High-Yield synthesis of the novel E,E-2, 5-dimethoxy-1,4-bis[2-(4- ethylcarboxylatestyril)] benzene by the heck reaction

Article abstract of DOI:10.1080/00397911.2012.699579

The novel E,E-2,5-dimethoxy-1,4-bis[2-(4-ethylcarboxylatestyril)]benzene, 4, was obtained in good yield (92%), by the Heck cross-coupling reaction using Pd(dba)₂ and P(OPh)₃ like catalytic system. The high trans specificity of the product produced by the Heck reaction was confirmed by Fourier Transform-infrared and NMR. The methodology reported can be used as synthetic route for precursors to phenylenevinylene target systems with highly desired functional groups in their molecular structure, such as carboxylic, to build metal-organic frameworks and other applications within the supramolecular chemistry. Copyright

Full text of DOI:10.1080/00397911.2012.699579

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High-Yield Synthesis of the Novel
E,E-2,5-Dimethoxy-1,4-bis[2-(4-
ethylcarboxylatestyril)]benzene by the
Heck Reaction
Diego Alzate a , Juan P. Hinestroza b & Cesar A. Sierra a
a Departamento de Química , Universidad Nacional de Colombia ,
Bogotá , Colombia
b College of Human Ecology , Cornell University , Ithaca , New York ,
USA
Accepted author version posted online: 26 Jan 2013.Published
online: 03 Jun 2013.
To cite this article: Diego Alzate , Juan P. Hinestroza & Cesar A. Sierra (2013): High-Yield Synthesis
of the Novel E,E-2,5-Dimethoxy-1,4-bis[2-(4-ethylcarboxylatestyril)]benzene by the Heck Reaction,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 43:17, 2280-2285
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Synthetic Communications1, 43: 2280–2285, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.699579
HIGH-YIELD SYNTHESIS OF THE NOVEL E,E-2,
5-DIMETHOXY-1,4-BIS[2-(4-ETHYLCARBOXYL-
ATESTYRIL)]BENZENE BY THE HECK REACTION
Diego Alzate,1 Juan P. Hinestroza,2 and Cesar A. Sierra1
1
´
Departamento de Quımica, Universidad Nacional de Colombia,
´
Bogota, Colombia
2College of Human Ecology, Cornell University, Ithaca, New York, USA
GRAPHICAL ABSTRACT
Abstract The novel E,E-2,5-dimethoxy-1,4-bis[2-(4-ethylcarboxylatestyril)]benzene, 4,
was obtained in good yield (92%), by the Heck cross-coupling reaction using Pd(dba)2
and P(OPh)3 like catalytic system. The high trans specificity of the product produced by
the Heck reaction was confirmed by Fourier Transform–infrared and NMR. The method-
ology reported can be used as synthetic route for precursors to phenylenevinylene target
systems with highly desired functional groups in their molecular structure, such as
carboxylic, to build metal–organic frameworks and other applications within the supramol-
ecular chemistry.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords Heck reaction; phenylenevinylene; trans configuration
INTRODUCTION
Phenylenevinylene systems are commonly used as the luminescent section in
the construction of organic light-emitting diodes (OLEDs),[1] as antennae in the
fabrication of solar cells,[2] as chemosensors, because of their strong optoeletronic
changes in presence of specific analites,[3] and recently in the synthesis of metal–
organic frameworks (MOFs).[4,5]
Many routes have been reported for the synthesis of phenylenevinylene
systems. The Wittig reaction between an aldehyde with an appropriate triphenylpho-
sphoniumylide is the most used methodology to produce single- or multiple-carbon
couplings.[6] The Gilch reaction, which uses a strong base (generally sodium or pot-
assium tert-butoxide) and a benzyl halide at low temperature, generating a halo-vinyl
Received May 15, 2012.
Address correspondence to Juan P. Hinestroza, Departamento de Quımica, Universidad Nacional
´
´
de Colombia, Bogota, Colombia. E-mail: casierraa@unal.edu.co
2280

