Highly efficient epoxidation of vegetable oils catalyzed by a manganese complex with hydrogen peroxide and acetic acid

Article abstract of DOI:10.1039/c8gc03857k

Epoxidized vegetable oils (EVOs) are versatile building blocks for lubricants, plasticizers, polyvinyl chloride (PVC) stabilizers, and surface coating formulations. In this paper, a catalytic protocol for the efficient epoxidation of vegetable oils is presented that is based on a combination of a manganese catalyst, H₂O₂ as an oxidant, and acetic acid as an additive. This protocol relies on the use of a homogeneous catalyst based on the non-noble metal manganese in combination with a racemic mixture of the N,N′-bis(2-picolyl)-2,2′-bispyrrolidine ligand (rac-BPBP). The optimized reaction conditions entail only 0.03 mol% of the manganese catalyst with respect to the number of double bonds (ca. 0.1 wt% with respect to the oil) and ambient temperature. This epoxidation protocol is highly efficient, since it allows most of the investigated vegetable oils, including cheap waste cooking oil, to be fully epoxidized to EVOs in more than 90% yield with excellent epoxide selectivities (>90%) within 2 h of reaction time. In addition, the protocol takes place in a biphasic reaction medium constituted by the vegetable oil itself and an aqueous acetic acid phase, from which the epoxidized product can be easily separated via simple extraction. In terms of efficiency and reaction conditions, the current epoxidation protocol outperforms previously reported catalytic protocols for plant oil epoxidation, representing a promising alternative method for EVO production.

Full text of DOI:10.1039/c8gc03857k

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DOI: 10.1039/C8GC03857K
Journal Name
ARTICLE
Highly efficient epoxidation of vegetable oils catalyzed by a
manganese complex with hydrogen peroxide and acetic acid
Jianming Chen,a Marc de Liedekerke Beaufort,a Lucas Gyurik,a Joren Dorresteijn,a Matthias Otteab
Received 00th January 20xx,
Accepted 00th January 20xx
and Robertus J. M. Klein Gebbink*a
DOI: 10.1039/x0xx00000x
Epoxidized vegetable oils (EVOs) are versatile building blocks for lubricants, plastics plasticizers, polyvinyl chloride (PVC)
stabilizers, and surface coating formulations. In this paper, a catalytic protocol for the efficient epoxidation of vegetable oils
is presented that is based on a combination of a manganese catalyst, H2O2 as the oxidant, and acetic acid as additive. This
protocol relies on the use of a homogeneous catalyst based on the non-noble metal manganese in combination with a
racemic mixture of the N,N’-bis(2-picolyl)-2,2’-bispyrrolidine ligand (rac-BPBP). The optimized reaction conditions entail only
0.03 mol% of manganese catalyst with respect to the number of double bonds (ca. 0.1 wt% with respect to the oil) and
ambient temperature. This epoxidation protocol is highly efficient, since it allows most of the investigated vegetable oils,
including cheap waste cooking oil, to be fully epoxidized to EVOs in more than 90% yield with excellent epoxide selectivities
(> 90%) within 2 h of reaction time. In addition, the protocol takes place in a biphasic reaction medium constituted by the
vegetable oil itself and an aqueous acetic acid phase, from which the epoxidized product can be easily separated via simple
extraction. In terms of efficiency and reaction conditions, the current epoxidation protocol outperforms previously reported
catalytic protocols for plant oil epoxidation, representing a promising alternative method for EVO production.
www.rsc.org/
plasticizers,11–13 polyvinyl chloride (PVC) stabilizers,14–16 and
surface coating formulations.17–20
Introduction
O
Nowadays, the decreasing supply of fossil resources and
increasing environmental problems have triggered the search
for durable alternative raw materials for chemical production.
Renewable biomass has been identified world-wide as a prime
candidate to replace fossil resources as the feedstock for the
chemical industry.1 Among various biomass resources,
vegetable oils (VOs) represent one of the most promising
candidates due to their wide availability, biodegradable
properties, and low costs.2–4 Common VOs are mixtures of
triglycerides, which are composed of three fatty acid moieties
connected by a glycerol backbone (Figure 1). These fatty acids,
either saturated or unsaturated, normally have 14 to 22 carbons
in each hydrocarbon chain, resulting is a relatively high overall
carbon content.5 More importantly, the fatty acids in VOs are
mostly unsaturated with 0 to 3 double bonds per carbon chain.
O
O
O
O
O
Figure 1. A typical triglyceride in sunflower oil, derived from the fatty
acid linoleic acid (C18:2) and oleic acid (C18:1).
The Prilezhaev process is currently adopted in industry for the
production of EVOs. In this process double bonds are converted
with percarboxylic acids formed in situ from a carboxylic acid
(e.g. acetic acid) and hydrogen peroxide in the presence of a
mineral acid like H2SO4 or HCl (Scheme 1). This process has
several drawbacks, such as low epoxide selectivity due to
oxirane ring opening and corrosion issues, which are both
caused by the strongly acidic reaction conditions. In the past
decades, tremendous efforts have accordingly been spent on
developing new catalytic systems, both homogeneous and
heterogeneous, to form EVOs in a more selective and efficient
manner.
Modifications of these C═C bonds can produce new value-
added chemicals or monomers for polymers, which has
attracted research interests for many years.6,7 Functionalizing
the double bonds via epoxidation is one of the most common
approaches to produce epoxidized vegetable oils (EVOs), which
are versatile building blocks for lubricants,8–10 plastics
a. Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science,
Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands.
b. Current address: Institut für Anorganische Chemie, Georg-August-Universität
Göttingen, Tammannstraße 4, 37077 Göttingen, Germany.
This journal is © The Royal Society of Chemistry 20xx
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because of limited interaction between the catalyVtiiecwcAerntictleerOnalninde
the large triglyceride substrates due to sDteOrIi:c10h.i1n0d39ra/Cn8cGeC.403857K
O
O
R
O
H
O
O
Entries 4-6 in Table
1 show examples of utilizing
R1
R2
R1
R2
C
C
+
C
H
C
H
polyoxometalates as catalysts to epoxidize soybean oil with
H2O2 as the oxidant. These epoxidation processes were carried
out in a solvent-free medium, i.e., using a mixture of aqueous
H2O2 and the plant oil. A somewhat elevated reaction
H
H
R
OH
H
Scheme 1. Conventional vegetable oil epoxidation process (Prilezhaev
reaction).
o
temperature (40-60 C) and a high catalyst loading (5-33 wt%)
Some selected catalytic systems reported since 2000 for the
epoxidation of VOs are listed in Table 1. Gerbase and co-workers
were required in these examples. For instance, Cheng et al. used
5 wt% of [π-C5H5N(CH2)15CH3]3[PW4O16] to obtain 90 % of
epoxidized soybean oil at 60 oC reaction temperature.25
Immobilization of polyoxometalates onto an inorganic solid
support is mostly used in order to increase their stability and
reusability,4 nevertheless, this normally results in a lower
catalytic activity. For example, Jiang et al. reported the use of
the peroxophosphotungstate [MeN(n-C8H17)3]{PO4[WO(O2)2]4}
for the catalytic epoxidation of soybean oil, which provided 99%
yield at very high catalyst loading (31 wt%, entry 5).26
Supporting this catalyst on modified halloysite nanotubes
resulted in a diminished yield of 12%.26 Similarly, Chen et al.
supported this complex on acid-activated palygorskite, giving
rise to an epoxide yield of 79% (entry 6).27 The loading of this
catalyst can be optimized to 0.1 mol % in the epoxidation of
FAMEs, with 96% selectivity and 95% conversion at 60 oC.
