Chiral photoresponsive tetrathiazoles that provide snapshots of folding states

Article abstract of DOI:10.1002/chem.201302564

Herein, we designed chiral photoresponsive tetra(2-phenylthiazole)s, which induce a diastereoselective 6π-electrocyclization reaction in a helically folded structure to freeze the conformational interconversions. The folding conformation with one helical

Full text of DOI:10.1002/chem.201302564

DOI: 10.1002/chem.201302564
Chiral Photoresponsive Tetrathiazoles That Provide Snapshots of Folding
States
[a]
Takuya Nakashima,* Kyohei Yamamoto, Yuka Kimura, and Tsuyoshi Kawai*
Abstract: Herein, we designed chiral
photoresponsive tetra(2-phenylthia-
zole)s, which induce a diastereoselective
p-electrocyclization reaction in a heli-
cally folded structure to freeze the con-
formational interconversions. The fold-
ing conformation with one helical turn
of tetra(2-phenylthiazole)s was sup-
ported by multiple intramolecular non-
covalent interactions including vicinal
S···N interheteroatom interactions and
CH–p and p–p stacking interactions
between nonadjacent units, as found in
X-ray crystal structures as well as
quantum chemical calculations. The in-
troduction of a chiral group at both
ends of tetra(2-phenylthiazole) dictates
the preferential folding into a one-
handed helix conformation by the si-
multaneous operation of S···O and mul-
tiple CH–p interactions that involve
the chiral end groups. Since the
tetra(2-phenylthiazole)s possess two
equivalent photoreactive 6p-electron
systems and the folded conformation is
suitable for photoinduced electrocycli-
zation reaction, they undergo a photo-
cyclization reaction in a stereoselective
manner to memorize the chirality of
the helix in a resulting diastereomeric
closed form.
6
Keywords: chirality · foldamers ·
helical structures · photochromism ·
supramolecular chemistry
Introduction
which would expand the response behaviors of photorespon-
sive foldamers.
Foldamers have emerged as artificial oligomers that fold
into well-defined secondary structures in solution, thereby
Recently, we have proposed an a,b-linked triheteroaryl-
AHCTUNGTRENNUNG
[1]
mimicking biopolymers. Of particular interest are the stim-
[1–5]
[14–16]
uli-responsive chiral helices
with respect to biomimetics
(Scheme 1).
Since the photoinduced 6p-electrocyliza-
[11]
since the dynamic conformational processes of proteins cor-
relate with biological functions in natural systems. Mean-
while, light is one of the promising external inputs for vari-
ous practical applications, whereby one can control the
physicochemical properties of active materials with precise
tion reaction takes place within a few picoseconds, the re-
action efficiency is directly dependent on the geometry
around the 6p system at the ground state. Compared to con-
[11]
ventional diarylethenes, terarylenes offer better accessibil-
ity to the control of molecular folding in terms of the design
of intra- and intermolecular interactions through the combi-
nation of various heteroaromatics. Despite the presence of
the rotatable single bonds in the 6p system, well-designed
terarylenes were able to stabilize a folded photoreactive
spatiotemporal control in
a remote and noninvasive
[6]
manner. In this context, helical architectures that contain
azobenzene units in a main framework have been devel-
[
7–10]
oped.
unit serves to denature the helical structure in a dynamic
The E–Z photoisomerization of the azobenzene
conformation with C symmetry around the 6p system in so-
2
[7,8]
manner,
capability.
which was also integrated with guest-recognition
lution and achieved highly efficient photocyclization reactiv-
[
9,10]
[15–17]
Whereas photochromes that show 6p-electro-
ity with the topmost quantum yield.
Furthermore, the
[11]
cyclization reaction such as diarylethenes have been incor-
porated into helicenes with a rigid geometry,
photoreaction takes place in a stereospecific manner accord-
ing to the Woodward–Hoffmann rule, and the conformation-
al information of a precursor is directly recorded as the ste-
AHCTUNGTRENNUNGr eostructure of a photoproduct. Terarylenes with a chirally
controlled geometry exhibited a photoreaction not only with
high efficiency but with high stereoselectivity, which also of-
[12,13]
chiral heli-
cal structures with the photoresponsive 6p system have
never been demonstrated. The 6p-electrocyclization reaction
proceeds in a quite different way from that of azobenzene
with respect to the change in geometry and electronic state,
[
a] Dr. T. Nakashima, K. Yamamoto, Y. Kimura, Prof. T. Kawai
Graduate School of Materials Science
Nara Institute of Science and Technology, NAIST
8
916-5 Takayama, Ikoma, Nara 630-0192 (Japan)
Fax : (+81)743-72-6179
E-mail: ntaku@ms.naist.jp
tkawai@ms.naist.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302564.
Scheme 1. Reversible photoinduced 6p-electrocyclization reaction of ter-
arylene.
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Chem. Eur. J. 2013, 19, 16972 – 16980

FULL PAPER
fered an advantage over diastereoselective photoreactions
of diarylethenes.
We envisage that further extension of terarylenes to oligo-
ing a specific enantiomer and increasing the energy barrier
[18]
[21,22]
of helix inversion.
