Visible-Light Responsive Sucrose-Containing Macrocyclic Host for Cations

Article abstract of DOI:10.1021/acs.orglett.1c00590

Chiral photoresponsive host 1 was prepared by a high-yield Cs2CO3-templated macrocyclization. Trans-1 transforms into long-lived cis-1 (25 days) upon irradiation with green light, and the backward transformation is triggered by blue light. Both isomers pr

Full text of DOI:10.1021/acs.orglett.1c00590

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Letter
Visible-Light Responsive Sucrose-Containing Macrocyclic Host for
Cations
Patrycja Sokołowska, Kajetan Dąbrowa,* and Sławomir Jarosz*
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ABSTRACT: Chiral photoresponsive host 1 was prepared by a high-yield Cs2CO3-templated macrocyclization. Trans-1 transforms
into long-lived cis-1 (25 days) upon irradiation with green light, and the backward transformation is triggered by blue light. Both
1
isomers prefer potassium among alkali metal cations, and cis-1 binds cations stronger than trans-1 (Kcis/Ktrans ≤ 4.1). H NMR
titration experiments as well as density functional theory studies reveal that sucrose ring oxygen residues and azobenzene nitrogen
atoms in 1 contribute to cation coordination.
ne of the most investigated areas in supramolecular
Ochemistry concerns the development of macrocyclic
systems that can bind neutral or charged molecules.1 In
principle, the binding properties of such host−guest arrange-
ments could be adjusted toward a specific guest by changing
the geometry of the binding pocket as well as the type and
number of binding sites attached to the macrocyclic skeleton.
For example, incorporation of amino acids, binaphthyls, and
sugars introduces chirality into the binding pocket, allowing
discrimination of chiral guests. Carbohydrates are particularly
attractive among chiral building blocks due to their availability
and diversity of structures. On top of that, insertion of the
photochromic moiety allows one to change the shape and
geometry of the macrocyclic framework in a dynamic fashion
using light as a stimulus. From a variety of available
chromophores, azobenzene is particularly attractive due to its
robustness, excellent photochromic properties, and established
chemistry.2 The latter feature greatly facilitates the introduc-
tion of this scaffold into a desired location in the host platform.
Recently, a variety of macrocyclic compounds bearing the
azobenzene unit have been reported.3 They found broad
applications as host−guest systems,4 as molecular machines,5
or in self-organization processes.6 Despite that, only a limited
number of macrocyclic photoresponsive host−guest systems
incorporating chiral motifs have been reported.7 Moreover,
light-controlled chiral recognition was to date reported for only
phosphates8 and carboxylates9 using nonmacrocyclic hosts.
One of the main research topics of our team is the
development of sucrose-derived macrocyclic systems, includ-
ing, among others, cation10a,b and anion receptors,10c,d
cryptands,10e and molecular containers.10f On the contrary,
we are involved in the development of light-controlled
nonmacrocyclic receptors for anions.9,11
Herein, we synthesized the first dynamic chiral macrocyclic
host 1 that responds reversibly to visible light and shows
enhanced binding affinity toward achiral and chiral cations in
the metastable cis state. Preparation of host 1 was
accomplished in six steps starting from sucrose (Scheme 1
and Table 1).
Selective protection of the 6- and 6′-hydroxyl groups of
sucrose by trityl block provided 6,6′-di-O-tritylsucrose (3) in
51% yield. The remaining free hydroxyl groups were then
protected as benzyl ethers under standard conditions to afford
fully protected sucrose 4 in very good yield (80%). The trityl
Received: February 18, 2021
Published: March 17, 2021
© 2021 The Authors. Published by
American Chemical Society
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Org. Lett. 2021, 23, 2687−2692
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Scheme 1. Synthesis of Photoresponsive Chiral Host 1
Scheme 2. (a) Proposed Mechanism of Alkali Metal Cation-
Induced Stabilization of Intermediate Acyclic Polyether
Leading to 1 and (b) Dependence of Reaction Yield on the
Size of the Cation13
of these observations indicate that the bulky TBA cation lacks
the templating ability.