However, the epoxidation of VOs was not investigated in this
reported
a homogeneous CH3ReO3/H2O2/CH2Cl2 catalytic
system, in which a very high yield of epoxide (95%) was
obtained under mild reaction conditions (entry 1).21 However,
the use of an expensive noble metal (Re, 1 mol%) limits its
application in a large-scale process. A cheaper metal (Mo) has
been used by Farias et al. for the epoxidation of soybean oil.22
o
However, a relatively high reaction temperature of 110 C was
needed, to give only a moderate yield (54%) of epoxide (entry
2). Lipases have also been used in the chemoenzymatic
epoxidation of VOs. They have shown very high chemo-, regio-,
and stereoselectivity without the formation of undesired ring-
opening side-products.23 Vlček and Petrović used lipase Candida
antarctica (Novozyme 435) to epoxidize soybean oil with H2O2
in high yield (entry 3).24 The protocol is sensitive to the reaction
temperature; on one hand, a higher temperature is beneficial
for double bond conversion, on the other hand this leads to
deactivation of the lipase.4 Some other drawbacks of the use of
lipases are their high cost and their relatively low reactivity
study.28 Moores et al. developed
a
dipyridinium
peroxophosphotungstate catalyst, which is able to fully
epoxidize methyl oleate with a turnover number (TON) of 1868
Table 1. Selected catalytic systems for epoxidation of VOs.4
Entry
Oil
Oxidant
Catalyst (loading)
Solvent
Reaction
conditions
30 oC, 2 h
Epoxide
yield (%)
95
121
222
324
425
Soybean oil
Soybean oil
Soybean oil
Soybean oil
H2O2
TBHPa
H2O2
H2O2
CH3ReO3 (1 mol%)
CH2Cl2
Toluene
Toluene
-
[MoO2(acac)2] (1 mol%)
Novozym 435 (4.0 wt%)
110 oC, 2 h
50 oC, 4 h
60 oC, 4 h
54
90
90
[π-C5H5N(CH2)15CH3]3[PW4O16]
(5.0 wt%)
526
627
Soybean oil
Soybean oil
H2O2
H2O2
[MeN(n-C8H17)3]{PO4[WO(O2)2]4}
(31 wt%)
[MeN(n-C8H17)3]{PO4[WO(O2)2]4}
supported on palygorskite
(33 wt%)
-
-
40 oC, 3 h
50 oC, 2 h
99
79
730
Soybean oil
Soybean oil
Soybean oil
Castor oil
H2O2
TBHPa
H2O2
H2O2
TBHPa
H2O2
Amorphous Ti/SiO2 (2.5 wt%)
tert-Butanol
EtOAc
90 oC, >54 h
60 oC, 24 h
80 oC, 5 h
88
22
10
78
16
48
831
Meso-Ti-HMS (2.5 wt%)
Nb2O5–SiO2 (12 wt%)
932
Et2O
1033
1134
1235
Amberlite IR-120 (15 wt%)
Benzene
Toluene
Et2O
50 oC, 10 h
80 oC, 4 h
Soybean oil
Soybean oil
MoO3/Al2O3 (Mo/C═C = 1%)
γ-Al2O3 (12 wt%)
80 oC, 10 h
a TBHP = tert-butyl hydroperoxide.
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o
using 0.22 mol% catalyst after 5 cycles at 60 C.29 Also in this mechanism through which these Mn-catalysts operate starts
View Article Online
study the epoxidation of VOs was not reported.
with the oxidation of the starting MnII-DcoOmI: 1p0l.e10x39w/Cit8hGHC02O38257tKo
Many heterogeneous catalytic systems have also been form a MnIII-hydroperoxo species.41 Subsequently, this species
reported to be used in the epoxidation of VOs. Several binds a carboxylic acid, which aids in the formation of a MnV-
representative examples are listed in Table 1, entries 7-12. As oxo-carboxylato intermediate and water through a cyclic
can been seen from these examples, all reactions were hydrogen-bonded transition state. It is this MnV-oxo
o
performed at high temperatures (mostly > 80 C), and in most intermediate that converts the alkene substrate to the epoxide
of the cases relatively low reactivities were obtained (yields product. Based on these findings, the current study aimed to
mostly < 70%). For instance, reaction temperatures between explore the use of Mn(N2Py2) type complexes in combination
o
60-80 C were required in the reactions using meso-Ti-HMS, with H2O2 and AcOH for the catalytic epoxidation of VOs and
Nb2O5–SiO2, MoO3/Al2O3, or γ-Al2O3 as catalyst, giving rise to 10- their derivatives. This study has resulted in the development of
48 % yields of epoxidized soybean oil (entries 8, 9, 11, 12). Fierro a highly efficient, Mn-based protocol that can be conducted at
and co-workers have developed an amorphous Ti/SiO2/H2O2/t- room temperature in only 2 h of reaction time, generally
BuOH catalytic protocol for the epoxidation of soybean oil with providing EVOs in more than 90% yield and with over 90%
relatively low catalyst loading (2.5 wt%), achieving relatively epoxy-group selectivity. In addition, the use of MeCN solvent
high epoxide yield (88%, entry 7).30 However, a high reaction could be strongly limited and even avoided through further
temperature (90 oC) and long reaction time (> 54 h) were used. optimization of the reaction conditions. Details on the use of
Using Amberlite IR-120 as catalyst, 78% epoxidized castor oil this Mn-based epoxidation protocol for a series of different VOs
o
can be obtained at relatively mild reaction conditions (50 C, will also be discussed.
entry 10).33 However, the toxic solvent benzene and a large
amount of catalyst (15 wt%) were used in this case.
Results and discussion
To sum up, the conventional Prilezhaev process and
previously reported catalytic systems for the epoxidation of VOs
generally entail at least one of the following disadvantages: low
selectivity, low catalyst efficiency, usage of high-cost catalyst,
harsh reaction conditions, long reaction time, or the use of a
harmful organic solvent. In order to meet the increasing
demand for the production of EVOs on a large-scale, the
development of more efficient, practical catalytic systems for
the selective epoxidation of VOs and their derivatives under
mild reaction conditions is desirable. Ideally, such catalytic
systems would conform to the principles of green chemistry in
modern chemistry.4
A series of non-heme Mn-complexes bearing N2Py2 ligands
have been synthesized to study the epoxidation of UFAs
(FAMEs) and VOs (Figure 2). These N2Py2 ligands, including the
well-known BPMEN42, BPMCN,43 and BPBP44 (BPMEN = N,N’-
dimethyl-N,N'-bis(2- picolyl)ethylenediamine, BPMCN = N,N’-
dimethyl-N,N’-bis(2-picolyl)-cyclohexane-trans-1,2-diamine,
BPBP = N,N’-bis(2-picolyl)-2,2’-bispyrrolidine), can be readily
converted
[Mn(OTf)2(BPMEN)]45
[Mn(OTf)2(BPBP)]46
to
the
corresponding
), [Mn(OTf)2(R,R-BPMCN)]45
3), respectively, upon treatment with
Mn
complexes
(
1
(2) and
(
Spannring et al. have reported on a one-pot oxidative
cleavage protocol of unsaturated fatty acids (UFAs) and fatty
acid methyl esters (FAMEs) to form aldehydes as primary
products, with the catalytic epoxidation of double bonds as the
first and key step.36 This epoxidation step was carried out using
the abundant, environmentally benign first-row transition
metal iron, supported by a bis-alkylamine-bis-pyridine (N2Py2)
ligand, in the presence of H2O2 and acetic acid (AcOH). The
actual oxidizing species in this system is considered to be a
highly electrophilic FeV=O intermediate, that is generated
through lysis of the O–O bond of an FeIII–OOH species formed
upon exposure of the starting Fe(II) complex to H2O2.37 Even
though this catalytic system is relatively efficient, i.e., full
conversion of substrate can be achieved with 0.5 mol% catalyst
per double bond, a further reduction of the catalyst loading
seems necessary for the large-scale application of these
protocols. Furthermore, these catalytic protocols make use of
relatively toxic acetonitrile (MeCN) as the reaction solvent.
