Solvent- and light-controlled unidirectional transit of a nonsymmetric molecular axle through a nonsymmetric molecular wheel

Article abstract of DOI:10.1002/chem.201201625

The development of a pseudorotaxane motif capable of performing unidirectional threading and dethreading processes under control of external stimuli is particularly important for the construction of processive linear motors based on rotaxanes and, at leas

Full text of DOI:10.1002/chem.201201625

FULL PAPER
DOI: 10.1002/chem.201201625
Solvent- and Light-Controlled Unidirectional Transit of a Nonsymmetric
Molecular Axle Through a Nonsymmetric Molecular Wheel
Arturo Arduini,*[a] Rocco Bussolati,[a] Alberto Credi,*[b] Simone Monaco,[b]
Andrea Secchi,[a] Serena Silvi,[b] and Margherita Venturi*[b]
Abstract: The development of a pseu-
dorotaxane motif capable of perform-
ing unidirectional threading and de-
symmetric structure. Specifically they
are an axle containing a central elec-
tron-acceptor 4,4’-bipyridinium core
functionalized with a hexanol chain at
one side, and a stilbene unit connected
through a C6 chain at the other side,
and a heteroditopic tris(phenylureido)-
calix[6]arene wheel. In apolar solvents
the axle threads into the wheel from its
upper rim and with the end carrying
the OH group, giving an oriented pseu-
dorotaxane structure. After a stopper-
ing reaction, which replaces the small
hydroxy group with a bulky diphenyl-
A
ACHTUNGTRENNUNG
external stimuli is particularly impor-
tant for the construction of processive
linear motors based on rotaxanes and,
at least in principle, it discloses the
possibility to access to rotary motors
based on catenanes. Here, we report a
strategy to obtain the solvent-control-
led unidirectional transit of a molecular
axle through a molecular wheel. It is
based on the use of appropriately de-
signed molecular components, the es-
sential feature of which is their non-
AHCTUNGTRENNUNG
of the stilbene unit from the lower rim
of the wheel, that is, in the same direc-
tion of the threading process. The es-
sential role played by the stilbene unit
to achieve the unidirectional transit of
the axle through the wheel, and to tune
the dethreading rate by light is also
demonstrated.
Keywords: calixarenes · pseudoro-
taxanes · supramolecular chemistry ·
unidirectional transit · viologens
Introduction
ments can be controlled by external stimulation[8,9] makes
such systems appealing for the development of functional
materials.[10,11] Furthermore, studies on switchable pseudoro-
taxanes[12–17] are of great interest for the development of mo-
lecular machines based on rotaxanes, catenanes, and related
interlocked compounds. Specifically, the development of a
pseudorotaxane motif capable of performing unidirectional
threading and dethreading processes[18–25] under the control
of external stimuli (Figure 1a) would be important for the
The principles and methods of supramolecular chemistry ap-
plied to the construction of working devices and molecular
machines represent a powerful strategy for the development
of nanoscience and nanotechnology as well as for the com-
prehension of the several biological processes in which natu-
ral motors and machines operate.[1–5] Pseudorotaxanes, su-
pramolecular complexes minimally composed of a wheel-
type host surrounding an axle-type guest,[6,7] are the simplest
prototypes of artificial molecular level machines.[8] Their
working mechanism is based on the assembly/disassembly of
the axle and the wheel components and it resembles the
threading/dethreading of a needle. The fact that these move-
[a] Prof. A. Arduini, Dr. R. Bussolati, Prof. A. Secchi
Dipartimento di Chimica Organica e Industriale
Universitꢀ degli Studi di Parma
Parco Area delle Scienze 17 A, 43124, Parma (Italy)
Fax : (+39)0521905472
E-mail: arturo.arduini@unipr.it
[b] Prof. A. Credi, S. Monaco, Dr. S. Silvi, Prof. M. Venturi
Dipartimento di Chimica “G. Ciamician”
Universitꢀ di Bologna, Via Selmi 2, 40126 Bologna (Italy)
Fax : (+39)0512099456
E-mail: alberto.credi@unibo.it
Figure 1. Representation of unidirectional threading/dethreading of
a
margherita.venturi@unibo.it
[2]pseudorotaxane with a) nonsymmetric components, b) a processive
linear motor based on a [2]rotaxane, and c) a rotary motor based on a
[2]catenane.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201625.
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construction of processive linear motors based on rotaxanes
(Figure 1b) and, at least as a perspective, for rotary motors
based on catenanes (Figure 1c).[26–28]
The essential feature of molecular motors is indeed the di-
rectional control of the motion, which is achieved by modu-
lating not only the thermodynamics but also the kinetics of
the transition between the mechanical states of the device.
This result can be achieved by applying ratcheting concepts
to the design of the systems.[26–30] Although a few examples
of artificial molecular rotary motors and DNA-based linear
motors[31–34] have been described, only one prototype of a
fully synthetic linear motor molecule is available.[35] This
system, which is not based on a rotaxane, consists of a small
molecular fragment that can “walk” progressively along a
molecular “track” and its operation relies on a rather com-
plex sequence of chemical reactions. Therefore, the con-
struction of linear supramolecular motors is still an open
problem and an important challenge, because linear move-
ments are essential both in Nature and technology. In living
organisms movements related to intracellular trafficking and
muscle contraction are produced by linear motor proteins
such as kinesin, dynein, and myosin;[36] similarly, RNA poly-
merase moves along DNA while carrying out transcrip-
tions.[2] Indeed, biological supramolecular architectures are
themselves the premier, proven examples of the feasibility
and utility of nanotechnology, and constitute a sound ratio-
Figure 2. Formation of “up” and “down” orientated pseudorotaxane iso-
mers by self-assembly of the nonsymmetric wheel and axle components.
(Figure 2). In fact, we showed that in apolar media (e.g.,
benzene) compound 1 is able to act as a very efficient[38]
wheel that can be threaded exclusively from the upper rim
by axles derived from 4,4’-bipyridinium salts to yield orient-
ed pseudorotaxanes.[37,39] This behavior was explained by an-
alyzing the main chemical and structural features of com-
pound 1 as a host, which are 1) a p-donor macrocyclic cavity
that, because of its width, can include the positively charged
bipyridinium unit of the axle, but not together with its
countACTHNURTGNEUeGN r anions, 2) three efficient hydrogen-bonding donor
ureidic groups at the upper rim that, by complexing the
counter anions of the axle, can assist the insertion of the cat-
ionic portion of the latter into the cavity from this rim, and
3) three methoxy groups at the lower rim that, in apolar
media, are oriented towards the interior of the cavity in the
NMR timescale,[43] thereby hindering the access of the guest
from this direction. The use of a more polar solvent has a
profound effect. In fact, the solvent polarity affects both the
concentration of the active guest available in solution and
the binding ability of the wheel, by changing the extent of
ion pairing of the axle and decreasing the pivoting role of
the three ureidic groups of the host, respectively.