The efficient and clean formation of 1 with a highly crowded
hexa-O-benzyl-sucrose scaffold using alkali metal carbonates
suggests the preorganized looplike structure of the linear
intermediate that is formed after the first O-alkylation step
(Scheme 2). The preorganization facilitates the key ring-
closing step over the oligomerization and is probably driven by
the size-selective coordination of the alkali metal cation by
multiple O and N donor atoms (Scheme 2b).
Recently, we noticed a similar guest templation effect in the
synthesis of C2-symmetric sucrose ureas.10c The presence of
chloride anions leads to macrocyclic products in 90% yield,
whereas their absence results in a nearly 2-fold decrease in
yield (40%). Likewise, the addition of potassium iodine allows
the preparation of aza macrocyclic sucroses in good to
excellent yields (67−96%).15
Table 1. Optimization of the Base-Promoted
Macrocyclization between 7 and 8
a
b
entry
base
T (°C)
t (h)
yield (%)
1
2
3
4
5
6
7
8
Na2CO3
50
82
50
82
50
82
82
82
72
48
48
8
48
6
0
56.0
65.0
75.4
81.5
K2CO3
Cs2CO2
c
85.3 (37.0)
d
TBA2CO3
TBAOMe
24
24
trace
trace
d
a
Reaction conditions: anhydrous MeCN (20 mM, 1:1 7:8 molar
b
c
ratio), 8 equiv of M2CO3. Total yield for trans- and cis-1. With 14
equiv of TBACl added. Full consumption of starting materials.
d
Nonetheless, the efficient preparation of sucrose-containing
macrocycles such as trans-1 is rather surprising, because
generally macrocyclic azobenzenes containing sugar or
polyoxoethylene moieties are prepared in much lower yields
(as shown in Figure 1). For example, the 17-member
groups were then selectively removed using heterogeneous
silica-supported sodium hydrogen sulfate (NaHSO4/SiO2),12
which provided diol 5 in 60% yield. Hexa-O-benzyl sucrose 513
was then reacted with an excess of bis(2-chloroethyl)ether
under PTC conditions to provide bis-chloro derivative 6 in
74% yield. To facilitate the subsequent cyclization reaction, the
chlorine atoms were replaced with iodine atoms, which
afforded bis-iodo polyethyl ether 7 in 96% yield. Finally, key
intermediate 7 was subjected to the reaction with 2,2′-
dihydroxyazobenzene (8) to afford the target 29-member
macrocycle 1 in very high yield [85.3% under optimized
conditions, which included 6 h and 8 equiv of Cs2CO3 in
refluxing MeCN (see Table 1)]. The reaction virtually does
not occur at rt for alkali metal carbonates or even at 50 °C for
Na2CO3 (Table 1, entry 1).
Figure 1. Structures and macrocyclization yield (in parentheses) of
reported systems containing an azobenzene switch.7a,b,d,17−19
At reflux, the yields of the cyclization are clearly correlated
with the size of the alkali metal cation13 (see the inset in
Scheme 2). The smallest sodium (rion = 1.16 Å), intermediate
potassium (rion = 1.52 Å), and the largest cesium (rion = 1.81
Å) provide macrocyclic host 1 in 56%, 75.4%, and 85.3%
yields, respectively (entries 2, 4, and 6, respectively). Cs2CO3
was also effective at 50 °C, providing the cyclic product in
81.5% yield after 48 h (entry 5), whereas K2CO3 was much less
effective at a lower temperature (entry 3). In striking contrast,
tetrabutylammonium (TBA) salts of carbonate and meth-
anolate (TBA2CO3 and TBAOMe, entries 7 and 8,
respectively), despite the full consumption of substrate 8,
provide only traces of target macrocyclic host 1 along with
unidentified polar products.14 Furthermore, addition of an
excess of TBACl to Cs2CO3 markedly decreases the yield of 1
due to the cation-metathesis effect (entry 6 in parentheses). All
macrocyclic host with a phenyl thio-α-D-mannopyranoside
scaffold, reported recently by the Xie group,7a,b,d was prepared
in low 35% yield along with the nonmacrocyclic disubstituted
byproduct (32%) using a K2CO3/18-crown-6 system in
acetone (Figure 1a). A particularly striking example is the
preparation of simple azobenzene crown ethers structurally
related to 1 reported by Shinkai et al.16 (Figure 1b) and Shiga
et al.17 (Figure 1c). The azobenzo-crown platform was further
extended to a set of nitrogen analogues (Figure 1d) by Luboch
and Wagner-Wysiecka,18 yet the yields of the t-BuOK-
mediated macrocyclization were still low to moderate (23−
55%). These examples clearly highlight the potential advantage
of the utilization of a sucrose scaffold in the construction of
cation-templated synthesis of macrocyclic host−guest systems.
