Chiroptical inversion induced by rotation of a carbon-carbon single bond: An experimental and theoretical study

Article abstract of DOI:10.1021/jp410370q

We propose a new strategy to construct chiral molecular switches with highly reversible and sensitive chiroptical responses to variations in the external environment. Its fundamental concept involves a stimuli-triggered exchange of two conformations presenting significantly different chiroptical properties through the rotation of a carbon-carbon single bond, as demonstrated by chiral Schiff bases s-1, s-2, and a salicylamide analogue s-3. Upon addition of base in solution, the circular dichroism (CD) spectra of these molecular switches displayed unique changes featuring an inversion of the Cotton effect's signs, and the original CD profiles can be recoverd by acidification. Various spectroscopic studies as well as the conformational analysis combining with TDDFT computations allowed clear elucidation of the chiroptical inversion mechanism. It is expected that this kind of chiroptical switches is of great interest for molecular recognition, chemosensing, and the construction of molecular-scale devices. Furthermore, the present study indicates that the use of the conformational transition about a single bond may serve as the basis for designing chiroptical inversion systems.

Full text of DOI:10.1021/jp410370q

Article
pubs.acs.org/JPCA
Chiroptical Inversion Induced by Rotation of a Carbon−Carbon
Single Bond: An Experimental and Theoretical Study
Wei Lu, Ganhong Du, Keyuan Liu, Liming Jiang,* Jun Ling,* and Zhiquan Shen
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang
University, Hangzhou 310027, China
S
* Supporting Information
ABSTRACT: We propose a new strategy to construct chiral molecular switches with
highly reversible and sensitive chiroptical responses to variations in the external
environment. Its fundamental concept involves a stimuli-triggered exchange of two
conformations presenting significantly different chiroptical properties through the rotation
of a carbon−carbon single bond, as demonstrated by chiral Schiff bases s-1, s-2, and a
salicylamide analogue s-3. Upon addition of base in solution, the circular dichroism (CD)
spectra of these molecular switches displayed unique changes featuring an inversion of the
Cotton effect’s signs, and the original CD profiles can be recoverd by acidification. Various
spectroscopic studies as well as the conformational analysis combining with TDDFT
computations allowed clear elucidation of the chiroptical inversion mechanism. It is
expected that this kind of chiroptical switches is of great interest for molecular recognition,
chemosensing, and the construction of molecular-scale devices. Furthermore, the present
study indicates that the use of the conformational transition about a single bond may serve
as the basis for designing chiroptical inversion systems.
external environment, which is one feature desirable for many
applications. In fact, there have been recently a few reports
about molecular switches capable of producing chiroptical
inversion due to the rotation of a single bond induced by
solvent or metal coordination.74−76 However, these systems are
structurally complex and there is limited space for further
molecular design.
INTRODUCTION
■
Chiral and helical inversion phenomena are very common
signal transfers and molecular motions in natural living
organisms. External stimulus-responsive inversions of chirop-
tical properties are found to be important in many biological
processes, including DNA and gene-involved functional
expressions.1,2 A drastic change in chiroptical properties
makes dynamic chiroptical inversion systems especially suitable
for use in chemosensing3 as well as in molecular-level devices
such as molecular switches and molecular motors.4−8
Chiroptical inversion usually involves cleavage of a non-
covalent bond or weak interaction followed by rearrangement
to another optically pure diastereomer.9 For example, a lot of
chiral supramolecular aggregates,10−14 polymers,15−35 fol-
damers,36−46 and metal complexes47−64 have been shown to
offer helicity inversion induced by external stimuli, because in
these systems the hydrogen-bonding, π−π stacking, or metal
coordination does not have enough strength to fix the single-
handed helical or folded structures.