The tetra(2-phenylthiazole)s possess
[20]
two equivalent 6p systems, and the folded conformation is
suitable for a photoinduced electrocyclization reaction. Pho-
toirradiation immediately freezes the interconversion of
conformers to form a closed-ring isomer, which gives a sort
of snapshot of the folding status of the photoprecursor;
a right-handed helix (P helix) gives an R,S stereoisomer,
whereas a left-handed helix (M helix) forms an S,R stereo-
isomer (Scheme 2).
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
arylenes would give a stable helical structure with more
than one helical turn by the addition of p–p-stacking inter-
actions between an aryl unit and the next unit but two. a,b-
Connected oligoheteroarylenes could impart photoreactivity
in a main helical framework by the use of conjugated three
double bonds in a cis–cisoid backbone. Thus, the extension
of terarylenes is in line with the design of photoresponsive
foldamers. In this study, we describe tetrathiazoles as a mini-
mum building unit of a chiral photoresponsive helical motif
and a prototype of a photoresponsive foldamer with 6p sys-
tems. Enantiomers of (R,R)-1 and (S,S)-1 (Scheme 2) were
designed on the basis of tetra(2-phenylthiazole) (2), a single
A number of asymmetric reactions including diastereose-
[12,13,23]
[24]
lective
and enantioselective electrocyclizations have
been reported for diarylethenes in solutions, crystals, and
supramolecular assemblies. These diarylethenes often tether
chiral side units or adopt helicene structures to induce
a chiral structure in the backbone, mainly by repulsive intra-
molecular interactions. The stereoselective photochromic re-
action inspired by the foldamer design is demonstrated for
the first time.
[14a,19]
unit-extension of the terthiazole,
the photochromic per-
formance of which has been recently reported by Yu and
[20]
co-workers.
Scheme 2 depicts interconversion between representative
conformers followed by a stereoselective photoreaction
from the helically folded state. Steric repulsion between
nonadjacent 2-phenylthiazolyl units destabilizes the all-trans
conformer with a planar conformation and the rotation
about two single bonds with 4,5-connection is expected to
be harnessed by adjacent S···N interactions as demonstrated
Results and Discussion
X-ray crystal structures and quantum chemical calculations:
Recrystallization from the mixture of acetonitrile and
chloroform (95:5 v/v) successfully gave single crystals of
(R,R)-1 and (S,S)-1. The crystallographic data for (R,R)-
1 and (S,S)-1 together with reference compound 2 are sum-
marized in Table 1. Each enantiomer possessed a one-
handed helix with the all-cis conformation in the crystal as
we expected in Scheme 2. The space group was P2 2 2 for
[15a,16]
with several terarylenes.
Since the steric repulsion still
remains in the cis–trans–cis form with a planar structure, the
central single bond with a [4’,4’’]-connection rotates to adopt
an all-cis conformation. CH–p interactions between the end
alkyl substituents (R) and the thiazole ring in the next unit
but one, and the p–p interaction between both the end 2-
phenylthiazole units are expected to hold the all-cis
1
1 1
both the crystals, which is typical for a chiral crystal. Com-
pound (R,R)-1 formed a right-handed P helix, whereas
(S,S)-1 adopted a left-handed M one with a stereostructure
almost perfectly identical to the mirror image of (R,R)-
1 (Figure 1). A multitude of intra- and inter-unit noncova-
lent interactions were found in the crystal structure. Each 2-
phenylthiazole unit formed an almost planar structure with
the torsion angle below 108 sup-
conform ACHTUNGTRENNUNGe r with one helical turn. The quantum chemical cal-
culation indeed suggested that the helix conformation was
the most stable among these conformers (Figure S1 in the
Supporting Information). The introduction of a chiral group
at both end units could dictate the preferential folding into
a one-handed helix as reported in several papers by stabiliz-
ported by the CH···S and
CH···N weak hydrogen-bonding
[14–16,25]
interactions.
Compound
(
R,R)-1 built a P helix by twist-
ing the planar 2-phenylthiazole
units in a clockwise fashion
with angles of 55.18, 42.88, and
5
5.988 for fA–B, fB–C, and fC–D,
respectively (Figure 1). The
non-orthogonal dihedral angles
for fA–B and fC–D were support-
ed by inter-unit S–N interac-
[15a,16,26]
tions
with the atomic dis-
tances of 3.306 and 3.239 ꢁ for
N(A)ÀS(B) and S(C)ÀN(D), re-
spectively, which are shorter
than the sum of van der Waals
Scheme 2. Interconversion of conformers followed by reversible photochemical reactions of tetra(2-phenylthia-
zole)s (R,R)-1, (S,S)-1, and 2.
radii of
S
(1.8 ꢁ) and
N
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16973

T. Nakashima, T. Kawai et al.
Table 1. Crystallographic parameters and refinement details for (R,R)-1,
S,S)-1, and 2.
crystal structure of (R,R)-1 around the chiral end group.
