Efficient ruthenium-catalysed oxidative cleavage of methyl oleate with hydrogen peroxide as oxidant

Article abstract of DOI:10.1039/c2ra22370h

The oxidative cleavage of alkenes leads to the formation of carboxylic acids. One of the few technical processes using this reaction is the production of azelaic acid via the ozonolysis of oleic acid. Because of the need for stoichiometric amounts of the expensive oxidant ozone, together with safety hazards, there is still a requirement for a catalytic process using a cheap and environmentally friendly oxidant. In the present work, the oxidative cleavage of methyl oleate by hydrogen peroxide was catalysed by an easily available ruthenium precursor with dipicolinic acid as ligand. The systematic optimisation of the reaction led to the formation of azelaic acid monomethyl ester in high yields amounting to 86%. The investigation of the reaction pathway showed that the reaction proceeds via a tandem reaction of epoxidation and hydrolysis of the epoxide and oxidative cleavage of the vic-diol. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2ra22370h

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Efficient ruthenium-catalysed oxidative cleavage of
methyl oleate with hydrogen peroxide as oxidant
Cite this: RSC Advances, 2013, 3, 172
Arno Behr,* Nils Tenhumberg and Andreas Wintzer
The oxidative cleavage of alkenes leads to the formation of carboxylic acids. One of the few technical
processes using this reaction is the production of azelaic acid via the ozonolysis of oleic acid. Because of the
need for stoichiometric amounts of the expensive oxidant ozone, together with safety hazards, there is still
a requirement for a catalytic process using a cheap and environmentally friendly oxidant. In the present
work, the oxidative cleavage of methyl oleate by hydrogen peroxide was catalysed by an easily available
ruthenium precursor with dipicolinic acid as ligand. The systematic optimisation of the reaction led to the
formation of azelaic acid monomethyl ester in high yields amounting to 86%. The investigation of the
reaction pathway showed that the reaction proceeds via a tandem reaction of epoxidation and hydrolysis
of the epoxide and oxidative cleavage of the vic-diol.
Received 2nd October 2012,
Accepted 11th October 2012
DOI: 10.1039/c2ra22370h
www.rsc.org/advances
oxidant in the chemical industry. Hence, there is a need for a
selective, economical and energy-efficient, as well as safe
oxidation process for the production of azelaic acid with the
use of an environmentally friendly and cheap oxidant.
Introduction
The oxidative cleavage of alkenes is one of the paramount
reactions of organic chemistry, and is widely investigated.
Depending on the reaction conditions, aldehydes or ketones,
as well as carboxylic acids, can be formed.
The transition-metal catalysed oxidative cleavage of oleic
acid is often valued as a promising alternative process for the
industrial production of azelaic acid. Molecular oxygen seems
to be the obviously ideal oxidant, since it is the most important
and cheapest oxidant in the chemical industry. However,
oxidations with molecular oxygen are sometimes difficult to
control and are less selective in comparison to other oxidants,
especially in the direct oxidation of CLC bonds.57,58 Similarly,
the aerobic catalytic cleavage of oleic acid or methyl oleate is
only rarely described in the literature. Dapurkar et al.59 used
The common method for the direct oxidative cleavage of
alkenes is ozonolysis.1 Besides ozone, direct cleavage of
alkenes can also be performed with catalytic amounts of
transition metals such as ruthenium,2–18 osmium,19–24 palla-
dium,25 manganese,26 tungsten,13,27–37 molybdenum15,34,38–41
and rhenium13,42–44 in combination with numerous oxidants
for example NaOCl,3,4 NaIO4,3,5–7,20 peracetic acid,3,13 hydro-
gen peroxide3,13,15–18,24,27–39,42–44 or oxygen.14,23,25,26,45–51
One of the rare industrial applications of the oxidative
cleavage of alkenes is the production of azelaic acid from oleic
acid via ozonolysis (Fig. 1).52 Pelargonic acid is formed as a by-
product in stoichiometric amounts. The total amount of
azelaic acid produced is several 1000 tons/year.53 Azelaic acid
is industrially used in the manufacture of polyamides,
laminates, adhesives, plasticisers and hydraulic fluids.52
Pelargonic acid is also used in the manufacture of lubricants
and plasticisers.54
microporous and mesoporous catalysts containing chromium
23
¨
whereas Kockritz et al. utilised an osmium catalyst in
combination with an aldehyde as cooxidant.
