Palladium-catalyzed convergent synthesis and properties of conjugated dendrimers based on triarylethene branching

Article abstract of DOI:10.1002/anie.200600063

Branching out: Pd-catalyzed iterative triarylations of vinylboronate afford conjugated dendrimers with triarylethene branches (see picture) in a flexible and convergent manner. These highly branched dendrimers reveal good solubility in common organic solvents and exhibit high quantum yields of fluorescence, with the color of the emission ranging from blue to red depending on the extent of branching and hence π conjugation. (Figure Presented)

Full text of DOI:10.1002/anie.200600063

Communications
Dendrimers
DOI: 10.1002/anie.200600063
Palladium-Catalyzed Convergent Synthesis and
Properties of Conjugated Dendrimers Based on
Triarylethene Branching**
Kenichiro Itami,* Keisuke Tonogaki, Toshiki Nokami,
Youichi Ohashi, and Jun-ichi Yoshida*
Dendrimers are molecular architectures that offer an enor-
mous opportunity for creating new functional materials.[1]
Their highly branched, globular architectures give rise to a
number of interesting properties that contrast with those of
linear polymers. Although a number of dendrimers exist that
are based on various core and branching units, the develop-
ment of new core and branching units should pave the way for
a generation of new functional materials.[1] In particular, the
evolution of novel and effective p-branching units is neces-
sary in the emerging field of p-conjugated dendrimers,[2–6]
because most of the branches of known p-conjugated
dendrimers are based on benzene structures with 1,3,5-
linkages, which considerably interrupt any interbranch p con-
jugation.
Triggered by the tremendous progress in the field of
stilbenoid compounds, and in particular the oligophenylene-
vinylenes,[7] the development of electroactive and light-
emitting stilbenoid dendrimers is of great interest to many
researchers. However, all the stilbenoid dendrimers reported
to date are based on 1,3,5-benzene branching and thereby
suffer from disrupted interbranch conjugation (Figure 1).[8]
Over the last few years, we have explored the chemistry of
=
extended p systems based on highly substituted C C cores
(i.e., triarylethene- and tetraarylethene-based extended
p systems) which has involved methodological challenges
associated with selective synthesis and the potential applica-
tion of these compounds as functional materials.[9] We
[*] Prof. K. Itami
Research Center for Materials Science, Nagoya University
and PRESTO, Japan Science and Technology Agency (JST)
Chikusa-ku, Nagoya 464-8602 (Japan)
Fax: (+81)52-788-6098
E-mail: itami@chem.nagoya-u.ac.jp
K. Tonogaki, Dr. T. Nokami, Y. Ohashi, Prof. J. Yoshida
Department of Synthetic Chemistry and Biological Chemistry
Graduate Schoolof Engineering
Kyoto University
Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2727
E-mail: yoshida@sbchem.kyoto-u.ac.jp
[**] This work was supported in part by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science,
and Technology, Japan. K.T. is a recipient of the JSPS Predoctoral
Fellowships for Young Scientists. We thank Dr. Atsushi Wakamiya
and Professor Shigehiro Yamaguchi (Nagoya University) for
valuable discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2404
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2404 –2409

Angewandte
Chemie
À
either of the Ar X bonds of the aryl dihalides produced the
À
1:1 coupling adduct with unreacted Ar X (step B, selective
mono-cross-coupling). This simple sequence allowed us to
quickly prepare p-extended aryl halides 2. Thus, the repeti-
tion of the sequence of steps A and B resulted in the
production of highly p-extended alkenylboronates 3. I n
each cycle, the boronates 3 were doubly cross-coupled with
aryl dihalides (step C, double cross-coupling) to produce
triarylethene-based dendrimers Gn (where n is the gener-
ation number) in a convergent manner. The power of this
synthetic strategy is apparent, as all of these dendrimers are
prepared from common reagents using standard reactions.
