A novel, green 1-glycyl-3-methyl imidazolium chloride-copper(II) complex catalyzed CH oxidation of alkyl benzene and cyclohexane

Article abstract of DOI:10.1016/j.cclet.2012.04.014

A variety of alkyl-arenes and cyclohexane were converted to the corresponding ketones with NaClO as the oxidant in the presence of 1-glycyl-3-methyl imidazolium chloride-copper(II) complex. This method contains simplified product isolation and catalyst recycling, affording benzylic CH oxidation of alkyl-arenes imparting high yield of ketones. Furthermore, complex could be reused seven times without a significant loss of its catalytic activity.

Full text of DOI:10.1016/j.cclet.2012.04.014

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 681–684
www.elsevier.com/locate/cclet
A novel, green 1-glycyl-3-methyl imidazolium chloride–copper(II)
complex catalyzed C–H oxidation of alkyl benzene and cyclohexane
*
Parasuraman Karthikeyan, Pundlik Rambhau Bhagat , S. Senthil Kumar
Organic Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632 014, India
Received 16 February 2012
Available online 11 May 2012
Abstract
A variety of alkyl-arenes and cyclohexane were converted to the corresponding ketones with NaClO as the oxidant in the
presence of 1-glycyl-3-methyl imidazolium chloride–copper(II) complex. This method contains simplified product isolation and
catalyst recycling, affording benzylic C–H oxidation of alkyl-arenes imparting high yield of ketones. Furthermore, complex could
be reused seven times without a significant loss of its catalytic activity.
# 2012 Pundlik Rambhau Bhagat. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Copper; Alkane; Ionic liquid; Room temperature
The oxidation of hydrocarbons is an important task due to their wide applications in both laboratory and industry.
The significance of carbonyl compounds can serve as valuable building block for the manufacture of speciality
chemicals in pharmaceuticals, agrochemicals and various natural products containing aryl ketones [1]. Moreover
existing processes for the oxidation of hydrocarbons use toxic and often stoichiometric or excess oxidants eventually
producing wastes such as, Cr (VI) [2] or KMnO4 [3]. In the last few years, other methods have been devised for
introducing a carbonyl moiety at the alkyl/aryl position, for example, copper(II) complex/H2O2 [4], Ir-catalyzed C–H
activation/borylation [5], PhI (OAc)2 in [bmim]PF6/CH2Cl2 [6], poly(4-vinylpyridine) supported MTO [7] and HBr–
H2O2 [8]. Though all these methods provide good yield, but some have drawbacks such as lengthy work-up procedure,
harsh reaction conditions (organic co-solvents) and require absolutely dry and inert media. Thus, there is a need to
develop an operationally simple, safe and widely usable eco-friendly method.
Notably, more attentions have been given to the reusable ionic compounds as solvents and catalysts in the
development of cost-effective protocols. There is considerable interest in the use of room temperature ionic liquids as
promising substitutes due to their tuneable polarity hydrophobicity, thermal stability and low vapour pressure. These
properties of ILs lead to better solvating ability of various organic, inorganic and polymeric compounds, leading to
better solvents and catalysts for many reactions [9].
As a result of their green credential and potential to enhance reaction rates and selectivities, ionic liquids are finding
increasing applications in organic synthesis [10]. Recently, copper complex has been used for several organic
transformations such as the epoxidation of styrene [11], oxidation of alcohol [12], alkane [13], asymmetric synthesis
* Corresponding author.
E-mail address: drprbhagat111@gmail.com (P.R. Bhagat).
1001-8417/$ – see front matter # 2012 Pundlik Rambhau Bhagat. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.04.014

