Highly conjugated molecules from dibromonaphthyl derivatives and 4- vinylpyridine or 4-acetoxystyrene by the Heck reaction

Article abstract of DOI:10.1021/jo990697a

Palladium-catalyzed coupling of 1,6- and 2,6-dibromonaphthyl derivatives with 4-vinylpyridine or 4-acetoxystyrene resulted in mono- and bis- arylvinylation depending on the choice of reaction conditions. For the 1,6- dibromoarenes, the mono-arylvinylated derivative at position-6 was the sole product in the presence of (o-tol)₃P/Et₃N. The bis-arylvinylated derivative was the major product, accompanied by a minor product, arylvinylated at position-6 and reduced at position-1, in the presence of (o- tol)₃P/Et₃N/MeCN. For the 2,6-dibromoarenes, the bis-arylvinylated derivative was the sole product in the presence of either Ph₃P or (o- tol)₃P, if provided with a small volume of Et₃N/MeCN solvent, and the mono- arylvinylated derivative was the major product with larger solvent volume and higher haloarene ratio. Fluorescence intensities of 2,6-bis-arylvinylated products were 2 to 3 orders of magnitude higher than that of stilbene. Nematic phases, at relatively high temperatures, were observed for some of the rodlike molecules. Shown by ¹H NMR study, at the photostationary state of isomerization, smaller fractions of cis-form were obtained for molecules that exhibited larger fluorescence quantum yields. The results presented here are beneficial to the pursuit of nonlinear optical materials, fluorescent mesogens, photo- and electroactive materials.

Full text of DOI:10.1021/jo990697a

J . Org. Chem. 1999, 64, 5603-5610
5603
High ly Con ju ga ted Molecu les fr om Dibr om on a p h th yl Der iva tives
a n d 4-Vin ylp yr id in e or 4-Acetoxystyr en e by th e Heck Rea ction
C. C. Chang, K. J . Chen, and L. J . Yu*
Department of Chemistry, Tamkang University, Tamsui, Taipei 25137, Taiwan
Received April 26, 1999
Palladium-catalyzed coupling of 1,6- and 2,6-dibromonaphthyl derivatives with 4-vinylpyridine or
4-acetoxystyrene resulted in mono- and bis-arylvinylation depending on the choice of reaction
conditions. For the 1,6-dibromoarenes, the mono-arylvinylated derivative at position-6 was the sole
product in the presence of (o-tol)3P/Et3N. The bis-arylvinylated derivative was the major product,
accompanied by a minor product, arylvinylated at position-6 and reduced at position-1, in the
presence of (o-tol)3P/Et3N/MeCN. For the 2,6-dibromoarenes, the bis-arylvinylated derivative was
the sole product in the presence of either Ph3P or (o-tol)3P, if provided with a small volume of
Et3N/MeCN solvent, and the mono-arylvinylated derivative was the major product with larger
solvent volume and higher haloarene ratio. Fluorescence intensities of 2,6-bis-arylvinylated products
were 2 to 3 orders of magnitude higher than that of stilbene. Nematic phases, at relatively high
1
temperatures, were observed for some of the rodlike molecules. Shown by H NMR study, at the
photostationary state of isomerization, smaller fractions of cis-form were obtained for molecules
that exhibited larger fluorescence quantum yields. The results presented here are beneficial to the
pursuit of nonlinear optical materials, fluorescent mesogens, photo- and electroactive materials.
In tr od u ction
high hyperpolarizability,7 and large cross-section for two-
photon absorption.8 Enhancements of these properties
would be expected if the π-system were extended due to
a longer π-conjugated chain or more aromatic rings. It is
therefore interesting to study the variations in these
chemical and physical properties when the phenyl rings
of stilbene and stilbazole are replaced by a naphthyl
moiety.
The palladium-catalyzed Heck reaction has been widely
employed for the carbon-carbon bond formation in
organic synthesis ever since its original development by
Heck and co-workers.1 Several reviews2 and advanced
studies3 of the palladium-catalyzed reactions have ap-
peared recently. Since the palladium-catalyzed Heck
reaction is a coupling between aryl halides and olefins,
it is a particularly valuable method when an extended
π-system is desired. Molecules consisting of extended
π-systems are of interest for the development of new dyes
and nonlinear optical and photo- and electroactive ma-
terials. Stilbene and stilbazole are examples.4 While both
have been used as models for photochemical and photo-
physical studies, they have also led to derivatives that
exhibit large solvatochromism,5 intense fluorescence,6
While the Heck reaction has been applied amply to the
arylvinylation of halobenzene, which proceeds smoothly,
the studies on polyhaloarenes are limited,9 and similar
studies of fused ring halides are rare.10 In the present
study, the palladium-catalyzed Heck reaction was em-
ployed for the arylvinylation of dibromonaphthyl deriva-
tives. The olefins used were 4-acetoxystyrene and 4-vi-
nylpyridine and the haloarenes were 1,6-dibromo-2-
naphthol and 2,6-dibromo-1,5-naphthalenediol derivatives,
* To whom correspondence should be addressed. E-mail: ljyu@mail.
tku.edu.tw.
