Use of graphite oxide and graphene oxide as catalysts in the synthesis of dipyrromethane and calix[4]pyrrole

Article abstract of DOI:10.3390/molecules16097256

Graphite oxide and graphene oxides have been used as solid catalysts for the synthesis of 5,5-dialkyldipyrromethanes and calix[4]pyrroles in organic and aqueous solutions at room temperature.

Full text of DOI:10.3390/molecules16097256

Molecules 2011, 16, 7256-7266; doi:10.3390/molecules16097256
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Use of Graphite Oxide and Graphene Oxide as Catalysts in the
Synthesis of Dipyrromethane and Calix[4]pyrrole
Shive Murat Singh Chauhan * and Sweta Mishra
Bioorganic Research Laboratory, Department of Chemistry, University of Delhi, Delhi-110007, India;
E-Mail: sweta.chem19@gmail.com
* Author to whom correspondence should be addressed; E-Mail: smschauhan@chemistry.du.ac.in;
Tel.: +91-11-27666845; Fax: +91-11-27666845.
Received: 6 July 2011; in revised form: 17 August 2011 / Accepted: 19 August 2011 /
Published: 25 August 2011
Abstract: Graphite oxide and graphene oxides have been used as solid catalysts for
the synthesis of 5,5-dialkyldipyrromethanes and calix[4]pyrroles in organic and aqueous
solutions at room temperature.
Keywords: graphite oxide; graphene oxide; 5,5-dimethyldipyrromethane; calix[4]pyrrole;
solid acid
1. Introduction
Graphene is a two dimensional sheet of sp2 hybridized carbon with remarkable thermal, mechanical
and electronic properties [1-4]. Top down is an important method for the preparation of graphene by
oxidation of graphite [5], as compared to other methods [6,7]. Graphite oxides containing OH, epoxy
and carboxyl groups have been prepared by oxidation of graphite via minor modification of known
methods [8,9] and their structures have been characterized by different spectroscopic methods [10-12].
The presence of polar groups is responsible for the acidic solutions formed when graphite and
graphene oxides are suspended in aqueous media [13,14]. In organic synthesis polycarbon acids
[15,16] and polyacids on carbon nanostructures [17-20] are considered more robust in aqueous as well
as organic solvents than other solid acids [20-23] such as ion exchange resins [24], heteropolyacids
[25-27] and layer transition metal oxides [28,29].

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Molecules 2011, 16
One of the most important methods developed for the mass production of graphene is exfoliation of
graphite oxide in aqueous solution to single layers of graphene oxide. This exfoliation can be
performed in aqueous solution alone or in presence of surfactants [30], polymers [31,32], ionic liquids
[33] and polar solvents [34,35]. Aqueous suspensions of graphene oxide have been used in the
oxidation of alcohols and cis-stilbene and the hydration of various alkynes to the corresponding
aldehydes, acids and ketones under mild conditions [36,37]. The electrical conductivity of the
graphene oxide differs from that of pristine graphene, hence various attempts have been made to
deoxygenate graphene oxide to the maximum extent to regain the aromaticity and electrical
conductivity. The hydroxyl and the epoxy groups and some carboxylic groups can be removed by
reduction of graphite oxide by hydrazine, ascorbic acid and other reducing agents [38-40].
Porphyrinogens and more stable calix[4]pyrroles are important tetrapyrrolic macrocycles used in
biosynthesis of porphyrinoids [41], supramolecular chemistry [42], anion receptor [43,44] and material
chemistry [45-47]. They are synthesized by condensation of pyrroles with dialkylketones in the
presence of aqueous acids [48,49], Lewis acids [50,51] and solid acids including zeolites [52],
molecular sieves [53,54] and Amberlyst-15 [55] under different reaction conditions. Although various
solid acids have been used in the synthesis of different calix[4]pyrroles, to the best of our knowledge
graphite oxide and graphene oxides have not been used for this purpose. Hence, we report the
synthesis of selected calix[4]pyrroles in aqueous suspensions of graphite oxide and graphene oxides
under different reaction conditions (Scheme 1).
Scheme 1. Synthesis of dipyrromethane and calix[4]pyrrole.

