Palladium(0) catalyzed 2,2′-bipyrrole syntheses

Article abstract of DOI:10.1142/S1088424611003355

Several 2-iodopyrroles are used in Pd(0) catalyzed homocoupling reactions at room temperature in the presence of water to efficiently synthesize 2,2′-bipyrroles. These 2,2′-bipyrroles are strongly luminous materials and have high fluorescence quantum yiel

Full text of DOI:10.1142/S1088424611003355

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2011; 15: 433–440
DOI: 10.1142/S1088424611003355
Published at http://www.worldscinet.com/jpp/
Palladium(0) catalyzed 2,2′-bipyrrole syntheses
Lijuan Jiaoa,b, Erhong Haob, Frank R. Fronczeka, M. Graça H. Vicentea and
Kevin M. Smith*a
a Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA
b Anhui Key Laboratory of Functional Molecular Solids and Anhui Key Laboratory of Molecular Based Materials
College of Chemistry and Material Science, Anhui Normal University, Wuhu, Anhui 241000, China
Dedicated to Professor John A. Shelnutt on the occasion of his 65th birthday
Received 31 March 2011
Accepted 13 May 2011
ABSTRACT: Several 2-iodopyrroles are used in Pd(0) catalyzed homocoupling reactions at room
temperature in the presence of water to efficiently synthesize 2,2′-bipyrroles. These 2,2′-bipyrroles are
strongly luminous materials and have high fluorescence quantum yields.
KEYWORDS: bipyrrole, homocoupling, iodopyrrole, palladium.
stoichiometric amounts of the copper catalyst, and spe-
INTRODUCTION
cific requirements for successful substrates, e.g. at least
The bipyrrole motif occurs in natural products,
one electron-withdrawing group. Oxidative coupling [6]
pigments, and in porphyrin mimics. They are also
requires substoichiometric amounts of the Pd(II) catalyst,
well-known common components in molecular recogni-
special substrates, pyrrolinones or bipyrroles, and tedious
tion and self-assembly processes [1]. Recently, bipyrrole-
purification procedures associated with poor regioselec-
containing macrocycles have attracted increased attention
tivity of the reaction.
as therapeutic agents in combination with visible light
Recently, several improvements had been made in
for the treatment of tumors, especially in connection with
extended pyrrole systems [2] such as porphycenes [3, 4]
2,2′-bipyrrole syntheses [6, 7, 8d, 8e], but each has its
own limitations. Kita [6] and coworker used PIFA and
and sapphyrins [5]. However, a key potential synthetic
TMSBr as assisting reagents to improve the oxidative
precursor for these compounds, namely 2,2′-bipyrroles,
coupling, but the poor regioselectivity problem remains.
have only limited synthetic methods available [4–7].
Sessler [7a] and coworker protected the pyrrole nitrogen
The Ullmann coupling and oxidative coupling are
to improve the Ullmann coupling and Vogel [7b] further
the two most common synthetic methods [4–7]: Ull-
improved the method by substitution of DMF with tol-
mann coupling has found broad synthetic application
uene as a solvent; however, high temperatures are still
in the generation of biaryls [8a] and bipyrroles [4, 5, 7];
required and some functional groups can not survive.
oxidative coupling has been used for the generation of
Liebeskind [8c] and coworkers improved the Ullmann
2,2′-bipyrroles possessing electron-rich substituents [6].
However, only a limited array of 2,2′-bipyrroles can be
coupling by performing it at ambient temperature, but as
the authors stated “the most noticeable limitation of this
accessed using these two methods due, but not limited
process is the lack of aromatic halide substrates not pos-
to, the harsh reaction conditions: Ullmann coupling
typically requires high temperature (T ≥ 200 °C) [8b],
sessing a coordinating ortho-substituent”.
