Heteroaromatic cation-based chromophores: Synthesis and nonlinear optical properties of Alkynylazinium salts

Article abstract of DOI:10.1002/ejoc.201000816

A variety of alkynylazinium cationic (D-π-A⁺) chromophores were prepared in good yields by the reaction of bromoazinium (pyridinium, quinolinium, and isoquinolinium iodides) with alkynes under Sonogashira conditions. The analysis of the experimentally recorded spectra is supported by quantum chemical calculations using restricted configuration interaction and density functional methods. First-order hyperpolarizabilities of (D-π-A ⁺) azinium-based cations as a new class of second-order nonlinear optical (NLO) chromophores were also studied by hyper-Rayleigh scattering experiments and computational procedures. Copyright

Full text of DOI:10.1002/ejoc.201000816

FULL PAPER
DOI: 10.1002/ejoc.201000816
Heteroaromatic Cation-Based Chromophores: Synthesis and Nonlinear Optical
Properties of Alkynylazinium Salts
Tatiana Cañeque,[a] Ana M. Cuadro,*[a] Julio Alvarez-Builla,[a] Javier Pérez-Moreno,[b]
Koen Clays,[b] Gema Marcelo,[c] Francisco Mendicuti,[c] Obis Castaño,[c] José L. Andrés,[c]
and Juan J. Vaquero*[a]
Keywords: Heterocycles / Cations / Chromophores / Synthesis / Nonlinear optics
A variety of alkynylazinium cationic (D-π-A+) chromophores
were prepared in good yields by the reaction of bromoazin-
ium (pyridinium, quinolinium, and isoquinolinium iodides)
with alkynes under Sonogashira conditions. The analysis of
the experimentally recorded spectra is supported by quan-
tum chemical calculations using restricted configuration in-
teraction and density functional methods. First-order hyper-
polarizabilities of (D-π-A+) azinium-based cations as a new
class of second-order nonlinear optical (NLO) chromophores
were also studied by hyper-Rayleigh scattering experiments
and computational procedures.
Introduction
Most organic compounds with nonlinear optical (NLO)
properties are based on highly conjugated molecules with
donors and acceptors substituted at the two ends of the
molecule. These donor–acceptor (“push–pull”) systems are
amongst the most interesting structures for new compounds
with efficient NLO responses.[1] In these systems, particular
effort has been made to find the optimum combination of
the bridge, acceptor and donor groups with the aim of en-
hancing the NLO response, particularly with molecules that
can be chemically fine-tuned for desired applications.[2]
In this context, organic chromophores containing dif-
ferent acceptor units, such as nitro,[3] nitrile,[4] sulfonyl,[5]
polycyanovinyl[6] and phosphonate[7] salts, inter alia, have
been explored previously. The use of charged moieties as
acceptor units is restricted to diazonium salts[8] and some
heteroaromatic cations; for example, benzothiazolium[9] and
pyridinium salts[10] have been studied as cationic and dicat-
ionic acceptors in a few push–pull systems such as those
shown in Figure 1.
Figure 1. Examples of D-π-A+ NLO-phores based on heteroarom-
atic cation-acceptor units.
benzimidazolium) and azonia (quinolizinium and aza-quin-
olizinium) cations were studied as potential acceptor units
in a variety of NLO-catiophores. We report here our initial
results on the synthesis of a series of charged chromo-
phores, D-π-A+ (Figure 2), by Sonogashira[12] cross-cou-
Our research findings in the development of the chemis-
try of heteroaromatic cations[11] led us to undertake a com-
parative study in which different azinium (pyridinium, quin-
olinium and isoquinolinium), azolium (imidazolium and
[a] Departamento de Química Orgánica, Universidad de Alcalá,
28871 Alcalá de Henares, Madrid, Spain
Fax: +34-91-8854686
E-mail: juanjose.vaquero@uah.es
[b] Department of Chemistry, University of Leuven,
Celestijnenlaan 200 D, 3001 Leuven, Belgium
E-mail: Koen.Clays@fys.kuleuven.be
[c] Departamento de Química Física, Universidad de Alcalá,
28871 Alcalá de Henares, Madrid, Spain
Figure 2. General structure of donor–acceptor molecules based on
azinium heteroaromatic cations.
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000816.
View this journal online at
wileyonlinelibrary.com
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A. M. Cuadro, J. J. Vaquero et al.
