Cucurbiturils Monofunctionalized on the Methylene Bridge and Their Host-Guest Properties

Article abstract of DOI:10.1002/ejoc.202100705

Monofunctionalization of cucurbiturils is essential for transferring these potent supramolecular macrocyclic hosts into real-world application. Here, we present the synthesis of cucurbit[6]urils 1 and 2 in which one methylene bridge is modified by a single substituent containing a nitro or an ammonium group. We investigated host–guest properties in water and 0.2 M NaCl using ¹H?NMR and isothermal titration calorimetry, particularly for 2. The macrocycle 2 self-associated into dimeric aggregates in pure water, but readily disassembled in the presence of NaCl or organic cations. Cucurbit[7]uril was able to encapsulate the ammonium substituent of 2 inside its cavity resulting in a complex of 1 : 1 stoichiometry with an association constant of 3.1×10⁵ M^?1. The presented host–guest properties together with further possible derivatization showcase the potential of cucurbiturils modified in the methylene position such as 1 and 2 for the development of advanced supramolecular systems.

Full text of DOI:10.1002/ejoc.202100705

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Cucurbiturils Monofunctionalized on the Methylene Bridge and
their Host-Guest Properties
Authors: Marketa Kandrnalova and Vladimir Sindelar
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1/2020

European Journal of Organic Chemistry
10.1002/ejoc.202100705
COMMUNICATION
Cucurbiturils Monofunctionalized on the Methylene Bridge and
their Host-Guest Properties
[a]
[a]
Markéta Kandrnálová, and Vladimír Šindelář*
[
a]
Prof. Vladimír Šindelář, Markéta Kandrnálová
Department of Chemistry and RECETOX, Faculty of Science
Masaryk University
Kamenice 5, 625 00 Brno, Czech Republic
E-mail: sindelar@chemi.muni.cz
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: Monofunctionalization of cucurbiturils is essential for the
transfer of these potent supramolecular macrocyclic hosts into a
real-world application. Here, we present the synthesis of
cucurbit[6]urils 1 and 2 in which one methylene bridge is modified by
a single substituent containing a nitro or an ammonium group. We
1
investigated host-guest properties in water and 0.2 M NaCl using H
NMR and isothermal titration calorimetry, particularly for 2. The
macrocycle 2 self-associated into dimeric aggregates in pure water,
but readily disassembled in the presence of NaCl or organic cations.
Cucurbit[7]uril was able to encapsulate the ammonium substituent of
Figure 1. Structure of macrocycle 1 and 2 and supramolecular guests
used in this study.
2
inside its cavity resulting in a complex of 1:1 stoichiometry with an
5
-1
association constant of 3.1 × 10 M . The presented host-guest
properties together with further possible derivatization showcase the
potential of cucurbiturils modified in the methylene position such as 1
and 2 for the development of advanced supramolecular systems.
Cucurbituril 1 was prepared by the condensation reaction
of acyclic glycoluril hexamer with formaldehyde and
3-(p-nitrophenyl)propionaldehyde in conc. HCl at 90 °C for 3 h
(Scheme 1). Crude product was composed of 1 and the
unsubstituted cucurbit[6]uril, which were successfully separated
by column chromatography. Pure
1 was obtained after
Cucurbiturils are macrocycles composed of glycoluril building
recrystallization from 4.8 M HCl in 16% yield. Subsequently, the
nitro group in 1 was converted into the amino group. 1 was
dispersed in aqueous acetic acid and stirred with 5 wt%
palladium under hydrogen atmosphere overnight (Scheme 1),
blocks, which are connected via two rows of methylene
bridges.[
1,2]
The resulting macrocycles are very rigid with
well-defined cavities, soluble in water, and are chemically stable.
These properties make cucurbiturils suitable supramolecular
hosts for biologically relevant neutral and cationic organic guests.
