Synthesis of bis(pyrrol-2-yl)arenes by Pd-catalyzed cross coupling

Article abstract of DOI:10.1016/j.tetlet.2006.08.098

2-Borylpyrrole was prepared from 2-iodopyrrole almost quantitatively and then reacted with dihaloarenes under typical reaction conditions of Suzuki-Miyaura cross coupling to give bis(pyrrol-2-yl)arenes in good yields, while the cross coupling reaction of

Full text of DOI:10.1016/j.tetlet.2006.08.098

Tetrahedron Letters 47 (2006) 7541–7544
Synthesis of bis(pyrrol-2-yl)arenes by Pd-catalyzed cross coupling
Jun-ichiro Setsune,a,* Masayuki Toda,a Keigo Watanabe,a
Pradeepta K. Pandab and Takafumi Yoshidab
aDepartment of Chemistry, Faculty of Science, Kobe University, Nada-ku, Kobe 657-8501, Japan
bVenture Business Laboratory, Kobe University, Nada-ku, Kobe 657-8501, Japan
Received 20 July 2006; revised 23 August 2006; accepted 24 August 2006
Abstract—2-Borylpyrrole was prepared from 2-iodopyrrole almost quantitatively and then reacted with dihaloarenes under typical
reaction conditions of Suzuki–Miyaura cross coupling to give bis(pyrrol-2-yl)arenes in good yields, while the cross coupling reaction
of 2-iodopyrrole with 1,4-phenylenebisboronic acid was susceptible to oxidative self-coupling to produce 4,40-bis(pyrrol-2-yl)biphen-
yl as a byproduct. These bis(pyrrol-2-yl)arenes showed strong fluorescence.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Organic materials containing pyrrole are of great inter-
est and their application to p-conjugated polymers such
as polypyrroles is well known.1 Bis(pyrrol-2-yl)arenes
have been regarded as useful monomers for electroactive
polymeric materials.2 Another interesting application of
bis(pyrrol-2-yl)arenes is their use as building blocks for
expanded porphyrins.3 In this context, various bipyrrole
derivatives with an aromatic linker such as benzene,
pyridine, furan, thiophene, and pyrrole have been
reported.4–6 The five-membered heteroaromatic linker
between pyrroles was constructed by the cyclization of
1,4-bis(pyrrol-2-yl)-1,4-diketone.4,5 On the other hand,
pyrrole rings were constructed at both sides of the six-
membered aromatic core to produce bis(pyrrol-2-yl)-
arenes.6 However, these synthetic methods are not
satisfactory in the synthesis of bipyrrolic compounds
with various substituents.
Ar-X
(a)
Ar-M
(b)
M
Ar
X
N
R
N
R
N
R
Scheme 1. Pd-catalyzed cross coupling to 2-arylpyrrole.
reactions of N-unprotected halopyrrole and borylarene
were frequently reported to give arylpyrroles (Scheme
1. (b); R = H, X = halogen, M = B(OR0)2),8 the cou-
pling reaction using N-unprotected borylpyrrole
(Scheme 1, (a) R = H, X = halogen, M = B(OR0)2) has
never been reported except for a few examples.9a N-Pro-
tected borylpyrroles have usually been employed in the
protocol (a).9b–i Here, we will show a convenient prepa-
ration of N-unprotected borylpyrrole and its successful
application to the synthesis of bis(pyrrol-2-yl)-
arenes. The Pd-catalyzed cross coupling of N-unpro-
tected borylpyrrole and dihaloarene was found to be
much less susceptible to the oxidative self-coupling
reaction in comparison with the reverse combination
of halopyrrole and diborylarene.
The Pd-catalyzed cross coupling reaction is a straight-
forward synthetic method for arylpyrroles. Whereas
arylation of N-unprotected pyrrole through C–H activa-
tion has recently been reported (Scheme 1, (a) R = M =
H, X = halogen),7 conventional Suzuki–Miyaura cross
coupling reactions are favored practically in view of its
wide applicability and milder reaction conditions.8,9
There are two options of the reactant combination as
shown in Scheme 1, (a) pyrrolylmetal and haloarene
and (b) halopyrrole and arylmetal. While Pd-catalyzed
3,4-Dialkyl-2-ethoxycarbonylpyrrole 1 is the choice of
a pyrrole substrate in this letter since it is frequently
used in the porphyrinoid synthesis.10 It is well known
that the ester substituent stabilizes the pyrrole nucleus
against oxidative degradation and is readily removed
by the hydrolysis–decarboxylation procedure. When a
mixture of 3,4-diethyl-2-ethoxycarbonyl-5-iodopyrrole
2 (2.0 mmol), 1,4-phenylenebisboronic acid (1.0 mmol),
K2CO3 (6.6 mol), Pd(OAc)2 (0.10 mmol), and PPh3
(0.20 mmol) in ethanol (10 ml) was heated under argon
*
Corresponding author. Tel.: +81 78 803 5683; fax: +81 78 803
5770; e-mail: setsunej@kobe-u.ac.jp
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.08.098