For certain chiral molecular switches based on the sterically
overcrowded olefins, diarylethenes, and helicenes, the reversible
and stereochemically controlled transformation of one optically
pure diastereomer to another is realized through the cleavage
and formation of a π-bond, and even of σ-bond.26,65−73 The
lifetime of these molecular switches is frequently short. Their
inherent drawback stems from the well-known fact that no
chemical reaction can proceed with 100% conversion.9
Herein we describe a new approach to construct chiral
molecular switches with the features of stimuli-controlled and
reversible chiroptical inversion. Its fundamental concept
involves an interchange of two conformations presenting
diverse chiroptical properties by acid/base-mediated C−C
single bond rotation. As a proof of principle, a simple
chiroptical molecular switch has been established using N-
salicylidene Schiff base of chiral α-phenylethylamine (s-1) as a
peculiar skeleton motif (Scheme 1), which shows unique
chiroptical responses in different pH environments. To prove
the general applicability of the present strategy, we also
synthesized 2-hydroxyl-1-naphthaldehyde Schiff base derivative
s-2 and salicylamide analog s-3. The results showed that the
chiroptical properties of these compounds can be reversibly
modulated after sequential reactions with base and acid, and
thus a chiroptical molecular switch with nondestructive output
signal is realized. On the basis of the combined use of
spectroscopic measurement, conformational analysis, and time-
dependent density functional theory (TDDFT) computation,
The chiroptical switches based on a single bond movement
appear to be particularly attractive, because the molecular
conformation states are highly sensitive to changes in the
Received: October 20, 2013
Revised: December 2, 2013
Published: December 16, 2013
© 2013 American Chemical Society
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Scheme 1. Structures of Schiff Bases (s-1 and s-2, Including
the Enol and Keto Forms) and (S)-Salicylamide (s-3) as Well
as Their Deprotonated Species
Figure 1. CD and UV−vis spectra of s-1 (1 × 10−4 mol/L) in various
solvents at ambient temperature (DMSO, dimethyl sulfoxide; THF,
tetrahydrofuran; DCM, dichloromethane).
the working mechanism of chiroptical inversion is discussed in
detail.
RESULTS AND DISCUSSION
■
Schiff Base Chiroptical Molecular Switch. Chiral N-
salicylidene Schiff base (s-1 or r-1) was straightforwardly
synthesized by the condensation reaction of salicylaldehyde
with optically pure α-phenylethylamine. The structural
characterization data are provided in the Supporting
Information. On the basis of well-established stereochemistry
of N-salicylidene derivatives of chiral amines,77,78 it is
anticipated that the chiral compound would display evident
CD signals due to the electric dipole−dipole coupling
interaction of the salicylidenamino chromophore and the
phenyl group in the chiral unit. In view of the strong
dependency of keto−enol tautomerism on solvent nature, the
optical properties of s-1 were first examined by absorption and
CD spectrocopies in various media (Figure 1).
Figure 2. CD and UV−vis titration spectra of s-1 (1 × 10−4 mol/L) in
DMSO with increasing amount of TBAOH (0, 0.5, 1.0, and 2.0 equiv,
respectively).
disappeared and a new band around 400 nm concomitantly
appeared (Figure 2) due to the formation of deprotonated
species s-1−. The presence of a sharp isosbestic point at 340 nm
indicates that only two species coexists in the equilibrium over
the course of titration. Of great interest is that a complete
inversion of the Cotton effect (CE) occurred with a red shift of
∼80 nm when ∼2 equiv of TBAOH was added, although the
CD profiles of s-1 and s-1− are not mirror images of each other.
It is worth mentioning that such a chiroptical response is
completely reversible for the molecular switch. The CD spectral
changes of s-1 remain consistent after many deprotonation/
protonation cycles without a distinct decay in the signal
intensity, proving robust reversibility of this kind of chiroptical
molecular switch (Figure S1, Supporting Information). Mean-
while, its optical rotation values in solution could be also
reversibly tuned through cyclical addition of base and acid and
is accompanied by reversible inversion of the optical rotation
direction. As expected, the enantiomer of s-1 exhibited the
identical chiroptical inversion behavior (Figure S2, Supporting
Information).
