Facile synthesis of alternating benzene-pyrrole oligomers by cyclization of propargylic dithioacetals and imines and their fluorescent properties

Article abstract of DOI:10.1002/ejoc.201100111

Alternating benzene-heteroaryl oligomers possess fascinating optoelectronic properties and a wide range of applications. This article presents the facile synthesis of benzenepyrrole oligomers with diverse functional groups and the elongated alternating he

Full text of DOI:10.1002/ejoc.201100111

FULL PAPER
DOI: 10.1002/ejoc.201100111
Facile Synthesis of Alternating Benzene–Pyrrole Oligomers by Cyclization of
Propargylic Dithioacetals and Imines and Their Fluorescent Properties
Tong-Hui Huang,[a] Xiao-Lei Zhu,[a] Ai-Dong Zhang,*[a] and Hai-Yang Tu*[a]
Keywords: Alkynes / Cyclization / Fluorescence / Nitrogen heterocycles / Imines
Alternating benzene–heteroaryl oligomers possess fascinat-
ing optoelectronic properties and a wide range of applica-
tions. This article presents the facile synthesis of benzene–
pyrrole oligomers with diverse functional groups and the
elongated alternating heterocycle–benzene–pyrrole oligoar-
yls. The syntheses are based on a one-pot, three-step reac-
tion of propargylic dithioacetals and imines. The subtle in-
fluence of functional groups, such as ether, ester, hydroxy,
and dithiacetal groups on the peripheral benzene rings, on
the reaction, has been explored and the reaction conditions
were finely changed accordingly to achieve reasonable
yields. The elongation could be accomplished by using a sim-
ilar procedure. The fluorescent properties of original and
elongated alternating benzene–pyrrole oligomers were de-
termined. Theoretical calculations preliminarily reveal that
the fluorescent intensity is closely related to the molecular
geometry and to the HOMO/LUMO energy difference in the
ground state.
route for the synthesis of alternating teraryls with furan
rings.[9] Additional functions, such as alkyl chains, can be
introduced at the C-3 position of the furan ring to enhance
the solubility and hence simplify the procedures used to ob-
tain the oligomers. The synthesis is usually mild, and vari-
ous functionalities, such as carbon–carbon double and tri-
ple bounds, and even ester groups, can be tolerated. Thus,
elongation of the conjugation lengths of the oligoaryls can
be achieved by means of the Suzuki[7b] or Stille[7c] coupling
reactions, or by a convergent synthesis.[9b]
The pyrrol moiety, as an equivalent of furan and thio-
phene, can also be incorporated into oligoaryls through the
traditional Paal–Knorr cyclization of amines with 1,4-di-
ketones, which can be pre-formed through a necessary
multistep preparation.[10] Luh’s one-pot annulation protocol
is applicable and allows access of benzene–pyrrole oligomers
with a limited substituent scope by using imines instead of
arenecarbaldehydes.[9] Our initial study suggested that this
strategy could not be easily adopted in the synthesis of benz-
ene–pyrrole oligoaryls with diverse functionalities or in the
subsequent convergent synthesis of elongated analogues. In
this article, we report a facile, one-pot synthesis of these
oligoaryls through the annulation of propargylic dithioacet-
als with imines by modifying the reaction conditions. More-
over, the functional groups on the peripheral benzene rings
could be utilized further for the construction of elongated
oligoaryls with two or more five-membered heteroaromatic
ring units in a convergent synthetic protocol.
Introduction
Oligoaryls possess fascinating optoelectronic properties
that have been explored in the applications of photovoltaic
cells, light-emitting diodes (LEDs), field-effect transistors
(FETs), electrochromic devices, chemical sensors, and mi-
croelectronic actuators, among others.[1] The optoelectronic
properties of the oligoaryls can be enhanced and tuned to
satisfy the requirements of various applications by incorpo-
rating five-membered heteroaromatic moieties.[2] However,
although typical representative thiophene-containing oli-
goaryls have been extensively investigated,[3] the furan[4] or
pyrrole[5] analogues have been only sporadically explored,
mainly due to the limited synthetic methodologies available
and because of the incompatibility of the desired function-
alities of key intermediates in the synthesis.
