Selective synthesis of [6]-, [8]-, and [10]cycloparaphenylenes

Article abstract of DOI:10.1246/cl.130188

The selective synthesis of [6]-, [8]-, and [10]cycloparaphenylenes (CPPs) was achieved by a new synthetic route involving Ni(0)-mediated coupling of bis(para-haloaryl)dinuclear arylplatinum complexes and the reductive elimination of the complexes. Importantly, the highly strained [6]CPP was prepared in good overall yield.

Full text of DOI:10.1246/cl.130188

doi:10.1246/cl.130188
Published on the web June 5, 2013
621
Selective Synthesis of [6]-, [8]-, and [10]Cycloparaphenylenes
Eiichi Kayahara,1 Takahiro Iwamoto, Toshiyasu Suzuki, and Shigeru Yamago*1,2
,2
1
2,3
1
Institute for Chemical Research, Kyoto University, Kyoto 611-0011
CREST, Japan Science and Technology Agency, Tokyo 102-0076
Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8787
2
3
(
Received March 5, 2013; CL-130188; E-mail: yamago@scl.kyoto-u.ac.jp)
The selective synthesis of [6]-, [8]-, and [10]cycloparaphen-
ylenes (CPPs) was achieved by a new synthetic route involving
Ni(0)-mediated coupling of bis(para-haloaryl)dinuclear arylpla-
tinum complexes and the reductive elimination of the com-
plexes. Importantly, the highly strained [6]CPP was prepared in
good overall yield.
a)
b)
n
Figure 1. Structure of a) [n + 5]cycloparaphenylene (CPP)
and b) armchair CNT. The constituting [6]CPP unit of the
armchair CNT is highlighted in red.
Cycloparaphenylenes (CPPs; Figure 1a), being hoop-shaped
-conjugated molecules consisting of para-linked phenylene
³
rings, have recently gained much attention from a variety of
disciplines, which, besides their structural beauty, is mostly
associated with the topologically unique array of ³-orbitals.
a) Bertozzi/Jasti method
X
1
­5
Since CPPs are the smallest structural units of armchair carbon
nanotubes (CNTs; Figure 1b), their application in electronic and
optoelectronic devices6 and their use as a seed compound for
the synthesis of structurally uniform CNTs have been suggest-
MeO
MeO
MeO
MeO
OMe
OMe
[6]-[12], and [18]CPPs
0.2-4.5%)
Gram-scale synthesis
of [8] and [10]CPPs
(7.4-9.7%)
(
­9
2
,5,10
ed.
In addition, the cavity of CPPs offers a unique
n
opportunity in supramolecular chemistry to study concave­
convex ³­³ interactions and fabricate hierarchically ordered ³
X
b) Itami method
11,12
materials.
Despite their simple structure, however, the
X
synthesis of CPPs was only recently achieved by three groups
including our own despite intensive efforts that spanned more
than half a century.
MOMO
MOMO
MOMO
OMOM
OMOM
Selective synthesis of
Bertozzi and Jasti were the first to synthesize [9]-, [12]-, and
[
(
9]-[16]CPPs
4-13%)
[
18]CPPs utilizing cis-cyclohexa-2,5-diene-1,4-diyl as the key
MOMO
13
precursor (Scheme 1a). The subsequent extension of this
synthetic strategy by Jasti realized the selective synthesis of a
n
X
14­16
series of [6]­[12]CPPs,
as well as the gram-scale synthesis
of [8]- and [10]CPPs.17 So far, the smallest CPP synthesized
is [6]CPP. Jasti also reported the synthesis of substituted
c) Yamago method
M
(L)Pt
Pt(L)
n
1
8
19
CPPs, including CPP-dimers, based on the same synthetic
strategy. Itami used cis-cyclohexan-2,5-diol as a key precursor
Selective synthesis of
8] and [12]CPPs
(11-37%)
Random synthesis of
8]-[13]CPPs
[
PtCl2(L)
n
(
Scheme 1b) and succeeded in the size-selective synthesis of a
[
20­24
series of [9]­[16]CPPs.
