Graphene oxide as a recyclable phase transfer catalyst

Article abstract of DOI:10.1039/c3cc42787k

We demonstrated a simple and green chemical method to obtain Michael adducts and their derivatives by using GO as a phase transfer catalyst with different kinds of bases in water and dichloromethane, and we also used GO multiple cycles almost without redu

Full text of DOI:10.1039/c3cc42787k

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ARTICLE TYPE
Graphene oxide as a recyclable phase transfer catalyst†
Youngmin Kim‡, Surajit Some‡ and Hyoyoung Lee *
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
We demonstrated simple and green chemical method to
obtain Michael adduct and its derivatives by using GO as a
phase transfer catalyst with different kinds of bases in water
and dichloromathane, and we also used GO multiple cycles
with almost without reduction of reaction yields.
10
Graphene and its derivatives have great promise for diverse
electronic applications,1 and have also attracted great interest for
use in composite materials and catalysts2-4 due to their
remarkable physical,5 chemical6 and electrical properties,7
15 including their very high specific surface area.8 As part of wider
trends to develop green chemistry and to mimic nature, recent
efforts have been made to develop aqueous organocatalytic
reactions,9 and innovative synthetic approaches involving the use
of chemicals that reduce the risks to humans and the environment
20 have gained interest.10 To this end, the development and use of
catalysts that can be easily recovered and repeatedly recycled in a
heterogeneous organic reaction system is of tremendous
importance.11
Scheme 1 GO Phase-transfer catalyst in the Michael addition reaction.
55 Table 1 Michael addition reaction of trans-β-nitrostyrene and 2,4-
pentanedionea
O
O
NO2
O
O
Catalyst
NO2
Solvent, rt
1a
2
3a
Herein, we report a novel graphene oxide (GO) as a recyclable
25 phase transfer catalyst (PTC) for the Michael addition. The
Michael addition is one of the most useful and representative
methods for the mild formation of new C-C bonds.12 The
formation of new C-C bonds via Michael addition is an important
transformation in organic chemistry, and is used extensively in
30 the synthesis of a variety of molecules, including biologically
active natural products and antibiotics. For the Michael addition,
we selected trans-β-nitrostyrene and 2,4-pentanedione derivatives
as reactants, GO as a PTC, potassium hydroxide as a base, and
water-dichloromethane as an aqueous-organic co-solvent system.
35 We anticipated that the performance of the GO PTC in the
Michael addition would exceed that of the best-known crown
ether (CE), because GO has a large surface area with many
oxygen functional groups, including epoxide, alcohol, phenol,
carbonyl, carboxyl, lactone and quinone groups. These functional
40 groups are expected to easily hold the metal cations by forming a
metal-centered intercalated structure like GO-metal-GO layers.
We also expected that this intercalated structure would
accommodate alkali cations of any size, including Na+, K+ and
Cs+, because GO carries many oxygen functional groups
45 throughout its surface layer. However, the most commonly used
CE, 18-crown-6, carries only the K+ cation well in the Michael
addition (when KOH is used as the base). Furthermore, we
expected that the Michael addition using GO PTC should be
faster and give higher yield than that of the CE catalyst due to
50 GO’s unique characteristics. As expected, GO does not dissolve
in water or organic solvents such as methylene chloride (MC),
but rather is possible to disperse in aqueous solvent and MC as
Entry
Catalyst
Solvent
Time
[min]
40
Yield
[%]b
55
Recyclable
1
2
3
4
5
6
7
8
KOH
KOH
KOH
KOH
GO
GO
GO
KOH+GO
KOH+GO
KOH+GO
KOH+GO
KOH+CEe
NaOH
NaOH
NaOH
NaOH+GO
NaOH+CE
CsOH
CH2Cl2
2MTHFc
H2O
H2O+CH2Cl2
CH2Cl2
No
No
—
No
—
38
—
60
—
—
—
37
—
10
53
NDd
25
ND
ND
ND
59
ND
83
82
76
45
ND
15
75
41
60
ND
23
H2O
—
—
H2O+CH2Cl2
CH2Cl2
Yes
Yes
Yes
Yes
No
No
—
No
Yes
No
No
—
9
H2O
10
11
12
13
14
15
16
17
18
19
20
21
22
H2O+CH2Cl2
H2O+2MTHF 10
H2O+CH2Cl2
CH2Cl2
30
45
—
60
15
45
30
—
60
10
30
H2O
H2O+CH2Cl2
H2O+CH2Cl2
H2O+CH2Cl2
CH2Cl2
CsOH
CsOH
CsOH+GO
CsOH+CE
H2O
H2O+CH2Cl2
H2O+CH2Cl2
H2O+CH2Cl2
No
Yes
No
80
55
