3,3′- and 4,4′-dimethoxy-2,2′-bipyrroles: Highly electron-rich model compounds for polypyrrole formation

Article abstract of DOI:10.1002/chem.200390047

3,3′-Dimethoxy-2,2′-bipyrrole (1) and 4,4′- dimethoxy-2,2′-bipyrrole (2) were obtained in short sequences and good yields from N-benzyl-3-hydroxypyrrole-2,4-dicarboxylic acid. The key intermediate leading to 1 is an N-benzyl-3-methoxypyrrole, which is dim

Full text of DOI:10.1002/chem.200390047

FULL PAPER
3,3'- and 4,4'-Dimethoxy-2,2'-bipyrroles:
Highly Electron-Rich Model Compounds for Polypyrrole Formation.
Andreas Merz,*[a] Sergiy Anikin,[a] Bernd Lieser,[a] J¸rgen Heinze,[b] and Hermann John[b]
Abstract: 3,3'-Dimethoxy-2,2'-bipyrrole
(1) and 4,4'-dimethoxy-2,2'-bipyrrole (2)
were obtained in short sequences and
good yields from N-benzyl-3-hydroxy-
pyrrole-2,4-dicarboxylic acid.The key
intermediate leading to 1 is an N-benzyl-
3-methoxypyrrole, which is dimerized by
lithiation and oxidation with NiCl2.The
formation of 2 is achieved by a classical
Ullmann coupling of diethyl 1-benzyl-2-
bromo-4-methoxypyrrole-3,5-dicarbox-
ylate.The N-benzyl protection groups of
1 and 2 are cleaved under reducing
conditions with sodium in liquid ammo-
nia.Both isomeric bipyrroles are ex-
tremely sensitive toward air.Compound
1 has a very low oxidation potential of
0.09 V against AgCl but film formation
hardly occurs.On the other hand, com-
pound 2 with a potential of 0.35 V
readily forms stable polypyrrole films
with anodic waves at À0.51 and À0.35 V
and a cathodic wave at À0.77 V, the
lowest potential ever observed for a
p-doped polymer.
Keywords: cyclic voltammetry
¥
oxidation ¥ polypyrrole formation ¥
pyrrole dimerization
N-benzyl cleavage
coupling
¥
¥
reductive
Ullmann
Bipyrrole and thiophene dimers and oligomers have often
Introduction
been used to study the single steps of electrochemical
polymerization.[4, 11, 12] The position of substituents in the
dimer is of remarkable importance.Thus, 4,4 '-dimethoxy-2,2'-
bithiophene is a very potent monomer for polymerization
whereas 3,3'-dimethoxy-2,2'-bithiophene polymerizes quite
slowly.[4, 13] Due to the higher electronegativity of nitrogen as
compared with sulfur, the analogous 2,2'-bipyrroles 1 and 2
are expected to have extremely negative oxidation potentials.
Though, like in the thiophene case, both isomers should
display quite different properties.The syntheses of the title
compounds 1 and 2 and their chemical and electrochemical
behavior are reported.
Pyrrole is a fairly electron-rich aromatic compound.On
exposure to air, pyrrole gradually discolors.Aqueous pyrrole
forms a black powder of moderately conducting material on
treatment with various oxidants.Electropolymerized poly-
pyrrole[1] is the most-cited organic conducting polymer.To
date, about 8500 sources mention polypyrrole, and about 3900
sources can be found for polythiophene.[2] Thiophene can
likewise be polymerized by anodic oxidation, but 3-alkylthio-
phenes give better results.[3] Still more activating are alkoxy
substituents in the 3-position or in both the 3- and 4-posi-
tions.[4, 5] Ethylendioxythiophene (EDOT) is a commercial
chemical with a large range of applications.[6, 7] Dimethoxy-
pyrrole[8] and other 3,4-alkoxypyrroles, including 3,4-ethyl-
endioxypyrrole (EDOP),[9] are quite sensitive to air and must
be stored under inert conditions.Remarkably, and contrary to
3-alkylpyrroles, alkoxy substituents facilitate polymerization
and enhance the conductivity of the corresponding polypyr-
role.[10]
[a] Prof.Dr.A.Merz, S.Anikin, B.Lieser
Institut of Organic Chemistry, University of Regensburg
93040 Regensburg (Germany)
Results and Discussion
Fax (‡49)941-942-4121
E-mail: andreas.merz@chemie.uni-regensburg.de
Diethyl N-benzyl-3-hydroxypyrrole-2,4-dicarboxylate[14] 3 is
an excellent precursor molecule that is not only readily
prepared in large quantities from inexpensive chemicals, but
also allows for the preparation of both bipyrroles 1 and 2.The
3-hydroxy group in 3 is the basis of the later methoxy groups
[b] Prof.Dr.J.Heinze, Dipl-.Chem.H.John
Institute of Physical Chemistry, University of Freiburg
79104 Freiburg (Germany)
Fax (‡49)761-203-6237
E-mail: juergen.heinze@physchem.uni-freiburg.de
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449

A.Merz et al.
FULL PAPER
in 1 and 2.In solution, 3-hydroxypyrroles can exist as the
aromatic ™phenol∫ or in equilibrium with the pyrrolone
(™keto form∫) depending on substituents and solvents.For
instance, N-tert-butylpyrrole exists as 90% keto form in
CDCl3, but as 95% hydroxy form in [D6]DMSO, as deter-
mined by 1H NMR.3-Hydroxy- N-phenylpyrrole has the
pyrrolone structure[15] in the solid state; on the other hand,
the solid structure of N-tert-butylpyrrole-3-ol is the hydroxy
form.[16] Crystalline 3 is a hydroxypyrrole, as shown by the
X-ray structure (Figure 1).[17]
The thermal decarboxylation of both carboxylates 5 occurs at
around 2008C without a solvent, and the N-benzyl-protected
3-methoxypyrrole 6 can be isolated by vacuum distillation.
