Synthesis and dimerization of chloro[10]cycloparaphenylene: A directly connected cycloparaphenylene dimer

Article abstract of DOI:10.1021/ol500643c

The first synthesis and dimerization of monochlorinated [10]cycloparaphenylene (chloro[10]CPP) are described. By assembling a chlorinated 1,4-diborylbenzene unit with brominated and borylated cis-1,4-diphenylcyclohexane units by Suzuki-Miyaura coupling, a triangle-shaped chloro-containing macrocycle was synthesized. The acid-mediated cyclohexane-to-benzene aromatization afforded chloro[10]CPP without losing the chloro group. By the action of Ni(0) complex, chloro[10]CPP was converted to a directly connected [10]CPP dimer, which would be an ideal precursor for the carbon nanobelt .

Full text of DOI:10.1021/ol500643c

Letter
pubs.acs.org/OrgLett
Synthesis and Dimerization of Chloro[10]cycloparaphenylene: A
Directly Connected Cycloparaphenylene Dimer
Yuuki Ishii,† Sanae Matsuura,† Yasutomo Segawa,†,‡ and Kenichiro Itami*,†,‡,§
‡
§
†Graduate School of Science, JST ERATO, Itami Molecular Nanocarbon Project, and Institute of Transformative Bio-Molecules
(WPI-ITbM)Nagoya University, Chikusa, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: The first synthesis and dimerization of
monochlorinated [10]cycloparaphenylene (chloro[10]CPP)
are described. By assembling a chlorinated 1,4-diborylbenzene
unit with brominated and borylated cis-1,4-diphenylcyclohex-
ane units by Suzuki−Miyaura coupling, a triangle-shaped
chloro-containing macrocycle was synthesized. The acid-
mediated “cyclohexane-to-benzene” aromatization afforded
chloro[10]CPP without losing the chloro group. By the action
of Ni(0) complex, chloro[10]CPP was converted to a directly
connected [10]CPP dimer, which would be an ideal precursor for the “carbon nanobelt”.
ottom-up synthesis of structurally uniform carbon nano-
tube (CNT) structures is one of the grand challenges in
CPPs enables us to synthesize various CPP-based structures,
thereby bestowing diverse properties and functions. We herein
report the synthesis of monochloro[10]CPP and its dimeriza-
tion reaction to furnish a directly connected [10]CPP dimer for
the first time. Because functionalization reactions from C−Cl to
C−C or other bonds are already known,13 chloroCPP can be a
suitable platform for a variety of functional CPPs.
Since 2009, our group has accomplished the size-selective
synthesis of [7]−[16]CPP by utilizing sequential coupling
reactions of L-shaped units (cis-1,4-diarylcyclohexane) and
linear units (1,4-diborylbenzene or 4,4′-diborylbipheyl) as
shown in Figure 2(a). The C−H bonds of CPP are all equal
so that selective functionalization of CPP is very difficult. It can
be easily predicted that halogenation of CPP furnishes the
mixture of haloCPPs, and moreover, isolation of each haloCPP
is almost impossible. To avoid this problem, we decided to
synthesize chloroCPP from L-shaped units and a chloro-
containing linear unit (1,4-diboryl-2-chlorobenzene) (Figure
2b).
B
both nanocarbon science and synthetic organic chemistry.
Although there have been a number of attempts for the
synthesis of ring- and belt-shaped π-conjugated hydrocarbons
related to CNT structures,1 only very short structures have
been synthesized. Cycloparaphenylene (CPP),2 a molecule
consisting solely of benzene rings with para linkages to each
other, represents the shortest segment of armchair CNT
(Figure 1). Thus, scientists targeted CPP and considered it an
The established synthetic route of chloro[10]CPP is shown
in Scheme 1. As we already reported,4e C-shaped compound 3
can be prepared from two L-shaped units 1 and 2 via a Suzuki−
Miyaura cross-coupling reaction. The [9 + 1]-type cyclization,
which we used for the selective synthesis of [10]CPP,4e was
applied to form the cyclic precursor for chloro[10]CPP (6).
The chloro-containing linear unit (5) was synthesized by
palladium-catalyzed borylation of bromoarene14 from 1,4-
dibromo-2-chlorobenzene (4). Original conditions for our [9
+ 1]-type cyclization (Pd(OAc)2, 2-Cy2P-2′,4′,6′-iPr3biphenyl,
NaOH, 1,4-dioxane/water, 80 °C) were too reactive so that the
chloro group in 4 was also reacted during the cross-coupling.
Figure 1. Structures of [n]CPP and (n,n) armchair CNT.
ideal building block of CNT structures. Since 2008, Bertozzi
and Jasti,3 Itami,4 Jasti,5 and Yamago6 have successfully
synthesized [n]CPPs (n = 5−16, 18), where n is the number
of benzene rings. Structural analysis,7 photophysical proper-
ties,8 and guest-incorporation abilities9 of [n]CPPs have been
investigated. Very recently, our group has achieved a diameter-
controlled CNT synthesis using CPPs as templates.10
Despite this recent progress in CPP chemistry, however,
functionalization of CPPs and synthesis of functionalized CPPs
are still limited. Only arylated CPPs, polyaryl[n]CPPs,11 and
1,4-phenylene- or 1,5-naphthylene-bridged [8]CPP dimers12
have been reported. Introduction of a “foothold” group to
Received: March 2, 2014
Published: April 1, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
3
4
4
Hz), 7.10 (dd, JHH = 8 Hz, JHH = 2 Hz), 7.78 (d, JHH = 2
Hz)] are assigned to three hydrogen atoms of the 2-chloro-1,4-
phenylene moiety. To demonstrate further functionalization
