Cobalt complex in a room temperature ionic liquid: A convenient recyclable reagent for catalytic epoxidation of cyclic alkenes

Article abstract of DOI:10.1016/j.crci.2013.12.004

Co-catalyzed epoxidation of cyclic alkenes proceeds in ionic liquid media (1-ethyl-3-methylimidazolium hexafluorophosphate). Epoxidation of the alkenes to respective epoxides was greatly accelerated by the use of a cobalt-based catalyst in the presence of H₂O₂ as an oxidant. The catalyst in ionic liquid [Emim]PF₆ was recycled and reused for about seven times.

Full text of DOI:10.1016/j.crci.2013.12.004

C. R. Chimie 17 (2014) 1071–1074
Contents lists available at ScienceDirect
Comptes Rendus Chimie
www.sciencedirect.com
Preliminary communication/Communication
Cobalt complex in a room temperature ionic liquid: A
convenient recyclable reagent for catalytic epoxidation of
cyclic alkenes
Dileep Ramakrishna a, ,b, Badekai Ramachandra Bhat b,
*
Suresha Kumara T. Hanumanthappa a
a Department of Chemistry, GI TAM University, Bengaluru campus, NH 207, 561203 Doddaballapur Bangalore North, India
b Catalysis and Material Chemistry Division, Department of Chemistry, National Institute of Technology Karnataka, 575025 Surathkal,
Srinivasanagar, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Co-catalyzed epoxidation of cyclic alkenes proceeds in ionic liquid media (1-ethyl-3-
methylimidazolium hexafluorophosphate). Epoxidation of the alkenes to respective
epoxides was greatly accelerated by the use of a cobalt-based catalyst in the presence of
H2O2 as an oxidant. The catalyst in ionic liquid [Emim]PF6 was recycled and reused for
about seven times.
Received 14 October 2013
Accepted after revision 10 December 2013
Available online 22 July 2014
Keywords:
ß 2014 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Epoxidation
Co-complexes
Ionic liquid
Hydrogen peroxide
1. Introduction
liquids have recently gained recognition as environmen-
tally benign solvents due to their unique physical proper-
The homogeneous catalytic oxidation of hydrocarbons
under mild conditions continues to be a challenge in
catalysis. Particularly, it is difficult to achieve high
efficiency in the selective catalytic epoxidation of olefins
with H2O2. Macrocyclic complexes of biological impor-
tance have been extensively investigated as catalysts for
mild oxidation. Other oxidants such as urea hydrogen
peroxide [1] or oxone [2] instead of molecular oxygen [3]
are the reported systems usually employed for the
epoxidations of cyclic olefins.
One of the challenging issues for chemists in the present
times is the pursuit of ‘clean’ or ‘green’ chemical
transformations [4]. It is often taken for granted that the
use of volatile molecular solvents is a particular source of
chemical waste [5,6]. Ambient-room-temperature ionic
ties such as non-volatility, non-flammability, and thermal
stability [7–9]. They have been employed as solvents for
liquid–liquid separations, extractions and for recycling
homogeneous catalysts, etc. [10–12] Ionic liquids have also
been used as novel reaction media for economically and
environmentally attractive processes in aqueous mono-
and biphasic systems [13–17].
We had previously investigated the catalytic oxidation
of primary and secondary alcohols with the same catalyst
in an ionic liquid [15]. Hence, we started working towards
the oxidations of other organic compounds. The present
research work is one of them. Herein, we report the
oxidation of selected olefins with H2O2 catalysed by cobalt-
salen-triphenylphosphine complex, 1, in ionic liquid 2,
under different reaction conditions. The complex was
synthesized according to a reported procedure [15]. In
this work, the advantageous properties of the cobalt-
salen-triphenylphosphine–H2O2 oxidation system and
[Emim]PF6 have been combined to give an exceptionally
clean environment for catalytic oxidations. One major
*
Corresponding author.
E-mail addresses: dileepr@gitam.edu, dileep.618@gmail.com
(D. Ramakrishna).
1631-0748/$ – see front matter ß 2014 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.12.004

