Concentration-dependent supramolecular self-assembly of A1/A2-asymmetric-difunctionalized pillar[5]arene

Article abstract of DOI:10.1039/d1ra00078k

A series of A1/A2-bromoalkoxy-and-hydroxy-difunctionalized pillar[5]arenes were synthesized by the removal of the pillar[5]arene-bearing benzyl group using catalytic hydrogenation. The difunctionalized pillar[5]arene bearing 8-bromooctoxy and benzyloxy su

Full text of DOI:10.1039/d1ra00078k

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Concentration-dependent supramolecular self-
assembly of A1/A2-asymmetric-difunctionalized
pillar[5]arene†
Cite this: RSC Adv., 2021, 11, 2995
*
Talal F. Al-Azemi
and Mickey Vinodh
A series of A1/A2-bromoalkoxy-and-hydroxy-difunctionalized pillar[5]arenes were synthesized by the
removal of the pillar[5]arene-bearing benzyl group using catalytic hydrogenation. The difunctionalized
pillar[5]arene bearing 8-bromooctoxy and benzyloxy substituents at the A1/A2 positions formed pseudo
[1]rotaxane at low concentration and double-threaded supramolecular dimer at high concentration. The
supramolecular self-assembly behavior has been probed with multiple methods including varying
(variable) concentration 1H NMR spectroscopy, diffusion-ordered spectroscopy (DOSY), dynamic light
scattering (DLS) measurements, isothermal titration calorimetry (ITC), and single-crystal X-ray analysis.
Received 5th January 2021
Accepted 7th January 2021
DOI: 10.1039/d1ra00078k
rsc.li/rsc-advances
double-threaded dimer.14 In contrast, the concentration
dependence behavior of pseudo[1]rotaxane was demonstrated
Introduction
using monofunctionalized pillar[5]arene derivative containing
a viologen moiety, which self-complexation at low concentra-
tion, assembled into supramolecular daisy chain polymers and
eventually organogels upon the increasing of its concentra-
tion.15 More recent, concentration dependence nature between
the formation of pseudo[1]rotaxane and interlocked-dimer was
also been demonstrated using pillar[5]arene bearing ethylene
glycol bridged pyridine, in addition to the acid/base control of
the dimer between the shrinking state to extension state.16
In this work, we report the synthesis of series of A1/A2
asymmetric-difunctionalized pillar[5]arenes bearing bro-
moalkyl and hydroxy groups. The synthesized difunctionalized-
pillar[5]arene with long 8-bromooctoxy chain and benzyloxy
substituents formed two different size aggregates depending on
the concentration. The concentration-dependent supramolec-
ular transition between pseudo[1]rotaxane and double-threaded
dimer was investigated by variable concentration 1H NMR
spectroscopy, diffusion-ordered spectroscopy (DOSY), dynamic
light scattering (DLS) measurements, isothermal titration
calorimetry (ITC) and X-ray single crystal diffraction studies.
Mechanically interlocked molecules (MIMs) such rotaxanes/
pseudorotaxanes have been extensively synthesized and
studied in past decades and have found many applications in
the design of molecular machines,1 chemical probes2 and drug
delivery.3 The reversible exchange behavior of pseudorotaxanes,
which consist of macrocyclic systems with threaded linear
components, plays an important role in the design and fabri-
cation of MIMs based articial molecular machines and
supramolecular polymers.4 Thus, macrocyclic hosts such as
cyclodextrins,5 calixarenes,6 cucurbiturils,7 cyclophanes,8 and
more recent pillar[n]arenes9 plays an important role in
construction of MIMs. Pillar[n]arenes in particular, attracted
great interest for their unique structural features, versatile
synthetic manipulation, and exceptional ability to selectively
encapsulate different kinds of guest molecules.10,11
Pseudo[1]rotaxanes are the simplest form of threaded
structures which are mechanically self-interlocked macrocyclic
systems with reversible exchange behavior between threaded
and open forms. Many pseudo[1]rotaxane based pillar[5]arene
have been reported taking advantage of the highly symmetrical
rigid structure and electron-donating cavities.9,12 For example,
monofunctionalized pillar[5]arene bearing imidazolium moiety
that formed stable pseudo[1]rotaxane event at high concentra-
tion has been reported.13 The concentration independent
nature of self-inclusion was also shown with neutral mono-urea-
functionalized pillar[5]arene, however in solid state formed
Results and discussion
Synthesis
Macrocyclic compounds with different functionality enable
selective manipulations to the macrocycle frame and the
synthesis of materials with various architectures. Recently,
difunctionalized pillar[5]arene with hydroxy and amino groups
at the A1/A2 have been synthesized by controlled de-O-methyl-
ation of permethylated pillar[5]arene using oxidation-followed-
by-reduction strategy.17 Taking advantage of our previously
report approach for the synthesis of hydroxylated pillar[5]
Chemistry Department, Kuwait University, P. O. Box 5969, Safat 13060, Kuwait.