There are various traditional approaches available for the
synthesis of conjugated oligoaryls incorporating five-mem-
bered heteroaromatic rings. Cyclization of heteroatom com-
pounds with pre-formed 1,4-diketones[6] and cross-coupling
reactions catalyzed by transition metals[7] are two principal
methodologies used in the construction of oligoaryl back-
bones. In recent years, Luh reported an exquisite one-pot
method for the synthesis of oligoaryls containing 2,3,5-tri-
substituted furans by treating propargylic dithioacetals with
arenecarbaldehydes.[8] This strategy provides a versatile
[a] Key Laboratory of Pesticide & Chemical Biology of Ministry
of Education, Central China Normal University,
Wuhan 430079, P. R. of China
Fax: +86-27-67867141
Results and Discussion
E-mail: adzhang@mail.ccnu.edu.cn
haiytu@mail.ccnu.edu.cn
The cyclization of propargylic dithioacetals and imines
to give alternating benzene–furan or benzene–pyrrole oli-
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100111.
4588
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Synthesis and Fluoresce of Benzene–Pyrrole Oligomers
Scheme 1. Mechanistic illustration of the one-pot, three-step synthesis of alternating benzene–pyrrole oligoaryls.
goaryls by applying Luh’s protocol was proposed to be ac-
Unfortunately, no cyclization product was formed in any
complished through a one-pot, three-step reaction mecha- of the cases when the reaction was performed at room tem-
nism.[9a] Propargylic dithioacetal 1 reacts with butyllithium perature under the catalysis of BF3·Et2O in the third step
to form a lithiated allene intermediate that reacts with of the reported procedure. Prolonging the reaction time did
phenylenimine 2 and subsequently undergoes acid-catalyzed not benefit the cyclization. This failure may be attributed
ring-closure to afforded the desired product 3 (Scheme 1). to the different contribution of electronic effects from the
Two key processes that can reasonably be assumed to be N-butyl group in Luh’s case[9a] and the N-phenyl group in
involved in the mechanism are the resonance of the propar- our case to the nucleophilicity of the enamine nitrogen
gylic anion to the allene anion in the first step, and the atom. Clearly, the N-phenyl group decreases the nucleophi-
tautomerization of the allene anion to the conjugated en- licity of the enamine nitrogen atom to a certain extent be-
amine anion in the second step. The Lewis acid catalyzed cause of the increased delocalization compared to the N-
ring closure in the last step is believed to be carried out butyl group, which reduces its ability to attack the olefinic
through the association of BF3 with the alkyl sulfide, which carbon atom bearing the alkylsulfur group. However, the
increases its leaving ability and facilitates the ring closure.
reaction was found to proceed (indicated by 1H NMR
Application of Luh’s protocol worked smoothly for the analysis of the crude mixture) when heated at reflux for 6 h
cyclization for furan oligomers and for several cases of pyr- and then stirred at room temperature overnight for the last
role oligomers without functionalities.[9] With respect to the step. Indeed, these reaction conditions worked very well to
synthesis of pyrrole oligomers capable of convergent elong- produce 3a, 3b and 3g with appreciable yields, as shown in
ation and further functionalization, necessary functionali- Table 1; however, initially, the other expected products
ties such as ethers, esters, hydroxy, and even dithiacetal could not be obtained in reasonable yields.
groups need to be present in the corresponding benzene
rings prior the cyclization reaction. Thus, we initialized the
one-pot, three-step synthesis of pyrrole oligomers bearing
the desired functionalities according to Luh’s protocol as
shown in Scheme 2.
Attempts were then made to increase the yields in the
other cases. It was found that the reaction of propargylic
dithioacetal 1b with n-butyllithium in the presence of hexa-
methylphosphoramide (HMPA) was readily accomplished,
and the yield of 3c increased from 14% to 53% (Table 1).
The promotion by HMPA is presumably attributed to its
ability to stabilize the lithium counter ion,[11] because the
presence of an electron-withdrawing methoxycarbonyl
group at the para position of the phenyl dithioacetal renders
the lithium counter ion unstable in the medium. Thus, lith-
ium stabilization with HMPA was needed in the first step,
leading the smooth addition of the allene anion to the imine
in the second step. However, the addition of HMPA did not
affect the reaction in the remaining cases. In the cases of 3d
and 3e, because of the presence of the methoxymethyl
group at the para position, the dithioacetal becomes more
reactive to the nucleophile; it was therefore thought that the
Scheme 2. Cyclization of propargylic dithioacetals and imines in a
one-pot, three-step procedure.