Herein, we report a study in which the unidirectional
transit of a molecular axle through wheel 1 is clearly demon-
strated; the experimental results show indeed that the
threading of the axle occurs from the upper rim of the
wheel, and that the subsequent solvent-controlled dethread-
ing takes place from the lower rim of the wheel, that is, in
the same direction as the threading (Figure 1a).
ACHTUNGTRENNUNG
ditopic nonsymmetric tris(phenylureido)calix[6]arene deriv-
ative 1 (Scheme 1) as a three-dimensional macrocyclic com-
ponent[42] for obtaining oriented pseudorotaxanes and rotax-
anes brings about a significant increase in structural com-
plexity because, in principle, it is possible to selectively ad-
dress the threading of suitable axles from its “upper” or
“lower” rim leading to “up” and “down” oriented isomers
Results and Discussion
Design and synthesis: To obtain pseudorotaxane systems
based on wheel 1 capable of undergoing unidirectional
threading/dethreading motion, the axles 2 and 3 as tosylate
salts have been synthesized (see Scheme 2 and the Experi-
mental Section). They are composed by a central electron-
acceptor 4,4’-bipyridinium core functionalized with an hexa-
nol chain at one side, and a stilbene (2) or a tBu-substituted
stilbene (3) unit connected through a C6 chain at the other
side (Scheme 1). The terminal OH group was incorporated
because it is easily involved in stoppering reactions, whereas
the stilbene and the tBu -substituted stilbene head groups
Scheme 1.
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Unidirectional Transit of a Nonsymmetric Molecular Axle
FULL PAPER
Scheme 2. Reagents and conditions: a) PdACHTNUGTRNEGN(U OAc)2, K2CO3, N,N-dimethyl-b-alanine, DMF, heating to reflux, 6 h; b) oxalyl chloride, CHCl3, heating to
reflux, 2 h; c) NEt3, 4-dimethylaminopyridine (DMAP), CH2Cl2, 48 h; d) 4,4’-bipyridine, CH3CN, heating to reflux, 72 h; e) CH3CN, heating to reflux,
15 d; f) CH3CN, heating to reflux, 15 d.
were selected because 1) their dimensions are not too big to
prevent their slippage[44,45] through wheel 1, but large
enough to enable a kinetic control of the threading/de-
ACHTUNGTRENNUNGthreading of the axles in the wheel, as molecular mechanics
calculations indicate,[46] and 2) their capability to undergo
E–Z photoisomerization with a consequent change in struc-
ture and hampering effect[47–49] that can be exploited as a
further element to kinetically control the threading/de-
ACHTUNGTRENNUNG
threading movements.[50] For comparison purposes axles 4–5
were also synthesized (Scheme 2).
To verify the hypothesis that the stilbene-type head group
of the synthesized axles can act as a kinetic control element
during the threading process, wheel 1 and an excess of axle
2 or 3 were equilibrated in C6D6 at room temperature in
separate experiments (Scheme 3). A red solution was ob-
tained in both cases and, after removal of the excess of the
axle, a full NMR analysis was performed.
As expected, the threading of axles 2 or 3 into the wheel
results in a substantial rearrangement of the calix[6]arene
skeleton; for example a downfield shift of about 1.1 ppm of
the methoxy groups and the appearance of the AX system
of two doublets experienced by the six pseudo-axial and the
six pseudo-equatorial protons of the methylene bridges are
clearly evidenced.[37,39] In addition, the six ureido NH pro-
tons, because of their involvement in hydrogen bonding with
the two tosylate anions, are shifted downfield of about
3 ppm. The inclusion inside the wheel also affects the reso-
nances of axles 2 and 3. The NMR signals of both com-
pounds 2 and 3 in the pseudorotaxanes (Figure 3) are fully
assigned through 1D and 2D NMR experiments (Fig-
Scheme 3. Self-assembly of P[1ꢀ2]up and P[1ꢀ3]up
, and synthesis of
R[1ꢀ4]up and R[1ꢀ5]up
.
the protons of the two pyridinium moieties experience an
asymmetrical magnetic environment due to their embedding
into the truncated cone shape of the calix[6]arene macrocy-
cle. Moreover 2D TOCSY experiments evidence the pres-
ence of only one set of signals for the two alkyl chains
linked to the 4,4’-bipyridinium core in both pseudorotaxanes
(Figures S7 and S11 in the Supporting Information). These
findings suggest that one pseudorotaxane-type complex does
form predominantly in a benzene solution. 2D ROESY
NMR experiments (Figures S8, S12, and S13 in the Support-
ACHTUNGTRENNUNGures S5–S13 in the Supporting Information). In particular,
the aromatic protons of the 4,4’-bipyridinium core originate
four doublets denoted as (7), (7’), (8), and (8’) centered in
both pseudorotaxane complexes at d=8.1, 6.9, 7.9, and
6.8 ppm, respectively. As reported in previous studies,[37,39]
the presence of only two pairs of coupled doublets indicates
that the axles have effectively threaded the wheel, because
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A. Arduini, A. Credi, M. Venturi et al.
hydrogen bonds, and 4) hydro-
gen bonding between the coun-
ter anions of the axles and the
ureidic NH fragments of the
wheel.
To determine their stability
constants, titrations of axles 2
and 3 with wheel 1 have been
performed in CH2Cl2 at room
temperature. In both cases,
upon addition of the wheel to a
solution of the axle the absorp-
tion features in the l=320–
600 nm spectral region, charac-
teristic for the CT interac-
tions[38,41] (Figure S24 in the
Supporting Information). The
titration curves are satisfactori-
ly fitted with a 1:1 association
model and yield apparent sta-
bility constants of the order
106 mÀ1 for both pseudorotax-
anes, which are in agreement
with the results previously ob-
tained for similar systems.[38,40,52]
Also the energy (lmax ꢁ460 nm)
Figure 3. 1H NMR spectra (300 MHz, C6D6) of different pseudorotaxanes: a) P[1ꢀ2]up and b) P[1ꢀ3]up. The
most diagnostic resonances of axles 2 and 3 have been numbered according to the pseudorotaxane sketch
placed above the spectra (resonances of 1 have not been labeled for clarity, for further NMR characterizations
see Figures S5–S13 in the Supporting Information).