As part of our efforts to improve the porphycene syn-
thesis [9], an improved synthetic route for 2,2′-bipyrrole
was developed. We were attracted to the wide usage of
SPP full member in good standing
Pd catalysts in coupling reactions to form carbon-carbon
bonds [10a-c], such as the Stille [10b], Suzuki [10c],
*Correspondence to: Kevin M. Smith, email: kmsmith@lsu.
edu, tel: +1 225-578-7442
Copyright © 2011 World Scientific Publishing Company

434
L. JIaO et al.
Heck [10d], and Sonogashira [10e] couplings, and thus
introduced the Pd(0) catalyzed homo-coupling reaction
into 2,2′-bipyrrole syntheses. Although Pd-C has been
previously used as a catalyst in the Ullmann coupling,
phase-transfer reagents and 100 °C reflux conditions are
required [10]. Inspired by recent improved syntheses of
biaryls through Pd(0) catalyzed couplings at ambient tem-
perature [11], our improved syntheses reduced the syn-
thetic steps for most of the 5,5′-diformyl-2,2′-bipyrroles
studied [12] by using Pd-C and zinc as co-catalyst with
the presence of water. Considering the substituent array,
and especially those at the 2- or 5-position of pyrroles,
generally affect both the Ullmann and oxidative coupling
reactions, the versatility of this reaction and the photo-
physical properties of the resulting 2,2′-bipyrroles are
now discussed in the present paper.
where ΦS is the reported quantum yield of the standard, I
is the integrated emission spectra, A is the absorbance at
the excitation wavelength and η is the refractive index of
the solvent being used (η = 1 when the same solvent was
used for both the test sample and the standard). Subscript
X indicates the test sample, and subscript S the standard.
General procedure for the preparation of 2-iodopy-
rroles
2-iodopyrroles were obtained from successive deben-
zylation and iodination reactions of pyrrole esters. Deben-
zylation was performed using hydrogen and 6 mmol.%
Pd-C as catalyst in freshly distilled dry THF. Before add-
ing pyrrole, the catalyst was activated by stirring it under
a hydrogen atmosphere for 20 min. After completion of
the reaction, the mixture was passed through a Celite
plug and washed with THF several times. After evapora-
tion of the solvent under vacuum, the resulting pyrrole
carboxylic acid was used directly for the iodination pro-
cess, in a mixture of 1,2-dichloroethane and saturated
aqueous NaHCO3 under reflux with a mixture of I2/KI
as the iodination reagent. It usually took 1 h to complete
the reaction. After cooling to room temperature, sodium
thiosulfate was added in small portions before pouring
the mixture into a separatory funeral. The organic layer
was collected and the solvent was removed under vac-
uum. The resulting solid was purified by recrystallization
from methanol.
EXPERIMENTAL
General
Silica gel (32-63 µm) was used for flash column chro-
matography. All reactions were monitored by TLC using
0.25 mm silica gel plates with (60F-254) or without UV
1
indicator. H and 13C NMR spectra were obtained on
either a DPX-250 or a ARX-300 Bruker spectrometer.
Chemical shifts (δ) are given in ppm relative to CDCl3 as
indicated in each spectra. Mass spectra were obtained on
an Applied Biosystems QSTAR XL. All solvents (HPLC
grade) were obtained from Fisher Scientific (Houston,
TX) and used without further purification unless oth-
erwise indicated. Acetone (reagent plus, phenol free,
≥99.5%) and toluene (HPLC grade) were purchased
from Sigma-Aldrich. Palladium, 10% on activated car-
bon was purchased from Strem Chemicals. Zinc dust,
<10 micro, 98+%, was purchased from Sigma-Aldrich
and actived according to the literature [12]. Monopyr-
roles used to make 2-iodopyrroles were synthesized
according to the literature [13]. UV-visible absorption
spectra were recorded on a Hitachi U-3010 Spectro-
photometer (190-1100 nm scan range). Fluorescence
emission spectra were recorded on a Hitachi F-4600 FL
Spectrophotometer. The slit widths were 2.5 nm and 5.0
nm for excitation and emission respectively. Fluorescent
quantum yields were calculated using a secondary stan-
dard method by comparing the areas under the corrected
emission spectrum of the test sample in various solvents
with that of the standard. Dilute solutions (0.01 < A <
0.05) were used to minimize reabsorption effects and
correction for the solvent refractive index was applied.