Supramolecular complexes of cucurbiturils are appreciated for
their high stability, as well their high selectivity toward particular
guests.[3] For example, cucurbit[7]uril (CB7) is able to bind some
ferrocene and adamantane ammonium salts with binding affinity
yielding macrocycle
subsequently transformed to
2
as an acetate salt, which was
hydrochloric salt by
a
recrystallization from 4.8 M HCl. The yield of the hydrogenation
reaction was 72 %, where the loss of the product was probably
caused by the interaction of cucurbituril moiety with palladium
catalyst.
ranging from 101 to 10 M .
2
18
-1 [4–6]
The ultra-stable complexes
were utilized in protein fishing and imaging,[8] biorthogonal
catalysis, complexed therapeutic systems,[9] under-water
adhesive materials,[10] optical sensing[11] and others. All of the
[7]
above-mentioned
applications
require
the
use
of
monofunctionalized cucurbiturils as it enables selective
attachment of the macrocycle to the place of interest.
Monofunctionalized cucurbiturils can be prepared by
monohydroxylation of a single methine proton in a cucurbituril
molecule.[12–16] They can be also achieved by merging the
glycoluril hexamer with a monofunctionalized glycoluril,[17] or with
monofunctionalized phthalaldehydes.[18,19] Our group showed
Scheme 1. Synthesis of monofunctionalized cucurbit[6]urils 1 and 2; a:
concd. HCl, 90°C, 3 h; b: column chromatography HCO
c: H , Pd/C cat., dilut. CH CO H, RT, overnight; d: recrystallization from
.8 M HCl.
2 3 2
H/ CH CO H (1:1);
2
3
2
4
that cucurbiturils can also be substituted at the methylene bridge
position by reacting glycoluril with formaldehyde in the presence
of other aldehyde.[20] Using this approach, we prepared
cucurbit[6]uril (CB6) bearing one 2-phenylethyl group. In this
paper, we used similar synthetic strategy for the preparation of
macrocycles 1 and 2 (Figure 1), having nitro and amino
substituents, respectively. The self-assembly of these
monofunctionalized cucurbiturils was investigated, as well as
their ability to serve as both hosts and guests.
The macrocycles
MALDI-TOF MS and NMR spectroscopy. These measurements
were performed in and in the presence of
hexane-1,6-diammonium dichloride guest (HMDA, for structure,
see Figure 1), which served as a solubilizing agent as well to
prevent possible self-association interactions. Peaks at m/z of
1 and 2 were characterized by
H
2
O
2
D O
1
146.351 and 1116.377 in MS spectra of 1 (Figure S6) and 2
1
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European Journal of Organic Chemistry
10.1002/ejoc.202100705
COMMUNICATION
(
Figure S7) correspond to the desired monofunctionalized
increasing concentration of 2 (0.5 to 10 mM). This indicates that
macrocycles. The monofunctionalization was further evident
from the H NMR spectra of 1 and 2 (Figure 2a,b) by comparing
dimeric species are stable at least at millimolar concentrations in
D O and that binding mode of the dimer corresponds to that one
2
1
the intensities of the signals of 2-(p-nitrophenyl)ethyl
substituents and the 2-(p-aminophenyl)ethyl hydrogenchloride
depicted on Figure 2c. The same experiment rules out the
formation of supramolecular polymer or dimer in which both
macrocycles are occupied by the substituents from the adjacent
molecule (more discussion about possible binding modes is
given in ESI below Figure S19).
As previously demonstrated, the binding affinity of host-
guest complexes between cucurbiturils and aromatic guests can
be significantly weakened by the addition of an inorganic
salt.[22,23] Indeed, we found out that in the presence of 0.2 M
NaCl, signals of self-assembled aggregates of macrocycle 2 are
strongly reduced in the NMR spectra (Figure S5). Thus, we
decided to explore further host-guest properties of 2 in 0.2 M
NaCl. First, we turned our attention to the possibility of using
macrocycle 2 as a guest for cucurbit[7]urils (CB7). NMR titration
revealed that the addition of CB7 into the solution of 2 resulted
in the appearance of new upfield shifted aromatic (A’ and B’)
(
APE) substituents with those of the macrocycle repeating units.
