RESEARCH
| REPORT
ethanol (30). We envisioned that the spirosilane
moiety could similarly be used as a traceless
template for the synthesis of all-benzene cate-
nanes and trefoil knots, which would contain
no remnants of it.
reaction, desilylation, and reductive aromati-
zation reactions. The H-NMR signals of 1b in
0.2%; see fig. S1 for the plausible intermediate
of 1a from 10).
1
CD Cl
2 2
were observed at 7.29 and 7.38 ppm, with
Single crystals of 2 were obtained from a
hexane and 1,2-dichloroethane solution of 2,
and x-ray crystallography unambiguously cor-
roborated the trefoil-knot structure (Fig. 4A).
Similar to 1a and 1b, intramolecular p-p stacking
was observed (see the supplementary materials
for details). Trefoil knot 2 is chiral, and both
enantiomers (gray and orange) were incorporated
in a 1:1 ratio in the crystal (Fig. 4B). Separation
of the enantiomers of 2 was achieved by chiral
HPLC, and the circular dichroism (CD) spectra
of both enantiomers were successfully recorded.
Judging from the simulated CD spectra by den-
sity functional theory (DFT) calculations, the first
and second HPLC fractions could be assigned to
the left-handed (–) and right-handed (+) (35)
trefoil-knot structures, respectively (Fig. 4C).
The seemingly rigid knot 2 was demonstrated
the integral ratio of 9:12 indicating 1:1 catenated
structure of [9]CPP and [12]CPP in 1b.
The synthetic route to [12]CPP-based [2]
catenane (1a) is shown in Fig. 2A. According to
the synthetic protocol reported by Jasti and col-
leagues (31), dialkoxycyclohexadiene moieties
were selected as bent paraphenylene precur-
sors, whereas n-butoxy groups were used instead
of methoxy groups to increase the solubility of
the intermediates. The starting material, 2,2′-
dibromo-4,4′-diiodobiphenyl (3), was converted
into a dibrominated U-shaped unit 4a in four
steps that included an iodo-selective lithiation, a
nucleophilic addition to a p-quinone derivative,
a nucleophilic addition of p-halolithiobenzenes,
and an n-butylation of the resulting hydroxy
groups. A Suzuki–Miyaura coupling of 4a and
L-shaped unit 5, which bears chloro and boryl
groups (31), took place selectively at the iodo
moieties of 4a to form C-shaped unit 6a in mod-
erate yield. For the formation of spirosilane 7a,
conventional reaction conditions (dilithiation with
The catenated structures of 1a and 1b were
confirmed by x-ray crystallography (Fig. 3, A
and C). In the solid state, catenanes 1a and 1b
stack in one dimension to form void channels
(Fig. 3, B and D), wherein solvent molecules
used for recrystallization (hexane and chloro-
form for 1a and 1,4-dioxane and chloroform
for 1b) were incorporated. Intramolecular p–p
interactions are evident, with the C–C distance
of the nonbonding benzene rings ~3.4 to 3.6 Å
(see fig. S2 for details). Absorption and fluores-
cence measurements of 1b (Fig. 3E) revealed
unusual features of this heterocatenane com-
pared with [9]CPP and [12]CPP as reference
molecules (33). The fluorescence spectrum of
1b is almost identical to that of [9]CPP without
any trace of the fluorescence peaks that orig-
inated from [12]CPP, whereas the absorption
spectrum of 1b is the simple combination of
those of [9]CPP and [12]CPP. The fluorescence
lifetime of 1b (10.9 ns) is also similar to that of
[9]CPP (10.6 ns) (33). Judging from the fact
that the solution of the mixture of [9]CPP and
[12]CPP shows the fluorescence peaks of both
[9]CPP and [12]CPP (fig. S4), the complete fluo-
rescence quenching of the [12]CPP moiety in
1b seems to be a consequence of the catenated
structure. As depicted in Fig. 3F, fast energy
transfer from the excited [12]CPP moiety to the
[9]CPP moiety in 1b would account for these
observations (fig. S5). This result clearly dem-
onstrates the effect of the catenated structure
on the photophysical properties of fully con-
jugated rings, in that catenation is the only way
to connect the all-benzene rings without break-
ing their high symmetry.