As shown in Figure 1, two intensive positive CD signals near
316 nm (band I) and 252 nm (band II) are observed in DMSO,
THF, DCM, or n-hexane, which correspond to the UV−vis
absorption attributed to the π−π* transition of the intra-
molecularly hydrogen-bonded salicylidenamino chromophore
in the enol form (Scheme 1). For the ethanol solution of s-1, a
weak CD band in the range of 375−425 nm appears in addition
to bands I and II. This characteristic signal is believed to arise
from its keto form. According to 13C NMR analysis, s-1 exists
only in the enol form in DMSO.79,80 It should be noted that the
316 and 252 nm CD bands are both positive with no sign of
exciton splitting. Similar phenomena were previously observed
by H. E. Smith and considered to be related to the
configuration interaction or weak exciton coupling as a result
of vibrational effect cannot be ruled out.77
The change in CD and UV−vis spectra of s-1 upon
successive addition of tetrabutylammonium hydroxide
(TBAOH) was recorded in DMSO for the purpose of
removing interference from the keto tautomer. Upon titration
of s-1 with OH−, the absorption maximum at 316 nm gradually
A series of control experiments confirmed that the observed
chiroptical inversion for s-1 is hardly affected by different kinds
of base, counterion and solvent, although there exists a small
difference in the magnitude (see Figures S3−S5 in the
Supporting Information for details). Therefore, it is reasonable
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to speculate that the chiroptical switch triggered by acid/base
should work at a single-molecule level as a result of the changes
in the electronic and conformational states of the interacting
chromophores rather than the structure of a certain chiral
supramolecular assembly.
To gain insight into the chiroptical switching mechanism of
s-1, we performed the electronic circular dichroism (ECD)
calculations with the aid of time-dependent density functional
theory (TDDFT). Because CD spectroscopy is quite sensitive
to slight changes in the geometry and electronic displacement
of given chemical species, the combination of experimental and
theoretical CD data would possibly allow one to elucidate the
origin of observed chiroptical properties for the molecular
switch. In fact, this method has proved to be effective and
reliable for configurational and conformational analysis of
organic compounds.81−91
The B3LYP functional and the 6-311++G** basis set were
first used for geometry optimizations in vacuum. The computed
structures for the three energy miminum conformers of s-1
(denoted as s-1a∼c) are shown in Figure S6 (Supporting
Information). Among them, the conformer s-1c is preferred
over s-1a and s-1b by 2.25 and 1.41 kcal/mol, respectively. The
computed conformer of lowest energy is in good agreement
with the known single-crystal X-ray diffraction structure (Figure
3)92 and Smith’s conformational analysis as well.77
We then carried out the geometry optimizations of s-1 with
the PCM solvent model for DMSO in which the experimental
CD data are collected. As shown in Figure 4, the three most
stable conformers suggested by the solvent model have the
same structure as those optimized in vacumm. All the
conformers are stabilized by a strong intramolecular hydrogen
bond between the phenolic hydrogen and the imine residue;
their relative energies, Boltzmann weights, and selected
hydrogen-bonded parameters are listed in Table 1. The
Table 1. Relative Energies (ΔE0, kcal/mol), Boltzmann
Weights (BW, 298 K), and Intramolecular Distances C−
H···π (dC−H···π, Å) and O−H···N (rO−H···N, Å)
a
b
conformer
ΔE0 (kcal/mol)
BW (%)
dC−H···π (Å)
rO−H···N (Å)
s-1a
s-1b
s-1c
1.99
1.74
0.00
3.18
4.90
1.704
1.708
1.724
2.494
91.92
a
dC−H···π is the distance between methine hydrogen of salicylidene and
the carbon of the phenyl group directly attached to the chiral center.
b
rO−H···N is the distance between the hydrogen of phenolic hydroxyl
group and the imine nitrogen.
lowest-energy conformer s-1c has a Boltzmann weight of
91.92% owing to its less steric hindrance, which is preferred
over s-1a (3.18%) and s-1b (4.90%) by 1.99 and 1.74 kcal/mol,
respectively. Moreover, the fact that the stability of conformer
s-1b is slightly greater than s-1a could be attributed to the
intramolecular CH···π interaction between −NCH and
the phenyl group attached to the chiral center in the former.