Several derivatives have further-
(18-27% combined yields)
n
n
more been synthesized via this route.2
5­29
M
(L)Pt
Pt(L)
n
We have utilized cyclic cis-bisarylplatinum complexes
prepared by transmetallation of bismetallated aryls and platinum
Scheme 1. Previously reported synthetic routes to CPPs.
dihalides, as the key precursor (Scheme 1c). Once formed,
reductive elimination of Pt affords the CPPs,3
0,31
allowing the
selective synthesis of [8]- and [12]CPPs, as well as the random
synthesis of [8]­[13]CPPs. At that time (2010), [8]CPP was the
smallest CPP reported. As the CPPs could be obtained in high
overall yield, the isolation of CPPs with different ring sizes
enabled us to study the size-dependency of their physical
properties and size-selective host­guest chemistry between CPPs
Theoretical studies by our group have suggested that,31 in
sharp contrast to linear oligoparaphenylenes, the gap between
the highest-occupied and lowest-unoccupied molecular orbital
(HOMO­LUMO gap) of CPPs becomes narrower as the number
of phenylene rings decreases, which is due to increasing and
decreasing HOMO and LUMO energies, respectively. As a
result, compared to larger CPPs, CPPs of small ring size should
display a range of intriguing properties, including a higher redox
activity. Unfortunately, despite recent developments, the synthe-
sis of small CPPs still poses serious synthetic difficulties; the
1
2
and fullerenes. Isobe applied our method to the synthesis of the
shortest sidewall segments of helical CNTs.3
2,33
Very recently,
we reported the selective synthesis of [10]CPP through an L-
3
4
shaped cis-substituted bis(para-haloaryl)platinum complex.
Chem. Lett. 2013, 42, 621­623
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

6
22
(
(
L)Pt
L)Pt
Pt(L)
X
X
(dppf)Pt
(dppf)Pt
Br
Br
(
L)Pt
X
X
m
m
m
m
m
m
-
4"Pt"
(d)
(e)
2
CPPs
C-C bond
formation
Reductive
elimination
n
n
n
n
n
(
L)Pt
Pt(L)
3
: X = Br or I
4: X = SnMe3
: X = Pt(cod)Cl
: X = Pt(dppf)Cl
a: n = 1, b: n = 2, c : n = 3
(a)
b)
c)
1
a (45% from 6a)
b (71% from 6b)
1c (74% from 6c)
1
2
(
1
5
6
(
Scheme 2. The proposed new synthetic route to CPPs via
platinum squares.
(
(
dppf)Pt
Pt(dppf)
synthesis of [6]CPP as reported by Jasti requires long reaction
steps in a low overall yield (0.7% over 8 steps).15 So far, our
own investigations on the synthesis of CPPs smaller than [8]CPP
have been unsuccessful. Therefore, a new and high-yielding
synthetic route for small CPPs is highly anticipated.
We envisioned that para-haloaryl-substituted dinuclear
platinum complex 1 could serve as a precursor for tetranuclear
platinum complex 2 through double carbon­carbon bond
formation mediated by the carbon­halogen functionality
[8]CPP (74% from 2b)
[10]CPP (75% from 2c)
(
f)
n
n
dppf)Pt
Pt(dppf)
2
a (63% from 1a)
b (31% from 1b)
Br
Br
m
2
7
8
: m = 1 (33%)
: m = 3 (31%)
2c (20% from 1c)
Scheme 3. Synthesis of [8]- and [10]CPPs. Reagents and
conditions: a) 1) BuLi (2.0 equiv), THF, ¹78 °C, 1 h or Mg
(2.5 equiv), THF, reflux, overnight; 2) Me SnCl (2.5­3.0 equiv),
(Scheme 2). Reductive elimination by platinum in 2 should
3
then give the CPP. We here report the selective synthesis of
room temperature (rt), 3­6 h, 75­94%; b) [Pt(cod)Cl ] (2.0
2
[
6]CPP by this route, in which a Ni(0)-mediated Yamamoto
equiv), THF or THF/toluene, 60 °C, 8­12 h, 87­94%; c) dppf
(2.0 equiv), CH2Cl2, rt, 6­8 h, 87­92%; d) LiC6H4Br (3.0­
3
5,36
coupling
was utilized as the key step for the synthesis of 2.
5
.0 equiv), THF, ¹78 °C to rt, 12 h, 45­74%; e) [Ni(cod)2]
We further report an improved reductive elimination of 2 to
yield [6]CPP, as previously reported conditions did not give the
desired compound, probably because of the high ring strain. To
clarify the scope of this new route, we also report the selective
synthesis of [8]- and [10]CPPs.