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Table 1. Summary of the product yields and their fluorescent properties.
[a] Measured in EtOAc solution. [b] The quantum yield was determined in EtOAc by employing coumarin-1 (Φf = 0.99 in EtOAc)[13] as
the reference.
use of a weaker nucleophile from another alkanide source duce the yield of the reaction. The syntheses of these com-
may be helpful. Thus, the yield of 3d was increased from 36 pounds and their fluorescent properties are summarized in
to 63% when nBu2CuLi was used instead of nBuLi; Scheme 3 and Table 1.
whereas, for 3e, the yield was increased from 32 to 64%.
All the synthesized benzene–pyrrole oligoaryls fluoresce,
Interestingly, using either nBu2CuLi or nBuLi as the nucleo- but with significant differences; the maximum absorption
phile source had no significant effect, and 3a or 3b were wavelengths (λmax), maximum emission wavelengths (λem),
produced in similar yields.
and the fluorescence quantum yields (Φf) of these oligomers
When the reaction conditions (i.e., stirring at reflux for are listed in Table 1. As expected, introducing an electron-
6 h and then room temperature overnight in the last step) withdrawing group (in the cases of 3a and 3c) or expanding
were adopted for the synthesis of the hydroxy-bearing prod- the conjugated system (in the cases of 5, 6 and 8) could
uct 3f, no product was found. However, the crude NMR significantly bathochromically shift the maximum absorp-
spectra clearly indicated the presence of an acyclic product tion and emission wavelengths. All compounds exhibited
in the reaction medium, indicating that the one-pot, three- strong absorption (in all cases, log ε Ͼ 4), whereas only 3a,
step reaction stopped at the second stage. Hence, trifluoro- 3c, 6, and 8 showed strong emission in the visible region in
acetic acid (TFA) was finally added, and the reaction me- EtOAc solution, with fluorescence quantum yields of 64,
dium was heated to reflux for a further 4 h; this led to ring 88, 39, and 50%, respectively. Other oligoaryls produced
closure and the formation of 3f with a yield of up to 48%. fluorescence quantum yields of less than 10%. Compounds
A reasonable explanation for this result is that TFA, which 3a and 3c showed much stronger fluorescence, which clearly
is a stronger acid than BF3·OEt2, may improve the leaving arose from the contribution of the conjugated carbonyl
ability of the thiolate group in the ring-closure step. It is group to the delocalized system, regardless of whether the
worth mentioning that BF3·OEt2 cannot be omitted from group was located on the propargylic dithioacetal side or
the reaction, possibly because of its ability to promote the the imine side of the starting material.
resonance of the propargylic to the allene anion, which then
The fluorescent properties of oligoaryls, such as sexithio-
attacks the imine carbon atom in the second step. This phe- phene[14] and alternating benzene–furan oligomers,[2c] has
nomenon implies that BF3·OEt2 functions in the second been reported to be related to their structures and geome-
step in all cases. However, for 3f, a stronger acid catalysis tries. In our case, to preliminarily explore the origin of the
is needed in the last step because of the presence of the differences in fluorescence of the synthesized compounds,
hydroxy group.
theoretical calculations were performed by using
With the facile synthesis of oligomers bearing functional Gaussian 03 at the B3LYP/6-31+G(d) basis set level for the
groups established, the use of these functionalities to elongated systems 5, 6 and 8 in the ground state; the opti-
achieve elongation was attempted. In a similar manner, the mized geometries are illustrated in Figure 1. Incontrovert-
pyrrole-bearing dithioacetal 3g and the furan-bearing pro- ibly, all compounds displayed imperfect coplanar states
pargylic dithioacetal 4[8a] were separately treated with with nonplanar twists; the dihedral angles between the two
nBuLi, then treated with the imines 2c and subsequently heterocycles separated by the middle benzene ring in 5, 6
submitted to BF3·Et2O-catalyzed ring closure with stirring and 8 were 89.2°, 13.2° and 74.7°, respectively. With respect
first under reflux and then at room temperature overnight; to the conformations, 5 and 8 have a similar dihedral angle,
the desired products 5 and 6 were thus obtained in 52 and which is much larger than that of 6; however, the maximum
49% yield, respectively. Moreover, the propargylic di- absorptions of 6 and 8 are similar, both being bathochrom-
thioacetal 1a could be employed in the reaction with the ically shifted by approximately 30 nm compared with that
furan-bearing imine 7,[12] by a similar procedure, to obtain of 5. This result implies that the extent of conjugation is
8 in 44% yield. Interestingly, substituents either on the pro- not a critical factor. The imbalance in the electronegativity
pargylic dithioacetal side or on the imine side did not re- of these compounds may contribute to the difference, be-
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Synthesis and Fluoresce of Benzene–Pyrrole Oligomers
Scheme 3. Synthetic routes of pyrrole oligomers 5, 6 and 8.