Thus, TDDFT calculations on the three preferred conformers
followed by Boltzmann weighting resulted in an average ECD
spectrum in well consistent with the experimental except that
the position of the predicted Cotton effect is at a shorter
wavelength (306 nm) than the measured value (316 nm)
(Figure 5a). As for the deprotonated s-1, TDDFT calculations
reveal that only one dominant conformer, i.e., s-1d was
identified in the conformational searches with a 10 kcal/mol
energy window. This result seems to be reasonable because the
conformer adopts the least sterically hindered arrangement
compared with other possible isomers resulting from rotation
about the chiral carbon−nitrogen single bond. Similarly, there
is a good agreement between the theoretical ECD curve and
the experimental spectrum in DMSO for s-1−, wherein the
Figure 3. Comparison of (left) the computed structure for the most
stable conformer s-1c in vacuum obtained at the B3LYP/6-311++G**
level of theory and (right) the reported single-crystal structure of s-1
from ref 92.
Figure 4. Computed structures of most stable conformers for s-1 (s-1a−c) and the dominant conformer for s-1− (s-1d) obtained at the B3LYP/6-
311++G**/IEFPCM (DMSO) level of theory (with relative energies). Each conformer is shown as a side (above) and top (below) view. Note:
oxygen and nitrogen atoms are red and blue, respectively; the intramolecular hydrogen bond or C−H···π interaction is represented in a red dotted
line for clarity.
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Figure 5. Comparison of the calculated (B3LYP/6-311++G**/IEFPCM(DMSO), red line) and experimental (black line) ECD spectra in DMSO
for s-1 (a) and s-1− (b). Vertical bars represent the rotational strengths R in 10−39 cgs units.
former displays a strong negative Cotton effect near 388 nm
(Figure 5b). These observations indicate that the TDDFT
functional with the IEFPCM solvent model proves to be an
effective and reliable method for ECD calculations in the
present study.
On the basis of the above-mentioned facts, it is clear that the
conformers s-1c and s-1d are the most important contributors
to the dichroic absorption of free s-1 and its deprotonated
species, respectively. Between the two preferred conformers
there exists a distinct difference in the spatial arrangement of
groups at the chiral center with respect to the salicylidenamino
chromophore. Nevertheless, they can interconvert from one to
another by a 180°-rotation of the C−C bond connecting the
azomethine to phenyl group in the salicylidene moiety under
the stimulus of acid/base, as depicted in Figure 6. That is, the
chiroptical inversion mechanism. As anticipated, s-2 was found
to display the base/acid induced chiroptical response similar to
that of s-1 (Figure S8, Supporting Information). More
interestingly, the observed CD spectra appear to give a quasi-
mirror image for s-2 vs Et-s-2 as shown in Figure 7.94 At the
same time, TDDFT calculations revealed that the geometry-
optimized conformation of Et-s-2 is quite close to the most
Figure 6. Prediction of CD signs for s-1 and s-1− on the basis of their
respective preferred conformations (s-1c, s-1d) obtained at the
B3LYP/6-311++G**/IEFPCM(DMSO) level of theory. Each con-
former has a particular chirality of coupling as shown.
deprotonation/protonation of the phenolic hydroxy group and
accompanying conformational transition about the C−C bond
are responsible for the chiroptical inversion of the Schiff base
molecular switch. Furthermore, such a chiroptical inversion
may be interpreted in terms of the exciton chirality rule.93 From
the known transition moment directions of the two intra-
molecularly interacting chromophores,77,78 we can predict that
the most stable conformer s-1c makes a negative contribution
to the CE near 316 nm, whereas the contribution of s-1d is
negative to the 400 nm CE for the deprotonated species s-1−
(Figure 6).