The positions of hydrogen atoms were estimated theoretical-
ly. The close contact between S and O in the same unit was
observed with the distance below 3.1 ꢁ, thus indicating the
S···O inter-heteroatom interactions in both end units (A
and D). Trapping of the ether oxygen atom in the chiral end
group by thiazolyl–sulfur directed methylene, methine, and
methyl protons toward the p plane of the thiazole ring in
the next unit but one (broken lines in Figure 1c). The dis-
(
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(R,R)-1
A
T
N
T
E
N
N
(S,S)-1
2
formula
C
44
H
38
N
4
O
2
S
4
C
44
H
38
N
4
O
2
S
4
C
38 26 4 4
H N S
[27]
M
r
783.05
orthorhombic
P2
7.8578(2)
22.0339(4)
22.3295(4)
90.0000
90.0000
3866.1(2)
4
783.05
orthorhombic
P2
7.8449(2)
22.0277(4)
22.3413(4)
90.0000
90.0000
3860.7(2)
4
666.89
monoclinic
P2 /n
11.9748(3)
13.6322(3)
19.9841(4)
90.0000
96.0856(7)
3243.9(1)
4
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
a=g [8]
b [8]
V [ꢁ ]
1
2
1
2
1
1
2
1
2
1
1
AHCTUNGTRENNUNGt ances between these protons and the p plane were estimat-
3
ed to be in the range of 2.5–3.1 ꢁ (Table S1 in the Support-
ing Information), which corresponds to the effective range
Z
À3
[28]
1
[gcm
]
1.345
1.347
1.365
for CH–p interactions.
T [K]
123
123
123
To confirm the difference in the noncovalent interactions
on the chiral end group between P and M helices with the
identical chiral end group, we calculated the optimized ge-
reflns. measured
reflns. unique
67069
8841
66544
8824
31760
7432
R
1
[I>2s(I)]
0.0296
0.0765
0.0309
0.0746
0.0346
0.0880
wR (all data)
2
ACHTUNGTRENNUoGN metries of P and M helices of (R,R)-1 by the DFT method
[29]
using Gaussian 09. For the calculation, a DFT functional
[30]
wB97XD with a 6-31G(d) basis set was employed. Enan-
tiomeric structures picked out from the identical unit cell of
crystal 2 were used as initial structures after the substitution
of a methyl end group with an (R)-2-methoxypropyl group
to form both the P and M helices of (R,R)-1. The optimized
structure starting from the P-helical (R,R)-1 showed good
agreement with the crystal structure of (R,R)-1 (Figures S5
and S7 and Table S3 in the Supporting Information). The
noncovalent interactions including p–p stacking between
both the end units and S–O and CH–p interactions on the
chiral end groups were also well reproduced in the opti-
mized structure of P-helical (R,R)-1 (Figure S8 in the Sup-
porting Information). However, the simultaneous attainment
of S···O and multiple CH–p interactions was found to be im-
possible in the M-helical (R,R)-1, which resulted in the
higher energy of the M helix of (R,R)-1 than the P helix of
(
1.55 ꢁ). Despite the electrostatic repulsion between lone
[16]
pairs of nitrogen atoms, close contact of N(B)ÀN(C) was
observed (2.891 ꢁ). The parallel orientation of planar units
A and D with the distance of 3.45 ꢁ indicated the slipped
p–p-stacking interaction between these units. In addition,
multiple CH–p interactions between the chiral end group
and the next unit but one (Figure 1c) could force a decrease
in the torsion angle between units B and C, thereby result-
ing in the short atomic distance of N(B)ÀN(C). These intra-
and inter-unit noncovalent interactions mentioned above
guided the molecule to fold into compact structures with
one helical turn.
The chiral end groups apparently played a crucial role in
the induction of one-handed helicity since the reference
compound 2 gave a racemic crystal (P2 /n) that contained
1
À1
both-handed helices in a unit cell (Figure S4 in the Support-
ing Information). Interestingly, the mode of the CH–p inter-
actions between the chiral end group and the thiazole ring
in the next unit but one was most likely to determine the
preferential sense of helix. Figure 1c depicts the part of the
(R,R)-1 by 8.7 kcalmol . The DFT calculation also suggest-
ed that the P-helix conformation was the most stable geom-
etry among various conformers of (R,R)-1, and the helix in-
version needs to go through several conformers with higher
À1
energy than the P helix by 15.8 kcalmol at the most (Fig-
Figure 1. Crystal structures of a) (R,R)-1 and b) (S,S)-1. Hydrogen atoms are omitted for clarity. c) Part of the crystal structure of (R,R)-1 showing the in-
tramolecular interactions (red broken lines) involving the chiral end group. The red plane includes the p-conjugated plane of the inlying 2-phenylthiazole
unit (unit C).
