Microwave mediated facile one-pot synthesis of polyarylpyrroles from but-2-ene- and but-2-yne-1,4-diones

Article abstract of DOI:10.1016/j.tet.2003.11.087

Several pyrrole derivates with multiple aryl substituents were prepared conveniently in a one pot-reaction from but-2-ene-1,4-diones and but-2-yne-1,4-diones via hydrogenation of the carbon-carbon double bond/triple bond followed by amination-cyclization. The reaction could be performed with ammonium formate or alkyl/arylammonium formates under Pd/C in polyethylene glycol-200 (PEG-200) under microwave irradiation. Using this procedure, different aryl-substituted pyrroles were prepared. Furthermore, studies on microwave vs thermal conditions indicate faster heating under microwave conditions was responsible for rate enhancement.

Full text of DOI:10.1016/j.tet.2003.11.087

Tetrahedron 60 (2004) 1625–1630
Microwave mediated facile one-pot synthesis of polyarylpyrroles
from but-2-ene- and but-2-yne-1,4-diones
a
b
H. Surya Prakash Rao,a S. Jothilingam and Hans W. Scheeren
,
*
aDepartment of Chemistry, Pondicherry University, Pondicherry 605 014, India
bDepartment of Organic Chemistry, NSR Center for Molecular Structure, Design and Synthesis, University of Nijmegen, Toernooiveld 1,
Nijmegen 6525 ED, The Netherlands
Received 10 September 2003; revised 31 October 2003; accepted 28 November 2003
Dedicated to Professor Goverdhan Mehta, I.I.Sc., Bangalore, on the occasion of his 60th birthday
Abstract—Several pyrrole derivates with multiple aryl substituents were prepared conveniently in a one pot-reaction from but-2-ene-1,4-
diones and but-2-yne-1,4-diones via hydrogenation of the carbon–carbon double bond/triple bond followed by amination–cyclization. The
reaction could be performed with ammonium formate or alkyl/arylammonium formates under Pd/C in polyethylene glycol-200 (PEG-200)
under microwave irradiation. Using this procedure, different aryl-substituted pyrroles were prepared. Furthermore, studies on microwave vs
thermal conditions indicate faster heating under microwave conditions was responsible for rate enhancement.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
diarylbut-2-ene-1,4-diones can be prepared by Friedel–
Crafts acylation of arenes with fumaroyl chloride.6 The
1,2,4-triarylbut-2-ene-1,4-diones can be prepared by con-
densation of benzil and its derivatives with acetophenone.7
The 1,2,3,4-tetraarylbut-2-ene-1,4-diones can be prepared
by deoxygenative dimerization of benzil and its derivatives
via carbene intermediates.8 The 1,4-butanediones can also
be prepared by hydrogenation of the triple bond present in
the but-2-yne-1,4-diones. The but-2-yne-1,4-diones of
interest in the present study, the diaroylacetylenes, can be
prepared from the corresponding diaroylethylenes by
bromination followed by dehydrobromination.9
The pyrrole ring is one of the fundamental heterocycles. It is
a widely distributed structural unit in a variety of natural and
biologically important molecules such as porphyrins, bile
pigments, co-enzymes and alkaloids.1 Since fundamental
molecules for energy trapping from sunlight embody a
porphyrin nucleus with four pyrrole units, there has been a
continuous quest to synthesize this heterocycle with
different substituents efficiently and from readily available
starting materials.2 In recent years, there has been an
enhanced interest in the synthesis of pyrrole and its
oligomers due to their potential application as conducting
materials.3 Furthermore, pyrrole ring with multiple aromatic
ring substitutions have applications as electroluminescent
devices.4 For this reason we became interested in develop-
ing a versatile synthesis of pyrrole derivatives from readily
available starting materials.