Figure 7. Top: CD spectra of s-2 and Et-s-2 (1 × 10−4 mol/L) in n-
hexane/THF (4:1, v/v) solution. Notice that in the mixed solvent s-2
exists dominantly in the enol form estimated by UV−vis spectra
(Figure S7, Supporting Information). Bottom: optimized structures of
the most stable conformers of Et-s-2, s-2, and s-2− obtained at the
B3LYP/6-311++G**/IEFPCM(DMSO) level of theory, in which
oxygen and nitrogen atoms are red and blue, respectively. Each
conformer is shown as a side (above) and top (below) view.
2-Hydroxyl-1-naphthaldehyde Schiff base of (S)-α-phenyl-
ethylamine s-2 and its ethylated derivative Et-s-2 provide a
remarkable example to further account for the above-proposed
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stable conformer of the deprotonated s-2.95 In other words, Et-
s-2 takes a conformation in which the ethoxyl group is far away
from the imine nitrogen atom just as does the naphtholate
oxyanion in s-2− via the naphthyl group rotation about its
attachment bond. In addition, the theoretical ECD spectra are
in a good agreement with the main bands of the corresponding
experiment for the N-α-naphthal derivatives (Figures S9−S11,
Supporting Information).
Amide Chiroptical Molecular Switch. With the chirop-
tical inversion mechanism of the salicylaldehyde Schiff in mind,
we further investigated the chiroptical properties of salicylamide
of (S)-α-phenylethylamine (s-3) to examine whether the above
observation has general applicability. Choosing salicylamide
derivative is mainly owing to the following two reasons. First,
the intramolecularly hydrogen-bonded salicylamide moiety is
structurally similar to the salicylidenamino chromophore of s-1.
Thus, the rupture and formation of the intramolecular
hydrogen bond would trigger a conformational transition,
leading to the corresponding spectral changes in the same way
as the Schiff base systems. Second, compared to the azomethine
group, the amide linkage has a better tolerance to hydrolysis,
which will endow the amide-type molecular switches with
enhanced stability.
observed CEs in experimental CD spectra of s-3 and its
deprotonated species can be still predicted by using the simple
conformational analysis and TDDFT computations as done
above for s-1.
Geometry optimizations were conducted using the DFT
method at the B3LYP/6-311++G**/IEFPCM(DMSO) level of
theory, yielding the two most stable conformers (s-3a, s-3b) for
s-3 and one preferred conformer (s-3c) over all possible s-3−
isomers (Figure 9). For s-3a and s-3b, the former has a relative
Figure 9. Optimized structures of the most stable conformers s-3a and
s-3b for s-3 and s-3c for s-3−, obtained at the B3LYP/6-311++G**/
IEFPCM(DMSO) level of theory (with relative energies). Each
conformer is shown as a side (above) and top (below) view, in which
oxygen and nitrogen atoms are red and blue, respectively, and
intramolecular hydrogen bonds are represented asred dotted lines.
Figure 8 displays the CD and UV−vis titration spectra of s-3
upon successive addition of TBAOH in DMSO. It can be seen
energy of 0.39 kcal/mol with a population of 65.49%, whereas
the latter accounts for 34.07% of the population. Apparently,
these conformers are stabilized by the intramolecular hydrogen-
bonding interactions, where the amide group behaves as a
hydrogen-bond acceptor in s-3 and as a hydrogen-bond donor
in s-3−. Upon deprotonation of s-3, the original hydrogen bond
breaks to form a new internal hydrogen bond between the
amide N−H and phenolate oxyanion in the deprotonated
species. Such a rearrangement of the intramolecular hydrogen
bonding induced by the deprotonation of phenolic hydroxyl has
been fully demonstrated in the previous reports on achiral
salicylamide derivatives.96−100
To assess the reliability of theoretical ECD spectra, we
performed TDDFT computations at the B3LYP/6-311++G**/
IEFPCM(DMSO) level of theory for the amide-type molecular
switch and compared the results to experiment (Figure 10). It
can be seen that the averaged theoretical ECD agrees
reasonably well agree with the experimental spectra, although
the B3LYP functional slightly underestimates the CE’s
positions of s-3 and s-3− by 16 and 19 nm, respectively. Also,
the sign of the CEs of the amide derivative is in accord with
that predicted by the exciton chirality rule.101 As depicted in
Figure 11, the most important contributor s-3a makes a
negative contribution to the CD band of s-3 whereas the
contribution of the dominant conformer s-3c is positive for s-
3−. Accordingly, the reversible chiroptical switching behavior of
the acid/base-induced amide-type molecular switch may be
regarded as the consequence of the conformational transition
about the amide attachment bond driven by the hydrogen
bonding reorganization (Figure 11).
