An ionic liquid immobilized copper complex for catalytic epoxidation

Article abstract of DOI:10.1039/c5ra12175b

This article brings into focus an in situ strategy of immobilization of a copper complex onto an ionic liquid support. A practical method of olefin and terpene epoxidation by immobilizing a copper complex and 1-ethyl-3-methylimidazolium hexafluorophosphate and using H₂O₂ as the terminal oxidant is developed. The advantageous properties of this catalytic system redefine an exceptionally clean environment for catalytic epoxidations.

Full text of DOI:10.1039/c5ra12175b

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An Ionic Liquid immobilized Copper complex for catalytic
epoxidation
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
R. Dileep * and B. J. Rudresha
DOI: 10.1039/x0xx00000x
www.rsc.org/
This article brings into focus on an in situ strategy of The term terpene used to refer to some hydrocarbons from
turpentine, also called “resin of pine trees”. This term is now more
immobilization of a copper complex onto an ionic liquid support. A
practical method of olefin and terpene epoxidation by
immobilizing a copper complex and 1-ethyl-3-methylimidazolium
generally employed for all natural compounds composed by
7
isoprene units.
The terpenes are oligomeres composed by
isoprene units as monomer epoxides of terpenes are important
2 2
hexafluorophosphate and using H O as the terminal oxidant is
starting materials with a wide variety of uses such as in flavors,
developed. The advantageous properties of this catalytic system
redefine an exceptionally clean environment for catalytic
epoxidations.
5, 6
fragrances, and pharmaceuticals.
Pinene oxide is, for instance, a
starting material for the synthesis of the sandalwood fragrance.
Preparation of the oxidized terpenes under current industrial
methods requires percarboxylic acids, which may leave an
5
a, 6
Immobilization of transition metal Schiff-base complexes onto
zeolitie, polymeric or clay matrices has been adopted to prepare
equimolar waste product after the reaction
Hydrogen peroxide
2 2
(H O
) is an ideal oxidant because water is the only side product
7, 8
heterogeneous catalysts. One of the prominent achievements in and the atom efficiency is excellent.
this field is the use of ionic liquids as homogeneous immobilization metal-catalyzed H epoxidation reactions have been reported
the epoxidation of terpenes is still a challenging task
Although a number of
2
O
2
1
, 2
9, 10
of organocatalysts in organic synthesis.
catalysts can induce organic reactions and such reactions are
usually carried out in polar solvents, which can form
Ionic liquid immobilized thus far
due to their sensitivity toward acidic media.
5
b, 10
Hence a catalytic
system applicable to nearly neutral conditions without loss of
activity at ambient temperature is necessary for the epoxidation of
terpenes.
a
homogeneous system. After the reaction is complete, a less polar
solvent, such as diethyl ether, is added and the IL immobilized
catalyst can be easily separated from the organic phase due to Pursuing the reactivity of copper complexes with
a
solubility differences. As a result, the IL immobilized catalyst is triphenylphosphine and a ligand derived from 2-hydrazinopyridine
and salicylaldehyde and its 5-substituted derivatives is of our
regenerated and can be used for further reactions.
interest. We had reported earlier, the same complexes for the
1
1
oxidation of a variety of alcohols.
Herein, the oxidation of
catalyzed by Copper
Olefins are one of the most important starting materials for organic
synthesis and their oxidation leads to various value-added products
such as epoxides, alcohols, aldehydes, ketones, and carboxylic
acids. Hence these are important building blocks for the production
selected olefins and some terpenes with H
2 2
O
complexes, 1-5, in ionic liquid, 6, under different reaction
conditions is reported (Scheme 1). In this work, the advantageous
properties of the Copper-H O oxidation system and [Emim]PF6
have been combined to give an exceptionally clean environment for
catalytic oxidations. One major advantage of this system is that
both peroxide and the catalyst are soluble in the ionic liquid. This
gives an oxidation solution that is completely homogeneous. Based
on the synthesized catalyst, an extremely effective and reusable
oxidation system for the oxidation of olefins, performed in ionic
3
2
2
of bulk and fine chemicals. Epoxides are valuable and versatile
4
a-c
intermediates in organic synthesis
and useful starting materials
for the preparation of industrial chemicals such as epoxy resins,
4
d-f
surfactants, paints, adhesives, and surface coating agents.
They
can be efficiently obtained by metal-catalyzed oxidation of alkenes
using a variety of oxidants such as oxygen, peroxides, hypochlorites,
peracids etc. The catalyzed oxidations have the advantage of
generating less waste by-products than the stoichiometric
liquid [Emim]PF
6
with
a
well-proportioned CH
2
Cl
2
at room
4d
temperature is described.
processes.