A representative synthesis of 4-Gn (n = 1–3) dendrimers,
which include triphenylethene branching units, is shown in
Scheme 2. The use of the highly active [Pd(PtBu3)2] catalyst[14]
in combination with iPr2NH was useful in the double
Mizoroki–Heck arylation (step A in Scheme 1). These con-
ditions were particularly effective for the selective arylation
Figure 1. Triarylethene (right) as a p-conjugated branching unit for
conjugated dendrimers.
envisaged that a strategic modification of our triarylethene
synthesis would lead to the generation of novel conjugated
dendrimers that incorporate the triarylethene structure as a
new p-conjugated branching unit (Figure 1). We expected
such dendrimers to be different from previous stilbenoid
dendrimers with respect to their properties, which reflect the
effective delocalization of the p electrons as well as their
unique structure. Herein, we describe the convergent syn-
thesis of novel triarylethene-based conjugated dendrimers
using palladium-catalyzed iterative triarylation of vinylboro-
nate esters. The results of preliminary examinations of the
photophysical properties of the dendrimers are also de-
scribed.
À
À
of C H over C B, which is otherwise favorable (Suzuki–
Miyaura coupling). For selective mono-cross-coupling reac-
tions (step B in Scheme 1), the use of 1-bromo-4-iodobenzene
as the aryl dihalide under the catalytic influence of moder-
ately active [Pd(PPh3)4] was effective. When a highly active
catalyst such as [Pd(PtBu3)2] was used, then double cross-
coupling often took place to afford 4-G1 (see Figure 2) in
high yields. Therefore, we used [Pd(PPh3)4] for mono-cross-
coupling reactions (step B) and [Pd(PtBu3)2] for the final
double cross-coupling reactions (step C).[15] Gratifyingly, the
third generation dendrimer 4-G3 was obtained in reasonable
overall yield upon repeating these reactions.
We previously reported the rapid synthesis of triaryl-
=
ethenes using a catalytic one-pot triarylation on the C C core
[10]
=
of vinylboronate pinacol ester CH2 CHB(pin) (1). Under
the influence of a Pd catalyst and an amine, 1 undergoes a
double Mizoroki–Heck-type arylation[11,12] with two equiva-
lents of aryl halides to generate b,b-diarylated vinylboronates
=
Ar2C CHB(pin). The subsequent Suzuki–Miyaura cou-
pling[13] of the resultant boronates with aryl halides rapidly
[10]
=
affords triarylethenes Ar2C CHAr. We envisaged that a
When the double cross-coupling reactions (step C,
Scheme 1) were performed at earlier stages, such as, with 3-
G1 and 3-G2, then lower generation dendrimers 4-G1 and 4-
G2, respectively, were obtained in good yields (Scheme 2).
The application of various core units in the final double cross-
coupling rapidly afforded various conjugated dendrimers in a
convergent manner (Figure 2). When Mizoroki–Heck aryla-
tions (step A) were conducted in a mono fashion, then
dendrimers with fewer branches, such as 12, were easily
prepared. Such dendrimers are useful for elucidating the
effect of branching of the dendrimers on their properties.
Our strategy could be extended to the synthesis of
conjugated dendrimers with mixed branching units by
convergent synthesis of triarylethene-based dendrimers
would be possible by using these aryl-assembling reactions
À
À
(i.e., double C H arylation and C B arylation) with the
strategic use of di- and trifunctional aryl units (aryl dihalides
and trihalides). Our strategy that employs aryl dihalides is
shown in Scheme 1.
The treatment of vinylboronate 1 with two equivalents of
aryl halides 2 in the presence of a Pd catalyst and amine
afforded b,b-diarylated vinylboronates 3 (step A, a double
Mizoroki–Heck arylation).[10] Although the subsequent
double cross-coupling of 3 with aryl dihalides produced the
2:1 coupling adduct G1, the selective mono-cross-coupling at
Scheme 1. Convergent synthesis of triarylethene-based conjugated dendrimers. pin=pinacolyl.