PHENYLENEVINYLENE SYNTHESIS
2281
derivative, which in turn produces a carbon–carbon double bond, is also used to get
phenylenevinylene systems.[7] However, the main drawback of these reactions is the
poor stereoselectivity, which is very important for the appropriate luminescence
properties of the phenylenevinylene system.[8–10]
Because the configuration of phenylenevinylene systems plays an important
role in emerging technologies, the Heck reaction has been used to obtain systems
with total trans configuration.[8] In the past, we reported a modification of the classic
Heck methodology using phosphites and ionic liquids.[9] Continuing with our contri-
butions to this area, here we present the synthesis of E,E-2,5-dimethoxy-1,4-bis[2-(4-
ethylcarboxylatestyril)]benzene by the Heck cross coupling as a novel precursor for
phenylenevinylene target systems with highly desired functional groups in their mol-
ecular structure, such as carboxylic acids, to build MOFs and other applications
within the supramolecular chemistry.
The synthesis of phenylenevinylene systems directly from precursors containing
carboxylic or hydroxyl groups are sometimes difficult to carry out because of
chemical incompatibility of the reactants under the Heck reaction conditions. This
makes the protection of the reactive substituent groups necessary. Protected pheny-
lenevinylene systems have been previously synthesized using the Wittig reac-
tion.[[10],[11]] However, the reaction produces a mixture of cis and trans isomers,
which require additional steps to finally obtain the desired trans configuration, for
example, using a hot iodine solution, a typical isomerization reaction to produce
the trans from the cis phenylenevinylene system.[4] However, this extra step increases
the possibility of impurities in the final product and reduces the total yield of the
synthesis process. The approach described in this article, following the Heck reac-
tion, selectively produces trans phenylenevinylene with yields greater than other
conventional routes because of elimination of extra steps in the reaction.
DISCUSSION
The iodination reaction for obtaining 1 was carried out following a reported
procedure from 1,4-dimethoxybenzene.[8] The proposed synthetic route for the syn-
thesis of the precursor phenylenevinylene ester 3 (Scheme 1) started with commer-
cially available ethyl 4-(bromomethyl)benzoate, and it was accomplished in two
steps. The first step comprises the synthesis of the phosphonium salt by the reaction
of triphenylphosphine with the commercial bromobenzoate derivative. The second
step corresponds to the Wittig reaction, implemented by pouring the phosphonium
salt into a solution of paraformaldehyde (PFA) dissolved in absolute ethanol. After
the addition of the salt, a sodium ethoxide solution was added to yield the betaine
intermediate and finally producing the unsaturation of 3. The last step in the syn-
thesis of the phenylenevinylene system 4 by the Heck reaction involved a mixture
in dimethylformamide (DMF) of 1 and 3 (molar ratio 1:2 respectively), 1 mol% of
Pd(dba)2, 10 mol% of triphenylphosphite, and potassium carbonate. This mixture
was sealed under a nitrogen atmosphere and heated on oil bath at 100 ꢀC for 48 h
to obtain 4 as a green-yellowish powder. The yield of the final step was 92%, which
is greater than the 82% reported by White et al.,[4] who synthesized a similar
phenylenevinylene system and followed the Wittig reaction. In addition to the

2282
D. ALZATE, J. P. HINESTROZA, AND C. A. SIERRA
Scheme 1. Synthetic route for the obtention of molecule 4.
greater yield, the extra step reported by White et al. to convert the cis product into
the trans one is not required using the Heck route described in this article.
The final product of the Heck reaction was analyzed by modern spectroscopic
techniques. The Fourier transform–infrared (FT-IR) spectrum exhibited the charac-
teristic bands of the aliphatic (2962 and 2924 cmꢁ1), carbonyl group (1681 cmꢁ1), and
other characteristically strong signals of benzoates systems (1273 and 1103 cmꢁ1).
The trans configuration is revealed by the carbon–hydrogen bending mode of the
1
vinyl group in 4 at 972 cmꢁ1, characteristic for this isomer. The H NMR spectrum
showed signals for methyl, methoxy, and methylene groups at 1.41, 3.95, and
4.39 ppm, respectively. A doublet distinctive of vinyl group hydrogens centered at
7.17 ppm, with a coupling constant of 16.4 Hz, typical for trans configuration sys-
tems and not reported before for similar phenylenevinylene structures, corroborates
the stereoselectivity of the Heck reaction. Although Refs. 10 and 11 reported the
synthesis of a trans system, their results are only supported by FT-IR analysis. In
addition, a singlet at 7.15 ppm for aromatic hydrogens of the central benzene ring,
a multiplet centered at 7.59 ppm due to the overlapping of hydrogens of the aromatic
rings and the vinyl groups overlap, and a doublet at 8.03 ppm for the hydrogens in
orto position to the ester group confirmed the synthesis of 4. The 13C NMR dis-
played 13 signals corresponding to the total number of nonequivalent carbons found
in the molecule. Exhibiting signals for the carbonyl carbon observed at 166.6 ppm
and the aliphatic region for the methyl, methoxy, and methylene groups were obser-
ved at 14.5, 56.4, and 61.1 ppm respectively. The molecular structure of 4 was further
confirmed by high-resolution mass spectrometry with a molecular ion peak [M]þ at
m=z 486.2036 (calc. 486.2037).
The steady-state spectroscopy study confirms the presence of an electronic con-
jugated system. The absorption spectrum of 4 in CHCl3 (1 ꢂ 10ꢁ6 M) shows two
unusually intense absorption maxima bands at 327 and 379. The latter absorption
1
has been assigned to 1p ! pꢃ transition, while the unexpected first absorption band
=
is presumably due to the existence of the p-polarizable OC O groups at the ends of