Quinine sulfate in 5% H2SO4 solution (ΦF = 0.56) [14]
was used as a fluorescence standard for the fluorescence
quantum yield calculations according to the absorption of
the test sample. Quantum yields were determined using
the following equation [14]:
Diethyl 3-(2-ethoxyethyl)-5-iodopyrrole-2,4-dicar-
boxylate (1a) [13c]. 83% yield. mp 148–150 °C {lit [13c]
1
mp 150 °C}. H NMR (250 MHz, CDCl3): δ, ppm 9.30
(s, 1H), 4.40–4.33 (m, 4H), 3.55–3.52 (m, 2H), 3.44–3.41
(m, 2H), 3.37 (s, 3H), 1.43–1.36 (m, 6H). ESI-MS: m/z
calcd. for C14H20INO4, 393.04, found: 392.90.
Benzyl 5-iodo-3,4-dimethylpyrrole-2-carboxylate (1c)
[13d]. 81% yield. mp 127–129 °C {lit [13d] mp 126-
1
128 °C}. H NMR (250 MHz, CDCl3): δ, ppm 8.80 (s,
1H), 7.47-7.38 (m, 5 H), 5.34 (s, 2H), 2.35 (s, 3H), 2.00
(s, 3H). ESI-MS: m/z calcd. for [C14H14INO2], 355.01,
found: 355.10.
Benzyl 5-iodo-3-methyl-4-propylpyrrole-2-carbo-
xylate (1e) [13b]. 79% yield. mp 118–120 °C {lit [13b]
mp 119–120 °C}. 1H NMR (250 MHz, CDCl3): δ, ppm
9.01 (s, 1H), 7.45–7.34 (m, 5H), 5.32 (s, 2H), 2.37–
2.32 (m, 5H), 1.52–1.43 (m, 2H), 0.97–0.91 (m, 3H).
13C NMR (250 MHz, CDCl3): δ, ppm 160.77, 136.58,
131.23, 129.02, 128.66, 127.66, 123.82, 74.13, 66.33,
28.86, 23.84, 14.27, 11.40. ESI-MS: m/z calcd. for
[C16H18INO2], 383.22, found: 383.29.
Benzyl 5-iodo-3-(2-methoxycarbonylethyl)-4-methyl-
pyrrole-2-carboxylate(1f)[13a]. 87%yield.mp110–112°C
{lit [13a] mp 110–111 °C}. 1H NMR (250 MHz, CDCl3):
δ, ppm 7.38 (m, 5H), 5.30 (s, 2H), 3.64 (s, 3H), 3.10–
3.04 (m, 2H), 2.53-2.47 (m, 2H), 2.00 (s, 3H). ESI-MS:
m/z calcd. for [C17H19INO2 + Na]+, 450.03, found:
449.97.
ΦX = ΦS (IX/IS) (AS/AX) (ηX/ηS)
(1)
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PaLLaDIum(0) CataLyzeD 2,2′-bIPyrrOLe SyntheSeS
435
tert-butyl3-ethyl-5-iodo-4-methylpyrrole-2-carboxy-
late (1g) [13c]. 77% yield. mp 105–107 °C {lit [13c] mp
105–107 °C}. 1H NMR (250 MHz, CDCl3): δ, ppm 9.49
(s, 1H), 3.68 (s, 3H), 2.54–2.47 (m, 2H), 1.97 (s, 3H),
1.56 (s, 9H). ESI-MS: m/z calcd. for C14H20INO4, 393.04,
found: 392.90.
m/z calcd. for C28H29N2O4 [M + H]+, 457.2127, found
457.2124.