(1) (a) Heck, R. F. J . Am. Chem. Soc. 1968, 90, 5518. (b) Heck, R. F.
Acc. Chem. Res. 1979, 12, 146. (c) Heck, R. F. Org. React. 1982, 27,
345. And references therein.
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(b) Kanis, D. R.; Ratner, M. A.; Marks, T. J . Chem. Rev. 1994, 94, 195.
(c) Prasad, P. N.; Williams, D. J . Introduction to Nonlinear Optical
effects in Molecules and Polymers; J ohn Wiley & Sons: New York, 1991.
(d) Marder, S. R.; Sohn, J . E.; Stucky, G. D. Materials for Nonlinear
Optics: Chemical Perspectives; ACS Symposium Series 455; American
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Bhatt, J . C.; Kannan, R.; Yuan, L.; He, G. S.; Prasad, P. N. Chem.
Mater. 1998, 10, 1863. (b) Albota, M.; Beljonne, D.; Bre´das, J . L.;
Ehrlich, J . E.; Fu, J . Y.; Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M.
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M.; Subramaniam, G.; Webb, W. W.; Wu, X. L.; Xu, C. Science 1998,
281, 1653.
(2) (a) Heck, R. F. Palladium Reagents in Organic Synthesis;
Academic Press: London, 1985. (b) de Meijere, A.; Meyer, F. E. Angew.
Chem., Int. Ed. Engl. 1994, 33, 2379. (c) Tsuji, J . Palladium Reagents
and Catalysts: Innovations in Organic Synthesis; J ohn Wiley: Chich-
ester, 1995. (d) Cabri, W.; Candiani, I. Acc. Chem. Res. 1995, 28, 2. (e)
Shibasaki, M.; Boden, C. D. J .; Kojima, A. Tetrahedron 1997, 53, 7371.
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Ed. 1998, 37, 481. (b) Wagaw, S.; Yang, B. H.; Buchwald, S. L. J . Am.
Chem. Soc. 1998, 120, 6621. (c) Nakamura, H.; Sekido, M.; Ito, M.;
Yamamoto, Y. J . Am. Chem. Soc. 1998, 120, 6838. (d) Buezo, N. D.;
Alonso, I.; Carretero, J . C. J . Am. Chem. Soc. 1998, 120, 7129. (e)
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Old, D. W.; Wolfe, J . P.; Buchwald, S. L. J . Am. Chem. Soc. 1998, 120,
9722. (g) Kadota, I.; Shibuya, A.; Gyoung, Y. S.; Yamamoto, Y. J . Am.
Chem. Soc. 1998, 120, 10262. (h) Loiseleur, O.; Hayashi, M.; Schmees,
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(4) Go¨rner, H.; Kuhn, H. J . In Advances in Photochemistry; Neckers,
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(5) (a) Morley, J . O.; Morley, R. M.; Fitton, A. L. J . Am. Chem. Soc.
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10.1021/jo990697a CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/03/1999

5604 J . Org. Chem., Vol. 64, No. 15, 1999
Chang et al.
Sch em e 1
A and B, respectively. To the best of our knowledge, this
is the first report of arylvinylation of 1,6-dibromo- and
2,6-dibromonaphthyl derivatives by the Heck reaction.
It was found that depending upon the phosphines and
solvents used, the dibromoarenes could undergo either
mono- or bis-arylvinylation. These arylvinylated products
exhibit intense fluorescence and mesophase behaviors,
particularly the B-type molecules. Photoisomerization
was studied by NMR spectroscopy.
Exp er im en ta l Section
Compounds 1,6-dibromo-2-naphthol (A1) and 2,6-dibromo-
1,5-naphthalenediol (B1) were prepared from 2-naphthol
(Merck, as received) and 1,5-naphthalenediol (Aldrich and
Merck, either as received or recrystallized) according to refs
11 and 12, respectively. Their ether- and ester- derivatives
were synthesized by conventional methods. The catalysts
palladium(II) acetate (Pd(OAc)2, Strem), triphenylphosphine
(Ph3P, Merck), and tri(o-tolyl)phosphine ((o-tol)3P, Aldrich)
were used, as received. The olefin 4-acetoxystyrene (Aldrich)
was used, as received, and 4-vinylpyridine (4VP, Merck) was
vacuum-distilled before use.
Gen er a l P r oced u r es for Heck Rea ction .1c The amount
of phosphine used was 6-8 times (molar ratio) that of
Pd(OAc)2, and the latter was 1 mol % (per halo) of the
haloarene (10 mmol for each run). Either triethylamine (Et3N)
alone or Et3N/MeCN (volume ratio 1:2 to 1:9) were used as
solvents. The desired amounts of reagents were placed in a
thick wall Pyrex pressure bottle equipped with a cap and
gasket. After thoroughly mixing and flushing with nitrogen
gas, the bottle was capped and heated to 100 °C for the time
indicated in Tables. After workup, the final products were
separated by silica gel column chromatography with various
solvents. All the results of elemental analysis, molecular mass
determination, and proton NMR spectra confirmed the ex-
pected structures.