FULL PAPER
pling reactions. The basic strategy for the construction of units (D) through a π-bridge. The resulting D-π-A+ chro-
push–pull molecules is to combine charged heteroaromatic mophores, which bear electron-donating substituents on the
cations – employed as acceptor units (A+) – with donor benzene ring, were studied by hyper-Rayleigh scattering ex-
Table 1. Sonogashira reaction on bromo-1-methylpyridinium iodides 1a–c.
[a] Yields refer to isolated yields. [b] Dehalogenation. [c] Starting material.
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Heteroaromatic Cation-Based Chromophores
periments.[13] Other chromophores were also considered for these conditions, triisopropylsilylacetylene (1.2 equiv.) gave
comparative purposes in order to gain an understanding of 2a in 75% yield (Table 1, Entry 1) after exchange of the
the observed NLO properties. Furthermore, the experimen- iodide counterion by hexafluorophosphate in order to facil-
tal study is supported by a theoretical analysis combining itate the isolation of the product.[14] In light of this success,
a variety of computational procedures, including Density the same conditions were used to treat 3-bromopyridinium
Funtional Theory (DFT) and correlated Hartree–Fock- iodide with different arylacetylenes to give the coupling
based methods [RCIS(D)].
products (Table 1) in yields of up to 49%.
The differences in yield are probably due to the purifica-
tion process rather than to electronic effects associated with
the coupling partners. Thus, the best yields were obtained
Results and Discussion
–
for 2a and 2b as the PF6 salt. For the rest of the reactions,
The first series of pyridinium-based chromophores was
obtained from 3-bromo- (1a), 2-bromo- (1b), and 4-bromo-
1-methylpyridinium iodide (1c), which were coupled with
different alkynes under Sonogashira conditions.[13] The salt
1a, which was the least reactive substrate compared with 1b
and 1c, was initially used as a model compound to deter-
mine the best conditions for the reaction with triisoprop-
leading to 2c–f, better yields were obtained when the prod-
ucts were isolated as iodides. It is worth noting that com-
pound 2f was obtained in a better yield than expected for
a system with an electron-withdrawing substituent on the
benzene ring (Table 1, Entry 6).
Next, we examined the reactivity at the C-2 and C-4 posi-
ylsilylacetylene (TIPS). The best conditions found involved tions of the pyridinium ring with substrates 1b,c under the
the use of Pd2Cl2(PPh3)2 (5 mol-%), CuI (10 mol-%) and optimal conditions found for 1a. In general, yields of the
Et3N (1.5 equiv.) in DMF at room temperature. Under coupled compounds were lower than for 2a. In an analo-
Table 2. Sonogashira coupling on 3- and and 4-bromo-1-methylisoquinolinium salts 1d–e.
[a] Yields refer to isolated yields. [b] Starting material + homocoupling product. [c] Decomposition.
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A. M. Cuadro, J. J. Vaquero et al.
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gous way to reactions with 1a, better yields were obtained chromophores bearing electron-withdrawing substituents
when the coupling products were isolated as iodides, except were also considered for comparative purposes. Selected
for 3a. The main difference in the reactivity of 1a and 1b compounds are shown in Figure 3.
was observed in their reactions with 2-pyridylacetylene,
which in the case of 1b gave the dehalogenated pyridinium
salt as the main product. All attempts to overcome the de-
halogenation process, and the concomitant formation of the
1-methylpyridinium salt, proved unsuccessful.
The yields of coupled products obtained from 1c, as
hexafluorophosphates in all cases (Table 1), were in the
range 45–66%. In this case, the reaction with 2-ethynylpyr-
idine was also unsuccessful, affording a complex reaction
mixture along with the dehalogenated starting material and
1c as the main compounds. These unsuccessful results can
be explained in terms of the higher stabilization of α- and
γ-ylides formed in the dehalogenation process when com-
pared to the less stable β-ylide, formed in the dehalogena-
tion of 1a.
Having demonstrated that salts 1a–c can act as efficient
partners in Sonogashira reactions, we extended the reaction
to quinolinium and isoquinolinium derivatives. Thus, 3-
bromo-1-methylquinolinium iodide (1d) and 4-bromo-2-
methylisoquinolinium iodide (1e) reacted with certain
acetylenes to afford the coupling products 5 and 6 in mod-
erate or low yields (Table 2).
Figure 3. Selected chromophores for the study of linear and non-
linear optical properties.