Dibenzyl 3,3′-diethyl-4,4′-dimethyl-2,2′-bipyrrole-
5,5′-dicarboxylate (2d). After recrystallization from
DCM/hexane and washing with hexane (2 × 2 mL),
pure 2d was obtained as a milk-white solid in 77% yield
1
(92.2 mg, 0.19 mmol). H NMR (400 MHz, CDCl3): δ,
General procedure for the reductive coupling of
2-iodopyrroles
ppm 9.65 (s, 2H), 7.98 (s, 10H), 5.85 (s, 2H), 3.06–2.94
(m, 10H), 1.65 (s, 6H). 13C NMR (300 MHz, CDCl3):
δ, ppm 161.35, 136.23, 128.50, 128.02, 127.08, 126.82,
124.18, 119.04, 65.74, 17.69, 15.42, 10.59. HRMS (ESI):
m/z calcd. for C30H33N2O4 [M + H]+, 485.2440, found
485.2444.
Zinc metal was activated by washing with 3 M HCl.
After filtering through filter paper and successive wash-
ing with water, ethanol, and diethyl ether, it was dried
under vacuum. Pd-C (4 mmol.%) and activated Zn pow-
der (1.5 mmol) were placed in a dry 50 mL round bot-
tom flask. After removing air under vacuum, the flask
was filled with argon. Then 2-iodopyrrole (0.5 mmol)
was dissolved in 10 mL acetone or toluene, and added
through a syringe into the reaction flask. Finally, 10 mL
distilled water was added into the reaction mixture before
stirring at room temperature under argon. TLC was used
to monitor the reaction. Upon the completion of the reac-
tion, THF (20 mL) was added and the mixture was fil-
tered through a plug of Celite. After removing the THF
under vacuum, the mixture was poured into a separatory
funnel and DCM (50 mL) was added. The organic layer
was collected and dried under vacuum. The resulting
solid was purified either by recrystalization from ethanol
or by using column separation.
Dibenzyl
4,4′-bis(2-methoxycarbonylethyl)-3,3′-
dimethyl-2,2′-bipyrrole-5,5′-dicarboxylate (2e). After
recrystallization from DCM/hexane and washing with
hexane (2 × 2 mL), pure 2e was obtained as a milk-
white solid in 48% yield (72.1 mg, 0.12 mmol). 1H NMR
(250 MHz, CDCl3): δ, ppm 9.21 (s, 2H), 7.37–7.33 (m,
10H), 5.23 (s, 4H), 3.63 (s, 6H), 3.07–3.01 (m, 4H),
2.54–2.47 (m, 4H), 2.05–2.00 (m, 6H). 13C NMR (250
MHz, CDCl3): δ, ppm 173.89, 161.28, 136.28, 130.82,
128.98, 128.75, 128.66, 125.30, 120.14, 119.45, 66.56,
51.91, 35.07, 21.11, 10.16. HRMS (ESI): m/z calcd. for
C34H37N2O8 [M + H]+, 601.2550, found 601.2540.
Dibenzyl 4,4′-dimethyl-3,3′-dipropyl-2,2′-bipyrrole-
5,5′-dicarboxylate (2f). After recrystallizing from
MeOH/DCM and washing with hexane (2 × 2 mL),
pure 2f was obtained as a milk-white solid in 52% yield
Diethyl
3,3′,4,4′-tetramethyl-2,2′-bipyrrole-5,5′-
1
(66.7 mg, 0.13 mmol). H NMR (250 MHz, CDCl3):
dicarboxylate (2a). After separating on a silica gel col-
umn using a mixture of ethyl acetate and hexane as elut-
ing solvent, pure 2a was obtained as a milk-white solid
in 15% yield (13.3 mg, 0.04 mmol). 1H NMR (250 MHz,
CDCl3): δ, ppm 8.97 (s, 2H), 4.34–4.25 (m, 4H), 2.31
(s, 6H), 2.05 (s, 6H), 1.38–1.32 (m, 6H). 13C NMR (250
MHz, CDCl3): δ, ppm 161.70, 127.42, 124.55, 119.60,
119.25, 60.02, 14.47, 10.51, 9.87. MALDI-TOF MS: m/z
calcd. for C18H24N2O4, 332.17, found: 332.34.