(
2.0 equiv), dppf (2.0 equiv), THF, 50 °C, 12­18 h, 20­63%; f)
Br2 (4.0 equiv), toluene, 90 °C, 12 h.
P Br
Pt
P
Br
To address our hypothesis, 1 was first synthesized from
the commercially available dibromo (or diiodo) biaryls 3a­3c
(Scheme 3). The halogens in 3a­3c were transmetallated to a
trimethylstannyl group by treatment with BuLi or Mg (2.5 equiv)
9
in THF at ¹78 °C, followed by reaction with Me SnCl
3
(
3.0 equiv), giving 4a­4c in 75­94% yield. Compound 4 was
Figure 2. Structure of proposed intermediate 9, as formed by
treatment of complex 2 with Br2. DPPF ligand is abbreviated to
P­P.
then reacted with 2 equiv of [Pt(cod)Cl2] (cod: 1,5-cycloocta-
diene) giving bisplatinum complexes 5a­5c in 87­94% yield,
which were subsequently treated with 1,1¤-bis(diphenylphos-
phino)ferrocene (dppf) to give 6a­6c in 87­92% yield. Treat-
ment of 6a­6c with in situ generated 4-bromophenyllithium
afforded 1a­1c in 45­74% yield. The structure of 1 was
Table 1. Reaction optimization for the synthesis of [6]CPP via
reductive elimination of 2a
Entry
Additive (equiv)
Product (%)a
1
31
characterized by H and P NMR and ESI-TOF mass spec-
trometry.
[6]CPP (47) [40]b
1
2
3
4
5
6
7
XeF2 (4)
AgF (4)
TBAF (4)
P[OCH(CF3)2]3 (8)
acrylonitrile (4)
cod (4)
[6]CPP (29)
[6]CPP (44)
[6]CPP (37)
[6]CPP (25)
[6]CPP (22)
quaterphenyl (4)
Homocoupling of 1 was then attempted under Yamamoto
coupling conditions.35 Treatment of 5 mM 1a­1c in THF with
[Ni(cod)2] (2.0 equiv) and dppf (2.0 equiv) at 50 °C for 12­18 h
gave 2a­2c in 20­63% yield. A highly diluted solution of 1 and
a low temperature were required to achieve these high yields.
[
PPh (8)
3
8]- and [10]CPPs were obtained in 74 and 75% yield from 2b
a
1
Determined by H NMR using 1,1,2,2-tetrachloroethane as the
and 2c, respectively, using the Br -induced reductive elimination
2
b
adopted from previous studies.30 The yields were significantly
internal standard. Isolated yield.
improved by careful adjustment of the amount of added Br2 vs. 2.
In contrast, treatment of 2a with Br2 did not afford [6]CPP at
all. Careful analysis revealed that the major side products
formed were p-dibromobenzene (7) and p-dibromoterphenyl (8)
in 33 and 31% yield, respectively. The formation of 7 and 8
must be the result of undesirable C­Br reductive elimination
from Pt(IV) complex 9, formed by oxidation of 2 by Br2
desired reductive C­C bond coupling reaction could take place
3
9,40
when 2a was oxidized by a fluorine-based reagent.
Indeed,
treatment of 2a with XeF2 (4 equiv) at 90 °C afforded [6]CPP in
47% yield (as determined by NMR, isolated yield: 40%
(Table 1, Entry 1)). AgF was also found to be effective, giving
[6]CPP in moderate yield (Entry 2). Surprisingly, tetrabutylam-
monium fluoride (TBAF), which does not act as an oxidant but
rather is a fluorine anion source, was equally effective in
inducing the reductive elimination of 2a to give [6]CPP in 44%
3
7,38
(Figure 2).
Since C­F reductive elimination requires higher activation
energy than C­Br reductive elimination, we anticipated that the
Chem. Lett. 2013, 42, 621­623
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

6
23
yield (Entry 3). Several neutral and electron-deficient Pt ligands
Phys. Chem. Chem. Phys. 2012, 14, 14585.