E-mail: t.alazemi@ku.edu.kw; Fax: +965-2481-6482; Tel: +965-2498-554
† Electronic supplementary information (ESI) available. CCDC 2000224–2000226.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d1ra00078k
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arenes,11 a series of asymmetric A1/A2 difunctionalized pillar[5]
arenes were synthesized. The approach involves co-cyclization
of hydroquinone derivatives of 1,4-dimethoxybenzene and 1-
benzyloxy-4-bromoalkoxybenzenes with paraformaldehyde in
the presence of BF3$OEt2, as shown in Scheme 1. The copillar-
arenes, Pillar-1(a–c) were isolated by silica gel column chro-
matography, and their corresponding hydroxylated pillar[5]
arenes were obtained aer removal of the benzyl protecting
group by catalytic hydrogenation under mild reaction condi-
tions using palladium on charcoal in anhydrous ethyl acetate.
NMR studies
1H NMR spectra of pillar[5]arenes based on 2-bromoethoxy
(Pillar-1a) and 4-bromobutoxy (Pillar-1b) at 298 K show peaks
corresponding to different proton resonances (ESI†). In
contrast, 1H NMR spectrum of 8-bromooctoxy based pillar[5]
arene (Pillar-1c) show high upeld shi of resonances for
methylene protons of the long 8-bromooctoxy arm. In spectrum
of Pillar-1c at 298 K no signicant upeld shi for methylene
protons H1 and H2 were observed, whereas protons H4, H5, H6,
H7 and H8 (0.62, ꢀ0.35, ꢀ1.35, ꢀ0.88 and 1.29 respectively)
exhibit unusual high eld shi which indicate these protons
located in central position of pillar[5]arene cavity (Fig. 1a). The
methylene protons H5 and H6 show as broad signals due to
rapid movement of these groups with respect to NMR time
scale. To achieve better-quality signals, the movement was
reduced by lowering temperature to 233 K which resulted in
higher upeld shis of the 8-bromooctoxy substituent (Fig. 1b).
The peaks assignment was based on 1H–1H–COSY experiments
at 298 K and 233 K (ESI†). Further evidence of the encapsulation
of the octyl chain deep inside the macrocycle cavity in solution
was obtained from 2D ROSY experiment at 233 K in chloroform-
d3 which shows strong correlations between all of the octyl
protons H4-8 and the aromatic protons H6 as well as bridging
methylene protons H12 of the copillar[5]arene unit (ESI†).
Variable temperature 1H NMR experiments from ꢀ233 to 328 K
in chloroform-d3 show that the octyl chain is encapsulated in
the pillararene cavity in studied temperature rage (ESI†).
Fig. 1 Partial 1H NMR spectra (600 MHz, CDCl3) of Pillar-1c at 298 K
(a) and 233 K (b).
of 1H NMR spectra of Pillar-1c aromatic regions at 2- and 50 mM
concentrations in chloroform-d3 at 298 K. The chemical shi
data show the presence of two main size aggregates, one below
the 4 mM concentration and other around the 50 mM concen-
trations. At low concentration, the 1H NMR concentration study
implies that Pillar-1c exists predominantly as self-inclusion
complex “pseudo[1]rotaxane” and at higher concentration the
result indicate the existence of intermolecular complexation
“double threaded dimer”, which is evidence by the unusually
high chemical upeld shi observed for alkyl chain. In addi-
tion, the intermolecular threaded supramolecular formation at
high concentration is indicated by the observed downeld
chemical shit experienced by aromatic protons of pillaraene
frame as a result of the close proximity to the de-shielding
region of the adjacent pillararene system (Fig. 2). These obser-
vations are similar to results previously reported for pillar[5]
arenes with long side chains.15,16 The presence of monomer to
dimer in 1 : 1 ratio was estimated ca. 25 mM from a plot of
concentration versus chemical shi (ESI†).
The self-assembly behavior of compound Pillar-1c at
different concentrations in chloroform-d3 was further studied
To investigate the inclusion behavior of the 8-bromooctoxy
chain in pillar[5]arene cavity as self-inclusion or double-
1
threaded dimer (interpenetration structure) or n-mer, H NMR
spectra at different concentrations (1–100 mM) of Pillar-1c were
measured (ESI†). The data clearly demonstrate that Pillar-1c is
concentration dependent, suggesting that Pillar-1c exist in
different size aggregates. Fig. 2 shows representative examples
Scheme 1 Synthesis of A1/A2 difunctionalized bromo-hydroxy pillar Fig. 2 Partial 1H NMR spectra of pillararene aromatic frame (600 MHz,
[5]arenes.
CDCl3, 298 K) of Pillar-1c at 4 mM (a) and 50 mM (b).