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Molecules 2011, 16
2. Results and Discussion
The reaction of acetone (2a) with pyrrole (1) in a suspension of graphite oxide (GO) in
dichloromethane at room temperature gave dipyrromethane (3a) as the major product. The structure of
3a was confirmed by spectroscopic data, mixed melting point [56] and comparison of HPLC retention
times with an authentic sample (Table 1). The reaction of 1 with cyclohexanone (2b) gave the
corresponding dipyrromethane 3b in 70% yield. An analogous condensation of pyrrole and 2b in
dichloromethane in the presence of the solid acid zeolite HY gave dipyrromethane 3b in 63% yield,
along with other products, whereas the same reaction in the presence of HZSM in dichloromethane
gave dipyrromethane 3b and calix[4]pyrrole 4b in 53 and 11% yield, respectively [52]. The reaction of
1 with 2a in the presence of Al-MCM-41 zeolite gave 3a and 4a in 12 and 70% yield, respectively
[52]. The reaction of the above in the presence of AmberlystTM-15 gave 4a and N-confused
calix[4]pyrrole in 83 and 14% yield, respectively [55]. The high yield of 4a in acetone may be
explained by initial formation of 3a which on subsequent reaction with acetone leads to
calix[4]pyrrole. This was also confirmed by reaction of 3a with 2b with the formation of 5 in 17%
yield. (Table 1) [52]. The reaction of 1 and 2a was performed in other polar solvents and the results are
reported in Table 1. A moderate yield of calix[4]pyrrole 4 was obtained when the reaction was
performed in methanol or acetonitrile (Table 1). The graphite oxide is stable in organic solvents and it
may be used several times without much loss of catalytic activity (Table 1, entry 14).
Table 1. Graphite oxide catalyzed condensation of pyrrole and acetone in different solvents a.
Yield of product (wt%) b,c
Conversion of
pyrrole (%)
Entry
Solvent
Catalyst
2
3
4
Other
1
2
CH2Cl2
GO
GO
2a
2a
2b
2b
2b
2b
2a
99
99
99
-
-
d
CH2Cl2
99
-
-
3
CH2Cl2
GO
90
73
2
15
d
4
CH2Cl2
GO
86
70
-
9
16.2
5.9
12.4
14 e
-
d
5
CH2Cl2
Zeolite HY[52]
HZSM-5(30)[52]
Al-MCM-41
AmberlystTM-15[55] 2a
87.9
69.6
95
62.7
53.0
12.3
-
10.7
70.3
83
d
6
CH2Cl2
d
7
CH2Cl2
8
CHCl3
CHCl3
99
9
GO
GO
GO
GO
GO
GO
2a
2a
2a
2a
2a
2a
99
97
95
90
80
34
27
2
10
11
12
13
CCl4
99
3
-
Methanol
CH3CN
100
98
10
-
5e
13
Ethylene glycol
Acetone g
98
7
57
10e (6) f
14
99
62 (50) f
a
Reaction conditions: pyrrole (14.4 mmol), acetone (14.4 mmol); graphite oxide (50 mg); solvent
b
c
(20 mL); reaction time: 20h; yields were calculated by HPLC; other products may be
d
tripyrromethane, open chain tetramer or N-confused calix[4]pyrrole; reaction were performed in
e
f
g
10h in order to compare the results; N-confused calix[4]pyrrole; Isolated yield; Catalyst was
reused over three runs.