682
P. Karthikeyan et al. / Chinese Chemical Letters 23 (2012) 681–684
[Gmim]Cl-Cu (II)
O
R and R' = alkyl, aryl
R
R'
R
R'
NaClO
rt/10-12 h
Scheme 1. The oxidation of alkanes catalyzed by [Gmim]Cl–Cu(II) complex in NaClO.
[14], amination [15] and also for the Suzuki cross-coupling [16] reaction. From above the open literature the
development of benign, non-toxic and eco-friendly catalytic alkane oxidation, based on copper functionalized ionic
liquid, with NaClO is a major challenge. In this article we described a novel 1-glycyl-3-methyl imidazolium chloride–
copper(II) complex/NaClO (Scheme 1) as an effective catalyst for alkylarenes and cyclohexane oxidation. Herein, we
report an unprecedented, mild and simple synthetic protocol for the oxidation of alkylarenes and cyclohexane to their
corresponding carbonyl compounds at ambient temperature using [Gmim]Cl–Cu(II) complex as shown in Scheme 1.
1. Results and discussion
The [Gmim]Cl–Cu(II) complex catalyzed C–H oxidation reaction was carried out at 25 8C, using ethyl benzene for
model reaction to investigate different parameters, such as diverse oxidants and catalysts. Initially, the effect of different
oxidants to the model reaction was studied; these results are summarized in Table 1. Obviously, there is no appreciable
reaction in absence of catalyst (Table 1, entry 1). Futhermore, reaction carried with different oxidants such as t-BuOOH,
NaClO2, H2O2, and O2 that demonstrated poor conversions; while NaClO gave good results (Table 1, entries 2–6). After
conceivingthe best oxidant, we further optimized the reaction conditions in presence of [Gmim]Cl–Cu(II). The experiment
illustrated that on decreasing the time from 48 to 10 h gave the same results with full conversion and 85% yield of
acetophenone (Table 1, entries 7–10). However, the yields were dropped appreciably on decreasing the time from 10 to 1 h
(Table 1, entries 11–13). The influence of the amount of NaClO and [Gmim]Cl–Cu(II) were then investigated. When the
amount of NaClO decreased from 10 to 2 mmol, the C–H could still be oxidized with a satisfactory result. However, the
oxidizationofethyl benzenecould not becompleted with 1mmol NaClO (Table 1, entries 14–17). It is noted that increasing
the amount of ionic liquid led to increased yields of acetophenone. By all these experimental results and discussions it was
concluded that 2 mmol NaClO is sufficient to brought out the complete C–H oxidation reaction in 10 h at 25 8C.
Table 1
Effect of the oxidants on C–H oxidation of ethyl benzene.a
Entry
Oxidant
Time (h)
Yieldb (%)
1
2
–
>48
48
48
48
48
48
24
20
15
10
8
–
–
t-BuOOH
NaClO2
H2O2
3
–
4
40
53
85
85
85
85
85
80
72
64
85
85
85
70
5
O2
6
NaClO
NaClO
NaClO
NaClO
NaClO
NaClO
NaClO
NaClO
NaClOc
NaClOd
NaClOe
NaClOf
7
8
9
10
11
12
13
14
15
16
17
5
1
10
10
10
10
a
Reaction condition: a solution of the [Gmim]Cl–Cu(II) (0.1 mmol), ethyl benzene (2 mmol) and different oxidants (2 mmol) at ambient
temperature.
b
c
d
e
f
Yield determined by HPLC.
10 mmol NaClO was used.
5 mmol NaClO was used.
2 mmol NaClO was used.
1 mmol NaClO was used.