Having the macrocyclic host trans-1 in hand, we investigated
its photoswitching properties in MeCN (Scheme 3).
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found to be remarkably stable showing the t1/2 of 596 h (24.9
days) in MeCN at 298 K. For comparison, the structurally
related cis-tetra-o-OMe-azobenzene derivative reported by the
Wooley group19 is considerably less stable (t1/2 ≈ 2−14 days),
which suggests that a macrocyclic effect is responsible for the
observed high thermal stability of cis-1. The rotational
restrictions, forced by the macrocyclic ring strain, raise the
barriers of the thermal activation energy (Ea) and Gibbs free
energy (ΔG°) to 110.5 and 110.0 kJ mol−1, respectively.
To evaluate the solution binding properties of trans-1 and
cis-1, we performed 1H NMR titration measurements in
CD3CN using noncoordinating and highly soluble triflate salts
of alkali metal cations (Li+, Na+, K+, and Cs+). The
corresponding association constants (Ka’s) are listed in Table
2. Both trans-1 and cis-1 show a preference for K+ among all
Scheme 3. Photoswitching of Host 1 (a−c and d) and
Complexes of 1 with 10 equiv of Alkali Metal Triflates (b, c,
and e)[a] in MeCN (50 μM) at 298.0 0.1 K Using Green
Light (LED 530 nm) and Blue Light (LED 410 nm),
Respectively
Table 2. Stability Constants Ka (M−1) for Complexes of
a
Hosts trans-1 and cis-1 with Alkali Metal Cations
b
c
d
cation
rion
trans-1
cis-1
Kcis/Ktrans
Li+
Na+
K+
0.90
1.16
1.52
1.81
37
210
1023
90
21
446
2776
0.57
2.12
2.71
4.10
Cs+
370
a
Determined in MeCN-d3 by 1H NMR titrations at 303 K and
assuming a 1:1 binding model; HypNMR 2008 for nonlinear curve
−
fitting; anions added as triflate (CF3SO3 ) salts; estimated errors of
b
c
10%. Taken from ref 13. Titration was carried out in the dark
using pure trans-1 (see the Supporting Information for details).
d
Titration was carried for the cis-enriched mixture (see the
Supporting Information for details).
alkali cations (except LiOTf), like dibenzo-18-crown-6. cis-1
binds cations 2.1−4.1 times stronger than trans-1 and shows
the highest affinity for KOTf (Ka = 2776 M−1), whereas
triflates of Li+, Na+, and Cs+ are bound 79.3, 6.2, and 7.5 times
weaker, respectively.
The absorption spectrum of trans-1 in MeCN shows three
maxima (λ) at 308, 350, and 446 nm (Scheme 3b). The latter,
relatively intense band is located in the visible region and
corresponds to the n−π* transition of the azobenzene
chromophore. We envisioned that irradiation of trans-1 with
green light (495−570 nm) will allow the trans → cis
isomerization to occur, due to the blue-shifted absorption of
the cis isomer in this region. trans-1 readily undergoes
photoisomerization to produce almost 78% of cis-1 upon
irradiation with monochromatic green light at 530 nm [3 W
light-emitting diode (LED)] (Scheme 3a). The reverse cis →
trans isomerization using blue light at 410 nm (3 W LED)
trans-1 binds KOTf with a considerably lower affinity (Ka =
1023 M−1), rendering high cis/trans selectivity (Kcis/Ktrans
=
2.71). An even more pronounced light-controllable change in
selectivity is observed for CsOTf (Kcis/Ktrans = 4.10).
Furthermore, trans-1 shows lower binding selectivity than cis-
1; LiOTf, NaOTf, and CsOTf are bound 27.6, 4.9, and 9.1
times weaker than KOTf, respectively. The cation binding
selectivity of trans-1 and cis-1 is reflected in the trend and
1
magnitude of H NMR chemical shift changes observed upon
addition of triflate salts (Figure 2 and Figures S3−S26).
produced an 80% fraction of trans-1 at PSS410
.
