A simple and efficient synthesis of 1,3,5-tris[2-(3,5-diethynylphenyl) ethynyl]benzene from 1,3,5-triethynylbenzene

Article abstract of DOI:10.1007/s11164-011-0356-1

A concise and convenient three-step process is reported for preparation of 1,3,5-tris[2-(3,5-diethynylphenyl)ethynyl]benzene from 1,3,5-triethynylbenzene and 2-hydroxypropyl group-protected peripheral monomers via a Sonogashira coupling-deprotection react

Full text of DOI:10.1007/s11164-011-0356-1

Res Chem Intermed (2012) 38:403–411
DOI 10.1007/s11164-011-0356-1
A simple and efficient synthesis of 1,3,5-tris[2-(3,5-
diethynylphenyl)ethynyl]benzene from 1,3,5-
triethynylbenzene
•
Jie Li Pengcheng Huang
Received: 2 June 2011 / Accepted: 26 July 2011 / Published online: 7 August 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A concise and convenient three-step process is reported for preparation
of 1,3,5-tris[2-(3,5-diethynylphenyl)ethynyl]benzene from 1,3,5-triethynylbenzene
and 2-hydroxypropyl group-protected peripheral monomers via a Sonogashira
coupling–deprotection reaction sequence. This approach had several advantages, for
example high yields, short reaction time, and simple separation procedure. In the
key step a modified Sonogashira reaction was developed, under which conditions
the coupling of 1,3,5-triethynylbenzene and substituted iodobenzenes proceeded
rapidly, giving the desired products in good to excellent yields.
Keywords 1,3,5-Tris[2-(3,5-diethynylphenyl)ethynyl]benzene Á Dendrimer Á
Sonogashira cross-coupling Á 1,3,5-Triethynylbenzene Á Deprotection reaction
Introduction
Phenylacetylene dendrimers have been the subject of extensive interest in recent
years because of their unique shape-persistent molecular architecture, versatile
functionality, and wide range of potential applications in light-harvesting molecular
antennas, nonlinear optical materials, and organic light-emitting diodes [1–5]. As
one of the first generation of phenylacetylene dendrimers formed solely from 1,3,5-
triethynylphenyl units, 1,3,5-tris[2-(3,5-diethynylphenyl)ethynyl]benzene (1) acts
not only as a potential key building block in dendrimer synthesis but also as an
important model molecule for investigation of the physical and chemical properties
Electronic supplementary material The online version of this article (doi:
10.1007/s11164-011-0356-1) contains supplementary material, which is available to authorized users.
J. Li Á P. Huang (&)
Department of Polymer Materials and Composites, School of Materials Science and Engineering,
Beihang University, Beijing 100191, China
e-mail: huangpc@buaa.edu.cn
123

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J. Li, P. Huang
of these dendrimers [6–9]. Moore et al. were first to explore the synthetic
methodology and properties of phenylacetylene dendrimers. Their findings were an
important basis for subsequent investigation of related compounds [8, 10].
´
Rodrıguez et al. prepared trimethylsilyl group-protected 1 and its ethynylphenyl
homologues, and illustrated the relationship between their electronic properties and
the number of the conjugated ethynylphenyl units [11]. Yamaguchi et al.
synthesized methoxy group substituted analogs of 1, and achieved the first organic
molecules with light-emitting ability (in the violet–blue region with high
fluorescence quantum yield) [12]. Qiu and Shah et al. examined the photophysical
properties of the carbazole substituted analogs of 1, and demonstrated that white
electroluminescence could be obtained from a single phenylethynyl carbazole
emitter [13]. Nakano et al. investigated theoretically the structural dependence of
the exciton dynamics of the p-ethynylphenyl homologues of 1, and elucidated the
advantage of dendritic molecular systems for application in nano-optical and light-
harvesting devices [14]. Nguyen et al. prepared the DNA-hybrid analogs of 1. The
dendrimer–DNA aggregates had sharpened switch-like melting transitions, which
pointed the way for designing new DNA-based materials with enhanced recognition
properties [15].
Despite its attractive properties and potential applications, most classical
methods for synthesis of 1 were low yielding as they required a tedious process.