Melting points were determined without correction. Elemen-
tal analysis was carried out at the Tainan Instrumentation
Center of the National Science Council. 1H NMR spectra were
recorded with a 300 MHz spectrometer. Chemical shifts are
reported in ppm relative to residual nondeuterated solvent,
and J values are in hertz. Mass spectra were recorded under
electron impact at 70 eV. Absorption and emission spectra
were recorded with a UV-vis spectrometer and a fluorospec-
trometer, respectively. Mesophases were characterized by
optical polarizing microscopy in conjunction with a heating
stage for temperature variation. A medium-pressure mercury
lamp (450 W) was used for photochemical study.
Ta ble 1. Rea ction Con d ition s a n d Yield s of P r od u cts for
th e Heck Rea ction of 1,6-Dibr om o-2-n a p h th ol Der iva tives
a n d 4-Vin ylp yr id in e
yield, %
Et3N/MeCN reaction
haloarene
ligand
(mL)
time (h)
a
b
c
A2
A2
A2
A2
A2
A2
A3
A4
A5
A6
A6
A7
A7
A7
A7
A2b′
Ph3P
Ph3P
30/0
10/90
30/0
100/0
20/50
10/90
10/90
10/90
40/80
30/0
10/90
30/0
10/90
30/0
10/90
10/90
96
96
96
96
96
96
96
96
96
96
96
96
96
96
96
96
8
5
41
28
63
31
(o-tol)3P
(o-tol)3P
(o-tol)3P
(o-tol)3P
(o-tol)3P
(o-tol)3P
(o-tol)3P
Ph3P
(o-tol)3P
Ph3P
Ph3P
(o-tol)3P
(o-tol)3P
(o-tol)3P
46
43
33
37
48
13
11
4
7
25
41
45
3
7
6
25
22
47
48
14
12a
a
A2a , but with a 4-acetoxystyryl group at position-6, A2a ′′.
Resu lts a n d Discu ssion
ucts but in lower yields, 5% of A2a and 28% of A2b.
When (o-tol)3P was used instead of Ph3P, in Et3N alone,
A2b was obtained as the sole product in 63% yield. When
Et3N/MeCN was used together with (o-tol)3P, the highest
yield of 1,6-bis-arylvinylated product (A2a ) was obtained
(46%) but along with a 13% yield of 6-arylvinylated-2-
methoxynaphthalene (A2c), i.e., a product with reduction
1,6-Dibr om o-2-n a p h th ol Der iva tives a s th e Ha -
loa r en es. After several attempts, under various condi-
tions, the coupling of 4-vinylpyridine and 1,6-dibromo-
2-naphthol resulted in no isolable products, possibly due
to the instability of the naphthol in the slightly basic
medium and the electron-donating property of the hy-
droxyl group.1c The ether and ester derivatives were
therefore employed, Scheme 1, and the results are listed
in Table 1.
The coupling of 4VP with 1,6-dibromo-2-methoxynaph-
thalene (A2) resulted in only 8% yield of 1,6-bis-arylvi-
nylated product (A2a ) and 41% yield of 6-arylvinylated-
1-bromo-2-methoxynaphthalene(A2b;structureconfirmation
is discussed later) in Ph3P/Et3N condition. Replacing the
Et3N with Et3N/MeCN solvent resulted in similar prod-
1
at 1-position. The H NMR spectrum of this product is
identical to that of the product obtained from 6-bromo-
2-alkyloxynaphthalene reported previously.13 This reduc-
tion during the reaction was observed also by Heck and
co-workers although the situation was slightly different.9
Replacement of the methoxy group at position-2 with
a benzoate (A7) resulted in the same types of products
but in slightly different yields. The mono-arylvinylated
product (A7b) was the major product in the presence of
Ph3P (in either Et3N alone or Et3N/MeCN) and was the
(11) Koelsch, F. In Organic Syntheses; Horning, E. C., Ed.; J ohn
Wiley & Sons: New York, 1960; Coll. Vol. 3, pp 132-133.
(12) Carter, A. H.; Race, E.; Rowe, F. M. J . Chem. Soc. 1942, 236.
(13) Yu, L. J .; Chen, K. Y. Mol. Cryst. Liq. Cryst. 1995, 265, 89.

Highly Conjugated Molecules the Heck Reaction
J . Org. Chem., Vol. 64, No. 15, 1999 5605
Ta ble 2. Rea ction Con d ition s a n d Yield s of P r od u cts for
th e Heck Rea ction of 1,6-Dibr om o-2-n a p h th ol Der iva tives
a n d 4-Acetoxystyr en e
yield, %
Et3N/MeCN
(mL)
reaction
time (h)
haloarene
ligand
a ′
b′
c′
A2
A2
A2
A2b
(o-tol)3P 30/0
(o-tol)3P 10/90
(o-tol)3P 10/30
(o-tol)3P 10/30/10DMF
48
48
48
96
67
21
36
3
66
19a
a
A2a , but with 4-acetoxystyryl group at position-1, A2a ′′′.
sole product (in 47% yield) in the (o-tol)3P/Et3N case. In
the case of (o-tol)3P/Et3N/MeCN, 48% yield of 1,6-bis-
arylvinylated product (A7a ) and 14% yield of 6-arylvi-
nylated-1-H product (A7c) were obtained. It was noted
that 10% of the dibromoarene were recovered for the
cases with Et3N alone.