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2404 –2409
www.angewandte.org 2405

Communications
stilbenoid dendrimers. We believe that
the enhanced emission of the higher
generation dendrimers is attributable to
their unique structure. Owing to the
sterically congested environment, the
higher dendrimers such as 4-G2 probably
adopt
a nonplanar structure, which
reduces the probability of effective inter-
molecular fluorescence-quenching inter-
actions and molecular motions that may
trigger radiationless transitions, thereby
enhancing the overall fluorescence effi-
ciency.[16]
To examine the effect of branching on
the properties of these dendrimers, the
less-branched dendrimer 12 was pre-
pared as a reference for 4-G2. As the
absorption and emission maxima and
emission color of 12 are similar to those
of 4-G2, it is reasonable to assume that
the extent of the p conjugation seen in 4-
G2 is already established in the substruc-
ture 12 and that the remaining four p-
tolyl branches in 4-G2 contribute to its
enhanced
fluorescence
efficiency
Scheme 2. Synthesis of triarylethene-based dendrimers 4-Gn (n=1–3). a) 1, [Pd(PtBu3)2],
iPr2NH, toluene, 808C; b) 1-bromo-4-iodobenzene, [Pd(PPh3)4], aq. Cs2CO3, toluene, 908C;
c) 1,4-diiodobenzene, [Pd(PtBu3)2], NaOH, H2O, toluene, 908C.
(FF(12) = 0.32 vs FF(4-G2) = 0.58).
Moreover, the solubility of these
p systems was found to generally increase
as the degree of branching increases. For
simply applying aryl trihalides in place of the aryl dihalides in
the reaction scheme described in Scheme 1. For example,
when 1,3,5-tribromobenzene was used as a trifunctional unit,
then the highly branched dendrimer 13, which contains
triphenylethene and benzene branching units (Figure 2),
was obtained in high yield (Scheme 3).
Most of the dendrimers prepared were soluble in common
organic solvents and, thus, allowed us to characterize them
using standard methods such as 1H NMR and 13C NMR
spectroscopy and high-resolution FAB mass spectrometry
(HRMS). In many cases, the methyl groups at the surface of
the dendrimers were well distinguished (although not fully) in
the 1H and 13C NMR spectra. In addition, analysis of the
samples by gel permeation chromatography showed that all
the dendrimers were monodispersed, as would be expected
from their well-ordered construction.
Interestingly, many of the conjugated dendrimers pre-
pared emitted fluorescence in both solution and the solid
state. The photophysical properties (UV/Vis and fluores-
cence) of these conjugated dendrimers in chloroform are
listed in Table 1. As expected, the wavelength of the emission
maximum (lem) increases with increasing dendrimer gener-
ation (i.e., lem(4-G3) > lem(4-G2) > lem(4-G1)) and is ascri-
bed to the extension of the p conjugation. We also found that
the quantum yield of fluorescence (FF) also increased with
increasing dendrimer generation (FF(4-G1) = 0.024, FF(4-
G2) = 0.58, FF(4-G3) = 0.62). Note the 24-fold increase from
4-G1 to 4-G2. Such a dramatic effect of the dendrimer
generation in the color and efficiency of the fluorescence
emission has not been reported for 1,3,5-benzene-branched
example, the solubility of 4-G2 in chloroform (61.4 mm) and
benzene (63.8 mm), as saturated solutions, is 6–10 times
higher than that of 12 in the same solvents (11.3 and
6.7 mm, respectively). The increased solubility of 4-G2 is
also thought to be attributable to its unique nonplanar
structure, which reduces intermolecular interactions and
thereby facilitates solvation. The current focus of research
into oligophenylenevinylenes and stilbenoid dendrimers lies
in enhancing their solubility and processability while preserv-
ing the planarity and rigidity of the phenylenevinylene unit,
which is responsible for their unique functional properties.
Therefore, the exploitation of p branching as observed in our
work may be a useful approach toward this goal.