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Molecules 2011, 16
The formation of calix[4]pyrrole 4a in excess acetone may be explained by initial formation of 3a
which on subsequent reaction with acetone leads to 4a. The reaction of 3a with cyclohexanone lead to
cailx[4]pyrrole 5 (Scheme 1).
The reaction of 1 and 2a in the presence of graphite oxide suspended in aqueous solution gave 3a as
major product in 92% yield (Table 2). The reaction of pyrrole and ketones regardless of the ratio of
starting compounds, forms the dialkyldipyrromethanes in the presence of weak acid in aqueous
solutions [48]. The increase in the ratio of acetone decreases the yield of 3a and a minor amount of 4a
was observed (Table 2).
Table 2. Reaction of pyrrole and acetone in presence of graphite oxide in aqueous solution a.
Yield of product (wt%) b,c
Conversion of pyrrole
Entry Pyrrole:Acetone
(%)
93
3
4
-
Other
1
2
3
4
5
6
1:1
2:1
92
97
98
97
96
84
1
-
97
-
5:1
99
-
1
1
1
1:1.5
1:5
98
-
100
100
3
8
1:10
2 d
a
b
Reaction conditions: water (20 mL); Graphite oxide (50 mg); reaction time: 20h; yields were
c
calculated by HPLC peak area; other products may be tripyrromethane, open chain tetramer or
N-confused calix[4]pyrrole; d N-confused calix[4]pyrrole.
The reaction of 1 and 2a in the presence of graphite oxide and SDS gave dipyrromethane (48%),
calix[4]pyrrole (35%) and N-confused calix[4]pyrrole in 10% yield at room temperature in 1.5 to 3 h
(Table 3).
Table 3. Reaction of pyrrole and acetone in the presence of graphite oxide and surfactant
and salts in aqueous solution a.
Yield of product (wt%) b,c
Conversion of
Entry
Surfactant and salts
Time
pyrrole (%)
No reaction
3
-
4
-
Other
1
2
3
4
5
SDS alone
SDS+GO
24 h
1.5–3 h
24 h
-
10 d
3
93
96
99
89
48
93
96
84
35
-
CTAB + GO
PEG + GO
24 h
-
3
Poly(sodium 4-styrenesulfonate)
+ GO
24 h
5
-
6
7
Sodium methanesulfonate + GO
Methanesulfonic acid
(2 × 10−2 M)
24 h
24 h
99
99
99
79
-
-
-
2
8
Sodium trifluoromethane sulfonate
+ GO
24 h
99
99
-
-
a Reaction conditions: water (20 mL); graphite oxide (50 mg); surfactant and salts (50 mg); b yield
were calculated by HPLC peak area; c other products may be tripyrromethane, open chain tetramer
or N-confused calix[4]pyrrole; d N-confused calix[4]pyrrole.

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Molecules 2011, 16
The addition of SDS to graphite oxide in aqueous solution forms the corresponding acid which
catalyzes the formation of dipyrromethane in low and calix[4]pyrrole in high yields. The graphite
oxides have been dispersed in SDS and poly(sodium 4-styrenesulphonate) in aqueous solution [32].
The reactions of 1 with 2a in the presence other surfactants and salts are given in Table 3. The reaction
of 1 and 2a in the presence of reduced graphene oxide [57,58] in aqueous solution gave 3a as major
product and 4a as minor product (Table 4).
Table 4. Reaction of pyrrole and acetone in presence of reduced graphene oxide a.
Yield of product (wt%) b,c
Conversion of
pyrrole (%)
Entry
Solvent
Time
(3)
98
94
52
82
69
(4)
1
Other
-
-
10 d
10
22
1
2
3
4
Dichloromethane
Methanol
24 h
24 h
24 h
24 h
24 h
99
99
100c
92
5
Acetone
25
Water
-
5
a
Water + SDS
92
1
b
Reaction conditions: solvent (20 mL); catalyst: reduced graphene oxide (50 mg); yields were
c
calculated by HPLC peak area; other products may be tripyrromethane, open chain tetramer or
N-confused calix[4]pyrrole; d N-confused calix[4]pyrrole.
The reduced graphene oxide agglomerizes in aqueous solution, but in the presence of organic
solvents [59] and ionic liquids [60] it may be dispersed in aqueous solution. Suitable chemical
modifications of reduced graphene oxides have been used to solubilize the graphene oxides in aqueous
[60] and organic solvents [61]. The above results indicate that graphite and graphene oxide may be
used as solid acids in various organic transformations.
3. Experimental
3.1. General
1
The infrared spectra (IR) were recorded on a Perkin-Elmer FT-1710 spectrophotometer. H-NMR
spectra were recorded in CDCl3 on a Bruker Avance 400 MHz spectrophotometer with TMS as
internal standard. UV-Vis spectra were recorded on Perkin-Elmer Lambda 35 spectrophotometer.
Raman spectra were recorded on inVia Renishaw Raman spectrophotometer using a green (514 nm)
laser. Powder XRD were recorded on a Bruker Discover 8 X-ray diffractometer. HPLC analysis was
performed on a Waters 2998 using a Waters PAH C18 HPLC column (4.6 × 250 mm) and methanol as
the eluent. Starting materials such as pyrrole (1) and acetone (2) were obtained from Acros USA and
distilled immediately prior to use. The experimental operations were performed under ambient
conditions. Neutral alumina was used for all the chromatographic purifications. Graphite powder was
obtained from Alfa Aesar, USA.
3.2. Preparation of Authentic Samples
5,5-Dimethyldipyrromethane (3a) was prepared by following the literature procedure starting from
acetone and pyrrole in ionic liquid [56] and aqueous solution [48]; m.p. 56 °C (lit [56] 55–57 °C);

