Concise synthesis and crystal structure of [12]cycloparaphenylene

Article abstract of DOI:10.1002/anie.201007232

Crystal clear: The title macrocycle was constructed by a nickel-mediated shotgun macrocyclization. The X-ray crystallographic structure of [12]CPP revealed a circular structure incorporating two cyclohexane molecules within the ring. The [12]CPP molecules

Full text of DOI:10.1002/anie.201007232

Communications
DOI: 10.1002/anie.201007232
Carbon Nanorings
Concise Synthesis and Crystal Structure of [12]Cycloparaphenylene**
ˇ
Yasutomo Segawa, Shinpei Miyamoto, Haruka Omachi, Sanae Matsuura, Petr Senel,
Takahiro Sasamori, Norihiro Tokitoh, and Kenichiro Itami*
Cycloparaphenylene (CPP) represents the shortest sidewall
segment of armchair carbon nanotube structures, and thus has
greatly captivated scientists (Scheme 1).[1–3] Even though the
Scheme 1. Unsolved issues in CPP chemistry.
bottom-up chemical synthesis of this simple molecular entity
had been recognized as a Holy Grail in synthetic chemistry,[1,4]
three groups including our own have recently succeeded in
synthesizing some [n]CPPs.[5–8] Although these studies from
the three research groups established the synthetic viability of
the long-awaited CPPs, important issues remain unresolved
(Scheme 1). For example, any synthetic route must be more
concise, cost-effective, and scalable to provide CPP in useful
quantities and to ensure that this interesting molecular entity
is studied further. In addition, the molecular structure of CPP
must be concretely validated by X-ray crystallographic
analysis. We herein report a concise nickel-based synthesis
of [12]CPP and the first X-ray crystal structure of [12]CPP.
Some of the key features of the previous methods of
making CPPs are summarized in Scheme 2. Both the group of
Bertozzi[5] and ours[6] utilized the palladium-catalyzed
Scheme 2. Key features of the reported synthetic methods of CPPs.
Suzuki–Miyaura coupling of terphenyl-convertible bent
monomers[9] to access the macrocycles, which were converted
[*] Dr. Y. Segawa, Dr. S. Miyamoto, H. Omachi, S. Matsuura,
ˇ
Dr. P. Senel, Prof. Dr. K. Itami
into CPPs by aromatization reactions.[10] Bertozzi and co-
workers[5] took a pseudo “shotgun” approach,[11] which
involves using a one-pot multicomponent cyclization of
small monomers, to access the macrocycle. This shotgun
method however often suffers from low yield and lack of
control over the ring structure.[11] Indeed, three different
macrocycles were obtained in their synthesis.[5] Our group[6]
took a stepwise approach[11] in the palladium-catalyzed cross-
coupling reaction and only one intermediate macrocycle was
synthesized en route to [12]CPP. More recently, we have
reported a new strategy using cyclohexane-inserted U-shaped
units for the modular and size-selective synthesis of [n]CPPs
(demonstrated for n = 14–16).[7] The use of the cyclohexane
unit is important in our CPP synthesis not only because the L-
shaped structure of cyclohexane attenuates the build-up of
strain energy during the macrocyclization but also because
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
Prof. Dr. T. Sasamori, Prof. Dr. N. Tokitoh
Institute for Chemical Research, Kyoto University, Uji
Kyoto 611-0011 (Japan)
[**] This work was supported by a Grant-in-Aid for Scientific Research
from MEXT and JSPS. H.O. thanks the JSPS for predoctoral
fellowship. S.M. and P.S. thank the MEXT Project of Integrated
Research on Chemical Synthesis for postdoctoral fellowships. The
computations were performed at the Research Center for Compu-
tational Science, Okazaki (Japan). We thank Prof. Yasuhiro Ohki for
useful discussions and technical assistance.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007232.
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the ring-flipping aptitude of cyclohexane provides a strategic
basis for accessing both even- and odd-numbered [n]CPPs.
