4,4'-bipyridine as a unidirectional switching unit for a molecular pushing motor

Article abstract of DOI:10.1002/chem.201304166

A chirality switch in which the intrinsic chirality of a 4,4'-bipyridine is combined with a metal-ion-induced switching principle is described. In the uncomplexed state the 4,4'-bipyridine unit, which is linked to an S,S,S,S-configured cyclic imidazole peptide, is P-configured. The addition of zinc ions leads to a rotation around the C-C bond axis of the 4,4'-bipyridine and the Misomer of the metal complex is formed. By addition of a stronger complexing agent the metal ions are removed and the switch returns to its initial position. The combination of the chirality switch with a second switching unit allows the construction of a molecular pushing motor, which is driven chemically and by light. Motor drive: The combination of the intrinsic chirality of a 4,4'-bipyridine with a metal-ion-induced switching principle leads to a metal-ion-induced chirality switch that can also be used as a basic unit for a molecular pushing motor. Rotation around the C-C bond axis of the bipyridine, which is linked to a cyclic imidazole peptide, leads to switching between P and Misomers (see scheme).

Full text of DOI:10.1002/chem.201304166

DOI: 10.1002/chem.201304166
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&
Molecular Motors
4,4’-Bipyridine as a Unidirectional Switching Unit for a Molecular
Pushing Motor
Christine Kallweit, Gebhard Haberhauer,* and Sascha Woitschetzki[a]
Dedicated to Professor Reiner Sustmann on the occasion of his 75th birthday
Abstract: A chirality switch in which the intrinsic chirality of
a 4,4’-bipyridine is combined with a metal-ion-induced
switching principle is described. In the uncomplexed state
the 4,4’-bipyridine unit, which is linked to an S,S,S,S-config-
ured cyclic imidazole peptide, is P-configured. The addition
of zinc ions leads to a rotation around the CꢀC bond axis of
the 4,4’-bipyridine and the M isomer of the metal complex is
formed. By addition of a stronger complexing agent the
metal ions are removed and the switch returns to its initial
position. The combination of the chirality switch with
a second switching unit allows the construction of a molecu-
lar pushing motor, which is driven chemically and by light.
Introduction
2,2’-Bipyridines and their derivatives have been successfully
used as switching elements in supramolecular chemistry for
several decades.[1–3] Their switching process is induced by
metal ions or a change in pH.[1] In the uncomplexed state 2,2’-
bipyridines exhibit an N-C-C-N dihedral angle of 1808 (see
Scheme 1a). By addition of a metal ion a rotation around the
CꢀC bond between the two pyridine units takes place due to
the coordination of the metal ion by the nitrogen free-electron
pairs. The resulting complex has an N-C-C-N dihedral angle of
about 08. If the 2,2’-bipyridine is substituted in both ortho posi-
tions to the nitrogen atoms, a relative amplitude change of
1808 can be obtained during the switching process
(Scheme 1a). However, the 2,2’-bipyridine unit is achiral in
both states (uncomplexed and complexed) so that it cannot
be directly used as a central element for a chirality switch.[3]
The isomeric 4,4’-bipyridine unit features an intrinsic chirality
(see Scheme 1b), but it cannot be used as a metal-ion-induced
switch because it is not possible to obtain a simultaneous co-
ordination of one metal ion by both nitrogen lone pairs. In-
stead, the coordination of two metal ions leads to the forma-
tion of coordination polymers the topology of which ranges
from one-dimensional chains (linear or zigzag) to complex
three-dimensional networks.[4,5] Our idea was to combine the
Scheme 1. a) Metal-ion-induced switching process of 2,2’-bipyridine 1.
b) Enantiomers of 4,4’-bipyridine 2. c) Concept for a metal-ion-induced uni-
directional switching process of 4,4’-bipyridine derivative 3.
intrinsic chirality of a 4,4’-bipyridine with a metal-ion-induced
switching principle like that in 2,2’-bipyridines to design
a metal-ion-induced chirality switch that can also be used as
a basic unit for a molecular pushing motor.