The above results indicated that the variations of
products and yields were mainly due to the phosphine
and solvent chosen, the ether and ester linkages played
a minor role if any. Similar results were also obtained
when the methoxy group at position-2 was changed to
longer chains (propyloxy, A3, and octyloxy, A5), chiral
((S)-2-methylbutyloxy, A4) and bulkier (benzyloxy, A6)
groups, as shown for the case of (o-tol)3P/Et3N/MeCN, i.e.,
1,6-bis-arylvinylation was the major product and 6-arylvi-
nylated-1-reduced product was the minor. It was noted
that higher yields were obtained, in the presence of (o-
tol)3P, when the ratio of Et3N/MeCN was in the range of
1/2 to 1/3. Also noted was that lower yields were obtained
with larger volumes of solvents (vide infra).
When 4-acetoxystyrene was used as the olefin, as in
Scheme 1 but replacing the 4VP moieties by 4-acetoxy-
styrene (and the corresponding products were designated
by a ′, b′, and c′), no product could be isolated for the cases
with Ph3P. For the cases with (o-tol)3P, shown in Table
2, however, 67% yield of 6-arylvinylation, A2b′, was
obtained as the sole product with Et3N as solvent, and
21% yield of 1,6-bis-arylvinylated product, A2a ′, and 36%
yield of 6-arylvinylated-1-reduced product, A2c′, were
obtained with Et3N/MeCN(10 mL/90 mL) as solvent.
When this ratio was changed to 10 mL/30 mL, higher
yields were obtained, 66% yield of A2a ′ and 3% yield of
A2c′. With 4-acetoxystyrene as the olefin, the reaction
time was shortened to 48 h.
The mono-arylvinylated products, i.e., 1-bromo-6-
arylvinylated derivatives, could be subjected to Heck
reaction again, but DMF was needed in order to reach a
homogeneous solution. Thereafter, 1,6-bis-arylvinylated
product could be obtained along with the 6-arylvinylated-
1-reduced product. The total yield was lower. Further-
more, for the second arylvinylation, a different olefin
could be used. Thus, different arylvinyl groups could be
attached to the 1- and 6-positions and resulted in
different final products as shown for A2b′ in Table 1 and
A2b in Table 2. These results indicated that stepwise
arylvinylation could be achieved by a proper choice of
reaction conditions, and the first arylvinylation always
occurred at the 6-position.
F igu r e 1. The structural representation and peak assign-
ments of the 1H NMR spectra, 1D and 2D COSY, for the
aromatic protons of 1-bromo-2-methoxy-6-(2′(4′′-pyridyl)ethe-
nyl)naphthalene (A2b) in CDCl3.
overlapped with one of the Hb doublet (due to Ha). The
other half of the Hb doublet is overlapped with the singlet
(compounded by a smaller coupling due to Hd) of Hc. If
the bromo group were located at position-6 (of naphtha-
lene), and the pyridylvinyl group at position-1, the most
downfield signals (of naphthalene protons) should be the
doublet-doublet of Hd (due to He and Hc) and the singlet
(compounded by a smaller coupling due to Hd) of Hc
located nearby. However, these are not observed in the
spectrum.
A further confirmation is obtained by analysis of the
COSY spectrum. The strongest cross-peaks associated
with Hf (the vinylic proton) observed are a pair of
contours located at 7.83 ppm, indicating that the associ-
ated signal should be a singlet, which should be of Hc.
Those of Hd are barely observed, since they are doublet-
doublet in nature and should be weaker. The contours
of weaker cross-peaks are observed also for Hf-He
interaction. If the vinyl group were located at position-
1, the strongest cross-peaks should be of two pairs of
contours for He (with Hf), and also for Hd but weaker.
Those of Hc, one pair of contours, would not be observ-
able. This is not the spectrum observed.
Th e F ir st Ar ylvin yla tion Occu r r ed a t P osition -
1
6. The H NMR spectra for the aromatic protons of A2b,
1D and 2D COSY, are shown in Figure 1 along with the
peak assignments (confirmed by proton decoupling). The
doublet signals at 8.23 ppm are of He. The signals of Hd,
doublet-doublet due to He(ortho) and Hc(meta), are
located at 7.80 and 7.82 ppm. The latter signal is
The third confirmation is assisted by the consideration
of molecular geometry. The molecular shapes of A2b and
A2c are rodlike and that of the 1-substituted analogue
(see next section for its preparation) is more like a T. It

5606 J . Org. Chem., Vol. 64, No. 15, 1999
Chang et al.
Ta ble 3. Mesop h a se a n d Tr a n sition Tem p er a tu r e (°C)
for Mixtu r es of p-Bu tyloxyben zoic Acid w ith A2b, A2c,
a n d 1-(2′(4′′-p yr id yl)eth en yl)-2-m eth oxyn a p h th a len e
(1-A2c) in 1:1 Mola r Ra tio
Sch em e 2
compound
mpa
mesophase behaviorb
161
181
A2b
162-164
C y z N y z Iso
131
177
150
189
A2c
189-191
109-111
C y z N y z Iso
130
186
129
1-A2c
C y z Iso
115
a
b
Melting points of A2b, A2c, and 1-A2c. C: Crystal, N:
Nematic, Iso: Isotropic.
has been well-known that pyridine could act as a proton
acceptor, and formation of a hydrogen bond would be
expected in the presence of a proton donor. Therefore,
mesophases should be observed for the hydrogen-bonded
complexes of A2b and A2c, for they were more like
rodlike mesogens, and not for the 1-substituted analogue.