Various substituted 3-methoxypyrroles have been prepared
by Wong and Clezy,[18] and the first N-alkyl 3-alkoxypyrroles
with free 2, 4, and 5-CH positions, were reported by Kochlar
and Pinnock.[19] Among these was 6, which was only margin-
ally characterized.The facile syntheses of 3,4-dialkoxy
pyrroles[20, 21] suggested the analogous condensation of a
N-benzyliminodiacetic ester with formylacetic ester to give
3a. Compound 6 was obtained in a similar sequence to the one
above; however this approach suffers from a very poor yield
(7%) due to the competing Cannizzarro disproportion of the
formylacetic ester in the first step.A completely different
source of 3-methoxypyrrole 6 is derived from a retro-Diels
Alder product of a diazine.[22] The present procedure, how-
ever, is far superior to all others.
The methoxy group and the nitrogen in 6 are in an activated
position for lithiation at the 2-position.Compound 6 was
lithiated at low temperature with sec-BuLi in THF/ether/
TMEDA by following a Kaufmann protocol.[23] Subsequently,
anhydrous NiCl2 was employed for oxidative dimerization.
Bipyrrole 7 was isolated in 35 to 45% yield, but more than
50% starting material 6 can be recovered by distillation.More
favorable conditions have not yet been established.Thus, the
benzyl-protected bipyrrole 7 with the ™inner∫ methoxy groups
is established for deprotection.Compound 7 shows atropic
isomerism as shown by the AB splitting of the benzyl-CH2
group.
À
À
Figure 1.Compound 3: selected bond lengths: O1 C9 1.3457, N1 C1
À
À
À
À
1.4597, N1 C8 1.3942, N1 C11 1.3370, C8 C15 1.434, C8 C9 1.388,
À
À
À
C9 C10 1.407, C10 C11 1.3910, C10 C12 1.4592; bonds angles: C1-N1-
C11 123.82, C8-N1-C11 108.85, C1-N1-C8 127.18, N1-C1-C2 112.76, N1-C8-
C9 107.12, O1-C9-C10 126.85, C8-C9-C10 108.01, O1-C9-C8 125.13, C9-
C10-C11 106.08, C9-C10-C12 131, C11-C10-C12 122.66, N1-C11-C10 09.93;
torsion angles: C1-N1-C8-C9 176.62, C8-N1-C11-C10 À0.47, N1-C1-C2-C3
À27.70, N1-C8-C9-C10 À1.11, C8-C9-C10-C11 0.0, C9-C10-C11-N1 À0.23.
Preparation ofthe 3,3 '-dimethoxybipyrrole skeleton: The
preparation of 1 starts with the methylation of the 3-hydroxy
group of 3 with dimethyl sulfate to give 4, followed by a
classical ester cleavage to give the free diacid 5 (Scheme 1).
Sessler et al.reported the dimerization of known pyrrole 8
to bipyrrole 9 (Scheme 2), which is a building block for
sapphyrines.[24] The dimerization of pyrrole 4, which is similar
À
Scheme 2.i) NaI/I 2. E ˆ COOEt.
to 9 with a single hydrogen at the 5-position, appeared to
provide easy access to 2.By using Sessler×s protocol, 4 was
treated with NaI/I2 in dichloroethane/aqueous buffered NaH-
CO3.Dimerization does indeed occur, however at the 2-
position, to give the 2,2'-bipyrrolidone as the crystalline
racemic Æ-10 and the achiral resinous meso form 11. The
assignment of the stereochemistry has been confirmed by an
X-ray structure (Figure 2[17]).The 1H NMR spectra of 10 and
11 display manifold diasterotopic -CH2- groups of the ethyl
esters and benzyl groups.Both ethyl -CH 2 groups in 10 show a
Scheme 1.i) Me 2SO4, acetone, K2CO3, reflux; ii) KOH, EtOH; iii) py-
rolysis, 2408C; iv) THF/Et2O, sec-BuLi, TMEDA, À60 to 08C, then NiCl2.
450
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Bipyrrole Formation
449 455
Scheme 3.i) Br 2; ii) Me2SO4, acetone, K2CO3, reflux; iii) Cu powder,
DMF (trace), 3008C; iv) KOH, EtOH; v) pyrolysis, 2408C.
Preparation and isolation of1 and 2 : As long as the pyrroles
contain electron-deficient substituents, it is possible to cleave
the N-benzyl protecting group with CF3-COOH.[20] Electron-
rich bipyrroles such as 6, 7, and 16, however, need reliable
procedures in a reducing environment.Solvated electrons in
liquid ammonia are excellent for this purpose.[21, 28] In the
present case, the reduction products are the bipyrrolyl sodium
salt and benzyl sodium.Aqueous-organic workup under
argon with adequately adapted conditions guarantees very
clean pyrroles.
The twin bipyrroles 1 and 2 are very sensitive to air and to
any other oxidation agents.Therefore, each compound is
taken up in a toluene solution, from which, at low temper-
ature, the bipyrroles crystallize.Both compounds can be
stored under argon at ꢁ À 208C in the dark.The crystals of 2
blacken within minutes on exposure to air, acquiring a soot-
like appearance, apparently a sort of polypyrrole.Freshly
prepared samples of 1 can be handled in air for some minutes,
but the samples begin to turn greenish-blue.The different
behavior of the two bipyrroles was expected by analogy with
the corresponding thiophenes.[4, 13]
À
À
Figure 2.Compound 10: selected bond lengths: N1 C1 1.48(18), N1 C8
À
À
À
À
1.3312, N1 C11 1.4717, N2 C18 1.4722, N2 C21 1.3249, N2 C28 1.4878,
À
À
À
À
À
C8 C9 1.3783, C9 C10 1.433, C10 C11 1.5990, C11 C18 1.580, C18 C19
À
À
1.5972(19), C19 20 1.435 (2), C20 C21 1.380(2); bond angles: N1-C11-C18
114.48, C10-C11-C15 107.58, C10-C11-C18 107.22, C15-C11-C18 131.7, N2-
C18-C11 113.18, N2-C18-C19 101.86, N2-C18-C22 110.96, C11-C18-C19
81.4, C11-C18-C22 115.24, C19-C18-C22 106.29, O10-C19-C18 122.67, O10-
C19-C20 132.20, C18-C19-C20 105.12, C19-C20-C21 108.04, C19-C20-C25
124.64, C21-C20-C25 127.20, N2-C21-C20 114.74(12); torsion angles at C11/
C18: C10-C11-C18-N2 37.44, C10-C11-C18-C19 74.62, C10-C11-C18-C22
À166.65, C15-C11-C18-N2 81.03, C15-C11-C18-C19 À166.91, C15-C11-
C18-C22 À48.18, N1-C11-C18-N2 49.04, N1-C11-C18-C19 36.98, N1-C11-
C18-C22 81.75.