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Chiral Photoresponsive Tetrathiazoles
FULL PAPER
ure S7 in the Supporting Information), which is much larger
than the thermal energy at room temperature (0.6 kcal
mol ). Thus, the DFT calculations thoroughly supported
Tetra(2-phenylthiazole)s possess two equivalent photo-
reactive 6p systems for the pericyclization reaction in a heli-
cally folded structure (Scheme 2). Irradiation of (R,R)-1 and
(S,S)-1 solutions with UV light (l=365 nm) gave rise to the
progression of a new absorption band at around 600 nm
owing to the formation of pericyclized photoproducts (Fig-
À1
the induction of chirality in the helical handedness by the
chiral end group to stabilize the helical conformation with
a specific handedness and increase the energy barrier of
helix inversion. It should also be noted that the even less
stable M helix of (R,R)-1 could be categorized as a photo-
reactive structure with close distances between the photo-
reactive carbon atoms shorter than 0.4 nm (0.346 and
[14,20]
ure 2d).
Meanwhile, a drastic change occurred in the
CD spectra, in which broad CD signals that corresponded to
the visible absorption band of photoproducts emerged after
UV irradiation (Figure 2c). This CD response is characteris-
tic of a chiral 6p system, which also demonstrated the differ-
[31]
0.345 nm).
[7,8]
ence to azobenzene-based chiral foldamers.
The preferen-
Circular dichroism (CD) and photochemical reaction study:
We studied the chiral induction in (R,R)-1 and (S,S)-1 on
the basis of CD spectra as demonstrated in several re-
tial formation of R,S- and S,R-type photoisomers
(Scheme 2) from (R,R)-1 and (S,S)-1, respectively, were sug-
gested, and the CD signs in the visible region were in good
agreement with those simulated on the basis of time-depen-
dent (TD)-DFT calculations (Figure S10 in the Supporting
[1–3,21,22]
ports.
Compounds (R,R)-1 and (S,S)-1 exhibited sym-
metrical mirror images of their CD profiles with bisignate
Cotton effects in the region of p–p-transition absorption
[32]
Information).
As demonstrated above, the photoirradia-
(
Figure 2a,b). Since the chiral end group itself has no p-con-
tion to the helical tetrathiazoles successfully transferred and
memorized their chiral information in the cyclized photo-
isomers. One might consider here whether the observed CD
spectra are the results of the uneven existing ratios of P and
M helices or R,S- and S,R-type photoproducts. To make this
point clearer, we evaluated the conformational change of
chiral helices in solution by means of temperature-depen-
1
dent H NMR spectroscopy, CD, and photoreaction studies.
Conformational change of tetrathiazoles studied by temper-
1
ature-dependent CD and H NMR spectroscopy: Figure 3
shows the temperature-dependent CD spectra of (S,S)-1 in
CH Cl from 213 to 293 K. The CD peaks became promi-
2
2
nent with a decrease in temperature. The CD intensity at
the first Cotton band (350 nm) almost doubled from 293 to
2
13 K. This observation suggests the stabilization of the M
helix of (S,S)-1 at low temperature, thus leading to the in-
Figure 2. a,c) CD and b,d) UV/Vis spectra of (R,R)-1 (red traces) and
À5
(
S,S)-1 (blue traces) in acetonitrile (c=1.5ꢂ10 m) before (a and b) and
after UV (l=365 nm) irradiation to achieve the photostationary state (c
and d) at 298 K.
jugation system that is responsible for the absorption above
250 nm, the observed CD spectra suggested the induced
chirality in the p system of the tetra(2-phenylthiazole)
framework. Neither clear absorption nor a CD band was ob-
served in the visible region before UV irradiation. Accord-
ing to the X-ray crystal and quantum chemical calculation
studies, (R,R)-1 and (S,S)-1 were expected to form P and M
helices, respectively. Each Cotton effect sign was positive
for (R,R)-1 and negative for (S,S)-1, which clearly indicated
the helical arrangement or distortion of chromophores in
clockwise and anticlockwise fashions, respectively. The CD
spectral features were in accordance with the simulated CD
profiles based on the calculated structures of the P helix of
(
R,R)-1 and the M helix of (S,S)-1 (Figure S6 in the Sup-
porting Information). Thus the CD study clearly supported
the chiral induction of helix handedness in solution.
2 2
Figure 3. Temperature-dependent CD spectra of (S,S)-1 in CH Cl . Each
trace was measured at 293, 273, 253, 233, and 213 K.
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T. Nakashima, T. Kawai et al.
crease in the population of M helices or the tightening of
helix conformation to make it more CD-active. The helix in-
version between P and M helices might fail to account for
this CD spectral change since the apparent isointensity
point (indicated by a gray arrow) at 338 nm was found
below the baseline. If the CD change was based on the equi-
librium that involves the helix inversion between P and M
helices with mirror-symmetrical CD profiles, the isointensity
point should be found as a zero value.
surface of unit C to avoid the gauche conformation, thereby
resulting in lower upfield shifting (Dd=À0.11 ppm from 293
c
to 193 K). In a similar manner, when proton H comes close
e
to the thiazole in unit C, the methoxy protons (H ) could be
affected by the deshielding effect of the phenyl ring in unit
D to show slight downfield shifting with decreasing tempera-
ture (Figure S11 in the Supporting Information). Thus, upon
1
decreasing temperature, the H NMR spectral change in the
aliphatic region demonstrated enhanced interaction between
the end groups and the non-adjacent p units.