The results are listed in Table 3; the melting points of
the pure components are also included. Nematic phases,
with extended temperature ranges, are observed for
mixtures of p-(butyloxy)benzoic acid (crystal 147 °C,
Nematic 160 °C, isotropic) with either A2b or A2c, in
1:1 molar ratio, but not with the 1-substituted analogue.
All the above results indicate that A2b should have the
structure as shown.
Red u ction Occu r r ed a t P osition -1. The occurrence
of reduction at position-1 required, as the above results
indicated, the presence of Pd(OAc)2, (o-tol)3P, Et3N, and
MeCN. In a parallel run, but with 1-bromo-2-methoxy-
naphthalene (obtained by methylation for 1-bromo-2-
naphthol (Aldrich)) as the haloarene, the sole product
obtained was the 1-arylvinylated derivative (47% yield),
and no reduced product, 2-methoxynaphthalene, was
detected. This result further indicates that the presence
of a substituent at positon-6 would facilitate the reduc-
tion. This situation resembles to the reduction of 1,6-
dibromo-2-naphthol, by tin (Sn) in the presence of acids,
to 6-bromo-2-naphthol during its preparation.11 The
reducing agent would be the palladium because it changed
back and forth between Pd(II)-complex and Pd(0)-complex
states during the catalytic cycle;2b,d however, both (o-
tol)3P and MeCN were required to furnishing the reduc-
tion with proton. Further study would be helpful to reveal
the mechanism.
2,6-Dibr om o-1,5-n a p h th a len ed iol Der iva tives a s
th e Ha loa r en es. The dimethoxy and bis(4′-alkyloxyben-
zoyloxy) derivatives of 2,6-dibromo-1,5-naphthalenediol
were used for coupling with 4VP and 4-acetoxystyrene,
Scheme 2, because the diol did not give isolable product.
The results are listed in Tables 4 and 5. For the coupling
of the 4VP and dimethoxy derivatives, with Et3N alone
as the base and also as solvent, both mono- and bis-
arylvinylated products, B2d and B2e, were obtained
along with 17-35% recovery of the dibromoarene. How-
ever, the bis-arylvinylated product was the predominant
in the presence of Ph3P and the mono-arylvinylated
product was predominant in the presence of (o-tol)3P. In
either case the total yield was about 30%. When Et3N
was replaced by Et3N/MeCN (10 mL/90 mL), the same
as that for 1,6-dibromo-2-methoxynaphthalene to obtain
the bis-arylvinylated product, the mono-arylvinylated
product was obtained as the sole product in only 10%
Ta ble 4. Rea ction Con d ition s a n d Yield s of P r od u cts for
th e Heck Rea ction of 2,6-Dibr om o-1,5-Na p h th a len ed iol
Der iva tives a n d 4-Vin ylp yr id in e
yield, %
Et3N/MeCN reaction
haloarene ligand
(mL)
time (h)
d
e
recovery, %
B2
B2
B2
B2
B2
B2
B2
B2
B2
B2a
B3
B3
B4
Ph3P
Ph3P
(o-tol)3P 30/0
(o-tol)3P 10/90
Ph3P
Ph3P
(o-tol)3P 4.5/40.5
(o-tol)3P 15/28
30/0
30/0
96
120
96
96
96
96
96
96
96
96
48
72
48
17
17.4
35
17
17.4
40
40.5
trace
trace
trace
trace
57.2
trace
trace
trace
8.7 28.7
30.3 2.2
10
10.8
8
10/90
6/54
66.3
80.9
81.5
80.3
3
38
28
30
Ph3P
15/30
(o-tol)3P 33.5/66.5
(o-tol)3P 30/30/10DMF
(o-tol)3P 15/30
38.2
(o-tol)3P 20/40
a
Haloarene:4-vinylpyridine ) 3:1 mole ratio.
Ta ble 5. Rea ction Con d ition s a n d Yield s of P r od u cts for
th e Heck Rea ction of 2,6-Dibr om o-1,5-Na p h th a len ed iol
Der iva tives a n d 4-Acetoxystyr en e
yield, %
Et3N/MeCN
(mL)
reaction
time (h)
haloarene
ligand
d ′
23
21.7
8.2
e′
B2
B2
B2
B2d
(o-tol)3P
(o-tol)3P
(o-tol)3P
(o-tol)3P
30/0
10/90
15/30
15/30
48
48
48
48
50.1
62.6
70a
a
B2e, but with 4-acetoxystyryl group at position-2, B2e′′.