1072
D. Ramakrishna et al. / C. R. Chimie 17 (2014) 1071–1074
Table 1
advantage of this system is that both the peroxide and the
catalyst are soluble in the ionic liquid. This gives an
oxidation solution that is completely homogeneous.
Epoxidation of styrenea in [Emim]PF6–H2O2 system.
Entry
Reaction time (min)
% Conversion
1
60
360
360
120
120
99.0
<10
NRd
29.9
72.6
2b
3c
4e
5f
a
Unless otherwise indicated, all reactions were carried out with
1 mmol of styrene with 2.4 mmol of catalyst, 2 mmol of H2O2.
P
b
The reaction was carried out under oxygen instead of H2O2.
c
Co
O
Reaction without catalyst.
Cl
NH
d
NR: no reaction.
e
1.5 mmol of catalyst.
N
f
1 mmol of H2O2.
N
3. Results and discussion
The epoxidation of styrene with hydrogen peroxide
catalyzed by 1 was carried out in 2, affording, under
stirring in the presence of hydrogen peroxide, styrene
epoxide (3) and phenylacetaldehyde (4) in 78.8% and 21.2%
yields, respectively. The oxidation of cyclohexene (5) with
hydrogen peroxide and 1 in 2 gave epoxycyclohexane (6),
2-cyclohexenol (7) and 2-cyclohexenone (8) in 87.6, 7.1,
5.3% yields, respectively (Scheme 1), but cyclooctene gave
only epoxycyclooctane in 86% yield.
1
-
N
N
PF6
2
Styrene was used for the optimization of the reaction
conditions in ionic liquid (Table 1). The initial study was
carried out using styrene as the substrate and 2 mmol of
aqueous H2O2 at room temperature in the presence of
2.4 mmol of catalyst. The oxidation proceeded smoothly,
and a 90% isolated yield of benzaldehyde was obtained
with 99% conversion and 79% selectivity after 1-h stirring.
The oxidation occurred only in poor yield by simply
bubbling molecular oxygen through the reaction mixture
under similar reaction conditions (Table 1, entry 2). To
evaluate the catalytic effect of the catalyst, the oxidation of
styrene was carried out under similar reaction conditions
in the absence of catalyst and no conversion was observed
(Table 1, entry 3). The reaction was complete after 60 min
at room temperature (Table 1, entry 1). Furthermore, when
the oxidation was carried out using a lesser amount of the
catalyst, the yield was low (Table 1, entry 5). Also the
reaction was carried out by varying the oxidant’s
2. Experimental
2.1. General procedure for the catalytic reaction
In a typical reaction, the catalyst (8 mg, 2.4 mmol) was
dissolved in [Emim]PF6 (1 cm3). After addition of the
substrate (1 mmol), hydrogen peroxide (30% in water)
(2 mmol) was added. The reaction mixture was stirred at
room temperature for about 1 h. The completion of the
reaction was monitored by GC–MS until the reactants have
disappeared. After completion of the reaction, the reaction
mixture was extracted with diethyl ether (3 Â 3 cm3). The
ether layer was collected and then a definite amount of
toluene was added as an internal standard for GC analysis.
The ionic liquid phase including the catalyst was washed
with diethyl ether and dried prior to recycling. The
epoxidation of the other olefins was carried out similarly
with a little variation in reaction time (Table 2).
O
CHO
Co-Salen-PPh3/H2O2
[Emim]PF6
+
3
4
OH
O
Co-Salen-PPh3/H2O2
[Emim]PF6
O
+
+
6
8
5
7
Scheme 1. Epoxidation of alkenes by hydrogen peroxide catalyzed by 1 immobilized in 2.

D. Ramakrishna et al. / C. R. Chimie 17 (2014) 1071–1074
1073
Table 2
Epoxidation of cyclic alkenes catalyzed by complex 1a.
Entry
Substrate
Time (h)
1
Conversion (%)
95.6
Selectivity (%)
Epoxideb
87.6
Others
1
12.4
2
3
1
1
92.2
95.4
89.2
31.2
10.8
CH3
68.8c
4
2.5
94.1
92
8d
OH
5
6
1
1
99.1
95.6
78.8
82.1
21.2e
17.9
7
1
96.7
87.6
12.4
CH3
8
1
2
96.5
94.6
86.4
86.7
13.6
9
13.3f
10
2.5
36
97.1
12
90.2
92.8
9.8
7.2
11
a
All the reactions were carried out at room temperature.
Epoxides were identified by using authentic samples for comparison.
The major product was a diol.
b
c
d
e
f
The major products were in the form of epoxyhexanol and epoxyhexanone.
The minor product was aldehyde.
The minor product was a monoepoxide.