According to the limited selectivity of the oxidation of CLC
bonds with molecular oxygen, different two step processes
were also examined for the oxidative cleavage of unsaturated
fatty acids with both oxygen and H2O2 as oxidants. The first
step is usually the dihydroxylation of the unsaturated fatty acid
to the vic-diol with H2WO4/H2O2 or formic acid/H2O2, followed
by the aerobic cleavage of the vic-diol with H2WO4/Co(acac)3/
N-hydroxyphthalimide60,61 or Co/Mn/Br62 as catalysts.
Apart from molecular oxygen, H2O2 is the cheapest, and
also an environmentally friendly oxidant since only water is
formed as a by-product. Therefore, it is also a suitable oxidant
for the catalytic cleavage of oleic acid. For instance, Re2O7,13
H2WO4,13,37 a tungsten heteropoly acid27 or molybdenum
compounds39 were used as a catalyst with H2O2 as oxidant.
The cleavage of oleic acid with a two-metal system containing
MoO3 and RuCl3 under phase transfer conditions is described
Although ozonolysis has been established as an industrial
process for a long time,55,56 the oxidative cleavage of oleic acid,
as well as the ozonolysis itself, is still the subject of research
and development. Because of the need for stoichiometric
amounts of the expensive oxidant ozone, and the safety
hazards associated with the process, ozone is not a large-scale
¨
¨
Technische Universitat Dortmund, Lehrstuhl fur Technische Chemie A, Emil-Figge
Str. 66, Dortmund, 44227, Germany. E-mail: behr@bci.tu-dortmund.de;
Fax: +49-231-755-2311
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Fig. 1 Ozonolysis of oleic acid.52
by Service et al.15 It is assumed that MoO3 catalysed the
dihydroxylation of oleic acid, whereas RuCl3 catalysed the
cleavage of the formed vic-diol. Recently, a review about the
were systematically optimised with respect to precursor,
ligand, temperature, Ru/ligand ratio, solvent, reaction time
and pH-value.
¨
synthesis of azelaic acid was published by Kockritz and
Martin.53 However, up to the present time, no alternative to
industrially employed ozonolysis of oleic acid has been
developed.
Results and discussion
Since we were interested in the research and development of a
new catalyst for the oxidative cleavage of methyl oleate 1 by
H2O2, we decided to develop the desired catalyst by performing
the reaction with the formation of the epoxide 2 and the vic-
diol 3 as intermediate (see Fig. 2). This reaction pathway is
also known as hydrolytic pathway (see Fig. 5, pathway A).27
With respect to the reaction mechanism, we assumed that we
needed a catalyst which can be used for the epoxidation and/or
dihydroxylation of olefins, as well as for the oxidative cleavage
of vic-diols. Thus we were also interested in these individual
reactions.
In our investigations concerning the oxidation of methyl
oleate 1 with H2O2, we found that the catalyst system
Ru(acac)3/dipicolinic acid (pyridine-2,6-dicarboxylic acid)
(Ru(acac)3/PDC) is an efficient catalyst for the epoxidation of
methyl oleate 1.63 Under our optimised reaction conditions for
the oxidation of methyl oleate 1, the epoxide 2 was obtained in
quantitative yield with acetonitrile (CH3CN) as solvent and at
92% with tert-butyl alcohol (t-BuOH).
The ruthenium catalysed cleavage of alkenes is usually
performed with catalytic amounts of high-valent RuO4 and
NaOCl,3,4 NaIO4
or peracetic acid3,13 as stoichiometric
3,6,7
oxidants. In contrast, the ruthenium catalysed cleavage of
alkenes with H2O2 as an oxidant is only rarely described in the
literature, because H2O2 is unable to oxidise low-valent
ruthenium compounds into the catalytic active species RuO4.
Only the cleavage of alkenes to carboxylic acids with [(Me3tacn)
(CF3CO2)2Ru(III)(OH2)]CF3CO2 (Me3tacn = 1,4,7-trimethyl-1,4,7-
cyclononane)16 and the cleavage of primary alkenes to
aldehydes with [cis-Ru(II)(dmp)2(H2O)2][PF6]2 (dmp = 2,9-
dimethylphenanthroline),17 are described. To the best of our
knowledge, with the exception of cleavage of fatty acids with
the two-metal system MoO3/RuCl3, the ruthenium catalysed
cleavage of fatty acids by H2O2 has not been described.15
In the present study, we report the selective oxidative
cleavage of methyl oleate to azelaic acid monomethyl ester by
H2O2, with a simple ruthenium salt and with a special
emphasis on the reaction mechanism. The reaction conditions
Fig. 2 Suggested reaction pathway for the oxidative cleavage of methyl oleate by Ru/PDC with optimized reaction conditions.