We hypothesized that our traceless synthetic
strategy could be extended to the generation of
an even more challenging, all-benzene trefoil
knot 2, as shown in Fig. 1C. Inspired by the
report of Dietrich-Buchecker and Sauvage, who
constructed the trefoil-knot–type topology using
two phenanthroline-Cu moieties as template
units (34), we designed 11 (Fig. 2C) as the key
intermediate, in which a loop consisting of 16
paraphenylenes and 8 cyclohexadiene-diyl units
is knotted through two spirosilane joints. As the
key intermediate 11 was a dimerized product of
spirosilane 10, we started with the synthesis
of 10. The U-shaped unit 4c was lithiated and
silylated under the aforementioned conditions
to have rapid dynamic motion in solution. The
1
2 2
H-NMR spectrum for 2 in CD Cl exhibited a
sharp singlet at room temperature (7.14 ppm),
and even at –95°C, a slightly broadened peak
was present, similar to those of 1a and [12]CPP
(Fig. 4D). This result clearly indicates that all
benzene rings of 2 were equivalent on the NMR
time scale even at –95°C because of the rapid
dynamic motion of the molecule (see fig. S7 for
simulated NMR spectra). The dynamic behavior
of 2 was simulated by density-functional tight-
binding (DFTB) with molecular dynamics (MD)
methods. In this simulation, we observed the
intrinsic dynamics whereby the paraphenylene
chains of 2 coil around a thin torus like the
typical motion of trefoil-knot vortices (36). Be-
cause this dynamic motion seamlessly shifts the
benzene rings (orange in Fig. 4E) from the under-
crossing to the overcrossing regions of the knot
(see movie S1), it plausibly accounts for the av-
eraged NMR signals of 2.
These catenane and trefoil-knot molecules rep-
resent the cornerstone objects for topological
molecular nanocarbons. The properties of these
p-conjugated molecules raise further scientific
questions such as how the structural features
(size, components, substituents) affect their dy-
namic motions, physical properties, p-conjugation,
and optoelectronic properties. We anticipate that
this traceless synthetic method will generate a
broader variety of topological molecular nano-
carbons and open the door to expanded research
areas in nanocarbon science.
n-BuLi, followed by spirosilylation with SiCl
were not applicable because of the high Lewis
acidity of SiCl , which reacts quickly with the
4
)
4
n-butoxy moieties. After screening the reaction
conditions using a model reaction (compare table
S1), we discovered that the complex of SiHCl
TMEDA (N,N,N ', N'-tetramethylethylenediamine)
32) was suitable for this spirosilylation step.
Sequential lithiation of 6a and addition of
SiHCl and TMEDA afforded spirosilane 7a
3
and
(
3
in 57% yield.
Catenane 1a was synthesized from 7a in three
steps including a Ni(0)-mediated intramolecular
aryl–aryl coupling reaction at the C–Cl moieties,
a fluoride-mediated desilylation, and a reductive
aromatization of the dialkoxycyclohexadiene
units (Fig. 2A). As a result, the desired catenane
1a was isolated in 16% yield (9.0 mg) from 7a by
column chromatography and preparative thin-
layer chromatography (PTLC), along with the
generation of pristine [12]CPP (14% yield, 8.0 mg).
Although intermediates were neither isolated
nor characterized, precursors 8a and 9a were
expected to be generated from the aryl–aryl cou-
pling step and the desilylation step, respectively
(
see fig. S1 for the plausible intermediate of [12]
1
CPP). The H–nuclear magnetic resonance (NMR)
spectrum of 1a in CD Cl showed a singlet peak
2
2
at 7.35 parts per million (ppm), which indicates
fast mutual rotation of the two [12]CPP compo-
nents of 1a in solution.
REFERENCES AND NOTES
1.
H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, R. E. Smalley,
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By applying this strategy, the all-benzene
catenanes consisting of two different-sized rings
could also be synthesized (Fig. 2B). Starting from
C-shaped unit 6b in addition to 6a, hetero-
spirosilylation successfully occurred to furnish 7b
in 41% yield together with relatively small amounts
of the homo-spirosilylation products 7a and 7c,
which is a possible precursor for [9]CPP catenane.
The thus-obtained hetero-spirosilane 7b was con-
verted into [9]CPP–[12]CPP heterocatenane 1b in
3
(SiHCl and TMEDA) to afford 10 in 86% yield.
2.
S. Iijima, Nature 354, 56–58 (1991).
In a manner similar to the synthesis of 1a and
1b, 10 was subjected to a sequence of Ni(0)-
mediated aryl-aryl coupling, desilylation, and
reductive aromatization without isolating the
intermediates. Fortunately, after extensive chro-
matographic separation of the crude mixture,
the all-benzene trefoil knot 2 was isolated by
PTLC and preparative high-performance liquid
chromatography (HPLC) in 0.3% yield (0.8 mg),
along with [12]CPP (17.3 mg, 6.1%) and 1a (0.5 mg,
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1.5% yield (2.0 mg) by sequential homocoupling
Segawa et al., Science 365, 272–276 (2019)
19 July 2019
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Segawa, Yasutomo