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by two-dimensional diffusion-ordered 1H NMR (DOSY) exper- scattering intensities at the two measured concentrations (2
iments.4c,18-20 The DOSY spectrum at 2, 50 and 100 mM and 50 mM) of Pillar-1c. Fig. 3 shows histograms of the Dh
concentrations revealed one set of signals, manifesting only one distributions for the supramolecular self-assembly at the two
aggregate size in solutions. A change in the diffusion coefficient different concentrations. In the 2 mM solution, the measured
(D) should be observed in the DOSY spectra because larger Dh distribution centered at 0.72 nm which is consistent with the
aggregates have larger hydrodynamic radii (R), and R is inversely Dh value of self-inclusion complex. While, the centered Dh
proportional to D according to the Strokes–Einstein equation [D distribution for the 50 mM solution doubled to 1.50 nm indi-
¼ kBT/(6phR)], where T denotes the temperature, kB is the cating the presence of double-threaded supramolecular dimer.
Boltzmann constant, and h is the dynamic viscosity of the The DLS measurements clearly indicate the supramolecular
solvent. As the concentration of Pillar-1c increased from 2 mmol self-assembly of Pillar-1c is pronouncedly concentration
Lꢀ1 to 50 mmol Lꢀ1, the value of weight-averaged diffusion dependent, which is in agreement with NMR data.
coefficients (D) decreased from 1.08 ꢁ 10ꢀ9 to 5.17 ꢁ 10ꢀ10 m2
sꢀ1 (D2/D50 ¼ 2.01). This decrease was sufficient to suggest the
change of the average dimension of aggregates with increase the
ITC studies
concentration. The calculated hydrodynamic diameters at 2-
The ITC technique has been successfully employed to study the
reversible noncovalent interactions of the supramolecular
assemblies in solution.21–25 Recently, we reported self-
assembled linear supramolecular polymer based on pillar[5]
arene driven by guest halogen–halogen interactions,4c the
supramolecular assembly in solution was investigated by dilu-
tion ITC experiments which involve sequential injections of
concentrated solution of supramolecular system into a cell
containing pure solvent. Similarly, the concentration-
dependent double threaded supramolecular dimer was
studied by dilution ITC experiments. At high concentration (50
mM), the dimer is present predominately and the exchanged
heat is a measure of the dissociated dimer and it is character-
istic of the dissociation event. When the concentration is
sufficiently low, there is no further disassembly of the supra-
molecular dimer and only the heat of dilution remains.
and 50 mM are 0.76 and 1.58 nm, which correspond to mono-
meric and dimeric copillar[5]arene assemblies, respectively. At
higher concentration of 100 mM, no signicant change of the D
(4.86 ꢁ 10ꢀ10 m2 sꢀ1) was observed which indicated no further
change of average dimension of the aggregates (Table 1).
The number-averaged degree of aggregation “supramolecular
assembly” could be roughly approximated using the diffusion
coefficient obtained from DOSY measurements by assuming that
all assemblies are hydrodynamically spherical according to the
Stokes–Einstein equation [N z 1/(D/Dref)3].4c The number-
averaged degree of aggregation “N” is proportional to the recip-
rocal of the cubic root of the diffusion coefficient, where D and
Dref are the diffusion coefficients of dimer and monomer,
respectively. The average degree of supramolecular assembly was
calculated to be 2, which corresponds to dimer formation. This
simple but rough calculation conrms the presence of double
threaded dimeric supramolecular assembly at high concertation.
Even though the hydroxy derivative Pillar-2c show threaded 8-
bromooctoxy chain as evidence from the high upeld shi in the
1H NMR, analyses were complicated by the presence of the
hydroxy functional group on the cavity rim (ESI†).
DLS measurement
DLS experiments were conducted in chloroform at 298 K to
investigate the supramolecular assembly of Pillar-1c at two
different concentrations. DLS measurements are mainly suit-
able for spherical models and can only give qualitative results
about supramolecular assemblies. However, the DLS measure-
ments of the hydrodynamic diameter (Dh) show pronounced
Table 1 The hydrodynamic diameter calculated from DOSY experi-
ments for Pillar-1c
Sample conc.
(mM)
log D
D
Diametera
(nm)
(log m2 sꢀ1
)
(10ꢀ10 m2 sꢀ1
)
2
50
100
ꢀ8.97
ꢀ9.29
ꢀ9.31
10.8
5.17
4.86
0.76 (0.72)b
1.58 (1.50)b
1.68
a
b
Diameter calculated using Strokes–Einstein equation. Centered
Fig. 3 Hydrodynamic radii distributions of Pillar-1c in chloroform at
two different concentrations (2, and 50 mM).
hydrodynamic diameter calculated from DLS measurement.
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The ITC dilution data for the dissociation of Pillar-1c dimer
into chloroform at 2 and 50 mM are shown in Fig. 4. A plot of
heat rate (mJ sꢀ1) against time (min) shows a series of peaks
where the area under the endothermic heat peaks gives the
enthalpy (DH) of dissociation (Fig. 4a). At high concertation (50
mM), the integrated heat data t to a dimer dissociation model
with positive value of DHdiss ¼ 11.53 kJ molꢀ1 and dissociation
equilibrium constant of Kdiss ¼ 2.27 ꢁ 10ꢀ2 M (Fig. 4b). In
addition, the positive value of Gibbs free energy (DG ¼
11.42 kJ molꢀ1) obtained from ITC dilution experiment indicate
the formation of doubl-threaded supramolecular dimer at high
concentration is a spontaneous process. In contrast, ITC dilu-
tion experiments at low concentration (Pillar-1c at 2 mM), no
signicant dimer dissociation was observed as evidence of the
endothermic heat pulses (Fig. 4a). The ITC experiment results
are in agreement with data obtained from DOSY experiments
and DLS measurements.