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Table 1 Influence of the pH value on the acid catalysed hydrolysis of epoxide 2a
reaction conditions of the three individual reactions, are
shown in Fig. 2.
As a starting point for the reaction conditions for the
oxidative cleavage of methyl oleate 1, we modified the reaction
pH
1.5
2.0
2.4
3.1
4.0
Conversion 2 [%]
Yield 3 [%]
81
85
97
62
4
24
79
97
61
1
conditions of the epoxidation.
A metal–ligand ratio of
Ru(acac)3 : PDC = 1 : 10 and a mixture of t-BuOH : H2O =
3 : 1 as solvent was chosen, because the water is necessary for
the hydrolysis of the epoxide 2. Due to the fact that H+ is
already formed by dissolving the Ru(acac)3, and PDC in the
solvent (pH-value # 2.9), no additional H2SO4 was added. The
H2O2 (3.3 eq. or 8 eq.) was slowly added to the reaction mixture
over a period of 12 h by a syringe pump to avoid unproductive
decomposition of the H2O2. A total reaction time of 16 h was
chosen. At first, the reaction was performed at different
temperatures (Table 3).
6.8
4
4
5
4
0
0
0
0
10.1
11.2
13.2
14.0
8
0
a
Reaction conditions: 2.5 mmol 2, 10 ml solvent mixture
(t-BuOH : H2O = 3 : 1), H2SO4 or NaOH, 80 uC, 4 h.
In agreement with the suggested reaction pathway, the
conversion of methyl oleate 1 under the chosen reaction
conditions led to the formation of the carboxylic acids 4 and 5.
At 80 uC, the carboxylic acids 4 and 5 were formed as the major
products with yields of 51% each. With eight equivalents of
H2O2 (40 mmol), the carboxylic acids 4 and 5 were observed
with a yield of 66%. In contrast, no carboxylic acids were
formed at room temperature or 40 uC, since the hydrolysis of
the epoxide 2, as well as the oxidative cleavage of vic-diol 3, is
preferred at higher temperatures. As expected at lower
temperature, epoxide 2 is formed as a major compound with
yields of up to 74%, whereas vic-diol 3 yields only small
amounts, because the hydrolysis of epoxide 2 proceeds slowly
under these reaction conditions.
Thus, we were able to present Ru(acac)3/PDC as a new
catalyst system for the oxidative cleavage of methyl oleate 1 by
analysing the reaction mechanism and investigation of the
three single reactions of the hydrolytic reaction pathway (e.g.
epoxidation, hydrolysis, cleavage of vic-diol). After establishing
the oxidative cleavage with Ru(acac)3/PDC, the reaction
conditions were optimised.
We also found that epoxide 2 can be easily converted into
the diol 3 in nearly quantitative yields in an aqueous t-BuOH
solution, with H2SO4 as catalyst. As shown in Table 1, with a
pH-value from 2.0 to 3.1, vic-diol 3 could be obtained with high
yields and high selectivity. In contrast, the epoxide 2 seemed to
be stable under basic conditions because the hydrolysis does
not occur in significant amounts at pH ¢ 4.0.
Ruthenium, especially Ru(PPh3)3Cl2, has been observed to
be an effective catalyst for the cleavage of vic-diols by H2O2, as
well as for the cleavage of vic-diol 3 into carboxylic acids 4 and
5.64 Our investigation shows that RuCl3 as well as the catalyst
system RuCl3/PDC, which we already used for the epoxidation
of methyl oleate 1, could also cleave the vic-diol 3 into
carboxylic acids 4 and 5 (Table 2). As by-products, the
aldehydes nonanal 6 and 9-oxononanoic acid methyl ester 7
as well as the acyloins 8 and the 9,10-diketostearic acid methyl
ester 9, are also formed (structures are shown in Fig. 5).