cause compound 5 consists of only pyrrole rings, whereas ported to be related to the coplanarity and energy gaps not
compounds 6 and 8 contain both pyrrole and furan rings. only in the ground state but also in excited state,[16] at pres-
Furan is a stronger electron-donating group than pyr- ent it is difficult to obtain a quantitative structure–effect
role;[15] thus, a partial movement of electron density relationship. Nonetheless, we were still able to preliminarily
through the middle benzene ring favors a bathochromic determine that smaller twist angles and narrower HOMO/
shift of the maximum absorptions in the systems of 6 and LUMO energy gaps benefit the fluorescence quantum yield.
8. The energy differences between the HOMO and LUMO Further investigations aimed at determining the correlation
orbitals for compounds 5, 6, and 8 in the ground state were between the excited-state energy and fluorescence is un-
estimated to be 4.11, 3.65 and 3.56 eV, respectively. These derway.
differences result in the corresponding fluorescent quantum
yields of 7, 39, and 50% for 5, 6 and 8, respectively. Because
the fluorescent quantum yield of a conjugated system is re-
Conclusions
This work presents the development of the propargylic
dithioacetal/imine cyclization strategy for the facile synthe-
sis of 2,3,5-trisubstituted pyrroles bearing ether, ester, hy-
droxy, and dithiacetal groups on the peripheral benzene
rings. The substituents on the peripheral benzene rings of
propargylic dithioacetal can impart subtle changes to the
electron distribution, leading to the requirement for modi-
fied reaction conditions to make the reactions proceed with
a reasonable yield. Nevertheless, a simple guideline can still
be derived from the listed examples: (1) allene anion forma-
tion with attack of organolithium is workable in the first
step, with the exception of compounds such as 3d and 3e,
for which the use of lithium dialkylcuprate is more suitable
because of the presence of the methoxymethyl group in the
propargylic dithioacetal; (2) the electron-withdrawing group
methoxycarbonyl slightly decreases the electron density of
the resulting allene anion, therefore stabilization of the lith-
Figure 1. Geometries of oligomers 5, 6 and 8, calculated at the
B3LYP/6-31G* level.
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3 mmol) in THF (20 mL), nBuLi (2.5 m in hexane, 1.4 mL,
3.6 mmol), HPMA (0.64 g, 3.6 mmol) in the first step, and N-
benzylideneaniline (2c; 0.65 g, 3.6 mmol) were used to afford 3c as
white crystals (0.65 g, 53%); m.p. 105–106 °C. 1H NMR (400 MHz,
CDCl3): δ = 0.90 (t, J = 7.3 Hz, 3 H, nBu CH3), 1.34–1.39 (m, 2
H, nBu CH2), 1.59–1.63 (m, 2 H, nBu CH2), 2.50 (t, J = 7.6 Hz, 2
H, nBu CH2), 3.86 (s, 3 H, OCH3), 6.53 (s, 1 H, Pyrrole-H), 6.92–
7.19 (m, 12 H, Ar-H), 7.81 (dd, J1 = 8.3, J2 = 2.0 Hz, 2 H, Ar-H)
ppm. 13C NMR (100 MHz, CDCl3): δ = 14.0 (nBu CH3), 22.7,
25.9, 33.5, 51.9 (OCH3), 111.6, 124.0, 126.7, 127.0, 127.6, 127.7,
ium cation is needed and addition of HMPA is helpful; (3)
for hydroxy-bearing propargylic dithioacetal, at least
1 equiv. of TFA is needed in the ring-closure step. This
guideline is suitable for the one-pot, three-step synthesis of
pyrrole oligomers bearing functional groups, which are nec-
essary for the synthesis of elongated derivatives. Moreover,
the synthetic protocol is applicable to the convergent syn-
thesis of elongated alternating heterocycle–benzene–furan
oligomers regardless of which heterocycle is attached (e.g.,
for the synthesis of 5 and 8). Theoretical calculations pre- 128.6, 128.7, 129.2, 130.8, 132.5, 132.6, 134.0, 137.7, 138.9 (Ar-C),
167.0 (C=O) ppm. HRMS: calcd. for C28H27NO2 409.2042; found
409.2045.
liminarily suggest that the fluorescence intensity is closely
related to the molecular geometry and to the HOMO/
LUMO energy difference.