4 Â 2 Â 2 line signal, whereas only the inner groups split into a
4 Â 2 signal in 11).
Surprisingly, the demethylation at the methoxy groups is
faster than the direct oxidation.The slightly alkaline aqueous
phase together with the organic solvent 1,2-dichloroethane
seems to be a mild phase-transfer medium that favors the
Finkelstein type SN2 reaction of the iodide or polyiodide ion
to give iodomethane and the mesomeric anion of 3.Oxidation
The structure of 1 is related to the well known indigo
system, in the sense that 1 is an O-alkyl leuko-pyrrolindigo.
O-alkyl and O-acyl leukoindigo structures have been de-
scribed in the literature.[29] In pursuit of the unsubstituted
of 3À by the I2/In system can give rise to the resonance-
stabilized neutral radical 3 , which then dimerizes to 10 and
11.A possible route from 10 and/or 11 to bipyrrole 1 by ester
hydrolysis, decarboxylation, and re-methylation could not be
realized, however.
À
.
Preparation ofthe 4,4 '-dimethoxybipyrrole skeleton: The
known bromohydroxypyrrole 12[25] derived from 3 is methyl-
ated at the OH group to give 13 in high yield (Scheme 3).
Compound 13 is a classical Ullmann precursor[26] with typical
electron deficient substituents.The bipyrrole 14 is formed
in the melt at ꢀ3008C with copper powder and a trace of
DMF in a quite remarkable yield of 60%.After removal
of the four carboxylic ester groups by hydrolysis (to give
15) and decarboxylation, the N-benzyl-protected 16 is
obtained in good yield as a fairly stable compound under
N2.Compounds 14 and 15 show atropic isomerism due to
the splitting of the benzyl methylene groups, but 16 does
not.
pyrrolindigo chromophore, Bauer was the first to prepare 3,3'-
diethoxy-2,2'-bipyrrole, the homologue of 1.[30] Furthermore,
Bauer obtained several substituted pyrrolindigo structures,
for example 4,5,4',5'-tetramethyl-3,3'-dioxobipyrrole,[31] but
the unsubstituted pyrroloindigo has not yet prepared.Com-
pound 1 is certainly a candidate precursor to obtaining this
goal.
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A.Merz et al.
FULL PAPER
Electropolymerization behavior of1 and 2 : The electro-
polymerization experiments that were carried out with
compounds 1 and 2 in acetonitrile in the presence of a small
amount of water give results in perfect agreement with the
expectations.Figure 3 shows a voltammetric multisweep
current peak in the cyclic voltammogram increases with each
scan.The reason for the fast polymerization process lies in the
.
‡
reactivity of the radical cation 2 , which has its highest spin
density at the nonblocked a-positions; this favors a fast
coupling reaction and evidently also facilitates proton elim-
ination.The further products of the coupling steps again have
substituents in the ™outer∫ b-positions of the growing
oligomer; these make it very reactive.
The corresponding polymer film of 2 was characterized by
using solid state voltammetry (Figure 5).The oxidation
potential of the main first wave lies at À0.51 V and a second
Figure 3.Multisweep voltammogram of 1 (c ˆ 8  10À4 m) at a 1 mm
platinum disk in Bu4NPF6 (0.1m) in acetonitrile ‡1% water.
experiment with 1.The oxidation of
1 occurs at the
remarkably low reduction potential of ‡0.09 V vs Ag/AgCl,
as an chemically irreversible process.However, no generation
or deposition of an extended oligomer or polymer is observed
during potentiodynamic cycling, only two small redox waves
at potentials of À0.25 and À0.0 V can be seen. Evidently 1, in
accordance with the findings obtained for 3,3'-dimethoxybi-
thiophene,[4, 5, 13] forms a s-dimer that slowly eliminates
protons to give a neutral tetramer.This can be charged up
to a dication without further follow-up reactions.The coupling
tendency of the tetrameric species is low due to the fact that
the spin density at the outer a-positions is low.
By contrast, compound 2 polymerizes rapidly in acetonitrile
in the presence of 1% water.It is oxidized at a peak potential
of Ep ˆ ‡0.35 V, and shows a following broad cathodic wave
in the reverse scan.In a multisweep experiment, fast growth of
a conducting polymer film can be observed (Figure 4).The
Figure 5.Voltammogram of a PPy- 2 film generated during the potentio-
dynamic cycling of 2 in acetonitrile in the absence of 2.The two anodic
peaks at À0.51 and À0.35 (a, b) and the cathodic peaks at À0.59 and À0.77
(c, d) indicate the formation of two structurally different polymers.
wave appears at À0.35 V. In contrast to Figure 4, the first
oxidation wave is higher than the second one.As in the case of
the unsubstituted polypyrrole,[32, 33] we assume that two
structurally different polypyrrole systems are generated
during the electrochemical oxidation.As usual in conducting
polymers, the reduction peak is more negative (peak at
À0.56 V). A second reduction wave, at À0.77 V vs. Ag/AgCl,
however, is the lowest potential ever observed for the
discharging of a p-doped polymer.The strong hysteresis
between oxidation and reduction gives evidence that during
charging a thermodynamically stabilized network with s
interchain bonds is formed.The s-bond generation produces
a marked stabilization of the charged polymer and, in
addition, results in localized charges.