The approaches often involved Sonogashira cross-coupling between a 1,3,5-
triiodobenzene core and 3 equiv. ethynyl-terminated peripheral monomers with
trialkylsilyl-protected ethynyl groups at the meta positions, and subsequent
deprotection of the trialkylsilyl groups [6, 8, 10, 11, 16]. For example, by using
an eight-step sequence, 1 was synthesized in an overall yield of only 19% [16]. On
the other hand, by using a 1,3,5-triethynylbenzene core and iodine terminated
peripheral monomers a simple three-step approach seemed very attractive
(Scheme 1). However, Pesak found that Sonogashira cross-coupling of the
monomers (R = Sii-Pr3) and 1,3,5-triethynylbenzene was low-yield, which could
R
R
I
I
I
R
a
b
c
I
3
+
R
R
2
R
Core molecule
Peripheral monomer
2a, 3a R=-C(CH3)2OH
R
3
R
1
Scheme 1 A simple synthetic route to 1. For R = –C(CH3)2OH: a 2-methyl-3-butyn-2-ol, tris(diben-
zylideneacetone)dipalladium, PPh3, CuI, piperidine, 25 °C, 48%; b Pd2(dba)3, tris(2,4,6-trimethyl-
phenyl)phosphine, CuI, N,N-diisopropylethylamine, tetrabutylammonium iodide, DMF, -20 °C, 92%;
c tetrabutylammonium hydroxide, CH3OH, toluene, 75 °C, 83%
123

A simple and efficient synthesis
405
be attributed to the relative instability of 1,3,5-triethynylbenzene. A likely side
reaction was the self-coupling of triethynylbenzene [16].
In the work discussed in this paper, we developed the simple synthetic approach
toward 1 described in Scheme 1, which reduced the number of reaction steps and
improved the overall yield (37%). The crucial step was improvement of the
Sonogashira cross-coupling of 1,3,5-triethynylbenzene with iodide-terminated
monomers. In addition, 2-methyl-3-butyn-2-ol was used as the acetylene source
for the peripheral monomers at the meta positions. Compared with alkylsilylacet-
ylenes, 2-methyl-3-butyn-2-ol is much less expensive, and it couples with aryl
halides in nearly quantitative yield. Furthermore, the coupling products can be
easily purified by chromatography because of the very different chromatographic
polarities of the products and the starting materials. To the best of our knowledge,
this is the first report of the synthesis of phenylacetylene dendrimers bearing
2-hydroxypropyl protecting groups.
Results and discussion
Initially 1,3,5-triiodobenzene was treated with 2-methyl-3-butyn-2-ol under Sono-
gashira coupling conditions by using the tris(dibenzylideneacetone)dipalladium
(Pd2(dba)3)–PPh3–CuI catalyst system in piperidine at 25 °C to afford 1-iodo-3,5-
bis[2-(2-hydroxypropyl)ethynyl]benzene (2a) in 48% yield. The high polarity of
2-methyl-3-butyn-2-ol facilitated separation of the desired bisethynylated product
from the starting materials, and from the monoethynylated and the trisethynylated
byproducts. We tried different amounts of 2-methyl-3-butyn-2-ol and found the best
2-methyl-3-butynl-2-ol to 1,3,5-triiodobenzene molar ratio was 1.8:1.
For synthesis of 3-cascade:benzene[3-1,3,5]:5-ethynyl-1,3-bis-[(2-hydroxyprop-
2-yl)ethynyl]benzene (3a), Sonogashira cross-coupling of 1,3,5-triethynylbenzene
and 2a was performed under different reaction conditions (Table 1). When
Pd2(dba)3–PPh3–CuI was used as the catalyst system with Et3N as the base and
solvent, the reaction was carried out at 65 °C to afford 3a in 29% yield (Table 1,
entry 1). Using piperidine [17] as the base and solvent instead of Et3N, 3a was
obtained in 67% yield after stirring at 25 °C for 3.5 h (Table 1, entry 2). Inspired by
these results, we attempted to carry out the reaction at a lower temperature of
-20 °C [18], and found that 3a could be prepared in a higher yield. Tetrabutyl-
ammonium iodide (Bu4NI) was reported to be an important additive for the cross-
coupling reaction. Bu4NI greatly accelerated the Pd-catalyzed coupling reaction,
probably because it not only increased the ionic strength of the solvent but also
stabilized the highly active Pd catalyst, which underwent oxidative addition of the
aryl halide [19]. Unfortunately, the Pd2(dba)3–PPh3–CuI–piperidine system had no
activity at -20 °C, even with the addition of Bu4NI, and no cross-coupling products
could be detected by TLC even after stirring for 24 h (Table 1, entry 3). We
examined the applicability of different bases and solvents and found that the
Sonogashira cross-coupling reaction rate at -20 °C was highly dependent on the
strength of the base and the polarity of the solvents. Compared with piperidine, N,N-
diisopropylethylamine (i-Pr2NEt) was a more effective base in DMF, giving 3a in a
123

406
J. Li, P. Huang
Table 1 Sonogashira cross-coupling of 1,3,5-triethynylbenzene and 2a to afford 3a under different
reaction conditions
Entry
Base
Solvent
Additive
Phosphines ligand
Temp.
