A Nanosized Phenylene-Ethynylene-Butadiynylene [2]Catenane

Article abstract of DOI:10.1002/chem.201802567

In a convergent, template-directed synthesis, an efficient route to a phenylene-ethynylene-butadiynylene based [2]catenane is described. The key step is performed by the aminolysis of the corresponding precatenane, which is obtained by a sequence of metal-catalyzed cross-coupling and desilylation reactions. The cyclization reaction leads besides the [2]precatenane to a variety of larger precatenanes and offers an attractive approach to mechanically interlocked structures of different size.

Full text of DOI:10.1002/chem.201802567

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Title: A Nanosize Phenylene-Ethynylene-Butadiynylene [2]Catenane
Authors: Christopher Schweez and Sigurd Höger
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A Nanosize Phenylene-Ethynylene-Butadiynylene [2]Catenane
Christopher Schweez,[a] and Sigurd Höger*[a]
Dedicated to Professor Manfred T. Reetz on the occasion of his 75th birthday
Abstract: In a convergent, template-directed synthesis, an efficient
route to a phenylene-ethynylene-butadiynylene based [2]catenane is
described. The key step is performed by the aminolysis of the
Results and Discussion
corresponding precatenane, which is obtained by a sequence of
metal-catalyzed cross-coupling and desilylation reactions. The
cyclization reaction leads besides the [2]precatenane to a variety of
larger precatenanes and offers an attractive approach to
mechanically interlocked structures of different size.
Here, we report on the synthesis of a [2]catenane consisting
shape-persistent phenylene-ethynylene-butadiynylene
of
macrocycles. These macrocycles are highly attractive building
blocks due to their structural and optical properties.[8] The
emerging synthetic progress improved their availability in high
yields, so that they are convenient building blocks for functional
molecular nanoarchitectures with predictable structures in bulk,
solution, or at interfaces.
Introduction
Our approach toward the catenanes is based on a covalent
template strategy, which we have already used successfully in
the synthesis of two [2]rotaxanes.[7] The approach is very flexible
and modular so that small variations led to the catenane
synthesis as described here and in principle even more complex
architectures can be obtained rather easily. Additionally, the
[2]catenane can help to elucidate the rotaxane isomer
hypothesis suggested by Morin et al.[6] and also discussed by
ourselves (see below and Figure 1).[7]
The covalently bound template (Scheme 1a, blue) takes over
two tasks in this strategy. First, it supports the cyclization
reaction towards the first macrocycles (Scheme 1a, left) and
second, it enables the further attachment of the building blocks
for the second macrocycle by its two additional binding sites
(Scheme 1a, middle). In the final step, the auxiliary bonds
between both macrocycles are cleaved giving the mechanically
interlocked molecule (Scheme 1a, right).
The synthesis of the precatenane (Scheme 1b) started with
the coupling reaction of macrocycle 1 and the half-ring 2. 1 was
already used in our previous rotaxane synthesis and details on
its synthesis can be found elsewhere.[7] It contains the p-
terphenyl template, which is bound inside the ring via phenolic
ester bonds and features two sterically non-congested aryl
iodides. These allowed the transition-metal catalyzed coupling
with the ethynylenes of the mono (3-cyanopropyl)diisopropylsilyl
(CPDIPS)-protected “half-rings” 2 using the standard catalyst
system PdCl2(PPh3)2 and CuI in toluene and NEt3 at 100 °C. The
use of the CPDIPS group allowed a rather simple purification of
2 (see Supporting Information).[9] Removing of the CPDIPS
groups of 3 with TBAF in THF in the presence of water gave 4.
Adding water to this reaction mixture is crucial to avoid the
undesired cleavage of the phenolic ester bonds at this point of
the synthesis.[10]
Molecular nanotechnology has attracted considerable
attention over the last years. Particularly, the design and
assembly of artificial molecular architectures that mimic the
function of biological systems[1] has drawn great interest.
Mechanically interlocked molecules, e.g. catenanes and
rotaxanes have proved appropriate to build these artificial
devices, and significant developments have been made for the
last years.[2]
Catenanes contain at least two macrocycles that are linked
together. Their unique feature is the mechanical bond, a
noncovalent linkage between the components. This special
bonding situation offers unusual properties that make catenanes
high potential materials for the construction of nanomachines.
Especially the large amplitude motions and rotations of the
components relatively to each other favor catenanes for
utilization in artificial devices. Focus of the scientific efforts is the
question how to exploit and to address the dynamics of such
molecules and how to combine them with other structures or
devices. There are numerous examples demonstrating the
future applications, e.g. molecular motors, switches or shuttles.[2]
In the 1980’s, the development of mechanically interlocked
molecules was accelerated by the introduction of template
methods.[3] Numerous synthetic approaches based on
noncovalent interactions have been published during the last
thirty years, whereas the covalent template approach was
rejected, as it was regarded as less versatile and inefficient.[4]
However, in the last 15 years some examples were published
that illustrate the performance of this method.[5, 6] Recently we
reported on the successful syntheses of [2]rotaxanes via this
strategy.[7]
The precatenanes 5a-c were formed by palladium catalyzed
Glaser coupling[11] of 4 under pseudo high-dilution conditions.
This step is similar to the synthetic pathway of Sauvage et al.
and Anderson et al., respectively.[12, 13] High-dilution conditions
should favor the formation of the [2]precatenane by an
intramolecular coupling. However, according to the GPC
[a]
C. Schweez, Prof. Dr. S. Höger*
Kekule-Institut für Organische Chemie und Biochemie