As a first-row transition metal, manganese has similar
advantages as iron in terms of cost, availability, and low toxicity.
In addition, several studies have reported that Mn(N2Py2)
complexes generally demonstrate higher conversions and yields
as compared to their Fe(N2Py2) analogs in both aliphatic C–H
oxidation and alkene epoxidation.38–41 The generally accepted
N
N
N
N
N
N
N
N
Mn
N
N
N
Mn
N
Mn
TfO
OTf
TfO
OTf
TfO
OTf
[Mn(OTf)2(BPMEN)]
[Mn(OTf)2(R,R-BPMCN)]
[Mn(OTf)2(S,S-BPBP)]
S,S-3
1
2
N
N
N
N
N
N
N
N
Mn
N
N
N
N
+
Mn
Mn
TfO
OTf
TfO
OTf
TfO
OTf
[Mn(OTf)2(R,R-BPBP)]
[Mn(OTf)2(R,S-BPBP)]
R,S-3
[Mn(OTf)2(S,S-BPBP)]
rac-3
N
N
N
N
N
N
Mn
N
N
Mn
TfO
OTf
TfO
OTf
[Mn(OTf)2(mix-BPBP)]
mix-3
[Mn(OTf)2(S,S-BPBI)]
4
Figure 2. Non-heme Mn(II) complexes with N2Py2 ligands used in this study.
This journal is © The Royal Society of Chemistry 20xx
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Mn(OTf)2 in THF (Figure 2; see experimental section for details). using BPBP-based Mn-complexes as catalyst. As reported
View Article Online
DOI: 10.1039/C8GC03857K
Amongst the BPBP-based complexes, Mn-complexes derived previously,44,52 adding the oxidant slowly improves the
from different BPBP stereoisomers,47 i.e., S,S-, R,S-, rac-, and substrate conversion in aliphatic C–H oxidations. A slow
mix-BPBP, were synthesized and tested for their epoxidation addition protocol, i.e., dropwise addition of H2O2 over 30 min
activity. In contrast to the enantiomerically pure versions of the using a syringe pump, was therefore tested in oleic acid
ligand, less costly non-enantiomerically pure versions of the epoxidation. Considering that S,S-3 shows very high epoxide
BPBP ligand (such as rac- and mix-BPBP) would be preferably selectivities and that N2Py2-based iron complexes were shown
used in the epoxidation of VOs, in which enantioselectivity to decompose under the oxidizing conditions47, 52, the oxidant
issues are not at stake from a product application point of view. loading was lowered to 1.5 equiv. Using these conditions,
The development and application in Fe-catalyzed epoxidation substrate conversion and product yield both significantly
reactions of mix-BPBP was earlier reported by Spannring et al.36 increased to 99 % (entry 8). Further lowering of the catalyst
Mix-BPBP constitutes a mixture of R,R-BPBP, S,S-BPBP, and loading to 0.1 mol% still gave quantitatively conversion of oleic
(meso) R,S- BPBP. In the current study, rac-BPBP was also used, acid into its epoxide product (entry 9). In comparison, using the
which constitutes a mixture of R,R- and S,S-BPBP (see same amount of S,S-3-Fe ([Fe(OTf)2(S,S-BPBP)]), only 70% of
experimental section for details). Another N2Py2 ligand used in conversion and yield were obtained (entry 10). This observation
this study is BPBI (N,N’-bis(2-picolyl)-2,2’-bis-isoindoline), is consistent with previously reported results by Bryliakov and
which was previously developed in our group for the synthesis Talsi, that show that the Mn(N2Py2) complexes exhibit higher
of the Fe(BPBI) complex employed in aliphatic C–H oxidations.48 reactivities than the corresponding Fe(N2Py2) complexes in
Recently, this ligand and its derivatives have also been reported epoxidation reactions.41,53 Meso-complex
R,S-3 turned out to be
for the synthesis of the corresponding Mn- complexes, for almost inactive in the epoxidation reaction, with only 10%
which good reactivities have been observed in alkene substrate converted and product formed (entry 11), which is in
epoxidation.49
line with the catalytic behavior of the corresponding iron
complex [Fe(OTf)2(R,S-BPBP)] in both alkene epoxidation and
aliphatic C–H oxidation.47 Furthermore, using 0.1 mol% of mix-
Epoxidation of UFAs and FAMEs
3
provided an identical reaction outcome to the reaction with
S-3 (quantitative yield, entry 12). This observation
Oleic acid (C18:1) was chosen as the model substrate to
optimize reactions conditions for the epoxidation of UFAs by the
Mn-complexes in combination with H2O2 as the oxidant and
AcOH as additive (Table 2). Initial experiments were carried out
in the absence of Mn-complex to evaluate the crucial role of
manganese in catalysis. Adding only H2O2 (2 equiv.)/AcOH, no
substrate conversion was observed at all after 1 h (Table 2, entry
1). In contrast, 2 equiv. of m-chloroperoxybenzoic acid (mCPBA)
were able to fully epoxidize the substrate, yielding the epoxide
quantitatively (entry 2). This result is in line with a previous
report on the high efficiency of mCPBA in the epoxidation of
UFAs and VOs.50 When Mn(OTf)2, the Mn salt used for the
synthesis of the Mn complexes, together with H2O2 (2 equiv.)
and AcOH (9 equiv., undiluted) was used, no conversion of oleic
acid was detected (entry 3). Subsequently, using similar
reaction conditions to the reported Fe(mix-BPBP)/H2O2/AcOH
catalytic protocol for the epoxidation of UFAs,36 catalytic
experiments were performed with several Mn-complexes as
S
,
corroborates the notion that
active components in mix-3 and that
to the activity of mix-3. In accordance with this notion, the use
of 0.1 mol% of rac-3 in the reaction provided a quantitative yield
of epoxide as well (entry 13). Since the rac-BPBP ligand mixture
can be readily isolated from the mix-BPBP mixture via flash
S,
S-3 and
R,
R-3 are the catalytically
R,S-3 does not contribute
Table 2. Screening of reaction conditions for epoxidation of oleic
acid using Mn-catalysts.a)
O
O
7
O
Cat.
OH
OH
H2O2, AcOH
CH3CN, RT
7
7
7
Entry
Cat. (mol%)
H2O2 equiv.
(addition
time)
Conv.
(%)b)
Yield
(%)b)
catalyst (0.5 mol%). Complex
32% of epoxidized oleic acid (entry 4). Similar to
epoxide was found in the reaction with chiral complex
with a somewhat higher conversion (48%; entry 5). The epoxide
yield was found to increase to 46% with S-3, even though
substrate conversion did not further increase (48%, entry 6).
The generally increased catalytic performance from to to
S,S-3 seems related to the enhanced stability of the manganese
complexes, as a result of the increased rigidity of their ligand
backbone.44,51 Using complex
4, a significant drop was found in
1
gave 40% conversion and yielded
, 32% of
, albeit
1
-
-
2 (at once)
-
2 (at once)
0
>99
0
0
99
0
2c)
3
1
2
Mn(OTf)2
(0.5)
1
2
S
4
S,
4
5
6
7
8
9
10d)
(0.5)
(0.5)
S-3 (0.5)
(0.5)
S-3 (0.5)
2 (at once)
2 (at once)
2 (at once)
2 (at once)
1.5 (30min)
1.5 (30min)
1.5 (30min)
40
48
48
28
>99
>99
70
32
32
46
28
99
99
70
1
2
,
S
S
,
,S-3 (0.1)
both the conversion and yield (28% and 28%, respectively;
entry7).
