Catalytic applications of nano β-PbO in Paal-Knorr reaction

Article abstract of DOI:10.1016/j.cclet.2010.12.053

Several 2,5-dimethyl-N-substituted pyrroles were prepared by the condensation of different substituted anilines with 2,5-hexanedione using nano lead oxide as an efficient and recyclable catalyst. All the synthesized compounds are confirmed through IR, ¹H NMR, ¹³C NMR and mass spectral data. Nano lead oxide β-PbO (P85) was prepared by dissolving lead acetate dihydrate in 1-propanol at a pH 9.0 under stirring at 85 °C. The structural study and surface morphology of the lead oxide (PbO) were characterized using X-ray diffraction (XRD), Scanning electron microscopy (SEM) and the functional groups of the PbO sample were investigated using infrared spectrophotometer.

Full text of DOI:10.1016/j.cclet.2010.12.053

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Chinese Chemical Letters 22 (2011) 891–894
www.elsevier.com/locate/cclet
Catalytic applications of nano b-PbO in Paal–Knorr reaction
Sk. Khadeer Pasha a, V.S.V. Satyanarayana b, A. Sivakumar b,
*
,
K. Chidambaram a, L. John Kennedy a
a Physics Division, School of Advanced Sciences, VIT University, Vellore 632014, India
b Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632 014, India
Received 27 October 2010
Available online 19 May 2011
Abstract
Several 2,5-dimethyl-N-substituted pyrroles were prepared by the condensation of different substituted anilines with 2,5-
hexanedione using nano lead oxide as an efficient and recyclable catalyst. All the synthesized compounds are confirmed through IR,
1H NMR, 13C NMR and mass spectral data. Nano lead oxide b-PbO (P85) was prepared by dissolving lead acetate dihydrate in 1-
propanol at a pH 9.0 under stirring at 85 8C. The structural study and surface morphology of the lead oxide (PbO) were characterized
using X-ray diffraction (XRD), Scanning electron microscopy (SEM) and the functional groups of the PbO sample were
investigated using infrared spectrophotometer.
# 2011 A. Sivakumar. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: 2,5-Dimethyl-N-substituted pyrroles; Nano b-PbO; Spectral data; Catalyst characterization
Among the various classes of heterocyclic compounds, pyrroles have broad synthetic utility in medicine, materials
science and organic synthesis [1]. Consequently, the enormous number of procedures has been developed for the
construction of pyrroles. The classic methods for pyrrole synthesis are as follows: (i) the Hantzsch reaction [2], (ii) the
Knorr reaction [3], (iii) the Paal–Knorr reaction [4] and (iv) some other methods were also reported [5]. The Paal–Knorr
reaction, oneofthemost commonapproachesinwhichg-diketoneisconvertedtopyrrolesfromthereactionwithprimary
amines (or ammonia) in the presence of various promoting agents such as HCl, p-TSA, HOAc, H2SO4 [6], I2, Al2O3,
montmorillonite [7], different metal complexes [8] as catalysts. Additionally, the above cyclocondensation process could
proceed in ionic liquid [9], microwave irradiation [10], or ultrasound irradiation using ZrCl4 as catalyst [11]. However,
there are some limitations with these methodologies such as prolonged reaction times, elevated temperatures, moisture
sensitive/hazardous catalysts, use of costly ionic liquids and use of an additional microwave oven or ultrasonic processor.
Thus, the development of new catalysts with great efficiency, environmental friendly, convenient procedure, and delivery
of better yield methods for the synthesis of pyrroles is of great interest.
Generally red colored lead oxide is called as litharge. This litharge is used primarily in the manufacture of various
ceramic products. Moreover, lead oxide has important applications in the production of many lead chemicals, dry
colors, soaps (i.e., lead stearate), and driers for paint. Lead oxide (PbO) has been used as a catalyst, with excellent
results, in one pot synthesis of quinoxalines [12]. Along this line, we found that lead oxide is an effective catalyst for
the one pot synthesis of substituted pyrroles by using Paal–Knorr reaction. Herein, we report a simple and efficient
* Corresponding author.
E-mail address: profaskumar09@gmail.com (A. Sivakumar).
1001-8417/$ – see front matter # 2011 A. Sivakumar. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.12.053