We reasoned that a simple and versatile route for the
synthesis of pyrrole derivatives from 1,4-enediones could be
developed if the two steps, viz. reduction and amination–
cyclization can be combined in a single pot operation. For
this purpose, ammonium formate can be employed as it
behaves as a reducing agent of the double bond and also as a
source of ammonia. There are several reports in the
literature on the utility of ammonium formate as a source
of hydrogen in the transfer hydrogenation reaction.10 In the
preliminary communication we disclosed the utility of
ammonium formate for one-pot transformation of (E)-1,4-
diarylbut-2-ene-1,4-diones to 2,5-diaryl-1H-pyrroles
(Scheme 1).11 We found that the reaction was greatly
accelerated under microwave irradiation and required only
2–3 min. for completion. We also found that polyethylene
glycol-200 (PEG-200) can be employed as a convenient
solvent for the microwave promoted reactions. Now, we
wish to report that this versatile method can be extended for
the preparation of 2,5-diaryl and 1,2,5-triaryl-1H-pyrrole
The 2,5-disubstituted pyrroles can be synthesized by the
classical Paal–Knorr method involving the reaction of 1,4-
butanediones with amines.5 Even though this procedure is
versatile and applicable for the synthesis of a wide variety of
pyrrole derivatives, it is limited to the availability of 1,4-
diketones. On the other hand, but-2-ene-1,4-diones, the
precursors to 1,4-butanediones can be prepared easily from
readily available starting materials. For example, 1,4-
Keywords: Pyrrole synthesis; Microwave assisted organic synthesis; Alkyl
and aryl ammonium formates; Reduction; Amination–cyclization.
*
Corresponding author. Tel.: þ91-413-2655991x411; fax: þ91-413-
2655265; e-mail address: hspr@yahoo.com
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.11.087

1626
H. S. P. Rao et al. / Tetrahedron 60 (2004) 1625–1630
Scheme 1.
derivatives from 1,4-diarylbut-2-yne-1,4-diones and also for
the synthesis of 2,3,5-triaryl, 1,2,3,5-tetraaryl and 2,3,4,5-
tetraaryl-1H-pyrroles. We also report our studies on
comparison of yield of the selected pyrrole derivative
when synthesized by microwave or conventional heating
techniques.
conducted under microwave irradiation in PEG-200 the
transformation of 1,4-diphenylbut-2-ene-1,4-dione 1a into
2,5-diphenyl-1H-pyrrole 3a was over in 30 s whereas the
reaction required 30 min for completion in methanol reflux.
However, the transformation of 1,4-di(4-methoxyphenyl)-
but-2-ene-1,4-dione 1e into 2,5-di(4-methoxyphenyl)-1H-
pyrrole 3e required comparatively longer time both under
microwave conditions or under methanol reflux indicating
the influence of the electron donating 4-methoxy group in
the transformation. The yield of the pyrrole derivative 3e
was 89% under microwave irradiation but only 37% in
methanol reflux. In addition to the pyrrole product, the
double bond reduced product 1,4-diphenyl-1,4-butanedione
was obtained in 45% yield in refluxing in methanol.
Prolonged reflux did not increase the yield of the pyrrole
product. However, the yield of 3e rose to 85% when the
reaction was conducted in diethylene glycol at 150 8C for
10 min. This result indicated that the reduction of the double
bond in 1e with ammonium formate was efficient and was
not affected by the electron donating methoxy substituent on
the phenyl ring. On the other hand, subsequent steps in the
sequence, the formation of the hemiaminal intermediates
and cyclisation to pyrrole rings appears to be highly
influenced by the presence of the methoxy group. Amarnath
and co-workers made similar observations in the mechan-
istic studies on Paal–Knorr pyrrole synthesis.12
2. Results and discussion
The 1,4-diarylbut-2-ene-1,4-diones (dibenzoyl ethylenes)
1a–f were smoothly converted into 2,5-diaryl-1H-pyrroles
3a–f when they were subjected to reductive amination–
cyclization with ammonium formate 2a in the presence of
5% Pd/C in PEG-200 under microwave irradiation for 2 min
(Scheme 1). In this method the ene-dione was being reduced
to the 1,4-dione in the initial transformation via palladium
catalyzed catalytic transfer hydrogenation. The resulting
1,4-dione moiety was further transformed into 2,5-diaryl-
1H-pyrroles in a domino fashion through amination–
cyclization by utilizing in situ generated ammonia. The
results of the transformation of differently phenyl sub-
stituted 1,4-diarylbut-2-ene-1,4-diones 1a–f to pyrrole
derivatives 3a–f are gathered together in Table 1. It can
be seen from the Table that when the reaction was
Table 1. Transformation of ene-diones 1a–e to pyrroles 3a–i
The N-alkyl or N-aryl pyrrole derivatives 3g–i was prepared
by employing alkyl/aryl ammonium formates 2b–d as
reagents for the reduction, amination–cyclization of
enedione 1a. In general the reaction of alkyl/aryl
ammonium formates was found to be difficult and only
moderate yields of the pyrrole products 3g–i were realized
in the reaction (Table 1, entry 7–9). Whereas with the
microwave heating conditions, the yield of the pyrroles 3g–i
was around 60%, the yield was only about 28% in methanol
reflux. It appears that the steric and electronic effects on the
amino group of the alkyl/aryl amines play a role in the rate-
determining step during the pyrrole formation. Similar to our
observationpreviously, Sammesand Chiuhaveshown thatthe
Paal–Knorr reaction may follow different pathways depend-
ing on whether the reactant is ammonia or an alkyl amine.19
Entry Enedione Formate derivatives Pyrrole Time Power Yield
(min) (W)
(%)
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1a
1a
1a
2a
2a
2a
2a
2a
2a
2b
2c
2d
3aa
3bb
3cc
3db
3ed
3fe
0.5
1.0
1.0
1.5
2.0
2.0
2.0
2.0
2.0
200
200
200
200
200
200
200
200
200
92
80
85
85
89
84
56
63
60
3gf
3hg
3ih
a
b
c
d
e
f
Ref. 13.