Since S,S-3 outperforms the other Mn-catalysts in terms of
S,S-3-Fe
(0.1)
R
mix-3 (0.1)
rac-3 (0.1)
rac-3 (0.05)
rac-3 (0.05)
rac-3 (0.05)
rac-3 (0.01)
rac-3 (0.02)
11
12
13
14
,
S-3 (0.1)
1.5 (30min)
1.5 (30min)
1.5 (30min)
1.5 (30min)
1.5 (30min)
10
10
99
99
99
7
50
80
99
epoxide yield, reaction condition optimization was carried out
>99
>99
>99
10
52
80
15e)
16f)
17
4 | J. Name., 2012, 00, 1-3
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1.5 (30min)
1.5 (30min)
1.5 (45min)
18g)
>99
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a) General reaction conditions: Oleic acid (0.5 mmol), catalyst, and AcOH
(9 equiv.) were mixed and stirred in MeCN (2 mL) at room temperate
(RT), subsequently H2O2 (1 M solution in MeCN) was added, 1 h total
reaction time. b) Determined by NMR analysis. c) 2 equiv. of mCPBA was
of a series of other UFAs and FAMEs (Table 3). Elaidic acid, the
trans-isomer of oleic acid, undergoes epoxidation quantitatively
View Article Online
DOI: 10.1039/C8GC03857K
under the protocol conditions (entry 2). The same catalytic
outcome was obtained when changing the carboxyl group in
oleic acid to a carbomethoxy group as is methyl oleate (entry 3).
Notably, a very high TON of 4950 was obtained in this case,
which largely outperforms a previously reported benchmark in
the epoxidation of methyl oleate (TON = 1868).29 Erucic acid
(C22:1), on the other hand, seems more difficult to epoxidize
using the current reaction conditions: only 36% of the substrate
was converted in this case, yielding 36% yield of the epoxide
(entry 4). Erucic acid is a solid at room temperature (melting
d)
used as oxidant without the presence of AcOH. [Fe(OTf)2(S,S-BPBP)]
e)
f)
was used as catalyst. 1 equiv. of AcOH was used. 5 equiv. of AcOH
was used. g) 18 equiv. of AcOH was used, 1.25 h total reaction time.
column chromatography and no ligand resolution is need, rac-3
represents a much cheaper catalyst than an enantiopure
Mn(BPBP) complex. In addition, the use of rac-3 as opposed to
mix-3 could be advantageous for practical applications, since
rac-3 is devoid of inactive metal-containing components which
could facilitate regulatory registration and since a lower weight
amount of catalyst could be used because rac-3 contains more
active catalyst per gram of catalyst material.
o
point = 34 C) and not well soluble in MeCN. The resulting
biphasic solid-liquid reaction medium is likely to limit catalytic
activity, leading to poor catalytic results. Upon increase of
o
reaction temperature to 36 C, the resulting biphasic liquid-
No drop in catalytic conversion and yield were found when
further reducing the amount of catalyst by 50% (0.05 mol% rac-
, entry 14). Decreasing the loading of AcOH to either 1 or 5
liquid reaction medium allowed for more effective catalysis to
occur and, accordingly, erucic acid was fully converted to give
90% of the epoxide product under these conditions (entry 5). In
turn, methyl erucate undergoes the epoxidation process
smoothly using the standard protocol, forming the epoxide in
quantitative yield (entry 6). This catalytic system can also be
perfectly applied to UFAs with more double bonds, as shown in
the cases of linoleic acid with two double bonds and linolenic
acid with three double bonds (both with quantitative yields,
entries 7 and 8). For the small set of UFAs and FAMEs tested,
excellent epoxide yields and high turnover numbers (TON>4500)
were obtained, meaning that the rac-3/H2O2/AcOH catalyst
system is more efficient than previous examples28,29 and is
promising to be widely applied in the epoxidation of a wide
range of UFAs and FAMEs.
3
equiv. led to incomplete substrate conversions (10% or 52%,
respectively, entries 15 and 16). Costas et al. have earlier shown
the beneficial effect of the addition of larger amounts of acetic
acid in epoxidation reactions catalyzed by S,S-3 54
The use of
.
increased amounts of AcOH does furthermore improve the
solubility of fatty acid substrates in the AcOH/CH3CN medium,
as was noted earlier in Fe-catalyzed oxidative cleavage
reactions.36 Further lowering of the catalyst loading to 0.01
mol% rac-3 using 9 equiv. of AcOH gave 80% of substrate
conversion and product yield within the standard reaction time
of 1 h, providing a boundary of catalyst activity per time unit
(entry 17). Variation of the reaction parameters based on these
combined observations finally led to a protocol that uses 0.02
mol% of rac-3 catalyst loading, 45 min of H2O2 (1.5 equiv.)
addition time, and an AcOH loading of 18 equiv., respectively;
these reaction settings lead to full oleic acid conversion and
quantitative formation of its epoxide (entry 18).
Epoxidation of VOs
Next, the Mn-catalyzed epoxidation of VOs was explored using
the rac-3/H2O2/AcOH catalytic protocol (Table 4). In order to
obtain the optimal reaction conditions, sunflower oil was
chosen as the benchmark oil. The Wijs method55 was used to
Using the catalytic protocol optimized for the epoxidation of
oleic acid, the protocol was tested for the catalytic epoxidation
Table 3. Epoxidation of UFAs (FAMEs) using the rac-3/H2O2/AcOH catalytic system.a)
O
O
0.02 mol% rac-3
O
1.5 equiv. H2O2,
18 equiv. AcOH
CH3CN, RT, 1.25 h
OR
OR
z
z
x
x
y
y
Entry
1
2
3
4
5d)
6
7
8
Substrate
Oleic acid
Elaidic acid
Methyl oleate
Erucic acid
x, y, z
6, 1, 5
6, 1, 5
6, 1, 5
6, 1, 9
6, 1, 9
6, 1, 9
3, 2, 5
0, 3, 5
Lipid number
C18:1 cis-9
C18:1 trans-9
C18:1 cis-9
C22:1 cis-13
C22:1 cis-13
C22:1 cis-13
C18:2 cis-9,12
C18:3 cis-9,12,15
R
H
H
Me
H
H
Me
H
Conv. (%)b)
>99
>99
>99
36
>99
>99
>99
>99
Yield (%)b)
TONc)
4950
4950
4950
1800
4500
4950
4950
4950
99
99
99
36
90
99
99
99
Erucic acid
Methyl erucate
Linoleic acid
Linolenic acid
H
a) Unless stated otherwise, reaction conditions are substrate (0.5 mmol
H2O2 (1.5 equiv, 1 M solution in MeCN) and AcOH (18 equiv) in CH3CN at room temperature, the oxidant was added over 45 min, and the
C═C, 1 equiv.), rac-3 (0.02 mol %, added as 10 mM solution in MeCN),
reaction mixture stirred for additional 30 min. All reagent loadings are with respect to (w.r.t.) the amount of C
═
C bond. b) Determined by
NMR analysis. c) TON = [mol epoxide]/[mol rac-3
]
d) Reaction temperature was 36 oC.
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ARTICLE
Journal Name
determine the iodine value (I.V.) of sunflower oil, providing catalyst to 0.04 mol% under these conditions increases both
View Article Online
DOI: 10.1039/C8GC03857K
information on its C
loadings listed in Table 4 are w.r.t. the amount of C
the oils. Iodine values were determined again after the
epoxidation reaction in order to calculate the conversion of C
═C bond content (I.V. = 130). All reagent conversion (80%) and yield (45%) at the same epoxidation
═C bond in selectivity of 56% (entry 4).
