Synthesis of a Strained Spherical Carbon Nanocage by Regioselective Alkyne Cyclotrimerization

Article abstract of DOI:10.1002/anie.201903422

The smallest spherical carbon nanocage so far, [2.2.2]carbon nanocage, has been synthesized by the cationic rhodium(I)/H₈-binap complex-catalyzed regioselective intermolecular cyclotrimerization of a cis-1-ethynyl-4-arylcyclohexadiene derivative followed by the triple Suzuki–Miyaura cross-couplings with 1,3,5-triborylbenzene and reductive aromatization. This cage molecule is highly strained, and its ring strain is between those of [6] and [5]cycloparaphenylenes. A significant red-shift of an emission maximum was observed, compared with that of known [4.4.4]carbon nanocage. The sequential cyclotrimerizations of a cis-1,4-diethynylcyclohexadiene derivative with the same rhodium(I) catalyst followed by reductive aromatization failed to afford [1.1.1]carbon nanocage; instead, a β-graph-shaped cage molecule was generated.

Full text of DOI:10.1002/anie.201903422

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Synthesis of A Highly Strained Spherical Carbon Nanocage via
Regioselective Alkyne Cyclotrimerization
Authors: Norihiko Hayase, Juntaro Nogami, Yu Shibata, and Ken
Tanaka
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903422
Angew. Chem. 10.1002/ange.201903422
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http://dx.doi.org/10.1002/ange.201903422

10.1002/anie.201903422
Angewandte Chemie International Edition
COMMUNICATION
Synthesis of a Strained Spherical Carbon Nanocage by
Regioselective Alkyne Cyclotrimerization
Norihiko Hayase,[a] Juntaro Nogami,[a] Yu Shibata,[a] and Ken Tanaka*[a]
Mes
Abstract: The smallest spherical carbon nanocage so far,
[2.2.2]carbon nanocage, has been synthesized via the cationic
rhodium(I)/H8-binap complex-catalyzed regioselective intermolecular
cyclotrimerization of a cis-1-ethynyl-4-arylcyclohexadiene derivative
followed by the triple Suzuki-Miyaura cross-couplings with 1,3,5-
triborylbenzene and reductive aromatization. This cage molecule is
highly strained, and its ring strain is between those of [6] and
n
Mes
Mes
n
n
n = 1–3
[5]cycloparaphenylenes.
maximum was observed, compared with that of known [4.4.4]carbon
nanocage. The sequential cyclotrimerizations of cis-1,4-
A significant red-shift of an emission
Mes
MOMO
a
OMOM
B
diethynylcyclohexadiene derivative with the same rhodium(I) catalyst
followed by reductive aromatization failed to afford [1.1.1]carbon
nanocage; instead, a β-graph-shaped cage molecule was generated.
[6.6.6], [5.5.5], and [4.4.4]
carbon nanocages
[C (n = 3), D (n = 2), E (n = 1):
Itami, 2013, 2014]
multiple couplings
&
Mes
Mes
tripodal-[2]HBC
(F: Du, 2018)
multiple couplings
&
oxidative aromatization
of template B
oxidative aromatization
of template B
The synthesis of highly strained cyclic π-conjugated
molecules, possessing bent benzene rings, has been attracting
much attention as a challenging target in organic synthesis.[1] In
order to achieve this synthesis, overcoming molecular strain is
crucial. In 2008, Jasti reported the elegant synthesis of cyclic
paraphenylene rings, [9], [12], and [18]cycloparaphenylenes
(CPPs), via the multiple coupling reactions followed by reductive
aromatization of cyclohexadiene template A (R = Me).[2] In this
synthesis, the reductive aromatization can overcome the
molecular strain to produce CPPs. Independently, Itami reported
the synthesis of [12]CPP via the multiple coupling reactions
followed by oxidative aromatization of cyclohexane template B.[3]
Subsequently, Yamago reported a different route to CPPs via
the formation of cyclic platinum complexes followed by reductive
elimination.[4] Following these pioneering works, [6]–[16] and
[18]CPPs were synthesized until 2012.[5,6] Finally, more strained
[5]CPP was synthesized by Jasti in 2014.[7] This synthesis
consists of the intramolecular boronate homocoupling and
stepwise reductive aromatization of template A (R = Me) by
treatment with sodium naphthalenide, methanol, and lithium
diisopropyl amide. Independently, Yamago also reported the
synthesis of [5]CPP, which employs the SnCl2/HCl-mediated
reductive aromatization of template A (R = H).[8] This molecule is
a fragment of C60, and its calculated strain energy is significantly
larger (119 kcal•mol–1) than that of [6]CPP (96 kcal•mol–1).[7,8]
RO
OR
A
[2.2.2]carbon nanocage
(1: this work)
cyclotrimerization
&
coupling
&
reductive aromatization
of template A (R = Me)
ball-shaped molecule
(G: Yamago, 2013)
complexation
&
reductive elimination
Figure 1. Spherical carbon nanocages C–G and 1. MOM = methoxymethyl.