[a] C. Kallweit, Prof. Dr. G. Haberhauer, S. Woitschetzki
Institut fꢀr Organische Chemie, Fakultꢁt fꢀr Chemie
Universitꢁt Duisburg-Essen
Results and Discussion
Universitꢁtsstrasse 7, 45117 Essen (Germany)
E-mail: gebhard.haberhauer@uni-due.de
Concept
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304166. It contains synthetic procedures
for the compounds 5, 7–9, 11, and 13–16, molecular structures of 8 and
8*Zn2Cl4, titrations of 3b, 8, 12, 13, and 15 with Zn2+, spectra to prove the
isomerization of the molecular motor 12, computational details, Cartesian
coordinates, and absolute energies for 3a,b and 3a,b*Zn2Cl4.
The envisaged principle is shown in Scheme 1c. A central 4,4’-
bipyridine unit is linked to two nitrogen-containing aromatics,
which in turn are connected to each other through a chiral
clamp. The latter predetermines the spatial orientation of the
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two nitrogen-containing aromatics.[6] To avoid repulsive inter-
actions between the lone pairs of the 4,4’-bipyridine and the
nitrogen-containing aromatics, the 4,4’-bipyridine preferably
adopts that kind of configuration in which all lone pairs are
pointing away from each other; in our example this is the
P configuration (Scheme 1c; (P)-3). The addition of metal ions
leads to a coordination of two lone pairs at each metal center.
To achieve this, a rotation around the CꢀC bond axis of the
4,4’-bipyridine has to take place and the M isomer of the metal
complex is obtained (Scheme 1c; (M)-3*M2X4). The removal of
the metal ion by a stronger complexing agent restores the ini-
tial configuration. Thus, a selective switching between the M
and P isomers is possible. Furthermore, the chiral clamp pre-
vents a rotation from one isomer to the other from proceeding
bidirectionally (clockwise and anticlockwise); in our case
(Scheme 1c) only a clockwise rotation from the uncomplexed
to the complexed state is possible. This means that the move-
ment is unidirectional, which is the basic requirement for the
design of a molecular motor.[7]
inductions.[10] Moreover, clamp 10 can be built up in a one-
step synthesis from a commercially available compound. As ni-
trogen-containing aromatics we chose quinolines.
The synthesis of 4,4’-bipyridine switch 3 is presented in
Scheme 2. Hydroxyquinoline 5 can be synthesized in one step
from hydroxyaniline 4 through Skraup quinoline synthesis.[11]
The linkage of the hydroxyquinoline with the central 4,4’-bipyr-
idine unit to compound 8 is possible by nucleophilic substitu-
tion of phenol 5 with tetrabromide 7. The latter is accessible
from dichloroiodopyridine 6 in two steps by palladium-cata-
lyzed indium coupling[12] and nucleophilic substitution. Subse-
quent double bromination of bipyridine 8 leads to tetrabro-
mide 9. Coupling of compound 9 with chiral clamp 10 is ach-
ieved by alkylation using cesium carbonate in acetonitrile and
results in the desired 4,4’-bipyridine switch 3b (R=Br).
Proof for a zinc-ion-induced switching process
In the first step we wanted to verify that a double quinoline-
substituted 4,4’-bipyridine actually shows the previously pro-
posed switching process (the lone pairs of the bipyridine and
quinoline units point away from each other in the uncom-
plexed state whereas they are orientated towards each other
in the complexed state). For simple test modeling we used the
bisquinoline bipyridine compound 8 and two further reference
systems, which were treated with Zn(OTf)2 (OTf=triflate) in di-
chloromethane and analyzed by UV spectroscopic measure-
ments (see the Supporting Information, Figures S1 and S2).
Synthesis
For the realization of the concept explained above, the relative
spatial adjustment of the two nitrogen-containing aromatics
by a chiral clamp is highly important. We chose cyclic peptide
10 to be this central element (see Scheme 2), because we al-
ready established chiral cyclic imidazole peptides[8] successfully
within other unidirectional switching processes[9] and chirality
Scheme 2. Synthesis of 4,4’-bipyridine switch 3b (R=Br). Reaction conditions: i) glycerine, sodium p-nitrobenzene sulfonate, H2O/H2SO4, D, 59%; ii) Pd(OAc)2,
In, LiCl, NMP, D, 57%; iii) HBr/AcOH, AcOH, D, 90%; iv) 5, NaH, DMSO, D, 54%; v) NBS, CCl4, D, 71%; vi) Cs2CO3, 9, acetonitrile, D, 18%. NMP=N-methylpyrroli-
done, NBS=N-bromosuccinimide.