PHENYLENEVINYLENE SYNTHESIS
2283
Figure 1. Absorption and emission spectra of molecule 4.
the chromophore system, which seems to be very influenced by the position of the
substituent, because our previous studies showed no presence of this absorption in
phenylenevinylene systems with ester groups attached directly to the middle of the
conjugated system.[12] The fluorescence spectrum shows typical features of phenyl-
enevinylene systems with a maximum at 471 nm and a tailing shoulder at 500 nm.
Measurements of fluorescence quantum yield, as well as solvent and concentration
behavior and other optoelectronic studies, are under experimental and theoretical
evaluation.
In summary, an efficient synthetic route for phenylenevinylene systems with
pure trans configuration has been developed, in particular, for the novel phenylene-
vinylene compound E,E-2,5-dimethoxy-1,4-bis[2-(4-ethylcarboxylatestyril)]benzene
(4). Additionally, two novel intermediates are reported: 4-(ethylbenzoate)triphenyl-
posphonium bromide (2) and ethyl 4-vynilbenzoate (3). All compounds were cha-
1
racterized using FT-IR, H NMR, 13C NMR, and electrospray ionization–mass
spectrometry (ESI-MS). Compound 4 was synthesized as a precursor for a carbo-
xylic acid phenylenevinylene system, which is a very promising candidate as a linker
in the synthesis of fluorescent MOFs, chemosensors, and photocatalyst.
EXPERIMENTAL
Commercially available reagents were used as received without further purifi-
cation. DMF and the CH3CN were dried according to standard procedures.[13] All
melting points were observed in a Stuart SMP10 hot block and are uncorrected.
FT-IR spectra were obtained in a Shimadzu IR Prestige-21 FT-IR spectrophot-
ometer. NMR spectra were recorded in CDCl3 or dimethylsulfoxide (DMSO-d6)
on a Bruker Advance 400 using tetramethylsilane (TMS) as internal standard.
High-resolution mass spectrum was measured using a Q-T of 6540 UHD (Agilent