Mes = mesityl.
Following the successful synthesis of CPPs, three-
dimensional spherical carbon nanocages have been synthesized.
In 2013, Itami reported the synthesis of cage C, possessing two
trisubstituted benzene units, via the multiple coupling reactions
followed by oxidative aromatization of template B (Figure 1, top
left).[9a] The cage C is a junction unit of branched carbon
nanotubes and named as [6.6.6]carbon nanocage, in which “6”
is the number of benzene rings between the two trisubstituted
benzenes. High conformational flexibility of template B allowed
the size-selective synthesis of [6.6.6], [5.5.5], and [4.4.4]carbon
nanocages C–E,[9b] and additionally large cage F, possessing
two hexabenzocoronene units (Figure 1, top right).[10]
Independently, Yamago reported the synthesis of cage G,
possessing four trisubstituted benzene units, via the formation of
an octahedral platinum complex followed by reductive
elimination (Figure 1, bottom right).[11] However, smaller and
more strained cages have not been synthesized presumably due
to the difficulty of aromatization of template B with the large
strain energy.[12]
[a]
N. Hayase, J. Nogami, Dr. Y. Shibata, Prof. Dr. K. Tanaka
Department of Chemical Science and Engineering, Tokyo Institute
of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan
E-mail: ktanaka@apc.titech.ac.jp
http://www.apc.titech.ac.jp/~ktanaka/
Supporting information for this article is given via a link at the end
of the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201903422
Angewandte Chemie International Edition
COMMUNICATION
In this paper, we disclose the synthesis of the smallest
spherical carbon nanocage so far, [2.2.2]carbon nanocage 1, via
128.1, 127.4, 127.2 ppm. These data are consistent with the D3
symmetric structure of 1. In the APCI-TOF mass spectrum,
[M+H]+ was clearly observed at m/z = 607.2430 (calcd value =
607.2420) with an isotopic distribution pattern identical to the
theoretical pattern (Figure S1). As with [5]CPP,[7] this cage 1 is
air-sensitive and decomposed into an insoluble white solid
compound presumably by oxidation. Although we failed to obtain
a single crystal of 1 due to its poor crystallizability and instability,
the structure of 7 (precursor to cage 1) was confirmed by an X-
ray crystallographic analysis (Figure S2).[18]
the
cationic
rhodium(I)/H8-binap
complex-catalyzed
regioselective intermolecular homo-cyclotrimerization of terminal
alkynes[13–15] followed by the triple Suzuki-Miyaura cross-
couplings with 1,3,5-triborylbenzene and reductive aromatization
of template A (Figure 1, bottom left). Properties of this highly
strained cage molecule 1 and the unexpected synthesis of a β-
graph-shaped cage molecule are also disclosed.
Scheme 1 displays the synthesis of [2.2.2]carbon nanocage
1. The stepwise 1,2-additions of lithium (trimethylsilyl)acetylide
and (4-bromophenyl)lithium to 1,4-benzoquinone (2) afforded
cis-diol 3 in good yield. Dimethylation followed by desilylation
afforded terminal alkyne 4 in high yield. The intermolecular
homo-cyclotrimerization of 4 proceeded in the presence of a
cationic rhodium(I)/H8-binap complex (5 mol %) at room
temperature to give tribromide 5 in good yield as a single regio-
and stereoisomer.[16] The triple Suzuki-Miyaura cross-couplings
of 5 with 1,3,5-triborylbenzene 6 afforded cage molecule 7,
although the yield was low. Finally, the Yamago’s reductive
aromatization of 7 with SnCl2/HCl[8,17] proceeded at room
temperature to give 1 in 18% yield. Importantly, the SnCl2/HCl-
mediated reduction is crucial to obtaining 1. The treatment of 7
with sodium naphthalenide did not afford 1 at all and led to an
unidentified complex mixture.