δ, ppm 8.73 (s, 2H), 7.44–7.33 (m, 10H), 5.31 (s, 4H),
2.44–2.33 (m, 10H), 1.49–1.39 (m, 4H), 0.89–0.83
(m, 6H). 13C NMR (250 MHz, CDCl3): δ, ppm 161.09,
136.24, 128.56, 128.13, 127.64, 124.85, 124.44, 118.90,
65.80, 29.70, 26.56, 24.07, 13.95, 10.65. HRMS (ESI):
m/z calcd. for C32H37N2O4 [M + H]+, 513.2753, found
513.2759.
Di-tert-butyl 4,4′-diethyl-3,3′-dimethyl-2,2′-bipyrrole-
5,5′-dicarboxylate (2g). After separating by using a sil-
ica gel column eluted with ethyl acetate and hexane, pure
2g was obtained as a milk-white solid in 32% yield (33.4
Tetraethyl 4,4′-di(2-ethoxyethyl)-2,2′-bipyrrole-3,3′,
5,5′-tetracarboxylate (2b). After recrystallization from
DCM/hexane and washing with hexane (2 × 2 mL), pure
2b was obtained as a milk-white solid in 45% yield (59.0
1
mg, 0.08 mmol). H NMR (250 MHz, CDCl3): δ, ppm
1
8.66 (s, 2H), 2.81–2.72 (m, 4H), 2.05 (s, 6H), 1.58 (s,
18H), 1.19–1.13 (m, 6H). 13C NMR (250 MHz, CDCl3):
δ, ppm 161.37, 133.51, 124.49, 120.30, 118.95, 81.10,
28.85, 18.84, 15.52, 10.08. HRMS (ESI): m/z calcd. for
C24H37N2O4 [M + H]+, 417.2753, found 417.2737.
mg, 0.11 mmol). H NMR (250 MHz, CDCl3): δ, ppm
9.42 (s, 2H), 4.49–4.37 (m, 8H), 3.54–3.48 (s, 8H), 3.38
(s, 6H), 1.47–1.40 (s, 12H). FAB MS: m/z calcd. for
C26H36N2O10, 536.24, found: 536.30. MALDI-TOF MS:
m/z calcd. for C26H36N2O10, 536.24, found: 536.41.
Di-tert-butyl 4,4′-bis(2-methoxycarbonylethyl)-3,3′-
dimethyl-2,2′-bipyrrole-5,5′-dicarboxylate (2h). After
separating by using a silica gel column eluted with ethyl
acetate and hexane, pure 2h was obtained as a milk-white
solid in 34% (45.3 mg, 0.09 mmol). 1H NMR (250 MHz,
CDCl3): δ, ppm 8.75 (s, 2H), 3.67 (s, 6H), 3.08-3.01 (m,
Dibenzyl 3,3′,4,4′-tetramethyl-2,2′-bipyrrole-5,5′-
dicarboxylate (2c). After recrystallization from MeOH/
DCM and washing with hexane (2 × 2 mL), pure 2c was
obtained as a milk-white solid in 76% yield (86.8 mg,
0.19 mmol). mp 218-20 °C. 1H NMR (250 MHz, CDCl3):
δ, ppm 9.08 (s, 2H), 7.39–7.34 (m, 10 H), 5.25 (s, 4H),
2.31 (s, 6H), 2.02 (s, 6H). 13C NMR (250 MHz, CDCl3):
δ, ppm 161.42, 136.25, 128.51, 128.07, 128.03, 127.90,
124.90, 119.87, 119.00, 65.78, 10.67, 9.89. HRMS (ESI):
2H), 2.60–2.54 (m, 2H), 2.05 (s, 6H), 1.56 (s, 18H). 13
C
NMR (250 MHz, CDCl3): δ, ppm 173.59, 160.66, 128.92,
123.98, 120.55, 119.12, 81.12, 51.48, 34.90, 29.67, 20.77,
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436
L. JIaO et al.
9.68. HRMS (ESI): m/z calcd. for C28H41N2O8 [M + H]+,
533.2862, found 533.2854.