( °C)
Time
(h)
Yield
(%)a
1
2
3
4
5
6
7
8
9
a
Et3N
Et3N
None
PPh3
65
25
6
29
67
0
Piperidine
Piperidine
Piperidine
i-Pr2NEt
i-Pr2NEt
i-Pr2NEt
i-Pr2NEt
i-Pr2NEt
Piperidine
Piperidine
DMF
None
PPh3
3.5
24
24
6
Bu4NI
Bu4NI
Bu4NI
Bu4NI
Bu4NI
Bu4NI
Bu4NI
PPh3
-20
-20
-20
-20
-20
-20
-20
PPh3
0
DMF
PPh3
78
92
0
DMF
P(2,4,6-triMeC6H2)3
P(2,4,6-triMeC6H2)3
P(2,4,6-triMeC6H2)3
P(2,4,6-triMeC6H2)3
2.5
24
24
24
THF
Toluene
Dioxane
0
0
Isolated yields
satisfactory yield of 78% after 6 h (Table 1, entries 4 and 5). Using tris(2,4,6-
trimethylphenyl)phosphine (P(2,4,6-triMeC6H2)3) as a bulky, electron-rich ligand
[18] instead of PPh3, compound 3a was obtained in a high yield of 92% (Table 1,
entry 6). Among all the solvents we used here, the polar solvent DMF [20] resulted
in good reactivity, giving 3a in 92% yield within 2.5 h (Table 1, entry 6), whereas
other solvents, for example tetrahydrofuran (THF), toluene, and dioxane gave no
cross-coupling products even after 24 h (Table 1, entries 7–9).
Thus the optimal reaction conditions at -20 °C required Pd2(dba)3–P(2,4,
6-triMeC6H2)3–CuI as the catalyst system, DMF as the solvent, and i-Pr2NEt as the
base with the addition of Bu4NI. Dendrimer 3a was obtained in 92% yield within 2.5 h.
Besides 2a, this method also tolerated the coupling of 1,3,5-triethynylbenzene with
peripheral monomers 2b and 2c, giving 3b and 3c in satisfactory yields (Table 2). In
comparison with 2b and 2c, 2a bearing hydroxylisopropylethynyl instead of
trialkylsilylethynyl groups at the meta positions yielded 3a in the highest yield within
the shortest time. The solubility of 2c in DMF was poor, so the unfavorable solvent
THF had to be added to improve the solubility (the optimized volume ratio was 1:4 for
THF–DMF), and the yield of 3c was lower than those of 3a and 3b.