Our investigation of these three individual reactions
showed that the epoxidation of 1, and the oxidative cleavage
of the vic-diol 3, can be catalysed by the same catalyst, and also
that the hydrolysis of epoxide 2 could be performed under
similar reaction conditions. We therefore assumed that
Ru(acac)3/PDC would also be an effective catalyst for oxidative
cleavage by performing the epoxidation, the hydrolysis and
oxidative cleavage of vic-diol 3 consecutively in situ as an auto-
tandem reaction. The suggested reaction pathway for the
oxidative cleavage of 1 with Ru(acac)3/PDC, and the optimised
Different ligands instead of PDC were tested, but with
respect to the fact that the epoxidation of methyl oleate 1 only
works quite well with PDC or 2,6-pyridinedimethanol,63 we
only obtained the carboxylic acids 4 and 5 with these two
ligands (Table 4). Therefore, DPC or 2,6-pyridinedimethanol as
Table 3 Oxidation of methyl oleate 1: Variation of the reaction temperaturea
Yield of oxidation products [%]
Table 2 Ruthenium-catalysed oxidative cleavage of the vic-diol 3a
T/uC Conversion 1 [%] Y 2
Y 3
Y 4
Y 5
Yield of oxidation products [%]
rt
94
100
99
99
99
74
65
20
5
5
18
24
1
0
0
25
50
66
0
0
20
51
65
40
60
80
80b
Catalyst
Conversion 3 [%] Y 4 Y 5 Y 6 Y 7 Y 8 Y 9
Ru(PPh3)3Cl2 100
RuCl3/PPh3 100
RuCl3/PDC
RuCl3
63
64
58
66
74
75
71
79
0
0
6
0
0
0
3
0
1
0
4
0
12
12
21
10
2
2
a
98
100
Reaction conditions: 5 mmol 1, 0.05 mmol Ru(acac)3, 0.5 mmol
dipicolinic acid, 20 ml solvent mixture (t-BuOH : H2O = 3 : 1), pH =
b
2.9, 12 h: slow addition of 16 mmol 35% H2O2. 40 mmol 35%
H2O2 in 4 ml t-BuOH, total reaction time: 16 h.
a
Reaction conditions: 2 mmol 3, 0.004 mmol Ru (0.2 mol%), 0.012
mmol ligand, 10 ml t-BuOH, 80 uC, 120 min: slow addition of 16
mmol 35% H2O2, total reaction time: 135 min.
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Table 4 Oxidation of methyl oleate 1: Variation of the liganda
for the quantitative hydrolysis of epoxide 2. Accordingly,
carboxylic acid 4 and 5 are only formed in minor amounts. The
optimal reaction mixture t-BuOH : H2O = 3 : 1 presents a
compromise between an effective epoxidation which is
preferred by a small amount of water, and the effective
hydrolysis of the epoxide 2 to diol 3 which is benefited by
higher amounts of water.
Yield of oxidation products [%]
Conversion
Ligand
1 [%]
Y 2
Y 3
Y 4
Y 5
PDC
2,6-Pyridine-
dimethanol
99
99
2
4
2
4
66
44
65
34
Subsequently the Ru : PDC ratio (Table 7) and the catalyst
concentration were optimised. Good yields of carboxylic acids
4 and 5 are obtained with an excess of the ligand (M : L ,
1 : 10). The highest yields of carboxylic acids 4 and 5 are
observed with a Ru : PDC ratio of 1 : 20, which conforms to
the optimised reaction condition for the epoxidation of methyl
oleate 1. With small amounts of PDC (M : L = 1 : 2), epoxide 2
was formed as the main product, whereas carboxylic acids 4
and 5 are only obtained in small amounts. We believe that an
increasing concentration of PDC leads to lower pH-values,
which will have a positive effect on the hydrolysis of epoxide 2.
The effect of the pH-value on the reaction is described later.
Variation of the catalyst concentration shows that the
reaction could produce good yields with low catalyst concen-
trations from 0.50 mol% to 1.00 mol% of Ru(acac)3.
Further exploration showed that the yield of carboxylic
acids 4 and 5 could be clearly increased by increasing the time
t for adding the H2O2 as well as the total reaction time tt
(Table 8). High yields of carboxylic acids 4 and 5 up to 86%
were achieved by adding the H2O2 slowly over about 20 h. In
contrast with a low total reaction time of 6 h, only 40% of 4
and 5 together with broad amounts of epoxide 2 and vic-diol 3
as by-products, were formed. The results indicated that the
slow addition of the oxidant by a syringe pump leads to a
better efficiency, and avoids the nonproductive decomposition
of H2O2. Also, a higher selectivity towards the desired
products, the carboxylic acids 4 and 5, is achieved.