Fig. 5 Supramolecular double-threaded dimer of Pillar-1c. (a) The top
view, and (b) the side view. Red, grey (or green) and yellow denote
oxygen, carbon and bromine, respectively. Hydrogen atoms are
omitted for clarity.
evaporation method using dichloroethane Single crystals of
Pillar-1c suitable for X-ray diffraction studies were grown by
slow vapor diffusion of acetone into chloroform solution. No
suitable crystals of the hydroxy-pillar[5]arene derivatives (Pillar
2a–c) could be obtained for X-ray analysis. For Pillar-1a, the 1-
Bromooctane molecules were included in the pillar[5] arene
cavity and the pillararenes are aligned in a face-to-edge manner
in the crystal network (ESI†). These host–guest complexes were
stabilized by C–H/p and C–H/O interactions. The asym-
metric unit of Pillar-1b crystal is observed to be half of the
molecular structure and it was complicated by positional
disorder for both benzyloxy and 1-bromobutoxy substituents.
The crystal renement was carried out by assigning half occu-
pancy for constituent atoms of two fragments. Since the occu-
pancy of the substituents are half at each site, the completed
structure has both substituents (benzyloxy and 1-bromobutoxy)
at both of the pillararene rim (ESI†).
Crystallography
Single crystals of Pillar-1a suitable for X-ray diffraction studies
were grown by slow vapor diffusion of hexanes into CH2Cl2/1-
bromooctane solutions (10/1 v/v), whereas X-ray single crystal
structures of pillar[5]arene Pillar-(1b) were obtained by the slow
The crystal structure of Pillar-1c shows the formation of
a supramolecular double-threaded dimer where the bromo-
octyl chains is threaded inside the cavity of the adjacent pillar
[5]arene (Fig. 5). The formation of interlocked structure in the
solid state were previously reported with mono-urea-
functionalized pillar[5]arene and copillar[5]arene with long
bromodceyl group in the solid state.14,26 The dimer formation is
induced by C–H/p and C–H/O interactions between the octyl
chain and the host. The encapsulation characteristics demon-
strated in the solid state by the single-crystal X-ray diffraction
technique agree with the results obtained from 1H NMR
concentration experiments, DOSY and DLS measurements
which indicate the formation of a supramolecular double-
threaded dimer or [c2]daisy chain type structure at high
concertation.
Experimental
Materials and methods
Nuclear Magnetic Resonance (NMR) spectroscopy measure-
ments were carried out by Bruker Avance II 600 MHz (Germany),
Fig. 4 ITC dilution raw heats for sequential injection in chloroform at
298 K for 2 (red) and 50 (black) mM of Pillar-1c (a), and the integrated
heat data for Pillar-1c at 50 mM fitted to a dimer dissociation model (b). spectrometer. Electron impact ionization (EI) mass
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spectrometry was performed using Thermo Scientic DFS High was added, and the mixture was heated at 80 ꢂC overnight. The
Resolution GC/MS (Germany) mass spectrometer. Electro-spary product was precipitated in ice cold water and puried by
ionization in high resolution mode was done using Waters Xevo column chromatography using dichloromethane/hexane
G2-S Qtof, (Germany) LC MS/MS mass spectrometer. The single mixture (65 : 35 v/v). Yield 2.4 g, 78%. 1H NMR (600 MHz,
crystal data analysis was made by R-AXIS RAPID II, Rigaku, CDCl3) d: 3.63 (t, J ¼ 6.0 and J ¼ 6.6 Hz, 2H), 4.26 (t, J ¼ 6.0 and J
(Japan) diffractometer. The data were collected at 150 K (Oxford ¼ 6.6 Hz, 2H), 5.04 (s, 2H), 6.87 (m, 2H), 6.92 (m, 2H), 7.33 (m,
cryosystems, UK). Flash column chromatography was per- 1H), 7.40 (m, 2H), 7.44 (m, 2H). 13C NMR (150 MHz, CDCl3), d:
formed using silica gel (Silica gel 60, 40–60 mesh ASTM, EMD 29.5, 69.0, 70.9, 116.1, 116.3, 127.7, 128.1, 128.8, 137.4, 152.6,
Millipore, Merck KGaA, Germany). 4-Benzyloxyphenol was 153.8. HRMS: (m/z): calcd for [M+]: 306.0250 (for C15H15O2Br);
synthesized according to literature procedure.11 DMF, acetoni- found 306.0250.
trile and dichloroethane were distilled before use. All other
1-(Benzyloxy)-4-(4-bromobutoxy)benzene (2). 4-Benzylox-
reagents and solvents were of reagent grade purity and used yphenol (2 g, 10 mmol) was dissolved in dry DMF (30 mL) at
without further purication.