3-Butyl-5-[4-(methoxymethyl)phenyl]-1,2-diphenyl-1H-pyrrole (3d):
In a procedure similar to that for the preparation of 3a, a solution
of 2-(1-hexyn-1-yl)-2-(4-methoxymethylphenyl)-1,3-dithiolane (1c;
0.92 g, 3 mmol) in THF (20 mL), a solution of nBu2CuLi [prepared
from nBuLi (3.5 mmol) and CuI (0.34 g, 1.8 mmol) in THF
(100 mL)], and N-benzylideneaniline (2c; 0.65 g, 3.6 mmol) were
used to give 3d as white crystals (0.75 g, 63%); m.p. 105–106 °C.
1H NMR (400 MHz, CDCl3): δ = 0.90 (t, J = 7.3 Hz, 3 H, nBu
CH3), 1.34–1.39 (m, 2 H, nBu CH2), 1.59–1.63 (m, 2 H, nBu CH2),
2.51 (t, J = 7.7 Hz, 2 H, nBu CH2), 3.37 (s, 3 H, OCH3), 4.37 (s, 2
H, OCH2), 6.41 (s, 1 H, Pyrrole-H), 7.20–6.92 (m, 14 H, Ar-H)
ppm. 13C NMR (100 MHz, CDCl3): δ = 14.0 (nBu CH3), 22.7,
26.0, 33.6, 58.2 (OCH3), 74.5 (OCH2), 110.3, 123.5, 126.4, 126.6,
127.4, 127.6, 128.3, 128.4, 128.8, 130.8, 132.5, 132.7, 132.8, 133.6,
135.6, 139.1 (Ar-C) ppm. HRMS: calcd. for C28H29NO 395.2249;
found 395.2245. C28H29NO (395.2): calcd. C 85.02, H 7.39, N 3.54;
found C 85.09, H 7.53, N 3.50.
Experimental Section
General: Mass spectra were measured with a Finnigan Trace MS
spectrometer. NMR spectra were recorded in CDCl3 with a Bruker
400 MHz spectrometer and resonances are given in ppm (δ) relative
to TMS. UV/Vis absorption spectra were measured with a Shim-
adzu UV-1601PC UV/Vis spectrophotometer. Emission spectra
were recorded with an Aminco-Bowman Series 2 luminescence
spectrometer.
Methyl 4-(1,3,5-Triphenyl-1H-pyrrol-2-yl)benzoate (3a): Under ni-
trogen, to a solution of 2-phenyl-2-(2-phenylethynyl)-1,3-dithiolane
(1a; 0.3 g, 1 mmol) in THF (60 mL) cooled to –78 °C, was added,
dropwise, nBuLi (2.5 m in hexane, 0.5 mL, 1.2 mmol). The mixture
was stirred at –78 °C for 1 h, and then a solution of methyl 4-
(benzylideneamino)benzoate (2a; 0.3 g, 1.2 mmol) in THF (20 mL)
was introduced. The mixture was stirred at –78 °C for 0.5 h, and
BF3·OEt2 (0.3 mL, 2.3 mmol) was then added. The mixture was
stirred at reflux for 1 h, then cooled to room temperature overnight,
and quenched with saturated aqueous NH4Cl. The organic layer
was dried with MgSO4, and the solvent was removed in vacuo to
give a residue that was purified by chromatography on silica gel
(hexane/ethyl acetate, 20:1) to give 3a as white crystals (0.18 g,
3-Butyl-5-[4-(methoxymethyl)phenyl]-1-phenyl-2-(p-tolyl)-1H-pyr-
role (3e): In a procedure similar to that for the preparation of 3d,
a solution of 2-(1-hexyn-1-yl)-2-(4-methoxymethylphenyl)-1,3-di-
thiolane (1c; 0.92 g, 3 mmol) in THF (100 mL), a solution of nBu2-
CuLi [prepared from of nBuLi (3.5 mmol) and CuI (0.34 g,
1.8 mmol) in THF (100 mL)], and N-benzylideneaniline (2c; 0.70 g,
3.6 mmol) were used to give 3e as white crystals (0.79 g, 64%); m.p.