N-Methylimino- 24
7
2
0
0
0
0
0
diacetic acid
Picolinic acid
24
10
N-oxide
a
Reaction conditions: 5 mmol 1, 0.05 mmol Ru(acac)3, 0.5 mmol
ligand, 20 ml solvent mixture (t-BuOH : H2O = 3 : 1), 80 uC, 12 h:
slow addition of 40 mmol 35% H2O2 in 4 ml t-BuOH, total reaction
time: 16 h.
ligands as well as Ru(acac)3 or RuCl3 as ruthenium sources are
essential for this reaction.
Further investigation shows that different organic solvents
can be used for the oxidative cleavage of methyl oleate 1
(Table 5). Best results are obtained with aqueous mixtures of
t-BuOH and CH3CN, which are also the preferred solvents for
ruthenium catalysed epoxidation of methyl oleate 1,63 whereas
with 1,4-dioxane smaller amounts of the carboxylic acids 4 and
5 are obtained. In aqueous dimethylformamide (DMF), no
carboxylic acids are observed, because the hydrolysis of
epoxide
2
does not occur in this solvent mixture.
Accordingly, epoxide 2 is formed as a major product.
Next, the effect of different concentrations of H2O in the
solvent mixture was investigated, with the knowledge that H2O
is mandatory for the hydrolysis of epoxide 2, but inhibits the
epoxidation of methyl oleate. Table 6 shows the important
influence of the water content in the solvent mixture towards
the selectivity of the desired acids 4 and 5. The highest yields
of the carboxylic acids
t-BuOH : H2O ratio of between 4 : 1 and 2 : 1.
In contrast with the small amount of water (t-BuOH : H2O =
19 : 1), only 36% of carboxylic acids are formed. In pure
t-BuOH, epoxide 2 is formed as a major compound with an
amount of 32%, because there is probably not enough water
An interesting and important parameter in the oxidative
cleavage of methyl oleate 1 is the pH-value (Fig. 3). As already
described in Table 1, the hydrolysis of the epoxide 2 to the vic-
diol 3 only occurs in high yields at pH-values , 4.0. This
explains why also at pH-values , 4.0 high amounts of
carboxylic acids 4 and 5 are formed, whereas with pH-values
¢ 4.0 epoxide 2 is formed as the main product.
4 and 5 are obtained with a
Table 6 Oxidation of methyl oleate 1: Variation of H2O content in the solvent
mixturea
Table 5 Oxidation of methyl oleate 1: Variation of the organic solventa
Yield of oxidation products [%]
Yield of oxidation products [%]
Ratio
Solvent
Conversion 1 [%] Y 2
Y 3
Y 4
Y 5
t-BuOH : H2O Conversion 1 [%] Y 2
Y 3
Y 4
Y 5
t-BuOH : H2O
CH3CN : H2O
1,4-Dioxane : H2O 98
DMF : H2O
99
99
2
0
5
2
4
4
5
66
57
48
0
65
65
50
0
pure t-BuOH 98
32
13
3
2
1
10
6
3
2
2
14
35
64
66
63
54
10
36
64
65
64
57
19 : 1
4 : 1
3 : 1
2 : 1
1 : 1
98
99
99
99
99
88
42
a
Reaction conditions: 5 mmol 1, 0.05 mmol Ru(acac)3, 0.5 mmol
dipicolinic acid, 20 ml solvent mixture (3 : 1), 80 uC, 12 h: slow
addition of 40 mmol 35% H2O2 in 4 ml t-BuOH, or respective
organic solvent, total reaction time: 16 h.
6
2
a
Reaction conditions: 5 mmol 1, 0.05 mmol Ru(acac)3, 0.5 mmol
dipicolinic acid, 20 solvent mixture, 80 uC, 12 h: slow addition of 40
mmol 35% H2O2 in 4 ml t-BuOH, total reaction time: 16 h.
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Table 7 Oxidation of methyl oleate 1: Variation of the Ru/PDC ratioa
Yield of oxidation products [%]
Ru:PDC Conversion 1 [%] Y 2
Y 3
Y 4
Y 5
1 : 2
1 : 5
1 : 10
1 : 20
1 : 30
97
98
99
100
100
37
11
2
1
1
4
5
2
3
3
11
46
66
70
67
12
49
65
73
68
a
Reaction conditions: 5 mmol 1, 0.05 mmol Ru(acac)3, dipicolinic
acid, 20 ml solvent mixture (t-BuOH : H2O = 3 : 1), 80 uC, 12 h: slow
addition of 40 mmol 35% H2O2 in 4 ml t-BuOH, total reaction time:
16 h.