1
Figure 4 shows temperature-dependent H NMR spectra
of (S,S)-1 in CD Cl . All peaks were reasonably assigned as
depicted in Figure 4. In particular, the peak assignment for
Given the fact that all the phenyl groups were under
a similar local electronic environment, the number of
H NMR spectroscopic peaks was supposed to be three at
2
2
1
most. The five peaks observed in the aromatic region clearly
indicated that there were two types of phenyl rings with dif-
ferent environments in a molecule, in which the inlying units
(
B and C) and both the end units (A and D) could be distin-
guished. Three peaks that showed continuous upfield shift-
ing with decreasing temperature could be assigned to the
f
g
h
protons (H , H , and H ) in both the end units (A and D),
which are anticipated to interact with each other by means
of p–p stacking as observed in the crystal structure as well
as the calculated structure. In comparison with the chemical
i
j
k
shifts of H , H , and H that were unaltered with tempera-
f
g
h
ture change, those of H , H , and H , respectively, appeared
upfield by d=0.5–0.2 ppm at 293 K. These chemical shifts
suggested that the p–p interaction between units A and D
operated in solution even at room temperature to give
a shielding effect to these phenyl protons.
1
D NOE measurements at 293 and 203 K further con-
firmed the presence of a p–p interaction. The differential
f
h
NOE spectra showed positive signals at peaks H and H
g
when peak H was irradiated, and also a positive signal at
g
h
peak H when H was irradiated at 293 K (Figure S12 in the
Supporting Information). Meanwhile, these differential
NOE peaks became negative at 203 K. This change from
positive to negative in the differential NOE signals with de-
creasing temperature was clear evidence that the intra-unit
correlations between vicinal protons, which are also active
1
Figure 4. Temperature-dependent H NMR spectra of (S,S)-1 in CD
2
Cl
2
.
The parts of the chemical structure of (S,S)-1 are also shown for the peak
assignment.
1
1
for H, H COSY (homonuclear proton–proton correlation),
were not responsible for these NOE signals. Moreover, it
was a typical indication of the increase in the molecular cor-
relation time owing to the stabilization of a specific molecu-
lar conformation and the slowing of molecular rotation at
the protons on phenyl groups was on the basis of differential
nuclear Overhauser effect (1D NOE) measurements. A
peak that emerged at d=1.7 ppm, which showed a downfield
shift below 233 K, was due to the dew condensation water
when the sample tube was chilled at low temperature. In the
aliphatic region, almost all peaks except for methoxy pro-
[33]
1
low temperature. Therefore, H NMR spectroscopic mea-
surements in the aromatic region clearly supported the
AHCTUNGTRENNUNG
inter-unit spatial correlation of phenyl protons between both
end units A and D by means of p–p interactions, which con-
tributed to the stabilization of a left-handed helix of (S,S)-
1 in solution. It should be noted that no peak splitting due
to the formation of minor diastereomers that corresponded
to the right-handed conformation of (S,S)-1 was observed in
both aliphatic and aromatic regions in the range of tempera-
ture studied. According to the results of DFT calculations
(Figure S8 in the Supporting Information), the protons in
the chiral end groups were expected to have different chem-
ical shifts between P and M helices when the inversion rate
e
tons (H ) exhibited upfield shifting to varying degrees upon
decreasing the temperature. The observed upfield shift for
a
d
H ÀH clearly indicated the enhancement of CH–p interac-
tions between these protons and thiazole ring in the next
unit but one (unit C in Figure 4). The upfield shift was the
c
most prominent for the methine proton (H ) directly at-
tached to the chiral carbon, and its chemical shift changed
from d=3.10 ppm at 293 K to d=2.83 ppm at 193 K. When
c
proton H approaches the face of the thiazole ring in unit C,
a
b
the methylene protons (H and H ) move away from the p
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Chiral Photoresponsive Tetrathiazoles
FULL PAPER
was slow enough on the NMR spectroscopic timescale. The
broadening of several peaks observed at 230 K might be at-
tributed to the local conformational fluctuation in the M
helix, since the peak broadening was found only for the pro-
tons associated with CH–p and p–p interactions mentioned
above. Furthermore, taking the result of the temperature-
dependent CD spectral change in Figure 3 into consider-
ation, the structural fluctuation within chiral helix conforma-
tions between less and strong CD-active conformers rather
than the helix inversion is most likely responsible for the
conformational change. To make this point clearer, the tem-
perature-dependent photochemical reaction was monitored
by CD spectra.
Conformational change of foldamers probed by photoreac-
tion: UV irradiation to tetra(2-phenylthiazole)s resulted in
the stereoselective photochemical formation of closed-ring
isomers, thus reflecting the folding status of photoprecursors
in solution. Compound 2 has been reported to show an elec-
trocyclization reaction with a quantum yield of 55% and
Figure 5. Plots of the relative CD peak intensity normalized by CD inten-
sity at 293 K (CD293K) for (S,S)-1 (circles, CD monitored at 350 nm) and
after 1 min UV irradiation at given temperatures (squares, CD monitored
at 600 nm).