Complexes 1-5 were tested for their catalytic activities on the
epoxidation of some olefins (Table 1) under similar conditions as
mentioned in the experimental. It was found that complexes 1, 2
and 5 gave a better result compare to complexes 3 and 4. This may
be due to the electronegative nature of the substituents in the
complexes. Complex 1 gave the best result (conversion) followed by
a.
Department of Chemistry, GITAM University, Bengaluru campus, Doddaballapur,
Bangalore North-561203, India.Email. dileepr@gitam.edu; dileep.618@gmail.com
Department of Chemistry, Brindavan College of Engineering, Yelehanka,
Bangalore- 560063, India
b.
Electronic
DOI: 10.1039/x0xx00000x
Supplementary
Information
(ESI)
available:
[See
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complexes 5 and 2. Also, this may be due to the fact that the mixed proportion. As shown in Figure 1, the reaction was almost
catalytic activity of complexes varies with the size of substituent. It complete within 30 hours in the mixed solvent [Emim (1 ml) +
was observed that the activity decreases with increase of bulkiness CH
of the substituents. This may be due to steric hindrance caused by effect in the mixed solvent [Emim (4 ml) + CH
2
Cl
2
(1 ml)], however, there was no obvious rate acceleration
Cl (1 ml)].
2
2
the substituent, which can affect the planarity of the ligand in the
complexes. Further the recycling of the catalyst-ionic liquid
mixture was found to be easier with respect to 1.
a
1
2
Table 2 Epoxidation of styrene in [Emim]PF
6
-H
2
O
2
system
b
Entry
Solvent
% Conversion
% Yield
1
2
3
4
5
Water
0
0
24
78
97
0
0
12
27
78
Isopropyl alcohol
Acetonitrile
Acetone
P
Cl
N
N
PF -
6
O Cu
N
Dichloromethane
N
a
all reactions were carried out with 1 mmol of Styrene with 8 mmol
N
H
6
R
2 2 3 4
of catalyst 1, 2.6 equiv. of H O , 4 mmol H PO , pH adjusted to near
neutral with NaOH solution
1
-5
b
The reaction was carried out with 1:1 Emim: Solvent.
Scheme1. Structures of Copper complexes [R= -H (1), -OCH
4), -NO (5)] and ionic liquid (6)
3
(2), -Cl (3), -Br
(
2
a
Table 1 Epoxidation of cyclic alkenes catalyzed by copper complexes 1-5
b
Entry Substrate
Time
h)
Conversion (%)
(
1
2
3
4
5
1
2
Cyclohexene
3
3
3
96 90 78 76 89
89 87 65 67 80
92 87 54 49 82
1-methylcyclohexene
Cyclohexen-1-yl
benzene
3
4
5
a
Cyclohex-2en-1-ol
Styrene
4
3
93 89 69 72 85
97 91 78 71 81
All the reactions were carried out at room temperature
Epoxides were identified by using authentic samples for
b
comparison.
Figure 1 Study on the rate of the oxidation reaction of Styrene
The epoxidation of styrene with hydrogen peroxide was catalyzed
by 1 was carried out in 6 to give styrene epoxide (7) and phenyl
acetaldehyde (8) in 78.8% and 21.2% yield, respectively. The
oxidation of cyclohexene (9) with hydrogen peroxide and 1 in 6
gave epoxycyclohexane (10), 2-cyclohexenol (11) and 2-
cyclohexenone (12) in 87.6, 7.1, 5.3%, respectively (Scheme 2), but
cyclooctene gave only epoxycyclooctane in 86% yield.
Imidazole plays a crucial role in biological systems especially in
13
metalloenzymes.
transformations involving metal complexes.
It is also used as solvent for many catalytic
1
4, 15
Preliminary
screening started with using a mixture of appropriate amounts of
complex 1 and Emim in different solvents (Table 2). Styrene was
chosen as the substrate for the epoxidation process using H O as
2 2
oxidant. No reaction was observed with water (Table 2, entry 1) and
isopropyl alcohol (Table 2, entry 2) due to the high dielectric
constants. With acetonitrile a little conversion of 24% was observed
Styrene was used for the optimization of the reaction conditions in
ionic liquid (Table 3). The initial study was carried out using styrene
as substrate and 2 mmol of aqueous H O at room temperature in
2 2
the presence of 8 mmol of catalyst. The oxidation proceeded
smoothly and 90% isolated yield of benzaldehyde was obtained
with 99% conversion and 79% selectivity after one hour stirring.