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The UV titration spectra of 8 clearly show the coordination of shift from 240 to 250 nm, the absorption decrease at 295 nm,
zinc ions caused by the nitrogen lone pairs of the bipyridine and the formation of a new band at values above 350 nm. The
and quinoline units: the p!p* bands of quinoline (230 nm) latter is, however, a very broad band. The simulated spectra
and bipyridine (295 nm) decrease due to the addition of zinc (see Figure 1b and Figure S4 in the Supporting Information)
ions (see Figure S2 in the Supporting Information). If both basically exhibit the same band shifts, even though the extents
functional units were not involved in coordination, the absorp- are slightly different. However, the intensity of the transitions
tion of the two bands would hardly change by stepwise addi- is not well reproduced in the area between 300 and 380 nm.
tion of zinc salt. Saturation of compound 8 is obtained after ti- For example, the intensity of the absorption band at 315 nm is
tration of 2.5 equivalents of Zn2+. The manner of complexation overestimated by the calculations whereas the intensity of the
is analogous to the titrations of the structurally similar di-2- very broad band at values above 350 nm is underestimated.
picolylamine-substituted quinoline sensors.[13]
Furthermore, bipyridine 8 and its complex 8*Zn2Cl4 were
geometrically optimized using B3LYP/6-31G* and the corre-
Proof for a unidirectional zinc-ion-induced switching
process
sponding UV spectra were simulated using time-dependent
density functional theory (TDDFT) at the B3LYP/6-31G* level.
The chlorine ligands were used in the calculations instead of
the triflate groups to reduce the computational cost. The DFT-
optimized structures show the behavior explained above: in
the uncomplexed state 8 the nitrogen lone pairs are pointing
away from each other, but in the complexed state 8*Zn2Cl4
they point towards each other due to the coordination of zinc
ions (see Figure S3 in the Supporting Information).
For unidirectional switching it is most important that switch 3
and complex 3*Zn2X4 each adopt only one configuration. To
prove this, calculations (B3LYP/6-31G*) were carried out for de-
livering the energy profiles of the ligands 3a (R=H) and 3b
(R=Br) and the metal complexes 3a*Zn2Cl4 (R=H) and
3b*Zn2Cl4 (R=Br) dependent on the dihedral angle qC1-C2-C2’-C1’
(Figure 2; for numbering, see Scheme 2).
For direct comparison the simulated and experimentally ob-
4+
tained UV spectra of ligand 8 and its complex 8*Zn2 are
shown in Figure 1. Regarding Figure 1a the most obvious
changes caused by complexation are the bathochromic band
Figure 2. Calculated energy profiles (B3LYP/6-31G*) of 4,4’-bipyridine deriva-
tives 3a (R=H; triangles, dashed line), 3a*Zn2Cl4 (rectangle, solid line), 3b
(R=Br; rhombus, dotted line), and 3b*Zn2Cl4 (circles, dash-dotted line) de-
pendent on the dihedral angle qC1-C2-C2’-C1’ (for numbering, see Scheme 2).
In the case of ligands 3a (R=H) and 3b (R=Br) only one
minimum is found for each compound. For 3a the dihedral
angle at the minimum amounts to +428 and for 3b it is +438,
which means that both exist as P isomers. The corresponding
M isomers represent no minima on the potential hypersurface
and the relative energy values of 3a and 3b with dihedral
angles (qC1-C2-C2’-C1’) of ꢀ408 amount to almost 38 kJmolꢀ1. This
means that even if the M isomers exhibited minima, the M to
P ratio would be 1 to 3ꢁ105 according to the Boltzmann distri-
bution at 258C. Thus, only the P isomers of 3a and 3b are
present in solution. For the metal complexes 3a*Zn2Cl4 (R=H)
and 3b*Zn2Cl4 (R=Br) it is vice versa: the M isomers with dihe-
dral angles (qC1-C2-C2’-C1’
)
of ꢀ408 (3a*Zn2Cl4) and ꢀ268
Figure 1. a) UV spectrum of 8 before (solid line) and after addition (dashed
line) of Zn(OTf)2 in CH2Cl2. b) Calculated (TDDFT B3LYP/6-31G*) UV spectra of
8 (solid line) and 8*Zn2Cl4 (dashed line).