In both cases the reaction with 2-pyridylacetylene failed
to give the coupled product. Moreover, 1e seemed to be less
reactive with all the acetylenes tested, and the reaction was
Linear Optical Properties
The absorption spectra of the selected chromophores
also unsuccessful with 1-ethynyl-4-(trifluoromethyl)benz- were recorded in the range 200–1000 nm. The absorption
ene, which gave a complex reaction mixture. An NMR spectra for a group of representative molecules [2c, 3c and
study of the reaction mixtures allowed the identification of 4c in (a); 5b and 6b in (b)] are shown in Figure 4. The com-
the homocoupling product of 1e.
pounds did not show any absorption above 450 nm.
A comparative study of linear and non-linear optical
In general, a good correlation was found between the
properties of chromophores was carried out on a selection electron-donating substituents used and the stabilization
of the chromophores shown in Tables 1 and 2. A range of energy for all the different types of chromophores: the bro-
systems bearing electron-donating substituents on the ben- mophenyl substituent is the weakest donor and, as ex-
zene ring were considered, although whenever possible, pected, yielded the smalles λmax value; the tolyl substituent
Figure 4. Absorption and emission spectra for some selected chromophores (in methanol at 25 °C). Fluorescence quantum yields and
lifetime averages are superimposed.
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Heteroaromatic Cation-Based Chromophores
is an intermediate donor and yielded an intermediate λmax for 2c the maximum is at 537 nm. The emission spectra of
value; the methoxyphenyl substituent is the best donor, 5b and 6b are significantly different, with maxima at 447
yielding the largest λmax value. A good correlation was also and 546, respectively. The fluorescence quantum yields (Φf)
found for all compounds between the position of the pyridi- and intensity decay profiles for all of the systems were char-
nium nitrogen atom and the wavelength of the absorption acterized in methanol at 25 °C (superimposed in Figure 4
maximum: a nitrogen atom in the γ-position resulted in the and/or included in the Supporting Information). In general,
most efficient conjugation, whereas a nitrogen atom in the most of the samples exhibit Φf Ͻ 0.1. Strong donor/ac-
β-position resulted in a less efficient conjugation. The ef- ceptor groups usually lead to efficient transfer, such as
fects of the α-position varied from group to group, but in HOMO–LUMO transitions, and fluorescence is sub-
general the efficiency of the conjugation was somewhere be- sequently quenched in relatively polar solvents.[15] Fluores-
tween the effects of the γ- and the β-positions, probably due cence intensity profiles for all the samples fitted to bi-ex-
to a combination of electronic and steric effects.
ponential decays.
The use of different electron-donating/withdrawing sub-
stituents also affects the stabilization energy of the chromo-
phores. A bathochromic effect is exerted on the absorption
spectrum of chromophores bearing electron-donating sub-
stituents in comparison to those containing electron-with-
drawing ones. Thus, the maximum of the π-π* absorption
bands for compounds containing methoxy substituents is
shifted towards longer wavelengths by approximately 30–
40 nm with respect to those containing bromo substituents.
Comparison of the absorption maxima (data collected in
Table 3) and/or the spectra for compounds 2c, 3c and 4c,
which contain the same electron-donating methoxy group
at a different location with respect to the pyridinium nitro-
gen atom, reveals a better stabilization when the substituent
is at the α-position compared to the γ- and β-positions.
Nonlinear Optical Properties – Hyper-Rayleigh Scattering
(HRS) Studies
Femtosecond hyper-Rayleigh scattering measurements
performed at 800 nm on selected compounds are compiled
in Table 3. Congruent with the trends already observed in
the linear optical (absorption) experiments, the first hyper-
polarizability value for the compounds with a pyridinium
acceptor increases when the methoxyphenyl group is pres-
ent as the aryl donor (2c, 3c and 4c), and the most marked
effect is seen when the pyridinium nitrogen atom is in the
γ-position (4c). The value for the first hyperpolarizability
of this group of compounds compares favorably with values
for other short chromophores,[9,10a,10c,10d,10g] with absorp-
tion maxima trending to longer wavelengths. This trend,
however, has not been confirmed in the isoquinolinium de-
rivatives.