In the final step 1 was prepared by removing the 2-hydroxypropyl protecting
groups. Although the 2-hydroxypropyl groups were attractive in consideration of
separation, the deprotection reaction always required harsh conditions (e.g., strong
base, high temperature, and a long reaction time) [21–23]. Furthermore, for
molecules bearing more than one 2-hydroxypropyl group, the deprotection yields
were often low or long reaction time was required [24–26]. Compound 1 was
obtained in only 10% yield when 3a was treated with NaOH in toluene under reflux
for 8 h (Table 3, entry 1). With addition of H2O and tetrabutylammonium iodide
(Bu4NI, the phase-transfer catalyst), the complete deprotection reaction was
achieved at 85 °C after 20 h, affording 1 in 42% yield (Table 3, entry 2). Most
recently we explored the deprotection reaction and found that tetrabutylammonium
hydroxide (Bu4NOH) with methanol was a highly efficient catalyst system [27]. The
123

A simple and efficient synthesis
407
Table 2 Sonogashira cross-coupling of 1,3,5-triethynylbenzene with iodide terminated monomers at
-20 °C
R
R
R
R
Pd2(dba)3, CuI
P(2,4,6-triMeC6H2)3
I
3
+
DMF, i-Pr2NEt
-20 oC
2
R
R
R
2a, 3a: R= -C(CH3)2OH
2b, 3b: R= -SiMe3
2c, 3c: R= -Sii-Pr3
3
R
Entry
Ar–I
Time (h)
Yield (%)a
1
2
3
2a
2b
2c
2.5
3
92
73
55
3.5
Reaction conditions: triethynylbenzene (0.2 mmol), 2 (0.6 mmol), Pd2(dba)3 (0.015 mmol), CuI
(0.12 mmol), P(2,4,6-triMeC6H2)3 (0.12 mmol), i-Pr2NEt (2 mmol), n-Bu4NI (1.2 mmol), and DMF
(8 mL), -20 °C
a
Isolated yields
deprotection reaction was carried out in toluene in the presence of 10 mol %
Bu4NOH with 1.2 equiv. CH3OH at 75 °C, giving the corresponding product 1 in a
high yield of 83% after 5 min without any incompletely deprotected products
(Table 3, entry 3).
Conclusion
In summary, we report a simple procedure with high overall yield (37%) for synthesis
of 1,3,5-tris[2-(3,5-diethynylphenyl)ethynyl]benzene from 1,3,5-triethynylbenzene
and 2-hydroxypropyl group-protected peripheral monomers via a Sonogashira
coupling–deprotection reaction sequence. A modified Sonogashira reaction was
developed for coupling 1,3,5-triethynylbenzene to substituted iodobenzenes. By
Table 3 Removal of the 2-hydroxypropyl groups to give 1 under different reaction conditions
Entry
Base (equiv.)
Temp. ( °C)
Time (h)
Yield (%)a
1
2
3
a
NaOH (5)
110
85
8
10
42
83
5 M aqueous NaOH (75) with Bu4NI (0.1)
Bu4NOH (0.1) with CH3OH (1.2)
20
0.08
75
Isolated yields
123

408
J. Li, P. Huang
using Pd2(dba)3–P(2,4,6-triMeC6H2)3–CuI as the catalyst system, i-Pr2NEt as the
base, and DMF as the solvent in the presence of Bu4NI, Sonogashira coupling of 1,3,5-
triethynylbenzene with aryl iodides performed rapidly with good to excellent yields at
-20 °C. The short reaction time and satisfactory isolated yields would make this
method very suitable for further synthesis of new arylacetylene dendrimers from
1,3,5-triethynylbenzene.
Experimental
Unless otherwise indicated, all materials were obtained from commercial suppliers
and were used without further purification. Before use, piperidine was distilled over
calcium hydride, THF, toluene, and dioxane were distilled in the presence of
sodium, and DMF was vacuum distilled over MgSO4. Compound 2c was prepared
in accordance with the literature [16]. All reactions were performed under an
atmosphere of nitrogen.
1
Melting points were determined on an XT5 hot-plate microscope apparatus. H
NMR and 13C NMR spectra were acquired on JNM-AL300 and Varian-VNMRS500
NMR spectrometers. IR spectra were measured on a Nicolet NEXUS-470 FTIR
spectrometer. Mass spectra were recorded on a Shimadzu GC/MS-QP5050A mass
spectrometer (EI) or a Bruker Daltonics Autoflex III MALDI-TOF mass spectrom-
eter, using 2,5-dihydroxybenzoic acid (DHB) as the matrix. Elemental analysis was
performed with a vario EL III elemental analyzer.
