β-Cyclodextrin as a recyclable catalyst: Aqueous phase one-pot four-component synthesis of polyfunctionalized pyrroles

Article abstract of DOI:10.1039/c6ra08335h

An elegant, mild and straightforward methodology was explored for the first time towards the synthesis of polyfunctionalized pyrroles using β-cyclodextrin as a reusable supramolecular catalyst in aqueous medium. The present method provides various diversely substituted pyrroles from the commercially available synthons in good to excellent yields. The advantages of this novel protocol are use of environmentally benign reaction medium and recyclability of β-cyclodextrin over the existing methods.

Full text of DOI:10.1039/c6ra08335h

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b-Cyclodextrin as a recyclable catalyst: aqueous
phase one-pot four-component synthesis of
polyfunctionalized pyrroles†
Cite this: RSC Adv., 2016, 6, 43339
a
b
b
c
Received 1st April 2016
Accepted 26th April 2016
*
*
Karnakar Konkala, Rakhi Chowrasia, Padma S. Manjari, Nelson L. C. Domingues
c
*
and Ramesh Katla
DOI: 10.1039/c6ra08335h
www.rsc.org/advances
An elegant, mild and straightforward methodology was explored for workers11 employed modied Paal–Knorr method involving
the first time towards the synthesis of polyfunctionalized pyrroles molecular iodine and montmorillonite KSF-clay as catalysts to
using b-cyclodextrin as a reusable supramolecular catalyst in aqueous afford substituted pyrroles. Shi and co-workers12 used low-
medium. The present method provides various diversely substituted valent titanium reagent (TiCl4) to access the highly regiose-
pyrroles from the commercially available synthons in good to excel- lective polysubstituted pyrroles using three-component reac-
lent yields. The advantages of this novel protocol are use of environ- tion. Yu et al.,13 demonstrated a facile method for the synthesis
mentally benign reaction medium and recyclability of b-cyclodextrin of polysubstituted pyrroles via the coupling of phenyliodonium
over the existing methods.
ylides and enamine esters using BF3$Et2O. Liang and co-
workers14 described a straightforward method by the oxidative
cyclization of b-enamino ketones or esters and alkynoates using
CuI in the presence of oxygen. Jia and co-workers15 have used
AgOAc mediated one-pot condensation strategy to obtain 3,4-
disubstituted pyrroles from commercially available aldehydes
and amines under mild conditions. Narasaka and co-workers16
synthesized regioisomeric polysubstituted N–H pyrroles from
vinylazides and 1,3-dicarbonylcompounds using catalytic
amount of Cu(NTf2)2. Intermolecular cyclization of alkylidene-
cyclopropyl ketones with amines to yield 2,3,4-trisubstituted
pyrroles using Mg2SO4 was reported by Ma and co-workers.17
Scheidt et al.,18 described a multi-component strategy for highly
substituted pyrroles utilizing acylsilanes (sila-Stetter) for in situ
generation of 1,4-dicarbonyl functionality and subsequent Paal–
Knorr strategy involving various amines. Smith and co-
workers19 have reported one-pot synthesis of 3,4-diaryl-(1H)-
pyrroles from arylalkenes and tosylmethyl isocyanide. Zhan and
co-workers20 have used zinc(II) chloride as a multifunctional
catalyst in one-pot process for the synthesis of substituted
Introduction
The pyrrole structural skeleton has gained prominence in
heterocyclic chemistry,1 due to the numerous applications
associated with it as well as its presence as an important
structural motif in a wide variety of natural, biological and
pharmacologically potent molecules (Fig. 1)2 such as porphy-
rins, bile pigments, coenzymes and alkaloids. Of these afore-
mentioned class of compounds, polysubstituted pyrroles
exhibit signicant role as synthetic intermediates,3 pharmaco-
phores,4 and functional materials.5 Traditional methods for the
synthesis of polysubstituted pyrroles include Knorr reaction,6
Hantzsch reaction7 and Paal–Knorr condensation reaction.8
Even though many alternate methodologies have been
described in the literature, the synthesis of functionalized
pyrroles is challenging tasks to the researchers worldwide
because of lack of selectivity and the problems associated with
the polymerization.9 Rao and co-workers have developed a facile
one-pot synthesis of polyarylpyrroles from 2-butene and but-2-
yne-1,4-diones under microwave irradiation.10 Bimal and co-
aMCP-Division, Indian Institute of Chemical Technology (IICT), Hyderabad-500607,
India
bDepartment of Chemistry, Osmania University, Tarnaka, Hyderabad, India-500004.