Other evidence of the monofunctionalized cucurbiturils can be
drawn from the triplet belonging to the proton E of the methylene
carbon atoms bearing the substitution, which was also observed
previously on the first monosubstituted cucurbituril.[20] Downfield
shift of proton E indicates its apical position along the carbonyl
oxygen atoms, for both 1 and 2. Transformation of nitro group in
1
into the amino group in 2 is clearly evident from an upfield shift
of aromatic protons A from 8.29 to 7.41 ppm.
and aliphatic (C’ and D’) signals of the APE substituent (Figure
3
).
Figure 3. Cut of stacked 1H NMR spectra (300 MHz, 0.2 M NaCl/D
O) of a)
2
pure 2, and after the addition of b) 0.3 equiv, c) 0.6 equiv and d) 1.1 equiv
of CB7, and e) pure CB7.
Figure 2. 1H NMR spectra of a) 1 and b) 2 in the presence of HMDA, and
c) 2 in the absence of HMDA. All spectra were recorded in D2O at 30 °C.
*
Signals of free HMDA. xSignals of bound HMDA.
Complete disappearance of the original signals upon the
addition of 1 equivalent of CB7 is in agreement with the
formation of a stable complex between 2 and CB7 at 1:1 ratio.
2
NMR spectra of the macrocycle 2 in D O in the absence of
any guest (Figure 2c) showed additional signals when compared
to the spectra recorded in the presence of HMDA (Figure 2b,c).
These broad signals at 6.99-6.30 ppm and 2.69-2.46 ppm were
recognized as aromatic (A’, B’) and aliphatic (C’, D’) signals of
the APE substituent, which is included inside the cavity of
another molecule of 2. The exchange between free and bound
substituents was slow on the NMR time scale, allowing
assessment of the degree of aggregation using diffusion ordered
5
-1
The association constant of the 2∙CB7 complex of 2.9 × 10 M
1
was determined by H NMR competition experiment using
methylviologen diiodide (MV, for structure see Figure 1) as a
competitor in 0.2
M NaCl (Figure S10). The host-guest
exchange during this competition experiment was slow on the
NMR time scale. Therefore, the association constant was
determined based on the integration of signals belonging to the
free and bound forms of the guests (for calculation see SI). We
also performed direct titration using isothermal titration
calorimetry (ITC), which resulted in a very similar association
spectroscopy (DOSY) (Figure S8). Diffusion coefficient of the
-1
free macrocycle, (2.618
±
0.072)
×
10-10 m2 s , was
approximately 1.23 times higher than for the aggregates,
2
-1
5
-1
(
1
2.139 ± 0.026) × 10-10 m s , which is very close to the value of
constant of 3.1 × 10 M for the 2∙CB7 complex (Figure S11).
Formation of this complex between CB7 and 2 was confirmed by
MALDI-MS spectrum by a signal at m/z of 2300.712 (Figure
S14).
.26 obtained by theoretical calculation for a cucurbit[6]uril
dimer.[
19,21]
Dimeric aggregates were also observed by
MALDI-MS by signal at m/z of 2231.733 (Figure S7). Further
NMR investigations (Figure S19) revealed, that ratios of the free
and bound forms of the APE substituent do not change with
Stability of the 2∙CB7 complex could be influenced by
possible mutual interaction of both host and guest
2
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European Journal of Organic Chemistry
10.1002/ejoc.202100705
COMMUNICATION
macrocycles.To investigate such a possibility, we used p-
propylanilinium chloride (pPA, for structure, see Figure 1) as a
model compound for APE substituent of 2 and investigated its
interaction with CB7. We found that pPA formed inclusion
complex with CB7 (Figure S13) with binding affinity of 2.5 × 105
M-1 (determined by ITC, Figure S15) and 2.3 × 105 M-1
In
conclusion,
we
prepared
monofunctionalized
cucurbit[6]urils 1 and 2, in which substituents bearing nitro and
amino groups were attached to one methylene bridge of the
macrocycle. The macrocycle
macrocyclization reaction in 16 % yield, and its subsequent
hydrogenation led to 2 in 72 % yield. The macrocycle 2
containing ammonium function underwent self-association into a
1
was obtained by the
(
determined by NMR competitive titration, Figure S14) in 0.2 M
NaCl. These values did not differ significantly from those
obtained for the 2∙CB7 complex, indicating a marginal influence
of the macrocyclic core of 2 on the binding with CB7.