To the best of our knowledge, this is a first example of the
photoresponsive host for cations that can undergo the trans ↔
cis isomerization without employing UVA light (315−400
nm).
Furthermore, hosts 1 (cis or trans, Scheme 3b−d) as well as
complexes of 1 with alkali metal triflates (Scheme 3b,c,e and
Figures S2−S7) showed no signs of photodegradation after
several cycles of alternate irradiation with green and blue light.
This is in marked contrast to the azobenzene crown ethers
reported by Schiga,17b which decompose during photo-
isomerization (Figure 1c). Further investigation by UV−vis
revealed that the trans/cis ratio of 1 at PSS530 is affected by the
type and concentration of the added alkali metal triflate, which
is attributed to the cation-induced shift of absorption maxima
of both isomers (Figures S3−S7). Furthermore, host cis-1 was
Figure 2. Comparison of the chemical shift changes (Δδ) for
anomeric proton CH(1) upon addition of cation triflates to the
solution of hosts (a) trans-1 and (b) cis-1 in CD3CN. Fitted binding
isotherms (gray lines). For the proton label, see Scheme 1.
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Analysis of the 1H NMR spectra reveals that the addition of
both the smallest LiOTf and the largest CsOTf salts has a
marginal effect on the chemical shifts of both isomers,
suggesting that the binding of the cation is realized outside
the macrocyclic cavity, most likely near azobenzene scaffold
(see the Supporting Information for details). Interestingly, the
trans and cis isomers exhibit contrasting changes in chemical
shifts of the α-glucose anomeric proton CH(1). Namely, near
1.0 ppm upfield and 0.32 ppm downfield a shift of the CH(1)
hydrogen signal is observed after the addition of 4 equiv of
KOTf to the solutions of trans-1 and cis-1. At the same time,
the aromatic CH resonances of the azobenzene moiety were
shifted downfield by ≤0.55 and ≤0.28 ppm for the trans and cis
isomers respectively, exemplifying the indirect involvement in
cation binding. Among other factors, the shift behavior of the
CH(1) resonance results from (i) the cation-induced
conformational change of the binding pocket, (ii) the
deshielding effect of the cationic guest localized in the
proximity, and (iii) geometry and distance-dependent
through-space effects associated with the ring currents of
benzyl moieties. For the cis isomer, effect (ii) has a decisive
impact on the proton shift, whereas for the trans isomer, effects
Figure 3. Models of the energy-minimized host−guest complexes (a)
[K⊂trans-1]+ and (b) [K⊂cis-1]+, where ⊂ denotes encapsulation.
M−1 for the R and S enantiomers, respectively) and cis-1 (Ka =
38 and 42 M−1 for the R and S enantiomers, respectively)
hosts, plausibly due to their large size and more dispersed
charge of the primary ammonium cation. The discrimination of
the PEA enantiomers by trans-1 (KR/KS = 1.15) and cis-1 (KR/
KS = 1.11) is relatively weak, yet the cis isomer exhibits near 2
times better affinity for (R)-PEA (Kcis/trans = 1.93) and (S)-PEA
(Kcis/trans = 1.97) cations as compared with trans-1. Both trans-1
an cis-1 have a slight preference for the R-PEA enantiomer over
the S-PEA enantiomer, in contrast to other host−guest systems
based on a sucrose scaffold developed in our laboratory.10b,20
Only in one case has the 21-membered host, bearing two
amide groups, shown the complexing ability with a preference
for the R enantiomer of α-PEA.10a Notably, cis-1 exhibits a
different binding mode for PEA as compared with alkali metal
cations; i.e., addition of PEA enantiomers [clearly the effect of
(R)-PEA is stronger than that of (S)-PEA] causes the upfield
shift and metal cations the downfield shift of the signal
corresponding to the anomeric sugar moiety (H1). This might
be attributed to the π−π stacking between the aromatic part of
PEA and benzyl groups of the sugar moiety.
1
(i) and (iii) prevail. On top of that, careful analysis of the H
NMR titration spectra reveals that cis-1 displays a distinct
binding behavior for KOTf versus NaOTf. In particular, the
CHsp2(11) and CHsp2(11′) protons (for numbering, see
Scheme 1) exhibit upfield and downfield shifts upon addition
of NaOTf and KOTf salts, respectively (Figures S23−S26).