The disadvantages of our palladium-based synthetic methods
include the necessity to prepare two cross-coupling partners
and the requirement for stepwise reactions to access the
macrocyclic precursors for [n]CPPs.[6,7] Yamago et al.
reported the synthesized [8]CPP through the application of
Bꢀuerle’s cycloarene synthesis[12] that uses platinum-contain-
ing macrocycles.[8] Even though this procedure allowed the
synthesis of the smallest CPP to date, the stoichiometric use of
expensive platinum complexes is a bane for large-scale
synthesis.[10]
Although we were aware of a number of unsuccessful
examples of the shotgun approach in the synthesis of shape-
persistent macrocycles,[11] we dared to investigate a one-pot
shotgun macrocyclization using the simplest diphenylcyclo-
hexane monomer as a key step for a more concise CPP
synthesis (Scheme 2). In particular, we postulated that a
nickel-promoted one-pot macrocyclization of the L-shaped
cis-1,4-bis(4-halophenyl)cyclohexane monomer (1a or 1b)
might proceed to give the box-shaped macrocycle 2 directly
(Scheme 3A).[13] We expected that this shotgun approach to
macrocycle construction using a single monomer and an
inexpensive nickel promoter would streamline the synthesis
of [12]CPP significantly. Notably, however, no nickel-medi-
ated homocoupling reaction has been utilized in the synthesis
of relatively large macrocycles, and this is presumably
because of the mechanistic complexity[13] in aryl–aryl bond
formation.[14] Nevertheless, we were encouraged by the X-ray
crystal structure of 1a (Scheme 3B),[15] which revealed its
nearly ideal “included” angle (ca. 808, see the Supporting
Information).
The requisite L-shaped monomers (1a and 1b) were
prepared by a two-step reaction from 1,4-dihalobenzene
(Scheme 3A). The twofold addition of 4-halophenylcerium
reagents, prepared from 1,4-dihalobenzene, nBuLi, CeCl3,
and LiCl, to cyclohexane-1,4-dione furnished cis-1,4-bis(4-
halophenyl)cyclohexane-1,4-diol in high yields. The forma-
tion of both the monoaddition product and the unfavorable
trans isomer was suppressed under these cerium-based car-
bonyl addition conditions.[16,17] The resultant diols were
protected with MOM groups to give 1a and 1b in 74% and
81% overall yields, respectively, from 1,4-dihalobenzene. By
using this method, we synthesized in total 50 g of the L-
shaped monomers (1).
With the requisite L-shaped monomers 1 in large quanti-
ties, we next investigated the nickel-mediated shotgun macro-
cyclization of 1 to produce the box-shaped macrocycle 2 in
one step. We began by employing the original procedure of
Semmelhack et al. using [Ni(cod)2] as a promoter.[18] How-
Scheme 3. A) Concise synthesis of [12]CPP. Reaction conditions: a) 1. 1,4-X2C6H4 (X=I or Br), nBuLi, THF, 2. CeCl3, LiCl, 3. cyclohexane-1,4-dione,
4. CH3OCH2Cl, iPr2NEt, CH2Cl2; b) [Ni(cod)2], bpy, (with or without cod), THF; c) NaHSO4·H2O, m-xylene/DMSO, under air. B) X-ray crystal structures of
1a, 2·2CHCl3, and [12]CPP·2cyclohexane. Thermal ellipsoids are shown at 50% probability and hydrogen atoms and minor parts of disordered moiety are
omitted for clarity. bpy=2,2’-bipyridyl, cod=1,5-cyclooctadiene, DMSO=dimethyl sulfoxide, MOM=methoxymethyl, THF=tetrahydrofuran.
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ever, the desired macrocycle 2 was not obtained under these
reaction conditions. Similarly, procedures using NiCl2, triphe-
nylphosphine, and zinc dust did not furnish 2 at all.[19] After
extensive investigations (see the Supporting Information), we
determined that 1a (1.0 equiv) underwent a shotgun macro-
cyclization in the presence of [Ni(cod)2] (2.0 equiv), cod
(1.7 equiv), and 2,2’-bipyridyl[20] (0.5 equiv) in THF (16 mmol
of 1a) at 608C and 2 was isolated in a 27% yield. For the
macrocyclization of 1b we found that 2 could be obtained in a
22% yield when the additional cod was excluded from the
reaction. Gratifyingly, these reaction conditions were ame-
nable to relatively large-scale reactions (see the Supporting
Information). Notably, we synthesized, in total, 5 g of 2 using
these methods. Although the yield could be improved, this
nickel-based shotgun method already outperforms our palla-
dium-based stepwise method in terms of overall yield and
cost.[10]
Scheme 4. Equilibrium of two different conformations of 2.
