several recycling experiments (Figure 1;
see also the Supporting Information). Similarly, a hot filtra-
tion test revealed no significant leaching.
Entry Support
Co cat. [mol%] Ligand Conv. [%][h] Yield [%][i]
1[a]
2[a]
3[a]
4[b]
5[b]
6[c]
7[c]
8[c]
9[c]
–
–
–
–
4
4
4
4
4
4
4
4
4
4
4
4
4
–
–
L1
–
2
3
10
5
15
>99
70
63
75
65
8
0
0
0
Vulcan XC72R
Vulcan XC72R
Vulcan XC72R
Vulcan XC72R
Vulcan XC72R
BN
0
L1
L1
L2
L3
L1
L1
L1
L1
L1
L1
5[j]
97
65
60
70
61[j]
5
10[c] TiO2
11[d] Vulcan XC72R
12[e] Vulcan XC72R
13[f] Vulcan XC72R
4
16
10
0
0
14[g]
–
7[j]
[a] Reaction under homogeneous conditions: trans-stilbene (0.5 mmol),
Co(OAc)2·4H2O (4 mol%), ligand (8 mol%), CH3CN (6 mL), tert-
BuOOH (5 equiv). [b] Reaction performed without pyrolysis. [c] Reaction
under heterogeneous conditions: trans-stilbene (0.5 mmol), catalyst
(40 mg; 4 mol% Co; 3 wt% Co), CH3CN (6 mL), tert-BuOOH (5 equiv).
[d] Reaction with H2O2 (30%). [e] Reaction with NaOCl. [f] Reaction
without tert-BuOOH. [g] Reaction was performed with carbon-supported
mixed Co-oxides. [h] Conversions were determined by GC-MS analysis of
the crude reaction mixture. [i] Yield of isolated product. [j] In the case of
lower yields, 5–10% of benzaldehyde was detected as a minor side
product. L1=1,10-phenanthroline, L2=terpyridine, L3=pyridine bis-
(imidazoline).
around 64% of all N atoms are bound to the metal ions. In
a similar manner, new cobalt oxides supported on boron
nitride and on TiO2 were also prepared.
Figure 1. Recycling of the active CoxOy–N/C (x=1,3; y=1,4) catalyst
for the epoxidation of trans-stilbene, according to the reaction con-
ditions of the model reaction (Scheme 1). Run 1 is with fresh catalyst.
For our initial catalytic tests, we choose trans-stilbene as
our model substrate. As a commercially available oxidant,
aqueous tert-butyl hydroperoxide (70%) was used (Table 1).
Not surprisingly, epoxidation under homogeneous conditions
.
using Co(OAc)2 4H2O did not form any of the desired trans-
To understand the influence of the oxidant as well as
reaction conditions on the structure of the active material,
transmission electron microscopy (TEM) analyses of both the
freshly pyrolyzed and the recycled catalyst after the first and
the fifth run were performed. These investigations indicated
no significant structural change during the oxidation proce-
dure, although a carbon support is used. In the active catalyst,
oxidic particles of different size coexist with larger ones
containing a metallic Co core and an oxidic shell. After the
first oxidation reaction, these core–shell particles are more
abundant. In addition to TEM measurements (Supporting
Information, Figure S1), XRD powder patterns show narrow
peaks of metallic cobalt, which point to a rather large
crystallite size. The presence of ferromagnetic Co metal
particles is suggested by temperature-dependent EPR meas-
urements (Figure S3). Furthermore, XRD analysis shows very
weak and broad reflections of CoO, suggesting that divalent
CoII is more predominant after recycling (Figure S2). This is
also confirmed by XPS data, which indicate the exclusive
presence of oxidized Co (mainly Co2+; Figure S4). Thus, it is
very probable that at least the larger particles consist of
stilbene oxide 2a (Table 1, entries 1–3). Similarly, the use of
unpyrolyzed phenanthrolin–CoII–acetate supported on
Vulcan XC72R showed only poor conversion into 2a
(Table 1, entries 4 and 5). Gratifyingly, the model reaction
resulted in nearly quantitative yield of trans-stilbene oxide in
the presence of the pyrolyzed carbon-supported phenanthro-
lin–CoII–acetate complex (Table 1, entry 6). Scale-up of this
reaction by a factor five led to trans-stilbene oxide in 94%
yield. Further reactions of different nanostructured cobalt
oxides, for example, carbon-supported terpyridine or pyridi-
nebisimidazoline Co-catalysts also gave significant yields (60–
65%) of the desired product (Table 1, entries 7 and 8). Next,
we investigated the effect of different support materials. To
our delight, pyrolysis of CoII-L1 under standard conditions
(8008C) on boron nitride (BN) and TiO2 also resulted in
active materials (Table 1, entries 9 and 10). When performing
the model reaction with hydrogen peroxide (30% aqueous
solution) or NaOCl as oxidant, only poor or no conversion to
the desired product was observed (Table 1, entries 11 and 12).
The reaction with commercially available mixed CoII,III oxide
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Table 2: CoxOy–N/C (x=1,3; y=1,4)-catalyzed epoxidation of aromatic
a metallic Co core covered by a CoO shell. This is also
supported, by TEM-EDX elemental mapping, which shows
the enrichment of oxygen on the large Co-containing particles
(Figure S1b). In agreement with our previous studies,[11] XP
spectra in the N1s region indicate the presence of three
different N species: a) N bound to Co ions at 399.0 eV
(comprising 65–70% of the N content of the sample),
and aliphatic olefins.[a]
Entry Olefin 1
Product 2
Yield
[%][b]
+
+
b) pyrrolic nitrogen at 400.8 e.V, and c) NH4 or R–NH3
species at 403.2 eV. Interestingly, almost no change in the
structural properties were observed after repeated use of the
catalysts, apart from slightly more pronounced agglomeration
of particles after the 5th run (Figure S1). A more detailed
description of all characterization data can be found in the
Supporting iInformation.
