Synthesis of C3-symmetric nano-sized polyaromatic compounds by trimerization and Suzuki-Miyaura cross-coupling reactions

Article abstract of DOI:10.1002/ejoc.200400257

Various C₃-symmetric molecules were prepared by trimerization of acetyl aromatic compounds and subsequently coupled with various boronic acids under Pd⁰ catalysis conditions to generate oligoaryl/-heteroaryl C₃-symmetric molecules. Several furan- and thiophene-containing star-shaped molecules were prepared by the use of Suzuki-Miyaura cross-coupling as a key step. Structural and conformational details were explored by semi-empirical molecular orbital theory using the AM1 method. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejoc.200400257

FULL PAPER
Synthesis of C3-Symmetric Nano-Sized Polyaromatic Compounds by
Trimerization and Suzuki؊Miyaura Cross-Coupling Reactions
Sambasivarao Kotha,*[a] Dhurke Kashinath,[a] Kakali Lahiri,[a] and Raghavan B. Sunoj[a]
Keywords: CϪC coupling / Cyclotrimerizations / Nanostructures / Polycycles / Semi-empirical calculations
Various C3-symmetric molecules were prepared by trimeriz-
ation of acetyl aromatic compounds and subsequently
coupled with various boronic acids under Pd0 catalysis condi-
tions to generate oligoaryl/-heteroaryl C3-symmetric molec-
ules. Several furan- and thiophene-containing star-shaped
molecules were prepared by the use of Suzuki−Miyaura
cross-coupling as a key step. Structural and conformational
details were explored by semi-empirical molecular orbital
theory using the AM1 method.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
Before elaboration on the synthesis of C3-symmetric mol-
ecules, it is best to discuss some applications of C3-sym-
metrical compounds in diverse areas of chemistry. Miller
and co-workers reported the synthesis of mono-dispersed
1,3,5-phenylene-based hydrocarbon dendrimers 1 (Figure 1)
Symmetrical starting materials are useful building blocks
for designing complex target molecules, and synthetic
chemists might reap enormous rewards by paying attention
to symmetry in this regard.[1] Structural symmetry in the
target molecule can easily be exploited to reduce the length
of synthetic sequences. The classical, one-directional con-
vergent synthetic approach is most frequently found in the
literature, but an alternative strategy based on two-direc-
tional syntheses involving sequential or simultaneous
homologation is a powerful tool for the efficient construc-
tion of a large number of natural products. While symmetri-
cal substances are readily constructed from intermediates
of high symmetry, the synthesis of unsymmetrical molecules
may also be achieved by employing symmetrical precursors
and vice versa.[2] Symmetrical molecules can also be used
as central cores for the design of dendrimeric frameworks
with increased diversity, and higher levels of structural or-
der in the globular morphology of dendrimers are indeed
an attractive feature.[3] Interestingly, several applications of
dendrimers in the forms of chemical sensors, optical
switches, etc., are believed to be dependent on higher order
as well as on the possibility of controllable conformations.
In this regard, conformational control has often been a
challenge in most commonly reported dendrimers, primar-
ily because of the presence of groups such as methylene
linkages.[4] Here we have employed the restricted rotation
around biphenyl units as a tool with which to achieve better
control over the conformational mobility.[5]
˚
with molecular diameters of 15Ϫ31 A and up to 46 benzene
rings.[6] The luminescent star-shaped molecule 1,3,5-tris[p-
(2,2Ј-bipyridylamino)phenyl]benzene (2) was synthesized in
order to study fluorescent properties by coordination with
zinc chloride.[7] In another investigation, compound 3 was
prepared in order to study its electron-transfer properties in
complexation with ruthenium and osmium metals.[8] Some
of the 2,4,6-trisubstituted-1,3,5-triazine derivatives behave
as liquid crystalline materials and find useful applications
in coordination chemistry and crystal engineering.[9,10]
Symmetric tetrathiafulvalene system 4 has been prepared in
order to examine the interactions of tetrathiafulvalene rad-
ical cations and was used as a neutral donor in measure-
ment of the electrolytic conductivity of charge-transfer
complexes.[11] Shirota and co-workers reported the synthesis
of 1,3,5-tris[5-(dimesitylboryl)thiophen-2-yl]benzene (5),
useful in designing organic electroluminescent devices,[12]
Tour and co-workers have synthesized symmetrical conju-
gated oligo(phenylene ethynylene)s used for various three-
dimensional molecular wires and devices,[13] and the sym-
metrical 1,3,5-triphenylbenzene core has also been used in
syntheses of dendritic chromophores[14] and hyperbranched
conjugated molecules.[15] Since the applications of C3-sym-
metric compounds are increasing at an accelerating rate in
several areas of chemical sciences, there is a great need to
develop simplified methods for their synthesis.
In view of the applications of C3-symmetric molecules
described above, we sought a general and simple method-
ology for the synthesis of various C3-symmetric aromatics
through the employment of readily available starting mate-
rials. Our strategy relies on the use of acid-catalyzed trimer-
[a]
Department of Chemistry, Indian Institute of Technology,
Bombay, Powai, Mumbai 400 076, India
Fax: (internat.) ϩ 91-22-2572-3480
E-mail: srk@chem.iitb.ac.in
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DOI: 10.1002/ejoc.200400257
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S. Kotha, D. Kashinath, K. Lahiri, R. B. Sunoj
FULL PAPER
Figure 1. Various recently reported useful C3-symmetric molecules
ization and SuzukiϪMiyaura (SM) cross-coupling reac- fragments.[17,18] Oxidation of 7 followed by a DielsϪAlder
tions[16] as key steps (Figure 2).
reaction with an appropriately substituted acetylenic dieno-
phile, for example, would be expected to deliver the other-
wise poorly accessible 1,3,5-triphenylbenzene derivatives
(e.g., 9, Scheme 1). Raney nickel desulfurization of the thio-
phene derivative 8 would provide C3-symmetric compounds
possessing long alkyl chains suitable for liquid crystalline
materials. On the other hand compound 8 (R ϭ CH3) is
also an attractive starting material for the synthesis of C3-
symmetric amino acid derivatives.