In separate catalyst tests the Mn-catalyst was found to be
insoluble in sunflower oil but slightly soluble in AcOH, whereas
═
C
bonds (see experimental section). The amount of epoxy-groups the oil is insoluble in AcOH. This results in a biphasic reaction
formed in the reaction is described as the percentage of oxirane medium (oil:AcOH = 1:2.4, v/v; 9 equiv. AcOH), with the Mn-
oxygen in the resulting oils, which was determined according to catalyst residing in the AcOH layer. Addition of more AcOH (18
AOCS Official Method Cd 9-57.56 The formal yields were equiv., oil:AcOH = 1:4.8, v/v) was considered to increase the
calculated from the measured oxirane oxygen content and the total liquid-liquid interface under vigorous stirring. Entry 5
theoretical maximum oxirane oxygen content (see clearly shows that the reaction benefited significantly from the
experimental section).
additional amount of AcOH. Compared to entry 3, considerably
Using similar reaction conditions to the optimized ones for more double bonds were converted (82% over 45%) and a
epoxidation of UFAs and FAMEs, except for half the amount of moderate yield of epoxide was obtained (53%). Yet, the epoxide
AcOH used, the epoxidation of sunflower oil (100 mg) was selectivity under these conditions is still relatively low (65%). In
performed with 0.02 mol% of rac-3 (added as a 10 mM solution order to suppress the formation of side-products, H2O2 was
in MeCN), 1.5 equiv. of H2O2 (added as a 1 M solution in MeCN delivered to the reaction mixture more slowly over a period of
over 45 min), and 9 equiv. of AcOH in MeCN (2 mL) under 90 min. This resulted in a significant improvement in epoxide
vigorous stirring (1000 rpm) at ambient temperature. Full selectivity to 87%, at almost the same conversion (84%, entry
conversion of double bonds in the oil was obtained, with 95% of 6). A further increase in H2O2 delivery time (120 min) and
epoxy-group yield (Table 4, entry 1). To make this epoxidation extension of the overall reaction time to 3 h lead to a drop in
more practical, the reaction scale was increased to 1 g of epoxide selectivity (to 73%), even though a slightly increased
sunflower oil in the same amount of MeCN (i.e., 2 mL, entry 2). conversion was observed (89%, entry 7). 1H NMR analysis of the
Both the conversion and yield dropped to 83% and 75%, resulting mixture obtained from extraction with diethyl ether
respectively, albeit with a similar epoxide selectivity (95% for and subsequent condensation showed the presence of a
entry 1, 90% for entry 2). For 1 g scale reactions like this, H2O2 remarkable amount of diol compounds (~ 20%). Apparently, the
was added as a commercial 35% aqueous solution. With the aim longer overall reaction time (3 h) allows for a more pronounced
of making the epoxidation protocol more environmentally ring-opening of the initially formed epoxides under the acidic
friendly, the reaction conditions were further optimized not to conditions.
use MeCN as the organic solvent. Of note is that in all the
reactions in Table 4, the catalyst was added as a 10 mM stock slightly increased to 0.03 mol% and the delivery time of H2O2
solution in MeCN (ca. 100 – 200 L), for the purpose of easy brought back to 90 min. In this case, the double bonds were fully
To maximize the epoxide yield, the catalyst loading was
µ
operation. As shown in entry 3, the epoxidation process converted and an excellent epoxide yield (90%) was found
performs very poorly in the absence of MeCN solvent: only 45% (entry 8). Considering the potential epoxide ring-opening side
of the double bonds were converted, forming 25% of epoxide reaction under acidic condition, using these altered conditions
with an epoxide selectivity of 56%. Doubling the loading of Mn- but at a lower AcOH loading (9 equiv.) lead to a decrease of both
Table 4. Screening of reaction conditions for epoxidation of sunflower oil catalyzed by rac-3 a)
.
Entry
MeCN Catalyst loading H2O2 addition
AcOH
(eq.)
Overall reaction
time (h)
Conv. Epoxide
Epoxide
selectivity (%)
(mol%)
time (min)
(%)
yield (%)
1b)
2
2 mL
0.02
0.02
0.02
0.04
0.02
0.02
0.02
0.03
0.03
0.03
45
45
45
45
45
90
120
90
90
90
9
1.75
1.75
1.75
1.75
1.75
2
>99
83
45
80
82
84
89
>99
80
83
95
75
25
45
53
73
65
90
70
75
95
90
56
56
65
87
73
90
88
90
2 mL
9
3
-
-
-
-
-
-
-
-
9
4
9
5
18
18
18
18
9
6
7
3
8
2
9
10c)
2
18
2
a) General reaction conditions: sunflower oil (1 g), rac-3 (0.02-0.04 mol %, w.r.t. double bonds, added as 10 mM solution in MeCN, ca. 100 - 200 µL) and
AcOH (2.6 mL for 9 equiv., 5.3 mL for 18 equiv.) were mixed in the absence or presence of MeCN (2 mL), and stirred vigorously (1000 rpm) at room
temperature, H2O2 (1.5 equiv., w.r.t. double bonds, 35% aqueous solution) was added over 45-120 min, and the reaction mixture stirred for additional
30-60 min. b) 100 mg of sunflower oil was used, H2O2 was added as a 1 M solution in MeCN (0.8 mL) c) Stirring rate = 500 rpm.
6 | J. Name., 2012, 00, 1-3
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Page 7 of 12
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Journal Name
the conversion and yield (80% and 70%, respectively, entry 9).
Finally, the impact of vigorous stirring was examined by bringing achieved using a small increase in catalyst loading to 0.04 mol%
down the stirring rate to 500 rpm, which resulted in a decrease (entry 8). At 0.03 mol% catalyst, only 81% conversion was found
of the conversion from 99% to 83% (entry 10). The necessity of for rice oil. However, under these conditions some 20% of
using a larger amount of AcOH and a high stirring speed double bonds remain in sesame oil, with 75% epoxides being
indicates that the epoxidation reaction is enhanced with more formed (entry 9). Overall, entries 1-9 show that the
pronounced mixing, which suggests that the catalytic reaction performance of this catalyst system is independent of the iodine
ARTICLE
Full epoxidation of the double bonds in rice oil VcieowuAldrticalelsOonlibnee
DOI: 10.1039/C8GC03857K
takes place at the biphasic liquid-liquid interface.
values of the starting VOs. Not only vegetable oils with a low
Overall, the reaction conditions described in entry 8 provide iodine number (like peanut oil, I.V. = 91), but also the ones with
an optimized reaction parameter set for the epoxidation of a high iodine number (like linseed oil, I.V. = 165) give full
sunflower oil in the absence of MeCN and lead to full conversion conversion and very high epoxide yields (90% for peanut oil,
of all double bonds in the oil and to 90% of epoxide formation 99% for linseed oil).
with the corresponding diols as likely byproducts. These
From a cost point of view, waste cooking oils are potentially
conditions even provide a better reaction outcome than the promising feedstocks for the production of EVOs since they are
original conditions using acetonitrile as a solvent (compare generally 30-60% cheaper than regular vegetables oils.6
entries 2 and 8 for 1 g scale reactions).
Applying the current epoxidation protocol to cooked sunflower
To further investigate the substrate tolerance and validation oil gave full conversion of double bonds and 98% epoxide yield
of this epoxidation strategy, the epoxidation of various VOs was (entry 10). Next, a scale-up experiment with 5 grams of
examined using this optimized rac-3/H2O2/AcOH catalytic sunflower oil was carried out (entry 11). No drop in conversion
system (Table 4, entry 8). All reactions were done without using and yield were observed in comparison with the 1-gram scale
MeCN as solvent. Of note is that in the cases of 1-gram scale experiment (entry 1 vs. entry 11). Notably, the Mn-catalyst was
reactions, the catalyst was added as a 10 mM stock solution in added as a solid in this case, meaning that the reaction can be
MeCN for the purpose of easy operation. In addition, a fixed performed with the same efficiency in the complete absence of
amount of AcOH (5.3 mL) was used for all 1-gram scale MeCN. Furthermore, separation of the resulting epoxidized
reactions, regardless of the different iodine values of the oils. As sunflower oil was conducted straightforwardly by simple
shown in entries 1-7 in Table 5, most of the VOs can be extraction of the reaction mixture with Et2O and follow-up
epoxidized using these standard reaction conditions with full removal of the organic solvents. The obtained epoxidized
conversions and excellent yields (up to 99%). For olive oil a sunflower oil showed high purity according to 1H NMR analysis.