Figure 8. CD and UV−vis titration spectra of s-3 (1 × 10−4 mol/L) in
DMSO with increasing amount of TBAOH (0, 0.5, 1.0, and 2.0 equiv,
respectively).
that free s-3 displays a CD spectrum with a negative Cotton
effect associated with the absorption near 309 nm. On addition
of increasing amounts of TBAOH (0−2.0 equiv), the negative
CD signal reduces gradually and finally turns into a new band
with positive CE at 349 nm. The positive CD maximum is
∼1.8-fold more intensive than that of the original negative CE,
which is different from s-1. As expected, for the amide system
the base-induced CD change could also be totally reversed to
the original state by acidification. Upon alternate addition of
base and acid, such a reverse of CE signs can be repeated five
times without a noticeable loss of signal intensities (Figure S1,
Supporting Information), which is nearly the same as that
observed in the case of s-1.
Also noteworthy is that the sense of Cotton effect in the CD
spectrum of s-3 changes from negative to positive upon
deprotonation (Figure 8), whereas for s-1 the CD band inverts
from positive to negative (Figure 2). In spite of this, the sign of
CONCLUSION
In the present work, we demonstrated a new strategy to
construct stimuli-responsive molecular switches that show a
■
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Figure 10. Experimental and Boltzmann-averaged ECD solution spectra of s-3 (a) and s-3− (b) optimized by the B3LYP/6-311++G**/
IEFPCM(DMSO) method. Vertical bars represent the rotational strengths R in 10−39 cgs units.
chiroptical molecular switches triggered by stimuli other than a
change in pH, for instance, temperature, light, or electro-
chemical signals. Related work is now in progress.
EXPERIMENTAL SECTION
■
General Information. Compounds s-2 and Me-s-2 were
prepared following the reported procedure.17 All the solvents
used were dried and distilled by usual procedures before use.
Melting points were taken with a micro melting point apparatus
1
and are uncorrected. H and 13C NMR spectra were recorded
with tetramethylsilane as an internal standard and CDCl3 as a
Figure 11. Prediction of CD signs for s-3 and s-3− on the basis of their
solvent, respectively. UV−vis and CD spectra were obtained at
respective most stable conformations (s-3a, s-3c) obtained at the
20 °C using a quartz cell of 1 cm. The optical rotation and ESI-
B3LYP/6-311++G**/IEFPCM(DMSO) level of theory. Each con-
former has a particular chirality of coupling as shown.
MS were measured.
Synthesis of (S)-N-Salicylidene-α-phenylethylamine (s-1).
To a solution of salicylaldehyde (1.22 g, 10.0 mmol) in ethanol
(50 mL) was added (S)-α-phenylethylamine (1.21 g, 10.0
mmol). After 12 h of reflux, the solvent was removed under
reduced pressure to give the crude product. The product was
recrystallized from n-hexane to give a yellow solid in 83% yield
reversible chiroptical inversion through the controlled C−C
single bond rotation. The theoretical calculations provided
useful insight into the conformational transition and intra-
molecular interaction and allow for elucidation of the observed
chiroptical inversion behaviors. To the best of our knowledge,
despite several examples of chiroptical inversion due to the
single bond rotation,74−76,102 no chiroptical molecular switches
based on the proposed working mechanism have been reported
until now. The feasibility of the approach has been successfully
confirmed by simple chiral salicylaldehyde Schiff bases and a
salicylamide analog. Compared to the existing molecule-based
chiral inversion systems, this promising new type of chiroptical
switching molecule is relatively unique as it does not involve
any covalent bond formation/breakage and thus possesses
distinct advantages, such as fast switching rate, high reversibility
and fatigue resistance, and the nondestructive readout. Thus,
such dynamic chiroptical inversion systems are expected to find
potential applications in molecular recognition, chemosensors,
or the construction of molecular-scale devices. More
importantly, these findings suggest that the use of the
conformational transition about a single bond may serve as
the basis for designing chiroptical inversion systems.