The photophysical properties of the dendrimers were
found to depend significantly on the nature of the core
structure. For example, the second generation dendrimers
based on 1,2- and 1,3-benzene cores (5 and 6) emitted blue
light with a low efficiency, which was significantly different to
the yellow light emitted by 4-G2 with FF = 0.58. The low
emissions of the “half-dendrimer” 7 and dendrimer 8 with a
nonconjugated core also support the importance of the core
structure with regard to the fluorescence properties. Such
“core-tuning” is also useful in the generation of materials with
interesting properties. For example, the second generation
dendrimer 9 with a fluorene core emitted yellow light with an
extremely high quantum yield (FF = 0.82). The finding of a
hydrocarbon that strongly emits yellow light is of great
interest. Moreover, the replacement of the above core
structure with bithiophene to give 10 and fluorenone to give
11 gives rise to orange and red light emissions, respectively.
2406 www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2404 –2409

Angewandte
Chemie
Figure 2. Various conjugated dendrimers prepared in this study.
These results clearly show that the photophysical properties
can be conveniently tuned at the final double cross-coupling
stage (step C in Scheme 1) rather than at the earlier stages.
In summary, we have developed an extremely useful and
flexible method for the convergent synthesis of novel
conjugated dendrimers based on triarylethene branching. A
simple yet powerful strategy using common reagents (aryl
halides) and reactions (Pd-catalyzed arylations) allowed us to
quickly and systematically prepare various fluorescent den-
drimers. Moreover, our highly branched dendrimers revealed
both a high efficiency of fluorescence emission and high
solubility in common organic solvents, which are useful
attributes for future applications. This branching effect may
find many uses in the emerging field of stilbenoid materials[7,8]
Angew. Chem. Int. Ed. 2006, 45, 2404 –2409
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 2407

Communications
[1] a) S. M. Grayson, J. M. FrØchet, Chem. Rev. 2001, 101, 3819;
b) Dendrimers V: Functional and Hyperbranched Building
Blocks, Photophysical Properties, Applications in Materials and
Life Sciences (Eds.: C. A. Schalley, F. Vögtle), Springer, Berlin,
2003; c) Dendrimers II: Architecture, Nanostructure and Supra-
molecular Chemistry (Ed.: F. Vögtle), Springer, Berlin, 2000.
[2] Early important conjugated dendrimers based on 1,3,5-benzene
branches: a) T. M. Miller, T. X. Neenan, Chem. Mater. 1990, 2,
346; b) T. M. Miller, T. X. Neenan, R. Zayas, H. E. Bair, J. Am.
Chem. Soc. 1992, 114, 1018; c) J. Zhang, J. S. Moore, Z. Xu, R. A.
Aguirre, J. Am. Chem. Soc. 1992, 114, 2273; d) Z. Xu, J. S.
Moore, Angew. Chem. 1993, 105, 1394; Angew. Chem. Int. Ed.
Engl. 1993, 32, 1354.
[3] Selected recent examples of conjugated dendrimers based on
benzene branches: a) C. D. Simpson, G. Mattersteig, K. Martin,
L. Gherghel, R. E. Bauer, H. J. Räder, K. Müllen, J. Am. Chem.
Soc. 2004, 126, 3139; b) X. Shen, D. M. Ho, R. A. Pascal, Jr., J.
Am. Chem. Soc. 2004, 126, 5798; c) T. W. Kwon, M. M. Alam,
S. A. Jenekhe, Chem. Mater. 2004, 16, 4657; d) D. Liu, S. D.
Feyter, M. Cotlet, A. Stefan, U.-M. Wiesler, A. Herrmann, D.
Grebel-Koehler, J. Qu, K. Müllen, F. C. De Schryver, Macro-
molecules 2003, 36, 5918; e) J. S. Melinger, Y. Pan, V. D. Klei-
man, Z. Peng, B. L. Davis, D. McMorrow, M. Lu, J. Am. Chem.
Soc. 2002, 124, 12002.