somewhat lower reactivity was observed (90% conversion and
It can be concluded from Table 5 that the Mn-based catalytic
85% yield, entry 6). Notably, amongst these VOs rapeseed oil, protocol is capable of epoxidizing a variety of VOs, mostly
linseed oil, and soybean oil are promising feedstocks to produce leading to epoxide yields of 90% or higher (only sesame oils
epoxidized VOs in industry, because of their wide availabilities showed a 75% yield) with low catalyst loadings (0.03 – 0.04
and low prices.4 The present epoxidation protocol provides an mol%). As a result, high TON values were obtained (1875 –
excellent tool to produce EVOs from these three VOs, with 3300), which are much higher than the previously reported
nearly quantitative epoxide yields in all cases (98%, 99% and homogeneous systems21,22 summarized in Table 1 (54 – 95,
99%, respectively, entries 4, 5 and 7).
entries 1 and 2). In addition, the system provides very high
Table 5. Epoxidation of VOs using the rac-3/H2O2/AcOH catalyst system.a)
Entry
Oil
Iodine
value
Catalyst loading
(mol%)
Conversion Epoxide
Epoxide
Selectivity (%)
TONb)
(%)
>99
>99
>99
>99
Yield (%)
1
2
3
4
Sunflower oil
Walnut oil
Peanut oil
130
144
91
0.03
0.03
0.03
0.03
90
96
90
98
90
96
90
98
3000
3200
3000
3267
Rapeseed oil
121
5
6
7
Linseed oil
Olive oil
Soybean oil
165
88
128
0.03
0.03
0.03
>99
90
>99
99
85
99
99
94
99
3300
2833
3300
8
9
Rice oil
Sesame oil
Cooked sunflower oil
Sunflower oil
105
115
118
130
0.04
0.04
0.03
0.03
>99
80
>99
>99
92
75
98
90
92
94
98
90
2300
1875
3267
3000
10
11 c, d)
a) Unless stated otherwise, reaction conditions are: oil (1 g), rac-3 (0.03 mol % w.r.t. double bonds, added as 10 mM solution in MeCN, ca. 100 - 180 µL),
and AcOH (5.3 mL) at room temperature; H2O2 (1.5 equiv. w.r.t. double bonds) was added over 90 min, and the reaction mixture was stirred for additional
30 min, stirring rate = 1000 rpm. b) TON = [mol epoxide]/[mol rac-3]. c) 5 g of oil was used, 26.5 mL of AcOH was added. d) Solid catalyst was added.
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ARTICLE
Journal Name
selectivities for the epoxide products in the range of 90-99% for respectively, entries 1, 3, 5). A moderate nonanal yield of 70%
View Article Online
DOI: 10.1039/C8GC03857K
all oils tested in this study.
was observed in the oxidative cleavage of elaidic acid (entry 2).
1H-NMR analysis of the resulting reaction mixture showed that
in this case there was still ca. 20% of intermediate epoxide
remained, which is likely due to the fact that a trans-epoxide,
formed from epoxidation of the trans double bond in elaidic
acid, is harder to hydrolyze than a cis-epoxide.57 Oxidative
cleavage of erucic acid yielded only 55% of nonanal (entry 4),
whereas neither over-oxidized nonanoic acid nor unreacted
intermediates (epoxide and diol) were detected by GC or NMR
analyses. In all these reactions, no formation of nonanoic acid
was observed, indicating that this one-pot cleavage
methodology is highly selective for aldehyde products. In
general, this Mn-initiated protocol for one-pot oxidative
cleavage of UFAs and FAMEs outperforms the reported Fe-
based catalytic system in terms of catalytic efficiency. In the
later protocol, 0.5 mol% of [Fe(OTf)2(mix-BPBP)] was used to
yield 69-96 % of nonanal in a general reaction time of 20-72 h.36
One-pot oxidative cleavage of UFAs and FAMEs
As mentioned earlier, epoxidized UFAs, their ester derivatives,
and EVOs can be further transferred into other industrially
valuable compounds.4 One example is through the oxidative
cleavage of the C–C bond in the oxirane ring to yield a
monofunctional aldehyde and an α,w-aldehyde fatty acid (or
methyl ester) as the primary products (Scheme 2).5 The primary
aldehyde product, e.g. nonanal in the case of oleic acid, can be
used as
petrochemically-derived nonanal in plasticizer production.5 On
the other hand, the aldehyde moiety in an α, -aldehyde fatty
a
direct, yet renewable replacement for
w
acid (ester) can readily be converted to either an acid, hydroxy,
or amino group. These bifunctional fatty acids (or esters) can be
used for the production of nylons and polyesters, both large
volume and high value products.5 Spannring et al. have
previously developed the Fe-catalyzed one-pot oxidative
cleavage of UFAs and FAMEs into aldehydes with H2O2 and
sodium periodate.36 This protocol comprizes consecutive
reaction steps involving double bond epoxidation catalyzed by
the [Fe(OTf)2(mix-BPBP)]/H2O2/AcOH catalyst system (in which
0.5 mol% of Fe-catalyst was employed), hydrolysis of the
epoxide in the presence of H2SO4 to form a vicinal diol, and
cleavage of the diol to give the two aldehydes with NaIO4 as the
oxidant (Scheme 2).36
Similarly, the Mn-based epoxidation protocol developed in
this paper was used in the one-pot cleavage of a number of
UFAs and FAMEs. The cleavage reaction followed the same
procedure as before: 1) epoxidation of the double bond using
the optimized reaction conditions with the rac-3/H2O2/AcOH
(0.02 mol%, 1.5 equiv. and 18 equiv., respectively) catalyst
system as used in Table 3; 2) hydrolysis of the oxirane ring by
addition of 0.5 equiv. of H2SO4 (1 equiv. of H+); and 3) cleavage
of the diol with 1 equiv. of NaHCO3 and 1 equiv. of NaIO4. GC
analyses were conducted after the one-pot procedure to
determine the amount of nonanal and detect the formation of
nonanoic acid (over-oxidized product from nonanal). Of note is
that only the formation of nonanal was quantified by GC, and
Table 6. One-pot oxidative cleavage of UFAs and FAMEs
initiated by rac-3.a)
O
O
O
0.02 mol% rac-3
O
+
1) 1.5 equiv. H2O2, 18 equiv. AcOH,
CH3CN
2) 0.5 equiv. H2SO4 in H2O
3) 1 equiv. NaHCO3 and NaIO4
H
H
OR
OR
7
n
n
7
Nonanal
Entry
Substrate
n
R
Conv. Nonanal
(%)b)
>99
>99
>99
>99
>99
yield (%)c)
1
Oleic acid
7
H
H
Me
H
98
70
94
55
91
2 d)
3 d)
4 e)
5 d)
Elaidic acid
Methyl oleate
Erucic acid
7
7
11
Methyl erucate 11 Me
a) General reaction conditions at ambient temperature: step 1: substrate
(0.5 mmol), rac-3 (0.02 mol%), AcOH (9 mmol), and H2O2 (0.75 mmol,
added in 45 min) in CH3CN (2 mL), 1.25 h reaction time; step 2: 1 mL of
CH3CN and 1 mL of H2SO4 (0.25 M) in water were added, 3 h reaction
time; step 3: 0.5 mmol of NaHCO3 and 0.5 mmol of NaIO4 were added,
1.5 h reaction time. b) Determined by NMR analysis. c) Determined by GC
analysis. d) The reaction time for step 3 was 3 h. e) Same as d), in addition
the reaction in step 1 was heated to 36 ⁰C.
that the formation of the α,
w-aldehyde fatty acid (ester) was
considered to have taken place in equal amounts.
Conclusions
O
7
O
7
O
O
OH
HO
A highly efficient catalytic protocol has been developed for the
epoxidation of UFAs, FAMEs, and VOs based on the use of the
abundant first-row transition metal manganese in combination
with aqueous H2O2 as the oxidant. Using [Mn(OTf)2(rac-BPBP)]
cat.
