Selective synthesis of [12]cyeloparaphenylene

Full text of DOI:10.1002/anie.200902617

Communications
DOI: 10.1002/anie.200902617
Aromatic Rings
Selective Synthesis of [12]Cycloparaphenylene**
Hiroko Takaba, Haruka Omachi, Yosuke Yamamoto, Jean Bouffard, and Kenichiro Itami*
In memory of Professor Kazuhiko Seki
Shape-persistent, nanosized macrocycles that are composed
of only sp- and sp2-hydridized carbon atoms on their
perimeter (conjugated molecular loops and belts) have
attracted significant attention because of their potential
applications in materials science and supramolecular chemis-
try.[1,2] Particularly interesting and challenging among these
are aromatic belts[1] and rings as exemplified by the Vꢀgtle
belts,[3,4] cyclophenacenes,[5,6] cycloparaphenylenes,[7–9] and
cyclacenes [10,11]. Adding to their sheer aesthetic appeal,[1]
these unusual hydrocarbons represent structural models for
carbon nanotube segments, and can be envisioned as potential
precursors in the preparation of structurally uniform armchair
or zigzag carbon nanotubes (Scheme 1).[12,13] However,
despite extensive trials the synthesis of these molecules
remains a formidable challenge.[1]
In 2005, as a part of our research program exploring new
synthetic methods and properties of oligoarenes[14] and nano-
carbons,[15] we initiated a synthetic study of these aromatic
belts/rings that aims at contributing to a bottom-up organic
synthesis of structurally uniform single-walled carbon nano-
tubes. We selected cycloparaphenylene[7–9] as a first target in
view of a comparatively straightforward approach to their
synthesis through aryl–aryl bond formation. Despite its
structural simplicity, no successful synthesis had been
reported at the inception of our work. Very recently, Bertozzi
and co-workers have accomplished the elegant first synthesis
of [9]-, [12]-, and [18]cycloparaphenylenes and coined them as
“carbon nanohoops”.[9] Herein, we report a selective synthesis
of [12]cycloparaphenylene (1) through stepwise palladium-
catalyzed coupling reactions.
Scheme 1. Aromatic belts/rings related to the carbon nanotube struc-
ture.
[*] H. Takaba, H. Omachi, Y. Yamamoto, Dr. J. Bouffard,
Prof. Dr. K. Itami
Department of Chemistry, Graduate School of Science
Nagoya University, Chikusa, Nagoya 464-8602 (Japan)
Fax: (+81)52-788-6098
E-mail: itami@chem.nagoya-u.ac.jp
Homepage: http://synth.chem.nagoya-u.ac.jp
[**] This work was supported by the PRESTO program of the Japan
Science and Technology Agency (JST), and a Grant-in-Aid for
Scientific Research from MEXTand JSPS. J.B. is a recipient of a JSPS
Postdoctoral Fellowship. We thank Dr. Kin-ichi Oyama (Nagoya
University) for assistance in MALDI-TOF MS analysis, Prof.
Yasuhiro Ohki and Prof. Kazuyuki Tatsumi (Nagoya University) for
assistance in X-ray crystal structure analysis, and Prof. Atsushi
Wakamiya and Prof. Shigehiro Yamaguchi (Nagoya University) for
useful discussion and technical assistance. Last but not least, we
thank the late Kazuhiko Seki (Nagoya University) for his encour-
agement.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902617.
Scheme 2. Strategy for the synthesis of [12]cycloparaphenylene (1).
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The challenge for synthesizing cycloparaphenylenes lies in
the increased strain energy that results from ring closure.[16]
For example, the strain energy for aryl–aryl bond-forming
cyclization at the two ends of -(p-C6H4)12- to furnish [12]cyclo-
paraphenylene (1) is calculated to be 55 kcalmolꢀ1.[17] Thus,
polymerization is, as expected, the observed outcome of these
reactions (Scheme 2). Inspired by the insightful works of
Vꢀgtle and co-workers,[7b] we hypothesized that a suitably
substituted cyclohexane moiety could function as a bent
benzene-convertible unit in the synthesis of [12]cyclopara-
phenylene (1). By inserting four cis-cyclohexane-1,4-diyl
units, the cyclization of B to give C becomes synthetically
viable (calculated strain energy = 5 kcalmolꢀ1).[17] The trans-
formation of cyclohexane moieties of C to give benzene rings
(aromatization) would then provide 1, but the balance
between aromatic stabilization energy and macrocyclic
strain energy would be critically important in this final step.
The important elements in this synthetic strategy are:
accessed? 2) What could be a suitable cyclohexane unit?