a Reactions were carried out with 1a (25 mg, 0.1676 mmol), 2 (1.5 equiv.),
60 base (1.1 equiv.), 0.5 mg ml−1 GO aqueous (0.5 ml) and catalyst 18-
d
b
c
crown-6 ether (1.1 equiv.). Isolated yield. 2-methyl tetrahydrofuran.
Almost no product was detected. e CE = 18-crown-6 ether.
This journal is © The Royal Society of Chemistry [year]
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well. The insolubility of GO has been a serious drawback in other
applications, but GO as a PTC is expected to have a strong
solution to a reaction mixture of trans-β-nitrostyrene and 2,4-
pentanedione in MC solvent or other derivative systems. All
advantage, which will be a superior to the conventional CE 40 experiments were carried out at RT and reactions were vigorously
catalyst. After the reaction is completed, the GO can be collected
simply by filtering, washing and drying the GO-metal composites.
This character of GO is superior to CE, which can carry the
potassium cation from the aqueous phase to the organic phase,13
stirred. To evaluate the GO catalyst in the Michael addition, we
used 2,4-pentanedione as a nucleophile and trans--nitrostyrene
as an electrophile, using 0.5 ml of aqueous solution containing
0.5 mg ml−1 GO catalyst and 1 ml of MC solvent at RT (Table 1).
5
and can also be easily dissolved in organic solvents like MC. And 45 The catalytic properties of GO were studied under various
due to high solubility of CE it is difficult to recover CE after the
conditions including the use of different solvents and bases
(Table 1). Same reactions using 18-crown-6 ether were also
carried out for comparison. Control experiments included
10 reaction is over. For the experiment, GO was prepared from
graphite powder (Bay carbon, SP-2) using a modified Hummers
method and was purified as previously reported.14 Abundant
oxygen functional groups are present at both edges and at defects
in the plane of the GO sheet.15 The abundant oxygen functional
15 groups of GO should be the key elements to act as a cation holder
which makes base, hydroxide anion, stronger and also make the
reaction related to the high surface areas faster to enhance the
activities. The possible reaction process is illustrated in Scheme 1.
For efficient catalytic performance, GO nanosheets need to be
20 fully dispersed in the reaction medium in order to maximize the
number of catalytic sites provided by various oxygen functional
groups, especially epoxy and hydroxyl groups. XPS, Raman and
AFM analysis were used to characterize the GO (see ESI† Figure
S1, S2, S3 and S4). As-made GO was first
50 Table 3 1,3-dicarbonyl compounds as Michael donorsa
O
O
R3
O
O
R1
R2
NO2
KOH+GO
NO2
H2O+CH2Cl2, rt
R1
R2
R3
4a-h
1a
5a-h
Entry
Product
Time [min]
Yield [%]b
O
O
1
35
80
NO2
5a
O
O
2
3
4
5
6
7
8
2880
25
50
79
80
83
66
82
81
25 Table 2 Trans-β-nitroolefins as Michael acceptorsa
O
O
NO2
O
R
O
KOH+GO
O
O
5b
O
NO2
R
H2O+CH2Cl2, rt
NO2
O
O
1a-g
2
3a-g
O
Entry
1
R
Time [min]
Yield [%]b
83
NO2
5c
O
10
16
16
30
23
25
20
20
1a
1b
O
NO2
2
3
4
5
6
80
77
69
79
75
78
5d
O
O
15
O
NO2
1c
5e
MeO
O
60
COMe
1d
NO2
5f
F
Ph
H
O
1e
15
Cl
CO2Me
NO2
5g
Ph
H
1f
Br
O
25
CO2Et
7
NO2
5h
Ph
S
1g
H
a
Reactions were carried out with 1a-g (0.1676 mmol), 2 (1.5 equiv.),
KOH (1.1 equiv.), and 0.5 mg ml−1 GO aqueous (0.5 ml) in MC (1 mL). b
Isolated yields.
a
Reactions were carried out with 1a (25 mg, 0.1676 mmol), 4a-h (1.5
equiv.), KOH (1.1 equiv.), and 0.5 mg ml−1 GO aqueous (0.5 mL) in MC
(1mL). b Isolated yields.
30
dispersed in water to create an aqueous solution. The electrostatic
repulsion among negatively charged carboxylate groups on the
edges of the GO sheets prevented aggregation and stabilized the
dispersion. In this way, most of the surface area of each single-
35 layer GO sheet was available to provide catalytic reaction sites.
After the GO solution was prepared, we dissolved one of three
alkali bases for each individual experiment. We then added each
55
single-phase organic reactions using bases of differently sized
metal ions without a PTC (Table 1, entries 1, 2, 13 and 18),
single-phase aqueous reactions without a PTC (entries 3, 14 and
19), and two-phase reactions without a PTC (entries 4, 15 and 20).
60 No product were observed in the aqueous-only systems (entries 3,
6, 9, 14 and 19), and also in absence of bases (entries 5, 6 and 7).