Ref. 14.
Ref. 15.
Ref. 12.
Ref. 11.
Ref. 16.
Ref. 17.
Ref. 18.
g
h
Next, we sought to demonstrate utility of the present
procedure in the one-pot synthesis of 2,5-disubstituted

H. S. P. Rao et al. / Tetrahedron 60 (2004) 1625–1630
1627
Scheme 2.
Having demonstrated a facile 2,5-diarylpyrrole synthesis
from enediones 1a–f and ynediones 4a–f, we extended the
method for the preparation of pyrroles with multiple aryl
substitution. The reaction of (E)-1,2,4-triphenylbut-2-ene-
1,4-dione (1,2-dibenzoylstyrene) 5a with ammonium
formate 2a and Pd/C in PEG-200 under microwave
irradiation resulted in the known20 2,3,5-triphenyl-1H-
pyrrole 6a in near quantitative yield within 1 min
(Scheme 3; Table 3, entry 1). We conducted this reaction
on a 10 mmol scale and found smooth transformation to the
pyrrole derivative in 92% yield. Thus, the present procedure
is amenable for scaling up. The reaction of 5a with
anilinium formate 2d resulted in 1,2,3,5-tetraphenyl-1H-
pyrrole21 6b within 1 min (Scheme 3; Table 3, entry 2).
Similarly, the reaction of (E)-1,2,3,4-tetraphenylbut-2-ene-
1,4-dione (1,2-dibenzoylstilbene) 5b with ammonium
formate furnished 2,3,4,5-tetraphenyl-1H-pyrrole22 6c
(Scheme 3; Table 3, entry 3). However, the reaction of 5b
with anilinium formate did not yield the expected 1,2,3,4,5-
pentaphenyl-1H-pyrrole23 6d. The 1H NMR and IR spectra
of the crude product from this reaction indicated that
reduction of the double bond and formation of imine
Table 2. Transformation of yne-diones 4a–f to pyrroles 3a–i
Entry Ynedione Formate derivatives Pyrrole Time Power Yield
(min) (W)
(%)
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
4a
4a
4a
2a
2a
2a
2a
2a
2a
2b
2c
2d
3a
3b
3c
3d
3e
3f
3g
3h
3i
0.5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
200
200
200
200
200
200
200
200
200
95
92
94
90
91
90
60
65
61
pyrroles from 1,4-diphenylbut-2-yne-1,4-diones wherein
complete hydrogenation of the triple bond in ynedione to
1,4-dione is followed by an amination cyclization reaction.
Thus, in the palladium mediated microwave assisted
reaction of ammonium formate with 2a, 1,4-diarylbut-2-
yne-1,4-diones 4a–f resulted in the 2,5-disubstituted
pyrrole derivatives 3a–f in over 90% yield within one
min (Scheme 2). Similar reaction with alkyl/aryl
ammonium formate 2b–d also resulted in the pyrrole
derivatives 3g–i in 60–95% yield (Table 2). It appears that
the reactivity of ynediones 4a–f towards the formation of
pyrrole derivatives is similar to enediones 1a–f. Since there
is little difference in the reactivity of similarly substituted
ynediones and enediones it can be concluded that hydro-
genation of the triple bond to the corresponding fully
saturated derivatives is facile and the steps involving
amination–cyclization determine the rate of formation of
pyrrole derivatives.
Table 3. Transformation of ene-dione 5a,b to pyrroles 6a–c
Entry Enedione Formate derivatives Pyrrole Time Power Yield
(min) (W)
(%)
1
2
3
5a
5a
5b
2a
2b
2a
6aa
6bb
6cc
1.0
1.0
5.0
200
200
200
95
65
95
a
b
c
Ref. 20.