Synthesis of 1-iodo-3,5-bis[2-(2-hydroxypropyl)ethynyl]benzene (2a)
Pd2(dba)3 (44 mg, 0.048 mmol), CuI (18 mg, 0.095 mmol), and PPh3 (125 mg,
0.475 mmol) were added to a solution of 1,3,5-triiodobenzene (1.140 g, 2.5 mmol)
in piperidine (15 mL). After addition of 2-methyl-3-butyn-2-ol (378 mg, 4.5 mmol)
in piperidine (10 mL) the mixture was stirred at room temperature for 2 h followed
by removal of the solvents under reduced pressure. The residue was dissolved in
CH2Cl2 (50 mL). The solution was washed with 5% HCl and brine, then dried over
MgSO4. After evaporation of the solvent under vacuum, the crude product was
purified by silica gel column chromatography (petroleum ether–ethyl acetate 4:1) to
afford 2a as a white solid (441.8 mg, 48%). Mp: 123.6–125 °C; IR (KBr): 3334 (O–
H), 3064 (Ar C–H), 2980 (Ar C–H), 2931 (C–H), 2868 (C–H), 2216 (C:C), 1581
(Ar C=C), 1544 (Ar C=C), 1442, 1414, 1371 (C–H), 1240, 1150, 959, 863 (Ar C–H)
cm-1; 1H NMR (500 MHz, CDCl3): d 7.71 (s, 2H, Ar–H), 7.44 (s, 1H, Ar–H), 1.99
(s, 2H, –OH), 1.61 (s, 12H, –CH3); 13C NMR (125 MHz, CDCl3): d 139.85, 133.89,
124.68, 95.62, 92.90, 79.73, 65.51, 31.32; MS (EI) m/z: 368 (M?), 353, 335, 43
(100). Anal: calcd. for C16H17IO2 (368.21): C 52.19, H 4.65; found C 52.10, H 4.96.
Synthesis of 1-iodo-3,5-bis(2-trimethylsilylethynyl)benzene (2b)
Following the method for the synthesis of 2a (trimethylsilyl)acetylene (441 mg,
4.5 mmol) was added instead of 2-methyl-3-butyn-2-ol. Purification was carried out
123

A simple and efficient synthesis
409
by silica gel column chromatography (petroleum ether), giving 2b as a colorless
viscous oil (456 mg, 46%). IR (KBr): 2958 (Ar C–H), 2924 (C–H), 2855 (C–H),
2158 (C:C), 1579 (Ar C=C), 1542 (Ar C=C), 1411 (C–H), 1251, 1162, 974, 849
1
(Ar C–H) cm-1; H NMR (500 MHz, CDCl3): d 7.72 (s, 2H, Ar–H), 7.49 (s, 1H,
Ar–H), 0.21 (s, 18H, –CH3); 13C NMR (125 MHz, CDCl3): d 140.21, 134.46,
125.01, 102.26, 96.60, 92.86, -0.21; MS (EI) m/z: 396 (M?), 381 (100). Anal:
calcd. for C16H21ISi2 (396.42): C 48.48, H 5.34; found C 49.44, H 5.81.
General procedure for synthesis of 3
Compound 2 (0.6 mmol), Bu4NI (443 mg, 1.2 mmol), P(2,4,6-triMeC6H2)3 (46 mg,
0.12 mmol), Pd2(dba)3 (14 mg, 0.015 mmol), CuI (23 mg, 0.12 mmol) and i-
Pr2NEt (259 mg, 2 mmol) were dissolved in 8 mL DMF in a dry Schlenk flask (for
3c 2 mL THF was added simultaneously as the co-solvent). After cooling to -20 °C
1,3,5-triethynylbenzene (30 mg, 0.2 mmol) was added and the mixture was stirred
at -20 °C for the time indicated in Table 1. Then saturated aqueous NH4Cl was
added to quench the reaction. The resulting mixture was extracted with CH2Cl2
(50 mL). The extracts were washed with 5% HCl and brine, dried over MgSO4, and
concentrated under reduced pressure. The product was purified by silica gel column
chromatography.