yield, along with 40% recovery of the dibromoarene,
regardless of the phosphines used. When the solvent
volume was reduced, 6 mL/54 mL of Et3N/MeCN, higher
yields were obtained, 8% of B2d and 66.3% of B2e. A
further reduction of solvent volume resulted in 80.9%
yield of B2e as the sole product. When the solvent ratio
was changed to 15 mL/28 mL (and a further reduction
of volume), 81.5% yield of B2e was obtained as the sole
product. A similar result was obtained when Ph3P was
used. For the case of 33.5 mL/66.5 mL of Et3N/MeCN,
with molar ratio of haloarene/4VP ) 3/1, mono-arylvi-
nylation was the major product (38.2% of B2d ), and a
large amount of haloarene was recovered (57.2%). Under
the condition of Et3N/MeCN ) 1:2, the diester derivatives

Highly Conjugated Molecules the Heck Reaction
J . Org. Chem., Vol. 64, No. 15, 1999 5607
F igu r e 3. The absorption (left) and fluorescence (right)
spectra of stilbazole analogues in chloroform. Concentration
for absorption, 4.4 ×10-5 M. Concentration (× 10-7 M, in
parentheses) for fluorescence, B2e ‚-‚- (1.0); B2e′′ ‚‚-‚‚-
(0.8); B2d - - - (4.0); A2c ‚‚‚‚ (10.0); stilbazole s (28.0).
F igu r e 2. The absorption (left) and fluorescence (right)
spectra of stilbene analogues in chloroform. Concentration for
absorption, 4.4 × 10-5 M. Concentration (× 10-7 M, in
†
parentheses) for fluorescence, B2e′ ‚‚-‚‚- (0.65); B2e′ ‚-‚-
†
(1.0); B2d ′ ‚‚‚‚ (8.0); A2c′ - - - (2.0); stilbene s (30.0).
Ta ble 6. Th e P a r a m eter s of Absor p tion , F lu or escen ce,
a n d P h otoisom er iza tion P r od u cts for Stilben e a n d
Stilba zole An a logu es in Ch lor ofor m
(bis(4′-octyloxybenzoyloxy) and bis(4′-dodecyloxybenzoyl-
oxy)) gave the bis-arylvinylation as the sole products in
30-40% yields.
absorption ext fluorescencea
coef ꢀ, 104 cm-1
Φfb
photoisomerization
cis/trans (%, ss)d
With 4-acetoxystyrene as the olefin (as in Scheme 2
but replacing 4VP with 4-acetoxystyrene, and the corre-
sponding products were designated by d ′ and e′) and
employing (o-tol)3P, the dimethoxy derivative resulted in
bis-arylvinylation as the sole product (B2e′) in 23% yield
in Et3N. However, in Et3N/MeCN (10 mL/90 mL) mixed
solvent, the mono- and bis-arylvinylation (B2d ′ and B2e′)
were obtained in 21.7% and 50.1% yields, respectively.
These yields were changed to 8.2% and 62.6%, respec-
tively, in Et3N/MeCN (15/30 mL) mixed solvent. When
this olefin was used for the second coupling with 2-bromo-
1,5-dimethoxy-6-(2′(4′′-pyridyl)ethenyl)naphthalene (B2d),
a dissymmetrical bis-arylvinylated product (B2e′′) was
obtained in 70% yield in Et3N/MeCN (15 mL/30 mL). It
was found that the 1,5-diester derivatives resulted in only
recovery of p-(alkyloxy)benzoic acids, side products due
to dissociation of the ester linkages, with the above
various conditions.
The above results indicated that mono- or bis-arylvi-
nylation for derivatives of 2,6-dibromo-1,5-naphthalene-
diol could be obtained by a proper choice of reaction
conditions. It was noted that under all of the above
conditions there was no product found with Br replaced
by H for the mono-arylvinylation, the case observed for
the 1,6-dibromoarenes.
Absor p tion a n d Em ission . The UV-vis absorption
and emission spectra for some of these highly conjugated
molecules, in chloroform, are shown in Figures 2 (stilbene
analogues) and 3 (stilbazole analogues). Also included are
those of stilbene and stilbazole for comparison. It can be
seen clearly that the absorption bands are wider and red-
shifted by about 60-100 nm, and extinction coefficients
are about two times larger for the bis-arylvinylated
products. These spectra display vibronic structures spaced
by energies of ∼1300, ∼1200, and ∼1100 cm-1 from the
long wavelength side. The wavelengths of maximum
emission of these bis-arylvinylated products are also red-
compound M-1 (λmax, nm) (λmax,em, nm)c
B2e′
B2e
7.0 (357)
5.8 (357)
6.2 (360)
6.7 (373)
5.1 (363)
5.6 (337)
3.7 (330)
4.3 (330)
3.3 (337)
3.8 (333)
3.3 (330)
3.0 (300)
2.7 (295)
0.982 (405)
0.716 (440)
0.607 (441)
0.564 (408)
0.358 (460)
0.195 (430)
0.070 (395)
0.040 (411)
0.033 (408)
0.020 (397)
0.015 (435)
0.016 (355)
0.040 (353)
11.2/88.8
14.5/85.5
40.7/59.3
e
B2e′′
B2e′
B2e′′
A2a ′
B2d ′
B2d
A2c
A2c′
A2a
† f
† f
† f
63.5/36.5
80.1/19.9
87.4/22.6
† f
stilbazole
stilbene
90.4/9.6
77.2/22.8
a
Concentration was 1.0 × 10-7 M for recording the fluorescence
b
spectrum. The fluorescence quantum yield, quinine sulfate (Φf
) 0.55) used as standard (ref 14). c The wavelength of the
maximum fluorescence intensity, excited at the λmax of excitation
spectrum (uncorrected). Percentages evaluated from the 1H NMR
d
(CDCl3) spectra at the photostationary state of isomerization.
