Synthesis of an extended hexagonal molecule as a highly symmetrical ligand

Article abstract of DOI:10.1016/j.tetlet.2005.01.014

An extended hexagonal molecule that has high symmetry and rigidity was designed and synthesized by a cobalt-catalyzed cyclotrimerization reaction. The ligand possesses six benzoic acids at its periphery, which were connected to a central hexakis(2,5-dodesyloxyphenyl)hexaphenylbenzene core with a maximum intramolecular carboxyl group distance of approximately 3.5 nm.

Full text of DOI:10.1016/j.tetlet.2005.01.014

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 1739–1742
Synthesis of an extended hexagonal molecule as a
highly symmetrical ligand
Masayoshi Takase,a Atsushi Nakajimab and Toshifumi Takeuchia,b,
*
aGraduate School of Science and Technology, Kobe University, 1-1 Rokkodai-cho, Kobe 657-8501, Japan
bPRESTO, Japan Science and Technology Agency (JST), 4-1-8Honmachi, Kawaguchi 332-0012, Japan
Received 13 October 2004; revised 23 December 2004; accepted 7 January 2005
Available online 28 January 2005
Abstract—An extended hexagonal molecule that has high symmetry and rigidity was designed and synthesized by a cobalt-catalyzed
cyclotrimerization reaction. The ligand possesses six benzoic acids at its periphery, which were connected to a central hexakis(2,5-
dodesyloxyphenyl)hexaphenylbenzene core with a maximum intramolecular carboxyl group distance of approximately 3.5 nm.
Ó 2005 Elsevier Ltd. All rights reserved.
One of the growth areas of supramolecular chemistry is
the construction of organic and inorganic networks.
They have attracted wide interest because they provide
an efficient novel route towards the synthesis of well-de-
fined porous materials that have applications in molecu-
lar recognition, catalysis, gas storage and separation.1 A
common motif is the use of benzene and its derivatives
as rigid frameworks.2 Among these, highly symmetrical
molecules with multiple interaction sites can serve to
prevent the formation of interpenetrated networks to
yield hexagonal networks.2c,d In this letter we describe
the synthesis of an extended hexagonal ligand molecule
1b by a cobalt-catalyzed cyclotrimerization reaction of
ethynyl-bridged di(triphenylene)s. Ligand 1 possesses
six benzoic acids at its periphery, which can serve to di-
rect the formation of well-defined porous materials, and
a diameter of 3.5 nm making it one of the largest hexa-
gonal ligands. In addition, preliminary studies of the
solution behaviour of the complex between Sn(IV) por-
phyrin and 1b are reported.
dodesyloxy groups were introduced in the middle of
the triphenylene moiety to increase its solubility in com-
mon organic solvents. According to the molecular mod-
elling calculations (MOPAC), the carboxyl groups of 1
all lie in a plane with a maximum and minimum intra-
molecular carboxyl group distance of approximately
3.5 and 1.6 nm, respectively (Scheme 1).
The synthesis of 1 is illustrated in Scheme 2. The route
relies on the stepwise Suzuki coupling protocols3 and
transition metal catalyzed trimerization reaction4 of
ethynyl-bridged di(triphenylene) 8.5 First, 4,40-diboro-
natetolane (4) was prepared by a one-step Miyaura aryl-
boronate synthesis catalyzed by PdCl2(dppf).6 Coupling
of 5 with 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)benzoic acid methyl ester(6)6 followed by coupling
of the product 4 with 7 were conducted under standard
Suzuki coupling conditions. Our first trial to synthesize
8a from 7a started with the reaction of 1,4-dibromo-
2,3,5,6-tetrakis-hexyloxy-benzene (5a)7a and 6. In the
presence of an excess amount of 5a, 7a was obtained
in 72% yield from 5a and 6 (conditions: Pd(PPh3)4,
Cs2CO3, DMF, 95 °C, 11 h). However, the reaction of
4 and 7a gave 8a in low yield (at most 20%) along with
the recovery of 7a (52%), debrominated derivative of 7a,
and a other complicated mixture of side products (con-
ditions: Pd(PPh3)4, Cs2CO3, DMF, 100 °C, 24h). In
addition, in our hands, pure 8a could not be obtained
by standard purification methods such as recrystalliza-
tion, silica gel column and size exclusion chromatogra-
phy. Based on the hypothesis that the steric congestion
around the bromo group was the main cause for the
low yield using 7a, we changed the reactant to 7b, which
As a starting point for this study, we designed an ex-
tended hexagonal ligand molecule 1 constructed from
19 benzene rings. The 6 carboxyl groups of 1 were ar-
ranged around the periphery to direct the formation of
hexagonal interaction networks and 24hexyloxy or 12
Keywords: Cyclotrimerization; Supramolecular chemistry; Hexagonal
ligand; Tin porphyrin.
*
Corresponding author. Tel./fax: +81 78 803 6158; e-mail: takeuchi@
scitec.kobe-u.ac.jp
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.01.014