Results and Discussion
Our initial objective was to accomplish the trimerization
reaction with several acetylated aromatic systems to gener-
ate C3-symmetric oligo-aryl dendrimers. Towards this end,
trimerization of acetophenone 10 by treatment with SiCl4
Figure 2. Our strategy for C3-symmetric molecules by the SM
cross-coupling and acid-catalyzed trimerization sequence
It is generally known that trimerization involving bulky in ethanol was attempted, and furnished the compound 11
substrates is not a trivial task. To address the problems as- in 86% yield.[19a] FriedelϪCrafts acylation of 11 with acetyl
sociated with the sterically demanding systems, we intended chloride and anhydrous aluminium chloride in nitrobenzene
to use a combination of trimerization and the SM cross- yielded the acetylated product 12, which on further treat-
coupling reaction. Some of the C3-symmetric building ment with SiCl4 in ethanol failed to give the trimerized
blocks prepared by this methodology can be further ma- product 13 (Scheme 2). Aspects concerning the solubility of
nipulated to provide a variety of otherwise poorly accessible the oligomers were our major concern at each stage of the
C3-symmetric derivatives such as dendrimers and fullerene dendrimer synthesis, the solubility of the starting ketone
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Synthesis of C3-Symmetric Nano-Sized Polyaromatic Compounds
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Scheme 1
Scheme 2
Scheme 3
being crucial for the success of the trimerization reaction. that earlier attempts to trimerize 2-acetylthiophene by
Even the acetylated derivative 12 is insoluble in ethanol at treatment with Nafion-H had given no condensation
room temp. To circumvent this problem we decided to use product.[24a] Moreover, compounds of type 7 were also pre-
acetylated heterocyclic molecules as a starting materials, as pared by an independent strategy.[24b]
solubility aspects may be dealt with here thanks to the
To generalize the trimerization reaction with other thio-
presence of the heteroatoms. In this context, SiCl4-mediated phene-related systems, various acetylated thiophene deriva-
trimerization of 2-acetylfuran was attempted, and it was tives (17Ϫ19) were prepared by the alkylation[25a] and
found that a complex mixture of products was obtained.[20] acetylation[25b] sequence (Scheme 3). Because of the volatile
In view of the facile ring-opening reactions of furan moiet- natures of the alkylated thiophene derivatives (14Ϫ16) they
ies under acidic conditions, this result was not surprising. were used in subsequent reactions without additional puri-
Since substituted thiophenes are valuable building blocks in fication. Treatment of the acetylated thiophene derivatives
electronic devices and also enjoy potential applications in (17Ϫ19) with SiCl4 gave the trimerized compounds
the flavor and pharmaceutical industries,[21,22] the trimeriz- (20Ϫ22). The formation of the C3-symmetric trimerized
ation of 2-acetylthiophene 6 was attempted. Treatment of 6 products 20Ϫ22 was confirmed by 1H and 13C NMR. Simi-
with SiCl4 at room temp. gave compound 7 in 42% yield larly, 2-acetyl-5-chlorothiophene 23 and acetylated bithio-
after column chromatography.[23] It is worth mentioning phene[26,27] 25 were also treated with SiCl4 in ethanol to
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S. Kotha, D. Kashinath, K. Lahiri, R. B. Sunoj
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deliver the trimerized compounds 24 (60%) and 26 (26%)
Later on, SM cross-coupling of compound 28 was at-
respectively. The trimerization results for various acetyl tempted with other arylboronic acids [4-methylphenylbo-
thiophene derivatives are included in Table 1.
ronic acid, 4-acetylphenylboronic acid, 4-fluorophenylbo-
ronic acid, 3-trifluoromethylphenylboronic acid, 4-formyl-
phenylboronic acid, 4-methoxyphenylboronic acid and het-
erocyclic boronic acids (thiophene-2-boronic acid, furan-2-
boronic acid)] and (PPh3)4Pd catalyst, and the results are
included in Table 2. Shirota and co-workers have recently
also reported the synthesis of compound 32.[28] Typically,
47Ϫ75% yields of the coupling products were obtained. All
the coupling products were characterized by their spectro-
scopic data.
Table 1. List of C3-symmetric thiophene derivatives prepared by
trimerization
It is worth noting that we also considered the preparation
of 29 and 37, with thiophene units present either in the
outer or in the inner core of these star-shaped molecules,
by an alternative route: SM cross-coupling followed by tri-
merization (Scheme 5). Retrosynthetic analysis identifies 38
and 39 as possible precursors for the synthesis of 29 and
37. Recently, Hiyama et al. prepared compound 38 by pal-
ladium-catalyzed cross-coupling of organosilicon com-
pounds.[29a,29b] However, we have adapted Pd-catalyzed SM
cross-coupling of thiopheneboronic acid and 4-bromoace-
tophenone to obtain 38 in 92% yield. Along similar lines,
coupling of 2-acetyl-5-bromothiophene and phenylboronic
acid gave compound 39 in 88% yield. The preparation of
39 through palladium-catalyzed coupling of aryl halides
was also recently reported.[29c] Compounds 38 and 39 gave
the trimerized compounds 29 and 37, respectively, on treat-
ment with SiCl4. The structures of the trimerized com-
pounds 29 and 37 have been established by 1H and 13C
NMR spectroscopic data. This idea was next extended to
furan derivatives; triiodobenzene[30] (40), prepared by
known procedures, was treated with furanboronic acid un-
der SM cross-coupling conditions to give compound 41
(66%) (Scheme 6).
Having successfully developed synthetic strategies for the
C3-symmetric polyaromatic molecules, we turned our atten-
tion to structural and conformational features of these mol-
ecules. One of the broad objectives of our study is to design
molecular entities of desired shape and size. Molecular di-
ameter in conjunction with conformational flexibility is a
useful parameter with which to identify the propensities of
molecules towards spherical shapes. In order to unravel the
key structural features of these polyaromatics, we per-
formed standard semi-empirical calculations by use of an
AM1 Hamiltonian.[31]
Having generalized the trimerization reaction with vari-
ous acetylated thiophene derivatives, we next turned our at-
tention towards the preparation of mixed heterocyclic/poly-
aromatic C3-symmetric derivatives by use of trimerization
and the SM cross-coupling as key steps. To this end we
prepared 1,3,5-tris(4-bromophenyl)benzene (27) and 1,3,5-
tris(4-iodophenyl)benzene (28) from 4-bromoacetophenone
and 4-iodoacetophenone, respectively, by the SiCl4-me-
diated trimerization reaction.[19a] Next, we tried palladium-
mediated SM cross-coupling of thiophene-2-boronic acid
with the tribromo derivative 27, and the coupled product
29 was obtained in 14% yield. During the preparation of
29, we also isolated mono- and difunctional derivatives in
13% and 67% yields. Since iodo aromatics are better part-
ners in SM cross-coupling reaction, we tried the above reac-
tion on compound 28 and, as expected, an improved yield
(60%) was obtained (Scheme 4).
Computational studies provided useful insights into the
conformations of these molecules. Geometry optimizations
of the compounds listed in Table 2 revealed interesting
structural features. Except for furan and thiophene systems
(29 and 36), the inter-ring torsional angles between the cen-
tral trisubstituted aryl rings and the adjacent aryl rings were
consistently found to be around 41 degrees. Similarly, the
torsional angles between second and third rings were found
to be around 40 degrees. The inter-ring torsional angles be-
tween the second aryl rings and the thiophene rings were
found to be lower than all other systems by 17 degrees
(Table 3).