(3b*Zn2Cl4) are minima on the potential hypersurface. The cor-
responding P isomers with dihedral angles of +408 are not
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Zn2+ leads to the appearance of a second set of signals in the
1H NMR spectrum, which is a distinct proof for the formation
of a single zinc complex. After further zinc ion addition, the ini-
tial set of signals disappears showing that complexation is
complete. A proof for the rotation around the CꢀC bond axis
of the 4,4’-bipyridine is the shift of the proton signals H3 and
H3’ (for numbering, see Scheme 2). For the uncomplexed spe-
cies (P)-3b, proton resonance is found at 5.60 ppm. This high-
field shift can be explained by the position of the protons that
point towards the quinoline ring. During complexation, which
leads to a rotation around the CꢀC bond axis of the 4,4’-bipyri-
dine, the protons H3 and H3’ move away from the quinoline
rings and their signal is shifted low-field to 5.87 ppm (Dd=
0.27 ppm). All other aromatic proton signals exhibit a smaller
shift (Dd<0.14 ppm). By addition of cyclam, which is a stronger
complexing agent, the metal ions are removed and the initial
Figure 3. Molecular structures of (P)-3b (R=Br; left) and (M)-3b*Zn2Cl4
(right) calculated using B3LYP/6-31G*. All hydrogen atoms are omitted for
clarity.
1
set of signals in the H NMR spectrum is obtained.
stable. Here the extent of destabilization for 3a*Zn2Cl4 is
higher than that for 3b*Zn2Cl4.
UV and CD spectra of (P)-3b were monitored before and
after addition of Zn(OTf)2 in dichloromethane (see Figure 5; for
titration curves, see Figure S7 in the Supporting Information).
Furthermore, the UV and CD spectra of (P)-3b and (M)-
3b*Zn2Cl4 were simulated by TDDFT calculations using the
B3LYP functional and the 6-31G* basis set. Several spectra are
given in Figure 5. The complexation of zinc ions can be recog-
nized by the bathochromic shift of the absorption band at
233 nm in the measured as well as in the simulated UV spectra
(Figure 5c,d). In the experimentally obtained spectrum the
bathochromic shift of the band at 230 nm is, however, stronger
than in the simulated spectrum. The absorption maximum at
297 nm decreases during zinc addition whereas a new broad
band appears at 333 nm. Moreover, there are three isosbestic
points at 239, 285, and 312 nm. Here again the intensity of the
absorption band at 315 nm is overestimated by the calcula-
tions, whereas the intensity of the very broad band at values
above 350 nm is underestimated.
In Figure 3 the molecular structures calculated by means of
B3LYP/6-31G* for (P)-3b and (M)-3b*Zn2Cl4 are shown. In (P)-
3b the nitrogen atoms are clearly pointing away from each
other, but in (M)-3b*Zn2Cl4 they are orientated towards each
other. A metal-ion-induced transition from (P)-3b to (M)-
3b*Zn2Cl4 therefore leads to an inversion of configuration and
the rotation around the CꢀC bond axis of the 4,4’-bipyridine
has to take place unidirectionally due to chiral clamp fixation.
Notably, the motion about the bipyridine axis (698 for 3a
and 818 for 3b) is not the one available for possible work, but
a net amplitude change with regard to the peptidic clamp
would describe the motion. If the four methine groups of the
tetrapeptide macrocycle are defined as a scaffold, the motion
relative to this plane can be calculated: the angle between the
pyridine ring and the scaffold amounts to 67.88 in (P)-3b; in
the complex 3b*Zn2Cl4 a value of 102.38 is found. Thus the net
amplitude amounts to 358.
The inversion of chirality caused by complexation can be
verified by comparing the measured CD spectra with the simu-
lated ones in the area between 230 and 270 nm. For ligand
(P)-3b negative Cotton effects are obtained at 241 and
251 nm. The simulated CD spectrum of (P)-3b exhibits in this
region a similar curve pattern: it shows a shoulder at 230 nm
and a minimum at 245 nm. Due to zinc complex formation the
curve progression changes drastically: the experimental CD
spectrum of (M)-3b*Zn2(OTf)4 no longer shows a negative but
a strong positive Cotton effect in the area around 242 nm. The
same behavior is found for the simulated spectrum of (M)-
3b*Zn2Cl4, which has to be caused by configurational change
from P to M. The unidirectional zinc-ion-induced inversion of
the configuration in system 3 is thus additionally confirmed.
