Synthesis of N-alkyl pyrroles via decarboxylation/dehydration in neutral ionic liquid under catalyst-free conditions

Article abstract of DOI:10.1039/c4ra09797a

A general route to N-alkyl pyrroles by reacting aromatic, heteroaromatic or aliphatic aldehydes with 4-hydroxyproline in ionic liquid under neutral condition was developed. The ionic liquid can be readily recovered and reused for up to 5 reaction cycles without any effect on the yield of the product formed. The utility of the protocol for the one-pot synthesis of 9H-benzo[e]pyrrolo[2,1-b][1,3]oxazines is also presented.

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Synthesis of N-alkyl pyrroles via decarboxylation/ dehydration in neutral ionic liquid
under catalyst-free condition
Veena D. Yadav, Shashikant U. Dighe and Sanjay Batra*
A catalyst-free benign route to N-alkyl pyrroles by reacting aromatic, heteroaromatic or aliphatic
aldehydes with 4-hydroxyproline in neutral ionic liquid under microwave irradiation is
presented.

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DOI: 10.1039/C4RA09797A
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ARTICLE TYPE
Synthesis of N-alkyl pyrroles via decarboxylation/ dehydration in
neutral ionic liquid under catalyst-free condition
Veena D. Yadav,a Shashikant U. Dighea and Sanjay Batra*a,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A general route to Nꢀalkyl pyrroles by reacting aromatic, heteroaromatic or aliphatic aldehydes with 4ꢀ
hydroxyproline in ionic liquid under neutral condition was developed. The ionic liquid can be readily
recovered and reused up to 5 reaction cycles without any effect on the yield of the product formed. The
utility of the protocol for oneꢀpot synthesis of 9Hꢀbenzo[e]pyrrolo[2,1ꢀb][1,3]oxazines is also presented.
performed in a suitable ionic liquid there is a possibility that the
use of acid is eliminated and the reaction could be performed at a
10 Introduction
lower temperature under neutral conditions. Working towards this
50 objective we have discovered that the reaction between 4ꢀ
hydroxyproline and the aldehyde is successful in neutral ionic
liquid under microwave conditions at 130 oC to efficiently
produce the Nꢀalkyl pyrroles and we report details of this study
herein. Remarkably the ionic liquid can be recovered from the
55 reaction and can be reused without any loss of reactivity in
subsequent runs.
Nꢀsubstituted pyrroles are important class of compounds due to
significant medicinal and material properties associated with
them.1ꢀ2 Therefore the synthesis of diverse Nꢀsubstituted pyrroles
had been an area of continuous research in synthetic chemistry.3
15 The classical methods of synthesis of Nꢀalkyl pyrrole involve
harsh acidic/ basic conditions such as refluxing amine with 2,5ꢀ
dimethoxy tetrahydrofuran in glacial acetic acid or direct heating
of pyrrole with benzyl halide in ionic liquid in the presence of
KOH or K2CO3.4ꢀ5 In 2007, Tunge et al. reported the redoxꢀ
20 neutral approach to the synthesis of Nꢀalkyl pyrroles from 3ꢀ
pyrroline and aldehyde or ketone in catalytic amount of acid.6
Subsequently, Pan’s and Seidel’s groups independently reported
Results and Discussion
We commenced our investigations by screening the reaction of 4ꢀ
chlorobenzaldehyde (1.0 equiv) with 4ꢀhydroxyproline (1.2
the synthesis of Nꢀsubstituted indole via reaction between 60 equiv) in 1ꢀbutylꢀ3ꢀmethylimidazolium bromide ([bmim]Br)
indoline and aldehyde in the presence of acid.7ꢀ8 The reaction
25 proceeded via intramolecular hydride transfer to the azomethine
ylide that was generated as intermediate during the reaction to
afford product. In further advancement of the methodology
under microwave irradiation (Table 1). The reaction was initially
performed at 90 oC to be completed in 45 min to afford a product
(70%) which was identified to be the required pyrrole derivative
2a (entry 1). Next we investigated the reaction at higher
Kumar and Rao reported synthesis of Nꢀalkyl pyrroles by reaction 65 temperatures and found that at 130 oC the reaction was completed
between 4ꢀhydroxy proline and aldehyde in DMSO which was
30 required to be added slowly for efficient reaction.9 Later Seidel et
al. reported the synthesis of Nꢀalkyl pyrrole via reaction of
similar substrates in the presence of benzoic acid at 240 oC under
in 25 min to afford the product in 80% yield (entry 2,3).