Scheme 4
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Synthesis of C3-Symmetric Nano-Sized Polyaromatic Compounds
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Table 2. List of the C3-symmetric derivatives obtained by SM cross-coupling of compound 28
The higher degree of planarity between these two rings in Figure 3. Since the systems studied in this work have little
in 29 is due to the presence of weak hydrogen bonding in- conformational flexibility, the lengths D1 and D2 (Table 3)
teractions between the thiophene sulfur atoms and the hy- provide details on the dimensions of the molecule. It was
drogen atoms of the aryl ring. The interaction distance was found that the longest arm and largest distances were found
computed to be 0.27 nm in this case (Figure 3). Since oxy- to be for acetyl- (31) and formyl-substituted (34) systems.
gen is a better hydrogen-bond acceptor, such interactions Furan- and thiophene-containing systems were found to
would be expected to be stronger with the furan system, have smaller distances than other polyaryls.
and this was indeed found to be the case in 36, with a
shorter O···H distance of 0.24 nm. More effective hydrogen
bonding is found to be responsible for the reduction in the
inter-ring torsional angle, resulting in a completely flat mo-
lecular geometry for 36. This should be a useful tool in
Conclusion
To conclude, we have shown that various acetylated thio-
making templates while larger dendritic molecules are de- phene derivatives undergo trimerization reactions to gener-
signed; better conformational control of the dendritic arms ate C3-symmetric building blocks with potential for appli-
through the use of suitably crafted heterocycles as building cation in catalysis,[32] materials science, and organic syn-
blocks can be envisioned. Such strategies using heterocycles thesis. We have prepared star-shaped thiophenes bearing
have not been widely reported in the literature. It is worth alkyl groups (e.g., 21 and 22) which may be important in
pointing out that attachment of polyaryl groups to the the design of liquid crystalline materials.[33] In addition,
outer heterocyclic rings can induce angularity.
availability of mixed aryl-thienyl oligomers in which the
Another interesting parameter is the molecular size. We thiophene unit is present either in the outer or in the inner
have determined important ‘‘end-to-end’ distances as shown core of the molecule (e.g., 29 and 37) by this strategy may
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S. Kotha, D. Kashinath, K. Lahiri, R. B. Sunoj
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Scheme 5
Scheme 6
rigidity as well as distances to polyaryls. We plan to extend
this approach, in concert with the SM cross-coupling reac-
tion, towards the design of dendrimeric frameworks with
increased diversity and predictable distances.
Table 3. Optimized geometrical parameters for substituted C3-sym-
metric compounds
Substituent[a]
D1[b]
[nm]
D2
[nm]
Dihedral 1
(1,2,3,4)
Dihedral 2
(5,6,7,8)
ϪH
ϪF
ϪCHO
ϪOCH3
ϪCF3
ϪCOCH3
Furan
1.91
1.95
2.31
2.28
2.20
2.31
1.85
1.91
1.10
1.13
1.33
1.31
1.21
1.33
0.94
0.95
41
41
41
41
41
41
41
41
Ϫ40
Ϫ40
Ϫ40
Ϫ40
Ϫ40
Ϫ39
0
Experimental Section
General Remarks: Analytical TLC was performed on (10 ϫ 5 cm)
glass plate coated with silica gel G or GF 254 (containing 13%
CaSO4 as a binder). Viewing of the spot on the TLC plate was
achieved by exposure either to I2 vapor or to UV light. Flash chro-
matography was performed on silica gel (100Ϫ200 mesh) and the
columns were usually eluted with EtOAc and petroleum ether (bp,
60Ϫ80 °C) mixtures. Melting points are uncorrected. 1H NMR and
13C NMR spectroscopic data were recorded on Varian VXR 300
spectrometers with TMS as internal standard and CDCl3 as sol-
vent. The coupling constants (J) are given in Hertz (Hz). Mass
Thiophene
Ϫ23
[a]
Except for furan and thiophene derivatives substituent refers to
groups at the para-position of the biphenyl attached to the central
aryl unit. [b] D1 represents maximum end-to-end total length of the
molecule. D2 is the length of one arm, measured from center of
aryl ring and the farthest atom. Dihedral 1 and dihedral 2 are the
inter-ring dihedral angles as indicated in Figure 3.
spectral measurements were carried out on
a GCD 1800
HewlettϪPackard GC-MS spectrometer. Micromasses were calcu-
lated on a Q-TOF micromass machine. UV spectroscopic data were
obtained (in CHCl3) on Shimadzu UV-2100 or UV-260 instru-
ments. FT-IR spectra were recorded as KBr pellets unless otherwise
mentioned. Anhydrous Et2O and THF were obtained by distil-
lation over sodium/benzophenone ketyl. Anhydrous MgSO4 was
used as drying agent after workup in all the reactions. Tetrachloro-
silane, absolute ethanol, and n-butyllithium were purchased from
provide easy access to novel polymers and dendrimers. We
have also prepared 1,3,5-trifurylbenzene (41) by SM cross-
coupling, this compound being difficult to prepare by acid-
catalyzed trimerization of 2-acetylfuran. The structural fea-
tures of the compounds synthesized here clearly indicate
that biphenyl linkages in combination with heterocyclic
units can be exploited to impart the desired conformational Aldrich Chemical Co., Inc. (Milwaukee, WI, USA), Farco Chemi-
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Synthesis of C3-Symmetric Nano-Sized Polyaromatic Compounds
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Figure 3. AM1-optimized geometries of C3-symmetric polyaromatic compounds (distances given in nm)
cal Supplies (China), and Lancaster Chemical Co. (U.K.), respec-
stirred at room temp. for 1 h and cooled to Ϫ15 to Ϫ20 °C, and
tively. Acetic anhydride was prepared from acetyl chloride and so- the electrophile (0.3 mol) in dry Et2O was then added. The result-
dium acetate by the literature procedure.[34] Pd(PPh3)4 was pre-
pared by the reported procedure.[35] Dimethyl sulfoxide (Spectro-
chem), 1-bromobutane (Fluka), 1-bromooctane, 2-acetyl-5-
ant reaction mixture was then heated at reflux for 18 h, cooled
to room temp., and poured into crushed ice. The Et2O layer was
separated, washed with H2O, dried, and fractionated to give alkyl-
chlorothiophene (Lancaster), 4-iodoacetophenone (Lancaster ated thiophene derivatives.
chemicals), and 4-bromoacetophenone (SRL) were used as re-
ceived. Thiophene-2-boronic acid, furan-2-boronic acid, phenylbo-
ronic acid, and 4-methylphenylboronic acid were prepared by lit-
erature procedures.[36] 4-Methoxyphenylboronic acid, 4-acetylphen-
ylboronic acid, and 4-formylphenylboronic acid were purchased
from Aldrich and used as received. (4-Fluorophenyl)boronic acid
and 3-(trifluoromethyl)phenylboronic acid were obtained from
Optima Chemical Group, LLC, USA.
General Procedure for Acetylation of 2-Alkylthiophene Deriva-
tives:[25b] 2-Alkylthiophene (2.5 mmol) in acetic anhydride (0.2 mL,
2.2 mmol) was treated with a few drops of H3PO4 at 70Ϫ75 °C for
3 h. The reaction mixture was then cooled to room temp. and
poured into H2O. The aqueous layer was extracted with CH2Cl2,
and the organic layer was washed with NaHCO3 solution, H2O,
and brine, dried, and concentrated. The crude product was loaded
onto a silica gel column. Elution of the column with EtOAc/hexane
gave the acetylated product.