1
149–150 °C. H NMR (400 MHz, CDCl3): δ = 0.90 (t, J = 7.3 Hz,
1
43%); m.p. 134–135 °C. H NMR (400 MHz, CDCl3): δ = 3.86 (s,
3 H, nBu CH3), 1.37–1.39 (m, 2 H, nBu CH2), 1.61–1.63 (m, 2 H,
nBu CH2), 2.28 (s, 3 H, Ar-CH3), 2.49 (t, J = 7.7 Hz, 2 H, nBu
CH2), 3.36 (s, 3 H, OCH3), 4.37 (s, 2 H, OCH2), 6.40 (s, 1 H,
Pyrrole-H), 6.92–6.94 (m, 4 H, Ar-H), 6.99–7.05 (m, 4 H, Ar-H),
7.10–7.15 (m, 5 H, Ar-H) ppm. 13C NMR (100 MHz, CDCl3): δ =
14.0 (nBu CH3), 21.2, 22.8 (CH3), 26.1, 33.6, 58.2 (OCH3), 74.6
(OCH2), 110.3, 123.3, 126.5, 127.4, 128.2, 128.3, 128.4, 128.9,
129.8, 130.7, 132.6, 132.8, 133.4, 135.6, 136.0, 139.2 (Ar-C) ppm.
HRMS: calcd. for C29H31NO 409.2406; found 409.2408.
C29H31NO (409.2): calcd. C 85.04, H 7.63, N 3.42; found C 84.86,
H 7.58, N 3.39.
3 H, OCH3), 6.69 (s, 1 H, Pyrrole-H), 6.97–7.25 (m, 17 H, Ar-H),
7.78 (d, J = 8.2 Hz, 2 H, Ar-H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 52.0 (CH3), 110.5, 124.6, 125.8, 126.6, 127.4, 128.0,
128.1, 128.2, 128.4, 128.6, 129.0, 130.7, 131.2, 132.6, 135.6, 135.8,
137.4, 138.5 (Ar-C), 166.9 (C=O) ppm. HRMS: calcd. for
C30H23NO2 429.1729; found 429.1727. C30H23NO2 (429.1): calcd.
C 83.89, H 5.40, N 3.26; found C 83.78, H 5.31, N 3.44.
1,3,5-Triphenyl-2-(p-tolyl)-1H-pyrrole (3b): In a procedure similar
to that for the preparation of 3a, 2-phenyl-2-(2-phenylethynyl)-1,3-
dithiolane (1a; 0.85 g, 3 mmol) in THF (150 mL), nBuLi (2.5 m in
hexane, 1.4 mL, 3.6 mmol), and N-(4-methylbenzylidene)aniline
(2b; 0.58 g, 3 mmol) were used to produce 3b as pale-yellow crystals
4-Butyl-1-phenyl-5-(p-tolyl)-2-[(4-hydroxymethyl)phenyl]-1H-pyrrole
(3f): In a procedure similar to that for the preparation of 3a, a
solution of 2-(1-hexyn-1-yl)-2-(4-hydroxymethylphenyl)-1,3-dithiol-
ane (1d; 0.88 g, 3 mmol) in THF (150 mL), a solution of nBuLi
(2.5 m in hexane, 2.9 mL, 7.2 mmol), and a solution of N-benzyl-
ideneaniline (2c; 0.65 g, 3.6 mmol) in THF (40 mL) were used. Af-
ter the reaction, the solvent was removed in vacuo to give a residue,
which was dissolved in THF (50 mL), and TFA (0.75 mL) was
added. The mixture was heated at reflux overnight, then quenched
with saturated aqueous NaHCO3. The separated organic layer was
washed with saturated aqueous NaHCO3 (3ϫ 25 mL) and brine
(25 mL) and then dried with MgSO4. The solvent was removed in
vacuo to give a residue that was purified by chromatography on
1
(0.75 g, 65%); m.p. 172–173 °C. H NMR (400 MHz, CDCl3): δ =
2.27 (s, 3 H, CH3), 6.70 (s, 1 H, Pyrrole-H), 6.92–7.25 (m, 19 H,
Ar-H) ppm. 13C NMR (100 MHz, CDCl3): δ = 21.2 (CH3), 109.9,
123.3, 125.4, 126.3, 127.0, 127.9, 128.0, 128.1, 128.4, 128.5, 128.6,
129.1, 129.6, 131.3, 132.3, 133.0, 134.7, 136.3, 136.6, 138.9 (Ar-
C) ppm. HRMS: calcd. for C29H23N 385.1830; found 385.1834.