and
the
intensity
(emax
ing Information) finally confirm that the stilbene-based
head group of axles 2 and 3 is positioned at the upper rim of
the wheel in both pseudorotaxanes thereby suggesting that
both axles enter the wheel through the calixarene upper rim
with their OH terminus to give the oriented species
P[1ꢀ2]up and P[1ꢀ3]up (Scheme 3). This strict orientational
control during the formation of the pseudorotaxane can be
explained on the basis of the structural features of both the
wheel[40] and the axle. In fact, at least in principle, axles 2
and 3 could enter the wheel from its upper rim either
through the OH or the stilbene terminus. The fact that
under these experimental conditions only one orientational
pseudorotaxane isomer is formed clearly indicates that the
wheel pivots the threading of the axles from the upper rim
and that the latter components access the macrocycle
through the less bulkier OH terminus in a process that is ki-
netically controlled by the different size of the end groups
of the axles (for a simplified energy diagram describing the
possible threading modes see Figure S27 in the Supporting
Information). This also means that, in agreement with our
expectations, the presence of the stilbene-type units enables
the kinetic control of the threading process, leading to the
formation of the oriented pseudorotaxanes P[1ꢀ2]up and
P[1ꢀ3]up (Scheme 3). These pseudorotaxane structures, as
already evidenced for similar species,[51] are stabilized by a
variety of interactions, including 1) charge-transfer (CT) in-
teractions between the p-electron-accepting 4,4’-bipyridini-
um unit incorporated in the axles and the p-electron-rich ar-
ꢁ500mÀ1 cmÀ1) of the CT absorption band exhibited by
these pseudorotaxanes are consistent with those of similar
compounds previously investigated.[38]
The OH end group of pseudorotaxanes P[1ꢀ2]up and
P[1ꢀ3]up was then replaced by the bulky diphenylacetyl
moiety by submitting the pseudorotaxanes to the stoppering
reaction with diphenylacetyl chloride in toluene (Scheme 3).
Compounds R[1ꢀ4]up and R[1ꢀ5]up were obtained and after
separation from the reaction mixture they were fully charac-
terized (Figures S14–S22 in the Supporting Information).
NMR analysis confirms that in their structure the stilbene-
based group is still positioned at the upper rim of the wheel.
In the case of R[1ꢀ4]up the exact orientation of its axle with
respect to the wheel rims was further confirmed by compari-
son with the structure of compound R[1ꢀ4]down, which was
however obtained by threading wheel 1 with axle 4 in ben-
zene by heating to reflux (Scheme 4). The outcome of this
threading process gave a mixture of both isomers R[1ꢀ4]up
and R[1ꢀ4]down in which the latter was the main isomer
(ꢁ90%). The presence of the up isomer was verified by
2D NMR experiments (see the Supporting Information).
The down isomer was also prepared to verify whether the
stilbene units can pass through the calix[6]arene annulus.
Besides the NMR analysis, compounds R[1ꢀ4]up and
R[1ꢀ5]up were also characterized by UV/Vis absorption
spectroscopy in air-equilibrated CH2Cl2 solution at room
temperature. Their absorption spectra (Figure 4) show for
l<350 nm two bands that, by comparison with the separat-
ed molecular components, can be attributed as follows: the
intense band with maximum at around l=260 nm is as-
omatic cavity of the wheel, which are responsible of the red
.
À
À
color of the solution, 2) C H··· p interactions, 3) C H···O
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Unidirectional Transit of a Nonsymmetric Molecular Axle
FULL PAPER
AHCTUNGTRENNUNG
Dethreading of compounds R[1ꢀ4]up and R[1ꢀ5]up: As al-
ready mentioned, in apolar solvents compounds R[1ꢀ4]up
and R[1ꢀ5]up are mainly stabilized by 1) CT interactions be-
tween the aromatic rings of the wheel and the bipyridinium
ion contained in the axles, 2) hydrogen bonding between the
counter anions of the axles and the ureidic groups of the
wheel, and 3) solvophobic effects on account of the low sol-
ubility of the bipyridinium salts in apolar solvents. These in-
teractions are substantially weakened in polar solvents and,
therefore, it is expected that dissolution of compounds
R[1ꢀ4]up and R[1ꢀ5]up in such kind of solvents induces axle
Scheme 4. Synthesis of compound R[1ꢀ4]down
.
de
stopper at one end of the axles, it is also expected that de-
AHCTUNGTREGtNNNU hreading occurs by involving the other end side, that is,
ACHTUNGTRENtNUGN hreading. Because of the presence of the diphenylacetyl
through the slippage of the sufficiently slim stilbene-type
unit from the lower rim of the wheel.
Dethreading of R[1ꢀ4]up and R[1ꢀ5]up in the polar sol-
vent DMSO was indeed verified through NMR measure-
ments, which clearly show the presence of the axle and
wheel as separate components in solution (Figure 5). Sub-
stantial insight into the dethreading process was obtained by
monitoring the CT absorption band at around l=460 nm
over time, which is characteristic of the assembled structure,
after dissolution of R[1ꢀ4]up and R[1ꢀ5]up in DMSO. For
both compounds the intensity of this band decreases with
time (Figure 6a for R[1ꢀ4]up and Figure S23 in the Support-
ing Information for R[1ꢀ5]up); the decrease is faster for
R[1ꢀ4]up than for R[1ꢀ5]up. The absorption spectra ob-
served when the time-dependent changes are over coincide
with the sum of the spectra of the respective separated com-
ponents. This finding is fully consistent with the dethreading
of the compounds caused by the solvent-induced weakening
of the intercomponent interactions.
Figure 4. Absorption spectra (CH2Cl2, RT) of compounds R[1ꢀ4]up (solid
line) and R[1ꢀ5]up (dashed line). The inset shows an enlarged view of the
charge-transfer absorption bands.
signed to the absorption of the wheel component and the
4,4’-bipyridinium unit contained in the axle components,
whereas the slightly structured band at around l=320 nm is
ascribed to the stilbene-type end group of the axles.
Data fitting[53] shows that the absorbance decays at l=
460 nm take place with a first-order kinetic law and with
rate constants of 5.9ꢂ10À3 and 9.3ꢂ10À5 sÀ1 for R[1ꢀ4]up
(Figure 6b, full circles) and R[1ꢀ5]up (Figure 7, full circles),
respectively. The fact that the rate constants depend on the
stilbene structure (unsubstituted and tBu-substituted) is a
clear evidence that dethreading occurs through the slippage
of this group from the lower rim of the wheel. The two
order of magnitude lower rate constant of R[1ꢀ5]up com-
pared with R[1ꢀ4]up can indeed be explained on the basis of
the higher hampering effect of the tBu-substituted stilbene
present in axle 5 compared with the unsubstituted one com-
prised in axle 4.
The spectra of R[1ꢀ4]up and R[1ꢀ5]up also show a weak,
broad band with a maximum at around l=460 nm that is
not present in the spectra of the molecular components and
is responsible for the red color (Figure 4, inset). As already
discussed in the case of P[1ꢀ2]up and P[1ꢀ3]up, this visible
band can be ascribed to CT interactions between the elec-
tron-rich aromatic rings of the wheel and the electron-poor
4,4’-bipyridinium unit contained in the axles. Specifically,
the visible absorption bands of the examined compounds
are very similar, as far as energy and intensity are con-
cerned, to those of rotaxanes containing wheel 1 and 4,4’-bi-
pyridinium-based dumbbells carrying at both ends a diphen-
Further proofs of these dethreading mode and direction
are given by comparison with the behavior of the pseudoro-
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A. Arduini, A. Credi, M. Venturi et al.
Figure 5. 1H NMR spectra (300 MHz, [D6]DMSO) of a) axle 4, compound R[1ꢀ4]up b) 5, c) 10, and d) 30 min after dissolution, and e) wheel 1. The most
diagnostic resonances of axle 4 have been numbered according to the formula placed above the stacked plot.
taxanes P[1ꢀ2]up and P[1ꢀ3]up, and of rotaxanes formed by
dumbbells carrying the diphenylacetyl group at both ends.
As expected, dissolution of P[1ꢀ2]up and P[1ꢀ3]up in DMSO
causes their quantitative and fast dethreading. Indeed, the
absorption spectra of such solutions, monitored immediately
after dissolution (within 30 s), do not show any CT band, in-
dicating that disassembly occurs on a timescale faster than
that required for dissolution; in other words, the dethread-
ing rate constant is >0.1 sÀ1. This value, much higher than
that observed for R[1ꢀ4]up and R[1ꢀ5]up, indicates that for
the pseudorotaxanes a different dethreading mechanism op-
erates, that is, axles 2 and 3 dethread with the side carrying
the OH group from the upper rim of the wheel, that means
in a direction opposite to that of the threading. Conversely,
the rotaxanes containing dumbbells stoppered by the di-
bene-type unit in the dethreading process, and investigate
the possibility of photocontrolling the dethreading rate,
compounds R[1ꢀ4]up and R[1ꢀ5]up were irradiated in
CH2Cl2 solution with l=334 or 313 nm to induce the E–Z
isomerization of the stilbene end group of the axle (see Fig-
ure S25 in the Supporting Information). The photoreaction
did not cause any change in the CT absorption band, sug-
gesting that the configuration of the stilbene unit does not
affect the inclusion of the 4,4’-bipyridinium ion inside the
wheel. Only the band in the l=300–350 nm region, charac-
teristic for the stilbene unit, is affected, as also evidenced by
irradiation carried out on separate axles 4 and 5. For both
compound R[1ꢀ4]up and R[1ꢀ5]up we found that an irradia-
tion of 90 min causes about 70% conversion of the stilbene
unit from the E to the Z isomer.[54–56] After evaporation of
CH2Cl2 and addition of DMSO, the CT absorption bands of
ACHTUNGTRENNUNGphenACHTUNGTRENNUNGylacetyl group at both ends do not undergo any struc-
tural changes in DMSO, as evidenced by the fact that in this
solvent their CT absorption bands are still present and do
not change over time. This finding is consistent with the fact
that the diphenylacetyl moiety is too bulky to pass through
the rims of the wheel.
R[1ꢀ4]up and R[1ꢀ5]up were monitored. For the photo
ACHTUNGTRENNUNGisom-
AHCTUNGTRENNUNG
ed ones, the CT band decreases over time indicating the oc-
currence of axle dethreading.
In the case of R[1ꢀ4]up the best fitting of the absorption
values at l=460 nm clearly evidences that the disappear-
ance of CT band occurs according to two superimposed
first-order kinetics (Figure 6b, open circles) with rate con-
Photocontrol of the dethreading rate in R[1ꢀ4]up and
R[1ꢀ5]up: To further support the involvement of the stil-
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Unidirectional Transit of a Nonsymmetric Molecular Axle
FULL PAPER
agreement with the higher hampering effect of the Z isomer
compared with the E isomer, which results in a much more
difficult slippage of this unit through the lower rim of the
wheel. It is interesting to notice that the E–Z photoisomeri-
zation affects the dethreading rate constant more than the
incorporation of the tert-butyl group on the stilbene unit.
The rate constant observed upon isomerization is indeed
about one order of magnitude slower than that observed in
the case of the substituted stilbene.
In the case of R[1ꢀ5]up a decrease of the CT absorption
band of only about 30% is observed. The best fitting of the
absorption values at l=460 nm (Figure 7, open circles) evi-
dences that this decrease occurs according to a first-order
kinetics with a rate constant of 4ꢂ10À5 sÀ1, which is quite
similar to that obtained for the dethreading of the non-irra-
diated R
N
of the CT absorption band concerns the portion of the com-
pound with the axle carrying the E isomer of the tBu stil-
bene, and that the photoisomerized compound does not un-
dergo dethreading, accounting for the 70% of the CT ab-
sorption intensity that does not disappear in DMSO. Hence,
our results show that the Z isomer of the tBu stilbene is too
bulky to pass through the lower rim of the wheel, and that
R[1ꢀ5]up in its Z configuration behaves as a real rotaxane.
Unidirectional transit of the axle through the wheel: Taken
together the experiments reported here show that the strat-
egy we developed enables to obtain the unidirectional trans-
it of a molecular axle through a molecular wheel. Our strat-
egy is based on the use of appropriately designed molecular
components, an essential feature of which is their nonsym-
metric structure, and exploits the following steps.
Figure 6. a) Decrease of the CT absorption band of compound R[1ꢀ4]up
(2.2ꢂ10À4 m) after dissolution in DMSO at RT. b) Absorbance decrease
at l=460 nm upon dissolution in of R[1ꢀ4]up DMSO before (full circles)
and after (open circles) exhaustive irradiation at l=334 nm. The first-
order fitting curves are also shown.
1) In apolar solvents axle 2 or 3 threads into wheel 1 from
its upper rim and with the end carrying the -H group. An
oriented pseudorotaxane structure is obtained in which
the OH group is positioned at the lower rim of the
wheel. This threading mode experiences an energetic
barrier substantially lower than that of the stilbene
threading because of the small hampering effect of the
OH group (Figure 8a).
2) The stoppering reaction of the obtained pseudorotaxane
replaces the small OH group with a bulky diphenylacetyl
moiety. This reaction converts the pseudorotaxane in a
rotaxane-like species because one end of the axle carries
the diphenylacetyl stopper and the other end terminates
with the stilbene-type unit that prevents dethreading for
kinetic reasons (Figure 8b).
3) Replacement of the apolar solvent with the polar solvent
DMSO weakens the interactions that stabilize the assem-
bled structure. Such a destabilization substantially lowers
the energy barrier associated with the slippage of the stil-
bene unit and induces the axle dethreading from the
lower rim of the wheel (Figure 8c), that is, in the same
direction of the axle threading. The complete unidirec-
tional transit of the axle through the wheel is, therefore,
accomplished.
Figure 7. Absorbance decrease at l=460 nm upon dissolution of
R[1ꢀ5]up in DMSO (2.3ꢂ10À4 m) before (full circles) and after (open cir-
cles) exhaustive irradiation at l=313 nm. The first-order fitting curves
are also shown.
stants of 5.9ꢂ10À3 and 2.3ꢂ10À5 sÀ1, respectively. The faster
process, which accounts for 30% of the total decay, is attri-
ACHTUNGTRENNUNGbuted to the dethreading of the axle carrying the stilbene E
isomer because its rate constant coincides with the one ob-
tained for non-irradiated R[1ꢀ4]up. Consequently, the slower
process can be assigned to the dethreading of the axle com-
prising a Z stilbene. The low value of its rate constant is in
Chem. Eur. J. 2012, 18, 16203 – 16213
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16209

A. Arduini, A. Credi, M. Venturi et al.
AHCTUNGTRENNUNG
thesized according to literature procedures.
(E)-4-(4-tert-Butylstyryl)benzoic acid (8b): To a stirred solution of DMF
(250 mL) kept under argon atmosphere, 4-bromo benzoic acid (7) (1 g,
4.97 mmol), PdACHTNUGRTENUNG(OAc)2 (cat.), 1-tert-butyl-4-vinylbenzene (6b) (1.2 g,
7.5 mmol), K2CO3 (1.4 g, 10 mmol), N,N-dimethyl-b-alanine (cat.) were
added. The resulting heterogeneous mixture was heated to reflux for 6 h.
After this period, the reaction mixture was cooled to room temperature,
diluted with water (100 mL), and extracted with diethyl ether (2ꢂ
100 mL). The organic phase was separated, dried with Na2SO4, and
evaporated under reduced pressure. The orange oily residue was taken
up with ethyl acetate and the desired product 8b was precipitated as a
white solid (1 g, 71%) by addition of n-hexane. M.p. 296.0–297.18C;
1H NMR (300 MHz, CDCl3/CD3OD 1:1): d=7.96 (d, J=8 Hz, 2H), 7.49
(d, J=8 Hz, 2H), 7.42 (d, J=8 Hz, 2H), 7.33 (d, J=8 Hz, 2H), 7.15 (d,
J=16 Hz, 1H), 7.03 (d, J=16 Hz, 1H), 1.27 ppm (s, 9H); 13C NMR
(75 MHz, CDCl3/CD3OD 1:1): d=151.4, 146.1, 142.0, 133.8, 130.9, 130.2,
128.7, 126.7, 126.4, 126.0, 125.6, 77.2, 31.1 ppm; MS (ESI): m/z (%): 279
(100) [MÀH]À.
General procedure for the synthesis of compounds 11a and 11b: In a
100 mL round-bottomed flask, the appropriate stilbene derivate 8a or 8b
(4.5 mmol) and oxalyl chloride (13.4 mmol) were dissolved in dry CHCl3
(40 mL). The resulting solution was heated to reflux for two hours under
a nitrogen atmosphere, then the solvent was evaporated under reduced
pressure. The yellow solid residue of the acylic chloride (9a or 9b) was
dissolved in dry CH2Cl2 (50 mL). To the resulting solution, 6-hydroxyhex-
yl tosylate (10) (1.8 g, 6.7 mmol), triethyl amine (0.68 g, 6.7 mmol) and
DMAP (cat.) were added. The reaction mixture was stirred at room tem-
perature for 72 h, then the reaction was quenched by addition of water
(50 mL). The separated organic phase was extracted with a saturated sol-
ution of NaHCO3 in H2O (2ꢂ20 mL). After evaporation of the solvent
under reduced pressure, the oily residue was purified by column chroma-
tography (hexane/ethyl acetate 8:2).
Figure 8. Simplified potential energy curves representing the steps that
account for the unidirectional transit of the axle through the wheel. The
horizontal coordinate of the diagrams represents the axle–wheel distance
when they approach one another along the direction and with the orien-
tation shown in the cartoons. a) Threading of the axle through the upper
rim of the wheel in apolar solvents. b) The stoppering reaction to convert
the pseudorotaxane into a rotaxane-like species. c) Dethreading of the
axle from the lower rim of the wheel occurring in polar solvents.
Conclusion
Although our purpose was not to develop a system of practi-
cal use, it should be noted that the experiments described
here have several drawbacks. First of all, the chemical reac-
tions and the solvent replacement, which is necessary to
obtain the unidirectional transit, make the overall switch
procedure rather unpractical, and lead to the accumulation
of chemical debris. Full reset and cycling of the system is
possible, at least in principle, upon detachment of the di-
Compound 11a was isolated as a white solid (1.2 g, 56%). M.p. 69–718C;
1H NMR (300 MHz, CDCl3): d=8.01 (d, J=8 Hz, 2H), 7.79 (d, J=8 Hz,
2H), 7.57–7.52 (m, 4H), 7.4–7.3 (m, 5H), 7.22 (d, J=17 Hz, 1H), 7.12 (d,
J=17 Hz, 1H), 4.27 (t, J=6.5 Hz, 2H), 4.03 (t, J=6.5 Hz, 2H), 2.43 (s,
3H), 1.8–1.7 (m, 4H), 1.45–1.35 ppm (m, 4H); 13C NMR (75 MHz,
CDCl3): d=166.3, 144.6, 141.7, 136.6, 133.1, 131.1, 129.9, 129.7, 129.1,
128.7, 127.8, 127.5, 126.7, 126.2, 70.3, 64.6, 28.7, 28.5, 25.4, 25.0, 21.6 ppm;
MS (ESI): m/z (%): 501 (50) [M+Na], 517 (25) [M+K]+.
Compound 11b was isolated as colorless oil (1.42 g, 60%). 1H NMR
(300 MHz, CDCl3): d=8.01 (d, J=8 Hz, 2H), 7.78 (d, J=8 Hz, 2H), 7.55
(d, J=8 Hz, 2H), 7.47 (d, J=8 Hz, 2H), 7.39 (d, J=8 Hz, 2H), 7.33 (d,
J=8 Hz, 2H), 7.20 (d, J=16 Hz, 1H), 7.10 (d, J=16 Hz, 1H), 4.27 (t, J=
6.6 Hz, 2H), 4.03 (t, J=6.5 Hz, 2H), 2.44 (s, 3H), 1.75–1.50 (m, 6H) 1.4–
1.3 ppm (m, 11H); 13C NMR (75 MHz, CDCl3) d=166.4, 151.4, 144.6,
142.0, 133.9, 133.1, 131.0, 129.9, 129.8, 128.8, 127.8, 126.7, 126.5, 126.1,
125.7, 70.3, 64.6, 34.6, 31.2, 28.7, 28.4, 25.4, 25.1, 21.6 ppm; MS (ES): m/z
(%): 557 (40) [M+Na]+, 573 (15) [M+K]+.
ACHTUNGTRENNUNGphenACHTUNGTRENNUNGylacetate stopper from the axle and re-dissolution of
the latter, together with the wheel, in an apolar solvent. In
compounds 2 and 3 such a selective de-stoppering is pre-
vented by the nearly identical reactivity of the ester linkages
at the two ends of the axles. All these limitations, however,
could be removed with a more careful design of the axle
component. Finally, it is important to stress the essential
role played by the stilbene unit incorporated at one end of
the axle. It indeed enables 1) to achieve the unidirectional
transit of the axle through the wheel, because its dimensions
are not too big to prevent slippage through the wheel, but
big enough to induce a kinetic control of the axle threading/
dethreading processes and 2) to tune the dethreading rate
because of the possibility to modify its hampering effect
upon the use of stilbene unit substituted with relatively
bulky groups, or, more interesting, upon photoisomerization.
General procedure for the synthesis of compounds 12a and 12b: 4,4’-Bi-
pyridine (1.2 g, 7.7 mmol) and the appropriate stilbenic derivative 11a or
11b (2.5 mmol) were dissolved in CH3CN (50 mL). The resulting solution
was heated to reflux for 72 h, then the solvent was evaporated to dryness
under reduced pressure. The desired products were isolated by treating
the yellowish oily residue with hot ethyl acetate (2ꢂ25 mL) followed by
suction filtration.
Experimental Section
Compound 12a was isolated as a white solid (0.8 g, 52%). M.p. 189.0–
191.28C; H NMR (300 MHz, CD3OD): d =9.16 (d, J=6.5 Hz, 2H), 8.90
1
(d, J=6.5 Hz, 2H), 8.51 (d, J=6.5 Hz, 2H), 8.18 (d, J=8 Hz, 2H), 7.95
(d, J=8.1 Hz, 2H), 7.7–7.6 (m, 6H), 7.4–7.2 (m, 8H), 4.70 (t, J=7 Hz,
2H), 4.33 (t, J=7 Hz, 2H), 2.34 (s, 3H), 2.10 (t, J=7 Hz, 2H), 1.82 (t,
J=7 Hz, 2H), 1.65–1.31 ppm (m, 4H); 13C NMR (75 MHz, CD3OD): d=
166.6, 152.5, 148.2, 145.1, 144.2, 141.9, 140.1, 136.4, 131.2, 129.6, 128.5,
128.4, 128.0, 127.2, 127.0, 126.4, 126.2, 126.1, 125.4, 123.1, 64.4, 61.5, 31.0,
29.3, 28.1, 25.4, 25.1, 20.7 ppm; MS (ESI): m/z (%): 463 (100) [MÀTsO]+
(Ts=tosyl).
Materials and synthetic methods: Toluene and dichloromethane were
dried by using standard procedure, all other reagents were of reagent
grade quality obtained from commercial suppliers and were used without
further purification. NMR spectra were recorded at 400 and 300 MHz
for 1H and 100 and 75 MHz for 13C. Melting points are uncorrected.
Chemical shifts are expressed in [ppm] (d) by using the residual solvent
signal as internal reference. Mass spectra were recorded in ESI mode.
Calix[6]arene (1),[37] (E)-4-styrylbenzoic acid (8a),[57] 6-hydroxyhexyl tos-
16210
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 16203 – 16213

Unidirectional Transit of a Nonsymmetric Molecular Axle
FULL PAPER
Compound 12b was isolated as a white solid (1.3 g, 77%). M.p. 171.8–
173.88C; 1H NMR (300 MHz, CDCl3): d=9.29 (d, J=6.5 Hz, 2H), 8.74
(d, J=6.5 Hz, 2H), 8.18 (d, J=6.5 Hz, 2H), 7.94 (d, J=8 Hz, 2H), 7.73
(d, J=8 Hz, 2H), 7.57 (d, J=6 Hz, 2H), 7.51 (d, J=8 Hz, 2H), 7.45 (d,
J=8 Hz, 2H), 7.38 (d, J=8 Hz, 2H), 7.17 (d, J=16.5 Hz, 2H), 7.08 (d,
J=6 Hz, 2H), 7.04 (d, J=16.5 Hz, 2H), 4.77 (d, J=7 Hz, 2H), 4.20 (d,
J=7 Hz, 2H), 2.23 (s, 3H), 1.94 (d, J=7 Hz, 2H), 1.64 (d, J=7 Hz, 2H),
1.31 ppm (brs, 13H); 13C NMR (75 MHz, CD3OD): d=166.6, 154.1,
151.7, 145.9, 142.4, 141.3, 139.7, 134.3, 131.4, 130.2, 129.0, 126.8, 126.5,
126.2, 126.1, 121.9, 64.9, 62.1, 34.9, 31.8, 31.2, 28.7, 26.0, 25.9, 21.3 ppm;
MS (ESI): m/z (%): 519 (100) [MÀTsO]+.
(4 mL). After stirring at RT for 2 h, the resulting deep-red colored
hetero
submitted to NMR analysis with no further purification.
Compound P
1H NMR (300 MHz, C6D6): d=9.5 (brs, 6H), 8.44
ACHTUNGTRENgNUNG eneous solution was filtered off to remove the excess of axle and
A
:
(d, J=8 Hz, 2H), 8.27 (d, J=8 Hz, 4H), 8.2–7.9 (3br s, 10H), 7.70 and
7.7–7.5 (s and m overlapped, 9H), 7.42 (d, J=8 Hz, 6H), 7.2–6.8 (m, d,
J=8 Hz, m, 26H), 4.66 (d, J=14 Hz, 6H), 4.41 (brt, J=7 Hz, 2H), 4.1–
3.3 (s, brs, brs and m, 48H), 2.1–2.0 and 2.06 (brs and s overlapped, 8H),
1.80 and 1.69 (2s, 36H), 1.5–1.1 (2m, 22H), 1.1–0.9 ppm (brs, 6H).
Compound P
[1ꢀ3]up
:
1H NMR (300 MHz, C6D6): d=9.5 (brs, 6H), 8.44
(d, J=7.5 Hz, 2H), 8.26 (brd, J=6 Hz, 4H), 8.2–7.8 (3brs, 10H), 7.8
(brs, 2H), 7.69 (s, 6H), 7.6–7.5 (m, 12H), 7.2–7.1 (m, 12H), 7.03 (d, J=
8 Hz, 4H), 7.0–6.7 (m, 7H), 4.66 (d, J=15 Hz, 6H), 4.41 (brs, 2H), 4.1–
3.3 (s, brs, brs and m, 48H), 2.2 (brs, 6H), 2.0 (brs, 6H), 1.9–1.5 (2brs,
34H), 1.5, 1.36 and 1.20 (brs, s and brt, J=7 Hz, 38H), 1.1–0.9 ppm (brs,
6H).
General procedure for the synthesis of axles 2 and 3: 6-Hydroxyhexyl
tosACHTUNGTRENNUNGylate (10) (0.3 g, 1.1 mmol) and the appropriate salt 12a and 12b
(0.7 mmol) were dissolved in CH3CN (50 mL). The resulting reaction
mixture was heated to reflux for ten days and then cooled to room tem-
perature. The solvent was evaporated to dryness under reduced pressure
and the crude yellowish solid residue was triturated with hot ethyl ace-
tate (2ꢂ25 mL). After decantation of the ethyl acetate, the desired axles
2 and 3 were purified by recrystallization from CH3CN.
General procedure for synthesis of R
[1ꢀ4]up and R
ACHTUNGTRENNUNG
(0.04 mmol) was suspended in solution of compound
a
1
0.04 mmol) in dry toluene (10 mL). The resulting heterogeneous mixture
was stirred at RT until the solution turned to a deep-red color (2 h). Di-
phenylacetyl chloride (0.015 g, 0.06 mmol) and triethylamine (0.006 g,
0.06 mmol) were then added. After three days the solvent was completely
removed under reduced pressure. The red solid residue was dissolved in
CH2Cl2 (15 mL) and the solution extracted with an aqueous solution of
HCl (10% w/v, 2ꢂ15 mL). The separated organic phase was extracted
with an aqueous solution of NaOTs (2ꢂ15 mL) to regenerate the TsOÀ
ions. The separated organic phase was then dried with CaCl2 and evapo-
rated to dryness under reduced pressure.
Compound 2 was isolated as a white solid (0.45 g, 70%). M.p. 127.0–
128.28C; 1H NMR (300 MHz, CD3OD): d=9.21–9.17 (m, 4H), 8.60 (d,
J=6.5 Hz, 4H), 7.95 (d, J=8.5 Hz, 2H), 7.67 (d, J=8.5 Hz, 2H), 7.63 (d,
J=8.5 Hz, 2H), 7.56 (d, J=7.5 Hz, 2H), 7.39–7.27 (m, 5H), 7.19 (d, J=
8 Hz, 6H), 4.7–4.6 (m, 4H), 4.30 (t, J=6.5 Hz, 2H), 3.54 (t, J=6.5 Hz,
2H), 2.33 (s, 6H), 2.1 (brs, 4H), 1.8 (brs, 2H), 1.6–1.3 ppm (m, 10H);
13C NMR (100 MHz, CD3OD): d=172.9, 166.6, 149.7, 145.6, 142.3, 140.3,
138.9, 136.8, 131.3, 129.6, 128.7, 128.6, 128.4, 128.2, 128.0, 126.9, 126.6,
126.2, 125.6, 64.5, 61.8, 56.9, 30.9, 30.8, 27.9, 25.4, 25.2, 24.8, 21.1,
20.0 ppm; MS (ESI): m/z (%): 281 (50) [MÀ2TsO]2+
.
Compound R
ACHTUNGTRENNUNG
Compound 3 was isolated as a white solid (0.47 g, 70%). M.p. 131.3–
132.88C; 1H NMR (300 MHz, CD3OD): d=9.2 (brs, 4H), 8.6 (brs, 4H),
7.93 (d, J=8.3 Hz, 2H), 7.67 (d, J=8.3 Hz, 2H), 7.60 (d, J=8.3 Hz, 2H),
7.49 (d, J=8.3 Hz, 2H), 7.39 (d, J=8.3 Hz, 2H), 7.2–7.1 (m, 6H), 4.8–4.6
(m, 4H), 4.30 (t, J=6 Hz, 2H), 3.54 (t, J=6 Hz, 2H), 2.33 (s, 6H), 2.1
(brs, 4H), 1.8 (brs, 2H), 1.7–1.4 (m, 10H), 1.31 ppm (s, 9H); 13C NMR
(75 MHz, CD3OD): d=170.4, 155.2, 153.6, 149.5, 147.0, 146.3, 144.2,
137.9, 134.9, 133.41, 132.37, 130.7, 130.2, 130.0, 129.9, 129.4, 129.2, 68.3,
65.6, 65.1, 37.9, 35.7, 34.9, 34.8, 34.2, 31.9, 29.4, 29.3, 29.1, 28.8, 23.8 ppm;
MS (ESI): m/z (%): 986 (15) [M+Na]+, 791 (30) [MÀTsO]+, 310 (100)
(3ꢂ20 mL) to afford 0.06 g of RAHCTNUGTRENNNUG
122.88C; 1H NMR (300 MHz, C6D6): d=9.47 (s, 6H), 8.43 (d, J=8.1 Hz,
2H), 8.3 (brs, 4H), 8.2–7.9 (m, 10H), 7.8 (brs, 3H), 7.69 (s, 6H), 7.6–7.5
(m, 13H),7.45 (d, J=7.3 Hz, 2H), 7.3–7.1 (m, 16H), 7.1–6.9 (m, 5H), 6.8
(brs, 7H), 5.22 (s, 1H), 4.65 (d, J=14.5 Hz, 6H), 4.5–4.4 (m, 4H), 4.0
(brs, 15H), 3.7 (brs, 8H), 3.6–3.4 (m, 14H), 2.2 (brs, 2H), 2.06 (s, 6H)
1.9 (brs, 4H), 1.79 (s, 29H), 1.6 (brs, 2H), 1.40 (s, 9H), 1.4–1.1 (m, 11H),
0.9–0.8 ppm (m, 4H); 13C NMR (100 MHz, C6D6): d=172.1, 166.0, 153.5,
152.8, 148.1, 148.0, 144.4, 142.9, 141.2, 139.5, 139.0, 137.5, 137.1, 133.8,
132.1, 131.0, 130.0, 129.7, 129.3, 128.9, 128.8, 128.6, 128.4, 126.9, 126.5,
125.7, 124.8, 121.2, 118.1, 116.7, 72.4, 69.9, 66.3, 64.5, 61.0, 60.6, 57.3, 34.5,
31.5, 29.8, 29.2, 28.6, 28.3, 26.9, 25.7, 25.2, 20.8, 15.2 ppm; MS (ESI): m/z
[MÀ2TsO]2+
.
General procedure for the synthesis of dumbbells 4 and 5: The same pro-
cedure as employed for the synthesis of compounds 2 and 3 but by using
6-(tosyloxy)hexyl 2,2-diphenylacetate (13) (1.1 mmol) as alkylating agent.
(%): 1112 (100) [MÀ2TsO]2+
; elemental analysis calcd (%) for
C155H176O22S2N8 (2567.25): C 72.52, H 6.91, S 2.50, N 4.36; found: C
72.89, H 7.03, S 2.37, N 4.39.
Compound 4 was isolated as a white solid (0.57 g, 75%). M.p. 172.5–
173.58C; 1H NMR (300 MHz, [D6]DMSO): d=9.24 (d, J=7 Hz, 2H),
9.19 (d, J=7 Hz, 2H), 8.62 (d, J=6 Hz, 4H), 7.98 (d, J=8.5 Hz, 2H),
7.8–7.5 (m, J=8.5 Hz, 8H), 7.4–7.2 (m, 17H), 5.08 (s, 1H), 4.73 (t, J=
7.5 Hz, 2H), 4.63 (t, J=7.5 Hz, 2H), 4.32 (t, J=6.5 Hz, 2H), 4.14 (t, J=
6.5 Hz, 2H), 2.34 (s, 6H), 2.2–2.1 (m, 2H), 2.1–2.0 (m, 2H), 1.9–1.8 (m,
2H), 1.7–1.5 (m, 6H), 1.3 ppm (brs, 4H); 13C NMR (75 MHz,
[D6]DMSO): d=168.2, 151.5, 147.3, 144.2, 142.1, 140.7, 138.7, 132.9,
131.2, 130.4, 130.2, 130.0, 129.9, 129.6, 128.6, 128.5, 128.2, 127.8, 127.2,
66.1, 63.5, 64.4, 58.5, 32.6, 32.5, 29.8, 29.5, 27.1, 26.9, 26.8, 26.5, 21.6 ppm;
MS (ESI): m/z (%) 379.1 (100) [MÀ2TsO]2+, 929 (15) [MÀTsO]+.