Initial conformational sampling was performed by molecular
mechanics methods with the MM2 force field, with a fairly large
set of starting geometries. Low-energy structures were then sub-
jected to full geometry optimizations by semi-empirical method
through the use of an AM1 Hamiltonian as implemented in the
Gaussian 98 suite of quantum chemical programs.[37] All the
stationary points were characterized as true minima on their re-
spective potential energy surfaces by corresponding Hessian indi-
ces.
General Procedure for Trimerization of Acetylated Derivatives:[23]
SiCl4 (2Ϫ18 equiv.) was added dropwise with stirring at 0 °C to a
solution of acetylated derivative (0.04 mol) in absolute EtOH
(40 mL) and the reaction mixture was stirred at ambient tempera-
ture. At the conclusion of the reaction (TLC monitoring), the dark
reaction mixture was poured into ice-cold H2O and extracted with
CH2Cl2. The combined organic fraction was washed with H2O and
dried. Evaporation of the solvent and purification of the crude
product by column chromatography (silica gel) with hexane as an
eluent furnished the trimerized product.
General Procedure for Alkylation of Thiophene:[25a] A solution of
nBuLi (0.3 mol) in Et2O was added dropwise at 0 °C to a solution
of thiophene (0.3 mol) in dry Et2O. The reaction mixture was then
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1,3,5-Tris(2-thienyl)benzene (7): 2-Acetylthiophene (6, 5 g, 0.04
3.8 Hz, 3 H), 7.7 (s, 3 H). 13C NMR (75.4 MHz, CDCl3, ppm): δ ϭ
mol) in absolute EtOH (40 mL) was treated with SiCl4 (9 mL, 0.08 121.8, 123.9, 124.6 (3 C), 128.0, 135.4, 137.2, 137.4, 141.9. UV
mol) as described in the above general procedure for 6 h to deliver (CHCl3): λmax [nm] (ε, Ϫ1cmϪ1) ϭ 357 (82108). C30H18S6 (570):
7 as a white solid (1.8 g, 42%), m.p. 155Ϫ156 °C (ref.[24] m.p. calcd. C 63.16, H 3.16; found C 63.10, H 3.11.
156Ϫ158 °C). Rf ϭ 0.2 (petroleum ether). 1H NMR (300 MHz,
1,3,5-Tris[4-(2-thienyl)phenyl]benzene (29): 4-(2-Thienyl)aceto-
CDCl3, ppm): δ ϭ 7.12 (dd, J ϭ 3.6, 5.0 Hz, 3 H), 7.33 (dd, J ϭ
phenone (38, 65 mg, 0.322 mmol) in absolute EtOH (3 mL) was
1.1, 5.0 Hz, 3 H), 7.41 (dd, J ϭ 1.1, 3.6 Hz, 3 H), 7.73 (s, 3 H). 13
C
treated with SiCl4 (0.5 mL, 4.36 mmol) as described in the above
general procedure for 24 h to furnish 29 as a white solid (23 mg,
44%), m.p. 224Ϫ225 °C. Rf ϭ 0.7 (2% EtOAc/petroleum ether). 1H
NMR (300 MHz, CDCl3, ppm): δ ϭ 7.12 (dd, J ϭ 3.3, 4.9 Hz, 3
H), 7.32 (dd, J ϭ 1.1, 4.9 Hz, 3 H), 7.39 (dd, J ϭ 1.1, 3.7 Hz, 3
H), 7.74 (s, 12 H), 7.83 (s, 3 H). UV (CHCl3): λmax [nm] (ε,
NMR (75.4 MHz, CDCl3, ppm): δ ϭ 122.7, 123.9, 125.4, 128.1,
135.7, 143.5. UV (CHCl3): λmax [nm] (ε, Ϫ1cmϪ1) ϭ 296 (50359).
MS: m/z ϭ 324 [Mϩ].
1,3,5-Tris[5-(2-methylthienyl)benzene (20): 2-Acetyl-5-methylthio-
phene (17, 135 mg, 0.96 mmol) in absolute EtOH (2.5 mL) was
treated with SiCl4 (0.6 mL, 5.24 mmol) as described in the above
general procedure for 4 h to furnish 20 as a white solid (71 mg,
61%), m.p. 137Ϫ139 °C. Rf ϭ 0.6 (2% EtOAc/petroleum ether). 1H
NMR (300 MHz, CDCl3, ppm): δ ϭ 2.52 (s, 9 H), 6.74Ϫ6.76 (m,
3 H), 7.17 (d, J ϭ 3.5 Hz, 3 H), 7.57 (s, 3 H). 13C NMR (75.4 MHz,
CDCl3, ppm): δ ϭ 15.5, 121.4, 123.5 126.2, 135.7, 140.0, 141.3. UV
(CHCl3): λmax [nm] (ε, Ϫ1cmϪ1) ϭ 305 (71465). C21H18S3 (366):
calcd. C 68.85, H 4.92; found C 68.81, H 4.90.

Ϫ1cmϪ1) ϭ 315 (91175). C36H24S3 (552): calcd. C 78.26, H 4.35;
found C 78.22, H 4.37.
1,3,5-Tris[5-(2-phenylthienyl)]benzene (37): 2-Acetyl-5-phenylthio-
phene (39, 58 mg, 0.287 mmol) in absolute EtOH (3 mL) was
treated with SiCl4 (0.6 mL, 5.23 mmol) as described in the above
general procedure for 24 h to deliver 37 as a light yellow solid
(23 mg, 44%), m.p. 244Ϫ245 °C. Rf ϭ 0.5 (5% EtOAc/petroleum
1
ether). H NMR (300 MHz, CDCl3, ppm): δ ϭ 7.29Ϫ7.44 (m, 15
H), 7.67Ϫ7.70 (m, 6 H), 7.79 (s, 3 H). 13C NMR (75.4 MHz,
CDCl3, ppm): δ ϭ 121.9, 124.0, 124.7, 125.6, 127.6, 128.9, 134.1,
135.6, 142.5, 144.2. UV (CHCl3): λmax [nm] (ε, Ϫ1cmϪ1) ϭ 339
(72795). C36H24S3 (552): calcd. C 78.26, H 4.35; found C 78.32,
H 4.33.