room temperature, and anhydrous potassium carbonate (2.76 g;
20 mmol) was added. Then 1,4-dibromobutane (1.96ꢂmL, 14
mmol) was added, and the mixture was heated at 80 C over-
night. The product was precipitated in ice cold water and
puried by column chromatography using dichloromethane/
hexane mixture (65 : 35 v/v). Yield 3.1 g, 92%. 1H NMR (600
MHz, CDCl3) d: 1.94 (m, 2H) 2.09 (m, 2H), 3.51 (t, J ¼ 6.6 and J ¼
6.6 Hz, 2H), 3.97 (t, J ¼ 6.6 and J ¼ 6.0 Hz, 2H), 5.04 (s, 2H), 6.85
(m, 2H), 6.92 (m, 2H), 7.34 (m, 1H), 7.40 (m, 2H), 7.43 (m, 2H).
13C NMR (150 MHz, CDCl3), d: 28.2, 29.7, 33.7, 67.7, 70.9, 71.0,
115.6, 116.1, 116.1, 127.7, 128.1, 128.8, 137.5, 153.3, 153.5.
HRMS: (m/z): calcd for [M+]: 334.0563 (for C17H19O2Br); found
334.0564.
Preparation of single crystals for X-ray diffraction
The single crystal diffraction data were collected on an R-AXIS
RAPID diffractometer (Rigaku, Japan) at 150 K using the
Crystal-Clear soware package. The structures were solved by
direct methods and expanded using Fourier techniques. The
structure renement was performed using SHELXL-2017/1. All
non-hydrogen atoms were rened anisotropically, and
hydrogen atoms were placed at calculated positions and rened
using the riding model. The crystallographic data for the
structure reported in this paper have been deposited at the
Cambridge Crystallographic Data Centre as a supplementary
publication (CCDC 2000224–2000226).
1-(Benzyloxy)-4-(8-bromooctoxy)benzene (3). 4-Benzylox-
yphenol (2 g, 10 mmol) was dissolved in dry DMF (30 mL) at
room temperature, and anhydrous potassium carbonate (2.76 g;
DLS measurements
20 mmol) was added. Then 1,8-dibromooctane (2.64 mL, 14
ꢂ
Samples of the Pillar-1c at different concentrations (2 and mmol) was added, and the mixture was heated at 80 C over-
50 mM in CHCl3) were analyzed on a Zetasizer Nano Range, night. The product was precipitated in ice cold water and
Malvern PANalytical (UK) at 298 K. All DLS measurements were puried by column chromatography using dichloromethane/
performed at a scattering angle of 90ꢂ. Sample solutions were hexane mixture (60 : 40 v/v). Yield 3.4 g, 85%. 1H NMR (600
prepared by ltering each component solution through a 0.2 mm MHz, CDCl3) d: 1.38 (m, 4H), 1.47 (m, 4H), 1.77 (m, 2H), 1.87 (m,
polytetrauoroethylene (PTFE) syringe lter into a clean scin- 2H), 3.42 (m, 2H), 3.92 (t, J ¼ 7.2 and J ¼ 6.6 Hz, 2H), 5.04 (s,
tillation vial.
2H), 6.85 (m, 2H), 6.93 (m, 2H), 7.34 (m, 1H), 7.39 (m, 2H), 7.44
(m, 2H). 13C NMR (150 MHz, CDCl3), d: 25.8, 26.1, 28.2, 28.3,
28.8, 28.8, 28.9, 29.0, 29.2, 29.4, 29.5, 32.9, 33.0, 34.1, 34.2, 68.1,
68.7, 70.9, 115.6, 116.0, 127.7, 128.1, 128.7, 137.5, 153.1, 153.7.
HRMS: (m/z): calcd for [M+]: 390.1189 (for C21H27O2Br); found
390.1189.
ITC measurements
All ITC studies were carried out on an Affinity ITC, TA Instru-
ments (USA). The data were analyzed using NanoAnalyze
(Version 3.10.0). In the ITC dimer-dissociation experiments,
a 10 mM solution of the self-assembled Pillar-1c system in
CHCl3 was placed in a syringe and automatically titrated as 2 mL
per injection at a time interval of 240 s into a reaction cell
loaded with pure CHCl3 (300 mL). The dissociation of the self-
assembled aggregates was accompanied by non-constant heat
signals along with a constant heat of dilution. A control exper-
iment was performed using 2 mM Pillar-1c chloroform solution
under similar experimental conditions against pure CHCl3. All
the titrations were conducted at 298 K.