H
OH
OH
OH
H2O
H2O2
7
7
O
7
O
O
(
rac-3) as catalyst, the double bonds in most of the vegetable
NaIO4
+
oils tested can be fully converted with a very low catalyst
loading: 0.03 mol% w.r.t. double bonds or ca. 0.1 wt% w.r.t. oil.
In all the VOs tested, this catalytic system is highly selective for
epoxides (>90%), and most of the EVOs were obtained in more
than 90% yield within 2 h of reaction time at ambient
temperature. Therefore, this protocol has a wide applicability
for a variety of VOs, with a range of iodine values from 88 to
165. In the cases of walnut oil, linseed oil, rapeseed oil, cooked
H
H
OH
7
Scheme 2. Oxidative cleavage of oleic acid into aldehyde products.36
Table 6 shows that all reactions gave full substrate conversion
with 0.02 mol% of rac-3 in a total reaction time of 5.75 - 7.25 h.
In the cases of oleic acid, methyl oleate, and methyl erucate,
very high nonanal yields were achieved (98%, 94% and 91%,
8 | J. Name., 2012, 00, 1-3
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sunflower oil, and soybean oil, (nearly) quantitative epoxide
ARTICLE
View Article Online
yields can be achieved under standard reaction conditions. By Synthesis of ligands and Mn complexes DOI: 10.1039/C8GC03857K
increasing the addition time of the H2O2 oxidant to 90 min and
the volume ratio of oil:AcOH to ca. 1:4.8, it is possible to Rac-BPBP To a vigorously stirred 10 mL mixture of water and
perform the reaction with the use of MeCN only as a delivery CH2Cl2 (1/1, v/v) containing 320 mg of crude dl/meso-2,2’-
medium of the catalyst solution rather than a solvent in 1-gram bipyrrolidine58 (2.2 mmol) at room temperature (RT), sodium
scale reactions. Notably, the high catalytic efficiency was hydroxide (572 mg, 14 mmol) was added. Subsequently, 2-
retained in a scale-up experiment with 5 g of sunflower oil in picolyl chloride hydrochloride salt (750 mg, 4.5 mmol) was
which the use of MeCN could be completely omitted. The added at once, turning the mixture red. After stirring overnight,
volume of AcOH and the stirring rate are essential as the another 10 mL of water and CH2Cl2 (1/1, v/v) was added, and
reaction medium is biphasic due to the absence of MeCN. In the yellowish organic phase was separated and the aqueous
addition, the resulting EVOs can be obtained straightforwardly phase was extracted with CH2Cl2 (3 × 10 mL). Organic extracts
from the reaction crude via simple extraction with Et2O. Further were combined, dried over MgSO4 and the solvent was
transformation of epoxidized UFAs and FAMEs have also been removed under vacuum. The crude product was purified by
carried out via an oxidative cleavage procedure, providing neutral alumina column chromatography (petroleum ether/
nonanal and an α,w-aldehyde fatty acid (ester) with moderate EtOAc, 3/1 to 1/1 (v/v)) to provide 352 mg of rac-BPBP (50%
to excellent yields (55-98%). This process is performed in a one- yield). 1H NMR (400 MHz, CDCl3) δ 8.47–8.45 (m, 2 H), 7.56 (td,
pot manner, initiated by the rac-3/H2O2/AcOH catalytic system J = 7.7, 1.8 Hz, 2H), 7.37 (d, J = 7.8 Hz, 2H), 7.10–7.05 (m, 2 H),
in the first epoxidation step.
4.16 (d, J = 14.3 Hz, 2H), 3.48 (d, J = 14.3 Hz, 2H), 2.99–2.94 (m,
Overall, the present homogeneous Mn-catalyzed epoxidation 2 H), 2.79–2.74 (m, 2 H), 2.20 (q, J = 8.6 Hz, 2H), 1.83–1.62 (m, 8
protocol provides a highly efficient and practical tool for the H). 13C NMR (100 MHz, CDCl3) δ 160.62, 148.84, 136.45, 122.51,
production of EVOs under very mild reaction conditions in short 121.67, 65.46, 61.25, 55.34, 25.92, 23.67. HRMS (ESI-MS) calcd.
reaction times, and can be carried out using only minute m/z for C20H27N4 ([M+H]+): 323.2230, found 323.2235.
amounts of MeCN as solubilizing agent or even in the complete
absence of MeCN, allowing for facile product isolation. This [Mn(OTf)2(R,S-BPBP)] (R,S-3) To a vigorously stirred solution of
protocol is expected to represent a promising alternative to Mn(OTf)2 (107 mg, 0.3 mmol) in dry MeCN (1 mL) under a
conventional epoxidation methods and outperforms previously nitrogen atmosphere, a solution of R,S-BPBP (104 mg, 0.32
reported catalytic protocols for EVO production in terms of mmol) in dry MeCN (1 mL) was added. The resulting mixture was
efficiency and reaction conditions.
stirred at RT overnight, providing a light brown/white
precipitate. The precipitate was allowed to settle and the
supernatant was removed. The remaining precipitate was
washed with diethyl ether twice then dissolved in dry MeCN.
The resulting solution contained some black impurities, which
was removed via filtration through a filter paper. Subsequently,
diethyl ether was layered carefully to the remaining solution,
which was left to crystalize. In a few days, yellow crystals were
obtained in 65% yield. HRMS (ESI-MS) calcd. m/z for
C21H26F3MnN4O3S ([M-OTf]+): 526.1058, found 526.1108.
Experimental section
General
All the complexation reactions were performed under a
nitrogen atmosphere using standard Schlenk techniques and all
other reactions including catalytic reactions were conducted
under ambient conditions. The solvents diethyl ether and MeCN
were purified with an MBraun MB SPS-800 solvent purification
system. Demineralized water and technical grade CH2Cl2 were
used for reactions. Tetrahydrofuran for complexation reactions
was dried with sodium, and was distilled under nitrogen prior
to use. Vegetable oils were obtained from local supermarkets.
All other reagents, substrates, and reaction products were
obtained commercially and used without further purification.
Column chromatography was performed using neutral alumina.
1H and 13C NMR spectra were recorded with a 400 MHz Varian
spectrometer at 25 °C, chemical shifts (δ) are given in ppm
referenced to the residual solvent peak. Gas chromatography
(GC) was performed on a Perkin–Elmer Clarus 500 Gas
Chromatograph equipped with an Agilent HP 5 column with a
5%phenyl-95%methylpolysilaxane ratio and a flame-ionization
detector. ESI-MS measurements were performed with a Waters
LCT Premier XE KE317. The ligands S,S-BPBPM44, R,S-BPBP47,
Elemental analysis calcd. (%) for C22H26F6MnN4O6S2
37.14, H 4.25, N 7.87, found C 37.45, H 4.43, N 8.06.
×2H2O: C
[Mn(OTf)2(Rac-BPBP)] (rac-3) This complex was prepared in an
analogous manner to R,S-3 starting from rac-BPBP. Yellow-
white solid (55% yield). HRMS (ESI-MS) calcd. m/z for
C21H26F3MnN4O3S ([M-OTf]+): 526.1058, found 526.1129.
Elemental analysis calcd. (%) for C22H26F6MnN4O6S2: C 39.12, H
3.88, N 8.29, found C 39.71, H 3.75, N 8.36.
[Mn(OTf)2(mix-BPBP)] (mix-3) This complex was prepared in an
analogous manner to R,S-3 starting from mix-BPBP. Yellow-
white solid (51% yield). HRMS (ESI-MS) calcd. m/z for
C21H26F3MnN4O3S ([M-OTf]+): 526.1058, found 526.1187.
Elemental analysis calcd. (%) for C22H26F6MnN4O6S2
38.10, H 4.07, N 8.08, found C 38.31, H 4.15, N 8.26.