1,3,5-Tris[5-(2-butylthienyl)]benzene (21): 2-Acetyl-5-butylthio-
phene (18, 70 mg, 0.38 mmol) in absolute EtOH (1 mL) was treated
with SiCl4 (0.2 mL, 1.92 mmol) as described in the above general
procedure for 18 h to produce 21 as a white solid (45 mg, 72%),
1
m.p. 40Ϫ42 °C. Rf ϭ 0.6 (3% EtOAc/petroleum ether). H NMR
(300 MHz, CDCl3, ppm): δ ϭ 0.98 (t, J ϭ 7.3 Hz, 9 H), 1.43 (sept,
J ϭ 7.3 Hz, 6 H), 1.65 (pent, J ϭ 7.6 Hz, 6 H), 2.86 (t, J ϭ 7.6 Hz,
6 H), 6.79 (d, J ϭ 3.7 Hz, 3 H), 7.22 (d, J ϭ 3.3 Hz, 3 H), 7.62 (s,
3 H). 13C NMR (75.4 MHz, CDCl3, ppm): δ ϭ 13.9, 22.2, 30.0,
33.8, 121.4, 123.2, 125.1, 135.8, 141.0, 146.1. UV (CHCl3): λmax
[nm] (ε, Ϫ1cmϪ1) ϭ 306 (32472). EI-HRMS (C30H36S3): calcd.
492.1979; found 492.1966.
1,3,5-Tris(4-bromophenyl)benzene (27):[19a] 4-Bromoacetophenone
(500 mg, 2.51 mmol) in absolute EtOH (2.5 mL) was treated with
SiCl4 (890 mg, 5 mmol) as described in the above general procedure
for 4 h to produce 27 (351 mg, 77%) as a white solid m.p.: 263 °C
(ref.[19a] m.p. 264 °C). Rf ϭ 0.5 (petroleum ether). 1H NMR
(300 MHz, CDCl3, ppm): δ ϭ 7.53 (td, J ϭ 2.2, 8.4 Hz, 6 H), 7.61
(td, J ϭ 2.2, 8.7 Hz, 6 H), 7.69 (s, 3 H).
1,3,5-Tris[5-(2-octylthienyl)]benzene
(22):
2-Acetyl-5-octylthi-
1,3,5-Tris-(4-iodophenyl)benzene
(28):
4-Iodoacetophenone
ophene (19, 120 mg, 0.588 mmol) in absolute EtOH (1.5 mL) was
treated with SiCl4 (0.35 mL, 2.9 mmol) as described in the above
general procedure for 24 h to give 22 (70 mg, 63%), m.p. 38Ϫ39 °C.
Rf ϭ 0.8 (1% EtOAc/petroleum ether). 1H NMR (300 MHz,
CDCl3, ppm): δ ϭ 0.88 (t, J ϭ 6.9 Hz, 9 H), 1.28Ϫ1.38 (m, 30 H),
1.71 (pent, J ϭ 7.3 Hz, 6 H), 2.83 (t, J ϭ 7.6 Hz, 6 H), 6.77 (d,
J ϭ 3.6 Hz, 3 H), 7.20 (d, J ϭ 3.3 Hz, 3 H), 7.59 (s, 3 H). 13C
NMR (75.4 MHz, CDCl3, ppm): δ ϭ 14.2, 22.7, 29.2, 29.3, 29.4,
30.3, 31.7, 31.9, 121.4, 123.2, 125.0, 135.8, 141.0, 146.2. UV
(CHCl3): λmax [nm] (ε, Ϫ1cmϪ1) ϭ 306 (48543). EI-HRMS
(C42H60S3): calcd. 660.3857; found 660.3889.
(200 mg, 0.08 mmol) in absolute EtOH (3 mL) was treated with
SiCl4 (0.6 mL, 5.24 mmol) as described in the above general pro-
cedure for 12 h to furnish 28 as a white, crystalline solid (141 mg,
76%), m.p. 262Ϫ263 °C (ref.[38] m.p. 265 °C). Rf ϭ 0.6 (petroleum
1
ether). H NMR (300 MHz, CDCl3, ppm): δ ϭ 7.40 (AB part of
AAЈBBЈ system, J ϭ 8.4 Hz, 6 H), 7.68 (s, 3 H), 7.81 (AЈBЈ part
of AAЈBBЈ system, J ϭ 8.4 Hz, 6 H). 13C NMR (75.43 MHz,
CDCl3, ppm): δ ϭ 93.7, 124.9, 129.2, 138.1, 140.3, 141.7. EI-
HRMS (C24H15I3): calcd. 684.8386; found 684.8392.
General Procedure for the Suzuki؊Miyaura Cross-Coupling Reac-
tion:[16] A mixture of aryl halide (1 equiv.), arylboronic acid (6Ϫ7
equiv.), Pd(PPh3)4 (ca. 8Ϫ10 mol %), Na2CO3 (6 equiv.) in H2O,
and solvent [DME or THF and toluene (1:1)] was heated at 90 °C
under N2. At the conclusion of the reaction (TLC), the mixture
was diluted with H2O and extracted with EtOAc. The combined
organic layers were washed with H2O and brine and dried
(MgSO4). The solvent was evaporated, and the remaining crude
product was loaded onto a silica gel column. Elution of the column
with EtOAc/petroleum ether gave the desired cross-coupling prod-
uct.
1,3,5-Tris[5-(2-chlorothienyl)]benzene (24): 2-Acetyl-5-chlorothi-
ophene (23, 300 mg, 1.87 mmol) in absolute EtOH (6 mL) was
treated with SiCl4 (0.9 mL, 7.85 mmol) as described in the above
general procedure for 12 h to give 24 as a white solid (159 mg,
60%), m.p. 177Ϫ179 °C. Rf ϭ 0.9 (petroleum ether). 1H NMR
(300 MHz, CDCl3, ppm): δ ϭ 6.94 (d, J ϭ 4.0 Hz, 3 H), 7.15 (d,
J ϭ 3.7 Hz, 3 H), 7.50 (s, 3 H). 13C NMR (75.4 MHz, CDCl3,
ppm): δ ϭ 122.0, 123.3, 127.3, 130.2, 135.2, 141.4. UV (CHCl3):
λmax [nm] (ε, Ϫ1cmϪ1) ϭ 304 (52595). C18H9Cl3S3 (427): calcd. C
50.53, H 2.12; found C 49.99, H 2.07.
1,3,5-Tris[5-(2,2Ј-bithienyl)]benzene (26): 5-Acetyl-2,2Ј-bithiophene
(25, 80 mg, 0.38 mmol) in absolute EtOH (2.5 mL) was treated with
4-(2-Thienyl)acetophenone (38):
phenone (282 mg, 1.42 mmol), thiophene-2-boronic acid (300 mg,
A mixture of 4-bromoaceto-
SiCl4 (0.65 mL, 5.67 mmol) under reflux conditions as described in 2.34 mmol), Pd(PPh3)4 (20 mg, 0.0173 mmol), K2CO3 (320 mg,
the above general procedure for 4 h to produce 26 as a light yellow 2.13 mmol) in H2O (1 mL), and solvent DME (2 mL) was heated
solid (19 mg, 26%), m.p. 180 °C. Rf ϭ 0.2 (petroleum ether). 1H at 90 °C under N2. At the conclusion of reaction (TLC), the mix-
NMR (300 MHz, CDCl3, ppm): δ ϭ 7.06 (dd, J ϭ 3.7, 4.9 Hz, 3 ture was worked up as described in the general procedure. The
H), 7.20 (d, J ϭ 3.8 Hz, 3 H), 7.24Ϫ7.27 (m, 6 H), 7.35 (d, J ϭ crude product was purified by silica gel column chromatography.