(
Table 2, entry 3). High percentage conversions were observed with
acetone (Table 2, entry 4) and dichloromethane (Table 2, entry 5).
Between these two solvents, excellent conversions with good
selectivity resulted by using dichloromethane as the co-solvent.
In the present experimental conditions, phosphoric acid acts as a The oxidation occurred only in poor yield by simply bubbling
co-catalyst. It is noteworthy that the selectivity of the reaction molecular oxygen through reaction mixture under similar reaction
products decreased in absence of phosphoric acid (Table 3, entry 6). conditions (Table 3, entry 2). To evaluate the catalytic effect of
Also these reactions are effective at neutral pH. Most importantly, catalyst, the oxidation of styrene was carried out under similar
it is necessary to protect the epoxide from undergoing hydrolysis. reaction conditions in the absence of catalyst and no conversion
This is done by the addition of NaOH for weakening the Brønsted was observed (Table 3, entry 3). The reaction was complete at 60
acid reactivity. Without any pH alteration (without the addition of minutes at room temperature (Table 3, entry 1). Further, when the
Sodium hydroxide) the yield of the epoxide was found to be very oxidation of was carried out using less amount of the catalyst, the
low (Table 3, entry 7).
yield was low (Table 3, entry 4). Also the reaction was carried out by
varying the oxidant concentration to get the optimum yield (Table
Complex 1 was used as catalyst for the optimization of the reaction.
Three different fractions of [Emim]PF ionic liquid were chosen to
6
make a kinetic study, which could clearly illustrates that the
acceleration in the rate of the reaction results from the different
3
, entry 5). The organic phase containing product was separated
after the completion of the reaction leaving behind the catalyst
immobilized in ionic liquid. The recovered brown oily ionic liquid
containing 1 could be reused for further catalytic reactions.
2
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O
+
liquid used. This method of removing reactants and products is also
advantageous because catalyst and the ionic liquid are insoluble in
diethyl ether. Careful evaporation of the ether extracts gave the
reactant–product mixture, which was then analyzed by GC and
NMR spectra.
CHO
Cu-Salen-PPh
3
/H
2
O
2
[
Emim]PF
6
-CH
2
Cl
2
H
3
PO , NaOH
4
7
8
Table 5 shows the epoxidation of some terpenes. The reaction
conditions were the same as that used for the epoxidation of
alkenes, except for the temperature which was maintained at 25°C
and the reaction time was 12- 15 hours. Because the epoxidation of
terpenes is exothermic to a small extent, reaction temperature
control was found critical for a better yield of the product.
OH
O
Cu-Salen-PPh
3
/H
2
O
2
O
+
+
[
Emim]PF
PO
6
-CH
2
Cl
2
H
3
4
, NaOH
a
Table 5 Epoxidation of some terpenes catalyzed by complex 1
9
10
1
1
12
Time
h)
Conversion
(%)
b
Entry
Substrate
Yield (%)
(
Scheme 2. The epoxidation of alkenes by hydrogen peroxide catalyzed by 1
immobilized in 6
1
2
3
4
3-Carene
α-pinene
α -Terpinol
Limonene
Myrcene
13
15
12
12
14
87
93
82
86
82
79
81
74
74
75
a
Table 3 Epoxidation of styrene in [Emim]PF
6
-H
2
O
2
system
Entry
Reaction time
% Conversion
% Yield
87
(h)
5
a
All the reactions were carried out at 25°C
After distillation
1
3
99
b
b
2
10
10
5
5
3
<10
Nil
Nil
Nil
9
c
d
NR
3
4
e
30
73
99
It is noteworthy that the recycled copper catalyst 1 in ionic liquid 6
showed comparable activity even after 6–7 runs in the oxidation of
styrene with hydrogen peroxide (Fig 2). However, the addition of
excess oxidant to the reaction mixture resulted in the degradation
of catalyst. However, it should be pointed out that ILs are not
intrinsically “green”. Most of them are as toxic as other organic
solvents, whereas some demonstrate extreme toxicity. Recently,
the issues of biodegradability and toxicity of ILs have become of
primary interest. The toxicity of 1-alkyl-3- methyl Imidazolium ionic
liquids increases with increase in the size of the alkyl group . This
is due to the fact that the ionic nature decreases as the bulkiness of
6
the organic cation increases. Ionic liquids like Emim [PF ] have a
simple alkyl group which constitutes for a good ionic character and
hence their toxicity levels are very low. Moreover the complete
ionic liquid-catalyst mixture was recovered and reused.
f
5
g
6
14
h
7
3
23
8
a
Unless otherwise indicated, all reactions were carried out with 1
mmol of Styrene with 8 mmol of catalyst 1, 2.6 equiv. of H , 4
mmol H PO , pH adjusted to near neutral with NaOH solution.