Subsequently we evaluated other ionic liquids for the success of
the reaction and discovered that as compared to the butylꢀbased
ionic liquid 1ꢀhexylꢀ3ꢀmethylimidazolium bromide ([hmim]Br)
microwave irradiation.10 Subsequently, Hong and Wei and coꢀ 70 gave superior yield (82%) of 2a (entries 4ꢀ7). We also
workers reported synthesis of Nꢀalkyl pyrroles from identical
35 substrates in DMF in the presence of acid.11 In this case however,
addition of aldehyde was required to be carried out slowly in hot
DMF. In an ongoing project related to decarboxylative coupling
investigated the reaction in organic solvents and found that yields
of 2a were inferior as compared to the one in ionic liquid.
Besides the work up procedure with ionic liquid was easy as
partitioning the reaction mixture in ethyl acetate and evaporating
reaction we became interested to develop a more sustainable 75 the solvent gave the product. Thus the optimized conditions
alternative to the synthesis of Nꢀalkyl pyrroles under neutral
40 conditions. Analysis of the mechanism proposed by Seidel
reflects that the acid is required to catalyze the dehydration of the
enamine intermediate that is formed during the reaction. An
which worked efficiently for us were aldehyde (1.0 equiv), 4ꢀ
hydroxyproline (1.5 equiv) in [hmim]Br as medium under
microwave at 130 oC for 25 min.
Next we tested the scope of the protocol using a variety of
insight into the literature revealed that the dehydration of the 80 aldehydes and the results are summarized in Scheme 1. It was
activated hydroxyl group leading to an alkene can be readily
45 achieved via ionic liquid.12 Taking a cue from these reports, we
reasoned that if the reaction of 4ꢀhydroxyproline and aldehyde is
found that in all cases the reaction was successful to afford the
desired Nꢀalkyl pyrroles. The benzaldehydes bearing electron
withdrawing substituents such as chloro, nitro or cyano furnished
This journal is © The Royal Society of Chemistry [year]
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Table 1 Results of the optimization studya
10 the respective pyrroles (2a, 2c, 2e-2f, 2j-2k) in good yields. It
was observed that the yield of the product 2b prepared from
unsubstituted benzaldehyde was relatively low. Besides the
benzaldehydes carrying the electron donating substituent such as
methoxy group also afforded the product (2g-2h) in moderate
15 yields. Notably when terephthaldehyde was employed as
substrate the corresponding product 2d was isolated in 65%
yields. Perhaps the presence of a formyl group in 2d offers option
for further derivatisation. Gratifyingly, ferrocenecarbaldehyde
also smoothly afforded the pyrrole 2p in 68% yield. The reaction
20 worked well even with the polycyclic benzenes and heterocyclic
aldehydes to afford 2l-2o in 64ꢀ82% yields. We also investigated
the aliphatic aldehydes in the protocol which gave the respective
pyrrole derivatives 2q and 2r in 58ꢀ60% yields. Benzophenone
which was examined too gave the product 2s in 90% yield.
Entry
Solvent
Temp (oC)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
[bmim]Br
[bmim]Br
[bmim]Br
[bmim]BF4
[bmim]PF6
[bmim]OH
[hmim]Br
Toulene
90
45
30
25
25
25
25
25
25
25
25
70
77
80
42
41
48
82
26
38
18
110
130
130
130
130
130
130
130
130
DMSO
DMF
25
A plausible mechanism for the formation of Nꢀalkyl pyrroles is
delineated in Scheme 2. In the first stage the enamine I is
produced via decarboxylation and imine formation as proposed
by Seidel et al. This is followed by ionic liquid assisted
dehydration leading to the formation of the pyrrole to afford the
10
a All reactions were carried out with 100 mg (0.71 mmol) of 1a and 99
mg (0.85 mmol) of 4ꢀhydroxyproline in 1.0 mL of solvent under
microwave.
5
30 intermediate III that undergoes a deprotonation to furnish the
product. After completion of the reaction, the mixture was
extracted with ethyl acetate or ether (three times) and the residual
ionic liquid was dried under vacuum and reused for 5 cycles
without any significant effect on the yield of the product. This
35 result suggests that this approach for preparing Nꢀalkyl pyrroles is
environmentally benign compared to the processes reported
earlier.
