Trifluoroacetic acid: An efficient catalyst for paal-knorr pyrrole synthesis and its deprotection

Article abstract of DOI:10.14233/ajchem.2013.15185

In the present work, we demonstrated a simple and an efficient method for the condensation of substituted aryl/heteroaryl amines with acetonylacetone in the presence of trifluoro acetic acid to afford the corresponding 2,5-dimethyl-1-substitued pyrroles using Paal-Knorr synthesis in excellent yields. Trifluoroacetic acid was used under reflux condition for the deprotection of 2,5-dimethyl-1-substitued pyrroles to their corresponding substituted aryl/heteroaryl amines in moderate yields. 2,5-Dimethyl-1-substitued pyrrole were characterized by NMR and LC-MS. The yield of the compounds was found to be excellent.

Full text of DOI:10.14233/ajchem.2013.15185

Asian Journal of Chemistry; Vol. 25, No. 15 (2013), 8685-8689
http://dx.doi.org/10.14233/ajchem.2013.15185
Trifluoroacetic Acid: An Efficient Catalyst for Paal-Knorr Pyrrole Synthesis and Its Deprotection
1,*
2
1
K. NARAYANASWAMY VENUGOPALA , RENUKA T. PRASANNA and BHARTI ODHAV
1Department of Biotechnology and Food Technology, Durban University of Technology, Durban 4001, South Africa
2Department of Polymer Science & Technology, Sri Krishnadevaraya University, Anantapur-515 055, India
*Corresponding author: Fax: +27 86 2423534; Tel: +27 31 3736887; E-mail: katharigattav@dut.ac.za
(Received: 22 January 2013;
Accepted: 26 August 2013)
AJC-14027
In the present work, we demonstrated a simple and an efficient method for the condensation of substituted aryl/heteroaryl amines with
acetonylacetone in the presence of trifluoro acetic acid to afford the corresponding 2,5-dimethyl-1-substitued pyrroles using Paal-Knorr
synthesis in excellent yields. Trifluoroacetic acid was used under reflux condition for the deprotection of 2,5-dimethyl-1-substitued
pyrroles to their corresponding substituted aryl/heteroaryl amines in moderate yields. 2,5-Dimethyl-1-substitued pyrrole were characterized
by NMR and LC-MS. The yield of the compounds was found to be excellent.
Key Words: Deprotection, Amine, Trifluoroacetic acid, Paal-Knorr pyrrole synthesis, Protection, Acetonylacetone.
acidic alternative would extend the scope of the synthesis of
2,5-disubstituted pyrrole. Herewith, we report, trifluoroacetic
acid (TFA) as a simple and efficient catalyst for the synthesis
of 2,5-dimethyl-1-substituted pyrrole. The advantages of this
catalyst are: it is readily available, economical, homogenous
reaction conditions, simple product recovery process, easy to
scale up and results in high yields. In this research we show
that the substituted aryl/heteroaryl amine reacts efficiently with
acetonylacetone in methylene dichloride (MDC) at room tempe-
rature in the presence of trifluoroacetic acid (Scheme-I).
INTRODUCTION
Pyrrole is a five membered heterocyclic aromatic organic
compound and biosynthetic precursor to many natural products1.
It is widely distributed structural unit in many natural and biolo-
gically important molecules such as porphyrins2, bile pigments3,
co-enzymes4 and alkaloids5. Pyrrole derivatives also display
antibacterial6, antiviral7, antiinflammatory8, antioxidant activi-
ties9 and also inhibit cytokine mediated diseases10-13. Several
procedures have been developed for the synthesis of pyrroles.
The well-known methods were Paal-Knorr reaction14,15, Knorr
reaction16,17 and Hantzsch reaction18,19. The Paal-Knorr synthesis
involves the condensation of an excess of primary amine with
1-4-dicarbonyl compound and ammonia to provide a pyrrole.
The reaction can be performed under neutral or weakly acidic
conditions and organic acid such as acetic acid can be added
to accelerate the rate of reaction20.
O
TFA (2 mmol)
+
Ar
NH2
N
Ar
MDC, r.t.