Ref. 21.
Ref. 22.
Scheme 3.

1628
H. S. P. Rao et al. / Tetrahedron 60 (2004) 1625–1630
Table 4. The effect of reaction conditions on the transformation of 5a to 6a
S. No.
Vessel
Solvent
PEG-200
PEG-200
Methanol
Ethylene glycol
Time (min)
Power (W)
Temperature (8C)
Yield (%)
1
2
3
4
Glass (open)a
Glass (open)
Teflon (capped)
Glass (open)
1.0
5.0
2.0
2.0
200
–
200
200
126
126
–
96
90
96
88
116
a
The vessel was kept in silica gel bath.
intermediates have occurred but the anticipated cyclization
to the pyrrole ring did not take place. It appears that the
steric factors play a major role in preventing the open chain
intermediates to go through an entropically unfavorable
cyclization step.
(power source: 230 V, 50 Hz, microwave frequency:
2450 MHz) microwave oven. The TLC (pre-coated silica
gel 60 F254, Merck) method was used to monitor the
progress of the reaction and the products were isolated by
short column chromatography on silica gel (100–200 mesh,
Acme Synthetic Chemicals, India) using hexanes/dichloro-
methane (DCM) mixture as the eluent. Melting points were
noted using a Gallenkamp melting point apparatus. The IR
spectra were recorded as KBr pellets using Bomem MB104
Next, we sought to find out if there were any non-thermal
microwave effects in the conditions employed in the
conversion of 1,4-butanediones to the pyrrole derivatives.
The transformation of (E)-1,2,4-triphenylbut-2-ene-1,4-
dione (1,2-dibenzoylstyrene) 5a to 2,3,5-triphenyl-1H-
pyrrole 6a was taken as a representative example. The
results in the study are gathered together in Table 4. We
found that microwave irradiation at 200 W the temperature
of the reaction mixture in PEG-200 reached 126 8C in 1 min
(Table 4, entry 1). When the same reaction was conducted in
a preheated oil bath maintained at 126 8C it took 5 min. for
completion (Table 4, entry 2). However, the yield of the
desired product was marginally lower possibly due to
decomposition of the product. When the reaction was
conducted in methanol in a screw-capped Teflon vessel the
reaction took 2 min indicating that methanol is also a good
solvent for the reaction (Table 4, entry 3). The microwave-
mediated transformation of 5a to 6a was equally facile in
the high boiling solvent, ethylene glycol (Table 4, entry 4).
Thus, similar to the observations made by many groups,24
in the present transformation microwave energy is used
only for rapid heating of the reaction mixture to furnish
the desired pyrrole derivatives in near quantitative yield
within few min. Even tough the reaction worked well in
methanol; obviously, PEG-200 is a better solvent for
conducing the microwave-assisted reactions due to precau-
tions one need to take while conducing them in the sealed
vessels.
1
spectrometer. The frequencies at which the H NMR and
13C NMR spectra were recorded in CDCl3 or in CCl4:CDCl3
(1:1) with Bruker 300 MHz, Bruker 400 MHz, JEOL
400 MHz or Varian 300 MHz are noted in the spectral
data. TMS was used as internal standard. Mass spectra were
recorded on HP MS-engine S989A (EI, electron impact,
70 eV). We have given in the experimental section only
those spectral data (IR, 1H NMR, 13C NMR or MS), which
have not been described in literature.
4.1.1. General procedure for the synthesis of di, tri and
tetraarylpyrroles from enediones: 2,5-diphenyl-1H-pyr-
role (3a). A 25 mL conical flask, charged with enedione 1a
(100 mg, 0.42 mmol), ammonium formate 2a (267 mg,
4.2 mmol), 5% Pd/C (2 mg) and PEG-200 (2 mL), was
irradiated in the microwave oven at 200 W for 30 s. After
completion of the reaction (TLC) cooled (rt) contents of the
flask were charged on the short column of silica
(5 cm£1 cm) and eluted with hexane/DCM (90:10 to 50:
50). Removal of solvent from pooled fractions yielded
pyrrole 3a as a white solid (85 mg, 92%). An analytically
pure sample was obtained by recrystallization from DCM/
hexanes (2:98); mp 142-144 8C (lit:13 143 8C). 13C NMR
(CDCl3): d¼108.0, 123.8, 126.3, 128.9, 132.6, 133.0 ppm.