Finally, we investigated the aromatization of the fully
MOM-protected macrocycle 2. We previously reported that
the treatment of a partially MOM-protected macrocycle with
p-toluenesulfonic acid under microwave irradiation afforded
[12]CPP.[6] During our subsequent work on the modular
synthesis of [n]CPPs, we established non-microwave reaction
conditions for converting the fully MOM-protected macro-
cycles into the corresponding CPPs. The latter method
worked nicely in the present synthesis of [12]CPP. Treatment
of macrocycle 2 with NaHSO4·H2O (20 equiv) in refluxing m-
xylene/DMSO under air gave [12]CPP in a 65% yield. By
using this method, we synthesized in total 0.5 g of [12]CPP
(Scheme 3A).
In addition to improving the synthesis of [12]CPP, we also
strove to validate the molecular structures of [12]CPP and the
macrocyclic precursor 2 as well as to advance our under-
standing of them by using X-ray crystallography
(Scheme 3B). A single crystal of 2 was obtained from a
chloroform solution by the slow addition of n-hexane vapor at
258C. As shown in Scheme 3B, a squarelike D2 symmetric
structure of 2 that contains disordered chloroform molecules
in the cavity was identified. Interestingly, this squarelike
shape of 2 is different from the more rectangular structure
that we previously identified in the partially MOM-protected
macrocycle (similar to the right-hand structure shown in
Scheme 4).[6]
In solution, two conformations (2-squ and 2-rec,
Scheme 4) are likely to be in a rapid equilibrium through
the flipping of cyclohexane rings.[21] Indeed, the hydrogen
atoms on the MOM groups in 2 were observed as two singlet
Figure 1. Experimental 1H NMR spectra of 2 (500 MHz, CDCl3) at
different temperatures (left) and simulated spectra computed with the
specific rate constant, k (right).
1
peaks (d = 3.43 ppm, 4.46 ppm in CDCl3) by H NMR spec-
troscopy at 208C. Upon cooling to À308C, splitting of these
singlet peaks was observed.[22] We estimated representative
thermodynamic values of the chair-flipping process in 2 to be
DH° = 11.4 kcalmolÀ1, DS° = À3.4 calmolÀ1 KÀ1 by perform-
ing variable-temperature NMR experiments on the CDCl3
solution of 2 using the coalescence method (Figure 1; see the
Supporting Information). Interestingly, these values are
comparable to that of the chair-flip barrier of cyclohexane
(DH° = 11 kcalmolÀ1).[23] Notably a cyclohexane ring can
undergo rapid chair-flipping even within the macrocyclic
structure.
The structure determination of the CPP molecules by X-
ray crystal structure analysis remains a great challenge
because of some of their intrinsic properties, such as their
high solubility in most organic solvents and their tendency to
incorporate guest molecules within the ring. After extensive
investigation, we succeeded in obtaining the first X-ray
crystallographic structure of [12]CPP. Recrystallization of
[12]CPP from chloroform/cyclohexane gave pale yellow
crystals of [12]CPP·2cyclohexane.
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The ORTEP drawing is shown in Scheme 3B. Two
cyclohexane molecules were found to be highly disordered,
thus indicating that they might drift into the cavity of [12]CPP
even in the crystalline state. In the crystalline form, the center
of symmetry of [12]CPP is at the center of the ring. The
diameter of this ring is approximately 16.5 ꢁ as estimated by
taking the average of the intramolecular atom distances
structure of [12]CPP. From these structural data, the benze-
noid structure of [12]CPP has been firmly validated.
The packing mode of [12]CPP molecules in the crystal is
also interesting. As shown in Figure 2A, [12]CPP molecules
align in a herringbone manner. This type of packing has been
observed in a number of planar p-conjugated molecules. It
should be mentioned that there are no significant intermo-
lecular interactions between [12]CPP molecules. More strik-
ingly, in addition to the herringbone packing, the [12]CPP
molecules align nicely to form a tubular structure (Fig-
ure 2B). The emergence of such a surprising channel struc-
ture obviously provides various possibilities in CPP chemistry.