Conclusion
Starting from a readily available pyrrole precursor, bipyrroles
1 and 2 were both synthesized in a few steps.As expected, 1
and 2 are extremely sensitive towards oxygen, but both
compounds are stable under argon.Although 1 has the lower
oxidation potential, no polymerization was observed due to
the location of the electron hole in the inner portion of the
molecule.Only 2 is capable of forming a redox polymer.
Figure 4.Multisweep voltammogram of 2 (c ˆ 10  10À3 M) at a 1 mm
platinum disk in Bu4NPF6 (0.1m) in acetonitrile ‡ 1% water, showing the
development of the poly-3 film: a),b) anodic polymer peaks, c) oxidation of
2, d),e) cathodic polymer peaks.
452
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Bipyrrole Formation
449 455
(250 MHz, [D6]DMSO): d ˆ 3.76 (s, 3H; OCH3), 6.07 (s, 2H; CH2Ph), 6.69
(s, 1H; Py), 6.88 (m, 2H; o-Ph), 7.24 (m, 3H; m/p-Ph), 12.73 (brs, COOH);
13C NMR (63 MHz): d ˆ 47.90, 57.60, 101.98, 125.68, 126.64, 128.28, 139.48,
152.13, 161.38, 161.40; IR: nÄ ˆ 3250 2350, 1702, 1652; MS (70 eV, EI): m/z:
Experimental Section
General methods: NMR spectra were recorded on Bruker AC250 or
Bruker ARX400 instruments.IR spectra were recorded Bio-rad
a
‡
‡
‡
274 [M À H], 230 [M À CO2], 183 [M À C7H7]; elemental analysis calcd
(%) for C14H13NO5 (275.2): C 61.09, H 4.76, 5.09; found C 60.77, H 4.63,
N 4.99.
Excalibur Series FTIR spectrometer.Mass spectra were obtained with a
Finnigan Mat95 or Varian Mat311A.Ethyl N-benzylglycine ester was
prepared on a 100 g scale in 55% yield according to ref.[34]. Ethyl N-
benzyl-N-[2-bis(ethoxycarbonyl)vinyl]glycinate and diethyl 1-benzyl-3-hy-
droxypyrrole-2,4-dicarboxylic acid 3 was prepared according to Momose
et al.[10]
1-Benzyl-3-methoxypyrrole (6): Either compound 5 or 5a (5 g, 18 mmol)
was decarboxylated for 1 h in a round-bottomed flask for 1 h at 150 mbar
and 220 2308C by using a heat gun.The pressure was lowered to 2 mbar,
and 5 was distilled (b.p. 160 1708C) to give a colorless, air-sensitive oil
(3.4 g, 95%) to be stored under argon. 1H NMR (250 MHz, CDCl3): d ˆ
3.69 (s, 1H; OCH3), 4.95 (s, 1H; CH2), 5.86 (d, J ˆ 2.77 Hz, 1H; H4-
pyrrole), 6.23 (s, 1H; H2-pyrrole), 6.48 (d, J ˆ 2.77 Hz, 1H; H5-pyrrole),
7.08 7.37 (m, 5H; C6H5); 13C NMR (63 MHz, CDCl3): d ˆ 53.90, 57.89,
97.29, 103.11, 119.30, 127.03, 127.65, 128.69, 138.17, 149.34; IR (film): nÄ ˆ
Electrochemical measurements: The electrochemical measurements were
carried out in specially constructed cells containing an internal drying
column with highly activated alumina.The working electrode was a Pt disk
sealed in soft glass (1.0 mm diameter). This setup was controlled by a Jaissle
Potentiostat-Galvanostat IMP88 or IMP88PC.The potential scans were
performed with an EG&G PARC Model175 Universal Programmer scan
generator and the cyclovoltammetric response was recorded with an IMK
PSO8100 transient system.The measured potentials were referenced to the
Ag/AgCl electrode and were determined by an internal calibration with the
cobaltocinium/cobaltocene redox pair.
‡
2933, 1564, 1339, 1043, 736 cmÀ1; MS (70 eV, EI): m/z (%): 187 (45) [M ],
‡
‡
172 (9) [M À CH3], 91 (100) [C7H7 ]; elemental analysis calcd (%) for
C12H13NO (187.2): C 76.98, H 7.00, N 7.48; found: C 76.28, H 6.93, N 7.53.
1,1'-Dibenzyl-3,3'-dimethoxy-2,2'-bipyrrole (7):
A solution of 6 (1 g,
5.3 mmol) dissolved in dry THF/diethyl ether (1:1, 15 mL) and TMEDA
(0.4 mL, 2.8 mmol) was cooled to À708C with stirring and under argon. sec-
Butyllithium in cyclohexane/hexanes (92:8, 0.34 g, 5.3 mmol) was added.
The mixture was allowed to warm to room temperature for 2.5 h. The
mixture was cooled again to À408C, and dry NiCl2 was added (0.83 g,
6.4 mmol). With vigorous stirring, the mixture was allowed to warm up 08C
(30 h).MeOH (10 mL) and water (20 30 mL) were added with stirring.
Finally all solvents were removed in vacuo, and the dark oily residue was
distilled at 80 858C (0.013 mbar). The distillate was starting material 6 (ca.
50%), the dark residue was recrystallized from ethyl acetate to give
colorless crystals. Yield: 0.38 g (40%); m.p. 69 718C; 1H NMR (250 MHz,
CDCl3): d ˆ 3.68 (s, 6H; 2OCH3), 4.59 (AB, 4H; 2CH2Ph), 5.94 (d, J ˆ
3.17 Hz, 2H; 2H-pyrrole), 6.45 (d, J ˆ 3.17 Hz, 2H; 2H-pyrrole), 6.86 7.27
(m, 10H; 2C6H5); 13C NMR (63 MHz, CDCl3): d ˆ 50.91, 58.00, 95.24,
106.82, 118.73, 127.19, 127.83, 128.32, 138.27, 147.39; IR (KBr): nÄ ˆ 3463,
Dimethyl 1-benzyl-3-hydroxypyrrole-2,5-dicarboxylate (3a): A solution of
Na (7.46 g, 320 mmol) in MeOH (85 mL) and ethyl glyoxylic acid (29 mL,
146 mmol, 50% solution) was heated under reflux for 3 days.The mixture
was poured into ice-water, and the product was extracted with diethyl
ether.Recrystallization from MeOH gave 3a 2.98 g (7.05%); m.p. 898C;
1H NMR (250 MHz, CDCl3): d ˆ 3.79, 3.81 (2s, 3H; CO2CH3), 5.95 (s,
PhCH2), 6.53 (s, 1H; CH), 6.97 7,23 (m, C6H5), 8.16 (s, 1H; OH); 13C NMR
(63 MHz, CDCl3): d ˆ 49.48, 51.57, 51.86, 103.90, 111,43, 125.94, 126,11,
128.43, 138,81, 153.30, 160.95, 162.92; IR (KBr) nÄ ˆ 3480, 3035, 2957, 2950,
1735, 1662 cmÀ1; elemental analysis calcd (%) for C16H17NO5 (289.3): C
62.28, H 5.23, N 4.84; found C 62.16, H 5.28, N 4.83.