[20]
conversion of 82%. Since no chiral induction took place
for 2, unlike compound 1, it exhibited no CD peak before
and after photoirradiation. The electrocyclization reaction
of 2 was expected to give two types of stereoisomers, R,S
and S,R forms (Scheme 2). The relatively high thermal sta-
bility of 2 in the ring-closed isomer with a half-lifetime of
stationary state was conducted at room temperature. As
mentioned above, the CD peak intensity of (S,S)-1 linearly
increased with decreasing temperature, and the relative in-
tensity reached 1.77 at 213 K (see also Figure 3). The UV ir-
radiation for 1 min gave an identical photostationary state,
which was monitored by the output voltage of the photo-
multiplier for the DC component in the spectropolarimeter.
Since the reactivity of the cycloreversion reaction was
[20]
the backward reaction (24 days)
enabled us to separate
a one-enantiomer-rich solution with a chiral-phase HPLC by
using hexane/ethanol (1:9) as the eluent. The separated solu-
tion gave a similar CD profile to that of the R,S-type closed
form of 1 (Figure S13 in the Supporting Information). Pho-
toirradiation with visible light induced a backward reaction
[20]
almost negligible relative to the cyclization reactivity, the
conversion ratio at the photostationary state was indepen-
[20]
to the open-form 2. After visible-light irradiation, the CD
peaks disappeared even in the UV region, which showed an
immediate racemization. The small methyl end groups seem
insufficient to hinder the helix inversion and the transition
to the excited state with a higher energy might boost the
racemization.
AHCTUNGTRENNUNGd ent of temperature. Interestingly, the CD peak intensity
that corresponded to the photoproduct remained almost un-
changed relative to the temperature at which the photoirra-
diation was performed (Figure 5 and Figure S14 in the Sup-
porting Information). This result clearly indicated that the
de of the photoproduct was identical and the helix inversion
of (S,S)-1 did not take place in this temperature range. The
chiral-phase HPLC measurement for (S,S)-1 after UV irradi-
ation gave only one peak for the closed-ring isomer, and no
evidence of the formation of minor diastereomers was ob-
served.
Judging from the results of our various temperature-de-
pendent studies and DFT calculations (Figure S7 in the Sup-
porting Information) performed, Scheme 3 could be pro-
posed as a conclusive conformational behavior of (S,S)-1,
whereas achiral compound 2 is expected to follow Scheme 2.
The increase in CD intensity upon decreasing temperature
for (S,S)-1 could be attributed to the biased chiral induction
at low temperature, and the inversion to the P helix with
a higher energy did not take place. The induction of one-
handed helicity led to the photochemical reaction with an
extremely high diastereoselectivity. The induction of chirali-
ty in foldamers has often been evaluated by employing X-
ray crystallography, quantum chemical calculation, CD, and
Since the photoproducts of molecules (R,R)-1 and (S,S)-
1
showed the spontaneous reversion to the photoprecursor
states within one hour owing to the steric strain in the
closed-ring forms, we could neither isolate the photoprod-
ucts nor determine the photochemical reaction quantum
yield for compounds 1. Therefore, we evaluated the confor-
mational change of 1 by monitoring the CD intensity of the
photostationary state achieved at various temperatures. As-
suming that the helical inversion was responsible for the
temperature-dependent CD change (Figure 3) and both the
M and P helices of (S,S)-1 were equally photoreactive, as
suggested by DFT calculations, the UV irradiation at
a given temperature should lead to a definite diastereomeric
excess (de), and the relative CD intensity was supposed to
increase upon decreasing temperature. Figure 5 summarizes
the temperature-dependent change in the relative intensity
of the CD peak for (S,S)-1 together with that of the photo-
stationary state achieved with UV irradiation for 1 min at
given temperatures. The CD measurement for each photo-
Chem. Eur. J. 2013, 19, 16972 – 16980
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16977

T. Nakashima, T. Kawai et al.
ther extension of the oligoheteroarylene structure would
lead to a family of photoresponsive foldamers, the second-
AHCTUNGTRENNUNGa ry structures of which are controlled in a quick and dynam-
ic manner. The chiral photoresponsive foldamers could find
applications including as a chiral dopant for cholesteric
[34,35]
[36]
liquid crystals
as well as a ligand
of a chiral metal
complex to change its chiroptical properties. The controlla-
bility of the thermal cycloreversion reaction constant of ter-
[14a,b]
arylenes over seven orders of magnitude
would add an
option of duration control for those applications over a wide
range of timescales including real-time modulation of chi-
[37]
roptical properties.
Experimental Section
General: Compounds (R,R)-1 and (S,S)-1 were prepared according to the
reaction scheme depicted in Scheme 4. Reference compound 2 was pre-
[20]
pared according to the reported procedure. Their chemical structures
Scheme 3. Conformational change and photoreaction of (S,S)-1.