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Molecules 2011, 16
1H- NMR: 1.61 (s, 6H, –CH3), 6.08 (s, 2H, β-pyrrole), 6.1 (d, 2H, β-pyrrole), 6.5 (d, 2H, α-pyrrole),
7.64 (s, br, 2H, NH-pyrrole); HPLC retention time = 3.1 min.
Octamethylcalix[4]pyrrole (4a) was prepared by following a literature procedure [55] starting from
1
acetone and pyrrole; m.p. 294 °C (lit [55] 296 °C ); H-NMR: 7.01 (4H, br s, NH), 5.89 (8H, d,
J = 2.5 Hz, β-pyrrole), 1.50 (24 H, s); HPLC retention time = 3.8 min.
3.3. Preparation of Catalysts
3.3.1. Graphite Oxide
KMnO4 (9 g) was added in portions to a cooled (0 °C) solution of conc. H2SO4 (69 mL) containing
graphite (3 g) and NaNO3 (1.5 g). The mixture was stirred at room temperature for 5 days. Distilled
water (138 mL) was added slowly to the reaction mixture while the temperature was kept well below
98 °C for 3 h. The resultant bright-yellow suspension was diluted and a solution of H2O2 (6 mL, 30%)
was added dropwise. The reaction mixture was centrifuged and washed to remove the remaining salts.
The wet graphite oxide was dewatered by vacuum drying (50 °C). UV-Vis (λmax) (H2O) = 230 nm,
(DMF) = 269 nm. FTIR (cm−1) = 3447 (OH), 1740 (C=O), 1636 (OH bending), 1091 (C-O). Raman
spectra: 1350 (D band), 1584 (G band), D/G ratio: 0.85. XRD data: 10.5°.
3.3.2. Graphene Oxide
Aqueous colloids of single layer graphene oxide nanosheets were produced by exfoliation of
graphite oxide dispersed in deionized water with ultrasonication [32].
3.3.3. Preparation of Reduced Graphene Oxide
Graphite oxide (75 mg) was dispersed in water (75 mL) with sonication. Sodium borohydride
(600 mg) was added to the GO dispersion after the pH being adjusted to 9–10 with 5 wt% sodium
carbonate solution. The mixture was then kept at 80 °C for 1 h under constant stirring. During
reduction, the dispersion turned from dark brown to black accompanied by outgassing. UV-Vis (λmax
)
(H2O) = 270 nm. FTIR (cm−1) = 3440 (O-H), 1740 (C=O), 1091 (C-O). Raman spectra: 1350 (D
band), 1584 (G band), D/G ratio: >1.
3.4. The Reaction of Pyrrole and Acetone in Organic Solvents
Equimolar amounts of pyrrole (14.4 mmol) and acetone (14.4 mmol) were taken up in
dichloromethane (20 mL). Graphite oxide (10% w/w) was added in portions to the reaction mixture,
which was stirred at ambient temperature for the appropriate time (as indicated in Table 1). The
reaction progress was monitored by thin layer chromatography (TLC) with petroleum ether-chloroform.
After the completion of reaction, the catalyst was removed by filtration and washed thoroughly with
CH2Cl2 to dissolve all the contents. The filtrate was concentrated to give the crude product, which was
subjected to column chromatography over neutral alumina eluting with petroleum ether-chloroform to
afford pure calix[4]pyrrole and further elution of column gave dipyrromethane. The reaction mixture
was analysed by HPLC. The HPLC yields are given in Table 1.