2284
D. ALZATE, J. P. HINESTROZA, AND C. A. SIERRA
Technologies). Fluorescence spectra were measured in CHCl3 with an excitation
wavelength of 350 nm in a PTI-710 fluorometer and the ultraviolet–visible (UV-vis)
spectra were measured in a Thermo Scientific Evolution 300 UV-vis spectrophoto-
meter. 1,4-Diiodo-2,5-dimethoxybencene was prepared by a reported procedure.[8]
1,4-Diiodo-2,5-dimethoxybencene (1)
FT-IR (KBr pellet, cmꢁ1): 2954 (w), 2924 (w), 2893 (w), 2831 (w), 1627 (w),
1481 (s), 1442 (m), 1342 (m), 1273 (m), 1211 (s), 1056 (s), 840 (m), 748 (m), 617
1
(w). H NMR (400 MHz, CDCl3) d (ppm): 3.83 (s, 6H), 7.19 (s, 2H). 13C NMR
(100 MHz, CDCl3) d (ppm): 57.2, 85.5, 121.7, 153.4, mp 171–174 ꢀC (lit.[8] mp
171 ꢀC).
4-(Ethylbenzoate)triphenylposphonium Bromide(2)
FT-IR (KBr pellet, cmꢁ1): 3410 (m), 3039 (w), 2970 (w), 2870 (w), 2831 (m),
2762 (m), 1712 (s), 1607 (w), 1481 (w), 1435 (m),1365 (w), 1273 (s), 1189 (m), 1103
1
(s), 1018 (m), 871 (m), 694 (m), 632 (w), 509 (m). HNMR (400 MHz, DMSO-d6)
d (ppm): 1.29 (t, 3H, 3J ¼ 7.1 Hz), 4.28 (q, 2H, 3J ¼ 7.1 Hz), 5.34 (d, 2H,
3
4
2JHP ¼ 16.3 Hz), 7.14 (dd, 2H, J ¼ 8.4 Hz, JPH ¼ 2.3 Hz), 7.81–7.68 (m, 14H), 7.92
(m, 3H). 13C NMR (100 MHz, DMSO-d6) d (ppm): 14.6, 27.8 (d, JCP ¼ 46.8 Hz),
60.9, 117.5 (d, JCP ¼ 85.8 Hz), 129.3 (d, JCP ¼ 2.9 Hz), 129.7, 130.1 (d, JCP ¼ 12.5 Hz),
131.1 (d, JCP ¼ 5.4Hz), 133.5 (d, JCP ¼ 8.6 Hz), 133.9 (d, JCP ¼ 9.8Hz), 135.2, 165.1.
31P NMR (162MHz, DMSO-D6) d (ppm): 23.9, mp 237–239 ꢀC.
Ethyl 4-Vynilbenzoate (3)
FT-IR (liquid, cmꢁ1): 2985 (s), 2931 (m), 1712 (s), 1612 (m), 1566 (w), 1458
(m), 1404 (m), 1365 (m), 1273 (s), 1172 (m), 1103 (s), 1018 (m), 995 (m), 918 (m),
1
856 (m), 756 (m), 717 (m), 470 (w). H NMR (400 MHz, CDCl3)d (ppm): 1.39 (t,
3
3
3
3H, J ¼ 7.0 Hz), 4.37 (q, 2H, J ¼ 7.1 Hz), 5.37 (d, 1H, J ¼ 10.9 Hz), 5.85 (d, 1H,
3J ¼ 17.6 Hz), 6.74 (dd, 1H, J ¼ 17.6 Hz, 10.9 Hz), 7.45 (d, 2H, J ¼ 8.3 Hz), 8.00
(d, 2H, 3J ¼ 8.3 Hz). 13C NMR (100 MHz, CDCl3) d (ppm): 14.4, 61.0, 116.5,
126.1, 129.6, 129.9, 136.1, 141.8, 166.5.
3
3
E,E-2,5-Dimethoxy-1,4-bis[2-(4-ethylcarboxylatestyril)] Benzene (4)
FT-IR (KBr pellet, cmꢁ1): 2962 (m), 2924 (m), 2854 (m), 1697 (s), 1604 (m),
1496 (w), 1465 (w), 1411 (m), 1365 (w), 1273 (s), 1211 (m), 1103 (m), 972 (w), 763
1
3
(w), 478 (m). H NMR (400 MHz, CDCl3) d (ppm): 1.41 (t, 6H, J ¼ 7.1 Hz), 3.95
3
3
(s, 6H), 4.39 (q, 4H, J ¼ 7.1 Hz), 7.15 (s, 2H), 7.17 (d, 2H, J ¼ 16.4 Hz), 7.59 (m,
6H), 8.03 (d, 4H, J ¼ 8.2 Hz). 13C NMR (100 MHz, CDCl3) d (ppm): 14.5, 56.4,
3
61.1, 109.2, 125.6, 126.þ5, 126.6, 128.2, 129.2, 130.1 142.2, 151.6, 166.6. MS m=z
(rel. int.): 486.2037 (M) , mp 194–196 ꢀC.

PHENYLENEVINYLENE SYNTHESIS
2285
SUPPORTING INFORMATION
General experimental methodology and structural information are available
online.
ACKNOWLEDGMENTS
The authors wish to thank Professor Burkhard Koening, University of Regens-
burg, Germany, for the HR-MS experiments. Funding was supplied through grants
from Colciencias and DIB-Universidad Nacional de Colombia. Special thanks to
Dr. Fernando Aguirre for useful discussions.
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