Synthesis of A1/A2-benzyloxybromoalkoxy difunctionalized-
pillar[5]arenes 1(a–c)
Pillar-1(a). Paraformaldehyde (1.5 g, 50 mmol) was added to
a solution of 1,4-dimethoxybenzene (2.2 g, 16.0 mmol) and 1-
benzyloxy-4-(2-bromoethoxy)benzene (306 mg, 1 mmol) in dry
dichloromethane (120 mL) under nitrogen atmosphere. Boron
triuoride diethyl etherate (BF3$OEt2), (2.0 mL, 16 mmol) was
then added to the solution and the mixture was stirred at 30 ꢂC
for 1 h. Aer this time, methanol (50 mL) was poured into the
reaction mixture and the solution was concentrated then dis-
Synthesis of 1-benzyloxy-4-bromoalkoxybenzenes (1–4)
1-(Benzyloxy)-4-(2-bromoethoxy)benzene (1). 4-Benzylox- solved in dichloromethane (100 mL). The resulting organic
yphenol (2 g, 10 mmol) was dissolved in dry DMF (30 mL) at solution was washed with aqueous NaHCO3 (2 ꢁ 50 mL) and
room temperature, and anhydrous potassium carbonate (2.76 g; H2O (50 mL), then dried over anhydrous Na2SO4, concentrated
20 mmol) was added. Then 1,2-dibroethane (1.20 mL, 14 mmol) under reduced pressure, and subjected to purication by silica
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gel chromatography (dichloromethane/hexanes (75 : 25 v/v)) to 114.1, 114.4, 127.8, 127.9, 128.1, 128.2, 128.4, 128.6, 128.8,
give Pillar-1(a) as a white solid (240 mg, 26%), 1H NMR (600 138.3, 149.5, 149.6, 150.2, 150.2, 150.3, 150.3, 150.5, 150.7,
MHz, CDCl3) d: 3.37 (s, 3H), 3.55 (t, J ¼ 6.0, 6.6, 2H), 3.61 (s, 3H), 150.8. HRMS: (m/z): calcd for [M+]: 1002.3913 (for C58H67O10Br);
3.69 (s, 3H), 3.72 (m, 15H), 3.79 (s, 2H). 3.82 (m, 4H), 3.85 (d, J ¼ found 1002.3912.
8.4, 4H) 4.15 (t, J ¼ 6.0, 6.6, 2H), 4.97 (s, 2H), 6.72 (s, 1H), 6.80
(m, 3H), 6.82 (s, 1H), 6.85 (m, 4H), 6.93 (s, 1H), 7.36 (m, 3H),
Synthesis of A1/A2-benzyloxybromoalkoxy difunctionalized-
pillar[5]arenes 2(a–c)
7.43 (m, 2H). 13C NMR (150 MHz, CDCl3), d: 29.5, 29.8, 29.8,
30.1,30.2, 53.2, 55.6, 55.8, 55.9, 56.0, 56.2, 69.0, 70.5, 114.1,
114.1, 114.2, 114.2, 114.3, 115.1, 116.0, 127.4, 127.8, 127.9,
Pillar-2(a). To a solution of the starting material Pillar-1(a)
128.3, 128.4, 128.4, 128.5, 128.5, 128.6, 128.6, 128.8, 129.3, (100 mg, 0.11 mmol) in anhydrous ethyl acetate (30 mL) was
138.1, 149.4, 150.8, 150.9, 150.9, 151.0. HRMS: (m/z): calcd for added Pd/C (20 mg). The reaction mixture was stirred at room
[M+]: 918.2973 (for C52H55O10Br); found 918.2971.
temperature under an atmosphere of hydrogen for 24 h in the
Pillar-1(b). Paraformaldehyde (1.5 g, 50 mmol) was added to hydrogenation chamber. The catalyst was ltered through Cel-
a solution of 1,4-dimethoxybenzene (2.2 g, 16.0 mmol) and 1- ite. The Celite pad was washed with ethyl acetate (30 mL ꢁ 2).
benzyloxy-4-(4-bromobutoxy)benzene (335 mg, 1 mmol) in dry The combined ltrate was concentrated under reduced pres-
dichloromethane (120 mL) under nitrogen atmosphere. Boron sure. The residue was puried by silica gel column chroma-
triuoride diethyl etherate (BF3$OEt2), (2.0 mL, 16 mmol) was tography (99 : 1 v/v, CH2Cl2/methanol) to afford the desired
then added to the solution and the mixture was stirred at 30 ꢂC product Pillar-2(a) as a white solid (yield 83 mg, 92%). 1H NMR
for 1 h. Aer this time, methanol (50 mL) was poured into the (600 MHz, CDCl3) d: 3.53 (s, 3H), 3.62 (m, 15H), 3.73 (m, 6H),
reaction mixture and the solution was concentrated then dis- 3.81 (m, 12H), 4.19 (t, J ¼ 6.0, J ¼ 6.6, 2H), 6.62 (d, J ¼ 5.4, 2H),
solved in dichloromethane (100 mL). The resulting organic 6.65 (s, 1H), 6.69 (m, 2H), 6.75 (m, 4H), 6.90 (s, 1H). 13C NMR
solution was washed with aqueous NaHCO3 (2 ꢁ 50 mL) and (150 MHz, CDCl3), d: 28.2, 29.0, 29.6, 29.8, 29.9, 30.0, 30.0, 30.1,
H2O (50 mL), then dried over anhydrous Na2SO4, concentrated 30.3, 31.1, 32.1 55.6, 55.9, 56.0, 56.1, 56.2, 56.2, 56.3, 56.4, 56.6,
under reduced pressure, and subjected to purication by silica 69.9, 113.2, 114.0, 114.0, 114.3, 114.5, 114.6, 114.7, 114.7, 114.8,
gel chromatography (dichloromethane/hexanes (65 : 35 v/v)) to 115.5, 119.2, 123.7, 125.5, 126.8, 128.0, 128.3, 128.5, 128.5,
give Pillar-1(b) as white solid (330 mg, 35%). 1H NMR (600 MHz, 128.6, 128.6, 129.5, 129.7, 131.1, 133.6, 148.6, 148.8, 149.5,
CDCl3) d: 1.76 (s, 4H), 3.04 (s, 2H) 3.32 (s, 3H), 3.63 (d, J ¼ 8.4, 150.0, 151.0, 151.0, 151.1, 151.1, 151.2, 151.3, 152.1. HRMS: (m/
6H), 3.69 (m, 13H), 3.78 (s, 2H), 3.81 (s, 8H), 3.87 (s, 2H), 4.91 (s, z): calcd for [M + H2O]: 846.2615 (for C45H51O11Br); found
2H), 6.72 (s, 2H), 6.81 (m, 5H), 6.85 (s, 1H), 6.87 (s, 1H), 6.88 (s, 846.2672.