C29H23N (385.1): calcd. C 90.35, H 6.01, N 3.63; found C 90.08,
H 6.09, N 3.74.
Methyl 4-(4-Butyl-1,5-diphenyl-1H-pyrrol-2-yl)benzoate (3c): In a
procedure similar to that for the preparation of 3a, 2-(1-hexyn-1-
yl)-2-(4-methoxycarbonylphenyl)-1,3-dithiolane
(1b;
0.96 g,
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Synthesis and Fluoresce of Benzene–Pyrrole Oligomers
silica gel (hexane/ethyl acetate, 20:1) to give 3f as white crystals
(0.55 g, 48%); m.p. 153–154 °C. H NMR (400 MHz, CDCl3): δ =
133.5, 137.0, 139.2, 147.8, 151.6 (Ar-C) ppm. HRMS: calcd. for
C42H43NO2 593.3294; found 593.3281. C42H43NO2 (593.3): calcd.
1
0.90 (t, J = 7.3 Hz, 3 H, nBu CH3), 1.34–1.40 (m, 2 H, nBu CH2), C 84.95, H 7.30, N 2.36; found C 84.67, H 7.28, N 2.35.
1.58–1.64 (m, 2 H, nBu CH2), 2.51 (t, J = 7.7 Hz, 2 H, nBu CH2),
2-[4-(3,5-Diphenylfuran-2-yl)phenyl]-1,3,5-triphenyl-1H-pyrrole (8):
In a procedure similar to that for the preparation of 5, a solution of
4.60 (s, 2 H, OCH2), 6.42 (s, 1 H, Pyrrole-H), 6.93–6.94 (m, 2 H,
Ar-H), 7.03–7.07 (m, 4 H, Ar-H), 7.14–7.20 (m, 8 H, Ar-H) ppm.
the propargylic dithioacetal 1a (0.85 g, 3 mmol) in THF (100 mL),
13C NMR (100 MHz, CDCl3): δ = 14.0 (nBu CH3), 22.7, 26.0, 33.6,
nBuLi (2.5 m in hexane, 1.35 mL, 3.3 mmol), and the imine 7 (0.6 g,
65.2 (CH2OH), 110.4, 123.6, 126.4, 126.6, 127.6, 128.5, 128.8,
1
1.5 mmol) were used to give 8 (0.39 g, 44%); m.p. 119–120 °C. H
130.8, 132.6, 132.8, 133.5, 138.3, 139.2 (Ar-C) ppm. HRMS: calcd.
NMR (400 MHz, CDCl3): δ = 6.70 (s, 1 H, Pyrrole-H), 6.78 (s, 1
for C27H27NO 381.2093; found 381.2100. C27H27NO (381.2): calcd.
C 85.00, H 7.13, N 3.67; found C 84.74, H 7.13, N 3.83.