P. Karthikeyan et al. / Chinese Chemical Letters 23 (2012) 681–684
683
Table 2
Optimization of the [Gmim]Cl–Cu(II) complex catalyzed C–H oxidation of ethyl benzene.a
Entry
Catalyst (mmol)
Time (h)
Conversionb (%)
Yieldf (%)
1
2
3
4
5
6
7
8
–
48
24
24
24
10
10
8
Trace
37
Trace
34
[Gmim]Clc
CuCl2
30
26
CuCl2/[Gmim]Cl
[Gmim]Cl–Cu(II)
[Gmim]Cl–Cu(II)d
[Gmim]Cl–Cu(II)e
[Gmim]Cl–Cu(II)
51
48
89
85
90
86
90
87
11
89
86
a
Reaction condition: a solution of the catalyst (0.1 mmol), ethyl benzene (2 mmol), NaClO (2 mmol) at ambient temperature.
Determined by HPLC.
b
c
d
e
f
1-Glycyl-3-methyl imidazolium chloride ionic liquid.
0.2 mmol [Gmim]Cl–Cu(II) complex was used.
Reaction was carried out at 60 8C.
Isolated yield by flash chromatography.
Next, we have screened a number of different catalysts on the model reaction. At first, the oxidation reaction was
unsuccessful in absence of catalyst (Table 2, entry 1). Furthermore; the reaction was carried out in the presence of
[Gmim]Cl, CuCl2, CuCl2/[Gmim]Cl with different intervals, it gave 34–48% of carbonyl compounds (Table 3, entries
2–4). Finally, desired product was obtained in the presence of [Gmim]Cl–Cu(II), which have higher yield (Table 2,
entry 5). Moreover, the reaction was not accelerated obviously inspite of increased the amount of catalyst from 0.1 to
0.2 mmol (Table 2, entry 6). Almost similar yield was obtained when slightly increasing the temperature (60 8C) and
duration of the reaction (Table 2, entries 7 and 8).
Having optimized the reaction conditions, the oxidation of different alkyl-arenes and cyclohexane were examined
(Table 3). The oxidation of ethyl benzene and n-butyl benzene afforded the corresponding ketones with 85 and 82%
yield (Table 3, entries 1 and 2). As it could be expected, higher conversion (88%) was also obtained with
diphenylmethane (Table 3, entry 3). Treatment of 3-phenyl-1-propanol with NaClO in [Gmim]Cl–Cu(II) complex
system afforded the corresponding 2-benzoyl ethanol with 82% yield (Table 3, entry 4).
In the case of acetate as substrate for this reaction, modulator desired product were detected (Table 3, entries 5 and 6).
Further, this optimized condition applied in alicyclic system with [Gmim]Cl–Cu(II) complex with 70 and 88% yields
(Table 3, entries 7 and 8).
Subsequently, the recyclability of the used complex was demonstrated for the oxidation of ethyl benzene to
acetophenone. The catalytic system could be reused directly for the next cycle, after full extraction of the product
(3 Â 5 mL) diethyl ether per extraction and drying in vacuo. The results shown in Table 4 demonstrate that this
oxidative system was readily recyclable for seven runs without any significant loss of catalytic activity (Table 4).
Table 3
Benzylic C–H oxidation of alkyl-arenes using [Gmim]Cl–Cu(II) ionic liquid at ambient temperature.a
Entry
Substrate
Time (h)
Productb
Yieldc (%)
Boling point (8C)
1
2
3
4
5
6
7
8
Ethylbenzene
10
11
11
10
10
12
12
12
Acetophenone
85
82
88
82
80
80
70
88
202
228–230
47d
n-Butylbenzene
Butyrophenone
Diphenylmethane
Benzophenone
3-Phenyl-1-propanol
3-Phenylpropyl acetate
4-Phenylbutyl acetate
Cyclohexane
2-Benzoyl ethanol
2-Benzoylethyl acetate
3-Benzoyl propyl acetate
Cyclohexanone
289
306
326
155
1,2,3,4-Tetrahydronaphthalene
a-Tetralone
113
a
Reaction condition: a solution of the catalyst (0.1 mmol), ethyl benzene (2 mmol), NaClO (2 mmol) was stirred at ambient temperature.
All the products were characterized by spectroscopic analysis (IR, MS, 1H NMR, melting/boiling point) known compounds were compared with
b
authentic data.
c
Isolated yield by flash chromatography.
d
Melting point.

684
P. Karthikeyan et al. / Chinese Chemical Letters 23 (2012) 681–684
Table 4
Catalyst recyclability studies in oxidation of ethylbenzene at room temperature.a
Cycles
1
2
3
4
5
6
7
Yieldsb
85
85
83
83
83
82
80
a
Reaction condition: a solution of the catalyst (0.1 mmol), ethylbenzene (2 mmol), NaClO (2 mmol) at ambient temperature.
Isolated yield by flash chromatography.
b
In summary, we have successfully developed a novel, practical and eco-friendly method for the oxidation of alkyl-
arenes and cyclohexane by using [Gmim]Cl–Cu(II) complex at 25 8C under solvent-free condition. In addition, this
methodology offers a competitive recyclability of the catalyst without a significant loss of its catalytic activity and the
catalyst could be readily reused, thus making this procedure more environmentally acceptable whilst no catalyst leaching
was observed. Further investigation on the application of this kind of supported catalyst is still underway in our laboratory.
2. Experimental
The catalyst [Gmim]Cl–Cu(II) was synthesized according to literature [17] and structure conformation is available
in supplementary report.
2.1. Typical procedure for oxidation of alkenes
A solution of the alkane (2 mmol), Cu(II) complex (0.1 mmol) and NaClO (2 mmol) was stirred at 25 8C for the
appropriate time (see Table 3). After the completion of the oxidation, the mixture was extracted with diethyl ether
1
(3 Â 5 mL). The combined ether phase was concentrated in vacuo and monitored by HPLC and H NMR. Then the
residue was purified by flash chromatography (ethyl acetate/chloroform = 0.5:4.5 mL) to afford corresponding ketones
in 70–88% yields. The spectral data of the various ketones were found to be satisfactory in accord with the literature.
Acknowledgments
We gratefully acknowledge the management of VIT University for providing required facilities and SAIF (IITM)
for providing the spectral data.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/
j.cclet.2012.04.014.
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