O
The synthesis of 2,5-disubstituted pyrroles can be achieved
by the classical Paal-Knorr method involving the reaction of
1,4-butanediones with amines. Few methods have been reported
for Paal-Knorr synthesis with Sc(OTf)321, microwave irradia-
tion22 and Bi(NO)3·5H2O23, montmorillonite KSF-clay24. Many
of these methods have significant drawbacks e.g., they involve
expensive reagents, hazardous solvents, strongly acidic condi-
tions, require longer reaction time, high temperatures, diffi-
culties in work-up, use of stoichiometric quantities of reagents,
incompatibility with other functional groups, product isolation,
involve products that contribute to environmental pollution
and low product yield. Therefore, the development of a less
Ar
= substituted aryl/heteroaryl group
Scheme-I: Synthesis of 2,5-dimethyl-1-substituted pyrrole by Paal-Knorr
method
It was also observed that 2,5-dimethyl-1-substituted
pyrroles can deprotect to substituted aryl/heteroaryl amines
in excess of trifluoroacetic acid under reflux conditions in mod-
erate yields. Hence, acetonylacetone can be used as amine
protecting agent. Protection and deprotection of organic
functionalities play an essential role in the elegant art of multi-
step organic synthesis. The presence of an amine function in
many biologically active compounds makes its protection a

8686 Venugopala et al.
Asian J. Chem.
frequently needed exercise in medicinal chemistry. Generally
amine can be protected by di-tert-butyl dicarbonate (Boc
anhydride)25, benzyl chloroformate (CBZ chloride)26, allyl
chloride27,28, N-(9H-fluoren-2-ylmethoxycarbonyloxy) succi-
nimide27,29,30.According to reported literature, these protecting
groups are very sensitive to acidic, basic or catalytic hydroge-
nation conditions. Hence acetonylacetone could acts as an
amine-protecting agent in multistep organic synthesis due
to the stability of 2,5-dimethyl-1-substituted pyrroles (Scheme-
II).
MHz, CDCl3): δ = 13, 23, 44, 109, 125, 127, 128, 134, 140.
LC-MS: m/z = 200 (M+).
1-(4-Butyl-3-fluoro-phenyl)-2,5-dimethyl-1H-pyrrole
(1e): 1H NMR (400 MHz, CDCl3): δ = 1.0 (t, 3H), 1.33 (m,
2H), 1.62 (m, 2H), 2.0 (s, 6H), 2.5 (t, 2H), 5.7 (s, 2H), 6.9 (d,
2H), 7.1 (s, 1H). 13C NMR (400 MHz, CDCl3): δ = 11, 14, 22,
24, 34, 107, 110, 115, 123, 130, 131, 139, 162. LC-MS: m/z =
245 (M+).
[5-Chloro-2-(2, 5-dimethyl-1H-pyrrol-1-yl) phenyl](3-
fluorophenyl)methanone (1f): 1H NMR (400 MHz, CDCl3):
δ = 1.9 (s, 6H), 5.5 (s, 2H), 6.96-7.08 (m, 1H), 7.08-7.10 (q,
1H), 7.20-7.25 (m, 1H), 7.30-7.31 (m, 2H), 7.58-7.67 (q, 1H),
7.68 (s, 1H). 13C NMR (400 MHz, CDCl3): δ = 13, 110, 117,
119, 121, 125, 129, 130, 131, 133, 139, 140, 161, 187. LC-
MS: m/z = 328 (M+).
TFA (10 mmol)
N
Ar
Ar
NH2
Reflux, 48 h
Methyl [4-(2,5-dimethyl-1H-pyrrol-1-yl)phenyl]acetate
1
(1g): H NMR (400 MHz, CDCl3): δ = 2.04 (s, 6H), 3.5 (s,
Ar = substituted aryl/heteroaryl group
Scheme-II: Deprotection of 2,5-dimethyl-1-substituted pyrrole by
trifluoroacetic acid to substituted aryl/heteroaryl amines
3H), 3.7 (s, 2H), 5.91 (s, 2H), 7.16-7.18 (q, 2H), 7.27-7.38
(m, 2H). 13C NMR (400 MHz, CDCl3): δ = 12, 40, 52, 112,
121, 131, 132, 133, 140, 174. LC-MS: m/z = 244 (M+).
N-[2-(2,5-Dimethyl-1H-pyrrol-1-yl)-4-methoxy-
EXPERIMENTAL
Chemicals, solvents and catalysts were procured from
SigmaAldrich Company and used without further purification.