1
90
95
2
After having the optimized reaction conditions for the
model reaction in hand, we studied the selective epoxidation
of a series of aromatic alkenes, including stilbene derivatives,
1-aryl-2-alkyl, and trisubstituted olefins (Scheme 1). The
resulting epoxides are commonly used as intermediates for
3
4
5
95
70
95
6
60
Scheme 1. Catalytic epoxidation. Reaction conditions: stilbene
(0.5 mmol), catalyst (40 mg; 4 mol% Co), CH3CN (6 mL), tert-BuOOH
(5 equiv). Yields shown are of isolated products.
7
8
98
96
9[c]
85
the synthesis of various biologically active natural products
and drug molecules, for example, the corresponding amino
alcohols.[14] To our delight, epoxidation of cis- and trans-
stilbene, 1-phenylpropenes, and 1-phenyl-2-methylpropenes
proceeded in moderate to excellent yields to the correspond-
ing oxiranes 2b–2 f (Scheme 1). Notably, epoxidations of
a variety of substituted stilbene derivatives also afforded 2g–
2l in 60–95% yield (Table 2, entries 1–6).
10
11
30
95
[a] Reaction conditions: olefin (0.5 mmol), catalyst (40 mg; 4 mol% Co),
CH3CN (6 mL), tert-BuOOH (5 equiv). [b] Yield of isolated product.
[c] CH3CN (2 mL), 858C.
The high reactivity of the stilbene derivatives prompted us
to check our optimized method for the selective epoxidation
of more challenging cyclic and acyclic aliphatic alkenes
(Table 2). The reaction of norbornene and 2-methyl-1-hep-
tene resulted in selective formation of 2m and 2n in almost
quantitative yields (Table 2, entries 7 and 8). Further, cata-
lytic epoxidation of cyclooctene afforded 2o in 85% yield
(Table 2, entry 9). The reaction of 1-decene resulted in the
selective formation of 1,2-epoxidecene; however, owing to
the decreased reactivity of this linear terminal alkene, 2p was
obtained in only 30% yield (Table 2, entry 10). Gratifyingly,
functionalized alkenes such as cinnamyl acetate also afforded
almost quantitative yield of 2q (Table 2, entry 11).
In all reactions, we observed excellent chemoselectivity
(> 99%) for the respective epoxides, as shown by GC-MS
analysis of the crude reaction mixtures. When we obtained
a lower yield of epoxides, the corresponding olefins were
recovered from the reaction mixtures.
Next, our optimized method was applied to the epoxida-
tion of various renewables, including inexpensive terpenes,
such as a-pinene, (R)-(+)-limonene, geraniol acetate, and
methyl oleate (as an example of a fatty acid ester; Scheme 2).
Functionalization and epoxidation of this latter substrate
allowed access to industrially important intermediates for
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be useful for the oxidation of renewable olefins, as well as
vitamin and cholesterol derivatives. Characterization studies
using TEM, EPR, and XPS analysis did not show any
significant difference in the structure of the recycled catalyst
versus the fresh catalyst.
Experimental Section
The CoxOy–N/C (x = 1,3; y = 1,4) catalyst (Co-L1/C; 4 mol% Co,
40 mg) followed by an alkene (0.5 mmol) and acetonitrile (6 mL)
were added to a 25 mL glass tube under air. Then, tert-butylhydro-
peroxide (5 equiv; 70% aqueous solution) was added to the reaction
mixture and it stirred for 24 h in a preheated oil bath at 708C. The
reaction mixture was cooled to room temperature and diluted with
ethyl acetate. The catalyst was filtered off and concentrated under
reduced pressure. The crude reaction mixture was subjected to silica
gel column chromatography using an n-hexane/ethyl acetate mixture
to afford the desired epoxides.
Scheme 2. Epoxidation of renewable olefins. Reaction conditions:
alkene (0.5 mmol), catalyst (40 mg; 4 mol% Co), CH3CN (6 mL),
tert-BuOOH (5 equiv). Yields shown are of isolated products.
lubricants, basic chemicals, solvents, and biopolymers.[15]
Whereas epoxidation of a-pinene and (R)-(+)-limonene
resulted in 35–45% yield of 2r and 2s, geraniol acetate and
methyl oleate gave 75–93% yield of 2t and 2u.
Finally, we studied the epoxidation of biologically inter-
esting cholesterol and vitamin derivatives 3a–c (namely, 5a-
cholestan-3b-ol and 5-pregnen-3b-ol-20-one) of trans-styrylic
acid (4-phenyl-3-butenoic acid). To our delight, both reac-
tions afforded 80–85% yields of the corresponding epoxides
4a and 4b, without affecting the parent cholesterol moiety.
Similar reaction of (Æ)-a-tocopherol styrylicacetic ester
resulted in 78% yield of 4c (Scheme 3).
In summary, we have developed a convenient epoxidation
method using heterogeneous cobalt catalysts. Key to the
success of the reaction is the use of specific CoxOy–N/C (x =
1,3; y = 1,4) materials, which can be easily recycled. Various
aromatic and aliphatic epoxides are cost-effectively prepared
from the corresponding olefins with excellent selectivity and
often high yields. Apart from the epoxidation of standard
benchmark systems, this heterogeneous catalyst has proven to
Received: December 1, 2013
Published online: March 18, 2014
Keywords: cobalt oxides · epoxidation ·
.
heterogeneous catalysis · olefins · recycling
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