Diethyl 1-benzyl-3-methoxypyrrole-2,4-dicarboxylate (4): K2CO3 (33 g,
240 mmol) and dimethyl sulfate (9 mL, 95 mmol) were added to a stirred
solution of 3 (30 g, 95 mmol) in dry acetone (300 mL) under N2, and the
reaction mixture was refluxed overnight.The cooled mixture was filtered,
and the solvent was removed.Recrystallization from ethanol gave white
crystals. Yield: 26.6 g (85%); m.p. 69 718C; 1H NMR (250 MHz, CDCl3):
d ˆ 1.31 (t, J ˆ 7.13 Hz, 3H; CH2CH3), 1.34 (t, J ˆ 7.13 Hz, 3H; CH2CH3),
3.91 (s, 3H; OCH3), 4.27 (q, J ˆ 7.13 Hz, 2H; CH2CH3), 4.29 (q, J ˆ 7.13 Hz,
2H; CH2CH3), 5.46 (s, 2H; CH2Ph), 7.07 7.38 (m, 6H; Ph, H5-pyrrole);
13C NMR (100 MHz, CDCl3): d ˆ 14.24, 14.39, 53.45, 59.94, 60.18, 62.83,
108.47, 114.30, 127.13, 127.87, 128.78, 130.27, 136.86, 153.36, 160.56, 162.82;
IR (KBr): nÄ ˆ 2981 2870, 1698, 1551, 1449, 1388, 1295, 1247,1202, 1085,
3062 2831, 1606, 1550, 1479, 1452, 1411, 1330, 1091, 997, 711, 628 cmÀ1
;
HRMS (70 eV, PI-EI) calcd.for
372.1836.
C 24H24N2O2: 372.1838, found m/z:
Racemic- and meso-Tetraethyl 1,1'-dibenzyl-3,3'-dioxo-2,2',3,3'-tetrahy-
dro-2,2'-bipyrrole-2,2',4,4'-tetracarboxylates (10) and (11): A solution of
NaHCO3 (2.63 g, 30 mmol) in H2O (10 mL) was heated to 508C, and 1,2-
dichloroethane (115 mL) added, followed by 4a (3 g, 9.1 mmol). A mixture
of I2 (2.6 g, 10 mmol) and NaI (3.2 g, 20 mmol) in water (10 mL) was added
within 5 min, and the resulting mixture was heated for one hour.Some
undissolved I2 was washed into the mixture with H2O during the reaction.
The mixture was transferred to a separating funnel, the organic layer was
separated, and the aqueous phase was washed with CHCl3 (3 Â 30 mL).The
combined organic layers were washed with a 5% solution of Na2S2O3 (3 Â
30 mL), a 5% solution of NaHCO3 (3 Â 30 mL), and a saturated brine
solution (3 Â 30 mL), and then dried over Na2SO4.During the stripping off
of the solvent, 10 was precipitated.
‡
1028, 998, 787, 699 cmÀ1; MS (70 eV, EI): m/z (%): 331 (37) [M ], 91 (100)
‡
[C7H7 ]; elemental analysis calcd (%) for C18H21NO5 (331.4): C 65.24, H
6.39, N 4.23; found: C 65.25, H 6.33, N 4.23.
Dimethyl 1-benzyl-3-methoxypyrrole-2,5-dicarboxylate (4a): (Same pro-
cedure as given for 4.) Yield: 2.3 g (87%); m.p. 1308C; 1H NMR:
(250 MHz, CDCl3): d ˆ 3.79, 3.86 (2s, 3H; CO2CH3), 3.80 (s, 1H; -OCH3,),
6.11 (s, 2H; -CH2Ph), 6.63 7.21 (m, 6H; -C6H5, CH-pyrrol); 13C NMR
(63 MHz, CDCl3): d ˆ 49.16, 51.47, 51.74, 58.07, 101.66, 126.04, 113.54,
214.68, 126.86, 128.38, 138.90, 152.78, 160.66, 161.16; IR: nÄ ˆ 1718 cmÀ1; MS
(Æ)-10: white crystals, 0.7 g (25%); m.p. 198 1998C; 1H NMR (250 MHz,
CDCl3): d ˆ 1.18 (t, J ˆ 7.11 Hz, 6H; 2CH2CH3, 3,3'), 1.34 (t, J ˆ 7.15 Hz,
6H; 2CH2CH3, 4,4'), 4.10 (qdd, J ˆ 7.32, J ˆ 3.52, 4H; CH2CH3), 4.31 (qdd,
J ˆ 7.16, J ˆ 3.57, 2H; 2CH2CH3), 4,99, 5.12 (AB, J ˆ 14.02 Hz, 4H;
2CH2Ph), 7.35 7.56 (m, 10H; 2C6H5), 8.12 (s, 2H; 2H-pyrrole);
13C NMR (63 MHz, CDCl3): d ˆ 14.08, 14.34, 56.22, 59.78, 63.65, 80.06,
101.98, 129.01, 129.31, 130.49, 133.81, 161.75, 164.24, 169.72, 184.78; IR
(KBr): nÄ ˆ 3448, 3039, 2989, 2902, 1745, 1565, 1375, 1340, 1222, 1176, 1083,
‡
(70 eV, EI): m/z: 303 [M ], 272, 91; elemental analysis calcd (%) for
C16H17NO5 (303.3) C 63.36, H 5.65, N 4.62; found C 63.27, H 5.69, N 4.60.