4.1.2. 2,5-Di(4-chlorophenyl)-1H-pyrrole (3b). Following
the above general procedure, enedione 1b (100 mg,
0.33 mmol) was transformed to pyrrole 3b with
ammonium formate 2a (206 mg, 3.3 mmol) and 5% Pd/C
(2 mg) in PEG-200 (2 mL) under microwave irradiation
at 200 W for 1 min. Yield: 76 mg (80%; white solid);
3. Conclusion
In conclusion we have shown that various aryl substituted
pyrrole derivatives can be synthesized readily from
enediones and ynediones in a microwave mediated one-
pot synthesis using ammonium and alkylammonium
formates in the presence of palladium. In the future, we
plan to use the polyaryl pyrroles for the synthesis of flat
dendrimers incorporating the pyrrole core.
mp 180–182 8C (lit.14 181 8C). nmax¼3460 cm21
;
1H
NMR (CDCl3): d¼6.50 (d, J¼2.7 Hz, 2H), 7.33 (d,
J¼8.4 Hz, 4H), 7.42 (d, J¼8.4 Hz, 4H), 8.44 (br s, 1H)
ppm; 13C NMR (CDCl3): d¼108.6, 125.0, 129.2, 130.9,
132.3, 132.4 ppm.
4.1.3. 2,5-Di(4-bromophenyl)-1H-pyrrole (3c). Following
the above general procedure, enedione 1c (100 mg,
0.25 mmol) was transformed to pyrrole 3c with ammonium
formate 2a (160 mg, 2.5 mmol) and 5% Pd/C (2 mg) in
PEG-200 (2 mL) under microwave irradiation at 200 W for
1 min. Yield: 81 mg (85%, white solid); mp 212–214 8C
(lit.15 214–216 8C). 13C NMR (CDCl3): d¼108.1, 123.8,
126.4, 128.9, 132.7, 133.1 ppm.
4. Experimental
4.1. General
All reagents and solvents were purchased form E-Merck and
Sisco Chemicals, India. Microwave reactions were carried
out using BPL-Sanyo, India; mono-mode and multi-power

H. S. P. Rao et al. / Tetrahedron 60 (2004) 1625–1630
1629
4.1.4. 2,5-Di(4-methylphenyl)-1H-pyrrole (3d). Follow-
ing the above general procedure, enedione 1d (100 mg,
0.38 mmol) was transformed to pyrrole 3d with ammo-
nium formate 2a (238 mg, 3.8 mmol) and 5% Pd/C
(2 mg) in PEG-200 (2 mL) under microwave irradiation at
200 W for 1.5 min. Yield: 80 mg (85%, white solid);
d¼13.3, 19.3, 32.7, 44.9, 109.3, 126.8, 128.4, 129.0, 134.2,
136.5 ppm.
4.1.10. 1-Benzyl-2,5-diphenyl-1H-pyrrole (3h). Following
the above general procedure, enedione 1a (100 mg,
0.42 mmol) was transformed to pyrrole 3h with benzyl
amine (227 mg, 2.1 mmol), formic acid (0.1 mL, 2.1 mmol)
and 5% Pd/C (2 mg) in PEG-200 (2 mL) under microwave
irradiation at 200 W for 2 min. Yield: 81 mg, (63%); mp
144–146 8C (lit.17 144 8C).
1
mp 202–204 8C (lit.14 203 8C). H NMR (CDCl3): d¼2.35
(s, 6H), 6.45 (br s, 2H), 7.14 (d, J¼7.8 Hz, 4H), 7.37
(br d, J¼7.8 Hz, 4H), 8.42 (s, br, 1H) ppm; 13C NMR
(CDCl3): d¼21.3, 107.5, 123.8, 130.0, 132.0, 132.9,
135.7 ppm.
4.1.11. 1,2,5-Triphenyl-1H-pyrrole (3i). Following the
above general procedure, enedione 1a (100 mg, 0.42 mmol)
was transformed to pyrrole 3i with aniline (197 mg,
2.1 mmol), formic acid (0.1 mL, 2.1 mmol) and 5% Pd/C
(2 mg) in PEG-200 (2 mL) under microwave irradiation at
200 W for 2 min. Yield: 75 mg, (60%); mp 228–230 8C
(lit.18 229 8C).