HO
O
[hmim]Br, W,
130 oC, 25 min
N
CO2H
R1
R2
R1
N
H
R2
1
2
N
N
N
N
N
Br
N
N
CN
(78%)
2c
Cl
Me
CHO
C6H13
NO2
(80%)
HO
HO
+
(56%)
(82%)
2b
2a
(65%)
2d
OH
2e
[hmim]Br
Neutral ionic liquid
O
OH
N
N
-H2O, -CO2
R
H
N
N
N
H
N
N
O
imine formation/
decarboxylation
R
I
II
R
IL dried
and
recycled
NO2
OMe
MeO
OMe
(65%)
-H2O
O2N
N
R
H
OMe
N
IL assisted
dehydration
2i
(69%)
N
2f
OH
(59%)
2g
(64%)
2h
III
R
N
N
N
40 Scheme 2. Plausible mechanism for ionic liquid promoted formation of
Nꢀalkyl pyrrole under neutral conditions.
NO2
Cl
Cl
With a view to use this methodology for the synthesis of
pyrroleꢀfused system, we performed the reaction between
salicylaldehyde (A) and 4ꢀhydroxyproline in the presence of
45 iodine. The initially formed 2ꢀ((1Hꢀpyrrolꢀ1ꢀyl)methyl)phenol
undergoes an in situ electrophilic cyclization to afford 9Hꢀ
benzo[e]pyrrolo[2,1ꢀb][1,3]oxazine (3A) in 78 % yield (Scheme
3). Subsequently, 4ꢀbromosalicylaldehyde (B) and 3ꢀ
methoxysalicylaldehyde (C) were also subjected to identical
Cl
(88%)
N
(77%)
N
2j
(86%)
2l
2k
(82%)
2m
N
N
Fe
S
N
Me
(66%)
(64%)
2n
(68%)
2p
(60%)
2q
2o
N
N
Me
(58%)
2r
(90%)
2s
50
Scheme 1. Scope of the reaction. All reactions were performed at 100 mg
scale of aldehyde using 1.2 equiv of 4ꢀhydroxyproline under microwave
heating using 1 mL of ionic liquid.
Scheme 3. Synthesis of 9Hꢀbenzo[e]pyrrolo[2,1ꢀb][1,3]oxazine via
iodineꢀmediated electrophilic cyclization
2
| Journal Name, [year], [vol], 00–00
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reaction.
corresponding
Whereas,
4ꢀbromosalicylaldehyde
3B in 35%
gave
yield,
the
3ꢀ
δ = 52.7, 109.1, 121.3, 128.6, 129.1, 133.6, 137.1. MS (ESI+):
m/z = 192.0. ESIꢀHRꢀMS calculated for C11H10ClN (M++H):
192.0580, found: 190.0583.
product
methoxysalicylaldehyde gave a mixture of products from which
the product 3C could not be isolated in pure form.
Typical experimental procedure for the synthesis of 9H-
60 benzo[e]pyrrolo[2,1-b][1,3]oxazine 3A
An ovenꢀdried 2ꢀ5 mL microwave reaction vial was charged with
[hmim]Br (1.5 mL), salicylaldehyde (100 mg, 0.81 mmol), 4ꢀ
hydroxyproline (113 mg, 0.98 mmol) and molecular iodine (205
mg, 0.81 mmol). The reaction vial was heated under stirring in a
65 microwave reactor at 130 °C for 25 min. On completion, the
reaction was cooled with compressed air flow and quenched with
aq solution of Na2S2O3 (5.0 mL) and extracted with EtOAc (3 x
10 mL). The combined organic layer was dried over anhyd
Na2SO4 and concentrated under reduced pressure to obtain a
70 residue. The residue was purified through column
chromatography on silica gel using hexanes/ EtOAc (8.5:1.5, v/v)
as eluent to furnish 3 (0.109 g, 78%) as a brown solid.
5
Conclusion
In summary, we have demonstrated a sustainable approach for the
synthesis of Nꢀalkyl pyrroles in ionic liquid. This protocol is
attractive as the reaction is performed under neutral and mild
conditions and the ionic liquid used as the solvent could be
10 readily recovered and reused. We have also extended the protocol
for the oneꢀpot synthesis of 9Hꢀbenzo[e]pyrrolo[2,1ꢀ
b][1,3]oxazines.
Experimental
General. All experiments were monitored by thin layer
15 chromatography (TLC) performed on preꢀcoated silica gel plates.