Fig. 3 Influence of the pH-value on the ruthenium-catalysed oxidation of methyl
oleate. Reaction conditions: 5 mmol 1, 0.05 Ru(acac)3, 1.0 mmol dipicolinic acid,
20 ml solvent mixture (t-BuOH : H2O = 3 : 1), 80 uC, H2SO4 or NaOH, 20 h: slow
addition of 40 mmol 35% H2O2 in 4 ml t-BuOH, total reaction time: 24 h.
Thus, preceding the reaction with a pH ¢ 4.0, the reaction
stops at the epoxide stage because the hydrolysis of epoxide 2
does not take place in a significant amount and therefore only
small amounts of carboxylic acids or vic-diol 3 were formed.
The results showed clearly that the pH-value can affect the
product composition between the carboxylic acids 4 and 5 and
the epoxide 2. The highest yields are obtained at a pH-value of
3.0, which is achieved by preparing the reaction solution
without adding any acid or base. Similar high yields were only
achieved at pH = 3.2. When the pH-value was decreased from
3.0 to 1.7, the yield of the carboxylic acids 4 and 5 also
decreased from 86% to 32%. In more acidic reaction
mediums, more side reactions occur, leading to the formation
of undesired by-products and to a lower selectivity towards the
carboxylic acids 4 and 5.
With more than eight equivalents of H2O2, no higher yields
of carboxylic acids 4 and 5 were achieved. The reaction could
also precede with similar yields of carboxylic acids 4 and 5
using 60% H2O2 instead of 35% H2O2.
To extend the scope of this methodology, we tested other
unsaturated compounds such as oleic acid, high oleic sun-
flower oil (HOSO), as well as the assumed intermediates,
epoxide 2 and vic-diol 3 under our optimised reaction
conditions (Table 9). All compounds could be converted to
azelaic monomethyl ester 5 or azelaic acid, in good yields. In
some cases, nonanal 6 and 9-oxononanoic acid methyl ester 7
were formed as by-products in small amounts. The oxidative
cleavage of the ester 1 and HOSO led to comparable amounts
of cleavage products, whereas with oleic acid smaller amounts
of azelaic acid were formed. The existence of the free
carboxylic acid obviously led to more side reactions and to
the formation of higher molecular by-products.
Epoxide 2 and vic-diol 3 could also convert to carboxylic
acids
4 and 5 in high yields, proving that they are
intermediates in the oxidative cleavage of 1. 9-Decanoic acid
methyl ester could also be cleaved to azelaic monomethyl ester
5 and formic acid as by-product.
This demonstrated that not only unsaturated fatty acids
with internal double bonds, but also unsaturated compounds
with terminal double bonds, can be cleaved to carboxylic
acids.
Reaction mechanism
The reaction pathway of the oxidative cleavage of methyl oleate 1
was investigated with the help of a conversion-time-plot. Fig. 4
a
Table 8 Oxidation of methyl oleate 1: Variation of the time for adding H2O2
Yield of oxidation products [%]
Added H2O2 tt
Conversion
[h] 1 [%]
t [h]
Y 2
Y 3
Y 4
Y 5
3
6
12
16
20
6
10
16 100
20
24
98
99
16
6
1
1
0
9
4
3
3
4
37
55
70
77
81
40
58
73
78
86
99
99
Fig. 4 Oxidation of methyl oleate: Conversion-time-plot. Reaction conditions: 15
mmol 1, 0.15 Ru(acac)3, 3.0 mmol dipicolinic acid, 60 ml solvent mixture
(t-BuOH : H2O = 3 : 1), pH = 2.92, 80 uC, 20 h: slow addition of 120 mmol H2O2
(35% aq), in 12 ml t-BuOH, total reaction time: 24 h.
a
Reaction conditions: 5 mmol 1, 0.05 Ru(acac)3, 1.0 mmol
dipicolinic acid, 20 ml solvent mixture (t-BuOH : H2O = 3 : 1), slow
addition of 40 mmol 35% H2O2 in 4 ml t-BuOH, 80 uC.