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S.K. Pasha et al. / Chinese Chemical Letters 22 (2011) 891–894
method for the preparation of substituted pyrroles via condensation of g-diketone and different substituted primary
amines using nano level lead oxide as a catalyst and also the synthesis, characterization of PbO was discussed.
1. Results and discussion
The N-substituted 2,5-dimethylpyrroles (3a–i) were prepared according to the synthetic pathway described in
Scheme 1. g-Diketone compound 2,5-hexanedione (1) was treated with different substituted primary amines (2a–i) in
the presence of catalytic amount of nano level lead oxide under solvent free conditions to produce the corresponding
N-subsituted-2,5-dimethylpyrroles. The nano level lead oxide catalyzed synthesis of N-sustituted-2,5-dimethyl-
pyrroles (3a–i) was monitored by thin layer chromatography. The crude products were purified by recrystallization
with 10% aqueous methanol led to the substituted pyrroles (3a–i) in moderate to good yields. The structures of known
1
compounds were confirmed mainly based on their melting points, IR, H NMR spectral data and structures of
1
unknown compounds were confirmed by IR, H NMR, 13C NMR and mass spectral data.
In order to evaluate the catalytic efficiency of nano b-PbO and to determine the most appropriate reaction conditions;
initially a model study was carried out on the synthesis of 2,5-dimethyl-N-phenylpyrrole (3a) by the condensation of 2,5-
hexandione and aniline in different sets of reaction conditions (Table 1). Among the tested solvents such as acetonitrile,
ethanol, methanol and solvent free conditions using various catalyst ratios, the condensation of acetonyl acetone and
aniline was best catalyzed by 20 mol% of nano b-PbO in solvent free conditions under stirring at room temperature; it is
also found to be novel, one-pot combination and exclusively gave 2,5-dimethyl-N-phenylpyrrole (3a) in 92% in 60 min
(Table 1, entry 3). The recyclability of nano PbO was also investigated, and it could be recycled three times without
distinct loss of activity (Table 1, entry 3). Particularly, we became aware of another useful role of PbO catalyst, that is, the
coordinative activation of carbonyl functions of the 2,5-hexandione, which is trapped by different substituted amines to
give theproduct ingood yields. To extendthescope ofthis reaction, weapplied the optimal protocolto avariety ofamines
and the results are summarized in Table 2. In all cases, nano b-PbO catalyzed Paal–Knorr condensation reaction
proceeded smoothly and gave the corresponding products in good yield. Nano b-PbO was prepared by dissolving lead
acetate dihydrate in 1-propanol at a pH 9.0, the solution was continuously stirred for 1 h at room temperature and then
stirred at 85 8C for 12 h. The structural study, surface morphology and functionalgroupsof theb-PbO were characterized
by using X-ray diffraction pattern (Fig. 1), SEM (Fig. 2) and infrared spectrophotometer (Fig. 3), respectively.
2. Experimental
2.1. Typical procedure for the synthesis of nano b-PbO
Four grams of lead acetate dihydrate was dissolved in 150 mL of 1-propanol and pH is maintained at 9.0. The
solution was continuously stirred for 1 h at room temperature followed by heat treatment and the precursor solutions
[()TD$FIG]
O
nano β-PbO
H3C
CH3
RNH2
+
N
Solvent-free, rt
R
O
1
2a-i
3a-i
Scheme 1. Synthesis of 2,5-dimethyl-N-substitutedpyrroles catalyzed by nano b-PbO.
Table 1
Paal–Knorr reaction of acetonylacetone (1) and aniline (2a) under different reaction conditions.
Entry
Solvent
Catalyst loading (mol%)
Temperature (8C)
Time (min)
Yielda (%)
1
2
3
4
6
7
8
Solvent free
Solvent free
Solvent free
Acetonitrile
Ethanol
0
RT
RT
RT
RT
RT
RT
RT
300
120
60
20
10
78
92, 90, 87b
20
20
120
120
60
63
81
90
89
20
Methanol
20
Methanol
Excess
60
RT = room temperature.
a
Isolated yields.
b
Catalyst was reused for three times.