Table 3. Nonlinear optical properties of selected chromophores 2–
6 shown in Figure 3.[a]
Compound λmax
βHRS
βzzz
βzzz,0
τ
2b
3b
4b
5b
6b
2c
3c
4c
2f
Ͻ300
344
342
379
370
362
360
370
Ͻ300
331
–
–
–
–
140Ϯ10
55Ϯ2
230Ϯ20
92Ϯ9
176Ϯ7
129Ϯ4
210Ϯ10
25Ϯ2
350Ϯ30
135Ϯ5
550Ϯ60
220Ϯ20
430Ϯ20
312Ϯ10
500Ϯ30
60Ϯ5
74Ϯ7
29Ϯ1
44Ϯ5
25Ϯ2
61Ϯ2
47Ϯ2
56Ϯ3
–x-
5Ϯ2
4Ϯ1
Theoretical Studies
–
–
–
–
–
–
–
–
Depending on the spectroscopic property under study,
different theoretical methodologies have been used. The rel-
atively large size of the studied systems places most of the
ab initio techniques outside the area of applicability. The
configuration interaction with single- and double-excitation
methods [CIS(D)] as well as time-dependent density func-
tional theory (TDDFT) seem to be the most popular ap-
proaches for the calculation of excitation spectra. All theo-
retical calculations were performed by using the Gaussian
suite of quantum chemical programs.[16] For all the studied
cationic systems, the molecular structure used to compute
the nonlinear optical properties correspond to a minimum
energy calculated at the HF/6-31G(d) level. These struc-
3f
40Ϯ8
28Ϯ2
100Ϯ20
68Ϯ5
25Ϯ4
18Ϯ2
4f
331
[a] Wavelength of absorption maximum: λmax [nm]; resonance-en-
hanced HRS experimental first hyperpolarizability: βHRS
(10–30 esu); resonance-enhanced diagonal component of the molec-
ular first hyperpolarizability: βzzz (10–30 esu); off-resonance diago-
nal component of the molecular first hyperpolarizability: βzzz,0
(10–30 esu). The values of the fluorescence lifetime, τ [ns], are also
included for molecules that showed demodulation.
Replacement of the pyridinium acceptor in 2c, 3c or 4c tures have been checked to be minima by evaluation of the
by quinolinium (5b) or isoquinolinium (6b) results in a ba- harmonic vibrational frequencies, which were all real.
thochromic shift and a significant decrease in the intensity
For the first excited state of the studied cationic systems,
of the low-energy absorption band.
the calculated wavelengths have been compared to the ex-
Fluorescence emission for 2, 3 and 4, obtained upon exci- perimentally determined (λmax [nm]). These wavelengths
tation at the wavelength of the π-π* band, exhibits features have been estimated from two different approaches: (i) The
similar to those of the absorption spectra. The substituent CIS(D)[17] method, which generally predicts accurately ex-
groups that contribute to the stability of the systems by cited states that are mainly one-electron transitions from a
extending the conjugation show emission at longer wave- single reference ground state; (ii) the CAM-B3LYP[18] func-
lengths. For example, the fluorescence maxima for 3c and tional density, recently developed to account for long-range
4c are located at approximately 579 and 594 nm, whereas interactions (Table 4).
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A. M. Cuadro, J. J. Vaquero et al.
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Table 4. Wavelengths (λmax [nm]) of absorption maxima determined
experimentally and theoretically calculated at the CIS(D)/6-31G(d)
and CAM-B3LYP/6-31G(d) levels of theory.
Theoretical estimation of the second harmonic genera-
tion (SHG) of hyperpolarizability β(–2ω;ω,ω) was also per-
formed. This theoretical value has been compared to βHRS
,
Compound
λmax exp.
CIS(D)
CAM-B3LYP
which is experimentally determined by hyper-Rayleigh
scattering (HRS) for cationic systems. The resonant or
frequency-dependent hyperpolarizabilities obtained by
calculations at the MP2/6-31G(d) level of theory include
the electron-correlation effect in this NLO property
βMP2(–2ω;ω,ω). This property has been estimated from the
resonant or dynamic β(–2ω;ω,ω) at the HF/6-31G(d) level,
and the static or nonresonant value β(0;0,0), calculated at
both HF/6-31G(d) and MP2/6-31G(d) levels, following the
multiplicative approximation scheme where the frequency
dispersion is estimated to be similar at these two levels of
theory [Equation (1)].[19]
2b
3b
4b
5b
6b
2c
3c
4c
2f
Ͻ300
344
342
379
370
362
360
370
Ͻ300
331
289
311
336
377
343
325
342
374
267
290
310
359
349
363
394
400
389
371
386
355
352
367
3f
4f
331
Table 4 shows a comparison between experimentally ob-
served and theoretically predicted wavelengths for the first
excited electronic state. The CIS(D)-calculated wavelengths
are underestimated with respect to the experimental values.