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Table 9 Oxidative cleavage of different starting compoundsa
Yield of oxidation products [%]
Educt
Conversion [%]
Y 4
Y 5
Y 6
Y 7
Methyl oleate 1
Epoxide 2
vic-Diol 3
HOSO
Oleic acid
99
98
100
100
100
100
81
82
86
90
0
4
0
4
4
0
0
5
0
4
5
0
77
75
82b
59b
—
78b
66b
53
9-Decanoic acid methyl ester
a
Reaction conditions: 5 mmol educt, 0.05 mmol Ru(acac)3, 0.5 mmol dipicolinic acid, 20 ml solvent mixture (t-BuOH : H2O = 3 : 1), 80 uC, 20
b
h: slow addition of 40 mmol 35% H2O2 in 4 ml t-BuOH, total reaction time: 24 h. Cleavage products of high oleic sunflower oil (HOSO) and
oleic acid were analysed as methyl esters (see experimental section).
clearly shows that the reaction pathway proceeds mainly via the
presumed hydrolytic pathway, with the formation of epoxide 2 as
well as the vic-diol 3 as intermediates (Fig. 2). After a reaction
time of 3 h, a conversion of 94% of 1 was already observed. A
maximum yield of 28% of epoxide 2 was reached after 2 h, and
after 3 h vic-diol 3 was formed with a maximum yield of 48%,
indicating that the hydrolysis of epoxide 2 works fairly fast. After
3 h, the formation of the carboxylic acids 4 and 5 begins, and a
maximum yield was achieved after 12 h. In addition to the
already mentioned products, the aldehydes 6 and 7 and the
acyloins 8a and 8b were also observed to be formed in minor
amounts. These products were obviously formed as intermedi-
ates in the cleavage process. The acyloins 8 are formed by
oxidation of the vic-diol 3. Because acyloin 8 is only formed in
minor amounts, we are not able to estimate if it is an important
intermediate at the cleavage of ester 1 or not. Therefore, the
reaction mechanism for the cleavage of the vic-diol is hard to
interpret.
A supposable possibility is the cleavage of the acyloin 8 into
a carboxylic acid and an aldehyde which can also be further
oxidised to a carboxylic acid by hydrogen peroxide (Fig. 5, A).
The mechanism of the cleavage of acyloins into an aldehyde
and a carboxylic acid is described by Venturello65 and Ogata66
.
The formation of the aldehydes 6 and 7 is already observed
after a short reaction time of 2 h, and thus in advance of
carboxylic acids 4 and 5, as well as acyloins 8, which are not
formed until 3 h. Therefore, the hydrolytic pathway seems not
to be the only pathway for the formation of the aldehydes 6
Fig. 5 Oxidation of oleic acid methyl ester: Reaction mechanism.
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and 7. We assume that these small amounts of aldehydes 6
and 7 were formed via formation of the hydroxyperoxystearic
acid methyl esters 10a and 10b as intermediate, which are
formed by nucleophilic attack of H2O2 on the epoxide 2.
Thermal decomposition of the reactive peroxy-compounds 10
leads to the formation of the aldehydes 6 and 7, which can be
further oxidised to the carboxylic acid 4 and the monoester 5.
This pathway is known as perhydrolytic pathway (Fig. 5, B).27
Unfortunately, we were not able to observe the formation of
the unstable hydroxyperoxystearic acid methyl esters 10a and
10b.
use of aqueous hydrogen peroxide as an environmentally
friendly oxidant.
Experimental section
Methyl oleate (97%+) was prepared by transesterification of
high oleic sunflower oil (Emery oleochemicals) with methanol,
using catalytic amounts of sulphuric acid. 9,10-Epoxystearic
acid methyl ester 2 was prepared by epoxidation of methyl
oleate 1 with Ru(acac)3/dipicolinic acid.63 All other chemicals
were purchased from commercial suppliers at the highest
purity available. They were used as received without further
purification.
In general the conversion-time plot definitely showed that
the hydrolytic pathway is the preferred route for the oxidative
cleavage of methyl oleate 1 with Ru(acac)3/PDC, since both the
epoxide 2 and vic-diol 3 are formed as intermediates in
moderate yields during the reaction. However it is probable
that the hydrolytic and the perhydrolytic pathway are running
in parallel. There is no indication neither for the cleavage nor
The product samples were analysed by gas chromatography
on a Hewlett-Packard 6890 instrument with a HP-5 capillary
column (30 m 6 0.25 mm 6 0.25 mm) using a flame ionization
detector. Pentadecane was used as internal standard. Gas
chromatography-mass spectroscopy (GC-MS) chromatograms
were recorded using a Hewlett-Packard 5973 instrument (HP-5
column 30 m 6 0.25 mm 6 0.25 mm) with an ionization
energy of 70 eV.
22
for the cis-dihydroxylation of methyl oleate 1 by RuO4, RuO4
or any cis-dioxoruthenium species.