1074
D. Ramakrishna et al. / C. R. Chimie 17 (2014) 1071–1074
products is also advantageous, because the 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–MS.
It is noteworthy that the recycled cobalt-salen-PPh3
catalyst 1 in ionic liquid 2 showed comparable activity,
even after 6–7 runs, in the oxidation of styrene with
hydrogen peroxide (Fig. 1). However, the addition of excess
oxidant to the reaction mixture resulted in the degradation
of the catalyst.
4. Conclusion
In summary, the advantages of this oxidation system
are:
ꢀ
ꢀ
hydrogen peroxide and catalyst are completely soluble in
[Emim]PF6, giving a homogeneous oxidation solution;
the products are easily separated from the reaction
solution by extraction with an immiscible solvent.
Fig. 1. Effect of recycling on conversion.
concentration to get the optimum yield (Table 1, entry 6).
The organic phase containing the product was separated
after the completion of the reaction, leaving behind the
catalyst immobilized in the ionic liquid. The recovered
brown oily ionic liquid containing 1 could be reused for
further catalytic reactions (Table 2).
We also hope to expand the use of this oxidation
solution to substrates other than olefins, such as hydro-
carbons, amines, sulphides and other aromatic com-
pounds.
The results in Table 2 show that different olefinic
substrates have been oxidized to epoxides, with good
yields. In entry 3, to remove 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 11). 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, indi-
cating that the catalyst was still active. Also the reaction
mixture was stable in ionic media. The selectivity and the
yield was more and also the time required for the reaction
is quite less comparable to that required for previously
reported results [18–21]. The catalytic activity was
particularly compared with that of the methyltrioxoreh-
nium catalyst [22], and it was found that most of the
reactions were at least comparable if not higher than the
reported one. Further investigations on the catalytic
activity in different ionic liquids, their kinetics and the
mechanism are ongoing.
References
[1] G.S. Owens, M.M. Abu-Omar, Chem. Commun. 13 (2000) 1165.
[2] M.F.A. Adamo, V.K. Aggarwal, M.A. Sage, J. Am. Chem. Soc. 122 (2000)
8317.
[3] M.C.A. van Vliet, I.W.C.E. Arends, R.A. Sheldon, Chem. Commun. 9
(1999) 821.
[4] R.T. Baker, W. Tumas, Science 284 (1999) 1477.
[5] T. Welton, Chem. Rev. 99 (1999) 2071.
[6] M.J. Earle, P.B. McCormac, K.R. Seddon, Green Chem. 1 (1999) 23.
[7] R. Sheldon, Chem. Commun. (2001) 2399.
[8] P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. Eng. 39 (2000) 3772.
[9] A.E. Visser, R.P. Swatloski, W.M. Reichert, R. Mayton, S. Sheff, A. Weirz-
bicki, J.H. Davis, R.D. Roger, Chem. Commun. (2001) 135.
[10] M.L. Dietz, J.A. Dzielawa, Chem. Commun. (2001) 2124.
[11] L. Gaillon, F. Bedioui, Chem. Commun. (2001) 1458.
[12] G. Bar, A.F. Parsons, C.B. Thomas, Chem. Commun. (2001) 1350.
[13] P. Wasserscheid, C.M. Gordon, C. Hilgars, M.J. Muldoon, I.R. Dunkin,
Chem. Commun. (2001) 1186.
[14] J.D. Holbrey, K.R. Seddon, Clean Prod. Processes 1 (1999) 223.
[15] D. Ramakrishna, B. Ramachandra Bhat, Inorg. Chem. Commun. 13
(2010) 195.
[16] W.A. Herrmann, R.W. Fischer, D.W. Marz, Angew. Chem. Int. Ed. Engl. 30
(1991) 1638.
[17] H. Olivier-Bourbigou, L. Magna, D. Morvan, Appl. Catal. 373 (2010) 1.
[18] I.I.E. Markovits, W.A. Eger, S. Yue, M. Cokoja, C.J. Mu¨ nchmeyer, B. Zhang,
M.-D. Zhou, A. Genest, J. Mink, S.-L. Zang, N. Ro¨sch, F.E. Ku¨ hn, Chem. Eur.
J. 19 (2013) 5972.
[19] J. Rudolph, K.L. Reddy, J.P. Chiang, K.B. Sharpless, J. Am. Chem. Soc. 119
(1997) 6189.
[20] A.K. Yudin, K.B. Sharpless, J. Am. Chem. Soc. 119 (1997) 11536.
[21] W. Adam, C.M. Mitchell, Angew. Chem. Int. Ed. Engl. 35 (1996) 533.
[22] M. Crucianelli, R. Saladino, F. De Angelis, ChemSusChem. (2010) 524.
Two equivalents of hydrogen peroxide were used
relative to the substrate. Remaining reactants and products
were both easily removed from the reaction mixture via
extraction with diethyl ether, which is immiscible with the
ionic liquid used. This method of removing reactants and