E-mail: psmanjari76@gmail.com
cOrganic Catalysis and Biocatalysis Laboratory OCBL/FACET, Federal University of
´
Grande Dourados – UFGD, Dourados/Itahum rod. km 12 s/n, 79804-970, Dourados,
MS, Brazil. E-mail: rameshkchem@gmail.com; nelsondomingues@ufgd.edu.br
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra08335h
Fig. 1 Biologically active molecules with pyrrole as core skeleton.
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pyrroles and N-bridgehead pyrroles via regioselective prop-
argylation/amination/cycloisomerization.
Camp et al., reported
a concise synthesis of highly
substituted pyrroles via intermolecular addition of oximes to
activated alkynes and subsequent thermal rearrangement of in
situ generated O-vinyl oximes to form pyrroles via nucleophilic
catalysis.21 Hashemi and co-workers22 described a mild method
for the synthesis 2-alkyl-5-aryl-(1H)-pyrrole-4-ol in water at room
temperature via three component condensation reaction
between b-dicarbonyl compounds, arylglyoxals and ammonium
Scheme 1 Synthesis of polyfunctionalized pyrroles mediated by b-
cyclodextrin.
acetate. Rodrıguez et al.,23 synthesized tetrasubstituted pyrroles
´
In our initial study towards the development of the present
method, benzaldehyde (1.0 mmol) was taken, in preheated
aqueous solution of b-cyclodextrin (1.0 mmol), to which nitro-
methane (1.0 mL), aniline (1.5 mmol), and acetylacetone (1.0
mmol) were added successively at 70–80 ꢀC and stirred for 8 h.
Work up of the reaction medium resulted in polyfunctionalized
pyrrole in excellent yield (90%) (Table 2, entry 1). Although, we
optimized different kinds of supramolecular catalysts were
carried out by this present methodology such as a-cyclodextrin,
b-cyclodextrin, g-cyclodextrin, 2-hydroxy propyl-b-cyclodextrin
and methyl-b-cyclodextrin. Of these cyclodextrins (Table 1), b-
CD and g-CD were found to be superior mediators and both
gave moderate to excellent yield of the desired product. As b-
cyclodextrin was inexpensive and readily available when
compared to g-cyclodextrin. Therefore, b-cyclodextrin was
selected as a better choice. On the other hand, we examined the
progress of the four-component reaction in presence of sodium
dodecyl sulphate (SDS), PEG-400 and water. When SDS was used
as an additive the reaction resulted in corresponding poly-
functionalized pyrrole in only 20% yield whereas in case of PEG-
400 or water alone as reaction medium, the reaction did not
afford the desired product.
Aer having optimized the reaction conditions the scope of
this reaction has been extended to study the reactivity of
a variety of substituted aldehydes xing aniline, acetylacetone
and nitromethane as common reactants. It was observed that
the substitution pattern on aldehyde component played crucial
role in governing the product yield. Aldehyde with electron
donating substituent increases the product yield (Table 2, entry
5) whereas electron withdrawing substituents decrease the
product yield (Table 2, entry 14) due to electronic factors
via coupled domino process through mn-assisted rearrangement
of 1,3-oxazolidines to pyrroles. Liu and co-workers24 described
an efficient method for the synthesis of polysubstituted pyrroles
via [4C + 1N] cyclization of 4-acetylenic ketones with primary
amines using FeCl3. Recently Maiti et al. developed a direct
synthesis of polysubstituted pyrroles using FeCl3 as a catalyst
under reuxing conditions and very recently Dong et al.