e B2e but with 4-acetoxystyryl group at position-2, see footnote of
Table 5. f The superscript † indicates that the acetoxyl group was
replaced by hydroxyl group.
shifted by about 60-100 nm, and emission intensities are
about 2 to 3 orders of magnitude higher than that of
stilbene (compared with the same concentration, 1.0 ×
-6
10
M). These emission spectra also display vibronic
structures but with energy spacings different from those
of absorption. Smaller red shifts of wavelength (both
absorption and emission) and smaller increments of
extinction coefficient and fluorescence intensity are ob-
served for the mono-arylvinylated products. The results
are summarized in Table 6 for some of the molecules
synthesized.
Considering the fluorescent quantum yield, listed in
Table 6, the bis-arylvinylated products exhibit much
larger values than the mono-arylvinylated products. The
exception is A2a as compared to A2c. Both molecules

5608 J . Org. Chem., Vol. 64, No. 15, 1999
Chang et al.
have the same values of extinction coefficient. Appar-
ently, the second pyridylethenyl group in A2a played a
mysterious role, which did not help for photon absorption
but dissipated the excited-state energy. A further study
would help to elucidate this. For the same type of
chromophore attached, the fluorescent quantum yields
for the B type products are higher than A type products
†
(B2e vs A2a and B2e′ † vs A2a ′ ). This enhancement is
attributed to the resonance effect for chromophores
located at positions-2 and -6 (of naphthalene). It is worth
noting that the stilbenoid derivatives exhibit fluorescent
quantum yields larger than the stilbazolyl derivatives,
†
B2e′ vs B2e and A2a ′ vs A2a . Moreover, for the
stilbenoid derivatives, the existence of acetyl group
†
results in a larger enhancement (B2e′ vs B2e′ , and
B2e′′ vs B2e′′ †). These variations in fluorescent quantum
yield might have something to do with the characteristics
of electron-donating and -withdrawing properties of the
functional groups.8b It is also worth noting that the
enhancement of B2e′′ is the smallest among the B2e′s.
The B2e′′ molecule possesses different chromophores at
positions-2 and -6. These results are consistent with that
reported by Albota et al.,8b that the symmetrical chro-
mophores result in larger enhancements. The molecules
synthesized in the present study exhibit two-photon
absorption behavior. The details of these properties will
be reported later.
P h otoisom er ization . The photoisomerizations of these
compounds, 0.008 M in CDCl3, were monitored spectro-
1
scopically by H NMR spectrametry up to the photosta-
tionary states, and the ratios of cis/trans isomers are also
included in Table 6. Further photochemical processes
were observed for prolong irradiation and not included
here. The proton spectra at different irradiation stages
for B2d are shown in Figure 4 along with the schematic
representations of molecular conformations and the cor-
responding peak assignments. Upon irradiation of UV
light for 2 min, a doublet at 6.7 ppm with coupling
constant of 12.4 cps is clear evidence for the cis-
conformation of the ethenylic protons, and a doublet at
8.45 ppm is clearly due to the protons next to the nitrogen
atom of the pyridyl ring. The signals of cis-isomer grew
at the expenses of the trans-isomer with irradiation time.
At the photostationary state (12 min), the signals of the
cis-isomer dominate the spectrum, and the assignments
of remaining peaks are apparent and supported further
by the 2D COSY spectrum, as shown in Figure 5. The
cross-peaks at 6.68-8.46 ppm confirm the assignments
for Hg′ and Hi′. Stronger interaction should be expected
for Hg′-Hh′, but only half of the cross-peaks is observed,
at 6.7-7.14 ppm, the other half is buried under the Hg′
and Hf ′ cross-peaks. The cross-peaks at 7.1-7.2 and 7.1-
7.7 ppm are apparently due to interactions of Hf ′ with
Hd′ and He′, respectively. It is interesting to note that
nearly equally intense cross-peaks are observed for the
Hf ′-He′ interaction (through five chemical bonds). The
fact that the assignments for Hd′ and He′ are not
interchanged can be accounted for considering the cis-
conformation.