T. Nishihara, Y. Segawa, K. Itami, Y. Kanemitsu, J. Phys.
Chem. Lett. 2012, 3, 3125.
such as P[OCH(CF3)2]3,4
1,42
9
1
acrylonitrile, and cod, were also
effective in forming [6]CPP (Entries 4­6), while the reactions
without additive or with PPh , dppf, I , and tetrabutylammonium
0 B. D. Steinberg, L. T. Scott, Angew. Chem., Int. Ed. 2009, 48,
3
2
5
400.
bromide (TBABr) were ineffective. Under optimized conditions,
6]CPP was obtained in 8.9% overall yield from commercially
1
1
1 T. Kawase, H. Kurata, Chem. Rev. 2006, 106, 5250.
2 T. Iwamoto, Y. Watanabe, T. Sadahiro, T. Haino, S. Yamago,
Angew. Chem., Int. Ed. 2011, 50, 8342.
13 R. Jasti, J. Bhattacharjee, J. B. Neaton, C. R. Bertozzi, J. Am.
Chem. Soc. 2008, 130, 17646.
14 T. J. Sisto, M. R. Golder, E. S. Hirst, R. Jasti, J. Am. Chem. Soc.
[
available 3a. Even though the preparation of [6]CPP has already
been reported by Jasti, the overall yield was low (0.7%). In
contrast, the method reported here opens the way for the
production of large-scale quantities of [6]CPP in 8.9% overall
yield from commercially available 3a. The obtained [6]CPP was
2
011, 133, 15800.
1
13
15 J. Xia, R. Jasti, Angew. Chem., Int. Ed. 2012, 51, 2474.
fully characterized by H NMR (7.63 ppm in CDCl3), C NMR
1
1
1
1
6 E. R. Darzi, T. J. Sisto, R. Jasti, J. Org. Chem. 2012, 77, 6624.
7 J. Xia, J. W. Bacon, R. Jasti, Chem. Sci. 2012, 3, 3018.
8 T. J. Sisto, X. Tian, R. Jasti, J. Org. Chem. 2012, 77, 5857.
9 J. Xia, M. R. Golder, M. E. Foster, B. M. Wong, R. Jasti, J. Am.
Chem. Soc. 2012, 134, 19709.
20 H. Takaba, H. Omachi, Y. Yamamoto, J. Bouffard, K. Itami,
Angew. Chem., Int. Ed. 2009, 48, 6112.
21 H. Omachi, S. Matsuura, Y. Segawa, K. Itami, Angew. Chem.,
Int. Ed. 2010, 49, 10202.
(
127.03 and 134.90 ppm), MALDI-TOF MS (m/z 456.1908
+
[M ]), and UV­vis spectroscopy (­max = 340 nm, ¾ = 1.0 ©
5
¹1
¹1
1
0 M cm ), which fully agreed with the data reported by
1
5
Jasti.
However, electrochemical analysis of [6]CPP in 1,1,2,2-
¹
1
tetrachloroethane containing 0.1 mol L Bu4NPF6 solution (as
used in our previous studies) yielded a different result. Although
a reversible oxidation of [6]CPP was indeed observed by cyclic
1
/2
22 Y. Segawa, S. Miyamoto, H. Omachi, S. Matsuura, P. Šenel, T.
voltammetry (CV), the half-wave oxidation potential (Eox
)
+
43
Sasamori, N. Tokitoh, K. Itami, Angew. Chem., Int. Ed. 2011,
was 0.29 V vs. ferrocene/ferrocenium (Fc/Fc ) (Figure S2a).
Differential pulse voltammetry (DPV) also resulted in a similar
oxidation potential (0.30 V vs. Fc/Fc ) (Figure S2b). These
5
0, 3244.
2
2
3 Y. Segawa, P. Šenel, S. Matsuura, H. Omachi, K. Itami, Chem.
Lett. 2011, 40, 423.
+
43
values were considerably smaller than that obtained by Jasti
4 Y. Ishii, Y. Nakanishi, H. Omachi, S. Matsuura, K. Matsui, H.
Shinohara, Y. Segawa, K. Itami, Chem. Sci. 2012, 3, 2340.
25 H. Omachi, Y. Segawa, K. Itami, Org. Lett. 2011, 13, 2480.
26 A. Yagi, Y. Segawa, K. Itami, J. Am. Chem. Soc. 2012, 134,
2962.