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Molecules 2011, 16
3.5. The Reaction of Pyrrole and Acetone in Aqueous Solution
Equimolar amounts of pyrrole (14.4 mmol) and acetone (14.4 mmol) were taken up in water
(20 mL). Graphite oxide (10% w/w) was added in portions to the above reaction mixture that was
stirred at ambient temperature for the appropriate time (Table 2). The reaction progress was monitored
by thin layer chromatography (TLC) with petroleum ether-chloroform. After completion of reaction,
the catalyst was removed by extraction with CH2Cl2. The organic layer was separated dried over
sodium sulfate, filtered and the filtrate was concentrated to give the crude product, which was
subjected to column chromatography over neutral alumina eluting with petroleum ether-chloroform to
afford pure calix[4]pyrrole and further elution of column gave dipyrromethane. The reaction mixture
was analysed by HPLC. The HPLC yields are given in Table 2.
3.6. The Reaction of Pyrrole and Acetone in Aqueous Solution in Presence of Suphonate Salts
Pyrrole (14.4 mmol), acetone (14.4 mmol) and surfactant/salt (0.034 mmol) were taken up in water
(20 mL). Graphite oxide (10% w/w) was added in portions to the reaction mixture, which was stirred
at ambient temperature for the appropriate time (Table 3). The reaction progress was monitored by thin
layer chromatography (TLC) with petroleum ether-chloroform. After the completion of reaction, the
catalyst was removed by extraction with CH2Cl2. The organic layer was separated dried over sodium
sulfate, filtered and the filtrate was concentrated to give the crude product, which was subjected to
column chromatography over neutral alumina eluting with petroleum ether-chloroform to afford pure
calix[4]pyrrole and further elution of column gave dipyrromethane.The reaction mixture wasanalysed
by HPLC. The HPLC yields are given in Table 3.
3.7. The Reaction of Pyrrole and Acetone Catalyzed by Reduced Graphene Oxide under Different
Conditions
Equimolar amounts of pyrrole (14.4 mmol) and acetone (14.4 mmol) were taken up in different
solvents (20 mL). Graphene oxide (10% w/w) was added in portions to the above reaction mixture that
was stirred at ambient temperature for the appropriate time (Table 4). The reaction progress was
monitored by thin layer chromatography (TLC) with petroleum ether-chloroform. After completion of
reaction, the catalyst was removed by filtration or extraction. Filtrate was concentrated to give the
crude product, which was subjected to column chromatography over neutral alumina eluting with
petroleum ether-chloroform to afford pure calix[4]pyrrole and further elution of column gave
dipyrromethane.The reaction mixture was analysed by HPLC. The HPLC yields are given in Table 4.
4. Conclusions
Graphite oxide and reduced graphene oxides have been used as solid acid catalysts for the room
temperature preparation of dipyrromethanes and calix[4]pyrroles in organic and aqueous solutions.

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Molecules 2011, 16
Acknowledgements
S.M.S. Chauhan acknowledges the Department of Science and Technology and University of Delhi
for financial assistance. Sweta Mishra acknowledges the UGC for Jounior Research Fellowship.
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Sample Availability: Limited samples of compounds 3a and 4a are available from the authors.
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