The UV/Vis absorption and fluorescence spectra of 1 are
shown in Figure 2. The absorption maximum (302 nm) is blue-
shifted, compared to [4.4.4]carbon nanocage E (316 nm).[9b] On
the contrary, the emission maximum (499 nm) is red-shifted,
compared to E (427 nm).[9b] These observations are the same as
previously reported [6.6.6], [5.5.5], and [4.4.4]carbon nanocages
C–E, in which as the cage size becomes smaller, the absorption
and emission maxima become shorter and longer, respectively.
The maximum absolute fluorescence quantum yield (ΦF) of 1
was 5%, which is significantly smaller than that of E (66%),[9b]
but it is important to note that [6] and [5]CPPs show no
fluorescence despite their comparable degree of strain energies
to cage 1.[5e,7]
1) NaH, THF, –50 °C
Br
1)
2)
Li
SiMe3
2) MeI, THF, –50 °C to RT
3) K2CO3, MeOH, RT
Me3Si
THF, –78 °C to RT
O
O
Li
THF, –78 °C to RT
OMe
Br
2
HO
OH
3 / 48%
Br
OMe
5 mol %
[Rh(cod)2]BF4/
Br
Br
H8-binap
OMe
MeO
MeO
CH2Cl2, RT, 3 h
OMe
MeO
OMe
4 / 85%
PPh2
PPh2
Figure 2. UV/Vis absorption (in CHCl3; solid line) and fluorescence (excited at
302 nm in CHCl3; broken line) spectra of 1.
Br
5 / 70%
Bpin
H8-binap
The electronic structure of 1 was examined by DFT and
time-dependent DFT (TD-DFT) calculations using Gaussian 09
at the B3LYP/6-31G(d) level. In order to compare with previously
reported [6.6.6], [5.5.5], and [4.4.4] carbon nanocages C–E of
which the calculations were carried out with structures fixed in
D3 symmetry,[9b] the calculations of 1 were also carried out with a
structure fixed in D3 symmetry. The frontier molecular orbitals
(MOs) of 1 are depicted in Figure 3. The orbitals of HOMO and
HOMO–1 as well as those of LUMO and LUMO+1 are
degenerate (Figure 3, right). The energy diagram of 1 was
compared with E (Figure 3, left). The HOMO energy of 1 is
calculated to be higher in energy than that of E, on the contrary,
the LUMO energy of 1 is calculated to be lower in energy than
that of E. Thus, the HOMO–LUMO energy gap of 1 (3.43 eV) is
smaller than that of E (3.78 eV). A TD-DFT calculation revealed
the origin of the UV/Vis absorption bands. The strongest
absorption peak (302 nm) is assigned to the following transition:
HOMO–2 → LUMO+2, and the second strongest absorption
Bpin
Bpin
6
MeO
OMe
OMe
10 mol %
Pd2(dba)3/SPhos
conditions
A or B
in THF
MeO
K3PO4
MeO
toluene/H2O (9:1)
80 °C, 16 h
OMe
7 / 5%
1
PCy2
A (SnCl2•H2O, HCl, RT): 18%
B (Na naphthalenide, –78 °C): 0%
MeO
OMe
Me
O
O
Me
Me
Me
BPin =
B
Sphos
Scheme 1. Synthesis of [2.2.2]carbon nanocage 1. THF = tetrahydrofuran.
cod = 1,5-cyclooctadiene. dba = dibenzylideneacetone.
Compound 1 was identified by 1H, 13C, and 2D-NMR
spectroscopy, and HRMS (high-resolution mass spectrometry).
1
The H NMR signals of aromatic protons appeared at 7.32 (d,
12H), 7.26 (d, 12H), and 7.06 (s, 6H) ppm. In the 13C NMR
spectrum, six aromatic carbons appeared at 142.8, 137.2, 136.8,
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10.1002/anie.201903422
Angewandte Chemie International Edition
COMMUNICATION
peak (369 nm) is assigned to the following transitions: HOMO →
LUMO and HOMO–1 → LUMO+1.
aromatization, although we failed to synthesize an ultimately
small spherical carbon nanocage, [1.1.1]carbon nanocage.