27 K. Matsui, Y. Segawa, K. Itami, Org. Lett. 2012, 14, 1888.
8 K. Matsui, Y. Segawa, T. Namikawa, K. Kamada, K. Itami,
Chem. Sci. 2013, 4, 84.
+
(
0
0.44 V vs. Fc/Fc ), who conducted the CV analysis using
¹
1
.1 mol L Bu NPF in dichloromethane. When the oxidation
4
6
potential of the [6]-, [8]-, [10]-, and [12]CPPs, as obtained by
3
1
our analysis, was plotted against their corresponding HOMO
4
3
energies, a good linear correlation was observed (Figure S2c).
2
2
3
Therefore, the current data appears to be reasonable and suitable
for clarifying the size-dependence redox properties of CPPs. We
tentatively attribute this difference to an effect of the solvent, as
electrochemical measurements are generally sensitive to mea-
surement conditions.
9 T. Nishiuchi, X. Feng, V. Enkelmann, M. Wagner, K. Müllen,
Chem.®Eur. J. 2012, 18, 16621.
0 S. Yamago, Y. Watanabe, T. Iwamoto, Angew. Chem., Int. Ed.
2010, 49, 757.
In conclusion, we developed a new and efficient synthetic
route to [6]-, [8]-, and [10]CPPs based on cyclic platinum
intermediate 2 and subsequent reductive elimination. This new
approach is attractive because it gives [6]CPP, which is the most
strained CPP so far reported, in high overall yield. Apart from its
utility in the synthesis of CPPs, the reported reductive
elimination could be applicable in the development of more
efficient transition-metal-mediated coupling reactions.
31 T. Iwamoto, Y. Watanabe, Y. Sakamoto, T. Suzuki, S. Yamago,
J. Am. Chem. Soc. 2011, 133, 8354.
3
3
3
3
2 S. Hitosugi, W. Nakanishi, T. Yamasaki, H. Isobe, Nat.
Commun. 2011, 2, 492.
3 S. Hitosugi, T. Yamasaki, H. Isobe, J. Am. Chem. Soc. 2012,
1
34, 12442.
4 E. Kayahara, Y. Sakamoto, T. Suzuki, S. Yamago, Org. Lett.
012, 14, 3284.
2
5 T. Yamamoto, S. Wakabayashi, K. Osakada, J. Organomet.
Chem. 1992, 428, 223.
This work was partly supported by a CREST program from
the Japan Science and Technology Agency (S. Y.) and by a
Grant-in-Aid for Young Scientists (B) (E. K.).
36 K. Osakada, R. Sato, T. Yamamoto, Organometallics 1994, 13,
4645.
3
3
7 A. Yahav-Levi, I. Goldberg, A. Vigalok, J. Am. Chem. Soc.
006, 128, 8710.
8 A. Yahav-Levi, I. Goldberg, A. Vigalok, A. N. Vedernikov,
J. Am. Chem. Soc. 2008, 130, 724.
2
References and Notes
1
2
3
K. Tahara, Y. Tobe, Chem. Rev. 2006, 106, 5274.
R. Jasti, C. R. Bertozzi, Chem. Phys. Lett. 2010, 494, 1.
M. Iyoda, J. Yamakawa, M. J. Rahman, Angew. Chem., Int. Ed.
39 A. Yahav, I. Goldberg, A. Vigalok, J. Am. Chem. Soc. 2003,
125, 13634.
40 A. Yahav, I. Goldberg, A. Vigalok, Inorg. Chem. 2005, 44,
2
011, 50, 10522.
1547.
4
5
6
7
T. J. Sisto, R. Jasti, Synlett 2012, 483.
K. Itami, Pure Appl. Chem. 2012, 84, 907.
B. M. Wong, J. Phys. Chem. C 2009, 113, 21921.
Y. Segawa, A. Fukazawa, S. Matsuura, H. Omachi, S.
Yamaguchi, S. Irle, K. Itami, Org. Biomol. Chem. 2012, 10,
41 P. W. N. M. van Leeuwen, C. F. Roobeek, Tetrahedron 1981,
37, 1973.
42 S. Yanagisawa, T. Sudo, R. Noyori, K. Itami, J. Am. Chem. Soc.
2006, 128, 11748.
43 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
5
979.
8
M. Fujitsuka, D. W. Cho, T. Iwamoto, S. Yamago, T. Majima,
Chem. Lett. 2013, 42, 621­623
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/