2
dimer as indicated by DOSY measurements in D O. The
dimerization could be suppressed in the presence of 0.2 M NaCl.
The macrocycle 2 formed a supramolecular complex with CB7 in
which its ammonium substituent was included inside the CB7
Two portals of cucurbiturils 1 and 2 are unequal as the
substituent at the methylene bridge position is closer to one of
the two macrocyclic portals. Possible orientation of the
asymmetric guest after its inclusion inside the cucurbituril cavity
can be thus influenced by the presence of the APE substituent,
particularly in the case when part of the guest remains outside
the macrocycle. Such host-guest complexation was tested in the
case of N-pentyl-4-(4’-pyridyl)pyridinium (PV, for structure see
5
-1
5
-1
cavity. Association constants of 2.9 × 10 M and 3.1 × 10 M
were determined by 1H NMR and ITC, respectively. These
values were similar to the one determined for the complex of the
model compounds p-propylanilinium chloride and CB7,
indicating a minor influence of CB6 macrocycle of 1 on the
stability of the 1·CB7 complex. Finally, NMR study enabled the
observation of two diastereomers of the 1·PV inclusion complex,
which differed in the orientation of this unsymmetrical guest with
respect to the inequivalent portals of the macrocycle.
Figure 1) and 1 using NMR in 0.2M NaCl/D
S16). As expected, 1 and PV formed an inclusion complex in a
:1 ratio in which the aliphatic part of the guest is included inside
2
O (Figure 4 and
1
the macrocycle. The binding mode is clearly indicated by the
upfield shift of aliphatic protons of PV (H6-H9) upon
complexation with 1. On the other hand, the signals of the
aromatic part of the guest (H1) shift downfield, which is
consistent with its location outside of the macrocycle. Binding
mode is consistent with those previously reported for viologen
guests and CB6.[24] Closer investigation revealed that, upon
complexation, the signals H3 and H4 of guest PV not only shift
but also double (Figures 4 and S16). We believe that the
splitting is the result of the formation of two diastereomeric
complexes, which differ in the orientation of the guest molecule
with respect to the substituent of 1 (as depicted in Figure 4)
According to the integration of the split signals, we determine
that the diastereomers are present in the solution in 1:0.7 ratio.
Unfortunately, ROESY experiments did not reveal which of the
two diastereomers dominates as we did not observe any cross-
peaks between host and guest protons (Figure S17).
Acknowledgements
This work was supported by the Czech Science Foundation (No.
2
0-13922S), CETOCOEN EXCELLENCE Teaming 2 project
(
supported by MEYS CR: CZ.02.1.01/0.0/0.0/17_043/0009632)
and the RECETOX research infrastructure (LM2018121). We
acknowledge the CF Proteomic supported by the CIISB
research infrastructure (LM2018127) funded by MEYS CR.
Keywords: Macrocycles • Cucurbiturils • Monofunctionalization •
Self-assembly • Host-guest complexes
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Entry for the Table of Contents
Monofunctionalized bambusurils bearing a nitro or an ammonium group were prepared. The macrocycles formed dimeric self-
assembly, inclusion complexes with cucurbit[7]urils, and diastereomeric complexes with methylviologen.
5
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