Amidinium carboxylate salt bridges as a recognition motif for mechanically interlocked molecules: Synthesis of an optically active [2]catenane and control of its structure

Article abstract of DOI:10.1002/anie.201002382

(Figure Presented) Salty catenane: An optically active [2]catenane was synthesized by utilizing an amidinium carboxylate salt bridge (see picture). The relative motion of the two macrocyclic components was completely controlled by an acid/base or Zn²⁺/cryptand couple.

Full text of DOI:10.1002/anie.201002382

Angewandte
Chemie
DOI: 10.1002/anie.201002382
Supramolecular Chemistry
Amidinium Carboxylate Salt Bridges as a Recognition Motif for
Mechanically Interlocked Molecules: Synthesis of an Optically Active
[2]Catenane and Control of Its Structure**
Yuji Nakatani, Yoshio Furusho,* and Eiji Yashima*
Catenanes and related mechanically interlocked molecules[1]
have been regarded as intriguing candidates to serve as
indispensable components for nanomachines and nanodevi-
ces, since control over the relative motion of the components
is possible by external stimuli, such as heat and light, which
lead to changes in the catenanesꢀ physical properties, such as
conductivity, fluorescence, and circular dichroism (CD).[2]
The past three decades have witnessed substantial progress
in recognition motifs for the synthesis of mechanically
interlocked molecules, most of which rely on noncovalent
interactions,[3–7] such as metal coordination,[3] hydrogen
bonding,[4] hydrophobic effects,[5] and charge transfer inter-
actions,[6] which arrange their synthetic
by acid–base interactions.[8,9] Therefore, the amidinium car-
boxylate salt bridges can be regarded as promising candidates
for a recognition motif for a variety of mechanically
interlocked molecules. Moreover, the salt bridges can accom-
modate chiral substituents on the nitrogen atoms that could
regulate the twist sense of the salt-bridge-based supramole-
cules.[8–10] We have designed a new optically active [2]cate-
nane utilizing the amidinium carboxylate salt bridges. This
catenane is prepared from a chiral amidine strand and an
achiral carboxylic acid strand, both of which bear a crescent-
shaped m-terphenyl ligand with a terminal olefin at each end
(Figure 1). The two complementary molecules form a pre-
intermediates in the correct orientation
favorable for the formation of interlocked
molecules. The noncovalent interactions
used for the directed synthesis of the
interlocked molecules are not only of
significant importance for the synthesis,
but they are often utilized for control over
the relative location or motion of the
components, which can be changed by
external stimuli.[2] Consequently, the devel-
opment of new recognition motifs is very
important for advances in this emerging
research area.
We recently reported the rational
design and synthesis of artificial double
helical supramolecules and polymers con-
sisting of complementary molecular
Figure 1. Schematic illustration of the synthesis of [2]catenanes that relies on the amidinium
carboxylate salt bridges during the RCM reaction.
strands by utilizing amidinium carboxylate salt bridges,
which exhibit high association constants even in polar
solvents, have a well-defined geometry by the nature of the
double hydrogen bonding interactions, and can be controlled
catenane through the salt bridge, which is converted to the
[2]catenane by the ring-closing metathesis (RCM) reaction.[11]
Herein, we describe the synthesis of the optically active
[2]catenane based on the amidinium carboxylate salt bridges
as a new recognition motif and the control of the relative
motion of its macrocyclic components by acid–base interac-
tions together with metal coordination.
The synthesis of the [2]catenane was carried out according
to Scheme 1 (see also the Supporting Information). The
optically active amidine with two arms bearing vinyl groups
(1a) was synthesized by Sonogashira coupling between the
m-terphenyl amidine with two acetylene groups (4a)[9b] and
the m-iodobenzoate derivative with vinyl-group-terminated
oligo(ethylene oxide) side chains (5). The m-terphenyl-bound
carboxylic acid with vinyl-terminated oligo(ethylene oxide)
side chains (1b) was prepared in a similar manner. The
1H NMR spectrum (CDCl3) of an equimolar mixture of 1a
[*] Y. Nakatani, Dr. Y. Furusho, Prof. Dr. E. Yashima
Department of Molecular Design and Engineering
Graduate School of Engineering, Nagoya University
Chikusa-ku, Nagoya 464-8603 (Japan)
Fax: (+81)52-789-3185
E-mail: furusho@apchem.nagoya-u.ac.jp
yashima@apchem.nagoya-u.ac.jp
[**] This work was supported in part by Grants-in-Aid for Scientific
Research from the Japan Society for the Promotion of Science
(JSPS) and for Scientific Research on Innovative Areas, “Emergence
in Chemistry” (21111508) from the MEXT. We thank Dr. Masato
Ikeda for his preliminary work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002382.
