Highly efficient epoxidation of cyclic alkenes catalyzed by ruthenium complex

Article abstract of DOI:10.1039/b212899c

The epoxidation of cyclic alkenes with molecular oxygen was efficiently completed in excellent epoxide yield using a novel ruthenium complex as catalyst under mild reaction conditions.

Full text of DOI:10.1039/b212899c

Highly efficient epoxidation of cyclic alkenes catalyzed by ruthenium
complex
Jian Ying Qi, Li Qin Qiu, Kim Hung Lam, Chiu Wing Yip, Zhong Yuan Zhou and Albert S. C.
Chan*
Open Laboratory of Chirotechnology of the Institute of Molecular Technology for Drug Discovery and
Synthesis and Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic
University, Hong Kong, China. E-mail: bcachan@polyu.edu.hk; Fax: 852 2364 9932; Tel: 852 27665607
Received (in Cambridge, UK) 3rd January 2003, Accepted 20th March 2003
First published as an Advance Article on the web 7th April 2003
The epoxidation of cyclic alkenes with molecular oxygen was
efficiently completed in excellent epoxide yield using a novel
ruthenium complex as catalyst under mild reaction condi-
tions.
molecular structure of the complex with one molecule of
ethanol in the crystal lattice was determined by single crystal X-
ray diffraction† and the ORTEP drawing of it is shown in Figure
2.
The crystal structure revealed the deprotonation of the amide
group of one of the coordinated ligands and the coordination of
this anionic ligand (L) to the metal center through the binding of
the negative amide nitrogen and the pyridine nitrogen. The
negative amido ligand and two coordinated chloride ligands
The selective oxidation of hydrocarbons using molecular
oxygen as an oxidant is a highly attractive reaction because of
the low cost and environmentally friendly nature of the
1
3+
oxidant. In 1990, Mukaiyama et al. reported an important
electronically balanced the trivalent Ru metal center, while
the other ligand (HL) coordinated to Ru3 via a carbonyl oxygen
and a pyridine nitrogen. Therefore, the vibrational frequency of
the carbonyl (CNO) in the complex was lower than that in the
free ligand. Characteristic hydroxyl (O–H) vibration frequency
was also found due to the presence of hydroxyl of ethanol in the
complex.
+
breakthrough in the epoxidation of olefins using dioxygen as
2
oxidant under ambient conditions. The process involved the
2
+
2+
3+
use of b-diketonate complexes of Ni , Co , and Fe as
catalysts and an aldehyde as an oxygen acceptor. The yield for
1
,2-epoxycyclohexane reached 84% after carrying out the
reaction for 13 hours. Corain et al. used a copper b-
carbonylenolate catalyst and obtained similar conversion and
In our study of the epoxidation of cyclohexene with O
2
3
epoxide selectivity after 20–30 hours of reaction. Recently
catalyzed by the Ru complex (entry 1–9 in Table 1), we
discovered that the reaction proceeded with high conversion and
high selectivity for the desired 1,2-epoxycyclohexane product
by using a substrate-to-isobutyraldehyde molar ratio of 1+2 or
1+3 (entry 1–3). When the substrate/isobutyraldehyde molar
ratio was 1+2, catalyst concentration had a notable effect on the
result of the epoxidation (entry 4–7). The results indicated that
the Ru complex was highly efficient in the catalytic reaction.
High conversion was achieved even with a catalyst concentra-
Thomas et al. described a selective epoxidation of cyclohexene,
using Mn(III)- and Co(III)-containing molecular sieves as
catalysts. The conversion and epoxide selectivity were 62% and
7
7%, respectively, after 8 hours of reaction at 50 °C.4
The epoxidations of other cyclic olefins were relatively less
studied and the reported systems usually employed other
5
6
7
oxidants such as H
2 2
O , urea hydrogen peroxide or oxone
8
instead of molecular oxygen.
2
1
In the pursuit of economical and environmentally friendly
processes for the production of epoxides, the selective epoxida-
tion of olefins with molecular oxygen is particularly desirable.
In this communication we report a highly effective catalyst
system for the selective epoxidation of cyclic alkenes with
molecular oxygen using a new ruthenium catalyst, namely
tion as low as 0.22 mmol L . A control experiment without
catalyst (entry 8) indicated that the conversion was only
21.5%.
In the absence of isobutyraldehyde (entry 9), low conversion
rate and low selectivity were observed and the major product
was 2-cyclohexen-1-one besides the epoxide. These results
were consistent with a previous suggestion that there were two
possible types of active species in the reaction,10 one of which
produced the epoxide and the other induced the formation of
allylic products 2-cyclohexen-1-ol and 2-cyclohexen-1-one.
The presence of isobutyraldehyde in the reaction favored the
Ru(HL)(L)Cl
2
(where HL is a new ligand N-2A-chlorophenyl-
2
-pyridine-carboxamide and L is its corresponding anion).
Using this complex as catalyst, the epoxidation of cyclic alkenes
by molecular oxygen was usually completed within 3–9 hours
with excellent conversion and selectivity (e.g. 99.9% conver-
sion and 97.4% selectivity for the epoxidation of cyclooc-
tene).
The ligand and catalyst were prepared as shown in Figure 1.
9
The red-brownish Ru complex was characterized by FTIR. The
Fig. 1 Synthetic route of ligand HL 1 and the corresponding Ru-complex
Fig. 2 Molecular structure of complex 2 (thermal ellipsoids are set at 30%
2
.
probability; hydrogen atoms are omitted for clarity).
1
058
CHEM. COMMUN., 2003, 1058–1059
This journal is © The Royal Society of Chemistry 2003