After elution, plate was visualized under UV illumination at 254
nm for UV active materials. Further visualization was achieved
by staining with KMnO4 and charring on a hot plate. Column
chromatography was performed on silica gel (100ꢀ200 mesh) by
20 standard techniques eluting with solvents as indicated. IR spectra
o
9H-Benzo[e]pyrrolo[2,1-b][1,3]oxazine (3A). Mp 85ꢀ87 C; Rf
= 0.45 (Hexanes: EtOAc, 9:1, v/v); IR (KBr) νmax: 925, 1215,
75 1565, 3012 cmꢀ1. 1H NMR (400 MHz, CDCl3): δ = 4.84 (dd, J1 =
15.3 Hz, J2 = 15.3 Hz, 2H), 6.53 (d, J = 8.4 Hz, 1H), 6.97ꢀ7.02
(m, 2H), 7.24ꢀ7.28 (m, 2H), 7.48 (d, J = 8.0 Hz, 2H); 13C NMR
(100 MHz, CDCl3): δ = 55.7, 106.6, 108.5, 119.6, 121.6, 128.6,
128.9, 129.3, 132.0, 141.1, 144.4. MS (ESI+): m/z = 172.1. ESIꢀ
80 HRꢀMS calculated for C11H9NO (M++H): 172.0762, found:
172.0768.
1
were recorded using Perkin Elmer’s FTIR spectrophotometer. H
NMR and 13C NMR spectra were recorded on Bruker 400 MHz
spectrometer, using TMS as an internal standard (chemical shifts
in δ). Peak multiplicities of NMR signals were designated as s
25 (singlet), bs (broad singlet), d (doublet), dd (doublet of doublet), t
(triplet), m (multiplet) etc. The ESIꢀMS were recorded on Ion
Trap Mass spectrometer and the HRMS spectra were recorded as
Two authors (VD and SUD) acknowledge the financial assistance
in the form of fellowship from Council of Scientific and
85 Industrial Research, New Delhi. The authors acknowledge the
SAIF division of CSIRꢀCDRI for providing the spectroscopic
data. The financial assistance from the network project BSP0014
(HOPE) is acknowledged.
ESIꢀHRMS on
a QꢀTOF LCꢀMS/MS mass spectrometer.
Commercial grade reagents and solvents were used without
30 further purification. All reactions were carried out inflameꢀdried
reaction vessels with Teflon lined snap caps in Biotage initiator
2.5 microwave synthesizer. The identity of the Nꢀalkyl pyrrole
products was confirmed by comparison to the reported NMR
spectra of known compounds. The recovery of ionic liquid for
35 solid compounds was made with EtOAc whereas for oily
products diethylether was used.
Notes and references
90 a Medicinal & Process Chemistry Division, CSIRꢀCentral Drug Research
Institute, BSꢀ10/1, Sector 10, Jankipuram Extension, PO Box 173,
Lucknowꢀ226031, UP, India. Fax: +91ꢀ522ꢀ2771941; Tel: +91ꢀ522ꢀ
2772450/2772550 Extn. 4705, 4727; Eꢀmail: batra_san@yahoo.co.uk
b Academy of Scientific and Innovative Research, New Delhi, India.
95 † Electronic Supplementary Information (ESI) available: [1H and 13Cꢀ
General procedure for the synthesis of N-alkyl pyrroles as
exemplified for 1-(4-Chlorobenzyl)-1H-pyrrole 2a
NMR
DOI: 10.1039/b000000x/
‡ This manuscript is CDRI Communication No 271/2014/SB.
spectra
of
all
compounds
are
provided].
See
40 A clean ovenꢀdried 2ꢀ5 mL reaction vial was charged with 4ꢀ
chlorobenzaldehyde 1a (100 mg, 0.71 mmol), 4ꢀhydroxyproline
(99 mg, 0.85 mmol) and [hmim]Br (1.0 mL). The resulting
solution was stirred under microwave irradiation at 130 °C for 25
min. On completion, the reaction mass was allowed to cool to
45 ambient temperature and extracted with EtOAc (3 x 5 mL). The
combined organic layer was dried over anhyd. Na2SO4 and
concentrated under reduced pressure to obtain a residue. The
residue was purified through column chromatography on silica
gel using hexanes/ EtOAc (9.5:0.5, v/v) as eluent to furnish 2a
50 (0.11 g, 82%) as a colorless oil.
1
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1-(4-Chlorobenzyl)-1H-pyrrole (2a). Rf
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1676, 2926 cmꢀ1. H NMR (400 MHz, CDCl3): δ = 4.95 (s, 2H),
1
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This journal is © The Royal Society of Chemistry [year]
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