For the selected systems, the largest deviation from the ex-
perimental value is about 10%. The predicted values for
systems 2b and 2f are clearly smaller than 300 nm, in ac-
cordance with the experimental measurements. In the par-
ticular case of 5b, the experimental value of λmax is in very
good agreement with the theoretical result. On the other
hand, the wavelengths at the CAM-B3LYP level are overes-
timated with respect to the experimental values by about 5–
10%. CAM-B3LYP calculations predict the existence of a
second more intense peak at 339 nm for 2c as a result of a
HOMOǞLUMO+1 transition. For 5b, the theoretical
value of 394 nm obtained by using the CAM-B3LYP func-
tional also corresponds to a HOMOǞLUMO+1 transition.
With these exceptions, all the studied compounds have an
intense HOMOǞLUMO transition leading to an efficient
charge transfer between donor and acceptor moieties. This
can be seen in Figure 5, where the main contributions to
the HOMO and LUMO are due to the donor and acceptor
fragments, respectively. For both theoretical methodologies,
calculated λmax values for the methoxyphenyl substituents
(2c, 3c and 4c) present the largest values, as expected for
the best donors. The weaker donors – the bromophenyl
substituents (2f, 3f and 4f) – show the smallest λmax values,
and the bathochromic shift of the quinolinium and isoquin-
olinium (5b and 6b) λmax values with respect to the pyridi-
nium acceptors is also reproduced.
(1)
The first hyperpolarizability β has been calculated as a
second dipole moment derivative with respect to electric
fields. At the Hartree–Fock (HF) level, these dipole deriva-
tives are calculated analytically by solving the correspond-
ing CPHF equations. On the other hand, when the electron-
correlation effect has been included by means of the per-
turbative MP2 approach, these dipole derivatives are ob-
tained numerically. Calculated hyperpolarizabilities are ob-
tained in the molecular axes βijk and then transformed to
the laboratory axes βXYZ by applying the Civin et al. expres-
sions published elsewhere.[20]
In Table 5, two different kinds of data are presented:
HRS hyperpolarizabilities and dispersion. Although calcu-
lated βHRS values did not qualitatively reproduce the ob-
served values, the larger βHRS values are obtained when
methoxyphenyl substituents are the donors (2c, 3c and 4c)
with pyridinium acceptors. This is the most important effect
in cation 4c, in good agreement with the experimental data.
Dispersion ρ has a value of 5 in the limit of pure linear
molecules, as can be seen from the Civin et al. expressions.
It is sensitive to the different contributions of the nonzero
β tensor components, and therefore to the symmetry of the
scatterer. The calculated dispersion values ρ at 800 nm for
these cations are between 4.8 and 5.3, as it should be for
Table 5. Resonance-enhanced HRS experimental first hyperpolariz-
ability βHRS, the MP2/6-31G(d) calculations of βHRS at 800 nm,
and dispersion ρ at 400 nm (βHRS values are given in 10–30 esu).
Compound
βHRS
800 nm
ρ
Exp
400 nm
2b
3b
4b
5b
6b
2c
3c
4c
2f
–
140
55
230
92
176
129
210
25
53
77
139
108
102
88
130
234
34
2.5
4.2
3.8
2.4
2.5
4.4
2.9
2.5
5.3
2.4
3.6
3f
40
28
48
85
4f
Figure 5. HOMO (a) and LUMO (b) of 4c.
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Heteroaromatic Cation-Based Chromophores
(87 µL, 0.5003 mmol) were added, and the reaction mixture was
stirred at room temperature as indicated. The reaction mixture was
concentrated under reduced pressure, the residue was treated with
saturated aqueous NaHCO3 (10 mL) and extracted with CH2Cl2
(3ϫ15 mL). The organic phase was dried with Na2SO4, concen-
trated under reduced pressure and purified by treatment with a
mixture of CH2Cl2/Et2O (8:2).
these systems. On the other hand, at 400 nm the calculated
dispersion values are quite different, as shown in Table 5.
These changes can be due to the resonance between the
optical frequency and the λmax absorption, or to changes
induced in the electronic cloud for the optical frequency, or
both. They could probably explain the disagreement be-
tween calculated and observed HRS hyperpolarizabilities.