20
(2.02 g). Mp: 73.6−74.3 °C. [α]D = +121° (0.01 g/mL,
DMSO). 1H NMR (500 MHz, CDCl3): δ 13.48 (bs, 1H), 8.22
(s, 1H), 7.20−7.40 (m, 7H), 6.96 (d, 1H), 6.86 (t, 1H), 4.53
(m, 1H), 1.63 (d, 3H). 13C NMR (125 MHz, CDCl3): δ 163.7,
161.3, 143.9, 132.5, 131.3, 128.9, 127.3, 126.5, 119.1, 118.8,
68.7, 25.2. MS (ESI, m/z ([M + H]+)): calcd, 226.2; found,
226.1. (R)-N-Salicylidene-α-phenylethylamine (r-1) was pre-
pared by the same method as that for s-1 except for the use of
(R)-α-phenylethylamine as the starting material.
Synthesis of (S)-1-(1-Methylbenzyliminomethyl)-2-ethox-
ynaphthalene (Et-s-2). Ethyl bromide (1.5 g, 13.9 mmol), 2-
hydroxyl-1-naphthaldehyde (1.72 g, 10.0 mmol), and K2CO3
(5.0 g, 36.2 mmol) were mixed in dry acetone (50 mL) under a
N2 atmosphere. The resulting mixture was stirred at 40 °C for
36 h and then poured into a large amount of water to
precipitate the etherated product (2-ethoxy-1-naphthaldehyde)
as a white solid. The intermediate was recrystallized from n-
hexane to give a colorless crystal (1.8 g, 91%).
The mixture of 2-ethoxy-1-naphthaldehyde (0.8 g, 4.0 mmol)
and (S)-α-phenylethylamine (0.5 g, 4.2 mmol) in ethanol (50
mL) was refluxed for 12 h. After removel of solvent, the
resulting white solid was recrystallized from n-hexane to give
the desired product as a colorless crystal (1.1 g, 83%). Mp:
60.2−60.7 °C. [α]D20 = +85° (0.01g/mL, THF/n-hexane, v/v =
Taking into account the straightforward preparation of the
chiroptical molecular switches, as well as virtually unlimited
structural flexibility provided by the modular nature of our
finding, various chiral molecules containing imine or amide
skeleton motif could be prepared and fine-tuned to control
stability and rate of interconversion between the conformations
having different chiroptical properties. Also, these modular
structures make it possible to design a new generation of
1
1:4). H NMR (400 MHz, CDCl3): δ 9.32 (d, 1H), 9.20 (s,
1H), 7.88 (d, 1H), 7.76 (d, 1H), 7.51−7.60 (m, 3H), 7.35−
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(7) Guo, P. Z.; Liu, M. H.; Zhao, X. S. Studies of Chiroptical Switch.
Prog. Chem. 2008, 20, 644−649.
7.42 (m, 3H), 7.24−7.31 (m, 2H), 4.62−4.67 (m, 1H), 4.20−
4.26 (m, 2H), 1.71 (d, 3H), 1.48 (t, 3H). 13C NMR (125 MHz,
CDCl3): δ 157.4, 145.7, 132.2, 129.2, 128.3, 127.9, 127.7, 126.7,
126.6, 125.8, 123.9, 117.9, 114.1, 71.6, 65.3, 25.5, 15.0. MS
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Synthesis of N-(S)-1-Phenylethyl-2-hydroxybenzamide (s-
3). Methyl salicylate (1.52 g, 10.0 mmol) and (S)-α-
phenylethylamine (1.50 g, 12.4 mmol) were mixed and heated
at 100 °C for 20 h. The reaction mixture was cooled to room
temperature and the resulteing product was recrystallized from
n-hexane and ethyl acetate to give a white crystal (1.51 g, 62%).