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But in the two-phase system in the absence of a PTC, yield was
enantiomerically selective product even though the ee value is not
high.
In summary, we successfully demonstrated that GO sheets can
poor, presumably owing to the poor transfer of OH- ion into the
organic phase. For the control experiments using only one solvent
system, the reaction of KOH+GO in only water did not produce 70 be functioned as PTC for the Michael addition reaction. For
5
any product. On the other hand, the reaction of KOH+GO in only
MC (Table 1, Entry 8) was a slightly faster and produced 4%
higher yield than that of KOH in only MC (Table 1, Entry 1), but
much lower than that of KOH+GO in water and MC two phase
reactants, trans-β-nitrostyrene and 2,4-pentanedione and their
derivatives were used, as well as differently sized alkali metal
bases. GO was compared with 18-crown-6 ether, the well-known
conventional PTC. The GO promoted the formation of C-C bonds
system (Table 1, Entry 10). As a result, two-phase solvent system 75 in the Michael adducts, giving short reaction time and high yield
10 gives better yield and shorter reaction time than that of one-phase
system (entry 8, 9 and 10).The ability of GO to bind cations of
various sizes was tested through a series of reactions using KOH,
NaOH, and CsOH (entries 10, 16, and 21); these were mirrored
compared with CE. The used GO PTC could be recovered by
simple filtering and washing and could be reused many times,
while PTC such as CE are difficult to recover. Furthermore, the
GO PTC was effective with differently sized metal cation bases,
by experiments using 18-crown-16 ether (CE) as a comparison 80 while the CE catalyst worked effectively only with a specifically
15 PTC (entries 12, 17 and 22). When aqueous GO solution (0.5 ml)
was loaded with potassium cations using KOH, the reaction in
MC (1 ml) was completed within 10 min, affording the Michael
adduct in up to 83% yield (entry 10). The comparison reaction
sized metal cation. And also, GO have the potential to provide an
environmentally friendly, inexpensive and easy way to produce
commercial products on a large scale. Thus, the GO PTC
provides a novel method for the synthesis of new C-C bonds, and
with CE was more than 3 times longer (30 min) and gave poorer 85 can be used in an open system. This is the first observation of
20 yield (76%, entry 12). This difference was more pronounced
when sodium hydroxide was used; GO yielded 75% and CE
yielded 41% (entries 16 and 17). The yield from GO-catalyzed
reactions was nearly independent of the metal cation used: 83%
for KOH, 75% for NaOH, and 80% for CsOH, respectively. This
25 suggests that the cations were indeed intercalated between GO
layers as in Scheme 1. As expected, the CE catalyst performed
best with the correctly sized K+ cation (76% yield for KOH) and
more poorly with the other two alkali bases (41% for NaOH and
55% for CsOH, respectively). However, even in the best case, CE
30 did not perform well. The GO catalyst provided faster reactions
and higher yields in all experiments. We suggest that the
observed reactivity of GO was due to its hold on the cation,
making the hydroxide base stronger in the organic phase.
Furthermore, because yield was high and the reaction was faster
35 compared with that of CE, we could conclude that individual GO
sheets could carry more cations than that of CE. We also used 2-
methyl tetrahydrafuran (2MTHF) as organic solvent instead of
MC for the greenery reaction and it was acted similarly as MC
(entry 2 and 11) which could be replaced with MC in respect to
40 greenery. The used GO could be easily recovered and reused by a
simple process of filtering and washing with MC (see ESI† Table
S1). The recovered GO could be reused at least nine times almost
without reduction of reaction yields. It was assumed that the
recovered GO retained its catalytic activity due to the presence of
45 undamaged many oxygen functional groups on the GO surface.
To further explore our methodology, we carried out related
reactions using a series of trans--nitroolefin Michael acceptors,
with a variety of substituents on the benzene ring including
electron-donating groups such as methoxy and methyl, and
50 electron-withdrawing halogen groups (F, Cl, and Br). As
summarized in Table 2, the GO system showed fairly good yields
and short reaction times. Next, we evaluated the scope of the
reaction with a variety of 1,3-dicarbonyl compounds as Michael
donors, with substituents including methyl, methoxyl, tert-butyl,
55 and cycloolefins (Table 3). Like the derivative Michael acceptors,
most of the Michael donors showed fairly good yields and
reasonable reaction times. According to our experimental data,
we can suggest that oxygen functional groups in GO, including
carbonyl, carboxylic, lactone, and quinone, and especially epoxy
60 and hydroxyl groups, are responsible for interaction with the base
cations, increasing the catalyst’s ability to react strongly and
quickly with reactants. We also measured enantiomeric selective
property of 3a product with the GO PTC and it showed ~7%
enantiomeric excess. The CE PTC product and only base treated
65 product were racemic in comparison to GO PTC product (see
ESI†). The 2D template structure of GO helps to get an
GO’s ability to provide greatly enhanced phase transfer catalysis.
Now our current work is focused on improving the yield and
enantiomeric selectivity of GO PTC product.
This work was supported by the Creative Research Initiatives
90 research fund (project title: Smart Molecular Memory) of
MEST/NRF.
Notes and references
NCRI, Center for Smart Molecular Memory Department of Chemistry,
Sungkyunkwan University Suwon 440-746 (Republic of Korea). E-mail:
95 hyoyoung@skku.edu; Fax: (+82) 31-299-5934; Tel: (+82) 31-299-4566;
Homepage: http://home.skku.edu/~hyoyoung
† Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/b000000x/
‡ These authors contributed equally.
100
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