and 1b showed a downfield signal for the NH protons (Ha,a’
)
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at d = 13.45 ppm, which indicated the formation of the
precatenane (1a·1b) through the salt bridge (Figure 2). The
precatenane in chloroform was treated with a catalytic
amount of the first-generation Grubbs catalyst to afford the
[2]catenane (2) in 27% yield after chromatographic purifica-
tion; the yield was improved to 68% using toluene as the
solvent instead of chloroform. The individual macrocyclic
amidine and carboxylic acid (3a and 3b) were prepared by
the RCM reaction of 1a and 1b, respectively. The structure of
1
2 was first investigated by H NMR spectroscopy (Figure 2).
The RCM resulted in disappearance of the signals of the
terminal methylene groups and detection of the methine
protons of the internal olefin group. The resonance of the NH
protons was observed at d = 13.46 ppm, and the chemical
shifts of the aromatic protons were close to those of the
precatenane, thus indicating that 2 retained the intertwined
structure bound together by the salt bridge as in the
precatenane structure. The formation of 2 was also suggested
by electrospray ionization mass spectrometry (ESI-MS); the
ESI mass spectrum of a CHCl3/MeOH (1:1, v/v) solution
of 2 showed signals for the mono- and dicationic species
([2+Na]+ and [2 + 2Na]2+) at m/z 2274.10 and 1148.52,
respectively, of which the isotopic patterns were in good
agreement with the simulated ones (Figure S5 in the Support-
ing Information).
Scheme 1. Synthesis of the [2]catenane 2. 1) [PdCl2(PPh3)2], CuI, Et3N/
THF, RT; 2) Grubbs catalyst (first generation), toluene, RT.
A simple equimolar mixture of 3a and 3b and a larger
[1+1]macrocycle can be regarded as other possible structures,
as illustrated in Figure 3. Despite our efforts, we could not
obtain single crystals suitable for X-ray analysis. Final
confirmation of the interlocked nature of 2 was provided by
1
a H NMR spectroscopy comparison and chemical derivati-
zation of 2 in combination with ESI-MS. We first recorded the
1H NMR spectrum of an equimolar mixture of macrocycles
3a and 3b, which was almost identical to the sum spectrum of
those of 3a and 3b and was different from that of 2 (Figure 2).
In addition, the ESI mass spectrum (positive mode) of the
Figure 2. Partial 1H NMR spectra (500 MHz, CDCl3, 258C) of a) the
amidine (1a), b) the carboxylic acid (1b), c) the precatenane (1a·1b),
d) the [2]catenane (2), and e) an equimolar mixture of the macrocycles
(3a and 3b).
Figure 3. Schematic illustration of possible products from RCM reac-
tion of the precatenane (1a·1b) and subsequent degradation reaction
of the [2]catenane (2) and the [1+1]macrocycle through the ester
exchange reaction using tBuOK.
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mixture of 3a and 3b showed only signals attributable to the
macrocyclic species [3a + H]+ and [3b + Na]+ (Figure S6 in
the Supporting Information). Both the 1H NMR spectroscopy
and ESI-MS results unambiguously excluded the possibility of
an equimolar mixture of the macrocycles for the RCM
product. We also investigated the possible formation of the
[1+1]macrocycle. The treatment of 2 with a slight excess of
tBuOK in refluxing THF for 12 h afforded a mixture of
cleaved products through alcoholysis of the ester groups, and
this mixture was subjected to ESI-MS measurements. The
positive-mode ESI mass spectrum of the product mixture did
not exhibit any signals of products from the [1+1]macrocycle,
but rather showed those of the esters and the macrocyclic
amidine (3a), the last of which unambiguously indicates the
catenated structure of 2 (Figure S7 in the Supporting Infor-
mation).