4010
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2004, 4003Ϫ4013

Synthesis of C3-Symmetric Nano-Sized Polyaromatic Compounds
FULL PAPER
Elution of the column with 8% EtOAc/petroleum ether gave the
system, J ϭ 8.0 Hz, 6 H), 7.71 (AB part of AAЈBBЈ system, J ϭ
8.4 Hz, 6 H), 7.79 (AЈBЈ part of AAЈBBЈ system, J ϭ 8.4 Hz, 6 H),
desired cross-coupling product 38 (262 mg, 92%) as a white solid,
m.p. 115Ϫ117 °C (ref.[29b] m.p. 116Ϫ117 °C). Rf ϭ 0.7 (20% 7.87 (s, 3 H). 13C NMR (75.43 MHz, CDCl3, ppm): δ ϭ 21.3, 96.3,
1
EtOAc/petroleum ether). H NMR (300 MHz, CDCl3, ppm): δ ϭ
2.62 (s, 3 H), 7.13 (dd, J ϭ 3.7, 5.1 Hz, 1 H), 7.38 (dd, J ϭ 1.1,
124.9, 127.4, 127.7, 129.6, 136.9, 137.9, 139.9, 140.5, 142.1. UV (in
CHCl3): λmax [nm] (ε,

Ϫ1cmϪ1): 300 (108901). EI-HRMS
5.1 Hz, 1 H), 7.44 (dd, J ϭ 1.1, 3.7 Hz, 1 H), 7.70 (td, J ϭ 1.8, (C45H36): calcd. 576.2817; found 576.2813.
8.4 Hz, 2 H), 7.97 (td, J ϭ 1.8, 8.4 Hz, 2 H). 13C NMR (75.4 MHz,
CDCl3, ppm): δ ϭ 26.5, 124.6, 125.6, 126.4, 128.3, 129.1, 135.7,
1,3,5-Tris[4-(4Ј-acetylphenyl)phenyl]benzene (31): A mixture of 29
(30 mg, 0.043 mmol), 4-acetylphenylboronic acid (44 mg,
0.26 mmol), Pd(PPh3)4 (14.8 mg, 0.012 mmol), Na2CO3 (24 mg,
0.22 mmol) in H2O (1 mL), and solvent [THF (3 mL) and toluene
(3 mL)] was heated at 90 °C under N2. At the conclusion of reac-
tion (TLC) the mixture was worked up as described in the general
procedure. The crude product was purified by silica gel column
chromatography. Elution of the column with 16% EtOAc/petro-
leum ether gave the desired cross-coupling product 31 (15 mg, 52%)
as a white, crystalline solid, m.p. 196Ϫ198 °C. Rf ϭ 0.3 (25%
138.7, 142.9, 197.3.
5-Acetyl-2-phenylthiophene (39): A mixture of 2-acetyl-5-bromo-
thiophene (300 mg, 1.46 mmol), phenylboronic acid (291 mg,
2.39 mmol), Pd(PPh3)4 (44 mg, 0.038 mmol), K2CO3 (309 mg,
8.3 mmol) in H2O (1 mL), and solvent DME (2 mL) was heated at
90 °C under N2. At the conclusion of reaction (TLC), the mixture
was worked up as described in the general procedure. The crude
product was purified by silica gel column chromatography. Elution
of the column with 5% EtOAc/petroleum ether gave the desired
cross-coupling product 39 (261 mg, 88%) as a white solid, m.p.
109Ϫ112 °C (ref.[29c] m.p. 115 °C). Rf ϭ 0.3 (5% EtOAc/petroleum
1
EtOAc/petroleum ether). H NMR (300 MHz, CDCl3, ppm): δ ϭ
2.67 (s, 9 H), 7.77 (AB part of AAЈBBЈ system, J ϭ 8.4 Hz, 6 H),
7.78 (AЈBЈ part of AAЈBBЈ system, J ϭ 8.4 Hz, 6 H), 7.85 (AB
part of AAЈBBЈ system, J ϭ 8.0 Hz, 6 H), 7.91 (s, 3 H), 8.08 (AЈBЈ
part of AAЈBBЈ system, J ϭ 8.0 Hz, 6 H). 13C NMR (75.43 MHz,
CDCl3, ppm): δ ϭ 26.8, 125.4, 127.3, 127.9, 128.1, 131.1, 136.1,
139.3, 141.0, 142.0, 145.3, 198.1. UV (in CHCl3): λmax [nm] (ε,
1
ether). H NMR (300 MHz, CDCl3, ppm): δ ϭ 2.57 (s, 3 H), 7.33
(d, J ϭ 3.7 Hz, 1 H), 7.37Ϫ7.46 (m, 3 H), 7.64Ϫ7.67 (m, 3 H). 13
C
NMR (75.4 MHz, CDCl3, ppm): δ ϭ 26.5, 123.9, 126.2, 129.0,
129.1, 133.3, 133.4, 143.1, 152.8, 190.6.

Ϫ1cmϪ1): 310 (335757). IR (neat, cmϪ1): ν˜ ϭ 815.1, 1026.9,
1263.4, 1594.4, 1670.5, 2851.0, 2919.7 cmϪ1
EI-HRMS
(C48H36O3): calcd. 660.2664; found 661.2725 [M ϩ 1].
1,3,5-Tris[4-(2Ј-thienyl)phenyl]benzene (29) from 27: A mixture of
27 (100 mg, 0.184 mmol), thiophene-2-boronic acid (130 mg,
1.02 mmol), Pd(PPh3)4 (20 mg, 0.0173 mmol), Na2CO3 (189 mg,
1.7 mmol) in H2O (3 mL), and solvent [THF (3 mL) and toluene
(3 mL)] was heated at 90 °C under N2. At the conclusion of reac-
tion (TLC), the mixture was worked up as described in the general
procedure. The crude product was purified by silica gel column
chromatography. Elution of the column with petroleum ether gave
the desired cross-coupling product 29 (14 mg, 14%) as a white solid.
The physical characteristics and spectroscopic data of the com-
pound 29 obtained by this route were the same as those of the
product obtained by the trimerization reaction. Rf ϭ 0.3 (2%
EtOAc/petroleum ether).
.