1H), 7.34 (m, 3H), 7.40 (m, 2H). 13C NMR (150 MHz, CDCl3), d:
Pillar-2(b). To a solution of the starting material Pillar-1(b)
28.6, 29.7, 29.8, 30.0, 30.3, 33.6, 55.5, 55.9, 56.0, 56.1, 56.2, 56.3, (100 mg, 0.11 mmol) in anhydrous ethyl acetate (30 mL) was
56.4, 56.4, 67.6, 70.6, 114.1, 114.2, 114.2, 114.3, 114.3, 114.4, added Pd/C (20 mg). The reaction mixture was stirred at room
115.2, 115.3, 127.4, 127.8, 128.2, 128.4, 128.4, 128.4, 128.4, temperature under an atmosphere of hydrogen for 24 h in the
128.5, 128.5, 128.6, 128.6, 128.6, 138.2, 150.2, 150.3, 150.9, hydrogenation chamber. The catalyst was ltered through Cel-
150.9, 150.9, 151.0. HRMS: (m/z): calcd for [M + H2O]: 964.3397 ite. The Celite pad was washed with ethyl acetate (30 mL ꢁ 2).
(for C54H59O11Br); found 964.3345.
The combined ltrate was concentrated under reduced pres-
Pillar-1(c). Paraformaldehyde (1.5 g, 50 mmol) was added to sure. The residue was puried by silica gel column chroma-
a solution of 1,4-dimethoxybenzene (2.21 g, 16.0 mmol) and 1- tography (99 : 1 v/v, CH2Cl2/methanol) to afford the desired
benzyloxy-4-(n-bromo)octyloxy benzene (391 mg, 1 mmol) in dry product Pillar-2(b) as a white solid (yield 81 mg, 90%).1H NMR
dichloromethane (120 mL) under nitrogen atmosphere. Boron (600 MHz, CDCl3) d: 1.70 (s, 2H), 3.14 (S, 2H), 3.54 (s, 3H), 3.62
triuoride diethyl etherate (BF3$OEt2), (2.0 mL, 16 mmol) was (m, 4H), 3.65 (m, 10H), 3.76 (m, 11H), 3.81 (m, 10H), 6.69 (m,
then added to the solution and the mixture was stirred at 30 ꢂC 5H), 6.79 (s, 2H), 6.84 (m, 1H), 6.88 (s, 1H), 6.93 (s, 1H). 13C NMR
for 1 h. Aer this time, methanol (50 mL) was poured into the (150 MHz, CDCl3) d: 28.2, 28.5, 28.8, 29.2, 29.6, 29.6, 29.9, 30.3,
reaction mixture and the solution was concentrated then dis- 31.1, 33.5, 55.6, 55.9, 56.0, 56.1, 56.1, 56.2, 56.4, 56.6, 68.0,
solved in dichloromethane (100 mL). The resulting organic 113.3, 113.8, 113.9, 114.2, 114.3, 114.4, 114.5, 114.7, 119.1,
solution was washed with aqueous NaHCO3 (2 ꢁ 50 mL) and 123.6, 125.3, 127.1, 128.0, 128.0, 128.3, 128.4, 128.5, 128.5,
H2O (50 mL), then dried over anhydrous Na2SO4, concentrated 128.8, 129.5, 129.6, 130.0, 133.6, 146.7, 147.7, 148.8, 150.3,
under reduced pressure, and subjected to purication by silica 150.9, 150.9, 150.9, 151.0, 151.0, 151.1, 151.2, 151.2, 152.0.
gel chromatography (dichloromethane/hexanes (65 : 35 v/v)) to HRMS: (m/z): calcd for [M + H2O]: 874.2928 (for C47H55O11Br);
give Pillar-1(c) as a white solid (410 mg, 41%).1H NMR (600 found 874.2988.