4.1.5. 2,5-Di(4-methoxyphenyl)-1H-pyrrole (3e). Follow-
ing the above general procedure, enedione 1e (100 mg,
0.34 mmol) was transformed to pyrrole 3e with ammonium
formate 2a (213 mg, 3.4 mmol) and 5% Pd/C (2 mg) in
PEG-200 (2 mL) under microwave irradiation at 200 W for
2 min. Yield: 84 mg (89%, white solid); mp 230–232 8C
(lit.12 232 8C).
4.1.12. 1,2,3,5-Tetraphenyl-1H-pyrrole (6b). Following
the above general procedure, enedione 5a (624 mg, 2 mmol)
was transformed to pyrrole 6b with aniline (931 mg,
1 mmol), formic acid (0.45 mL, 10 mmol) and 5% Pd/C
(5 mg) in PEG-200 (3 mL) under microwave irradiation at
155 W for 8 min. Yield: 485 mg, (65%); mp 200–202 8C
(lit.21 200–1 8C).
4.1.6. 2,5-Di(4-chloro-3-methylphenyl)-1H-pyrrole (3f).
Following the above general procedure, enedione 1f
(100 mg, 0.3 mmol) was transformed to pyrrole 3f with
ammonium formate 2a (189 mg, 3.0 mmol) and 5% Pd/C
(2 mg) in PEG-200 (2 mL) under microwave irradiation at
200 W for 2 min. Yield: 80 mg (84%, white solid); mp 190–
192 8C (lit.11 190 8C).
4.1.13. General procedure for the synthesis of di and
triarylpyrroles from ynediones: synthesis of 2,5-di-
phenyl-1H-pyrrole (3a). A 25 mL conical flask, charged
with ynedione 4a (100 mg, 0.43 mmol), ammonium formate
(269 mg, 4.3 mmol), 5% Pd/C (2 mg) and PEG-200 (2 mL),
was irradiated in the microwave oven at 200 W for 30 s.
After completion of the reaction (TLC) the cooled (rt)
reaction mixture was loaded on a short column of silica
(5 cm£1 cm) and eluted with hexanes/DCM (90:10 to
50:50). Removal of solvent from pooled fractions resulted
in pyrrole 3a as the white solid (89 mg, 95%) after removal
of the solvent.
4.1.7. 2,3,5-Triphenyl-1H-pyrrole (6a). Following the
above general procedure, enedione 5a (1000 mg,
3.2 mmol) was transformed to pyrrole 6a with ammonium
formate 2a (2019 mg, 32 mmol) and 5% Pd/C (5 mg) in
PEG-200 (5 mL) under microwave irradiation at 200 W for
2 min. Yield: 901 mg (95%, white solid); mp 138–140 8C
(lit.20 135–137 8C).
4.1.8. 2,3,4,5-Tetraphenyl-1H-pyrrole (6c). Following the
above general procedure, enedione 5a (500 mg, 1.29 mmol)
was transformed to pyrrole 6c with ammonium formate 2a
(812 mg, 12.9 mmol) and 5% Pd/C (5 mg) in PEG-200
(5 mL) under microwave irradiation at 200 W for 7 min.
Yield: 453 mg (95%, white solid); mp 212–214 8C (lit.22
214–216 8C). 13C NMR (CDCl3): d¼123.4, 126.1, 126.7,
127.2, 128.0, 128.2, 128.6, 128.9, 130.1, 131.0, 132.9,
135.4, 138.6 ppm.
4.1.14. 2,5-Di(4-chlorophenyl)-1H-pyrrole (3b). Follow-
ing the above general procedure, ynedione 4b (100 mg,
0.33 mmol) was transformed to pyrrole 3b with ammonium
formate (207 mg, 3.3 mmol) and 5% Pd/C (2 mg) in PEG-
200 (2 mL) under microwave irradiation at 200 W for 60 s.
Yield: 87 mg (92%).
4.1.9. 1-Butyl-2,5-diphenyl-1H-pyrrole (3g). To butyl-
amine (155 mg, 2.1 mmol) and formic acid (0.1 mL,
2.1 mmol) were taken in a 25 mL conical flask at 5 8C,
enedione 1a (100 mg, 0.42 mmol), 5% Pd/C (2 mg) and
PEG-200 (2 mL), were added, This reaction mixture was
irradiated by microwaves in the microwave oven at 200 W
for 2 min. After completion of the reaction (TLC), the
mixture was cooled to room temperature and chromato-
graphed on silica (5 cm£1 cm) using hexanes/DCM as the
eluent. Removal of solvent from pooled fractions yielded
pyrrole 3g as white solid (65 mg, 56%). An analytically pure
sample was obtained by recrystallization from DCM/
4.1.15. 2,5-Di(4-bromophenyl)-1H-pyrrole (3c). Follow-
ing the above general procedure, ynedione 4c (100 mg,
0.26 mmol) was transformed to pyrrole 3c with ammonium
formate (161 mg, 2.6 mmol) and 5% Pd/C (2 mg) in PEG-
200 (2 mL) under microwave irradiation at 200 W for 60 s.