×H2O: C
mix-BPBP47, and S,S-BPBI45 and Mn complexes 145 245, and S,S-
,
346 were synthetized following reported procedures. Elemental
microanalyses were carried out by the Mikroanalytisches
Laboratorium Kolbe, Germany.
[Mn(OTf)2(S,S-BPBI)] (4) This complex was prepared in an
analogous manner to R,S-3 starting from S,S-BPBI. Light yellow
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
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solid (42% yield). HRMS (ESI-MS) calcd. m/z for conversion by measuring the iodine value (IV) according to the
View Article Online
DOI: 10.1039/C8GC03857K
C29H26F3MnN4O3S ([M-OTf]+): 622,1058 found: 622,1052. Wijs method.55 The other portion was used to determine the
Elemental analysis calcd. (%) for C30H26F6MnN4O6S2
44.62, H 3.74, N 6.94, found C 46.12, H 3.91, N 6.87.
×
2H2O: C yield of oxirane oxygens (OO) formed via the AOCS Official
Method Cd 9-57.56
The conversion of substrate was determined as follows:
Catalysis
IVo - IVp
Conversion =
× 100%
IVo
General procedure for optimization of the epoxidation of oleic
acid (Table 2).
Where IVo is the initial iodine value of starting vegetable oil, IVp
A 20 mL vial was charged with: oleic acid (0.5 mmol, 1 equiv.), is the iodine value of epoxidized vegetable oil after epoxidation.
catalyst, AcOH (9 or 18 equiv.), and MeCN (2 mL). Subsequently,
a 1.0 M H2O2 solution in CH3CN (2 or 1.5 equiv., diluted from a The yield of epoxidized products was determined as follows:
35% H2O2 aqueous solution) was added over the indicated time
at RT. Next, the resulting mixture was allowed to stir at RT for
the indicated time. At this point, a 1.0 M nitrobenzene or
OOp
× 100%
Yield =
OOthe
biphenyl solution in CH3CN (0.5 mL, 0.5 mmol, 1 equiv.) was Where OOp is the measured percentage of oxirane oxygen in
added as internal standard. The solution was diluted with Et2O epoxidized products, OOthe is the theoretical maximum
to precipitate the Mn-complex and passed through a cotton percentage of oxirane oxygen, which is calculated as follows:
wool filter to remove residual complex. Solvent was removed in
vacuo and a sample was submitted to 1H NMR analysis.
Conversion of oleic acid was determined by comparison of the
olefinic hydrogen signals at 5.3 ppm. Epoxide yield was
(IVo/253.8) × 16
× 100%
OOthe
=
100 + (IVo/253.8) × 16
determined by comparison of the oxirane hydrogen signals at Epoxidation of vegetable oils by rac-3 (Table 5).
2.9 ppm.
A 20 mL vial was charged with: oil (1 or 5 g), rac-3 (0.03 mol%,
w.r.t. double bonds in the starting oil, added as a 10 mM
solution in MeCN for 1 g scale reactions, and as a solid for 5 g
Epoxidation of UFAs (FAMEs) by rac-3 (Table 3).
A 20 mL vial was charged with: substrate (0.5 mmol, 1 equiv.), scale reactions), and AcOH (5.3 mL for 1 g scale, 26.5 mL for 5 g
rac-3 (10 L of a 10 mM solution in MeCN, 0.02 mol%), AcOH scale). The resulting mixture was stirred vigorously (1000 rpm)
µ
(0.5 mL, 9 mmol, 18 equiv.), and MeCN (2 mL). Subsequently, a at RT. Subsequently, 1.5 equiv. of H2O2 w.r.t. double bonds in
1.0 M H2O2 solution in CH3CN (0.75mL, 1.5 equiv., diluted from the starting oil (35% H2O2 aqueous solution) was added over 90
a 35% H2O2 aqueous solution) was added over 45 min, and the min, then the reaction mixture was allowed to stir for additional
reaction mixture stirred for additional 30 min. At this point, a 30 min. Upon completion of the reaction, H2O (20 mL) was
1.0 M nitrobenzene or biphenyl solution in CH3CN (0.5 mL, 0.5 added to the reaction mixture, followed by extraction with Et2O
mmol, 1 equiv.) was added as internal standard. The solution (3 × 20 mL). Organic extracts were combined and dried over
was diluted with Et2O to precipitate the Mn-complex, passed MgSO4 and the solvent was removed under vacuum. The
through a cotton wool filter to remove residual complex. residue was separated into two portions. One portion was used
Solvent was removed in vacuo and a sample was submitted to to determine conversion and the other portion was used to
1H NMR analysis. Conversion of oleic acid was determined by determine the yield of oxirane oxygens (OO) formed (see
comparison of the olefinic hydrogens at 5.3-5.4 ppm. Epoxide above). In the case of 5-gram scale reaction with sunflower oil,
yield was determined by comparison of the oxirane hydrogens the obtained product after work-up was also characterized by
at 2.7-2.9 ppm.
1H NMR.
General procedure for optimization of the epoxidation of One-pot oxidative cleavage of UFAs and FAMEs (Table 6).34
sunflower oil by rac-3 (Table 4). Substrate (0,5 mmol, 1 equiv.), rac-3 (10 L of a 10 mM solution
µ
Sunflower oil (1 g), rac-3 (0.02-0.04 mol %, w.r.t. double bonds in MeCN, 0.02 mol%), and AcOH (0.5 mL, 9 mmol, 18 equiv.)
o
in the starting oil, added as a 10 mM solution in MeCN), and were dissolved in CH3CN (2 mL) at RT (36 C for erucic acid).
AcOH (2.6 mL for 9 equiv., 5.3 mL for 18 equiv.) were mixed in Subsequently, a 1.0 M H2O2 solution in CH3CN (0.75 mL, 1.5
the absence or presence of MeCN (2 mL). The resulting mixture equiv., diluted from a 35% H2O2 aqueous solution) was added
was stirred vigorously (1000 rpm) at RT. Subsequently, 1.5 over 45 min. The resulting mixture was allowed to stir for
equiv. of H2O2 w.r.t. double bonds in the starting oil (35% H2O2 another 30 min. At this point, 1 mL MeCN and 1 mL 0.25 M
aqueous solution) was added over 45-120 min, then the H2SO4 aqueous solution (0.5 equiv.) were added (thus
reaction mixture was allowed to stir for additional 30-60 min. MeCN/H2O = 3/1, v/v). After stirring for 3 h, 42 mg of NaHCO3
Upon completion of the reaction, H2O (20 mL) was added to the (0.5mmol, 1 equiv.) and 107 mg of NaIO4 (0.5mmol, 1 equiv.)
reaction mixture, followed by extraction with Et2O (3 ×20 mL). were added, after which the reaction was stirred for 1.5 or 3 h.
Organic extracts were combined and dried over MgSO4 and the Upon completion of the reaction, 0.5 mmol biphenyl was added
solvent was removed under vacuum. The residue was separated as internal standard. The solution was diluted with Et2O to
into two portions. One portion was used to determine precipitate the catalyst, passed through a cotton wool filter to
10 | J. Name., 2012, 00, 1-3
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26 J. Jiang, Y. Zhang, L. Yan and P. Jiang, Appl. Surf. Sci. 2012, 258,
remove residual catalyst. Subsequently, samples were
submitted to 1H NMR and GC analysis. 1H NMR analysis provided
substrate conversions and GC analysis provided nonanal yields
by comparison with authentic samples.
View Article Online
DOI: 10.1039/C8GC03857K
6637–6642.
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Conflicts of interest
There are no conflicts to declare
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View Article Online
DOI: 10.1039/C8GC03857K
A highly efficient catalytic epoxidation of vegetable oils under
mild conditions was developed, using a homogeneous Mn
catalyst and oxidant H2O2.
12 | J. Name., 2012, 00, 1-3
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