1-Benzyl-3-methoxypyrrole-2,4-dicarboxylic acid (5): Compound 3 (19 g,
57 mmol) in ethanol (300 mL) was added to a solution prepared from
NaOH (13.8 g, 344 mmol) and H2O (300 mL), and the mixture was heated
under reflux for 12 h.The ethanol was removed in vacuo, and the rest was
acidified with H2SO4 (10%) to pH 2 3 with cooling.The acid 5 was
extracted with diethyl ether.Recrystallization from ethanol gave colorless
crystals. Yield: 10.5 g (92%); m.p. 142 1438C; 1H NMR (400 MHz,
[D6]DMSO): d ˆ 3.75 (s, 1H; OCH3), 5.48 (s, 2H; CH2Ph), 7.07 7.37 (m,
5H; C6H5), 7.65 (s, 1H; H-pyrrole), 12.17 (s, 2H; COOH); 13C NMR
(100 MHz, [D6]DMSO): d ˆ 52.05, 62.09, 107.72, 113.65, 126.79, 127.38,
128.49, 131.07, 138.10, 152.65, 161.09, 163.43; IR (KBr): nÄ ˆ 3434, 3032
2880, 2615, 1663, 1547, 1461, 1280, 1073, 1001, 924 cmÀ1; elemental analysis
calcd (%) for C14H13NO5 (275.2): C 61.09, H 4.76, N 5.09; found C 60.98, H
4.65, N 4.01.
‡
1018, 966, 771, 703 cmÀ1; MS (CI): m/z (%): 318 (68) [(M/2‡1)H ], 335
‡
‡
‡
(100) [(M/2‡1)‡NH4 ], 633 (3) [M H], 650 (8) [M ‡NH4]; elemental
analysis calcd (%) for C34H36N2O10 (632.67): C 64.55, H 5.74, N 4.43; found:
C 64.21, H 5.60, N 4.36.
rac-11: oil, 0.82 g (29%); 1H NMR (250 MHz, CDCl3): d ˆ 1.21 (t, J ˆ
7.11 Hz, 6H; 2CH2CH3), 1.26 (t, J ˆ 7.05 Hz, 6H; 2CH2CH3), 4.15 (qd, J ˆ
7.12 Hz, 2H; 2HCHCH3), 4.16 (q, J ˆ 7.16 Hz, 2H; 2HCHCH3), 4.22 (q,
J ˆ 7.11 Hz, 4H; 4CH2CH3), 4.93, 4.70 (AB, J ˆ 14.55 Hz, 4H; 2CH2Ph),
7.32 7.46 (m, 10H; 2C6H5), 8.33 (s, 2H; 2H-pyrrole); 13C NMR (63 MHz,
CDCl3): d ˆ 13.72, 14.48, 54.31, 60.04, 63.53, 79.32, 105.37, 128.86, 129.27,
129.38, 134.11, 162.20, 162.74, 171.47, 187.99; IR (KBr): nÄ ˆ 3454, 2982, 1733,
1-Benzyl-3-methoxypyrrole-2,5-dicarboxylic acid (5a): (Same procedure as
given for 5.) Yield: 0.64 g from 1.0 g of 4a (70%); m.p. 1508C; 1H NMR
Chem. Eur. J. 2003, 9, No. 2
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453

A.Merz et al.
FULL PAPER
1689, 1569, 1373, 1238, 1020, 769, 702 cmÀ1; MS (FI): m/z (%): 316 (7) [M/
2 ], 632 (100) [M ]; elemental analysis calcd (%) for C34H36N2O10 (632.67):
C 64.55, H 5.74, N 4.43; found: C 64.47, H 5.66, N 4.24.
3440, 3066 2825, 1565, 1405, 1326, 1132, 1035, 771, 698, 630 cmÀ1; HRMS
(70 eV, PI-EI) m/z: calcd for C24H24N2O2: 372.1838, found 372.1835.
‡
‡
3,3'-Dimethoxy-2,2'-bipyrrole (1): By using syringe techniques and strict
argon protection, sodium (50 mg, 2.2 mmol) was dissolved in liquid
ammonia (5 10 mL) at À60 to À708C.Compound 7 (0.1 g, 0.27 mmol)
was dissolved in THF (2 mL) and added dropwise to the blue solution.
After 2 h, the mixture was allowed to warm to ambient temperature, and a
solution of NaHCO3 (0.18 g, 2.2 mmol) in argon-saturated water (3 mL)
was added carefully.When the excess sodium was completely dissolved,
toluene (2 mL) was added, and the contents were stirred for 10 min.The
toluene phase was removed by syringe under argon, and the extraction was
repeated with toluene (2 mL).The total toluene solution was placed in a
freezer at ꢃ À 208C, and 1 was obtained as colorless crystals.The crystal
suspension is recommended for storage.Yield: 36 mg (70%); mp..173
1758C; 1H NMR (250 MHz, C6D6): d ˆ 3.44 (s, 6H; 2OCH3), 5.93 (t, J ˆ
3.07 Hz, 2H; 2H4-pyrrole), 6.16 (t, J ˆ 3.07 Hz, 2H; 2H5-pyrrole), 8.49 (s,
2H; 2NH);13C NMR (63 MHz, C6D6): d ˆ 58.19, 96.30, 113.35, 142.23; IR
(KBr): nÄ ˆ 3354, 2964, 1540, 1262, 1103, 1069, 1036, 803, 695 cmÀ1; UV/Vis
(CH3CN): lmax (e) ˆ 286 (17210); HRMS (70 eV, PI-EI) m/z: calcd for
C10H12N2O2: 192.0899, found 192.0896.