The presented catalyst system Ru(acac)3/PDC presents an
alternative method and a different reaction mechanism for the
ruthenium-catalysed oxidative cleavage of alkenes in compar-
ison to the RuO4-chemistry and enables also the use of H2O2 as
oxidant.
1H- and 13C-NMR spectra were recorded on a Bruker model
1
DPX500 spectrometer at room temperature. H- and 13C-NMR
chemical shifts were reported on the d-scale (ppm) relative to
Me4Si as an external standard.
General procedure for the hydrolysis of 9,10-epoxystearic acid
methyl ester 2
Conclusions
In this work, the oxidative cleavage of methyl oleate 1 by
hydrogen peroxide was investigated in detail. After discussing
the reaction mechanism of the transition-metal catalysed
cleavage of alkenes by hydrogen peroxide, a new ruthenium
catalyst for the oxidative cleavage of alkenes was developed.
The active catalyst is formed in situ from an easily available
ruthenium precursor, and a commercially available and cheap
ligand, dipicolinic acid. Systematic investigation of the
reaction parameters showed that the oxidative cleavage of
methyl oleate 1 was significantly influenced by the ligand, the
ratio of Ru-precursor to ligand, the temperature, the solvent
and the reaction time. The product distribution between the
azelaic acid monomethyl ester and the epoxidised methyl
oleate could be affected by changing the pH-value. Under
optimised reaction conditions, azelaic acid monomethyl ester
was obtained at a high yield of 86%. Investigations of the
reaction pathway showed that the reaction proceeds via a
tandem reaction with the corresponding epoxide and the vic-
diol of the unsaturated compound as intermediates. Besides
fatty esters, also fatty acids, fats and terminal olefins were
cleaved to carboxylic acids, demonstrating the general
application of this process.
In a typical experiment (0.78 g, 2.5 mmol) 9,10-epoxystearic
acid methyl ester 2 was dissolved in t-BuOH (5.85 g, 7.5 ml)
and H2O (2.50 g, 2.5 ml). The pH-value was adjusted to 2.4 by
adding H2SO4 (2 M), and the mixture was stirred magnetically
at 80 uC for 4 h. Conversion and yield analyses were performed
by GC.
General procedure for the oxidative cleavage of 9,10-
dihydroxystearic acid methyl ester 3
The oxidative cleavage of 9,10-dihydroxystearic acid methyl
ester 3 was performed according to Herrmann et al.64 9,10-
Dihydroxystearic acid methyl ester 3 (661 mg, 2.00 mmol),
Ru(acac)3 (1.05 mg, 0.004 mmol, 0.2 mol%) and dipicolinic
acid (2.01 mg, 0.012 mmol, 0.6 mol%) were dissolved in
t-BuOH (7.8 g, 10.0 ml). 35% aqueous H2O2 (1.55 g, 16 mmol)
was added slowly to the reaction solution by a syringe pump
(ISMATEC, REGLO Digital) with a flow rate of 11.5 ml min21
over a period of 120 min. The reaction mixture was stirred
magnetically at 80 uC for 135 min. Conversion and yield
analyses were performed by GC.
General procedure for the oxidative cleavage of methyl oleate 1
In a typical experiment, methyl oleate 1 (1.48 g, 5.00 mmol),
Ru(acac)3 (19.9 mg, 0.05 mmol, 1 mol%) and dipicolinic acid
(167.1 mg, 1.00 mmol, 20 mol%) were dissolved in t-BuOH
(11.7 g, 15.0 ml) and H2O (5.0 g, 5.0 ml). If required, the pH-
value was adjusted by adding H2SO4 (2 M or 5 M) or NaOH (2
M or 5 M). 35% Aqueous H2O2 (3.88 g, 40 mmol) was dissolved
in t-BuOH (3.1 g, 4.0 ml) and added slowly to the reaction
In summary, a simple procedure for the oxidative cleavage
of methyl oleate 1 to azelaic acid monomethyl ester, under
mild reaction conditions using hydrogen peroxide as an
oxidant, has been developed. The major advances of this
method are simplicity, easy preparation of the catalyst and the
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1.41–1.61
(m,
6H,
–CH2CH2CO2CH3,
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71 (12), 69 (24), 67 (17), 59 (13), 57 (30), 55 (53)
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Acknowledgements
This work was financially supported by the German Federal
Ministry of Food, Agriculture and Consumer Protection
(represented by the Fachagentur Nachwachsende Rohstoffe)
and Emery Oleochemicals. The authors thank Dr Alfred
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