Further proofs for the metal-ion-induced unidirectional
switching of 3 can be obtained by NMR and circular dichroism
(CD) spectroscopy. For 3b an NMR titration sequence was
monitored in deuterated acetonitrile (see Figure 4, and Figur-
es S5 and S6 in the Supporting Information). The addition of
Construction of a molecular pushing motor
The unidirectional zinc-induced switching of system 3 can be
used for the construction of a molecular pushing motor if it is
combined with a second switching unit. This motor imitates
the motion sequence of cilia,[7k,14] which are ubiquitous in
nature and can be used for transporting particles or the sur-
Figure 4. 1H NMR spectra of 3b before treatment (bottom), after addition of
Zn(OTf)2, and after addition of cyclam (diluted in CDCl3; top) in CD3CN in the
area from 9.0 to 5.5 ppm. The signals for the proton H3 are indicated (for
numbering, see Scheme 2).
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Figure 5. a) CD spectra of 3b before (solid line) and after addition of two (dashed line) and four (dotted line) equivalents of Zn(OTf)2 in CH2Cl2. b) Simulated
(TDDFT B3LYP/6-31G*) CD spectra of (P)-3b (solid line) and (M)-3b*Zn2Cl4 (dashed line). c) UV spectrum of 3b before (solid line) and after addition (dashed
line) of two equivalents of Zn(OTf)2 in CH2Cl2. d) Simulated (TDDFT B3LYP/6-31G*) UV spectra of (P)-3b (solid line) and (M)-3b*Zn2Cl4 (dashed line).
rounding media. The structure and the principle of such
a pushing motor are illustrated in Scheme 3. The Suzuki reac-
tion[15] of bipyridine switch 3b with boronic ester 11 leads to
pushing motor 12 in which an azo unit[16] is connected to the
central bipyridine. This azo unit is the pushing blade of the
motor and the 4,4’-bipyridine is the unidirectional switching
unit that controls the direction of movement.
merization that leads to a closing of the pushing blade and
a transition from state I to II (cis-(P)-12). The isomerization can
be observed by a decreasing p!p* band at 347 nm in the UV
spectrum but it does not lead to a significant change in the
CD spectrum, as the latter is dominated by the configuration
of the bipyridine and quinoline units (see Figure 6). The addi-
tion of Zn2+ ions induces a directed clockwise rotation around
the CꢀC bond axis of the 4,4’-bipyridine, which corresponds to
a pivoting of the closed pushing blade and the transition from
state II to state III (cis-(M)-12*Zn2X4). This transition leads to an
inversion of chirality at the 4,4’-bipyridine unit and causes dras-
tic changes in the UV and CD spectra. Irradiation of the com-
plex with visible light results in a cis!trans isomerization of
the azo unit and the pushing blade is opened. State IV (trans-
(M)-12*Zn2X4) is obtained, which is observable in the UV spec-
trum by an increase of the p!p* band at 347 nm. The remov-
al of the zinc ion by a strong complexing agent (for example
cyclam) initiates the so-called power stroke (IV!I) of the
opened pushing blade and state I is again adopted. The ob-
tained spectra are identical to the initial spectra, which is proof
for the supposed cyclic process. All in all, a pushing process in
one direction is achieved because both rotational movements
(II!III and IV!I) are unidirectional and more surrounding par-
ticles are moved anticlockwise during the power stroke (IV!I)
than are moved clockwise during the recovery stroke (II!III).
First the light-induced switching of the azo unit was ana-
lyzed. The obtained NMR and HPLC spectra demonstrate that
irradiation with UV light (l=366 nm) causes a trans!cis iso-
merization of the azobenzene (see Figures S8 and S11 in the
Supporting Information). As expected there is also a change in
the UV spectrum caused by isomerization (decrease of the p!
p* band) whereas there is no significant change in the CD
spectrum (Figure S9 in the Supporting Information). During the
zinc-induced switching process of the bipyridine unit there are,
however, strong changes of the spectra (see Figure S10 in the
Supporting Information). The light-induced switching of the
azobenzene can be reversed by irradiation with visible light,
and the zinc-induced bipyridine switching by addition of
cyclam. These preliminary investigations prove that all four
states can be adopted by system 12.
The experimental proof for a stepwise passing of all four
states is depicted in Figure 6. In state I the 4,4’-bipyridine is P-
configured and the pushing blade is opened as the azoben-
zene is trans-configured (trans-(P)-12, Scheme 3). If motor 12 is
irradiated with UV light (l=366 nm), there is a trans!cis iso-
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Scheme 3. a) Four states of molecular pushing motor 12. Reaction conditions: i) [Pd(PPh3)4], K2CO3, D, 14%. b) Schematic illustration of the four states of
pushing motor 12. The 4,4’-bipyridine unit is presented as a horizontally dashed pivot, and the azo group pivot is marked vertically dashed.