The CD spectrum of a solution of 2 in chloroform showed
first negative and second positive distinct Cotton effects for
the absorption of the m-terphenyl ligands around 300 nm,
thus indicating that the two m-terphenyl groups bound
together by the salt bridge were twisted in one direction by
the optically active (R)-phenylethyl substituents on the
amidine groups[9a] (Figure 4b and Figure S8 in the Supporting
Information). Upon the addition of one equivalent trifluoro-
acetic acid (TFA), the CD intensities were reduced by nearly
two thirds. After the addition of two equivalents TFA, the CD
spectrum became the same as that of the TFA salt of the
amidine macrocycle 3a and remained the same after the
addition of ten equivalents TFA, thus suggesting that the salt
bridge between the two macrocyclic components was
“unlocked” by TFA and that the two macrocyclic components
underwent a virtually free relative rotation around each
other. The 1H NMR spectrum of 2 changed upon the addition
of TFA and reached equilibrium with two equivalents TFA, as
is the case for the CD spectrum (Figure S9 in the Supporting
Information). In the 1H NMR spectrum of the TFA salt of 2,
the signals for macrocyclic components with amidine and
carboxylic acid were almost the same as those for the TFA
salts of 3a and 3b, respectively, supporting the deduction
from the CD results that the two macrocyclic components
were “unlocked” by TFA, undergoing free relative rotation.
The neutralization by the further addition of an equal amount
of diisopropylethylamine (iPr2NEt) completely restored both
CD and 1H NMR spectra, thus indicating that the macrocyclic
components were again “locked” in the twisted arrangement
as a result of the restoration of the salt bridge (Figure 4 and
Figure S9 in the Supporting Information).
Figure 4. a) Schematic illustration of control over on/off switching of
the salt bridge between the two macrocyclic components using an
acid/base or Zn2+/[2.2.1]cryptand system. b) CD and absorption spec-
tra (CDCl3, 0.1 mm, ca. 208C) of the [2]catenane (2) before (blue) and
after the addition of TFA (1 equiv (orange), 10 equiv (red), and further
neutralization with 10 equiv of iPr2NEt (dashed light blue). CD and
absorption spectra of the TFA salt of 3a are also shown (black).
c) Fluorescence spectra (CH2Cl2/THF (10:1, v/v), 0.01 mm, ca. 208C)
of 2 before (red) and after the addition of Zn(ClO4)2 (blue), with
subsequent removal of Zn2+ ion with [2.2.1]cryptand (dashed light
blue). d) Photographs of a solution of 2 in CH2Cl2/THF (10:1, v/v)
under irradiation at 254 nm.
became similar to those for free 3b, whereas those for the
macrocyclic components with amidine groups were totally
different from those of free 3a (Figure S11 in the Supporting
1
The switch between the “locked” and “unlocked” states of
the two macrocyclic components was also achieved by the
sequential addition of Zn(ClO4)2 and [2.2.1]cryptand,[12]
resulting in a similar change in the CD spectra (Figure S10
in the Supporting Information). The CD spectrum of 2
became identical to that of the Zn2+ complex with 3a after the
addition of one equivalent of the Zn2+ ion, thus suggesting
that the salt bridge was “unlocked” by the formation of the
Zn2+ complex through coordination to the amidine residue.
The addition of the ZnII ion caused a significant change in the
1H NMR spectrum of 2, in which the chemical shifts of the
signals for the macrocyclic components with carboxy groups
Information). The H NMR spectral change also supported
the switching between the “locked” and “unlocked” states.
The [2]catenane 2 has conjugated m-terphenyl units, which
emit light centered at 440 nm when irradiated at 300 nm
(Figure 4c). Interestingly, the addition of Zn2+ ions caused a
significant enhancement and red shift of this fluorescence,
leading to a change in color from faint purple to bright bluish
yellow (Figure 4c,d). The CD and fluorescent spectra were
restored after the addition of [2.2.1]cryptand, which trapped
the Zn2+ ion from 2, thus indicating that the salt bridge lock
was restored (Figure 4c and Figures S10 and S11 in the
Supporting Information).
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Communications
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In summary, we have designed and synthesized a new
optically active [2]catenane utilizing amidinium carboxylate
salt bridges as a new recognition motif and have successfully
controlled the relative motion of the two macrocyclic
components by acid–base interactions and metal coordina-
tion. Changes could be detected by CD spectra and fluores-
cence color. This methodology utilizing salt bridges may allow
access to a wide variety of mechanically interlocked mole-
cules owing to the complementarity of the salt bridges as well
as their compatibility with various functional groups.
Received: April 22, 2010
Published online: June 25, 2010
Keywords: amidines · catenanes · circular dichroism ·
nanostructures · salt bridges
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