synthesized substituted pyrroles using iron-containing metal–
organic frameworks (MOFs) as heterogeneous catalysts.25 In
view of these above mentioned reports were suffered with severe
draw backs. Therefore, we need to explore and development of
a mild, efficient and environmentally benign synthetic protocol
involving a recyclable organic catalyst in water medium is
highly desirable. Earlier our own research group has reported
a one-pot three component strategy to access 1,2,3,5-substitued
pyrroles catalysed by b-cyclodextrin in water.26 In continuation
of our efforts towards the synthesis of polysubstituted pyrroles,
we report here in for the rst time a facile one pot four
component condensation protocol for the synthesis of 1,3,4,5-
substituted pyrroles in aqueous medium under the supramo-
lecular catalysis using b-cyclodextrin.
Due to the signicant development achieved in the eld of
aqueous organic synthesis and water being inexpensive non-
toxic and environmentally benign reaction medium,
researchers are viewing organic reactions from different
perspective with new additions to the already existing synthetic
strategies by the invention of useful additives to water to
enhance the substrate solubility to carry forward the reaction.
Results and discussion
Cyclodextrins are cyclic oligosaccharides possessing hydro-
phobic cavities. They are torus-like macro rings consisting of six
(a-CD), seven (b-CD), and eight (g-CD) 1,4-linked a-D-glucopyr-
anose units. Cyclodextrins and modied cyclodextrins have
attracted much attention as aqueous based hosts for inclusion
complex phenomenon with a wide variety of guest molecules.
Inclusion complex formation occurs as a result of non-covalent
bond interaction between hydrophobic cavity of CD and
hydrophobic guest molecules. In continuation of our ongoing
research program involving cyclodextrins,27 as reusable organic
supramolecular catalysts, for the synthesis of a library of hetero
cyclic derivatives, the present one pot four component synthesis
of polyfunctionalized pyrroles, mediated by cyclodextrins
through host guest complexation phenomenon is reported, for
the rst time (Scheme 1).
Table 1 Optimization of different catalysts on this methodologya
Entry
Catalyst
Yieldb (%)
1
2
3
4
5
6
7
8
g-Cyclodextrin
a-Cyclodextrin
b-Cyclodextrin
2-Hydroxy propyl-b-cyclodextrin
Methyl-b-cyclodextrin
Sodium dodecyl sulphate
PEG-400
66
10
90
29
30
20
n.rc
n.rc
Water
a
All reactions were carried out using benzaldehyde (1.0 mmol), aniline
(1.5 mmol), and acetylacetone (1.0 mmol) with nitromethane (1.0 mL).
b
Yield obtained aer column chromatography. c n.r ¼ no reaction.
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Table 2 Synthesis of functionalized pyrroles with various aromatic Table 3 Synthesis of functionalized pyrroles with several aromatic
aldehydes using b-cyclodextrin in watera
amines using b-cyclodextrin in watera
a
Reaction conditions: aldehyde (1.0 mmol), nitromethane (1.0 mL),
amine (1.5 mmol), acetyl acetone (1.0 mmol) and b-CD (1.0 mmol) at
70–80 ꢀC.
observed that aromatic amines with electron donating groups
such as p-methoxy aniline (Table 3, entry 1), p-toluidine (Table
3, entry 2) reacted effectively and gave corresponding products
in excellent yields, whereas electron withdrawing aromatic
amines such as p-NO2 aniline (Table 3, entry 7) gave lower
product yield. Above all the optimized reaction conditions
worked well with aliphatic amine (Table 3, entry 10). Further
the present protocol was evaluated with different benzyl
amines also and the investigation revealed that benzylamines
reacted interestingly to afford corresponding pyrroles in
moderate to excellent yields (Table 4). The reaction of unsub-
a
Reaction conditions: aldehyde (1.0 mmol), nitromethane (1.0 mL),
amine (1.5 mmol), acetyl acetone (1.0 mmol) and b-CD (1.0 mmol) at
70–80 ꢀC.