RESULTS AND DISCUSSION
2-iodopyrroles 1a–h were synthesized from monopyr-
roles [13] in gram scale using literature procedures [15],
from which the desired 2,2′-bipyrroles were synthesized
based on a Pd(0) catalyzed homocoupling reaction under
argon (Table 1). Pd-C and zinc metal were used as co-
catalysts, a mixture of acetone/water was used as sol-
vent and the product yields are summarized in Table 1.
Substituents, especially those adjacent to the pyrrole
nitrogen atom, greatly affected the coupling reaction:
2-iodopyrroles 1c–f (R3 = PhCH2O2C) gave the high-
est yields of the desired 2,2′-bipyrroles (2c–f) while the
other 2-idopyrroles (1a–b and 1g–h) gave a lower yield
of 2,2′-bipyrrole with mainly the dehalogenated pyrrole
(such as 3b) as a by-product.
During an effort to improve the yields of (2a–b) and
(2g–h), the solvent was found to play an important role in
the coupling reaction. When a mixture of toluene/water
was used as solvent (Table 2), the major product from
the 2-iodopyrrole (1b) coupling changed from the deha-
logenated pyrrole (3b) to the desired 2,2′-bipyrrole (2b).
Benzene has been reported to stabilize the Pd(II) interme-
diate during the coupling, thus it is also possible for the
phenyl group of the 2-iodopyrroles benzyl esters to stabi-
lize the Pd(0) intermediate in our system [16]: 2-iodopy-
rroles (1c–f) (R3 = PhCH2O2C) most likely stabilized the
Pd(0) intermediate, and thus higher yields were obtained.
Among those 2-iodopyrroles with a PhCH2O2C attached
X-ray crystallographic data
For 2c: C28H28N2O4, triclinic space group P-1, a =
9.4902(12), b = 10.4909(15), c = 12.485(2) Å, α =
75.282(7), β = 85.399(8), γ = 79.163(8)°, V = 1180.1(3)Å3,
Z = 2, ρcalcd = 1.285 g cm-3, µ(MoKα) = 0.086 mm-1, R =
0.083 for 3940 data with Fo2 > 2σ(Fo2) of 4658 unique data
and 380 refined parameters. A total of 26,411 data was col-
lected at T = 90.0(5) K to θ = 26.1° with MoKα radiation
(λ = 0.71073 Å) on a Nonius KappaCCD diffractometer,
using a colorless crystal of dimensions 0.32 × 0.23 × 0.10
mm. Refinement was by full-matrix least squares using
SHELXL, with H atoms in idealized positions, except for
NH hydrogen atoms, which were located by difference
maps and their positions refined. Both phenyl groups are
rotationally disordered, each having a conformer with 63%
occupancy and another with occupancy 37%.
Crystallographic data have been deposited at the Cam-
bridge Crystallographic Data Center (CCDC) under depo-
sition number CCDC 817869. Copies can be obtained on
request, free of charge, via www.ccdc.cam.ac.uk/conts/
retrieving.html or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: +44 1223-336-033 or email: deposit@ccdc.cam.
ac.uk).
Table 1. 2,2′-bipyrroles 2a–h from reductive coupling of 2-idopyrroles under acetone/water
conditions
R1 R1
R2
R3
R1
R2
R3
R2
R3
Pd-C, Zn0
I
acetone/water
ambient temperature
N
N
N
H
H
H
(1)
(2)
O
O
O
O
R3 =
O
O
a,b
c-f
g,h
Compound 1
R1
R2
Compound 2
Yield, %a
1a
1b
1c
1d
1e
1f
Me
Me
2a
2b
2c
2d
2e
2f
4
CH2CH2OMe
CO2Me
5
Me
Et
Me
75
78
49
51
7
Me
Me
Pr
Me
Me
Me
CH2CH2CO2Me
Et
1g
1h
2g
2h
CH2CH2CO2Me
12
a Reaction conditions: 10% Pd-C/Zn, acetone/water, ambient temperature.