20
1
Mp: 108.5−109.4 °C. [α]D = +15° (0.01 g/mL, DMSO). H
NMR (400 MHz, CDCl3): δ 12.29 (s, 1H), 7.47−7.28 (m,
7H), 6.98 (d, 1H), 6.88 (t, 1H), 6.47 (b, 1H), 5.31 (m, 1H),
1.64 (d, 3H). 13C NMR (100 MHz, CDCl3): δ 169.1, 161.5,
142.4, 134.2, 128.8, 127.6, 126.1, 125.3, 118.6, 114.1, 49.0, 21.7.
MS (ESI, m/z ([M + H]+)): calcd, 242.1; found, 242.2.
Computations. Geometry optimizations and zero-point
energies of the compounds were performed by TDDFT
calculations at the B3LYP level103,104 with a 6-311++G**
basis set using “Gaussian03 program”.105 Vibration frequencies
were used to confirm intermediates (number of imaginary
frequencies = 0). This DFT calculation was proved economic
and reliable in our previous work.106,107
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ASSOCIATED CONTENT
* Supporting Information
■
S
Switchable behavior illustration, selected CD, UV−vis, and
ECD titration spectra, stick conformations, the optical rotation
values for s-1 and s-3 before and after addition of TBAOH, and
NMR and ESI−MS spectra of compounds. This information is
available free of charge via the Internet at http://pubs.acs.org.
(17) Fujiki, M.; Koe, J. R.; Motonaga, M.; Nakashima, H.; Terao, K.;
Teramoto, A. Computing Handedness: Quantized and Superposed
Switch and Dynamic Memory of Helical Polysilylene. J. Am. Chem. Soc.
2001, 123, 6253−6261.
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Induction of Chiral Helicity in Achiral Synthetic Hydrogen−Bonding
Oligomers. J. Am. Chem. Soc. 2004, 126, 2022−2027.
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J.; Meijer, E. W. Inversion of Optical Activity of Chiral Polythiophene
Aggregates by a Change of Solvent. Macromolecules 1998, 31, 6702−
6704.
(20) Maeda, K.; Morino, K.; Okamoto, Y.; Sato, T.; Yashima, E.
Mechanism of Helix Induction on a Stereoregular Poly((4-
carboxyphenyl)acetylene) with Chiral Amines and Memory of the
Macromolecular Helicity Assisted by Interaction with Achiral Amines.
J. Am. Chem. Soc. 2004, 126, 4329−4342.
(21) Maeda, K.; Yashima, E. Dynamic Helical Structures: Detection
and Amplification of Chirality. Supramolecular Chirality 2006, 265,
47−88.
(22) Nakashima, H.; Fujiki, M.; Koe, J. R.; Motonaga, M. Solvent and
Temperature Effects on the Chiral Aggregation of Poly-
(alkylarylsilane)s Bearing Remote Chiral Groups. J. Am. Chem. Soc.
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(24) Nonokawa, R.; Yashima, E. Detection and Amplification of a
Small Enantiomeric Imbalance in α-Amino Acids by a Helical
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(25) Pijper, D.; Feringa, B. L. Molecular Transmission: Controlling
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Molecular Motor. Angew. Chem., Int. Ed. 2007, 46, 3693−3696.
(26) Pijper, D.; Jongejan, M. G. M.; Meetsmia, A.; Feringa, B. L.
Light-Controlled Supramolecular Helicity of a Liquid Crystalline
AUTHOR INFORMATION
Corresponding Authors
*L. Jiang: e-mail, cejlm@zju.edu.cn.
*J. Ling: e-mail, lingjun@zju.edu.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are indebted to the financial support by the
National Natural Science Foundation of China (Grant No.
21074107).
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