OMe
orbital energy
1) 5 mol %
[Rh(cod)2]BF4/
H8-binap
CH2Cl2, RT, 4 h
(eV)
[4.4.4]carbon
nanocage E
(ref 9b)
[2.2.2]carbon
nanocage 1
iPr3Si
OMe
MeO
!1
!2
!3
!4
!5
!6
L+2
2) TBAF, THF, RT
L+2
L+2 (
!
1.267 eV)
MeO
OMe
MeO
8
L+1
L
MeO
OMe
L+1
L
9 / 69%
20 mol %
CH2Cl2, RT
[Rh(cod)2]BF4/ dropwise over 2 h
L+1 (
!
1.737 eV)
LUMO (
!1.737 eV)
H8-binap
MeO
then 1 h
3.78 eV
3.43 eV
OMe
OMe
OMe
MeO
HOMO (!5.169 eV)
H-1 (
!
5.169 eV)
MeO
OMe
OMe
OMe
OMe
H!1
H
H!1
H
MeO
MeO
MeO
OMe
H!2
OMe
H!2
OMe
MeO
10 / 29%
11 / 0%
H-2 (!
5.571 eV)
MeO
Figure 3. Energy diagrams of the frontier MOs of [4.4.4] and [2.2.2]carbon
nanocages E (ref 9b) and 1 (left and middle, respectively). Two-way arrows
conditions A or B
in THF
represent HOMO–LUMO gaps.
H
=
HOMO,
L
=
LUMO. Pictorial
representations of the frontier MOs of 1 (right).
MeO
OMe
MeO
The synthesis of an ultimately small spherical carbon
nanocage, [1.1.1]carbon nanocage,[19] was attempted as shown
in Scheme 2. The intermolecular cyclotrimerization of known
diyne 8 in the presence of the cationic rhodium(I)/H8-binap
catalyst followed by desilylation afforded terminal triyne 9 in
good yield as a single regio- and stereoisomer. We expected
that the subsequent intramolecular cyclotrimerization of 9 in the
presence of the same rhodium catalyst would afford cage
molecule 11 that is a precursor of [1.1.1]carbon nanocage.
Unexpectedly, the intermolecular cyclotrimerization of two
molecules of 9 proceeded to give cage molecule 10 in 29% yield.
Partial reductive aromatization of 10 proceeded by using
SnCl2/HCl as a reducing reagent to give β-graph-shaped[20] cage
molecule 12 in 38% yield, although fully aromatized cage
molecule 13[21] was not obtained. Also in this reduction, the use
of sodium naphthalenide did not afford 12 and 13 at all, and led
to an unidentified complex mixture. The structures of 10 and 12
were unambiguously determined by X-ray crystallographic
analyses (Figures S3 and S4).[18] The thus obtained cage
molecule 12 is a fluorescent compound (ΦF = 36%). The
emission maximum (403 nm) of 12 is markedly longer than that
(338 nm[22]) of p-terphenyl presumably due to the presence of
excimer emission of two p-terphenyl cores.
MeO
OMe
OMe
OMe
MeO
13 / 0%
12
A (SnCl2•H2O, HCl, RT): 38%
B (Na naphthalenide, –78 °C): 0%
UV-Vis absorption: !max = 277 nm
emission: !max = 403 nm (excited at 277 nm)
F = 36% (excited at 300 nm)
"
Scheme 2. Attempted synthesis of [1.1.1]carbon nanocage, which affords β-
graph-shaped cage 12. TBAF = tetrabutylammonium fluoride.
Acknowledgements
This research was supported partly by a Grant-in-Aid for
Scientific Research on Innovative Areas "π-System Figuration:
Control of Electron and Structural Dynamism for Innovative
Functions" (No. JP26102004) from the Japan Society for the
Promotion of Science (JSPS, Japan), and ACT-C (No.
JPMJCR1122YR) from the Japan Science and Technology
Agency (JST, Japan). We thank Takasago International
Corporation for the gift of H8-binap, and Umicore for generous
support in supplying the palladium and rhodium complexes.
In summary, we have achieved the synthesis of the smallest
spherical carbon nanocage so far, [2.2.2]carbon nanocage, via
Keywords: alkyne cyclotrimerization • carbon nanocages •
the
rhodium-catalyzed
regioselective
intermolecular
molecular strain • reductive aromatization • rhodium
cyclotrimerization, the triple Suzuki-Miyaura cross-couplings,
and reductive aromatization. This cage molecule is highly
strained, and its ring strain is between those of [6] and [5]CPPs.