Our approach to satisfying this first issue has been to connect
four 1,4-diphenylcyclohexane monomers by palladium-cata-
lyzed Suzuki–Miyaura cross-coupling reactions.[18] As for the
suitable cyclohexane moiety, we chose cis-1,4-dihydroxy-
cyclohexane-1,4-diyl because the corresponding monomers
can be readily made by two-fold carbonyl addition to the
commercially available cyclohexane-1,4-dione. We expected
the transformation of a cis-1,4-dihydroxy-cyclohexane-1,4-
diyl unit to give a benzene ring could be accomplished as a
final step by an acid-mediated eight-fold dehydration and
subsequent oxidation. In the synthesis by Bertozzi and co-
workers, cis-1,4-dimethoxy-2,5-cyclohexadiene-1,4-diyl was
utilized as a similar bent unit, where the final aromatization
step was accomplished with lithium naphthalenide; most
likely through a stepwise electron transfer mechanism.[9]
The selective synthesis of [12]cycloparaphenylene (1)
began with the preparation of 1,4-diphenylcyclohexane
monomers. The two-fold addition of 4-iodophenyllithium,
1) How can
a cyclohexane-inserted macrocycle C be
Scheme 3. Synthetic route toward 1. Conditions and reagents: a) 1. 1,4-diiodobenzene (3.0 equiv), nBuLi (3.0 equiv), THF, –788C, 1h;
2. cyclohexane-1,4-dione (1.0 equiv), RT, 2 h; b) cis-2 (1.0 equiv), CH3OCH2Cl (7.6 equiv), iPr2NEt (7.6 equiv), CH2Cl2, RT, 19h; c) cis-2 (1.0 equiv),
B2(pin)2 (2.4 equiv), [PdCl2(dppf)] (3 mol%), KOAc (6.0 equiv), DMSO, 808C, 13h; d) 3 (10 equiv), 4 (1.0 equiv), [PdCl2(dppf)] (10 mol%), NaOH
(5.0 equiv), H2O (16 equiv), 1,4-dioxane (8 mM with respect to 4), 608C, 24h; e) 6 (1.0 equiv), 4 (1.4 equiv), Pd(OAc)2 (20 mol%), X-Phos
(20 mol%), NaOH (5.0 equiv), H2O (26 equiv), 1,4-dioxane (2 mM with respect to 6), 808C, 24h; f) 7 (1.0 equiv), pTsOH (1.0 equiv), m-xylene,
1508C (microwave), 30min. DMSO=dimethyl sulfoxide, pin=pinacol, THF=tetrahydrofuran.
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prepared from 1,4-diiodobenzene and nBuLi, to cyclohexane-
trimer 6 and monomer 4 was investigated (Table 2). We first
screened several bases with the Pd(OAc)2/PPh3 catalyst in 1,4-
dioxane and identified NaOH/H2O to be a reasonable basic
1,4-dione afforded the common 1,4-diphenylcyclohexane
scaffold (cis-2) in 48% yield (Scheme 3). This common unit
was then converted into the methoxymethyl (MOM)-pro-
tected diiodide 3 (98% yield) and diboronate 4 (81% yield)
by using literature procedures.[19] Through the repetitive
cross-coupling of these units (3 and 4), the cyclic tetramer 7
was synthesized (Scheme 3). The combination of the free
hydroxy and MOM-protected monomers greatly simplified
the purification of the trimer and cyclic tetramer (vide infra).
The MOM group turned out to be beneficial in the mid-stage
synthesis (e.g., the silyl protecting groups are labile under
some cross-coupling reaction conditions).
Table 2: Palladium-catalyzed cross-coupling annulation of 4 and 6.[a]
Entry
Ligand (mol%)
4 [mm]
Yield of 7 [%][d]
1[b]
2
3
PPh3 (30)
5
5
2
0.5
2
15
16
34
17
46
51
With suitable monomers in hand, we first attempted the
synthesis of dimer 5 by the Suzuki–Miyaura coupling of 3 and
4 (Table 1). However, in early experiments we identified that
DavePhos (40)
DavePhos (40)
DavePhos (40)
X-Phos (40)
X-Phos (20)
4
5
6[c]
2
Table 1: Palladium-catalyzed cross-coupling of 3 and 4.[a]
[a] Reaction conditions:
6
(1.0 equiv),
4
(1.2 equiv), Pd(OAc)2
(20 mol%), ligand, NaOH (5 equiv), H2O (26 equiv), 1,4-dioxane,
808C, 24 hours. [b] Pd(OAc)2 (30 mol%). [c] 4 (1.4 equiv). [d] Yield of
isolated product. Cy=cyclohexyl.
Entry Pd catalyst
Base (equiv)
[3]/[4] Results
1[b]
2[c]
3[c]
4[c]
5[c]
6[c]
7[d]
[Pd(PtBu3)2] NaOH (5), H2O (5)
[Pd(PtBu3)2] NaOH (5), H2O (5)
[Pd(PtBu3)2] NaOH (5), H2O (5) 10
[Pd(PtBu3)2] NaOH (5), H2O (16) 10
1
1
5 (18%), 6 (27%)
oligomerization
6 (28%)
6 (43%)
6 (40%)
promoter (Table 2, entry 1). Further screening of phosphane
ligands and concentrations (Table 2, entry 2–5) led to the use
of Buchwaldꢁs X-Phos[22] as the optimal ligand to furnish the
target cyclic tetramer 7 in 51% yield (Table 2, entry 6). The
cyclic structure of 7 was confirmed by the X-ray crystal
structure analysis of a crystal grown from CHCl3/MeOH/n-
hexane (Figure 1).[23] It should be noted that unlike Bertozziꢁs
synthesis, where a mixture of three different macrocycles was
obtained,[9] our procedure provides 7 as the sole cyclic
product.