Diethyl 1-benzyl-2-bromo-4-methoxypyrrole-3,5-dicarboxylate (12): This
compound was prepared exactly as described for the corresponding N-
methyl compound.[25] Compound 3 (10 g, 30 mmol) was methylated to give
12. Yield: 11.8 g (95%); m.p. 87.5 888C (MeOH); 1H NMR (250 MHz,
CDCl3): d ˆ 1.26 (t, J ˆ 7.13 Hz, 3H; CH3), 1.40 (t, J ˆ 7.13 Hz, 3H; CH3),
4.27 (q, J ˆ 7.13 Hz, 2H; CH2CH3), 4.39 (q, J ˆ 7.13 Hz, 2H; CH2CH3), 5.68
(s, 2H; CH2Ph), 6.95 7.38 (m, 6H; C6H5, H5-pyrrole), 9.37 (s, 1H; OH);
13C NMR (63 MHz, CDCl3): d ˆ 14.30, 50.60, 60.57, 60.84, 103.31, 108.52,
115.15, 126.17, 127.50, 128.66, 130.27, 136.46, 154.63, 160.87, 164.32; IR
(KBr): nÄ ˆ 3294, 2984, 1698, 1493, 1205, 1096 cmÀ1; MS (70 eV, EI): m/z
‡
‡
‡
(%): 395 (18) [M ], 351 (25) [M À EtOH], 91 (100) [C7H7 ]; elemental
analysis calcd (%) for C17H18BrNO5 (396.24): C 51.53, H 4.58, N 3.53;
found: C 51.48, H 4.48, N 4.47.
Diethyl 1-benzyl-2-bromo-4-methoxypyrrole-3,5-dicarboxylate (13): Com-
pound 12 (4 g, 10 mmol) in dry acetone (60 mL), K2CO3 (3.5 g, 25 mmol),
and dimethyl sulfate (1.27 g, 10 mmol) were used in the same procedure as
for 4.Compound 13 was recrystallized from MeOH.White crystals, 39. g
(85%); m.p. 49 518C; 1H NMR (250 MHz, CDCl3): d ˆ 1.31 (t, J ˆ
7.13 Hz, 3H; CH2CH3), 1.39 (t, J ˆ 7.13 Hz, 3H; CH2CH3), 3.89 (s, 3H;
OCH3), 4.26 (q, J ˆ 7.13 Hz, 2H; CH2CH3), 4.36 (q, J ˆ 7.13 Hz, 2H;
CH2CH3), 5.74 (s, 2H; CH2Ph), 6.94 7.37 (m, 6H; C6H5); 13C NMR
(63 MHz, CDCl3): d ˆ 14.17, 14.29, 50.54, 60.44, 60.52, 63.20, 109.39, 115.06,
115.89, 126.31, 127.43, 128.63, 136.59, 153.33, 159.81, 162.12; IR (KBr): nÄ ˆ
2981, 1706, 1542, 1493, 1416, 1288, 1242, 1095, 1029, 696 cmÀ1; MS (CI): m/z
4,4'-dimethoxy-2,2'-bipyrrole (2): Compound 2 was prepared from 16 in
exactly the same manner as 1.Again, the product 2 crystallized from the
toluene solution at low temperature, and the suspension was put in the
freezer under argon.The crystal suspension is recommended for storage.
Colorless crystals, yield: 32 mg (70%); m.p. (under argon in a sealed
1
capillary) black melt ca.130 140 8C; H NMR (250 MHz, C6D6): d ˆ 3.51
(s, 6H; 2OCH3), 5.87 (s, 4H; 4H-pyrrole), 6.48 (brs, 2H; 2NH); 13C NMR
(63 MHz, C6D6): d ˆ 99.24, 93.59, 58,07; 1H (250 MHz, CD2Cl2): 3.70 (s, 6H;
2OCH3), 5.90 (d, J ˆ 1 Hz, 2H; 2-H, H2,2'), 6,25 (d, J ˆ 1 Hz, 2H; 2H;
H6,6'), 6.69 (brs, 2H; 2NH); 13C NMR (63 MHz, CD2Cl2): d ˆ 99.11, 93.68,
57.50. In both solvents, the fourth quaternary-carbon (C2,2') signal was too
weak to be observed in NMR.IR (KBr): nÄ ˆ 3354, 2964, 1540, 1262, 1103,
1069, 1036, 803, 695 cmÀ1; UV/Vis (CH3CN): lmax (e) ˆ 286 (11937); MS-
‡
‡
(%): 410 (100) [MH ], 330 (54) [MH À HBr]; elemental analysis calcd
(%) for C18H20BrNO5 (410.27): C 52.7, H 4.91, N 3.41; found: C 52.43, H
4.78, N 3.35.
Tetraethyl 1,1'-dibenzyl-4,4'-dimethoxy-2,2'-bipyrrole-3,3',5,5'-tetracarbox-
ylate (14): A mixture of 13 (4 g, 9.8 mmol) and copper powder (15.6 g,
240 mmol) with a small amount of DMF was heated at 3008C for 30 min in
an open rotating flask with argon flushing.After cooling, the contents were
taken up with ethyl acetate, the solvent was evaporated, and the brown-
black residue was recrystallized from methanol to gave light-yellow
crystals. Yield: 3.80 g (60%); m.p. 90 918C; 1H NMR (250 MHz, CDCl3):
d ˆ 1.03 (t, J ˆ 7.12 Hz, 6H; 2CH2CH3), 1.33 (t, J ˆ 7.12 Hz, 6H;
2CH2CH3), 3.79 (q, J ˆ 7.12 Hz, 2H; 2H3CHCHCO), 3.83 (q, J ˆ 7.12 Hz,
2H; 2H3CHCHCO), 3.95 (s, 6H; 2OCH3), 3.98 (q, J ˆ 7.15 Hz, 2H;
2H3CHCHCO), 4.02 (q, J ˆ 7.15 Hz, 2H; 2H3CHCHCO), 4.30 (q, J ˆ 7.09,
4H; 2H3CCH2CO), 4.88 (d, J ˆ 2.43, 4H; 2CH2Ph), 6.81 7.19 (m, 10H;
2C6H5); 13C NMR (63 MHz, CDCl3): d ˆ 13.91, 14.21, 49.89, 59.63, 60.53,
63.08, 110.58, 115.30, 127.10, 127.48, 128.31, 129.48,136.52, 153.10, 160.41,
161.69; IR (KBr): nÄ ˆ 2981 2821, 1714, 1492, 1413, 1286, 1241, 1193, 1083,
‡
‡
CI: m/z (%): 193 (100) [M H], 373 (100) [M H]; HRMS (70 eV, PI-EI)
m/z: calcd for C10H12N2O2: 192.0899, found 192.0896.