7542
J. Setsune et al. / Tetrahedron Letters 47 (2006) 7541–7544
Pd(OAc)2
K2CO3
+
I
+
(HO)2B
B(OH)2
EtOH
80 °C, 4 h
N
H
N
N
N
N
2n
n
n
EtO2C
EtO2C
CO2Et
EtO2C
CO2Et
H
H
H
H
3 (n=1), 80%
4 (n=2), 74%
4 (n=1), 5%
5 (n=2), 19%
2
Scheme 2. Pd-catalyzed cross coupling reactions of 2-iodopyrrole 2 and arenebisboronic acid.
for 24 h at reflux, 1,4-bis(pyrrol-2-yl)benzene 311 was
obtained in good yield. H NMR analysis after chro-
effective than PdCl2(dppf). The latter was reported to
be the best catalyst for the borylation of ordinary aro-
matic iodide.14a Borylation with bispinacolatodiborane
instead of pinacolborane did not work well in the case
of 2 either as shown in Table 1.
1
matographic purification showed that 3 (80% yield)
was contaminated with a byproduct, 4,40-bis(pyrrol-2-
yl)biphenyl, 411 (5% yield) originated from the self-cou-
pling of the pyrrolylbenzeneboronic acid intermediate
(Scheme 2). Compound 4 was identical with the major
product of the similar Pd-catalyzed cross coupling
reaction of 2 and 4,40-biphenylenebisboronic acid. The
major product 4 in 74% yield was also contaminated
with a self-coupling product, bis(pyrrol-2-yl)quarterphe-
nyl, 5 in 19% yield. The latter was characterized by the
MS signal at 693.33 (theory for C46H48N2O4 + H+:
693.37).
2-Borylpyrrole 6 (3.0 mmol) after separating ammonium
salts and most part of catalyst by filtration was subjected
to the Suzuki–Miyaura cross coupling reaction with
dihaloarenes (1.25 mmol) in the presence of Pd(OAc)2
(0.13 mmol), PPh3 (0.26 mmol), and K2CO3 (4.1 mmol)
in DMF–H2O (15–3 ml) at 80–90 ꢁC for 5 h. The cou-
pling products 3 and 4 free from the self-coupling
byproducts were obtained in 69% and 92% yield
from 1,4-diiodobenzene and 4,40-diiodobiphenyl,
respectively.
The purity of the bis(pyrrol-2-yl)arene is crucial if it is
used to generate oligomers and polymers. Therefore,
we examined the cross coupling of 2-borylpyrrole and
diiodoarenes as a reverse combination in the hope of
improving selectivity. 2-Borylpyrroles have been pre-
pared by the reaction of N-Boc-protected 2-lithiopyrrole
with alkoxyborane.12 Ir-catalyzed direct borylation of
pyrrole at the 2-position has recently been reported.13
Since the latter reaction is restricted to sterically unhin-
dered substrates and the former reaction needs protec-
tion–deprotection procedure, these methods would not
be suitable for 1. Thus, we have tried to convert 2 to
2-borylpyrrole 6 according to the procedure for Pd-
catalyzed borylation of aromatic halides.14 A mixture
It has been shown that the oxidative self-coupling of
areneboronic acid is accelerated by dioxygen by way
of diarylpalladium intermediate.15 Probably because of
the steric reason, double transmetallation of 2-borylpyr-
role 6 to Pd(II) seems to be suppressed, as compared
with that of pyrrolylareneboronic acid as an intermedi-
ate in the cross coupling reaction of 2 and arenebis-
boronic acid. Facile formation of 2-borylpyrrole and
its clean cross coupling reaction with dihaloarenes lead
to a reliable synthetic procedure for bis(pyrrol-2-yl)-
arenes and bis(pyrrol-2-ly)heteroarenes. Table 2 summa-
rizes the yields of the cross coupling reactions of 6 with
m-diiodobenezene, o-diiodobenezene, 2,5-diiodothio-
phene, and 2,6-dibromopyridine to give the coupling
products 7–10. It is worthy of note that 1,2-bis(pyrrol-
2-yl)benzene derivative could not be prepared by the
pyrrole ring forming reaction starting from phthaloyl
of
2 (4.6 mmol), pinacolborane (5.5 mmol), NEt3
(11.5 mmol), and PdCl2(PPh3)2 (0.23 mmol) in dioxane
(20 ml) was heated at 60 ꢁC for 3 h to give 611 almost
quantitatively. The amount of the dehalogenated prod-
uct 1 was negligible under these reaction conditions. It
is worthy of note that PdCl2(PPh3)2 was much more
dichloride.6a 2-Borylpyrrole 60 having
a sterically
Table 1. Pd-catalyzed borylation of iodopyrrole 2a
HB(pin) or B2(pin)2
Pd cat./ base
+
EtO2C
I
EtO2C
B(pin)
EtO2C
dioxane
N
H
N
H
N
H
2
1
6
Entry
Borane
Pd catalyst
PdCl2(dppf)
PdCl2(PPh3)2
PdCl2(dppf)
PdCl2(PPh3)2
Base
Time (h)
Yieldb (%)
6
1
1
2
3
4
HB(pin)
HB(pin)
B2(pin)2
B2(pin)2
Et3N
Et3N
KOAc
KOAc
4
3
21
2
5
4
2
8
2
98
10
12
a The molar ratio: 2/borane/Pd/base = 1.0/1.2/0.05/2.5.
b The yield was determined by 1H NMR.