S.K. Pasha et al. / Chinese Chemical Letters 22 (2011) 891–894
893
Table 2
Synthesis of pyrroles using 20 mol% of nano b-PbO as a catalyst.
Entry
R
Reaction time (min)
Yielda (%)
Melting point (8C)
3a
3b
3c
3d
3e
3f
C6H5
60
90
92
91
88
85
96
87
90
91
78
82
48–50
46–48
p-ClC6H4
p-BrC6H4
p-IC6H4
90
56–58
90
62–64
4-OCH3C6H4
4-NO2C6H4
4-CH3C6H4
PhCH2
60
60–62
180
90
144–146
44–46
3g
3h
3i
60
40–42
2-Naphthyl
NH2(CH2)2NH2
240
180
120–122
134–136
3j
a
Yields refer to pure products and were characterized by comparison of their mp, IR and 1H NMR spectra with those of authentic samples [8].
[()TD$FIG]
Fig. 1. X-ray diffraction pattern of P85.
[()TD$FIG]
Fig. 2. Room temperature SEM pattern of P85.
[()TD$FIG]
Fig. 3. FT-IR spectra of P85.

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S.K. Pasha et al. / Chinese Chemical Letters 22 (2011) 891–894
transferred to an oil bath at 85 8C for 12 h. The product was then washed several times with doubly distilled water,
filtered and dried at 60 8C overnight which is labeled as P85.
2.2. General procedure for the synthesis of pyrroles (3a–i)
To a solution of the aromatic amine 1 (1 mmol) and acetonylacetone 2 (1.2 mmol) at room temperature, nano level
lead oxide (20 mol%) was added. The mixture was allowed to stir at room temperature for a period of time specified in
Table 2. After completion of the reaction, as indicated by TLC, added few drops of methanol to the reaction mixture,
poured into a 5% HCl solution to remove the excess amine, filtered and washed with a small amount of cold water.
Dissolved the crude product in 10% aqueous methanol for recrystallization and filtered. The separated solid (PbO) was
washed with ethanol and water. Cooled the filtrate in ice, collected the product and then dried at room temperature.
The spectral data of some representative compounds are given below.
2,5-Dimethyl-1-phenyl-1H-pyrrole (3a): IR (KBr, nmax, cmꢀ1): 3053, 2922, 1596, 1491, 1400, 1316, 1218, 1037,
999, 747. 1H NMR (500 MHz, CDCl3): d 7.49 (m, 2H, m,m0-ArH), 7.43 (m, 1H, p-ArH), 7.25 (m, 2H, o,o0-ArH)
5.94 (s, 2H, pyrrole), 2.07 (s, 6H, 2CH3). 13C NMR (125.75 MHz, CDCl3): d 139.02, 129.05, 128.82, 128.27,
127.63, 105.63, 13.02. ESI-MS: m/z calcd for C12H13N: 171.10; found: 172.20 [M+H]+.
1-(4-Bromophenyl)-2,5-dimethyl-1H-pyrrole (3c): IR (KBr, nmax, cmꢀ1): 3078, 2917, 1519, 1484, 1435, 1402,
1
1320, 1065, 998, 840, 760. H NMR (300 MHz, CDCl3): d 7.57 (d, 2H, J = 8.4 Hz, m,m0-ArH), 7.08 (d, 2H,
J = 8.7 Hz, o,o0-ArH), 5.79 (s, 2H, pyrrole), 2.01 (s, 6H, 2CH3). 13C NMR (125.75 MHz, CDCl3): d 138.07, 132.32,
129.88, 128.67, 121.53, 106.13, 12.97. ESI-MS: m/z calcd for C12H12BrN: 249.02; found: 250.10 [M+H]+.
1-(4-Iodophenyl)-2,5-dimethyl-1H-pyrrole (3d): IR (KBr, nmax, cmꢀ1): 3073, 2916, 1583, 1484, 1434, 1403, 1321,
1053, 997, 838, 760. 1H NMR (300 MHz, CDCl3): d 7.77 (d, 2H, J = 8.4 Hz, m,m0-ArH), 6.95 (d, 2H, J = 8.4 Hz,
o,o0-ArH), 5.79 (s, 2H, pyrrole), 2.01 (s, 6H, 2CH3). 13C NMR (125.75 MHz, CDCl3): d 138.74, 138.34, 130.17,
128.66, 106.18, 92.98, 13.06. ESI-MS: m/z calcd for C12H12IN: 297.00; found: 297.10 [M]+.
Acknowledgment
Authors gratefully acknowledge the Management of VIT University, Vellore for encouraging and providing
necessary facilities for carrying out this work.
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