Scheme 3. Synthesis of conjugated dendrimer 13 with mixed branching units.
a) 1, [Pd(PtBu3)2], iPr2NH, toluene, 808C; b) 1,3,5-tribromobenzene, [Pd-
(PPh3)4], NaOH, H2O, toluene, 908C; c) 1, [Pd(PtBu3)2], iPr2NH, toluene,
reflux; d) 1,3,5-tribromobenzene, [Pd(PtBu3)2], NaOH, H2O, toluene, 908C.
Compounds 2-G2* and 3-G2* are related to 2-G2 and 3-G2, respectively
(see Scheme 2).
[4] Conjugated dendrimers based on enediyne branches: G. T.
Hwang, H. S. Son, J. K. Ku, B. H. Kim, J. Am. Chem. Soc.
2003, 125, 11241.
[5] Conjugated dendrimers based on phenylazomethine branches:
M. Higuchi, K. Yamamoto, Bull. Chem. Soc. Jpn. 2004, 77, 853.
[6] Conjugated dendrimers based on truxene branches: X.-Y. Cao,
X.-H. Liu, X.-H. Zhou, Y. Zhang, Y. Jiang, Y. Cao, Y.-X. Cui, J.
Pei, J. Org. Chem. 2004, 69, 6050.
[7] a) A. Kraft, A. C. Grimsdale, A. B. Holmes, Angew. Chem. 1998,
110, 416; Angew. Chem. Int. Ed. 1998, 37, 402; b) H. Meier,
Angew. Chem. 1992, 104, 1425; Angew. Chem. Int. Ed. Engl.
1992, 31, 1399.
[8] Stilbenoid dendrimers based on 1,3,5-benzene branches: a) S. K.
Deb, T. M. Maddux, L. Yu, J. Am. Chem. Soc. 1997, 119, 9079;
b) H. Meier, M. Lehmann, Angew. Chem. 1998, 110, 666; Angew.
Chem. Int. Ed. 1998, 37, 643; c) M. Halim, J. N. G. Pillow, I. D. W.
Samuel, P. L. Burn, Adv. Mater. 1999, 11, 371; d) J. N. G. Pillow,
M. Halim, J. M. Lupton, P. L. Burn, I. D. W. Samuel, Macro-
molecules 1999, 32, 5985; e) J. N. G. Pillow, P. L. Burn, I. D. W.
Samuel, M. Halim, Synth. Met. 1999, 102, 1468; f) H. Meier, M.
Lehmann, U. Kolb, Chem. Eur. J. 2000, 6, 2462; g) E. Díez-Barra,
J. C. García-Martínez, J. Rodríguez-López, R. Gómez, J. L.
Segura, N. Martín, Org. Lett. 2000, 2, 3651; h) J. L. Segura, R.
Gómez, N. Martín, D. M. Guldi, Org. Lett. 2001, 3, 2645; i) E.
Díez-Barra, J. C. García-Martínez, S. Merino, R. del Ray, J.
Rodríguez-López, P. Sµnchez-Verdffl, J. Tejeda, J. Org. Chem.
2001, 66, 5664; j) D. Ma, J. M. Lupton, R. Beavington, P. L. Burn,
I. D. W. Samuel, Adv. Funct. Mater. 2002, 12, 507; k) S. Sengupta,
S. K. Sadhukhan, R. S. Singh, N. Pal, Tetrahedron Lett. 2002, 43,
1117; l) A. R. Cano-Marín, E. Díez-Barra, J. Rodríguez-López,
Tetrahedron 2005, 61, 395.
[9] a) K. Itami, K. Mitsudo, T. Kamei, T. Koike, T. Nokami, J.
Yoshida, J. Am. Chem. Soc. 2000, 122, 12013; b) K. Itami, T.
Nokami, J. Yoshida, J. Am. Chem. Soc. 2001, 123, 5600; c) K.