formation of the first type of active species and resulted in
epoxide as a major product.
mixture, the epoxidation was stopped. This result is consistent
with those of Haber,11 Valentine, and Nolte using por-
phyrin, cyclam or b-diketonate complexes of metal as catalyst.
Further study on the mechanism of the reaction is in progress.
In conclusion, we have prepared a novel ruthenium complex
12
13
In the epoxidations of different cyclic alkenes (entry 10–13),
most substrates except 1-phenylcyclohexene gave > 99.9%
conversion and excellent selectivity. The highest selectivity for
cyclooctene reached 97.4%. The epoxidation of 1-phenyl-
cyclohexene did not give high selectivity, probably due to the
large spatial hindrance of the phenyl group attached to the
double bond.
Although the mechanism of the catalytic reaction and the role
of the metal complex are still not clear, it is reasonable to
speculate a mechanism involving radical species, with the metal
complex acting as an initiator of the reaction as well as a catalyst
for the epoxidation. When a radical trapping compound such as
2
Ru(HL)(L)Cl and its molecular structure has been determined
by X-ray diffraction. When cyclic alkenes were oxidized with
molecular oxygen using this complex as catalyst, the epoxida-
tion was highly efficiently completed giving up to 97.4% yield
of the desired epoxides.
We thank the Hong Kong Polytechnic University ASD Fund
and the University Grants Committee Areas of Excellence
Scheme (AOE P/10–01) for financial support of this study.
2,6-di-tert-butyl-4-methylphenol was added to the reaction
Notes and references
Table 1 Epoxidation of cyclic alkenes catalyzed by complex 2 a
†
17 l4 4 2 2 5
Crystal data for the complex 2: RuC24H C N O .C H OH, M = 699.87,
monoclinic, a = 16.706(2), b = 13.6138(17), c = 15.7145(19) Å, U =
3
3
219.7(7) Å , T = 294 K, space group P21/c, Z = 4, µ(Mo–Ka) = 0.854
1
2
mm , 21105 reflections collected, 7350 independent reflections (Rint
=
0
.0629). The final R indices [I > 2s(I)]: R1 = 0.0506, wR2 = 0.1287, R
indices (all data): R1 = 0.1021, wR2 = 0.1561. CCDC 183350. See http://
www.rsc.org/suppdata/cc/b2/b212899c/ for crystallographic data in .cif or
other electronic format.
Selectivity (%)
Reaction
Entry
Substrate
time (h)
Conv. (%)
Epoxideb
Others
1
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1 c
3
3
3
4.0
95.0
99.5
99.7
95.7
84.0
87.0
88.1
85.1
16.0
13.0
11.9
14.9
d
2
3
4
e
f
5 g
3.5
97.3
88.1
11.9
9.9
1
27; S.-I. Murahashi, Angew. Chem., Int. Ed. Engl., 1995, 34, 2443.
h
6
7
8
9
3.5
2.5
3.0
3.0
> 99.9
> 99.9
21.5
90.1
85.1
68.7
45.8
2 D. H. R. Barton, A. E. Martell and D. T. Sawyer, The Activation of
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2513.
i
j
14.9
k
31.3
54.2
l
j
3.4
1
1
1
0
3
9
6
7
> 99.9
> 99.9
> 99.9
> 99.9
90.1
9.9
2.6
3
4
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1
97.4
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000, 7, 537; J. Rudolph, K. L. Reddy, J. P. Chiang and K. B. Sharpless,
2
J. Am. Chem. Soc., 1997, 119, 6189.
2 m
90.2 n
78.0 n
9.8
6 G. S. Owens and M. M. Abu-Omar, Chem. Commun., 2000, 13,
1
165.
7
8
9
M. F. A. Adamo, V. K. Aggarwal and M. A. Sage, J. Am. Chem.Soc.,
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13
22.0
a
The reactions were carried out at room temperature under the following
The yield for this complex was 81%. The IR spectra shown
conditions: substrate = 0.93 M; catalyst = 0.88 mM; isobutyraldehyde =
2
1
21
b
characteristic bands for ligand at 3302 cm (nN–H), 1693 cm (nCNO)
2 2 2
1.85 M; O = 1 atm; 4mL ClCH CH Cl solvent. Epoxides were identified
21
21
c
and for complex at 3445 cm (nO–H), 1615 cm (nCNO).
by using authentic samples for comparison. Molar ratio of alkene to
d
10 Y. Tsuda, S. Matsui and K. Takahahi, J. Mol. Catal., 1999, 148, 183.
11 J. Haber, T. Mlodnicka and M. Witko, J. Mol. Catal., 1989, 52, 85; J.
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Muller, C. J. Burrows and J. S. Valentine, Inorg. Chem., 1996, 35,
isobutyraldehyde was 1:1. Molar ratio of alkene to isobutyraldehyde was
e
f
1
:2. Molar ratio of alkene to isobutyraldehyde was 1:3. Catalyst
g
concentration was 0.22 mM. Catalyst concentration was 0.44 mM.
h
i
Catalyst concentration was 0.88 mM. Catalyst concentration was 1.76
j
k
l
mM. Main by-product was 2-cyclohexen-1-one. No catalyst. No
aldehyde. m n-butyraldehyde was used. n The epoxides were identified by
GC (HP 5890 or 4890, column AT-1 30m 3 0.25 mm) and GC-MS (HP
G1800C).
6
632.
1
3 B. B. Wentzel, P. A. Gosling, M. C. Feiters and R. J. M. Nolte, J. Chem.
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