J. Setsune et al. / Tetrahedron Letters 47 (2006) 7541–7544
7543
Table 2. Pd-catalyzed cross coupling of 2-borylpyrrole 6 and 60 with
demanding iso-butyl group was prepared similarly in
more than 90% yield and then gave coupling products
11–14 in good yields.
dihaloarenesa
X-(arene)-X
Pd(OAc)2
PPh3
R2
R2
R2
R1
EtO2C
R1
EtO2C
R1
The newly synthesized dipyrrolic compounds are fluo-
rescent as shown in Figure 1 and the fluorescence inten-
sity is very high in some cases. For example, the
fluorescence efficiency of 4 in ethanol estimated relative
to anthracene (UF = 0.27)16 is 0.89. It is worthy of note
that protonation at the pyridine nitrogen of 10 caused
red shift of the absorption band from 352 nm to
432 nm and also of the fluorescence band from 392 nm
to 490 nm.
arene
B(pin)
DMF-H2O
K2CO3
N
H
N
H
N
H
CO2Et
80-90 °C, 24 h
6, 6'
3, 4, 7 - 14
X–(arene)–X
R1
Et
R2
Product
Yield (%)
69
Et
Et
3
4
I
I
I
Et
Et
92
84
I
In summary, 2-borylpyrrole was readily prepared
from 2-iodopyrrole and it was successfully applied to
the synthesis of bis(pyrrol-2-yl)arenes by the Pd-cata-
lyzed cross coupling with dihaloarenes. This is in con-
trast to the reverse combination of 2-halopyrrole and
arenebisboronic acid that was found to be more suscep-
tible to the oxidative self-coupling. In addition to good
solubility owing to the alkyl substituents at the
pyrrole-b positions, the strong fluorescence augments
the utility of these bis-(pyrrol-2-yl)arenes and bis(pyr-
rol-2-yl)heteroarenes as scaffolds for functional p-conju-
gated materials.
Et
7
I
I
Et
Et
Et
Et
Et
Et
8
81
70
74
I
I
I
9
I
S
10
Br
N
Br
I
I
Me
i-Bu
11
70
Acknowledgments
This work was supported by a Grant-in-Aid for
Scientific Research (No. 16033240, No. 18550058, and
No. 18Æ06071) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. The author is
also grateful to the CREST program (the Japan Science
and Technology Agent) and the VBL project (Kobe
University).
I
I
Me
Me
Me
i-Bu
i-Bu
i-Bu
12
13
14
92
97
76
Br
Br
O
N
N
Br
Br
References and notes
a Molar ratio: 6 (or 60)/dihaloarene/Pd(OAc)2/PPh3/K2CO3 = 2.4/1.0/
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and 7.5 Hz, CH2Me), 1.38 (t, 6H, J = 7.1 Hz, OCH2Me),
1.22, 1.21 (t · 2, 6H · 2, CH2Me, J = 6.7 and 6.9 Hz).
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14.53.
ESI-MS
(found/calcd
for
C28H36N2O4 + H+) 465.276/465.275. IR (cmꢀ1, in KBr
pellet) 1666 (m(CO)). Anal. Calcd for C28H36N2O4: C,
72.39; H, 7.81; N, 6.03. Found C, 72.36; H, 7.99; N, 5.63.
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1H NMR (400 MHz, d-value, CDCl3) 8.84 (br, 2H, NH),
7.71, 7.56 (d · 2, 4H · 2, J = 8.2 Hz, (C6H4)2-linker), 4.36
(q, 4H, J = 7.1 Hz, OCH2Me), 2.82, 2.64 (q · 2, 4H · 2,
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OCH2Me), 1.23, 1.22 (t · 2, 6H · 2, CH2Me, J = 7.5 Hz).
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14.44.
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