The reaction was carried out under oxygen instead of H
Reaction without catalyst.
NR: No reaction
.5 mmol of catalyst.
mmol of H
Reaction without H PO
Reaction without NaOH
2 2
O
3
4
b
2 2
O .
c
1
6
d
e
f
1
1
2 2
O .
g
h
3
4
The results in Table 4 show that different olefinic substrates have Conclusions
been oxidized to epoxides, with good yields. In entry 3, to remove
In summary, the advantages of this oxidation system are: (i)
hydrogen peroxide and catalyst are completely soluble in
Emim]PF , giving a homogeneous oxidation solution. (ii) Products
are easily separated from the reaction solution by extraction with
an immiscible solvent. We also hope to expand the use of this
oxidation solution to substrates other than olefins, such as
hydrocarbons, amines, Sulphides and other aromatic compounds.
water completely from the system, molecular sieves were used for
the reaction, and hence only the epoxide was obtained. One
interesting fact is the poor conversion of 1-decene (entry 15). This
may be due to the fact that 1-decene was almost insoluble in the
ionic liquid and hence heterogeneity was present in the solution.
Moreover after 36 h, we observed that the reaction mixture was
still intensely yellow-brown, indicating that the catalyst is still
[
6
active. Also the reaction mixture was stable in ionic media. The time Acknowledgements
required for the reaction is quite less. Remaining reactants and
products are both easily removed from the reaction mixture via
extraction with diethyl ether, which is immiscible with the ionic
RD thanks the NMR Research Centre, Indian Institute of
Science, Bangalore for NMR spectra.
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Fig 2 Effect of recycling on conversion
a
Table 4 Epoxidation of alkenes catalyzed by complex 1
Selectivity (%)
Entry
Substrate
Time (h)
Conversion (%)
b
Epoxide
87
Others
12
1
2
3
4
5
6
7
8
9
Cyclohexene
1-methylcyclohexene
3
3
3
4
3
3
4.5
5
4.5
4.5
6
3.5
3
96
90
92
93
97
86
94
86
91
92
89
89
88
97
9
89
11
c
Cyclohexen-1-yl benzene
Cyclohex-2en-1-ol
Styrene
31
69
d
92
8
e
79
21
f
2-chlorostyrene
Trans-Stilbene
Cis-Stilbene
76
82
81
89
88
65
86
24
18
19
11
12
35
14
(E)-(2-(cyclohexa-2,4-dien-1-yl)prop-1-en-1-yl)benzene
1
1
1
1
1
1
0
1
2
3
4
5
(E)-prop-1-en-1-ylbenzene
Cyclododecatriene
Cyclooctene
g
Cyclooctadiene
cyclopentene
87
13
5
90
10
7
1-decene
36
93
a
b
c
d
e
f
All the reactions were carried out at room temperature
Epoxides were identified by using authentic samples for comparison.
Major product was a diol
Major products were in the form of epoxyhexanol and epoxyhexanone
Minor product was aldehyde
Yield of ketone by-product
Minor product was a monoepoxide
g
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References
14. M. R. dos Santos, J. R. Diniz, A. M. Arouca, A. F.
Gomes, F.C. Gozzo, S. M. Tamborim, A.L. Parize, P. A.
1
2
3
. W. Miao and T.H. Chan, Acc. Chem. Res., 2006, 39, 897.
Z. Suarez and B. A. D. Neto, ChemSusChem, 2012, 5,
. S.T.Handy and M. Okello, J. Org. Chem., 2005, 70, 2874.
716.
. (a) J. E. B¨ackvall, Modern Oxidation Methods, Wiley-
VCH, Weinheim, 2004 pp 53-118; (b) M. Beller and C.
Bolm, Transition Metals for Organic Synthesis, Wiley-
VCH, Weinheim, 2004, pp 415-426.
15. K. A. Srinivas, Anil Kumar and S. M. S. Chauhan, Chem.
Commun., 2002, 2456.
16. K. S. Egorova and V. P. Ananikov, ChemSusChem, 2014,
7, 336.
4
. A. Somasekar Rao, Addition Reactions with Formation of
C-O Bonds: General Methods of Epoxidation,
Coomprehensive Organic Synthesis, B.M. Trost and I.