A significant bathochromic shift of an emission maximum was
observed, compared with that of known [4.4.4]carbon nanocage.
We have also achieved the synthesis of a unique β-graph-
shaped cage molecule via the rhodium-catalyzed sequential
regioselective cyclotrimerizations followed by reductive
[1]
[2]
[3]
For a recent review, see: M. A. Majewski, M. Stępień, Angew. Chem.
Int. Ed. 2019, 58, 86; Angew. Chem. 2019, 131, 90.
R. Jasti, J. Bhattacharjee, J. B. Neaton, C. R. Bertozzi, J. Am. Chem.
Soc. 2008, 130, 17646.
H. Takaba, H. Omachi, Y. Yamamoto, J. Bouffard, K. Itami, Angew.
Chem. Int. Ed. 2009, 48, 6112; Angew. Chem. 2009, 121, 6228.
This article is protected by copyright. All rights reserved.

10.1002/anie.201903422
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COMMUNICATION
[4]
[5]
S. Yamago, Y. Watanabe, T. Iwamoto, Angew. Chem. Int. Ed. 2010, 49,
757; Angew. Chem. 2010, 49, 757.
[20] A β-graph polymer topology has been reported. See: a) M. Igari, H.
Heguri, T. Yamamoto, Y. Tezuka, Macromolecules 2013, 46, 7303. For
systematic classification of nonlinear polymer topologies, see: b) Y.
Tezuka, H. Oike, J. Am. Chem. Soc. 2001, 123, 11570.
[21] We estimated the strain energy for 13 to be 243.5 kcal·mol–1. The strain
energy per carbon atom for 13 was calculated, which revealed that the
value (4.06 kcal·mol–1) is larger than that of [5]CPP (3.97 kcal·mol–1).
See the Supporting Information for details.
For the first synthesis of each size of CPPs, see: [16], [15], and
[14]CPPs: a) H. Omachi, S. Matsuura, Y. Segawa, K. Itami, Angew.
Chem. Int. Ed. 2010, 49, 10202; Angew. Chem. 2010, 122, 10400. [13],
[11], and [10]CPPs: b) T. Iwamoto, Y. Watanabe, Y. Sakamoto, T.
Suzuki, S. Yamago, J. Am. Chem. Soc. 2011, 133, 8354. [8]CPP: c) S.
Yamago, Y. Watanabe, T. Iwamoto, Angew. Chem. Int. Ed. 2010, 49,
757; Angew. Chem. 2010, 122, 769. [7]CPP: d) T. J. Sisto, M. R.
Golder, E. S. Hirst, R. Jasti, J. Am. Chem. Soc. 2011, 133, 15800.
[6]CPP: e) J. Xia, R. Jasti. Angew. Chem. Int. Ed. 2012, 51, 2474;
Angew. Chem. 2012, 124, 2524.
[22] G. Heimel, M. Daghofer, J. Gierschner, E. J. W. List, A. C. Grimsdale, K.
Müllen, D. Beljonne, J.-L. Brédas, E. Zojer, J. Chem. Phys. 2005, 122,
054501.
[6]
For recent reviews of the CPP synthesis, see: a) W. Di, C. Wei, B.
Xiangtao, X. Jianlong, Asian J. Org. Chem. 2018, 7, 1; b) Y. Segawa, A.
Yagi, K. Matsui, K. Itami, Angew. Chem. Int. Ed. 2016, 55, 5136;
Angew. Chem. 2016, 128, 5222.
[7]
[8]
P. J. Evans, E. R. Darzi, R. Jasti, Nat. Chem. 2014, 6, 404.
E. Kayahara, V. K. Patel, S. Yamago, J. Am. Chem. Soc. 2014, 136,
2284.
[9]
a) K. Matsui, Y. Segawa, T. Namikawa, K. Kamada, K. Itami, Chem. Sci.
2013, 4, 84; b) K. Matsui, Y. Segawa, K. Itami, J. Am. Chem. Soc. 2014,
136, 16452.
[10] S. Cui, G. Zhuang, D. Lu, Q. Huang, H. Jia, Y. Wang, S. Yang, P. Du.
Angew. Chem. Int. Ed. 2018, 57, 9330; Angew. Chem. 2018, 130, 9474.
[11] E. Kayahara, T. Iwamoto, H. Takaya, T. Suzuki, M. Fujitsuka, T. Majima,
N. Yasuda, N. Matsuyama, S. Seki, S. Yamago, Nat. Commun. 2013, 4,
2694.
[12] Recently, unique non-spherical all-benzene cages have been
synthesized. For a helicene cage, see: a) T. Matsushima, S. Kikkawa, I.
Azumaya, S. Watanabe, ChemistryOpen, 2018, 7, 278. For rod-shaped
cages, see: b) H. Sato, J. A. Bender, S. T. Roberts, M. J. Krische, J.
Am. Chem. Soc. 2018, 140, 2455.
[13] a) K. Tanaka, K. Toyoda, A. Wada, K. Shirasaka, M. Hirano, Chem. Eur.
J. 2005, 11, 1145; b) K. Tanaka, K. Shirasaka, Org. Lett. 2003, 5, 4697.
[14] For selected recent reviews of the rhodium(I)-catalyzed [2+2+2]
cycloaddition, see: a) K. Tanaka, Bull. Chem. Soc. Jpn. 2018, 91, 187;
b) K. Tanaka, Y. Kimura, K. Murayama, Bull. Chem. Soc. Jpn. 2015, 88,
375; c) K. Tanaka in Transition-Metal-Mediated Aromatic Ring
Construction (Ed. K. Tanaka), Wiley, Hoboken NJ, 2013, Chap. 4; d) Y.
Shibata, K. Tanaka, Synthesis 2012, 44, 3269; e) K. Tanaka,
Heterocycles 2012, 85, 1017; f) K. Tanaka, Chem. Asian J. 2009, 4,
508; g) K. Tanaka, Synlett 2007, 1977.
[15] For examples of the synthesis of CPPs via the rhodium(I)-catalyzed
alkyne cyclotrimerization, see: a) N. Hayase, Y. Miyauchi, Y. Aida, H.
Sugiyama, H. Uekusa, Y. Shibata, K. Tanaka, Org. Lett. 2017, 19,
2993; b) S. Nishigaki, Y. Miyauchi, K. Noguchi, H. Ito, K. Itami, Y.
Shibata, K. Tanaka, Eur. J. Org. Chem. 2016, 4668; c) Y. Miyauchi, K.
Johmoto, N. Yasuda, H. Uekusa, S. Fujii, M. Kiguchi, H. Ito, K. Itami, K.
Tanaka, Chem. Eur. J. 2015, 21, 18900; d) A.-F. Tran-Van, E. Huxol, J.
M. Basler, M. Neuburger, J.-J. Adjizian, C. P. Ewels, H. A. Wegner, Org.
Lett. 2014, 16, 1594.
[16] Perfect regioselectivity of the cyclotrimerization of 4 may arise from its
steric bulk because the cyclotrimerization of less bulky methyl propargyl
ether under the same reaction conditions afforded
regioisomers. See the Supporting Information for details.
a mixture of
[17] V. K. Patel, E. Kayahara, S. Yamago, Chem. Eur. J. 2015, 21, 5742.
[18] CCDC 1893157 (7), 1893161 (10), and 1893162 (12) contain the
supplementary crystallographic data for this paper. This data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html
(or from the Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
deposit@ccdc.cam.ac.uk).
[19] Although the synthesis of [1.1.1]carbon nanocage has not been
reported, the synthesis of a picotube consisting of four anthracene units
was reported. See: S. Kammermeier, P. G. Jones, R. Herges, Angew.
Chem. Int. Ed. Engl. 1996, 35, 2669.
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Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
MeO
Br
Norihiko Hayase, Juntaro Nogami, Yu
Shibata, and Ken Tanaka*
OMe
regioselective
alkyne
Br
1) Susuki-Miyaura
cross coupling
Br
cyclotrimerization
Page No. – Page No.
OMe
MeO
2) reductive
aromatization
OMe
Synthesis of a Strained Spherical
Carbon Nanocage by Regioselective
Alkyne Cyclotrimerization
MeO
OMe
MeO
[2.2.2]carbon nanocage
Br
The smallest spherical carbon nanocage ([2.2.2]carbon nanocage) so far has been
synthesized via the cationic rhodium(I)/H8-binap complex-catalyzed regioselective
intermolecular homo-cyclotrimerization of
a
cis-1-ethynyl-4-bromophenyl-
cyclohexadiene derivative followed by the triple Suzuki-Miyaura cross-couplings
with 1,3,5-triborylbenzene and reductive aromatization.
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