associated with the functionality. Sterically demanding alde- stituted benzylamine with benzaldehyde, acetylacetone and
hydes hamper the reaction and gave lesser yields (Table 2, nitromethane yielded the corresponding N-benzylpyrrole in
entries 6, 11, 13, 15). Hetero-aromatic aldehydes such as fur- excellent yield (Table 4, entry 1). When experiments were con-
furaldehyde (Table 2, entry 18), thiophene-2-carbaxaldehyde ducted with simple benzyl amine as a common reactant with
and propionaldehyde (Table 2, entry 17 & 19) reacted well to different substituted aldehydes (Table 4, entries 2, 3, 4, 5), it
afford corresponding products in encouraging yields in case of was observed that substitution pattern of aldehyde component
heteroaromatic aldehydes and gave lower yield in case of has a clear inuence on the overall product yield. In the case of
aliphatic aldehyde.
reaction with ve membered heteroaromatic aldehydes, the
We further examined the reaction with several substituted reaction proceeded satisfactorily to obtain corresponding
amines. In general all the reactions proceeded well with various products in moderate yields (Table 4, entries 6, 7). When we
substituted and unsubstituted amines and results were pre- carried out the reaction with ethylacetoacetate an unsymmet-
sented in Table 3. Here we also determined the electronic rical 1,3-dicarbonylcompound, the reaction yielded only one
factors associated with substituents on amine component product which was characterized and compared with reported
played a signicant role in governing the product yield. It is literature.28
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Table 4 Synthesis of functionalized pyrroles with various benzyl
amines using b-cyclodextrin in watera
Fig.
3
Recyclability of b-CD using benzaldehyde, nitromethane,
aniline and acetyl acetone in water at 70–80 ꢀC (Table 2, entry 1).
incorporation of guest molecule in to the hydrophobic cavity of
b-cyclodextrin.29 We conclude that the hydrophobic environ-
ment of b-CD facilitated the reaction by forming b-CD–aldehyde
inclusion complex, which reacts with nitromethane to form
nitrostyrene, which inturn reacts via Michael addition reaction
with in situ generated b-ketoenamine (B), followed by oxidative
aromatization affording the corresponding polyfunctionalized
pyrroles as shown in the Fig. 2. Aer completion of the reaction,
the reaction mass was cooled to rt, and b-CD was ltered off
from the reaction medium and reused further four-cycles with
same substrates (Table 2, entry 1) as we observed that the
desired product yield was slightly decreased aer three to fourth
cycle as shown in Fig. 3.
a
Reaction conditions: aldehyde (1.0 mmol), nitromethane (1.0 mL),
amine (1.5 mmol), acetyl acetone (1.0 mmol) and b-CD (1.0 mmol) at
70–80 ꢀC.
Conclusion
In conclusion, we have accomplished an efficient, environ-
friendly one-pot four-component condensation protocol for
the synthesis of polyfunctionalized pyrroles using b-cyclodex-
trin as a promoter in aqueous medium. In addition, we have
prepared many pyrrole motifs using different aromatic alde-
hydes, aromatic amines, and benzyl amines via a four-
component reaction in b-CD under aqueous medium. The
present strategy was an inexpensive useful addition to synthetic
organic chemistry, and utilizes the readily available starting
materials to access a wide variety of highly substituted pyrroles
with little effort.