[Pd(PPh3)4]
NaOH (5), H2O (16) 10
[PdCl2(dppf)] NaOH (5), H2O (16) 10
[PdCl2(dppf)] NaOH (5), H2O (16) 10
6 (67%)
6 (81%)[e]
[a] Reaction conditions: 3, 4, Pd catalyst (10 mol%), NaOH, H2O, 1,4-
dioxane, 608C, 24 hours. [b] [4]=110 mm (in toluene). [c] [4]=5 mm.
[d] [4]=8 mm. [e] 91% of unchanged starting material 3 was recovered
after the reaction.
this is not an easy task. For example, the use of [Pd(PtBu3)2]
catalyst[20] in combination with NaOH/H2O[21] in toluene gave
dimer 5 (18% yield), but non-negligible amounts of trimer 6
(27% yield) were also observed, even when using a 1:1 ratio
of the coupling partners (Table 1, entry 1). Although it was
found that the use of 1,4-dioxane as a solvent facilitated the
cross-coupling reaction, undesired mixtures of oligomers
were formed even under dilute conditions (5 mm; Table 1,
entry 2). At this point, we changed our target to trimer 6.
When an excess of 3 ([3]/[4] = 10) was employed, 6 was
obtained in reasonable yields (Table 1, entries 3 and 4). After
the screening of various Pd catalysts, we found that the use of
Finally, the transformation of cyclic tetramer 7 to give 1
was examined (Scheme 3). We anticipated that the treatment
of 7 with acid would lead to a sequence of 1) removal of the
[PdCl2(dppf)]
(dppf = 1,1’-bis(diphenylphosphanyl)ferro-
cene) led to a very clean 2:1 cross-coupling reaction and
afforded 6 in 81% yield after isolation (Table 1, entry 7).
Notably, 91% of the unchanged starting material 3 (based on
the consumption for trimer 6) was recovered (Table 1,
entry 7).
Next, the synthesis of cyclic tetramer 7 by the cross-
coupling annulation (inter/intramolecular cross-coupling) of
Figure 1. X-ray crystal structure of 7 (n-hexane and methanol mole-
cules in the crystal are omitted for clarity).
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M. Svoboda, M. Przybylski, Chem. Ber. 1994, 127, 767; e) W.
MOM groups, 2) eight-fold dehydration (dihydroxycyclohex-
ane!cyclohexadiene), and 3) oxidation (cyclohexadiene!
benzene) to provide the target [12]cycloparaphenylene (1).
Although we were aware of a number of unsuccessful
examples of acid-mediated aromatization in the synthetic
study of aromatic macrocycles,[10] we were finally able to
develop a satisfactory protocol for this transformation. Thus,
treatment of 7 with a stoichiometric amount of p-toluenesul-
fonic acid (pTsOH) in m-xylene under microwave irradiation
(1508C, 30 min) afforded 1 in 62% yield after isolation.[24] The
reaction when performed at a lower temperature (1008C) did
not give rise to the product. The use of protic solvents led to
complex mixtures of unidentified products. Gratifyingly, the
spectroscopic data of the product isolated from our synthesis
match those of 1 as reported by Bertozzi and co-workers
(1H NMR: d = 7.61 ppm in CDCl3; 13C NMR: d = 127.3,
138.5 ppm in CDCl3; MALDI-TOF MS, 912.37 for C72H48).[9]
In summary, an efficient method that selectively produces
[12]cycloparaphenylene (1) has been established. Our syn-
thesis capitalizes on the ability of cis-1,4-dihydroxycyclohex-
ane-1,4-diyl to attenuate the build-up of strain energy during
the macrocyclization, and exploits its benzene-convertible
nature. Our method, along with that of Bertozzi and co-
workers (utilizing 1,4-dimethoxy-2,5-cyclohexadiene-1,4-diyl
unit), can be categorized as belonging to a family of syntheses
inspired by the seminal work of Vꢀgtle.[7b] However, we stress
that putting these ideas into practice required extensive
optimization of the reaction conditions, even for modern
palladium-catalyzed cross-coupling reactions. Although here
we have focused on the selective synthesis of one specific
cycloparaphenylene, we believe that the strategy of stepwise
assembly would provide a synthetic platform for the prepa-
ration of other [n]cycloparaphenylenes. By carefully selecting
monomers with variable numbers of linear (arene) and bent
(cyclohexane) units in the trimerization and cross-coupling
annulation steps, a range of cycloparaphenylenes of discrete
ring size could be accessed. This synthetic campaign is now
the focus of our ongoing efforts.
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Received: May 16, 2009
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Published online: July 8, 2009
Keywords: aromaticity · cross-coupling · cycloparaphenylenes ·
.
macrocyclization · palladium
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[23] CCDC 738525 (7) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[24] The oxidant that is responsible for producing 1 in the final step is
yet to be identified.
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