1,3,5-Tris[4-(4Ј-fluorophenyl)phenyl]benzene (32): A mixture of 29
(50 mg, 0.073 mmol), 4-fluorophenylboronic acid (58 mg,
0.41 mmol), Pd(PPh3)4 (25.2 mg, 0.02 mmol), Na2CO3 (40 mg,
0.37 mmol) in H2O (1 mL), and solvent [THF (3 mL) and toluene
(3 mL)] was heated at 90 °C under N2. At the conclusion of reac-
tion (TLC), the mixture was worked up as described in the general
procedure. The crude product was purified by silica gel column
chromatography. Elution of the column with petroleum ether gave
the desired cross-coupling product 32 (23 mg, 53%) as a white, crys-
talline solid, m.p. 228Ϫ230 °C (ref.[28] m.p. 236 °C). Rf ϭ 0.4 (petro-
leum ether). 1H NMR (300 MHz, CDCl3, ppm): δ ϭ 7.14Ϫ7.11
(m, 6 H), 7.62Ϫ7.58 (m, 6 H), 7.65 (AB part of AAЈBBЈ system,
J ϭ 8.0 Hz, 6 H), 7.77 (AЈBЈ part of AAЈBBЈ system, J ϭ 8.0 Hz,
6 H), 7.85 (s, 3 H). 13C NMR (75.43 MHz, CDCl3, ppm): δ ϭ
115.7 (J ϭ 21.4 Hz), 115.8, 124.9, 127.4, 127.7, 128.6 (J ϭ 8.0 Hz),
136.6, 139.5, 139.9, 141.9, 162.5 (J ϭ 246.0 Hz), [values in parenth-
esis are C-F coupling constants]. UV (in CHCl3): λmax [nm] (ε,
1,3,5-Tris[4-(2Ј-thienyl)phenyl]benzene (29) from 28: A mixture of
28 (33 mg, 0.048 mmol), thiophene-2-boronic acid (33 mg,
0.25 mmol), Pd(PPh3)4 (20 mg, 0.0173 mmol), Na2CO3 (26 mg,
0.24 mmol) in H2O (1 mL), and solvent [THF (3 mL) and toluene
(3 mL)] was heated at 90 °C under N2. At the conclusion of the
reaction (TLC), the mixture was worked up as described in the
general procedure. The crude product was purified by silica gel col-
umn chromatography. Elution of the column with petroleum ether
gave the desired cross-coupling product 29 (16 mg, 60%) as a white
solid. The physical characteristics and spectroscopic data of the
compound 29 obtained by this route were the same as those of the
product obtained by the trimerization reaction. Rf ϭ 0.3 (2%
EtOAc/petroleum ether).

Ϫ1cmϪ1): 297 (268676). EI-HRMS (C42H27F3): calcd. 588.2064;
found 588.2072.
1,3,5-Tris[4-(3Ј-trifluoromethylphenyl)phenyl]benzene (33): A mix-
ture of 29 (50 mg, 0.073 mmol), 3-trifluoromethylphenylboronic
acid (84 mg, 0.41 mmol), Pd(PPh3)4 (25.2 mg, 0.02 mmol), Na2CO3
(40 mg, 0.37 mmol) in H2O (1 mL), and solvent [THF (3 mL) and
toluene (3 mL)] was heated at 90 °C under N2. At the conclusion
of reaction (TLC), the mixture was worked up as described in the
1,3,5-Tris[4-(4Ј-methylphenyl)phenyl]benzene (30): A mixture of 29
(30 mg, 0.043 mmol), 4-methylphenylboronic acid (35 mg, general procedure. The crude product was purified by silica gel col-
0.25 mmol), Pd(PPh3)4 (14.8 mg, 0.012 mmol), Na2CO3 (24 mg, umn chromatography. Elution of the column with petroleum ether
0.22 mmol) in H2O (1 mL), and solvent [THF (3 mL) and toluene gave the desired cross-coupling product 33 (40 mg, 75%) as a white,
1
(3 mL)] was heated at 90 °C under N2. At the conclusion of reac-
tion (TLC), the mixture was worked up as described in the general
procedure. The crude product was purified by silica gel column
chromatography. Elution of the column with petroleum ether gave
the desired cross-coupling product 30 (12 mg, 47%) as a white, crys-
crystalline solid, m.p. 222Ϫ224 °C. Rf ϭ 0.4 (petroleum ether). H
NMR (300 MHz, CDCl3, ppm): δ ϭ 7.65Ϫ7.85 (m, 9 H), 7.71 (AB
part of AAЈBBЈ system, J ϭ 8.4 Hz, 6 H), 7.84 (AЈBЈ part of
AAЈBBЈ system, J ϭ 8.4 Hz, 6 H), 7.85Ϫ7.91 (m, 6 H). 13C NMR
(75.43 MHz, CDCl3, ppm): δ ϭ 123.6 (J ϭ 3.7 Hz), 124.1, 124.2
talline solid, m.p. 184Ϫ186 °C. Rf ϭ 0.4 (petroleum ether). 1H (J ϭ 272.1 Hz), 125.2, 127.7, 127.9, 129.3, 130.3, 131.3 (J ϭ
NMR (300 MHz, CDCl3, ppm): δ ϭ 2.42 (s, 9 H), 7.29 (AB part 32.1 Hz), 139.0, 140.6, 141.4, 141.8, [values in parenthesis are C-F
of AAЈBBЈ system, J ϭ 8.0 Hz, 6 H), 7.57 (AЈBЈ part of AAЈBBЈ coupling constants]. UV (in CHCl3): λmax [nm] (ε, Ϫ1cmϪ1):
Eur. J. Org. Chem. 2004, 4003Ϫ4013
www.eurjoc.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4011

S. Kotha, D. Kashinath, K. Lahiri, R. B. Sunoj
FULL PAPER
294 nm (103925). EI-HRMS (C45H27F9): calcd. 738.1963; found
738.1956.
with MgSO4. Evaporation of the solvent and purification of the
crude product by neutral alumina column chromatography with
2% EtOAc/petroleum ether mixture as an eluent gave the desired
coupling product 41 (40 mg, 66%) as a white, crystalline solid, m.p.
125Ϫ128 °C(ref.[39] m.p. 125Ϫ126 °C). Rf ϭ 0.3 (5% EtOAc/
petroleum ether). 1H NMR (300 MHz, CDCl3, ppm): δ ϭ 6.52 (dd,
J ϭ 1.8, 3.3 Hz, 3 H), 6.78 (d, J ϭ 3.3 Hz, 3 H), 7.52 (d, J ϭ
1.8 Hz, 3 H), 7.88 (s, 3 H). UV (in CHCl3): λmax [nm] (ε, Ϫ1cmϪ1):
302 (38853). EI-HRMS (C18H12O3): calcd. 276.0786; found
276.0782.
1,3,5-Tris[4-(4Ј-methoxyphenyl)phenyl]benzene (35): A mixture of
29 (30 mg, 0.043 mmol), 4-methoxyphenylboronic acid (40 mg,
0.263 mmol), Pd(PPh3)4 (14.8 mg, 0.012 mmol), Na2CO3 (24 mg,
0.22 mmol) in H2O (1 mL), and solvent [THF (3 mL) and toluene
(3 mL)] was heated at 90 °C under N2. At the conclusion of the
reaction (TLC), the mixture was worked up as described in the
general procedure. The crude product was purified by silica gel col-
umn chromatography. Elution of the column with petroleum ether
gave the desired cross-coupling product 35 (15 mg, 55%) as a white,
crystalline solid, m.p. 182Ϫ184 °C. Rf ϭ 0.3 (15% EtOAc/petro-
Acknowledgments
1
leum ether). H NMR (300 MHz, CDCl3, ppm): δ ϭ 3.86 (s, 9 H),
We gratefully acknowledge the DST (NST) for the financial sup-
port. D.K. and K.L. thank CSIR, New Delhi and IIT-Bombay,
respectively, for the award of Research Fellowships. We would like
to acknowledge RSCI-Mumbai, for providing the spectroscopic
data. We thank Dr. J. Singh and Dr. G. Williams, Optima Chemical
group for providing gift samples of boronic acids.