All the products were characterized by 1H & 13C NMR, IR and
mass spectrometry. The catalytic efficiency of b-cyclodextrin was
established by conducting the reaction in water alone without
any additive, in which case product formation was not observed
even aer prolonged reaction times, highlighting the signi-
cant role of b-cyclodextrin as a promoter in carrying out the
reaction forward. This was further established by the formation
of b-CD–aldehyde inclusion complex when b-CD and aldehyde
1
were warmed in water in 1 : 1 ratio. Comparative study of H
NMR spectra of both aldehyde and b-CD–aldehyde inclusion
complex, recorded in DMSO-d6 clearly indicated that there was
an up eld shi of H-3 (0.02 ppm) and H-5 (0.02 ppm) due to the
Experimental section
General procedure for the synthesis of polyfunctionalized
pyrrole derivatives
b-Cyclodextrin (1.0 mmol, 1.135 g) was dissolved in water (15
ꢀ
mL) at 70 C and to this clear solution, aldehyde (1.0 mmol),
nitromethane (1.0 mL) followed by aromatic amine (1.5 mmol)
and acetyl acetone (1.0 mmol) aer which the reaction mixture
was heated at 70–80 ꢀC until completion of the reaction as
indicated by TLC. The reaction mixture was cooled to the room
temperature and b-CD was ltered, the aqueous phase was
extracted with ethyl acetate (3 ꢁ 10 mL). The organic layers were
washed with water, saturated brine solution and dried over
Fig. 2 Plausible mechanistic pathway.
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anhydrous Na2SO4. The combined organic layers were evapo-
rated under reduced pressure and the resulting crude product
was puried by column chromatography by using ethyl acetate
and hexane (1 : 9) as eluent to give corresponding poly-
functionalized pyrrole.
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1-(2-Methyl-1,4-diphenyl-1H-pyrrol-3-yl)ethanone (Table 2,
entry 1). Light brown solid, mp 106–109 ꢀC. IR (KBr) 3060, 2923,
1
1642, 1495, 1400, 1226 cmꢂ1; H NMR (300 MHz, CDCl3, TMS)
d ¼ 7.30–7.34 (m, 3H), 7.37–7.50 (m, 7H), 6.67 (s, 1H), 2.41 (s,
3H), 2.07 (s, 3H) ppm; 13C NMR (75 MHz, CDCl3, TMS) d 12.8,
30.9, 114.5, 120.5, 122.4, 126.1, 126.3, 126.7, 128.0, 128.1, 128.8,
129.2, 135.2, 135.9, 138.6, 197.5 ppm; HRMS: m/z calcd for
C
19H17NONa 298.1207; found 298.1200.
1-(2-Methyl-1-phenyl-4-p-tolyl-1H-pyrrol-3-yl)ethanone (Table
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1
cmꢂ1; H NMR (300 MHz, CDCl3, TMS) d ¼ 7.41–7.50 (m, 3H),
7.32 (d, J ¼ 7.7 Hz, 2H), 7.24–7.26 (m, 2H), 7.18 (d, J ¼ 7.7 Hz,
2H), 6.63 (s, 1H), 2.38 (s, 6H), 2.07 (s, 3H) ppm; 13C NMR (75
MHz, CDCl3, TMS) d 12.9, 29.7, 31.1, 110.8, 120.6, 122.4, 125.0,
126.2, 128.2, 128.4, 129.3, 130.4, 132.8, 134.5, 135.5, 138.6, 196.9
ppm; HRMS: m/z calcd for C20H20NO (M + 1) 290.1544; found
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Acknowledgements
We thank CSIR, New Delhi, India, for fellowship to K. K. and
Brazilian authors (R. K. and N. L. C. D) thanks to Conselho
´
´
Nacional de Desenvolvimento Cientıco e Tecnologico for BJT
fellowship and the nancial support process: 314140/2014-
0 and 400706/2014-8 CNPq – Brazil. The authors thanks to Dr
Y. V. D. Nageswar, Chief Scientist at Indian Institute of Chem-
ical Technology (IICT) Hyderabad, India for the spectroscopic
analysis.
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