1
were confirmed by high-resolution mass spectrometry, H NMR spectros-
1
copy, and elemental analysis. H NMR spectra were recorded using
a
JEOL AL-300 spectrometer (300 MHz). Temperature-dependent
1
NMR spectroscopic measurements, as we have demonstrat-
H NMR spectra were measured using a JEOL ECP 400 spectrometer
(400 MHz). Separative HPLC was performed using a JASCO LC-2000
Plus Series. Mass spectra were measured using a mass spectrometer
JEOL JMS-700. Absorption spectra in solution were studied using
a JASCO V-670 spectrophotometer. UV irradiation was carried out using
a Panasonic Aicure UV curing system (LED, l=365 nm) as the exciting
light source. CD spectra were recorded using a JASCO J-725 spectropo-
larimeter. For temperature-dependent CD studies, the temperature of
samples was controlled using a Unisoku CoolSpeK cryogenic cell holder
in the range from 213 K to room temperature. X-ray crystallographic
analyses were carried out using a Rigaku R-AXIS RAPID/s Imaging
Plate diffractometer with MoKa radiation. The structures were solved by
direct methods (SHELXL-97) and refined by full-matrix least-squares on
[1–5,21,22]
ed in the present study.
NMR spectroscopy is some-
times found to be inadequate for monitoring helix inversion
in the case that the diastereomers give identical NMR spec-
troscopic profiles or the rate of interconversion is faster
than the NMR spectroscopic timescale. Therefore, a combi-
nation of several methods mentioned above is often em-
ployed to discuss the chiral induction quantitatively. We
have herein demonstrated the photochemical conversion
that takes place quickly in a stereospecific manner as a snap-
shot or footprint of conformational state in foldable struc-
tures since the photoproduct basically never shows global
conformational change (Scheme 2). We also proved that the
present system was little affected by the thermal reversion
of photoproducts by CD monitoring combined with temper-
ature-dependent photochemical reaction.
Conclusion
We have demonstrated photoresponsive chiral helical archi-
tectures based on the foldamer design as the extension of
photochromic terarylenes with high 6p-electrocyclization ef-
ficiency. The X-ray crystallography, DFT calculations, and
1
H NMR spectroscopic study clearly suggested that the heli-
cally folded structure was stabilized by multiple intramolec-
ular noncovalent interactions. The introduction of chiral
groups at both the end units successfully induced the one-
handed helicity in the crystal as well as in solution. The con-
formational change in solution was investigated by means of
various temperature-dependent measurements, from which
we concluded that no helical inversion took place. The pho-
tochemical 6p-cyclization reaction that proceeded in a ste-
reospecific manner was proposed as a methodology for
taking snapshots of folding structures as a diastereomeric
photoproduct with a covalently bridged structure. The fur-
3
Scheme 4. Synthesis of (R,R)-1 and (S,S)-1: a) NBS, CHCl , 708C;
b) 1) LDA, THF, À788C, 2) (R)- or (S)-methyloxacyclopropane; c) NaH,
iodomethane, THF; d) nBuLi, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-di-
oxaborolane, THF, À788C; e) (R)- or (S)-5, [Pd
3 4 3 3 4
ACHTUNGTRENNGN(U PPh ) ], PPh , 2mK PO ,
water/1,4-dioxane.
16978
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 16972 – 16980

Chiral Photoresponsive Tetrathiazoles
FULL PAPER
2
F . All non-hydrogen atoms were refined anisotropically, and all hydro-
(m, 2H), 7.43–7.40 (m, 3H), 3.70–3.50 (m, 1H), 3.38 (s, 3H), 3.02–2.96
gen atoms were placed using AFIX instructions. DFT calculations were
(m, 2H), 1.38 (s, 12H), 1.24 ppm (d, J=7.2 Hz, 3H).
[
29]
performed with Gaussian 09
at the wB97XD/6-31G(d) level for the
[20]
A
H
U
G
R
N
N
(90 mg,
open-ring forms and the B3LYP/6-31G(d) level for the closed-ring forms
and TD-DFT calculations.
0
(
3
, 70 mg, 0.28 mmol), 1,4-dioxane
3
4
5
-Bromo-2-phenylthiazole: N-Bromosuccinimide (NBS; 42 g, 0.23 mol)
2
A
H
U
G
R
N
U
3 4
)
was added to a solution of 2-phenylthiazole (25 g, 0.16 mol) in chloro-
form (370 mL), and the mixture was stirred for 1 day at 708C. After
was then added to the reaction mixture and stirred for 1 day at 1108C.
After being quenched by adding aqueous HCl (2m), the reaction mixture
was extracted with ethyl acetate, and the combined organic layer was
dried over MgSO , filtered, and concentrated. The crude product was pu-
4
rified by recrystallization (acetonitrile/chloroform 30:1) to give (R,R)-
1 (0.13 g, 60%) as a white crystal. The enantiomer (S,S)-1 was also pre-
being quenched by adding an aqueous solution of Na
mixture was extracted with ethyl acetate, and the combined organic layer
was dried over MgSO , filtered, and concentrated. The crude product
2 2 5
S O , the reaction
4
was purified with silica-gel column chromatography (ethyl acetate/
hexane 1:4) to yield 5-bromo-2-phenylthiazole (15 g, 40%) as an off-
pared from (S)-5 (300 mg, 0.84 mmol) and 6 (130 mg, 0.42 mmol) in 55%
1
1
white crystalline solid. H NMR (CDCl
3
, 300 MHz, TMS): d=7.96–7.92
yield (0.18 g). (R,R)-1: H NMR (CDCl
3
, 300 MHz, TMS): d=8.10–8.06
(
m, 2H), 7.48–7.43 (m, 3H), 7.22 ppm (s, 1H).