The plausible explanation is that the larger K+ cation can
coordinate with lone pairs of the NN linkage, whereas the
smaller Na+ interacts with these donor groups to a lesser
extent. Furthermore, a specific through-space interaction
between O-benzyl residues and the azobenzene scaffold
might also be responsible for this effect. As depicted in Figure
S8, a clear dependence of the stability constants (log Ka) for
trans-1 and cis-1 on the alkali cation radius suggests that both
isomers possess cavities of similar sizes. This indicates that the
increased cation affinity of cis-1 presumably results from the
higher number of hydrogen bond interactions and/or a more
effective spatial arrangement of hydrogen bond donor atoms.
To gain better insights into the binding mode of trans-1 and
cis-1 toward alkali metal cations in solution, we performed
DFT calculations using B3LYP-D3 combined with the 6-
31G(d) basis set and C-PCM model (MeCN; ε = 37.5) to
approximate the solvent effects. The energy-minimized
conformations of the complexes of trans-1 and cis-1 with K+
are demonstrated in Figure 3. The results of DFT calculations
are in line with the experimental data showing that cis-1 binds
K+ stronger than trans-1 by 4 kJ mol−1 (see Table S3). For
both isomers, the potassium cation is encapsulated in a three-
dimensional cryptandlike cage by multiple interactions with
the O-donors and lone pairs of the NN bond.
In conclusion, we demonstrated the first example of a
macrocyclic host system in which cation binding properties
could be reversibly controlled by visible light. In contrast to the
photoresponsive macrocyclic systems reported to date, the key
ring closure step was remarkably efficient and cation-
dependent, allowing the preparation of trans-1 in 85.3%
yield, while Cs2CO3 was employed as a dual base and
templating agent. trans-1 is converted into long-lived cis-1 (t1/2
= 25 days) upon irradiation with green light (530 nm), and
reverse isomerization is driven by blue light (410 nm). Both
isomers have a preference for the potassium cation, and cis-1
exhibits higher binding affinity and selectivity for cations than
trans-1, including chiral phenylethylammonium guests (Kcis/
1
Ktrans ≤ 4.1). DFT calculations and H NMR titrations reveal
cis-1 displays a larger number of interactions with potassium
(coordination number of 8) compared to trans-1 (coordination
number 7). In addition, cis-1 exhibits a particularly short K+···
O bond length (d = 2.56 Å) originating from the glucose ring
oxygen atom, which might greatly contribute to stabilization of
the complex.
Furthermore, to elucidate the chiral recognition properties
of geometrical isomers of 1, we also tested triflates of R and S
enantiomers of the 2-phenylethylammonium (PEA) cation.
These chiral guests were bound with considerably lower
affinity than the achiral ones by both trans-1 (Ka = 19 and 22
that, besides the polyether residues, the sucrose ring oxygen
and azobenzene nitrogen atoms markedly contribute to cation
recognition.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00590.
Materials and methods, synthetic procedures and full
characterization of new compounds, spectroscopic data,
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copies of NMR spectra, and computational and 1H
NMR titration details (PDF)
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AUTHOR INFORMATION
Corresponding Authors
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persistent dendritic host with photoswitchable guest encapsulation
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Kajetan Dąbrowa − Institute of Organic Chemistry, Polish
Academy of Sciences, 01-224 Warsaw, Poland; orcid.org/
0000-0001-7767-5303; Email: kdabrowa@gmail.com
Sławomir Jarosz − Institute of Organic Chemistry, Polish
Academy of Sciences, 01-224 Warsaw, Poland; orcid.org/
0000-0002-9212-6203; Email: slawomir.jarosz@
icho.edu.pl
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cycles through intramolecular glycosylation. Chem. Commun. 2020,
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Patrycja Sokołowska − Institute of Organic Chemistry, Polish
Academy of Sciences, 01-224 Warsaw, Poland; orcid.org/
0000-0002-6464-1873
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00590
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by UMO-2016/21/B/ST5/03382
(S.J.) and UMO-2016/23/D/ST5/03301 (K.D.) projects
funded by Poland’s National Science Centre (NCN).
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■
The authors dedicate this paper to Professor Janusz Jurczak on
the occasion of his 80th birthday.
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