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M. Takase et al. / Tetrahedron Letters 46 (2005) 1739–1742
TOF MS (no matrix), m/z = 1337.2; calcd for
C90H127O8 = 1337.0 (MH++1)).
In the key step, a Co2(CO)8-catalyzed cyclotrimerization
reaction of 8b gave 1b0 in 68% yield (conditions:
10 mol % Co2(CO)8, toluene, 110 °C, 24h). Hydrolysis
of 1b0 with KOH aq in THF afforded the target ligand
1b in 75% yield. The molecular weight of 1b was deter-
mined by MALDI-TOF MS (2,5-dihydroxybenzoic
acid matrix) to be m/z = 3924.8 (calcd for
C264H367O24 = 3924.9 (MH++3)) and its 1H NMR spec-
trum in pyridine-d5 was quite simple, demonstrating the
hexagonal symmetry of the molecule; four doublets due
to the outer and inner 1,4-phenylene aromatic protons
at 8.57, 7.98, 7.76 and 7.64ppm and two singlets due
to the middle 1,4-phenylene spacer at 7.26 and
7.13 ppm.5,9 These signals in the aromatic region of
1b0 instead of 1b were fully assigned by using the
NOESY NMR technique because the aromatic region
of 1b was partially obscured from residual pyridine
peaks.5 Ring current effects were also apparent by com-
parison with ethynyl-bridged di(triphenylene) 8b; the
inner phenylene two protons of 8b were shifted upfield by
0.41 and 0.59 ppm. As reference molecules, triphenylene
Scheme 1. Extended hexagonal molecule 1.
was obtained from 1,4-bibromo-2,5-didodesyloxybenz-
ene (5b)7b and 6.8 Under the same conditions, 7b
afforded 8b in 67% yield as a pure form (MALDI-
Scheme 2. Reagents: (a) PdCl2(dppf), AcOK, DMF; (b) Pd(PPh3)4, Cs2CO3, DMF; (c) Pd(PPh3)4, Cs2CO3, DMF; (d) Co2(CO)8, toluene; (e) KOH
aq, THF.