Yield: 90 mg (94%).
4.1.16. 2,5-Di(4-methylphenyl)-1H-pyrrole (3d). Follow-
ing the above general procedure, ynedione 4d (100 mg,
0.38 mmol) was transformed to pyrrole 3d with ammonium
formate (240 mg, 3.8 mmol) and 5% Pd/C (2 mg) in PEG-
200 (2 mL) under microwave irradiation at 200 W for 60 s.
Yield: 85 mg (90%).
1
hexanes (2:98); mp 112–114 8C (lit.16 113 8C). H NMR
(CDCl3): d¼0.50 (t, J¼7.3 Hz, 3H), 0.81 (sextet, J¼7.3 Hz,
2H), 1.14 (pentet, J¼7.3 Hz, 2H), 4.03 (t, J¼7.3 Hz, 2H),
6.22 (s, 2H), 7.27 (br t, J¼7.2 Hz, 2H), 7.37 (br t, J¼7.2 Hz,
4H), 7.42 (br d, J¼7.2 Hz, 4H) ppm; 13C NMR (CDCl3):
4.1.17. 2,5-Di(4-methoxyphenyl)-1H-pyrrole (3e). Fol-
lowing the above general procedure, ynedione 4e

1630
H. S. P. Rao et al. / Tetrahedron 60 (2004) 1625–1630
Comprehensive heterocyclic chemistry; Katritzky, A. R.,
Rees, C. W., Eds.; Pergamon: Oxford, 1984; Vol. 4, p 370.
(c) Sundberg, R. J. Comprehensive heterocyclic chemistry II;
Katritzky, A. R., Rees, C. W., Eds.; Pergamon: Oxford, 1996;
Vol. 2, p 149. (d) Boger, D. L.; Boyce, C. W.; Labrili, M. A.;
Sehon, C. A.; Jin, Q. J. Am. Chem. Soc. 1999, 121, 54, and
references cited therein.
(100 mg, 0.34 mmol) was transformed to pyrrole 3e with
ammonium formate (214 mg, 3.4 mmol) and 5% Pd/C
(2 mg) in PEG-200 (2 mL) under microwave irradiation at
200 W for 60 s. Yield: 86 mg (91%).
4.1.18. 2,5-Di(4-chloro-3-methylphenyl)-1H-pyrrole (3f).
Following the above general procedure, ynedione 4f
(100 mg, 0.3 mmol) was transformed to pyrrole 3f with
ammonium formate (190 mg, 3 mmol) and 5% Pd/C (2 mg)
in PEG-200 (2 mL) under microwave irradiation at 200 W
for 60 s. Yield: 86 mg (90%).
2. The chemistry of heterocyclic compounds; Jones, R. A., Ed.;
Wiley: Toronto, 1990; part 1 and 2.
3. (a) Tietze, L. F.; Nordmann, G. Synlett 2001, 337.
(b) Groenendaal, L.; Meijer, E.-W.; Vekemans, J. A. J. M.
In Electronic materials: the oligomer approach; Mu¨llen, K.,
Wegner, G., Eds.; Wiley-VCH: Weinheim, 1997.
4. Sakon, Y.; Ohnuma, T.; Hashimoto, M.; Saito, S.; Tsutsui, T.;
Adachi, C. US Patent 5,077,142 A 19,911,231; 1991 CA
117:16862.
4.1.19. 1-Butyl-2,5-diphenyl-1H-pyrrole (3g). To butyl
amine (155.9 mg, 2.13 mmol) and formic acid (0.1 mL,
2.13 mmol) taken in a 25 mL conical flask at 5 8C, ynedione
4a (100 mg, 0.43 mmol), 5% Pd/C (2 mg) and PEG-200
(2 mL) were added. This reaction mixture was irradiated by
microwaves in the microwave oven at 200 W for 60 s. After
completion of the reaction (TLC) the mixture was cooled to
room temperature and chromatographed on silica
(5 cm£1 cm) using hexanes/DCM as the eluent. Removal
of solvent from pooled fraction yielded pyrrole 3g as white
solid (70 mg, 60%).