3-Cascade:benzene[3-1,3,5]:5-ethynyl-1,3-bis-[(2-hydroxyprop-2-
yl)ethynyl]benzene (3a)
Eluent, petroleum ether–ethyl acetate 1:1; yield: 165 mg, 92%; white solid; mp:
[260 °C (decomp.); IR (KBr): 3355 (–OH), 3062 (Ar C–H), 2978 (Ar C–H), 2931
(C–H), 2869 (C–H), 2220 (C:C), 2156 (C:C), 1587 (Ar C=C), 1421 (C–H), 1363
1
(C–H), 1240, 1164, 948, 877 (Ar C–H) cm-1; H NMR (300 MHz, DMSO-d6): d
7.83 (s, 3H, Ar–H), 7.55 (d, J = 1.5 Hz, 6H, Ar–H), 7.40 (s, 3H, Ar–H), 5.53 (s,
6H, –OH), 1.46 (s, 36H, –CH3); 13C NMR (125 MHz, DMSO-d6) d 134.45, 134.04,
133.33, 123.91, 123.25, 122.91, 97.88, 89.10, 88.59, 78.48, 63.61, 31.39; MS
(MALDI-TOF, DHB) m/z: 892.9 (M ? Na?). Anal: calcd. for C60H54O6 (871.09):
C 82.73, H 6.25; found C 79.79, H 6.99.
3-Cascade:benzene[3-1,3,5]:5-ethynyl-1,3-bis-(2-trimethylsilyethynyl)benzene
(3b)
Eluent, petroleum ether; yield: 140 mg, 73%; white solid; mp: 242–244 °C; IR
(KBr): 2958 (Ar C–H), 2921 (–CH3 C–H), 2853, 2156 (C:C), 1587 (Ar C=C),
1410 (C–H), 1249, 1162, 1021, 981, 879 (Ar C–H), 843 (Ar C–H) cm-1; 1H NMR
(500 MHz, CDCl3): d 7.58–7.54 (m, 12H, Ar–H), 0.23 (s, 54H, –CH3); 13C NMR
(125 MHz, CDCl3): d 135.22, 134.65, 134.38, 123.88, 123.71, 123.21, 103.04,
95.89, 89.02, 88.52, -0.18. Anal: calcd. for C60H66Si6 (955.70): C 75.41, H 6.96;
found C 75.23, H 7.01.
123

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J. Li, P. Huang
3-Cascade:benzene[3-1,3,5]:5-ethynyl-1,3-bis-(2-
triisopropylsilylethynyl)benzene (3c)
Eluent, petroleum ether; yield: 161 mg, 55%; white solid; mp: 262–263 °C; IR
(KBr): 2943 (Ar C–H), 2891 (C–H), 2864 (C–H), 2219 (C:C), 2154 (C:C), 1773,
1585 (Ar C=C), 1462, 1424, 1411 (C–H), 1383 (C–H), 1365 (C–H), 1161, 1072,
1017, 980, 881 (Ar C–H) cm-1; 1H NMR (500 MHz, CDCl3): d 7.66 (s, 3H, Ar–H),
7.58 (d, J = 1.5 Hz, 6H, Ar–H), 7.53 (t, J = 1.5 Hz, 3H, Ar–H), 1.28–1.16 (m,
126H, –CH(CH3)2); 13C NMR (125 MHz, CDCl3): d 134.92, 134.77, 134.39,
124.21, 123.71, 123.16, 105.11, 92.38, 89.11, 88.57, 18.60, 11.28. Anal: calcd. for
C96H138Si6 (1460.66): C 78.94, H 9.52; found C 78.66, H 9.25.
Synthesis of 1,3,5-tris[2-(3,5-diethynylphenyl)ethynyl]benzene (1)
Bu4NOH, 40 wt. % solution in CH3OH (78 mg, Bu4NOH 0.12 mmol) was added to a
solution of 3a (178.6 mg, 0.2 mmol) in toluene (60 mL). The mixture was stirred at
75 °C for 5 min. After cooling to room temperature, the mixture was washed with
5% HCl, brine, dried over MgSO4, and concentrated under vacuum. The mixture was
filtered through a short plug of silica gel, eluting with THF. After the product-
containing filtrate was evaporated, the residue was recrystallized from toluene to give
1 as a white fuzzy solid (87 mg, 83%). IR (KBr): 3285 (C:C–H), 3063 (Ar C–H),
2217 (C:C), 2156 (C:C), 2109, 1589 (Ar C=C), 1426 (Ar C=C), 1017, 880 (Ar
C–H) cm-1; 1H NMR (300 MHz, DMSO-d6): d 7.84–7.62 (m, 12H, Ar–H), 4.38 (s,
6H, C:C–H); 13C NMR (75 MHz, DMSO-d6): d 134.91, 134.60, 134.49, 123.10,
123.0, 88.85, 88.76, 82.69, 81.31; MS (EI) m/z: 522 (M?), 128, 101, 86 (100), 78, 58,