Fleming Vol 7 (Eds.), Pergamon Press, Oxford, 1991 pp
3
57. (a) K.A. Jørgensen, Chem. Rev., 1989, 89, 431; (b) T.
Punniyamurthy, S. Velusamy and J. Iqbal, Chem. Rev.,
005, 105, 2329; (c) B.M. Choudary, M. Lakshmi Kantam
2
and P. Lakshmi Santhi, Catal. Today, 2000, 57, 17; (d) K.
Srinivasan, P. Michaud and J.K. Kochi, J. Am. Chem. Soc.,
1986, 108, 2309; (e) W. Zhang, J.L. Loebach, S.R. Wilson
and E.N. Jacobsen, J. Am. Chem. Soc. 1990, 112, 2801.
. (a) M. Eggersdorfer, In Ullmann’s Encyclopedia of
Industrial Chemistry, 6th ed., Vol. 35 Wiley-VCH:
Weinheim, 2003, pp 653-671. (b) A. Corma, S. Iborra
and A. Velty, Chem. Rev., 2007, 107, 2411.
5
6
. G. Sienel and R. Rieth, In Ullmann’s Encyclopedia of
Industrial Chemistry, 6th ed., Vol. 12 Wiley-VCH:
Weinheim, 2003, 279.
7. (a) B. M. Trost, Science 1991, 254, 1471; (b) R. A.
Sheldon, Chem. Ind. 1992, 903.
8
. G. Strukul, In Catalytic Oxidations with Hydrogen
Peroxide as Oxidant; Kluwer Academic Publishers: The
Netherlands, 1992, pp 97-151. For recent examples,
see: (a) Rüsch gen. M. Klaas and S. Warwel, Org. Lett.,
1999, 1, 1025; (b) A. L. Villa, B. F. Sels, D. E. De Vos and
P. A. Jacobs, J. Org. Chem., 1999, 64, 7267; (c) L. Salles,
J.-M. Brégeault and R. Thouvenot, Surf. Chem. Catal.,
2000, 3, 183; (d) F. C. Skrobot, A. A. Valente, G. Neves, I.
Rosa, J. Rocha and J. A. S. Cavaleiro, J. Mol. Catal. A:
Chem., 2003, 201, 211; (e) C. B. Woitiski, Y. N. Kozlov, D.
Mandelli, G. V. Nizova, U. Schuchardt and G. B. Shul’pin,
J. Mol. Catal. A: Chem., 2004, 222, 103; (f) G. A. Eimer, I.
Díaz, E. Sastre, S. G. Casuscelli, M. E. Crivello, E. R.
Herrero and J. Perez-Pariente, Appl. Catal. A, 2008, 343
,
77.
9
. For recent reviews, see: (a) Applied Homogeneous
Catalysis with Organometallic Compounds, 2nd ed.; B.
Cornils and W. A. Herrmann, Eds.; Wiley-VCH:
Weinheim, 2002 pp 1-23; (b) B. S. Lane and K. Burgess,
Chem. Rev., 2003, 103, 2457. (c) J.-E.Bäckvall, Modern
Oxidation Methods Ed.; Wiley-VCH: Weinheim, 2004.
0. S. Sakaguchi, Y. Nishiyama and Y. Ishii, J. Org. Chem.,
1
1
1
1
1
996, 61, 5307.
1. D. Ramakrishna and B. Ramachandra Bhat, Inorg.
Chem. Commun., 2011, 14, 690.
2. D. Ramakrishna B. Ramachandra Bhat and
R
Karvembu, Catal. Commun. 2010, 11, 498
3. M. Costas, M. P. Mehn, M. P. Jensen and L. Que,
Chem. Rev., 2004, 104, 939.
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An Ionic Liquid immobilized Copper complex for catalytic epoxidation
a
b
R. Dileep* and B. J. Rudresha
a
Department of Chemistry, GITAM University, Bengaluru campus, Doddaballapur, Bangalore North-561203,
India
b
Department of Chemistry, Brindavan College of Engineering, Yelehanka, Bangalore- 560063, India
Graphical Abstract
A catalytic system consisting of Copper-Schiff base-triphenylphosphine complex and
hydrogen peroxide was effective in the epoxidation of olefins and terpenes. The oxidation
reactions were carried out in ethyl-methyl-imidazolium ionic liquid medium. The catalyst and
the ionic liquid mixture was recycled and reused consistently for about 7 times.
*Corresponding author: Department of Chemistry, GITAM University, Bengaluru campus,
Doddaballapur, Bangalore North-561203, India Ph: +91 9880245273
Email: dileepr@gitam.edu; dileep.618@gmail.com;