(m, 4H), 7.50–7.47 (m, 10H), 7.24–7.12 (m, 6H), 3.38 (s, 1H), 3.16–3.06
(
m, 2H), 2.68–2.45 (m, 4H), 0.87 ppm (d, J=6.0 Hz, 6H); HRMS (ESI):
(
R)- or (S)-1-(4-Bromo-2-phenylthiazol-5-yl)propan-2-ol ((R)-3, (S)-3):
+ +
m/z calcd for C44
H
38
N
4
NaO
2
S
4
[M+Na] : 805.17753; found: 805.17769;
: C 67.49, H 4.89, N 7.15;
found: C 67.19, H 4.65, N 7.12. (S,S)-1: H NMR (CDCl , TMS, 300 MHz,
TMS): d=8.10–8.06 (m, 4H), 7.50–7.47 (m, 10H), 7.24–7.12 (m, 6H),
Lithium diisopropylamide (LDA; 29 mL, 38 mmol) in anhydrous THF
30 mL) was added portionwise to a dried four-necked flask charged with
-bromo-2-phenylthiazole (3.0 g, 13 mmol) in anhydrous tetrahydrofuran
45 mL) under an Ar atmosphere at À788C. The mixture was stirred at
À788C for 10 min, and then (R)-methyloxacyclopropane (1.8 mL,
0 mmol) was added to the reaction mixture and stirred for 1 day at
elemental analysis calcd (%) for C44
38 4 4
H N S
(
5
(
1
3
3
6
8
.38 (s, 1H), 3.16–3.06 (m, 2H), 2.68–2.45 (m, 4H), 0.87 ppm (d, J=
+ +
[M+Na] :
.0 Hz, 6H); HRMS (ESI): m/z calcd for C44
05.17753; found: 805.17777; elemental analysis calcd (%) for
: C 67.49, H 4.89, N 7.15; found: C 67.23, H 4.64, N 7.10.
Crystallographic data: CCDC-944089 ((R,R)-1), 944090 ((S,S)-1), and
44091 (2) contain the supplementary crystallographic data for this
H
38
N
4
NaO
2
S
4
3
room temperature. After the reaction was quenched by adding aqueous
HCl (2m), the reaction mixture was extracted with chloroform, and the
combined organic layer was dried over MgSO , filtered, and concentrat-
4
ed. The crude product was purified with silica-gel column chromatogra-
44 38 4 4
C H N S
9
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
phy (ethyl acetate/hexane 1:4) to yield (R)-3 (1.3 g, 36%) as a yellow oil.
(
(
(
S)-3 (0.40 g, 44%) was also obtained from 5-bromo-2-phenylthiazole
2.0 g, 8.3 mmol) and (R)-methyloxacyclopropane (1.2 mL, 20 mmol).
1
R)-3: H NMR (CDCl
3
, 300 MHz, TMS): d=7.92–7.89 (m, 2H), 7.44–
7
1
.41 (m, 3H), 3.02–2.90 (dd, J=15.0, 7.2 Hz, 2H), 3.85–3.70 (m, 1H),
.27 ppm (d, J=7.2 Hz, 3H). (S)-3: H NMR (CDCl , 300 MHz, TMS):
3
1
Acknowledgements
d=7.92–7.89 (m, 2H), 7.44–7.41 (m, 3H), 3.85–3.70 (m, 1H), 3.02–2.90
(
dd, J=15.0, 7.2 Hz, 2H), 1.27 ppm (d, J=7.2 Hz, 3H).
We thank F. Asanoma, S. Katao, and Y. Nishikawa at NAIST for their
help in the measurements and analyses with low-temperature H NMR
(
4
R)- or (S)-4-Bromo-5-(2-methoxypropyl)-2-phenylthiazole ((R)-4, (S)-
): NaH ACHTUNGTRENNUNG( 60%) (0.35 g, 8.7 mmol) and anhydrous THF (5 mL) were
1
spectroscopy, X-ray crystallography, and HRMS, respectively. The au-
thors also thank L. McDowell for proofreading the entire text in its origi-
nal form. This research was partly supported by the Green Photonics
Project at NAIST sponsored by the Ministry of Education, Culture,
Sports, Science and Technology, MEXT (Japan).
added to a dried four-necked flask under Ar atmosphere at 08C and
stirred for 30 min. Compound (R)-3 (1.3 g, 4.4 mmol) in anhydrous THF
(
domethane (0.54 mL, 8.7 mmol) was added to the mixture and stirred for
another 12 h at room temperature. After being quenched by adding
aqueous HCl (2m), the reaction mixture was extracted with ethyl acetate,
5 mL) was added to the solution portionwise and stirred for 30 min. Io-
4
and the combined organic layer was dried over MgSO , filtered, and con-
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Chem. Eur. J. 2013, 19, 16972 – 16980
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Received: July 3, 2013
Published online: November 8, 2013
16980
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