5. (a) Paal, C. Chem. Ber. 1884, 17, 2756. (b) Knorr, L. Chem.
Ber. 1884, 17, 2863.
6. (a) Conant, J. B.; Lutz, R. E. J. Am. Chem. Soc. 1923, 45, 1303.
(b) Conant, J. B.; Lutz, R. E. J. Am. Chem. Soc. 1925, 47, 881.
(c) Coampaigne, E.; Foye, W. O. J. Org. Chem. 1952, 17,
1405.
7. Demirdji, S. H.; Haddadin, M. J.; Issidorides, C. H.
J. Heterocycl. Chem. 1985, 22, 495.
4.1.20. 1-Benzyl-2,5-diphenyl-1H-pyrrole (3h). Following
the above general procedure, ynedione 4a (100 mg,
0.43 mmol) was transformed to pyrrole 3h with benzyl
amine (228.6 mg, 2.13 mmol), formic acid (0.1 mL,
2.13 mmol) and 5% Pd/C (2 mg) in PEG-200 (2 mL)
under microwave irradiation at 200 W for 60 s. Yield:
80 mg, (61%).
8. Wilson, R. M.; Hengge, A. C.; Ataei, A.; Ho, D. M. J. Am.
Chem. Soc. 1991, 113, 7240.
9. Zhang, J.-J.; Schuster, G. B. J. Am. Chem. Soc. 1989, 111,
7149.
10. (a) Ram, S.; Ehrenkaufer, R. E. Synthesis 1988, 91. (b) Rao,
H. S. P.; Reddy, K. S. Tetrahedron Lett. 1994, 35, 171.
11. Rao, H. S. P.; Jothilingam, S. Tetrahedron Lett. 2001, 42,
6595.
4.1.21. 1,2,5-Triphenyl-1H-pyrrole (3i). Following the
above general procedure, ynedione 4a (100 mg, 0.43 mmol)
was transformed to pyrrole 3i with aniline (198.7 mg,
2.13 mmol), formic acid (0.1 mL, 2.13 mmol) and 5% Pd/C
(2 mg) in PEG-200 (2 mL) under microwave irradiation at
200 W for 60 s. Yield: 82 mg, (65%).
12. Amarnath, V.; Anthony, D. C.; Amarnath, K.; Valentine,
W. M.; Wetterau, L. A.; Graham, D. G. J. Org. Chem. 1991,
56, 6924.
13. Patterson, J. N.; Soedigdo, S. J. Org. Chem. 1968, 33, 2057.
14. Tsuge, O.; Tashiro, M.; Hokama, K.; Yamada, K. Kogyo
Kagaku Zasshi 1968, 71, 1667. CA 70:37587.
15. Alper, H.; Prickett, J. E. Inorg. Chem. 1977, 16, 67.
16. Schulte, K. E.; Reisch, J.; Walker, H. Chem. Ber. 1965, 98, 98.
17. Alickmann, D.; Frohlich, R.; Maulitz, A. H.; Wurthwein, E.
Eur. J. Org. Chem. 2002, 1523.
Acknowledgements
S. J. thanks CSIR for a fellowship. H. S. P. R. thanks the
UGC-SAP program and CSIR, India, for financial support.
We thank Professor A. Srikrishna, for spectra and helpful
discussions. We also thank RSIC, IITM, Chennai and SIF,
IISc, Bangalore, for recording the spectra.
18. Periasamy, M.; Srinivas, G.; Bharathi, P. J. Org. Chem. 1999,
64, 4204.
19. Chiu, P. K.; Samme, M. P. Tetrahedron 1988, 44, 3531.
20. Furstner, A.; Weintritt, H.; Hupperts, A. J. Org. Chem. 1995,
60, 6637.
21. Lee, C.; Yang, L.; Hwu, T.; Feng, A.; Tseng, J.; Luh, T. J. Am.
Chem. Soc. 2000, 122, 4992.
22. Sera, A.; Fukumoto, S.; Yoneda, T.; Yamada, H. Heterocycles
1986, 24, 697.
References and notes
1. (a) Jones, R. A.; Bean, G. P. The chemistry of pyrroles;
Academic: London, 1977; pp 1–5. (b) Sundberg, R. J.
23. Hong, P.; Yamazaki, H. J. Organomet. Chem. 1989, 373, 133.
24. Kuhnert, N. Angew Chem. Int. Ed. 2002, 41, 1863.