7.00 (AB part of AAЈBBЈ system, J ϭ 8.0 Hz, 6 H), 7.60 (AЈBЈ
part of AAЈBBЈ system, J ϭ 8.0 Hz, 6 H), 7.67 (AB part of AAЈBBЈ
system, J ϭ 7.3 Hz, 6 H), 7.77 (AЈBЈ part of AAЈBBЈ system, J ϭ
7.3 Hz, 6 H), 7.86 (s, 3 H). 13C NMR (75.43 MHz, CDCl3, ppm):
δ ϭ 55.3, 114.2, 124.8, 127.1, 127.6, 128.1, 133.1, 139.3, 140.0,
141.9, 159.2. UV (in CHCl3): λmax [nm] (ε, Ϫ1cmϪ1): 302 (104034).
EI Mass (QTOF): 625.2323 [M ϩ 1].
[1]
I. Hargittai, M. Hargittai, Symmetry Through the Eyes of a
1,3,5-Tris[4-(4Ј-formylphenyl)phenyl]benzene (34): A mixture of 29
(50 mg, 0.073 mmol) 4-formylphenylboronic acid (66 mg,
0.43 mmol), Pd(PPh3)4 (25.2 mg, 0.02 mmol), Na2CO3 (40 mg,
0.37 mmol) in H2O (1 mL), and solvent [THF (3 mL) and toluene
(3 mL)] was heated at 90 °C under N2. At the conclusion of the
reaction (TLC), the mixture was worked up as described in the
general procedure. The crude product was purified by silica gel col-
umn chromatography. Elution of the column with 13% EtOAc/
petroleum ether gave the desired cross-coupling product 34 (23 mg,
51%) as a white, crystalline solid, m.p. 174Ϫ176 °C. Rf ϭ 0.3 (20%
Chemist, VCH, Weinheim, 1987.
[2] [2a]
T.-L. Ho, Symmetry: A Basis for Synthesis Design, John
[2b]
Wiley & Sons, New York, 1995.
K. C. Nicolaou, E. J. Sor-
ensen, Classics in Total Synthesis, Wiley-VCH, Weinheim,
1996. [2c]S. R. Magnuson, Tetrahedron 1995, 51, 2167Ϫ2213.
[3] [3a]
G. R. Newkome, C. N. Moorefield, F. Vögtle, Dendritic
Molecules: Concepts, Synthesis, Perspectives; Wiley-VCH,
Weinheim, 1996. [3b] In: Dendrimers I, Topics in Current Chem-
[3c]
istry (Ed.: F. Vögtle); Springer, New York, 1998, vol. 197.
In: Dendrimers II, Architecture, Nanostructure and Supramol-
ecular Chemistry. Topics in Current Chemistry (Ed.: F. Vögtle),
Springer, New York, 2000; vol. 210.
A. B. Padias, Jr., H. K. Hall, D. A. Tomalia, J. R. McConnel,
J. Org. Chem. 1987, 52, 5305Ϫ5312.
E. L. Eliel, S. H. Wilen, L. N. Mander, Stereochemistry of Or-
ganic Compounds, John Wiley & Sons, New York, 1994.
T. M. Miller, T. X. Neenan, R. Zayas, H. E. Bair, J. Am. Chem.
Soc. 1992, 114, 1018Ϫ1025.
J. Pang, E. J.-P. Marcotte, C. Seward, R. S. Brown, S. Wang,
Angew. Chem. Int. Ed. 2001, 40, 4042Ϫ4045.
1
EtOAc/petroleum ether). H NMR (300 MHz, CDCl3, ppm): δ ϭ
[4]
[5]
[6]
[7]
[8]
7.52 (AB part of AAЈBBЈ system, J ϭ 8.0 Hz, 6 H), 7.87Ϫ7.80 (m,
12 H), 7.91 (s, 3 H), 8.00 (AЈBЈ part of AAЈBBЈ system, J ϭ 8.0 Hz,
6 H), 10.08 (s, 3 H). 13C NMR (75.43 MHz, CDCl3, ppm): δ ϭ
125.2, 127.5, 127.9 (2C), 130.3, 135.3, 139.1, 141.0, 141.8, 146.4,
191.8. UV (in CHCl3): λmax [nm] (ε, Ϫ1cmϪ1): 315 (103502). IR
(neat, cmϪ1): ν˜ ϭ 803.7, 1025.7, 1170.2, 1208.0, 1600.7, 1696.2,
2857.1, 2925.7 cmϪ1. EI Mass (QTOF): 619.1276[M ϩ 1].
P. Belser, A. von Zelewsky, M. Frank, C. Seel, F. Vögtle, L.
D. Cola, F. Barigelletti, V. Balzani, J. Am. Chem. Soc. 1993,
115, 4076Ϫ4086.
1,3,5-Tris[4-(2Ј-furyl)phenyl]benzene (36): A mixture of 29 (55 mg,
0.080 mmol), furan-2-boronic acid (42 mg, 0.3 mmol), Pd(PPh3)4
(27 mg, 0.02 mmol), Na2CO3 (40 mg, 0.37 mmol) in H2O (1 mL),
and solvent [THF (3 mL) and toluene (3 mL)] was heated at 90 °C
under N2. At the conclusion of the reaction (TLC) the mixture was
worked up as described in the general procedure. The crude prod-
uct was purified by silica gel column chromatography. Elution of
the column with petroleum ether gave the desired cross-coupling
product 36 (21 mg, 52%) as a white, crystalline solid, m.p. 140Ϫ142
°C. Rf ϭ 0.4 (2% EtOAc/petroleum ether). 1H NMR (300 MHz,
CDCl3, ppm): δ ϭ 6.51 (dd, J ϭ 3.4, 3.2 Hz, 3 H), 6.72 (d, J ϭ
3.2 Hz, 3 H), 7.51(d, J ϭ 1.3 Hz, 3 H), 7.74 (AB part of AAЈBBЈ
system, J ϭ 8.5 Hz, 6 H), 7.80 (AЈBЈ part of AAЈBBЈ system, J ϭ
8.5 Hz, 6 H), 7.83 (s, 3 H). 13C NMR (75.43 MHz, CDCl3, ppm):
δ ϭ 105.3, 111.7, 124.2, 124.6, 127.5, 130.2, 139.8, 141.8, 142.2,
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4013