F igu r e 4. The 1H NMR spectra as a function of irradiation
time for B2d in CDCl3 (0.008 M). The schematic representa-
tions of trans- and cis-isomers and the corresponding peak
assignments.
facing the edge of naphthalene. In the cis-conformation
the naphthalene plane is slightly rotated counterclock-
wise (viewing along the Br-C bond) with respect to the
ethenylic plane resulting in upfield shifts for both ethe-
nylic protons due to the shielding effects from either
rings. In this conformation, the adjacent methoxy group
is swung to a position having a better shielding from the
ethenylic π-electrons. These analyses are consistent with
those discussed by Steiner et al. in the study of isomer-
ization of N-methylated stilbazolium dye.15
The variation in compositions of cis- and trans-isomers,
obtained from the 1H NMR spectra, for B2d and B2e
during the irradiation and up to the isomerization
photostationary states are shown in Figure 6. The proton
spectra of B2e consisted of two sets of signals correlated
to the all-trans and all-cis conformations, thus only the
all-cis photoproduct was included for analysis. Also
shown in Figure 6 are the curves obtained with the fitted
rate constants (R ) 0.995 and 0.992 for B2d and B2e,
respectively). The ratio of kt-c/kc-t () K, the equilibrium
constant at the photostationary state) is 4 for B2d and
All the aromatic proton signals of the cis-conformation
are upfield shifted except those of Ha and Hb, which are
the farthest away from the ethenyl group and retain their
chemical shifts. The largest shift is observed for the
doublet of Hd, from 7.92 to 7.2 ppm, suggesting a
shielding effect due to the π-electrons of the pyridyl
moiety in the cis-conformation and the pyridyl ring plane
(14) Calvert, J . G.; Pitts, J . N., J r. Photochemistry; Wiley: New York,
1966.
(15) Steiner, U.; Abdel-Kader, M. H.; Fischer, P.; Kramer, M. E. A.
J . Am. Chem. Soc. 1978, 100, 3190.

Highly Conjugated Molecules the Heck Reaction
J . Org. Chem., Vol. 64, No. 15, 1999 5609
Ta ble 7. Mesop h a se a n d Tr a n sition Tem p er a tu r e (°C)
for th e Syn th esized Molecu les
compound
mesophase behaviorc
1
F igu r e 5. The H NMR 2D COSY spectrum of B2d in CDCl3
(0.06 M) at the photostationary state of isomerization (45 min
of irradiation). See Figure 4 for peak assignments.
a
The superscript † indicates that the acetoxyl group was
replaced by hydroxyl group. dec: decomposed. c C: crystal, N:
b
nematic, Iso: isotropic.
considerable amount of shorter wavelengths (<330 nm)
and the filtering was apparently not complete.
Mesop h a ses. Considering the shape of molecular
geometry of these highly conjugated molecules, the
existences of liquid crystal behavior would be expected,
particularly for the bis-arylvinylated products of 2,6-
dibromo-1,5-dimethoxynaphthalene (B’s). The results are
listed in Table 7. Either monotropic or enantiotropic
nematic phases were observed for the mono-arylvinylated
A type products. Enantiotropic nematic phases, at rela-
tive high temperatures with wide ranges and up to
decomposition, were observed for the B type products.
The conjugated moieties studied here could serve as the
cores of rod like mesogens, and a large variety of new
mesogens could be synthesized.
F igu r e 6. The percentages of trans- and cis-isomers of B2d
(2 and 4) and B2e (9 and 0), evaluated from the 1H NMR
spectra, as a function of irradiation time. The theoretical
curves are obtained with the fitted k values with R values of
0.995 and 0.992 for B2d and B2e, respectively.
Con clu sion
0.16 for B2e. This variation in rate constant ratios
correlates well with the fluorescent quantum yields as
shown in Table 6, the larger the fluorescent quantum
yield the smaller the fraction of cis-isomer at the pho-
toisomerization stationary state. This is quite reasonable
since a larger fraction of energy of the excited state is
radiated out as fluorescence a smaller fraction is left for
the chemical processes. It should be noted here that a
sample of trans-stilbene was next to the others during
the irradiation, the occurrence of cis-stilbene indicated
that the UV light, after filtering, was composed of a
Palladium-catalyzed coupling of dibromonaphthyl de-
rivatives with 4-vinylpyridine or 4-acetoxystyrene un-
derwent mono- and bis-arylvinylation dependent on the
choice of reaction conditions. For the 1,6-dibromoarenes,
the mono-arylvinylated derivative, shown to occur at
position-6, was the sole product in the presence of
(o-tol)3P/Et3N, and the bis-arylvinylated derivative was
the major product and accompanied by a minor product,
6-arylvinylated-1-reduced, in the presence of (o-tol)3P/
Et3N/MeCN. For the 2,6-dibromoarenes, the bis-arylvi-
nylated derivative was the sole product in the presence

5610 J . Org. Chem., Vol. 64, No. 15, 1999
Chang et al.
of either Ph3P or (o-tol)3P provided with a small volume
of Et3N/MeCN solvent and the mono-arylvinylated de-
rivative was the major product with larger solvent
volume and higher haloarene ratio.
Ack n ow led gm en t. This work was supported by the
National Science Council under contract number NSC
87-2113-M-032-003.
These molecules exhibit intense fluorescence, me-
sogenic behaviors, and photochemical properties. In view
of the current interests in the development of nonlinear
optical materials, two-photon dyes, and photo- and elec-
troactive materials, the results presented here are ben-
eficial to the pursuit of such materials.
Su p p or tin g In for m a tion Ava ila ble: Tables of results of
elemental analysis, melting point, molecular mass, and 1H
NMR peak assignments for molecules synthesized. This mate-
rialis availablefreeofchargevia theInternet at http://pubs.acs.org.
J O990697A