TLC was performed on Merck 60 F254 silica gel plates. 1H and
13C NMR spectra were recorded on 400 MHz Brucker spec-
trometer using CDCl3 as a solvent. Chemical shifts (δ) were
indicated in parts per million downfield from TMS. Mass spec-
tra were recorded using LC-MS-Agilent 1100 series with MSD
(ion trap) using 0.1 % aqueous trifluoo acetic acid in acetonitrile
system on C18-BDS column.
1
phenyl]benzamide (1h): H NMR (400 MHz, CDCl3): δ =
2.0 (s, 6H), 3.8 (s, 3H), 5.91 (s, 2H), 7.14-7.18 (q, 2H) 7.39-
7.41 (t, 2H), 7.63-7.69 (m, 2H), 7.67 (s, 1H), 7.7 (s, 1H), 8.02-
8.05 (q, 1H). 13C NMR (400 MHz, CDCl3): δ = 12, 57, 108,
112, 113, 122, 123, 128, 129, 132, 133, 134, 135, 159, 166.
LC-MS: m/z = 321 (M+).
1
3-(2, 5-Dimethyl-1H-pyrrol-1-yl)quinoline (1i)35: H
NMR (400 MHz, CDCl3): δ = 2.1 (s, 6H), 5.9 (s, 2H), 7.65-
7.67 (d, 1H ) 7.79-7.82 (q, 1H), 7.88-7.90 (d, 1H), 8.0 (d,
1H), 8.1-8.2 (d, 1H), 8.80-8.81 (d, 1H). 13C NMR (400 MHz,
CDCl3): δ = 13, 108,126, 127, 128, 129, 130, 133, 141, 144.
LC-MS: m/z = 222 (M+).
General procedure for the synthesis of 2,5-dimethyl-
1-substituted pyrrole (1a-m): Aromatic/heteroaromatic
amine (10 mmol), acetonylacetone (10 mmol) and
trifluoroacetic acid (2 mmol) were taken in MDC (5 mL)
under nitrogen atmosphere and stirred for about 1 h (moni-
tored by thin layer chromatography). After completion of the
reaction, it was quenched by 10 % sodium bicarbonate solu-
tion, extracted twice with MDC, dried and concentrated. The
crude product was purified by column chromatography by
eluting with n-hexane-ethyl acetate mixture (10-30 %). All
the synthesized products were characterized by spectral data.
4-(2, 5-Dimethyl-1H-pyrrol-1-yl)-2-fluorophenol (1a):
1H NMR (400 MHz, CDCl3): δ = 2.1 (s, 6H), 5.4 (s, 1H), 5.89
(s, 2H), 6.90-6.91 (m, 1H), 6.94-6.98 (d, 1H), 7.00-7.19 (m,
1H). 13C NMR (400 MHz, CDCl3): δ = 12, 109, 111, 117, 118,
132, 134, 141, 149. LC-MS: m/z = 212 (M+).
4-(2, 5-Dimethyl-1H-pyrrol-1-yl)-2-methyl-1H-indole
(1j): 1H NMR (400 MHz, CDCl3): δ = 2.10 (s, 6H), 2.47 (s,
3H), 5.90 (s, 2H), 6.20-6.21 (q, 1H), 6.93-6.95 (q, 1H), 7.32-
7.35 (m, 2H), 8.03-8.05 (s, 1H). 13C NMR (400 MHz, CDCl3):
δ = 13, 100, 104, 110, 110.1, 119, 121, 129, 131, 135, 136.
LC-MS: m/z = 225 (M+).
1
7- (2,5-Dimethyl-1H-pyrrol-1-yl)-1H-indole (1k): H
NMR (400 MHz, CDCl3): δ = 2.01 (s, 6H), 5.9 (s, 2H), 7.02-
7.05 (q, 1H), 7.39-7.40 (d, 1H), 7.80-7.81 (q, 1H), 8.20-8.21
(d, 1H), 11.0 (br, 1H). 13C NMR (400 MHz, CDCl3): δ = 13,
105, 109, 112.1, 122, 122.4, 129, 134, 137, 140. LC-MS: m/z
= 212 (M+).