V-550 spectrophotometers. CD absorption spectra were recorded
with a Jasco J-815 spectrophotometer. The starting material for
Conclusion
We have shown that it is possible to design a metal-ion-in-
duced chirality switch by combination of the intrinsic chirality
of a 4,4’-bipyridine unit with a metal-ion-induced switching
principle. In the uncomplexed state the 4,4’-bipyridine is P-con-
figured, and the complexation leads through unidirectional ro-
tation to the M-configured complex. By combining the chirality
switch with a light-induced switching unit, a molecular push-
ing motor can be developed that imitates the motion se-
quence of cilia.
compound 10 was purchased from SQUARIX GmbH; all other
chemicals were purchased from Sigma–Aldrich, ABCR, and TCI.
Switch 3b
Cs2CO3 (111 mg, 0.34 mmol) was added to a solution of dibromide
9 (27 mg, 0.034 mmol) and chiral clamp 10 (19 mg, 0.034 mmol) in
acetonitrile (30 mL) at room temperature. The mixture was stirred
under reflux for 3 h. The solvent was removed and the residue was
dissolved in dichloromethane and extracted with water and brine.
The combined organic phases were dried over MgSO4 and concen-
trated in vacuo. Purification of the crude product was accom-
plished by column chromatography over silica gel (CH2Cl2/AcOEt/
MeOH, 75:25:5) and by HPLC to give bipyridine system 3b (8.7 mg,
Experimental Section
General remarks
0.007 mmol, 18%) as a white residue. 1H NMR (500 MHz, CD3CN):
3
All chemicals were reagent grade and were used as purchased. Re-
actions were monitored by TLC analysis with silica gel 60 F254
thin-layer plates. Flash chromatography was carried out on silica
gel 60 (230–400 mesh). 1H, 13C, and 2D NMR spectra were mea-
sured with a Bruker Avance DRX 500 and 700 spectrometer. All
chemical shifts (d) are given in ppm. The spectra were referenced
to deuterated solvents indicated in brackets in the analytical data.
HRMS spectra were recorded with a Bruker BioTOF III Instrument.
UV/Vis absorption spectra were obtained with Jasco J-815 and
d=8.73 (dd, 3JH,H =4.3 Hz, 4JH,H =2.0 Hz, 2H; Har), 8.43 (d, JH,H
=
3
3
8.4 Hz, 2H; Har), 7.79 (d, JH,H =8.8 Hz, 2H; NH), 7.57 (dd, JH,H =8.2,
4
4.3 Hz, 2H; Har), 7.38 (d, 3JH,H =8.0 Hz, 2H; Har), 7.28 (d, JH,H
=
=
=
3
1.3 Hz, 2H; NH), 7.14 (d, 3JH,H =8.4 Hz, 2H; NH), 6.20 (d, JH,H
4
7.8 Hz, 2H; Har), 5.72 (d, 2JH,H =17.7 Hz, 2H; CH2), 5.60 (d, JH,H
3
1.3 Hz, 2H; Har), 5.58 (d, JH,H =17.7 Hz, 2H; CH2), 4.97–4.94 (m, 2H;
NHCH), 4.09–4.06 (m, 2H; NHCH), 2.29–2.24 (m, 4H; CH(CH3)2), 2.07
(s, 6H; imidazole-CH3), 1.13–1.09 (m, 18H; CH(CH3)2), 1.00 ppm (d,
3JH,H =6.8 Hz, 6H; CH(CH3)2); 13C NMR (125 MHz, CDCl3): d=171.29,
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3
NH), 6.95–6.91 (m, 2H; NH), 6.44 (d, JH,H =8.0 Hz, 1H; Har), 6.38 (d,
3JH,H =8.2 Hz, 1H; Har), 5.62–5.58 (m, 4H; CH2/2Har), 5.52–5.45 (m,
2H; CH2), 5.01–4.95 (m, 2H; NHCH), 4.41–4.37 (m, 2H; NHCH),
2.43–2.38 (m, 2H; CH(CH3)2), 2.32–2.27 (m, 2H; CH(CH3)2), 2.06 (s,
3H; imidazole-CH3), 2.04 (s, 3H; imidazole-CH3), 1.18–1.15 (m, 18H;
CH(CH3)2), 1.02 ppm (t, 3JH,H =6.3 Hz, 6H; CH(CH3)2); 13C NMR
(125 MHz, CDCl3): d=171.51, 171.47, 166.14, 165.63, 163.23, 163.18,