Itami, T. Nokami, Y. Ishimura, K. Mitsudo, T. Kamei, J. Yoshida,
J. Am. Chem. Soc. 2001, 123, 11577; d) K. Itami, Y. Ushiogi, T.
Nokami, Y. Ohashi, J. Yoshida, Org. Lett. 2004, 6, 3695; e) K.
Itami, M. Mineno, N. Muraoka, J. Yoshida, J. Am. Chem. Soc.
2004, 126, 11778; f) K. Itami, Y. Ohashi, J. Yoshida, J. Org.
Chem. 2005, 70, 2778.
Table 1: Photophysicalproperties of conjugated dendrimers.
Compounds
UV/Vis data[a]
Fluorescence data[a]
[b]
lmax [nm], (loge)
lem [nm]
Color
FF
4-G1
4-G2
359 (4.56)
402 (4.72),
343 (4.86)
343 (5.20),
248 (5.13)
340 (5.00)
340 (5.00)
340 (4.71)
345 (5.31)
406 (4.89),
340 (4.92)
451 (4.72),
335 (4.79)
473 (3.97),
388 (4.88),
332 (4.97)
344 (5.03)
473
518
blue
yellow
0.024
0.58
4-G3
520
yellow
0.62
5
6
7
8
9
492
489
471
484
497
blue
blue
blue
blue
yellow
0.15
0.050
0.015
0.099
0.82
10
11
559
616
orange
red
0.17
0.010[c]
12
511
yellow
0.32
[a] Measured in chloroform, unless otherwise stated. [b] Determined
with reference to 9,10-diphenylanthracene (lexc =350 nm), unless other-
wise stated. [c] Determined with reference to fluorescein in ethanol
(lexc =400 nm).
as well as in the development of novel functional materials,
where there remain problems with respect to solubility and
undesirable intermolecular interactions (aggregation) of the
currently used materials.
Received: January 7, 2006
Published online: March 9, 2006
À
Keywords: C C coupling · dendrimers · fluorescence ·
homogeneous catalysis · palladium
[10] K. Itami, K. Tonogaki, Y. Ohashi, J. Yoshida, Org. Lett. 2004, 6,
4093.
.
2408 www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2404 –2409

Angewandte
Chemie
[11] Reviews on the Mizoroki–Heck reaction: a) I. P. Beletskaya,
A. V. Cheprakov, Chem. Rev. 2000, 100, 3009; b) A. de Meijere,
F. E. Meyer, Angew. Chem. 1994, 106, 2473; Angew. Chem. Int.
Ed. Engl. 1994, 33, 2379.
[12] Multiple Mizoroki–Heck reaction: a) S. Bräse, A. de Meijere in
Handbook of Organopalladium Chemistry for Organic Synthesis
(Ed.: E. Negishi), Wiley, New York, 2002, p. 1179; b) P. Nilsson,
M. Larhed, A. Hallberg, J. Am. Chem. Soc. 2001, 123, 8217;
c) see Reference [9].
[13] Review on Suzuki–Miyaura couplings: N. Miyaura, A. Suzuki,
Chem. Rev. 1995, 95, 2457.
[14] Use of Pd/P(tBu)3 as a catalyst for Mizoroki–Heck reactions:
a) A. F. Littke, G. C. Fu, J. Am. Chem. Soc. 2001, 123, 6989; b) K.
Itami, D. Yamazaki, J. Yoshida, Org. Lett. 2003, 5, 2161; c) see
Reference [9].
[15] Use of Pd/P(tBu)3 as a catalyst for Suzuki–Miyaura couplings:
a) A. F. Littke, C. Dai, G. C. Fu, J. Am. Chem. Soc. 2000, 122,
4020; b) K. Itami, T. Kamei, J. Yoshida, J. Am. Chem. Soc. 2003,
125, 14670.
[16] It may also be possible that irradiation of the sample during
spectroscopy induces electrocyclization to give partially cyclized
products that are highly emissive. However, we could not detect
such products in the samples by UV or NMR spectral analysis
after irradiation.
Angew. Chem. Int. Ed. 2006, 45, 2404 –2409
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 2409