Rheinische Friedrich-Wilhelms-Universität Bonn
Gerhard-Domagk-Str. 1, 53121 Bonn, Germany
E-mail: hoeger@uni-bonn.de
analysis of the crude product
precatenanes ([2]precatenane 5a, [3]precatenane (5b),
a
series of oligomeric
Supporting information for this article is given via a link at the end of
the document.
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[4]precatenane (5c)) were formed besides the desired product
by intra- and intermolecular coupling reactions
Scheme 1. Schematic (a) and specific (b) synthesis of the [2]catenane.
a)
b)
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(see Supporting Information). A successful separation of the
products was performed by recycling GPC (recGPC) which
allowed the isolation and characterization of the respective
compounds.
Cleavage experiments with the higher precatenanes 5b and
5c have not yet been satisfactory and require larger product
amounts than prepared here. These should be available by
applying less diluted coupling conditions. However, those
experiments were beyond the scope of this work. Anyhow, the
[2]catenane 6a was obtained by aminolysis of the corresponding
[2]precatenane 5a with n-propyl amine. After purification by
recGPC, 6a was fully characterized by 1H NMR, mass
spectrometry and GPC analysis.
Besides the exploration of this synthetic access to
[2]catenanes and higher catenanes, the study of the so called
isomer hypothesis, which was discussed by Morin et al.[6] and
also by us,[7] was the impetus for our investigation. Specifically,
during the rotaxane synthesis by using the covalent template
approach under the usage of 1 arose the question if only the
prerotaxane is formed or also a side product where the axis
does not penetrate the wheel but touches the wheel tangentially.
Aminolysis of the latter compound does not lead to the rotaxane
but to the free wheel and axis. The free wheel and the free axis
have been observed in those experiments, however, they can be
also formed during the aminolysis by dethreading since it has
been independently shown that dethreading occurs at elevated
temperatures within the timescale of hours to days.[7]
Transferred to the catenane syntheses described here, it means
that in the synthesis of the precatenane (Scheme 1, steps 1 and
2) not only isomer A could be formed, which gives the desired
catenane after the ester cleavage, but also isomer B (Figure 1).
This topological isomer does not feature the topology of a
precatenane, as the rings do not penetrate each other, so that
cleavage of the auxiliary bonds leads to a separation of both
rings (Figure 1).
propenylidene]malononitrile (DCTB). This gives the evidence
that a separation or fragmentation of the [2]precatenane during
the course of the aminolysis does not take place. If both isomers
had been formed, additional signals of the free macrocycles
would have appeared in the spectrum at m/z = 2363.4 g/mol and
at m/z = 2790 g/mol.
Thus, it seems that no second isomer is formed during the
coupling of 1 and 2. This selectivity can be attributed to the
steric demand of the attached acetylenes. The Sonogashira
coupling reaction proceeds consecutively, this means that
initially one acetylene is coupled to the template followed by the
second. The first one shields one face of the macrocycle and
leads to a rotation of the template out of the plane of the
macrocycles so that the second acetylene must attack the
template from the other side of the ring. This geometry is fixed
for the cyclization, because the molecule cannot flip its
topological conformation, and therefore only one isomer is
formed.
We are quite aware of the fact that the synthesis of a
catenane via our covalent template can not be fully transferred
to our previous rotaxane synthesis. However, it seems to be
highly probable that the restricted yield in our rotaxane synthesis
is rather a result of the deslipping reaction proceeding in parallel
to the aminolysis of the bond to the template than the formation
of a topological isomer of the prerotaxane.
Conclusions
In conclusion, we have successfully applied our covalent
template approach to the synthesis of a phenylene-ethynylene-
butadiynylene based [2]catenane. The synthetic pathway is
convenient and benefits from the modular concept of our
approach that has already been used for the rotaxane formation.
The cleavage of the auxiliary template bonds of the precatenane
leads selectively and exclusively to the [2]catenane.
A
separation of both rings is not observed, thus, we have no hint
that another isomer (B) is formed during the reaction. The
[2]catenane was fully characterized by 1H NMR, MADLI-TOF
mass spectrometry and GPC analysis. Additionally, the route
easily opens access to larger catenanes (multicatenanes) via
their corresponding precatenanes that are formed in
intermolecular cyclization reactions.
Figure 1. Both possible isomers of the precatenane synthesis.
Conflict of interest
The authors declare no conflict of interest.
The crude product of the aminolysis was carefully analyzed by
GPC and MALDI-TOF mass spectrometry. The mass distribution
of the GPC analysis reveals only a broadened signal at m/z =
6995 g/mol, which gives no hint for a separation of the catenane
into its components. In addition, the mass spectrum of the crude
product (see SI) also only shows two signals at m/z = 5153 and
m/z = 5404.4 that result from the [2]catenane and its adduct with
E-2-[3-(4-tert-Butylphenyl)-2-methyl-2-
Acknowledgements
We gratefully acknowledge financial support by the Fonds der
Chemischen Industrie.
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Keywords: catenanes • rotaxanes • templates • macrocycles •
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Christopher Schweez, Sigurd Höger*
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A Nanosize Phenylene-Ethynylene-
Butadiynylene [2]Catenane
Text for Table of Contents
Text for Table of Contents
A covalent template connected to the reactants by ester bonds supports the synthesis of both rings of the precatenane and
subsequent aminolysis yields the corresponding catenane.
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