3-Methoxypyrrole (17): Compound 6 (0.25 g 11 mmol) was treated with Na/
liquid NH3 as described for 1 and 2.Vacuum distillation of the crude
material gave a colorless oil at RT under argon, which crystallizes in the
freezer.Yield: 84 mg (87%); 1H NMR (250 MHz, CDCl3): d ˆ 3.78 (s, 3H;
OCH3),5.99 (d, 1H; H4-pyrrole), 6.37 (s, 1H; H2-pyrrole), 6.59 (d, 1H; H5-
pyrrole), 7.86 (s, 1H; NH); 13C NMR (63 MHz, CDCl3): d ˆ 58, 97, 99, 117,
149; IR (film): nÄ ˆ 3402, 3000 2900, 2829, 1571 cmÀ1; MS (70 eV, EI): m/z
‡
‡
(%): 97 (75) [M ], 82 (100) [M À CH3]; elemental analysis calcd (%) for
C5H7NO (97.12): C 61.84, H 7.26, N 14.42; found: C 61.51, H 7.28, N 14.57.
‡
1025, 194, 711 cmÀ1; MS (FD): m/z (%): 660 (100) [M ]; elemental analysis
calcd (%) for C36H40N2O10 (660.73): C 65.44, H 6.10, N 4.24; found: C 65.28,
H 6.09, N 4.19.
Acknowledgement
1,1'-Dibenzyl-4,4'-dimethoxy-2,2'-bipyrrole 3,3',5,5'-tetracarboxylic acid
(15): Compound 14 (19 g, 29 mmol) was dissolved ethanol/water (1:1,
500 mL) containing NaOH (23 g, 580 mmol), and the mixture was heated
under reflux overnight.Most of the ethanol was stripped off, and the
aqueous solution was brought to pH ꢂ 2.The white precipitate was
dissolved in diethyl ether.Evaporation of the solvent gave 14.5 g (98%) of
the title compound.Mp..110 112 8C; 1H NMR (400 MHz, CDCl3): d ˆ
3.77 (s, 6H; 2OCH3), 4.84 (d, 4H; 2CH2Ph), 6.79 7.17 (m, 10H; 2C6H5),
12.25 (s, 4H; 4OH); 13C NMR (100 MHz, CDCl3): d ˆ 49.26, 62.45, 110.13,
114.97, 126.53, 126.97, 128.06, 129.37, 136.98, 152.19, 160.98, 162.78; IR
(KBr): nÄ ˆ 3446, 2940, 2550, 1681, 1494, 1278, 1143, 1079, 993, 898, 750,
This work was supported by the Deutsche Forschungsgemeinschaft
(Me409/27-1).
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1,1'-Dibenzyl-4,4'-dimethoxy-2,2'-bipyrrole (16): Tetracarboxylic acid 15
(5 g, 9.1 mmol) was decarboxylated without solvent in a similar manner to
6. The dark residue was recrystallized from ethyl acetate under argon to
give 2.34 g (70%) of 16.Mp..135 137 8C; 1H NMR (400 MHz, CDCl3):
d ˆ 3.37 (s, 6H; 2OCH3), 4.58 (s, 4H; 2CH2Ph), 6.08 (AB, 4H; H-pyrrole),
6.77 7.05 (m, 10H; 2C6H5); 13C NMR (100 MHz, CDCl3): d ˆ 50.49, 57.20,
100.50, 104.14, 122.99, 126.98, 127.29, 128.63, 139.41, 149.33; IR (KBr): nÄ ˆ
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¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0947-6539/03/0902-0454 $ 20.00+.50/0
Chem. Eur. J. 2003, 9, No. 2

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c
ˆ
18.6717(14), a
ˆ
908. b
ˆ
99.55498, g
ˆ
908; volume:
1592.42(18) ä3, Z ˆ 4, calculated density 1.324 MgmÀ3, absorption
coefficient: 0.098 mmÀ1, F(000): 672, m(MoKa), q range: 2.868 to 25.908,
data collected: 19810, independent data 2556 (Rint ˆ 0.0303); observed
data [I > 2s(I)] 2915, goodness-of-fit on F 2 ˆ 1.052.
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Crystal structure analysis of 10: C34H36N2O10, MW 632.65, crystal size:
0.36 Â 0.20 Â 0.10 mm, prism, translucent, colorless, monoclinic, space
group: P21/n; unit cell dimensions [ä]: a ˆ 10.1451(5), b ˆ 16.2191(9),
c
ˆ
19.0030(11), a
ˆ
908, b
ˆ
94.141(5)8, g
ˆ
908; volume:
3118.7(3) ä3, Z
ˆ
4, calculated density 1.347 MgmÀ3, absorption
coefficient: 0.098mmÀ1, F(000): 1336, m(MoKa), q range: 2.218 to
25.818, data collected: 31053, independent data 5861 (Rint ˆ 0.0303);
observed data [I > 2s(I)] 4182, goodness-of-fit on F 2 ˆ 0.052.
[34] E.Lorthiois, I.Marek, J.F.Normant,
2450.
J. Org. Chem. 1998, 63, 2442
Data collection was on
a STOE-IPDS diffractometer, method:
rotation, Tˆ 173 K, l ˆ 0.71073, graphite monochromator.
CCDC-187623 (3) and CCDC-187624 (10) contain the supplementary
crystallographic data for this paper.These data can be obtained free of
Received: June 24, 2002 [F4204]
Chem. Eur. J. 2003, 9, No. 2
¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0947-6539/03/0902-0455 $ 20.00+.50/0
455