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PaLLaDIum(0) CataLyzeD 2,2′-bIPyrrOLe SyntheSeS
437
at the 5-position, (1e) gave a relatively low yield, pos-
sibly due to steric hindrance from the 3-propyl group of
the 2-iodopyrrole.
45% within 6 h. However, in a toluene/water system, the
conversion rate of 1b was lower, with only 50% conver-
sion being achieved. Since the completion of the Ullmann
coupling for the generation of 2b requires more than two
days, extending the reaction time was expected to be help-
ful for increasing the conversion of 1b in toluene/water.
However, little improvement was observed even when
the reaction time was extended to 2 d. Water has been
reported [17] to be able to initiate or promote room tem-
perature reactivity, possibly by decreasing of the reaction
barrier. To test our hypothesis regarding toluene, toluene/
water was also used for 1a–h, as shown in Table 3.
It is also worth mentioning that water plays important
roles in the coupling reaction. As shown in Table 2, when
no water was added, the coupling of 2-iodopyrrole (1b)
gave no product, and 1b was recovered quantitatively
even when the reaction time was extended to 4 d. How-
ever, after adding 0.1% water into the reaction mixture,
within several hours most of 1b was consumed: in the
acetone system, most of 1b was converted into 2b within
2 h. In toluene, the yield of 2b was increased from 5% to
Table 2. Solvent effects on the coupling reaction
O
O
CO Et
CO2Et
N
2
EtO2C
H
O
O
EtO2C
I
Pd-C, Zn0
Pd-C, Zn0
CO2Et
(3b)
acetone/water
CO2Et
toluene/water
N
H
N
EtO2C
CO2Et
N
H
H
H
(1b)
(2b)
Entry
Solvents
Solvent ratio
Reaction times, h
Yield, %
2b
3b
1
2
3
4
5
acetone/water
acetone/water
acetone
1/1
1000/1
—
1
1
5
6
83
80
0
168
168
6
0
toluene
—
0
0
toluene/water
1/1
45
5
Table 3. Reductive coupling of 2-idopyrroles under toluene/water conditions
R1
R1
R2
R3
R1
R2
R3
R2
R3
Pd-C, Zn0
I
toluene/water
ambient temperature
N
N
N
H
H
H
(1)
(2)
O
O
O
O
R3 =
O
O
a,b
c-f
g,h
Compound 1
R1
R2
Compound 2
Yield, %a
1a
1b
1c
1d
1e
1f
Me
Me
2a
2b
2c
2d
2e
2f
15
45
76
77
48
52
32
34
CH2CH2OMe
CO2Me
Me
Et
Me
Me
Me
Pr
Me
Me
Me
CH2CH2CO2Me
Et
1g
1h
2g
2h
CH2CH2CO2Me
aReaction conditions: 10% Pd-C/Zn, toluene/water, ambient temperature.
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 437–440

438
L. JIaO et al.
Fig. 1. Molecular structure of 2,2′-bipyrrole (2c). Only the major conformer is shown for each of the disordered phenyl groups
Similar to the coupling results with 1b, substitution
of acetone with toluene improved the coupling yields
of 2-iodopyrroles (1a–b) and (1g–h) only from low to
modest: 4% to 15% for 1a, 5% to 45% for 1b, 7% to
32% for 1g and 12% to 34% for 1h, with little change
being observed for 1c–f. But the reaction became slow
and the conversion rate of 2-iodopyrrole was lower in
toluene compared with those in acetone. Thus for those
2-iodopyrroles with PhCH2O2C at the 5-position, the
mixture of acetone/water was shown to be more effi-
cient than toluene/water. The X-ray structure of 2,2′-
bipyrrole 2c is shown in Fig. 1. The two pyrrole rings
are not coplanar, but there is a twist about the central
C–C bond of the molecule. The N1–C4–C5–N2 tor-
sion angle is -56.7(4)°. The ester carbonyl O atoms
are nearly eclipsed with pyrrole N, N–C–C=O torsion
angles being 3.2(4) and 4.9(4)°. The N–H and C=O
groups form intermolecular hydrogen bonds in cyclic
pairs about inversion centers, the N O distances being
2.831(3) and 2.936(3) Å.