5-(2, 5-Dimethyl-1H-pyrrol-1-yl)-1-methyl-3-phenyl-
1H-pyrazole (1m): 1H NMR (400 MHz, CDCl3): δ = 1.99 (s,
6H), 3.5 (s, 3H), 5.9 (s, 2H), 6.59 (s, 1H), 7.2 (q, 1H), 7.41-
7.45 (m, 2H), 7.82-7.86 (m, 2H). 13C NMR (400 MHz, CDCl3):
δ = 13, 25, 105, 108, 127, 128, 130, 132, 133, 149. LC-MS:
m/z = 252 (M+).
2, 5-Dimethyl-1-[4-(trifluoromethyl)phenyl]-1H-
pyrrole (1b)31: 1H NMR (400 MHz, CDCl3): δ = 2.1 (s, 6H),
5.9 (s, 2H), 7.36-7.38 (d, 2H), 7.7-7.8 (d, 2H). 13C NMR (400
MHz, CDCl3): δ = 12, 111, 120, 121, 126, 128, 132, 144. LC-
MS: m/z = 240 (M+).
1-(4-Bromophenyl)-2, 5-dimethyl-1H-pyrrole (1c)32,33
:
1H NMR (400 MHz, CDCl3): δ = 2.01 (s, 6H), 5.91 (s, 2H),
7.09-7.18 (d, 2H), 7.57-7.60 (d, 2H). 13C NMR (400 MHz,
CDCl3): δ = 12, 111, 120, 123, 132, 133, 140. LC-MS: m/z =
251 (M+).
General procedure for the deprotection of 2,5-dimethyl-
1-substituted pyrrole to substituted aryl/heteroaryl amine
(2a-m): 2,5-Dimethyl-1-substituted pyrrole (1 mmol) was
taken in 50 mL round bottom flask, trifluoroacetic acid (10
mmol) was added and refluxed for 48 h. Completion of reaction
was monitored through thin layer chromatography. After
reaction completion, the reaction was quenched by 10 %
sodium bicarbonate solution and extracted twice with MDC,
2,5-Dimethyl-1-[(1R)-1-phenylethyl]-1H-pyrrole
(1d)21,34: 1H NMR (400 MHz, CDCl3): δ = 1.86-1.88 (d, 3H),
2.09 (s, 6H), 5.40-5.49 (m, 1H), 5.80 (s, 2H), 7.04-7.06 (t,
2H), 7.24-7.27 (m, 1H), 7.28-7.39 (m, 1H). 13C NMR (400

Vol. 25, No. 15 (2013)
Trifluoroacetic Acid: An Efficient Catalyst for Paal-Knorr Pyrrole Synthesis and Its Deprotection 8687
dried and then concentrated. The crude product was purified
by column chromatography by eluting with n-hexane-ethyl
acetate mixture (0-50 %). All the products were characterized
by spectral data.
RESULTS AND DISCUSSION
In this paper, we have reported an easy, efficient and
selective method for Paal-Knorr pyrrole synthesis. Substituted
aryl/heteroaryl amines smoothly reacted with acetonylacetone
in the presence of trifluoroacetic acid at room temperature.
MDC was found to be a good solvent for this reaction. To
demonstrate the protocol, we selected substituted anilines as
model substrates and treated them with acetonylacetone in the
presence of trifluoroacetic acid (2 mmol) to get substituted
pyrrole (Scheme-I) in excellent yields in Table-2 entry 1a-1m.
Interestingly substituted heterocyclic amines such as,
quinolone, indole, indazole and pyrrazole amines yielded the
corresponding pyrroles in excellent yields at 89, 98, 85 and
87 %, respectively (entry 1i, 1j, 1k and 1m). It was also observed
that substituted benzophenone amine and substituted benzo-
phenone amide undergo Paal-Knorr reaction smoothly to yield
TABLE-1
COMPARATIVE STUDY OF DIFFERENT CATALYST USED IN
THE PAAL-KNORR REACTION FOR PYRROLE SYNTHESIS
a
S. No.
Name of catalyst
Yield (%)
1
2
3
4
5
Trifluoroacetic acid
Iodine
92
40
60
80
40
Sulfamic acid
p-Toluene sulfonic acid
Sulfuric acid
aPresent method reaction conditions: p-bromo aniline (10 mmol),
acetonylacetone (10 mmol), catalyst (2 mmol) and MDC (5 mL) were
stirred for 1 h at nitrogen atmosphere to yield 1c.