156.67, 153.35, 152.89, 152.62, 150.90, 150.86, 149.89, 149.54,
149.17, 148.65, 148.61, 141.39, 141.09, 139.97, 139.39, 132.18,
132.09, 131.37, 131.07, 130.95, 130.93, 130.16, 129.11, 129.00,
128.93, 128.82, 128.23, 127.89, 127.17, 127.14, 125.07, 124.87,
122.49, 122,39, 122.35, 120.74, 116.92, 113.47, 104.94, 104.74, 71.95,
70.75, 60.29, 60.25, 52.71, 51.22, 51.18, 43.66, 43.52, 34.80, 34.74,
34.36, 32.08, 29.75, 29.28, 27.33, 22.84, 19.92, 19.48, 19.24, 14.28,
10.17, 10.15, 10.08 ppm; UV/Vis (CH2Cl2): lmax (loge)=347 (4.17),
318 (4.10), 304 (4.10), 234 nm (4.68); CD (CH2Cl2): lmax (De) 342
(ꢀ6.0), 298 (+0.4), 238 (ꢀ43.9), 228 nm (+33.1mꢀ1 cmꢀ1); HRMS
(ESI+): m/z: calcd (%) for [C70H6981BrN14O6 +Na]+: 1305.4585;
found: 1305.4626; HPLC: trans peak at 10.6 min, cis peak at
5.3 min (Nucleosil 100, C18, 5 mm, 250ꢁ8 mm; MeOH/H2O: 90:10).
Acknowledgements
This work was supported by the Deutsche Forschungsgemein-
schaft (DFG). We would like to thank Patrizia Chamier-Ciemin-
ski, Helma Kallweit, Petra Schneider, and Dr. Silvia Ernst for
helpful support.
Figure 6. CD and UV spectra for the four-stroke cycling process. CD (top)
and UV spectra (bottom) of the states I to IV in dichloromethane/acetonitrile
at 208C; the following direction is passed during the four-stroke cycle: I
(blue)!II (yellow)!III (green)!IV (red)!I (purple).
Keywords: chirality · isomerization · molecular modeling ·
molecular motors · molecular switches
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~
19.31, 10.07 ppm; IR (ATR): n=2966, 2934, 2874, 1710, 1673, 1587,
1523, 1504, 1470, 1372, 1303, 1272, 1222, 1159, 1068, 988, 909,
838, 792, 762, 717, 691, 668, 652 cmꢀ1; UV/Vis (CH2Cl2): lmax
(loge)=300 (4.04), 233 nm (4.77); CD (CH2Cl2): lmax (De)=307
(ꢀ5.4), 251 (ꢀ49.4), 230 nm (+45.0mꢀ1 cmꢀ1); HRMS (ESI+): m/z:
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1203.3015; HPLC: peak at 5.7 min (Nucleosil 100, C18, 5 mm, 250ꢁ
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Molecular motor 12
Boronic ester 11 (18 mg, 0.058 mmol) was dissolved in dioxane
(2.0 mL). Afterwards Pd(PPh3)4 (3 mg, 0.002 mmol), an aqueous so-
lution of potassium carbonate (saturated; 0.2 mL), and molecular
switch 3 (8 mg, 0.007 mmol) were added to the solution. The reac-
tion mixture was warmed to 608C and stirred overnight. After cool-
ing to room temperature, AcOEt (30 mL) and distilled water (5 mL)
were added. The phases were separated and the aqueous layer
was extracted with AcOEt twice. The organic layers were com-
bined, dried over MgSO4, the solvent was removed, and the crude
product was purified by column chromatography over silica gel
(CH2Cl2/AcOEt/MeOH, 75:25:3) to give 12 (1.3 mg, 0.001 mmol,
14%) as an orange solid. 1H NMR (500 MHz, CDCl3): d=8.84–8.82
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Chem. Eur. J. 2014, 20, 6358 – 6365
www.chemeurj.org
6364
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Received: October 24, 2013
Revised: February 21, 2014
Published online on April 8, 2014
Chem. Eur. J. 2014, 20, 6358 – 6365
www.chemeurj.org
6365
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim