Synthesis and characterization of [5]cycloparaphenylene

Article abstract of DOI:10.1021/ja413214q

The synthesis of highly strained [5]cycloparaphenylene ([5]CPP), a structural unit of the periphery of C₆₀ and the shortest possible structural constituent of the sidewall of a (5,5) carbon nanotube, was achieved in nine steps in 17% overall yield. The synthesis relied on metal-mediated ring closure of a triethylsilyl (TES)-protected masked precursor 1c followed by removal of the TES groups and subsequent reductive aromatization. UV-vis and electrochemical studies revealed that the HOMO-LUMO gap of [5]CPP is narrow and is comparable to that of C₆₀, as predicted by theoretical calculations. The results suggest that [5]CPP should be an excellent lead compound for molecular electronics.

Full text of DOI:10.1021/ja413214q

Communication
pubs.acs.org/JACS
Synthesis and Characterization of [5]Cycloparaphenylene
Eiichi Kayahara,†,‡ Vijay Kumar Patel,†,‡ and Shigeru Yamago*,†,‡
†Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan
‡CREST, Japan Science and Technology Agency, Uji 611-0011, Japan
S
* Supporting Information
was achieved recently by three groups including our own. Jasti,
Bertozzi, and co-workers reported the synthesis of [9]-, [12]-,
and [18]CPP featuring the use of a cis-1,4-dimethoxycyclohexa-
2,5-diene moiety as a masked paraphenylene unit.27 Jasti and co-
workers extended this synthetic strategy to realize the selective
synthesis of [n]CPP (n = 6−12).28−31 Particularly noteworthy
was the synthesis of [6]CPP by coupling of doubly masked
pentaparaphenylene unit 1a with 1,4-bis(pinacolylboryl)-
benzene followed by reductive aromatization (Figure 2a).31
While the overall yield was low (0.2%), [6]CPP is the smallest
CPP synthesized to date. Itami and co-workers subsequently
reported the selective synthesis of [12]CPP utilizing a cis-
cyclohexan-2,5-diol unit as a masked paraphenylene.32 Extension
ABSTRACT: The synthesis of highly strained [5]-
cycloparaphenylene ([5]CPP), a structural unit of the
periphery of C60 and the shortest possible structural
constituent of the sidewall of a (5,5) carbon nanotube, was
achieved in nine steps in 17% overall yield. The synthesis
relied on metal-mediated ring closure of a triethylsilyl
(TES)-protected masked precursor 1c followed by
removal of the TES groups and subsequent reductive
aromatization. UV−vis and electrochemical studies
revealed that the HOMO−LUMO gap of [5]CPP is
narrow and is comparable to that of C60, as predicted by
theoretical calculations. The results suggest that [5]CPP
should be an excellent lead compound for molecular
electronics.
onsiderable interest has recently been focused on hoop-
C
shaped π-conjugated molecules because of their great
potential in molecular electronics.1−8 In particular, cyclo-
paraphenylenes (CPPs) (Figure 1a) which consist of phenylene
Figure 1. Structures of (a) cycloparaphenylenes (n = 1 for [5]CPP), (b)
a (5,5) armchair CNT, and (c) C60. The [5]CPP units in the CNT and
C60 are highlighted in red.
units that are para-linked in a cyclic manner, have attracted
increasing attention in this area9−14 for not only their aesthetic
structure having radially extended unique π orbitals originating
from the curvature but also their applications in electronic and
optoelectronic materials, as CPPs are the smallest structural units
of the sidewalls of armchair carbon nanotubes (CNTs) (Figure
1b) and structural constituents of fullerenes (Figure 1c).15−23
Their use as seeds for the synthesis of structurally uniform CNTs
has also been suggested.5,24 In addition, the concave cavities of
CPPs can act as hosts for π-conjugated molecules with convex
surfaces, such as fullerenes,25,26 and such host−guest chemistry is
useful for studying concave−convex π−π interactions and
fabricating hierarchically ordered π materials.
Figure 2. Synthetic routes to (a) [6]CPP by Jasti and (b) [8]CPP by
Yamago and (c) a new synthetic route to [5]CPP.
Despite their simple structure, however, the synthesis of CPPs
has been a significant challenge because of the difficulty in
constructing the highly strained cyclic structure. After desperate
endeavors over more than a half century, the synthesis of CPPs
Received: December 30, 2013
Published: January 27, 2014
© 2014 American Chemical Society
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of this method led them to synthesize [n]CPP (n = 7−16) in a
size-selective manner.33−37
under Ni(0)-mediated Yamamoto coupling conditions42,43 by
employing Ni(cod)2 (2.0 equiv) and 2,2′-bipyridine (bpy) (2.0
equiv) in refluxing THF for 15 h. Despite the high efficiency of
the cyclization reaction, however, attempts at reductive
aromatization of 3a to [5]CPP using various reducing agents
were unsuccessful.
This result was probably due to the low reactivity of methoxy
group in 3a, and we expected that 3b having free hydroxyl groups
(R = H) would be sufficiently reactive and become a suitable
precursor for [5]CPP. As several attempts to deprotect 1a to give
1b or 3a to give 3b under standard conditions were unsuccessful
because of the extremely acid-sensitive character of 1a and 3a, we
synthesized TES-protected 1c as a precursor of 3b by following
the synthesis of 1a as reported by Jasti.31
We reported a synthesis of [8]CPP based on platinum-
mediated assembly of 4,4′-biphenyl units to form a square-
shaped cyclic platinum complex followed by reductive
elimination of the platinum (Figure 2b).38 [8]CPP was the
smallest CPP reported at the time (2010). In sharp contrast to
the methods developed by Bertozzi, Jasti, and Itami, our method
relied on the cis-coordinated bis(aryl)platinum species to serve
as the precursor for the cyclic structure. The method was
extended to the synthesis of [12]CPP and the random synthesis
of [n]CPP (n = 8−13).39 An extension of this method led to the
selective synthesis of [6]-, [8]-, and [10]CPP.40,41 As [6]CPP
was obtained in reasonable overall yield (10%), this method
would be suitable for the synthesis of highly strained CPPs.
Theoretical studies by our group suggested that in sharp
contrast to linear oligoparaphenylenes, for CPPs the HOMO−
LUMO energy gap (HOMO and LUMO refer to highest
occupied and lowest unoccupied molecular orbitals, respec-
tively) becomes narrower as the number of phenylene rings
decreases because of the increase and decrease of the HOMO
and LUMO energies, respectively.39 Therefore, CPPs of small
ring size should display a range of intriguing electronic properties
compared with larger CPPs. In particular, the calculated
HOMO−LUMO energy gap of [5]CPP, which is a constituent
of the equatorial belt of C60 (Figure 1c), is 2.71 eV, which is
almost identical to that of C60 (2.88 eV), suggesting that [5]CPP
would be an excellent lead compound for molecular electronics.
However, the synthesis of [5]CPP has not been reported to date.
Obviously, the challenge of synthesizing smaller CPPs arises
from the strain energy, which increases exponentially with
decreasing ring size. For example, the strain energy of [6]CPP
was computed to be 407 kJ mol−1, but that of [5]CPP jumped up
to 491 kJ mol−1.39 Furthermore, by analogy to the reactivity of
C60, [5]CPP would be highly reactive toward various reagents
and be unstable under certain conditions. Therefore, it was
unclear whether [5]CPP would be stable under the reaction
conditions used for the synthesis.
Considering these factors in mind, we accepted the challenge
of synthesizing [5]CPP by taking advantage of our own and
Jasti’s synthetic routes (Figure 2c). We envisioned that 1 would
directly cyclize to 3 via organometallic intermediate 2 if, for
example, a platinum complex (M = Pt) were employed. Upon
formation of 3, reductive aromatization of the masked para-
phenylene unit should give [5]CPP. We report here the first
synthesis of [5]CPP via this route. The cyclization of 1a to 3a
proceeded in high yield. However, the reductive aromatization of
3a did not give [5]CPP under various conditions. Therefore, we
developed a new precursor 1c having triethylsilyl (TES)
protecting groups. After the cyclization, removal of the TES
groups followed by reductive aromatization of the resulting free-
hydroxyl precursor afforded [5]CPP for the first time in nine
steps in 17% overall yield.
4-Bromophenyl-4-hydroxycyclohexadienone (4) was treated
with sodium hydride (1.3 equiv) and then with in situ-prepared
4-lithio-4′-(tert-butyldimethylsilyloxy)biphenyl (2.1 equiv), and
the resulting product was treated with TESCl (3.0 equiv) to
afford 5d (Scheme 1). The tert-butyldimethylsilyl (TBS)
a
Scheme 1. Synthesis of [5]CPP
a
Reagents and conditions: (a) (1) NaH (1.3 equiv), THF, −78 °C, 0.5
h; (2) LiC12H8OTBS (2.1 equiv), THF, −78 °C, 2 h. (b) TESCl (3.0
equiv), imidazole (4.0 equiv), DMF, room temperature (rt) to 40 °C,
9 h. (c) LiOH·H2O (2.5 equiv), DMF, rt, 11 h, 92% (three steps). (d)
PhI(OAc)2 (1.5 equiv), THF/CH3CN/H2O, rt, 2.5 h. (e) (1) NaH
(1.3 equiv), THF, −78 °C, 0.5 h; (2) LiC6H4Br (2.1 equiv), THF, −78
°C, 2.5 h. (f) TESCl (3.0 equiv), imidazole (4.0 equiv), DMF, rt to 40
°C, 10 h, 53% (three steps). (g) Ni(cod)2 (2.0 equiv), bpy (2.0 equiv),
THF, reflux, 15 h, 63%. (h) TBAF (4.2 equiv), THF, rt, 2 h, 94%. (i)
SnCl2·H2O (10 equiv), THF, 60 °C, 6 h, 58%.
protecting group in 5d was selectively removed by employing
LiOH·H2O (2.5 equiv), providing phenol 5e. It is worth noting
that the preparation of 5e from 4 was conducted in excellent
overall yield (92%) on a large scale (>20 g). Treatment of 5e with
PhI(OAc)2 (1.5 equiv) afforded hydroxy ketone 6, which was
successively treated with sodium hydride (1.3 equiv), 4-
bromophenyllithium (2.1 equiv), and TESCl (3.0 equiv) to
afford 1c in 53% yield over the three steps. Treatment of 1c with
Ni(cod)2 (2.0 equiv) and bpy (2.0 equiv) in refluxing THF for 15
h afforded the cyclized product 3c in 63% isolated yield. Removal
of the TES groups was successfully carried out by treatment with
tetrabutylammonium fluoride to afford tetraol 3b quantitatively.
Reductive aromatization of 3b was successfully carried out by
employing SnCl2·H2O (10 equiv) in THF at 60 °C for 6 h, and
[5]CPP was formed and isolated in 58% yield as a dark-purple
solid. [5]CPP was found to be stable under air and soluble in
many common solvents, including toluene, THF, CHCl3,
CH2Cl2, and acetone. SnCl2 reduction is usually carried out in
acidic media, but the same reaction under acidic conditions did
At first, Jasti’s compound 1a (R = Me, X = Br) was used as the
starting material.31 1a was transformed to the corresponding
stannane 1a′ (R = Me, X = SnMe3) in 93% yield by treatment
with BuLi (2.2 equiv) in THF at −78 °C followed by addition of
Me3SnCl (2.5 equiv). Next, 1a′ was treated with PtCl2(cod)
(0.50 equiv; cod = 1,5-cyclohexadiene) at 60 °C for 20 h and
PPh3 (2.2 equiv) at 95 °C for 18 h. The desired product 3a (R =
Me) was obtained in 52% yield after purification by silica gel
chromatography and preparative gel-permeation chromatogra-
phy. 3a was also synthesized in one step from 1a in 63% yield
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not give [5]CPP at all. Therefore, the neutral conditions were
essential to obtain [5]CPP.
The product was fully characterized by NMR spectroscopy
and mass spectrometry. In the 1H NMR spectrum in CDCl3 at 25
°C, [5]CPP showed one singlet at 7.85 ppm, which is consistent
with the highly symmetrical structure of [5]CPP. The result also
suggests that the paraphenylene units rotate freely at room
temperature on the time scale of the NMR measurement. The
singlet signal was shifted upfield by 0.22 and 0.37 ppm compared
with the corresponding protons of [6]- and [7]CPP (7.63 and
7.48 ppm, respectively). The observed shift is probably derived
from ring strain, as observed in the size dependence of the
chemical shift in cyclophanes.14,44−47 The 13C NMR spectrum
showed two signals at 126.61 and 131.97 ppm. In the MALDI-
TOF mass spectrum, a molecular ion peak was observed at m/z
380.1136 with an isotopic distribution pattern identical to the
calculated value based on the natural-abundance isotopic ratio.
The solution of [5]CPP in CHCl3 exhibited a dark-purple
color and had the strongest maximum absorption at λmax = 335
nm with absorption coefficient ε = 6.82 × 104 M−1 cm−1 (Figure
3). A weak absorption having λmax = 502 nm with ε = 652 M−1
Figure 4. Cyclic voltammograms of [5]CPP in C2H2Cl4 (for oxidation)
or THF (for reduction) containing 0.1 mol L−1 Bu4NPF6 at room
temperature at a scan rate of 0.1 V/s (for oxidation) or 0.05 V/s (for
reduction). The red curve represents the voltammogram obtained upon
scanning potential between −1.0 and −1.87 V.
positive direction in TCE exhibited two chemically reversible
oxidation processes with half-wave potentials (E1/2) of 0.13 and
0.27 V vs ferrocene/ferrocenium (Fc/Fc+). The same measure-
ment in THF also gave two oxidation waves with E1/2 = 0.10 and
0.30 V vs Fc/Fc+, but the reversibility was considerably decreased
(Figure S1 in the Supporting Information). The observation of
two oxidation waves is consistent with the formation of the
radical cation and dication of the CPP.22
Scanning in the negative direction in THF to −1.87 V vs Fc/
Fc+ gave a pseudoreversible reduction wave at E1/2 = −1.59 V vs
Fc/Fc+ (Figure 4, red curve). Further scanning gave a second
pseudoreversible reduction at E1/2 = −1.91 V vs Fc/Fc+. The
reversibility of the first wave significantly decreased under this
condition, suggesting that the reduced compound is unstable.
The HOMO−LUMO energy gap was estimated to be 1.69 eV
from the first oxidation and reduction potentials measured in
THF. This value is in good agreement with that obtained from
the absorption onset of the UV−vis spectrum, but it is much
smaller than that obtained from the theoretical calculation and
also that of C60 obtained electrochemically (2.34 eV).48 This
difference is most likely derived from inaccuracy of the oxidation
potential measured in THF. While further studies are needed to
obtain an accurate value of the energy gap, all of the data are
consistent with a low HOMO−LUMO gap in [5]CPP.
In summary, [5]CPP was synthesized for the first time by
metal-mediated ring closure of TES-protected masked precursor
1c followed by removal of the TES groups and subsequent
reductive aromatization. The use of hydroxyl groups instead of
methoxy groups considerably increased the efficiency of the
reductive aromatization, which enabled the use of a weak
reducing agent (SnCl2) at ambient temperature. The conditions
are in sharp contrast to the previous synthesis of larger CPPs by
Bertozzi and Jasti using a methoxy-protected precursor, which
required a strong reducing agent such as sodium naphtalenide at
low temperature. The TES protection method would be
applicable to the synthesis of larger CPPs under mild conditions,
and such a synthetic route should be suitable for large-scale
preparation. UV−vis and electrochemical studies revealed that
the narrow HOMO−LUMO gap of [5]CPP is comparable to
that of C60, as predicted by theoretical calculations. The results
suggest that [5]CPP should be an excellent lead compound for
molecular electronics. Furthermore, since [5]CPP is expected to
Figure 3. UV−vis spectrum of [5]CPP in CHCl3 along with the
oscillator strengths (red bars) obtained by TD-DFT calculations.
cm−1 was also observed. The λmax value of the strongest
absorption was almost same as those observed for other
CPPs.17,5−11 No fluorescence was observed when [5]CPP was
excited at <400 nm, and this result is also consistent with the
observed behavior of [7]- and [6]CPP, which exhibit negligible
or no fluorescence.28,31,41
Time-dependent density functional theory (TD-DFT)
calculations at the B3LYP/6-31G* level indicated that the
strongest absorption at λmax = 335 nm can be assigned as the
transitions from the nearly degenerate HOMO−1 or HOMO−2
to the LUMO and from the HOMO to the nearly degenerate
LUMO+1 or LUMO+2 at 319 and 320 nm with oscillator
strengths (f) of 0.6319 and 0.6446, respectively. The results are
consistent with the assignments for other CPPs.17,5−11 The weak
absorption at λmax = 502 nm was assigned as the almost-forbidden
HOMO−LUMO transition at 610 nm with f = 0.0014.17,5−11
The HOMO−LUMO gap of [5]CPP was calculated to be 2.47
or 1.91 eV (238 or 184 kJ mol−1) by taking the observed peak (λ
= 502 nm) or the absorption onset (λ = 649 nm), respectively. It
is worth noting that these values are smaller than that for C60.
Electrochemical analysis of [5]CPP was performed in 0.1 mol
L−1 Bu4NPF6 solutions in 1,1,2,2-tetrachloroethane (TCE) or
THF (Figure 4). Cyclic voltammetry (CV) by scanning in the
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(26) Iwamoto, T.; Watanabe, Y.; Takaya, H.; Haino, T.; Yasuda, N.;
Yamago, S. Chem.Eur. J. 2013, 19, 14061.
have high chemical reactivity, chemical modification of [5]CPP
for the synthesis of functionalized CPPs would be of great
interest.
(27) Jasti, R.; Bhattacharjee, J.; Neaton, J. B.; Bertozzi, C. R. J. Am.
Chem. Soc. 2008, 130, 17646.
(28) Sisto, T. J.; Golder, M. R.; Hirst, E. S.; Jasti, R. J. Am. Chem. Soc.
2011, 133, 15800.
ASSOCIATED CONTENT
* Supporting Information
Synthesis and characterization of all new compounds, electro-
chemical analysis, and results of the TD-DFT calculations on
[5]CPP. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
S
(29) Darzi, E. R.; Sisto, T. J.; Jasti, R. J. Org. Chem. 2012, 77, 6624.
(30) Xia, J.; Bacon, J. W.; Jasti, R. Chem. Sci. 2012, 3, 3018.
(31) Xia, J.; Jasti, R. Angew. Chem., Int. Ed. 2012, 51, 2474.
(32) Takaba, H.; Omachi, H.; Yamamoto, Y.; Bouffard, J.; Itami, K.
Angew. Chem., Int. Ed. 2009, 48, 6112.
(33) Omachi, H.; Matsuura, S.; Segawa, Y.; Itami, K. Angew. Chem., Int.
Ed. 2010, 49, 10202.
AUTHOR INFORMATION
Corresponding Author
yamago@scl.kyoto-u.ac.jp
̌
(34) Segawa, Y.; Miyamoto, S.; Omachi, H.; Matsuura, S.; Senel, P.;
■
Sasamori, T.; Tokitoh, N.; Itami, K. Angew. Chem., Int. Ed. 2011, 50,
3244.
̌
(35) Segawa, Y.; Senel, P.; Matsuura, S.; Omachi, H.; Itami, K. Chem.
Notes
Lett. 2011, 40, 423.
The authors declare no competing financial interest.
(36) Ishii, Y.; Nakanishi, Y.; Omachi, H.; Matsuura, S.; Matsui, K.;
Shinohara, H.; Segawa, Y.; Itami, K. Chem. Sci. 2012, 3, 2340.
(37) Sibbel, F.; Matsui, K.; Segawa, Y.; Studer, A.; Itami, K. Chem.
Commun. 2014, 50, 954.
(38) Yamago, S.; Watanabe, Y.; Iwamoto, T. Angew. Chem., Int. Ed.
2010, 49, 757.
(39) Iwamoto, T.; Watanabe, Y.; Sakamoto, Y.; Suzuki, T.; Yamago, S.
J. Am. Chem. Soc. 2011, 133, 8354.
(40) Kayahara, E.; Sakamoto, Y.; Suzuki, T.; Yamago, S. Org. Lett. 2012,
14, 3284.
ACKNOWLEDGMENTS
■
This work was partly supported by the CREST Program of the
Japan Science and Technology Agency (S.Y.) and by a Grant-in-
Aid for Young Scientists (B) from the Japan Society for the
Promotion of Science (E.K.). We thank Prof. Toshiyasu Suzuki
(Institute for Molecular Science) for valuable discussions and
suggestions.
(41) Kayahara, E.; Iwamoto, T.; Suzuki, T.; Yamago, S. Chem. Lett.
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