On TLC, the 2,2′-bipyrroles showed a strong fluores-
cence at 366 nm (UV-radiation). Photophysical properties
are shown in Table 4. It was found that the 2,2′-bipyrroles
are strongly luminescent materials at ambient tempera-
ture and the fluorescence quantum yields are between
0.45–0.72 in both non-polar solvent (DCM) and polar
solvent (DMSO) systems. Meanwhile, large Stokes shifts
were observed for these 2,2′-bipyrroles (varying between
63–84 nm). Thus, these 2,2′-bipyrroles might find appli-
cations in blue-light-emitting devices [18].
...
Table 4. Photophysical data for the 2,2′-bipyrroles in degassed dichloromethane (DCM) and dimethyl sulfoxide (DMSO) at
ambient temperature
abs
max
Compound
Solvent
λ
, nm (log ε)
λemmax, nma
ΦFb
Stokes shift, nm
2a
DCM
DMSO
DCM
229 (4.00), 317 (4.27)
315 (4.35)
382
379
401
402
386
381
387
377
387
381
387
378
384
381
384
381
0.55
0.65
0.56
0.58
0.52
0.72
0.45
0.56
0.56
0.67
0.52
0.49
0.50
0.71
0.58
0.62
65
64
84
87
67
63
75
84
68
64
72
84
68
68
67
68
2b
2c
2d
2e
2f
230 (4.47), 250 (4.56), 289 (4.32), 347 (4.48)
289 (4.31)
DMSO
DCM
229 (4.17), 319 (4.46)
318 (4.31)
DMSO
DCM
229 (4.11), 312 (4.26)
293 (4.44)
DMSO
DCM
209 (3.97), 319 (4.37)
317 (4.31)
DMSO
DCM
229 (3.85), 315 (3.95)
294 (4.12)
DMSO
DCM
2g
2h
229 (4.10), 318 (4.31)
313 (4.06)
DMSO
DCM
229 (4.02), 317 (4.22)
313 (4.38)
DMSO
aExcitation at 310 nm for all the compounds 2 in DCM and 290 nm for most of compounds 2 in DMSO except for 2a and 2b which
b
were excited at 340 nm. Quantum yields calculation for all the compounds 2 were based on quinine sulfate 5% H2SO4 solution
(ΦF = 0.56) [14].
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 438–440

PaLLaDIum(0) CataLyzeD 2,2′-bIPyrrOLe SyntheSeS
439
Cyr MJ, Lynch V, McGhee E and Ibers JA. J. Am.
Chem. Soc. 1990; 112: 2810. d) Schevchuk SV,
Davis JM and Sessler JL. Tetrahedron Lett. 2001;
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CONCLUSION
Based on a Pd(0) catalyzed coupling reaction, a series
of 2,2′-bipyrroles were synthesized at ambient tempera-
tures. Water plays important role in the reaction, and ace-
tone/water was shown to be a very efficient system for
the coupling of 2-iodopyrroles bearing a PhCH2O2C sub-
stituent at the 5-position to generate the 2,2′-bipyrroles.
These 2,2′-bipyrroles are highly luminescent materials
with high fluorescence quantum yields and a large Stokes
shift.
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The work described was supported by grants from the
US National Institutes of Health, (CA 132861; KMS)
and the US National Science Foundation (CHE 0911629;
MGHV).
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