TABLE-2
PAAL-KNORR REACTION OF ACETONYLACETONE WITH SUBSTITUTED
ARYL/HETEROARYL AMINE IN THE PRESENCE OF TRIFLUOROACETIC ACID
Time
(min)
Yieldb (%)
[ref]
Time
(min)
Yieldb
(%) [ref]
Entrya
2, 5-Dimethyl-1-substituted pyrroles
Entrya
2, 5-Dimethyl-1-substituted pyrroles
N
O
O
N
OH
1ª
40
90
1g
50
90
F
OCH3
O
8131
9232,33
8921,34
1h
1i
1j
45
45
30
89
8935
98
N
H
1b
1c
1d
50
47
45
N
CF3
N
N
N
Br
N
N
N
N
H
N
N
H
N
1e
50
90
1k
50
85
N
F
N
N
N
O
F
30
91
45
87
1f
1m
N
Cl
aNovel compounds, 1a, 1e, 1f, 1g, 1h,1j, 1k, and 1m werecharacterized by NMR (1H and 13C) and LC-MS analysis. Compounds 1b, 1c, 1d and 1i
were known compounds and were characterized by NMR (1H and 13C) and LC-MS analysis and compared with authentic sample.bIsolated yield.

8688 Venugopala et al.
Asian J. Chem.
TABLE-3
DEPROTECTION OF 2,5-DIMETHYL-1-SUBSTITUTED PYRROLE BY TRIFLUOROACETIC ACID UNDER REFLUX CONDITION
Time Yieldb
Yieldb
(%)
Entry
Substituted aryl/heteroaryl aminea
Entry
Substituted aryl/heteroaryl aminea
Time (h)
(h)
(%)
H2N
H N
2
OH
2a
48
49
2g
O
48
50
O
F
OCH3
O
H2N
CF3
2b
48
56
2h
48
49
N
H
NH2
NH2
H2N
Br
48
48
49
50
48
48
49
50
2c
2d
2i
N
NH2
NH2
2j
N
H
N
H2N
2e
2f
48
48
49
60
2k
48
48
50
60
N
H
NH
2
F
NH2
O
F
N
N
2m
H2N
Cl
aAll the compounds were characterized by NMR (1H and 13C) and LC-MS analysis and compared with authentic sample.bIsolated yield.
1f and 1h at 91 and 89 %, respectively. As shown in Table-1
out of five catalyst used for the synthesis of substituted pyrrole
in Paal-Knorr reaction, trifluoro- acetic acid emerged out as a
promising catalyst by yielding 92 % of 1-(p-bromophenyl)-2,
5-dimethyl-1H-pyrrole 1c when compared to other catalyst
such as p-toluenesulfonic acid, sulfamic acid, iodine and
sulfuric acid at 80, 60, 40 and 40 %, respectively.
compounds 1a, 1e, 1f, 1g, 1h, 1j, 1k, 1m and four known
compounds 1b, 1c, 1d and 1i were prepared and structurally
characterized using spectroscopic technique. The yield of the
products 1a-1m was found to be excellent at 81-98 %. The
deprotection of title compounds 1a-1m using trifluoroacetic
acid to yield the respective substituted anilines 2a-2m in the
satisfactory yield at 49-60 %.
Scheme-II also indicates that 2,5-dimethyl pyrroles can
deprotect and yield the corresponding aryl/heteroaryl amines
in the presence of trifluoroacetic acid at reflux condition. In
multistep organic synthesis acetonylacetone can be used as an
amine-protecting agent. Generally, we found that 10 mmol of
trifluoroacetic acid is required for the completion of reaction
over a period of 48 h. We have also found that some of the
substituted pyrroles are highly stable and noticed that traces
of the starting material remain after 2 days reflux condition.
Yields of the products 1a-1m were in the range of 49-60 % as
shown in Table-3. Heterocyclic substituted pyrroles such as
quinolone, indole, indazole and pyrrazole undergo deprotec-
tion smoothly and yielded 2i, 2j, 2k and 2m. We also found
that substituted benzophenone and substituted amide functional
groups survived under this experimental condition and yielded
2f and 2h.
ACKNOWLEDGEMENTS
The authors are grateful to the Durban University ofTechno-
logy for facilities. One of the authors (KNV) thanks NRF South
Africa for DST/NRF Innovation Postdoctoral Fellowship.
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