M. Takase et al. / Tetrahedron Letters 46 (2005) 1739–1742
1741
linear molecule 9 was prepared by Suzuki coupling reac-
tion of 6 with 5a followed by hydrolysis.5
Acknowledgements
The authors gratefully acknowledged the useful discus-
sions and the help of preparing this manuscript received
from Professor Ken D. Shimizu (University of South
Carolina). This work was partly supported by Mitsubi-
shi Chemical Corporation Fund (M.T.).
We have examined the complexation behaviour between
dichloro[5,10,15,20-tetrakis(3,5-di-tert-butyl phenyl)por-
phyrinato]tin(IV) (10) and 1b or 9 utilizing the trans-
bidentate coordination behaviour of the central metal
of the Sn(IV) porphyrin.10 The mixture of 9 and 10 could
form linear bead-like porphyrin arrays, whereas the mix-
ture of 1b and 10 could be expected to lead to honey-
comb-shaped arrays. The formation of a 1:1 complex
between 9 and 10, was observed by NMR from the car-
boxylate-ligand protons, Ha and Hb, and methylene pro-
tons, Hc, of 9 that were shifted far upfield by 2.55, 1.80
and 0.80 ppm, respectively, reflecting the porphyrin ring
current, and the molar ratio of complexed 9 to 10 was
determined to be approximately 1.0, as judged from the
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2005.01.014.
References and notes
1
integration of the H NMR spectrum in CDCl3. These
complexes were also detected using MALDI-TOF MS
spectrometer (9-nitroanthracene matrix); 9Æ10, 10Æ9Æ10,
10Æ9Æ10Æ9, 10Æ9Æ10Æ9Æ10, 10Æ9Æ109Æ10Æ9 and 10Æ9Æ10Æ9Æ10Æ9Æ10.
These results suggest that efficient coordination of car-
boxyl group to the Sn(IV) porphyrin occurred and the
desired bead-like structure was obtained. Unfortunately
in the case of the complex between 1b and 10, however,
the 1H NMR spectra were very broad and no assignable
peaks could be detected over a wide temperature range
(ꢀ50 to 40 °C). Only small assemblies could be detected
by MALDI-TOF MS spectrometer (3-hydroxypiconilic
acid matrix); 1bÆ9, 1bÆ9Æ9, 1bÆ9Æ9Æ9, 1bÆ9Æ9Æ9Æ9 and
1bÆ9Æ9Æ9Æ9Æ9.11 We are continuing the characterization of
this complex by other methods and will report these re-
sults in the future (Scheme 3).
1. (a) Davis, M. E. Nature 2002, 417, 813–821; (b) Seidel, S.
R.; Stang, P. J. Acc. Chem. Res. 2002, 35, 972–983; (c)
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In summary, extended ligand molecule 1b with hexago-
nal symmetry was synthesized by cobalt-catalyzed cyclo-
trimerization reaction and the structure was
3. Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457–2483.
4. Berresheim, A. J.; Muller, M.; Mullen, K. Chem. Rev.
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¨
¨
1
characterized by H NMR and MALDI-TOF MS anal-
5. Detailed experimental procedures and spectroscopic data
for all new compounds are shown in the Supplementary
data.
6. (a) Ishikawa, T.; Murata, M.; Miyaura, N. J. Org. Chem.
1995, 60, 7508–7510; (b) Zhu, L.; Duquette, J.; Zhang, M.
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yses. Initial studies on the Sn–porphyrin complexes may
suggest the novel method to construct porphyrin arrays.
Further details of characterization of the complexes as
well as self-assembly of 1 will be reported elsewhere
together with results on other systems.
7. (a) Keegstra, E. M. D.; Huisman, B.-H.; Paardekooper, E.
M.; Hoogesteger, F. J.; Zwikker, J. W.; Jenneskens, L. W.;
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H.; Ickenroth, D.; Stalmach, U.; Koynov, K.; Bahtiar,
A.; Bubeck, C. Eur. J. Org. Chem. 2001, 4431–4443.
8. Trial to synthesize arylboronate 7a (structure was not
shown), which was aimed to conduct Suzuki coupling
between 2 and arylboronate 7a, and similar reaction has
done to obtain 4, has been found to result in a debromin-
ation of 7a quantitatively.
9. Selected spectroscopic data for 1b0: 1H NMR (CDCl3,
300 MHz) d 8.03 (d, J = 8.3 Hz, 12H), 7.59 (d, J = 8.3 Hz,
12H), 7.20 (d, J = 8.2 Hz, 12H), 7.02 (d, J = 8.2 Hz, 12H),
6.87 (s, 6H), 6.81 (s, 6H), 3.92 (s, 18H), 3.79 (t, J = 6.2 Hz,
12H), 3.61 (t, J = 6.2 Hz, 12H), 1.48–1.09 (m, 240H), 0.92–
0.84(m, 18H); MALDI-TOF MS (no matrix) m/z calcd
for MH++3 (13C), C270H378O24: 4008.9; found 4009.0. For
1b: 1H NMR (pyridine-d5, 300 MHz) d 8.57 (d, J = 8.0 Hz,
12H), 7.98 (d, J = 8.0 Hz, 12H), 7.76 (d, J = 7.9 Hz, 12H),
7.64(d, J = 7.9 Hz, 12H), 7.26 (s, 6H), 7.13 (s, 6H), 3.91 (t,
J = 6.3 Hz, 12H), 3.84(t, J = 6.3 Hz, 12H), 1.78–1.63
Scheme 3. Reference molecule 9 and Sn(IV) porphyrin 10.

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M. Takase et al. / Tetrahedron Letters 46 (2005) 1739–1742
(m, 24H), 1.52–1.18 (m, 216H), 0.98–0.90 (m, 18H), 0.90–
1 2001, 2294–2302; (c) Hawley, J. C.; Bampos, N.;
Abraham, R. J.; Sanders, J. K. M. Chem. Commun.
1998, 661–662.
0.82 (m, 18H); MALDI-TOF MS (2,5-dihydroxybenzoic
acid matrix) m/z calcd for MH++3 (13C), C264H367O24:
3924.8; found 3924.9.
11. The molar ratio of 9/1b was estimated from UV–vis
spectrum in THF, to be approximately 1.2, as judged from
the average of molar absorbance coefficients of 1b and 9 at
285 and 335 nm.
10. (a) Arnold, D. P.; Blok, J. Coord. Chem. Rev. 2004, 248,
299–319, and references cited therein; (b) Crossley, M.;
Thordarson, P.; Wu, R. A.-S. J. Chem. Soc., Perkin Trans.