H, Furan-H), 7.01–6.93 (m, 4 H, Ar-H), 7.41–7.12 (m, 23 H, Ar-
H), 7.73 (d, J = 7.2 Hz, 2 H, Ar-H) ppm. 13C NMR (100 MHz,
2-{4-[2-(Hex-1-yn-1-yl)-1,3-dithiolan-2-yl]phenyl}-1,3,5-triphenyl-
1H-pyrrole (3g): In a procedure similar that for to the preparation
of 3a, a solution of 2-phenyl-2-(2-phenylethynyl)-1,3-dithiolane (1a;
1.02 g, 3.6 mmol) in THF (100 mL), a solution of nBuLi (2.5 m in
hexane, 1.4 mL, 3.5 mmol), and the imine 2d (1.10 g, 3 mmol) were
used to give 3g (0.72 g, 43 %); m.p. 134–135 °C. 1H NMR
(400 MHz, CDCl3): δ = 0.91 (t, J = 7.3 Hz, 3 H, nBu CH3), 1.39–
1.44 (m, 2 H, nBu CH2), 1.50–1.56 (m, 2 H, nBu CH2), 2.33 (t, J
= 7.1 Hz, 2 H, nBu CH2), 3.62–3.69 (m, 4 H, Dithiolane-H), 6.69
(s, 1 H, Pyrrole-H), 6.96–7.00 (m, 4 H, Ar-H), 7.25–7.10 (m, 13 H,
Ar-H), 7.70 (d, J = 8.4 Hz, 2 H, Ar-H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 13.6 (nBu CH3), 18.9, 22.1, 30.7, 41.2, 62.1, 82.3, 88.2
(Alkyne-C), 110.2, 123.9, 125.5, 126.4, 127.2, 127.3, 127.9, 128.2,
128.3, 128.5, 128.6, 129.0, 131.0, 131.5, 132.5, 132.9, 135.0, 136.0,
137.6, 138.7 (Ar-C) ppm. HRMS: calcd. for C37H34NS2 555.2054;
found 555.2054. C37H33NS2 (555.2): calcd. C 79.96, H 5.98, N 2.52;
found C 79.84, H 6.15, N 2.62.
CDCl3): δ = 109.6, 110.2, 123.8, 124.7, 125.3, 125.6, 126.4, 127.2,
127.3, 127.5, 127.9, 128.2, 128.3, 128.5, 128.6, 128.6, 128.7, 129.1,
129.3, 130.4, 131.3, 131.6, 132.8, 134.2, 135.0, 136.1, 138.8, 147.5,
152.5 ppm. HRMS: calcd. for C44H31NO 589.2406; found
589.2416. C44H31NO (589.2): calcd. C 89.61, H 5.30, N 2.38; found
C 89.31, H 5.34, N 2.47.
Supporting Information (see footnote on the first page of this arti-
cle): Copies of the 1H and 13C NMR spectra for all new com-
pounds.
Acknowledgments
The authors gratefully acknowledge financial support from the
National Natural Science Foundation of China (No. 20972056) and
the Program for Changjiang Scholars and Innovative Research
Team in University (PCSIRT) (No. IRT0953). We thank Prof. Tien-
Lau Luh from the National Taiwan University for helpful dis-
cussions.
3-Butyl-1,2-diphenyl-5-[4-(1,3,5-triphenyl-1H-pyrrol-2-yl)phenyl]-
1H-pyrrole (5): In a procedure similar to that for the preparation
of 3a, a solution of propargylic dithioacetal (3g; 0.6 g, 1.1 mmol)
in THF (100 mL), a solution of nBuLi (2.5 m in hexane, 0.5 mL,
1.25 mmol), and N-benzylideneaniline (2c; 0.29 g, 1.6 mmol) were
used. After the usual workup, the residue was recrystallized from
dichloromathene to give 5 (0.37 g, 52 %); m.p. 250–251 °C. 1H
NMR (400 MHz, CDCl3): δ = 0.88 (t, J = 7.3 Hz, 2 H, nBu CH3),
1.33–1.39 (m, 2 H, nBu CH2), 1.57–1.62 (m, 2 H, nBu CH2), 2.48
(t, J = 7.6 Hz, 2 H, nBu CH2), 6.39 (s, 1 H, Pyrrole-H), 6.68 (s, 1 H,
Pyrrole-H), 6.81–7.23 (m, 29 H, Ar-H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 13.8 (CH3), 22.6, 25.9, 33.5, 109.9, 110.7, 123.5, 125.3,
126.2, 126.3, 126.4, 126.9, 127.4, 127.8, 127.8, 127.9, 128.1, 128.3,
128.4, 128.7, 129.0, 130.8, 131.8, 132.1, 136.1, 138.8, 139.0 (Ar-
C) ppm. HRMS: calcd. for C48H40N2: 644.3191; found 644.3185.
C48H40N2 (644.3): calcd. C 89.40, H 6.25, N 4.34; found C 89.12,
H 6.28, N 4.35.
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Eur. J. Org. Chem. 2011, 4588–4594
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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In a manner similar to that used in the synthesis of 2c, aniline
(0.33 g, 3.6 mmol) was allowed to react with furancarbaldehyde
(0.58 g, 1.8 mmol) in benzene (25 mL) to give imine 7 (0.56 g,
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Received: January 26, 2011
Published Online: June 29, 2011
4594
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 4588–4594