MHz, CDCl3) d: ꢀ1.35 (s, 2H), ꢀ0.90 (s, 2H), ꢀ0.45 (s, 2H), 0.67
Pillar-2(c). To a solution of the starting material Pillar-1(c)
(s, 2H), 1.18 (s, 2H), 1.34 (s, 2H), 1.80 (s, 2H), 3.36 (s, 3H), 3.85 (100 mg, 0.11 mmol) in anhydrous ethyl acetate (30 mL) was
(m, 31H), 3.98 (s, 2H), 5.01 (s, 2H), 6.79 (s, 1H), 6.86 (m, 3H), added Pd/C (20 mg). The reaction mixture was stirred at room
6.92 (m, 2H), 6.95 (s, 1H), 6.99 (m, 2H), 7.16 (s, 1H), 7.36 (m, temperature under an atmosphere of hydrogen for 24 h in the
1H), 7.45 (t, J ¼ 7.8, 7.2, 2H), 7.63 (d, J ¼ 7.3, 2H). 13C NMR (150 hydrogenation chamber. The catalyst was ltered through Cel-
MHz, CDCl3), d: 26.8, 27.5, 29.1, 29.3, 29.6, 29.9, 34.0, 55.1, 55.4, ite. The Celite pad was washed with ethyl acetate (30 mL ꢁ 2).
55.4, 55.5, 55.5, 55.5, 55.8, 70.0, 113.1, 113.1, 113.2, 113.3, 113.6, The combined ltrate was concentrated under reduced
3000 | RSC Adv., 2021, 11, 2995–3002
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pressure. The residue was puried by silica gel column chro-
matography (99 : 1 v/v, CH2Cl2/methanol) to afford the desired
product Pillar-2(c) as a white solid (yield 85 mg, 93%).1H NMR
(600 MHz, CDCl3) d: ꢀ0.09 (s, 2H), 0.28 (s, 2H) 0.38 (s, 2H), 0.97
(s, 2H), 1.29 (m, 2H), 1.75 (m, 2H), 2.24 (s, 2H), 3.68 (s, 3H), 3.71
(m, 14H) 3.79 (m, 14H), 3.84 (s, 3H), 3.95 (m, 2H), 6.65 (s, 1H),
6.71 (s, 1H), 6.78 (m, 3H), 6.82 (m, 4H), 6.93 (s, 1H). 13C NMR
(150 MHz, CDCl3), d: 25.2, 27.2, 27.8, 28.7, 28.9, 29.1, 29.6, 29.8,
29.9, 31.0, 32,1, 34.0, 55.7, 55.8, 55.8, 55.8, 55.9, 55.9, 56.0, 56.3,
68.5, 113.0, 113.6, 113.8, 113.8, 113.9, 113.9, 113.9, 114.0, 114.2,
114.4, 119.4, 125.7, 127.3, 127.9, 128.2, 128.3, 128.3, 128.4,
128.5, 129.3, 129.9, 147.2, 149.0, 149.8, 150.7, 150.8, 150.8,
150.9, 150.9, 151.6. HRMS: (m/z): calcd for [M+]: 912.3443 (for
3 N. Pairault, R. Barat, I. Tranoy-Opalinski, B. Renoux,
M. Thomas and S. Papot, Rotaxane-Based Architectures for
Biological Applications, C. R. Chim., 2016, 19, 103–112.
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C
51H61O10Br); found 912.3448.
Conclusions
In summary, A1/A2-difunctionalized pillar[5]arenes bearing
bromoalkoxy and hydroxy substituents were synthesized by the
co-cyclization of hydroquinone derivatives followed by the
removal of the benzyl protected group using catalytic hydroge-
nation over palladium on charcoal in anhydrous ethyl acetate.
The 8-bromooctoxy substituent in the pillar[5]arene rim (Pillar-
1c) undergo self-inclusion as evidence by the unusual high
upeld proton chemical shi. 1HNMR concentration experi-
ments demonstrated that the inclusion behavior of long chain
8-bromooctoxy substituent is concentration-dependent, sug-
gesting the presence of two main size aggregates. The formation
of pseudo[1]rotaxane and double-threaded supramolecular
dimer at low and high concentrations respectively was
conrmed by single-crystal X-ray diffraction analysis, DOSY,
DLS and ITC measurements. Currently further studies and
6 G. Giuseppe, N. Anna, P. Andrea, P. Melchiorre, P. Ilenia,
P. Sebastiano, G. Domenico, M. Angela, C. Yoram and
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modications
of
the
A1/A2-bromoalkyloxy-hydroxy-
functionalized pillar[5]arenes host to develop new supramo-
lecular systems and functional materials are underway in our
laboratories.
9 T. Ogoshi, K. Demachi, K. Kitajima and T. Yamagishi,
Monofunctionalized
Pillar[5]arenes:
Synthesis
and
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
The support received from the Kuwait Foundation of Advance-
ment of Science (KFAS), made available through research grant
no. PR17-14SC-07, and the facilities of the RSPU (grant no.
GS01/01, GS01/03, and GS03/08) are gratefully acknowledged.
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3002 | RSC Adv., 2021, 11, 2995–3002
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