starting from chloro[10]CPP (7), we selected the dimerization
reaction. The Yamamoto-type nickel(0)-mediated homocou-
pling reaction of 7 took place smoothly to afford [10]CPP
dimer (8) in 37% yield. The NMR spectra and HRMS of 8 fully
supported the formation of [10]CPP dimer. This is the first
report on the synthesis of directly connected CPP dimer.15
Structural analysis of 8 was performed by DFT calculations.
The B3LYP method with 6-31G(d) basis set was used for all
calculations. The several thus found local minima structures of
8 can be categorized into two types: close and open as shown in
Figure 3.16 The most stable conformation of 8 belongs to the
Figure 2. Possible synthetic routes toward halogenated CPPs.
Scheme 1. Synthesis of Chloro[10]CPP (7) and [10]CPP
a
Dimer 8
Figure 3. Conformations of 8 and relative Gibbs free energies (kcal·
mol−1).
close type, whereas all open conformations have 5.1 kcal·mol−1
or higher Gibbs free energies (ΔG). Compound 8 can switch
between the close and open conformations through a transition
state (TS) where one of the [10]CPP rings revolves around the
connecting benzene rings. The lowest TS has 8.9 kcal·mol−1
higher energy compared to the lowest close conformation. This
computational study indicates that 8 forms close conformation
mainly with rapid isomerization between close and open
conformations.
Absorption and fluorescence spectra of 7 and 8 are shown in
Figure 4. Compared to those of [10]CPP, no significant
difference was found in both spectra while small red-shift and
a
R = CH2OMe, Bpin = 4,4,5,5-Me4-1,3,2-dioxaborolanyl, dppf = 1,1′-
(Ph2P)2ferrocene, cod = 1,5-cyclooctadiene.
We found mild conditions (PdCl2(PPh3)2, KOH, THF, reflux,
24 h) retained the chloro group during the cyclization. Under
these conditions, triangle-shaped chloro-containing macrocycle
6 was obtained in 41% yield. The macrocycle 6 was then
subjected to the acid-mediated aromatization reaction to afford
chloro[10]CPP (7) without dechlorination. Compound 7 was
1
identified by H and 13C NMR spectroscopies and HRMS.
Figure 4. Absorption (solid curves) and fluorescence (broken curves)
spectra of [10]CPP, 7, and 8.
1
3
Three characteristic H NMR signals [δ = 6.98 (d, JHH = 8
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Organic Letters
Letter
(3) Jasti, R.; Bhattacharjee, J.; Neaton, J. B.; Bertozzi, C. R. J. Am.
Chem. Soc. 2008, 130, 17646.
small blue-shift can be seen in the absorption of 8 and
fluorescence of 7, respectively. The fluorescence color of
[10]CPP, pale blue, is also the same as those of 7 and 8. The
result indicates that chloro group and dimerization have little
influence on the frontier orbitals of [10]CPP.
(4) (a) Takaba, H.; Omachi, H.; Yamamoto, Y.; Bouffard, J.; Itami, K.
Angew. Chem., Int. Ed. 2009, 48, 6112. (b) Omachi, H.; Matsuura, S.;
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̌
(c) Segawa, Y.; Miyamoto, S.; Omachi, H.; Matsuura, S.; Senel, P.;
In summary, we have achieved the selective synthesis of
chloro[10]CPP (7) in six steps from commercially available
materials. This is the first example of halogenated CPP.
Conditions of the Suzuki−Miyaura cross-coupling reaction
were modified to retain the chloro group during the
macrocyclization. Nickel-mediated C−Cl/C−Cl homocoupling
of 7 took place to obtain the [10]CPP dimer (8) as the first
example of the directly connected CPP dimer. Furthermore, 8
would be one of the most promising precursors for carbon
nanotube structure. Given that 19 C−C bonds between two
[10]CPPs of 8 are all connected (Scheme 2), a very short
carbon nanotube structures (9), which we name “carbon
nanobelt”, can be obtained. Conversion of 8 to 9 is ongoing in
our group.
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Scheme 2. Toward Carbon Nanobelt 9 via [10]CPP Dimer
(8)
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, characterization data for all new
compounds, and details of computational study. This material is
available free of charge via the Internet at http://pubs.acs.org.
Wagner, M.; Nishiuchi, T.; Mullen, K. Angew. Chem., Int. Ed. 2014, 53,
̈
■
1525.
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(b) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176.
(c) Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L. J. Am.
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AUTHOR INFORMATION
Corresponding Author
■
(14) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995, 60,
7508.
(15) The structure of a directly connected [8]CPP dimer was
optimized by DFT calculations. See ref 12.
(16) The three structures shown in Figure 3 are modified from
optimized structures for clarity. See the Supporting Information for the
original optimized structures.
*E-mail: itami@chem.nagoya-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the ERATO program from JST
(K.I.) and the Funding Program for Next Generation World-
Leading Researchers from JSPS (K.I.). Calculations were
performed using the resources of the Research Center for
Computational Science, Okazaki, Japan. ITbM is supported by
the World Premier International Research Center (WPI)
Initiative, Japan.
REFERENCES
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