1-Methyl-3-{[p-(trifluoromethyl)phenyl]ethynyl}pyridinium
Iodide
(2d): Compound 1a and 1-ethynyl-4-(trifluoromethyl)benzene
(57.0 µL, 0.4002 mmol) were allowed to react according to the Ge-
neral Procedure B by stirring the reaction mixture for 3.5 h to give
2d (82.4 mg, 69%) as a white solid, m.p. 187–189 °C. IR (KBr):
Conclusions
The synthesis of a series of D-π-A+ catiophores was
achieved by Sonogashira cross-coupling reactions of
azinium bromo derivatives. This process gave moderate to
good yields, and had a simple workup procedure. The first
hyperpolarizability values for this group of compounds
compare favourably with those of other short chromo-
phores. It is clear from this study that D-A systems contain-
ing the p-methoxyphenyl donor and the pyridinium ac-
ceptor show the largest second-order nonlinearity. Further
variations in the type of D/A+ groups should allow higher
values to be obtained.
ν
max = 3135, 2230, 1636, 1327 cm–1. 1H NMR (300 MHz, CD3OD):
˜
δ = 9.42 (s, 1 H, Ar-H), 9.15 (d, J = 6.2 Hz, 1 H, Ar-H), 8.85 (d,
J = 10.2 Hz, 1 H, Ar-H), 8.33 (dd, J = 8.0, 6.2 Hz, 1 H, Ar-H),
7.86 (s, 4 H, Ar-H), 4.70 (s, 3 H, N-CH3) ppm. 13C NMR (50 MHz,
DMSO): δ = 148.7, 147.2, 146.0, 133.3, 130.7 (q, J = 32.7 Hz),
128.4, 126.7 (q, J = 3.2 Hz), 125.3, 124.5 (q, J = 286.7 Hz), 122.7,
94.7, 85.4, 46.5 ppm. MS (ES+): m/z (%) = 262 (100) [M+].
C15H11F3N·I (388.98): calcd. C 53.59, H 4.50, N 4.17; found C
53.54, H 4.39, N 4.25.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details and spectroscopic characterization data
of all new compounds (2b–c, 2e–f; 3a–d, 3f; 4a–d, 4f; 5a–c; 6a–b),
tables of quantum yields and absorption spectra for 2–4b and 2–
4f.
Experimental Section
General Procedure for the Preparation of N-Methylazinium Salts
2–6
Acknowledgments
General Procedure A: A flame-dried round-bottomed flask (10 mL)
was charged under argon with the corresponding pyridinium salt
(100 mg, 0.3335 mmol), 10 mol-% of CuI (6.35 mg, 0.0333 mmol)
and 5 mol-% of Pd2Cl2(PPh3)2 (11.7 mg, 0.0167 mmol) in dry DMF
(5 mL). The corresponding acetylene (0.4002 mmol, 1.2 equiv.) and
dry Et3N (87 µL, 0.5003 mmol, 1.5 equiv.) were added, and the re-
action mixture was stirred at room temperature as indicated. The
reaction mixture was concentrated under reduced pressure, and the
residue was treated with saturated aqueous NH4PF6 and extracted
with CH2Cl2 (3ϫ15 mL). The organic phase was dried with
Na2SO4, the solvent was evaporated under reduced pressure, and
the product was purified by flash chromatography on silica gel by
using CH2Cl2/MeOH (different proportions) as eluent or by treat-
ment with a mixture of CH2Cl2/Et2O (8:2).
The authors acknowledge financial support from the Spanish Min-
isterio de Educación y Ciencia (projects: CTQ2008-04313/BQU
and CTQ2009-7120) and a grant from the Comunidad de Madrid
(TC). J. P. M. acknowledges the Fund for Scientific Research Flan-
ders (FWO) for a postdoctoral fellowship.
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˜
max
1H NMR (200 MHz, CD3OD): δ = 9.34 (s, 1 H, Ar-H), 9.10 (d, J
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and 5 mol-% of Pd2Cl2(PPh3)2 (11.7 mg, 0.0167 mmol) in dry DMF
(5 mL). The corresponding acetylene (0.4002 mmol) and dry Et3N
Eur. J. Org. Chem. 2010, 6323–6330
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Received: June 7, 2010
Published Online: October 12, 2010
6330
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 6323–6330