26. Anal: calcd. for C42H18 (522.60): C 96.53, H 3.47; found C 96.31, H 3.92.
References
1. D. Astruc, E. Boisselier, C. Ornelas, Chem. Rev. 110, 1857 (2010)
2. Y. Jiang, L. Wang, Y. Zhou, Y. Cui, J. Wang, Y. Cao, J. Pei, Chem. Asian J. 4, 548 (2009)
3. A.C. Grimsdale, K.L. Chan, R.E. Martin, P.G. Jokisz, A.B. Holmes, Chem. Rev. 109, 897 (2009)
4. S. Lo, P.L. Burn, Chem. Rev. 107, 1097 (2007)
´
5. J.M.J. Frechet, J. Polym. Sci., Part A 41, 3713 (2003)
6. T. Itoh, T. Maemura, Y. Ohtsuka, Y. Ikari, H. Wildt, K. Hirai, H. Tomioka, Eur. J. Org. Chem. 2004,
2991 (2004)
7. K. Sato, D. Shiomi, T. Takui, M. Hattori, K. Hirai, H. Tomioka, Mol. Cryst. Liq. Cryst. 376, 549
(2002)
8. D.J. Pesak, J.S. Moore, Macromolecules 30, 6467 (1997)
9. T.S. Ahn, A.L. Thompson, P. Bharathi, A. Mu¨ller, C.J. Bardeen, J. Phys. Chem. B 110, 19810 (2006)
10. Z. Xu, M. Kahr, K.L. Walker, C.L. Wilkins, J.S. Moore, J. Am. Chem. Soc. 116, 4537 (1994)
´
´
11. J.G. Rodrıguez, J. Esquivias, A. Lafuente, C. Dıaz, J. Org. Chem. 68, 8120 (2003)
12. Y. Yamaguchi, T. Ochi, S. Miyamura, T. Tanaka, S. Kobayashi, T. Wakamiya, Y. Matsubara,
Z. Yoshida, J. Am. Chem. Soc. 128, 4504 (2006)
13. R.M. Adhikari, L. Duan, L. Hou, Y. Qiu, D.C. Neckers, B.K. Shah, Chem. Mater. 21, 4638 (2009)
14. M. Nakano, R. Kishi, T. Minami, K. Yoneda, Molecules 14, 3700 (2009)
15. B.R. Stepp, J.M. Gibbs-Davis, D.L.F. Koh, S.T. Nguyen, J. Am. Chem. Soc. 130, 9628 (2008)
16. D. Pesak, Ph.D. Thesis, University of Illinois (1995)
123

A simple and efficient synthesis
411
17. U.H.F. Bunz, Chem. Rev. 100, 1605 (2000)
18. K. Nakamura, H. Okubo, M. Yamaguchi, Synlett 10, 549 (1999)
19. N.A. Powell, S.D. Rychnovsky, Tetrahedron Lett. 37, 7901 (1996)
20. M. Imoto, M. Takeda, A. Tamaki, H. Taniguchi, K. Mizuno, Res. Chem. Intermed. 35, 957 (2009)
21. E. Glimsdal, M. Carlsson, T. Kindahl, M. Lindgren, C. Lopes, B. Eliasson, J. Phys. Chem. A 114,
3431 (2010)
´
22. A. Nagy, Z. Novak, A. Kotschy, J. Organomet. Chem. 690, 4453 (2005)
23. Y. Jiang, B. Gao, W. Huang, Y. Liang, G. Huang, Y. Ma, Synth. Commun. 39, 197 (2009)
24. A.P. Melissaris, M.H. Litt, J. Org. Chem. 59, 5818 (1994)
25. D.L. Trumbo, C.S. Marvel, J. Polym. Sci. 24, 2311 (1986)
26. S. Chen, Q. Yan, T. Li, D. Zhao, Org. Lett. 12, 4784 (2010)
27. J. Li, P. Huang, Beilstein J. Org. Chem. 7, 426 (2011)
123