Compound R
ACHTUNGTRENNUNG
(3ꢂ20 mL) to afford 0.06 g of RAHCTNUGTRENNNUG
1H NMR (300 MHz, C6D6): d=9.5 (brs, 6H), 8.43 (d, J=8 Hz, 2H), 8.3
(brs, 4H), 8.1–7.9 (m, 10H), 7.8 (brs, 3H), 7.69 (s, 6H), 7.6–7.4 (m,
13H), 7.3–7.1 (m, 13H), 7.04 (brs, 5H), 6.9 (brs, 2H), 6,8 (brs, 5H), 5.22
(s, 1H), 4.65 (d, J=14.5 Hz, 6H), 4.5–4.4 (m, 4H), 4.1–3.9 (brs, 15H),
3.8–3.6 (brs, 8H), 3.6–3.4 (m, 14H), 2.2 (brs, 2H), 2.1 (s, 6H) 1.9 (brs,
4H), 1.79 (s, 29H), 1.6 (brs, 2H), 1.40 (s, 9H), 1.4–1.1 (m, 11H), 0.9 ppm
(brs, 4H); 13C NMR (100 MHz, C6D6): d=172.1, 166.0, 153.5, 152.8,
151.0, 148.1, 148.0, 144.3, 143.0, 142.0, 141.0, 139.6, 139.3, 139.0, 137.4,
134.4, 133.8, 132.1, 131.0, 130.0, 129.5, 129.2, 129.1, 129.0, 128.9, 128.7,
128.4, 127.0, 126.9, 126.8, 126.5, 125.7, 124.8, 121.2, 119.6, 119.1, 118.1,
116.7, 72.4, 69.9, 66.3, 64.9, 64.5, 61.0, 60.6, 59.7, 57.3, 57.1, 34.5, 34.3,
34.1, 31.5, 31.0, 29.8, 29.3, 29.2, 28.6, 28.2, 27.7, 26.8, 26.0, 25.2, 20.8,
15.2 ppm; MS (ES): m/z (%): 1140 (100) [MÀ2TsOÀ]2+; elemental analy-
sis calcd (%) for C159H188O22S2N8 (2623.35): C 72.79, H 7.07, S 2.44, N
4.27; found: C 73.51, H 7.30, S 1.69, N 4.27.
Compound 5 was isolated as a white solid (0.56 g, 70%). M.p. 176.5–
177.58C; 1H NMR (300 MHz, CD3OD): d =9.22 (d, J=8 Hz, 2H), 9.17
(d, J=8 Hz, 2H), 8.60 (d, J=6.5 Hz, 2H), 7.95 (d, J=8 Hz, 2H), 7.67 (d,
J=8 Hz, 4H), 7.62 (d, J=8 Hz, 2H), 7.51 (d, J=8 Hz, 2H), 7.41 (d, J=
8 Hz, 2H), 7.3–7.1 (m, 16H), 5.05 (s, 1H), 4.73 (t, J=7.5 Hz, 2H), 4.62 (t,
J=7.5 Hz, 2H), 4.31 (t, J=6.5 Hz, 2H), 4.13 (t, J=6.5 Hz, 2H), 2.33 (s,
6H), 2.12–2.08 (m, 2H), 2.02–1.96 (m, 2H), 1.81 (t, J=7 Hz, 2H), 1.7–1.5
(m, 6H), 1.32 ppm (s, 9H); 13C NMR (75 MHz, CD3OD): d=171.9,
165.9, 147.3, 142.0, 132.7, 131.2, 130.1, 129.9, 129.8, 128.0, 127.7, 127.2,
127.0, 66.1, 63.4, 58.5, 32.6, 32.5, 31.9, 29.9, 29.5, 27.1, 26.9, 26.8, 26.5,
Synthesis of R
[1ꢀ4]down: To a solution of wheel 1 (0.013 g, 0.009 mmol) in
C6D6 (0.6 mL) placed in a 5 mm NMR tube axle 4 (0.01 g, 0.009 mmol)
was added. The resulting heterogeneous mixture was heated to 708C for
12 h until the solution turned homogeneous and deep-red colored and
21.6 ppm; MS (ESI): m/z (%) 407.4 (100) [MÀ2TsO]2+
.
1
General procedure for synthesis of P
[1ꢀ2]up and P
[1ꢀ3]up: In a round-
analyzed by H and 13C NMR spectroscopy as well as mass spectrometry.
bottomed flask, the proper amount of the axle (2 or 3) (0.05 mmol) was
suspended in a solution of compound 1 (0.06 g, 0.04 mmol) in C6D6
1H NMR (300 MHz, C6D6): d=9.5 (brs, 6H), 8.4 (d, J=8 Hz, 2H), 8.28
(d, J=7 Hz, 4H), 8.2 (brs, 2H), 8.1 (brs, 6H), 7.95 (d, J=6.5 Hz, 2H),
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A. Arduini, A. Credi, M. Venturi et al.
7.8 (brs, 6H), 7.69 (s, 6H), 7.46 (d, J=7 Hz, 4H), 7.45 (d, J=7 Hz, 4H),
7.4–7.2 (m, 8H), 7.2–7.1 (m, 6H), 7.1–7.0 (m, 5H), 6.9 (brs, 4H), 6.78 (t,
J=7 Hz, 3H), 5.23 (s, 1H), 4.69 (d, J=14 Hz, 6H), 4.7 (brs, 2H), 4.13 (t,
J=7 Hz, 2H), 4.05 (s, 9H), 4.0 (brs, 8H), 3.7 (brs, 8H), 3.6–3.5 (m,
12H), 2.2 (brs, 2H), 2.04 (s, 8H), 1.8 (brs, 31H), 1.4 (brs, 4H), 1.24 (t,
J=7 Hz, 9H), 1.0 (brs, 2H), 0.9 ppm (brs, 2H); 13C NMR (100 MHz,
C6D6): d=172.0, 165.9, 153.5, 152.8, 148.1, 148.0, 144.3, 143.2, 143.0,
141.1, 139.4, 139.2, 137.5, 136.7, 133.8, 132.1, 131.6, 130.0, 129.3, 129.0,
128.7, 128.6, 127.2, 126.8, 126.5, 125.6, 125.3, 124.8, 121.2, 118.1, 116.7,
72.4, 70.0, 66.3, 64.7, 61.0, 60.4, 59.7, 57.4, 38.2, 34.6, 31.5, 31.3, 29.8, 29.6,
29.2, 28.3, 25.5, 24.9, 20.7, 15.2, 13.9 ppm; MS (ESI): m/z (%): 1112 (100)
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G
E
Received: May 9, 2012
Revised: August 6, 2012
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Published online: October 22, 2012
Chem. Eur. J. 2012, 18, 16203 – 16213
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www.chemeurj.org
16213