For example, the preorganized tubular alignment of CPP
molecules might pave the way for the formation of carbon
nanotube structures by oxidative assembly of CPP molecules
in their crystalline state. In addition, CPP molecules might
incorporate guest molecules or metal ions thus realizing their
one-dimensional alignment.
À
between two diagonal ipso carbon atoms (e.g. C(1) C(1)*,
see the Supporting Information). The small degree of
variation between the twelve measured atom distances
(16.24–16.84 ꢁ) supports the high circularity of [12]CPP.
Notably, the observed conformation is not the most stable D6d
symmetrical form that was calculated by DFT methods, but is
close to the D3d structure, the energy of which is 3.7 kcalmolÀ1
higher than that of D6d form (Scheme 5).[2a] Crystal-packing
forces are most likely responsible for the emergence of D3d-
like structure.
In summary, we have established a concise and cost-
effective synthesis of [12]CPP. A shotgun method for the
macrocycle construction using a single L-shaped diphenyl-
cyclohexane monomer and an inexpensive nickel complex
was developed. During this study, we successfully validated
the molecular structures of L-shaped monomer 1a, box-
shaped macrocycle 2, and [12]CPP by X-ray crystallography.
The benzenoid structure of [12]CPP has been firmly validated
by studying the bond lengths. Most excitingly, it was found
that [12]CPP molecules crystallize in a tubular and a
herringbone structure. Further studies on the properties of
[12]CPP will be outlined in due course.
Scheme 5. Two optimized structures of [12]CPP.
Although one of the benzene rings in half of [12]CPP was
À
found to be disordered as a result of the rotation of the C C
axis, the disorder was appropriately solved and the intrinsic
structural parameters can be discussed using these crystallo-
graphic data.[15] Summarized in Table 1 are the average bond
lengths of [12]CPP in the crystalline state. The results show
À
that within the range of experimental error the Cipso Cortho and
À
Cortho Cortho bond lengths are almost the same, that is
À
approximately 1.39 ꢁ, and the Cipso Cipso bonds exhibit
single-bond character, suggesting no considerable bond
alternation in the benzene rings of [12]CPP. Thus, it should
be emphasized that the benzenoid character is reasonably
preserved in [12]CPP, even though each benzene ring in the
crystal structure of [12]CPP is slightly folded as a result of the
inherent ring strain of the cyclic system. We also found that
these bond lengths are similar to those of the optimized D3d
Table 1: Average bond lengths of [12]CPP in crystalline state and the
optimized D3d structure of [12]CPP.
[a]
X-ray
D3d
À
Cipso Cipso
1.481 ꢀ
1.394 ꢀ
1.381 ꢀ
1.485 ꢀ
1.407 ꢀ
1.391 ꢀ
[b]
À
Cipso Cortho
[b]
À
Cortho Cortho
Figure 2. A) Herringbone packing of [12]CPP molecules. B) Tubular
alignment of [12]CPP molecules (hydrogen atoms and solvent mole-
cules are omitted for clarity).
[a] B3LYP/6-31G(d) level of theory. See, reference [2a]. [b] Bond lengths
including disordered Cortho atoms are not included.
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Communications
[10] The prices (per mol) of representative transition-metal com-
plexes and ligands used for CPP synthesis (Strem Chemicals,
Inc.) are as follows: Bertozziꢄs method: [Pd(PPh3)4] ($19,413).
Our previous method: [PdCl2(dppf)] ($12,021), [Pd(OAc)2]
($6,089), X-Phos ($13,730). Yamagoꢄs method: [PtCl2(cod)]
($40,752), dppf ($7,096). Present method: [Ni(cod)2] ($6,381),
2,2’-bipyridyl ($276).
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Received: November 17, 2010
Published online: March 2, 2011
Keywords: cycloparaphenylenes · macrocycles · nanotubes ·
.
nickel · x-ray crystal structures
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(DG8 = 0.7 kcalmolÀ1). See the Supporting Information for
details.
[22] The appearance of split peaks might be assigned to the hydrogen
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Chem. Phys. 1964, 41, 2041, and references therein.
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