Tuning catalysis of boronic acids in microgels by: In situ reversible structural variations

Article abstract of DOI:10.1039/c9ra10541g

The catalysis of boronic acids immobilized in polymer microgels can be modulated by bubbling with N₂/CO₂ gas, and in some cases by adding glucose, making their catalytic activity comparable or even superior to that of the corresponding free boronic acid monomers homogeneously dispersed in solutions and, more importantly, making these boronic-acid-containing polymer microgels able to catalyze alternate reactions that may extend the usefulness. This enhanced catalytic function of these boronic-acid-containing microgels as organoboron acid catalysts is plausibly achieved via in situ reversibly structural variations. Kinetic studies have been carried out on the model boronic-acid-catalyzed aza-Michael addition, aldol, amidation, and [4 + 2] cycloaddition reactions in order to better understand the catalytic process.

Full text of DOI:10.1039/c9ra10541g

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Tuning catalysis of boronic acids in microgels by in
situ reversible structural variations†
Cite this: RSC Adv., 2020, 10, 3734
c
Zhenghao Zhai,‡a Xue Du,‡a Qingshi Wu,b Lin Zhu,a Zahoor H. Farooqi,
Jin Li,a
a
Ruyue Lan,a Yusong Wangd and Weitai Wu
*
The catalysis of boronic acids immobilized in polymer microgels can be modulated by bubbling with N2/
CO2 gas, and in some cases by adding glucose, making their catalytic activity comparable or even
superior to that of the corresponding free boronic acid monomers homogeneously dispersed in
solutions and, more importantly, making these boronic-acid-containing polymer microgels able to
catalyze alternate reactions that may extend the usefulness. This enhanced catalytic function of these
boronic-acid-containing microgels as organoboron acid catalysts is plausibly achieved via in situ
reversibly structural variations. Kinetic studies have been carried out on the model boronic-acid-
catalyzed aza-Michael addition, aldol, amidation, and [4 + 2] cycloaddition reactions in order to better
understand the catalytic process.
Received 15th November 2019
Accepted 6th January 2020
DOI: 10.1039/c9ra10541g
rsc.li/rsc-advances
involves structural modication followed by laborious
screening, prior to performing the catalytic reactions for target
chemical transformations.1–13 This approach, by also combining
1. Introduction
The catalysis by organoboron acids is among the most active
elds in synthetic chemistry.1–4 Since organocatalysis highlights
the formation and breakage of temporary bonds, organoboron
acids possessing the unique ability of reversible covalent
binding acidic functional groups should display reactivity
patterns that are distinct from those of common organo-
catalysts.5–13 While a broad spectrum of transformations with
organoboron acids as reaction catalysts have been documented
in the literature, a problem that accompanies this exciting eld
from its early beginning, however, is the limited knowledge
about the possible action modes and, therefore, the obstacle in
the design and optimization of the catalysts, making it difficult
to ensure a meaningful and popularized use of organoboron
acid catalysis.3,4 The challenge of identifying organoboron acid
catalysts that can exhibit enhanced catalytic function remains
to be solved.
computational modeling, in principle could allow tailoring the
molecular factors that impact the catalysis, including the
charge, coordination, interatomic distance, bonding and
orientation of the catalytically active atoms, and thus should
enable empirical modulation of the attractive and repulsive
noncovalent interactions that determine the activity and selec-
tivity in the catalyzed processes.14 Such a tailored catalyst does
oen much better than others, and most of the time is quite
selective for one class of reactants, so that each transformation
might require its own tailored catalyst, and in some cases co-
catalysts, through a substantial process of design and optimi-
zation.4 However, in many successful instances, the catalysts
and co-catalysts have been found by chance rather than by
logical prediction.4,15,16 The catalytic reaction specicity and the
availability of suitable organoboron acids as organocatalysts
remain primary restrictions for the targets of interest. Naturally,
a question arises of how to appropriately introduce an in situ
structural variation to tune the catalysis of ready-prepared
organoboron acid catalysts, making it able to catalyze alter-
nate reactions that may extend their usefulness.17
Traditionally, the widely employed strategy for the design
and optimization of organoboron acids as organocatalysts
aState Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative
Innovation Center of Chemistry for Energy Materials, The Key Laboratory for
Chemical Biology of Fujian Province, Department of Chemistry, College of Chemistry
and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China.
E-mail: wuwtxmu@xmu.edu.cn
In this paper, as a proof of the concept, we report a new class
of organoboron acid catalysts (denoted as PBA@PM) by
immobilizing
a representative organoboron acid, phenyl-
bCollege of Chemical Engineering and Materials Science, Quanzhou Normal University,
boronic acid (PBA), in polymer microgels that are dispersible
but undissolvable colloids of three-dimensional crosslinked gel
network structures internally. To the best of our knowledge, so
far, catalysis with PBA-containing polymer microgels acting as
organoboron acid catalysts has not been recognized as a viable
option, and no example exists in the literature; PBA-containing
Quanzhou, Fujian 362000, China
cInstitute of Chemistry, University of the Punjab, New Campus, Lahore 54590, Pakistan
dHefei National Laboratory for Physical Sciences at the Microscale, University of
Science and Technology of China, Hefei, Anhui 230026, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra10541g
‡ These authors contribute equally.
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Scheme 1 Synthesis of the proposed PBA@PM.
polymer microgels are better known for molecular recognition, vibration at ca. 3260 cmꢁ1, and those of aliphatic C–H stretch-
biosensing, and drug delivery.18–24 Herein, our intention in ing vibrations at ca. 2850–2960 cmꢁ1 and aromatic ¼ C–H
exploiting PBA-containing polymer microgels is based on stretching vibrations at ca. 3000–3100 cmꢁ1 were observed. The
a combination of recent progresses in microgel synthesis, content of PBA groups immobilized in the microgels was
stimuli-responsive polymers, and supramolecular chemistry: determined (ca. 5.2 ꢂ 10ꢁ4 mol per (g dried microgels)) by the
PBA can anchor covalently in microgels and bind cis-diols of mannitol-assisted UV-vis spectrophotometric titration anal-
saccharides (e.g. glucose) to form boronates,18–24 where tetra- ysis,28 from which the molar ratio of 1 and 2 moieties incorpo-
hedral species would change into trigonal boronic acids upon rated into the microgels was estimated to be ca. 1 : 4.79. This is
CO2 bubbling,25 and recover to the original state without accu- close to the feeding molar ratio (1 : 4.86) of 1 and 2 for the
mulation of by-products by bubbling a suitable gas (e.g., N2) to microgel synthesis, together with the high yield of the microgels
expel CO2.26,27 With the catalytic, saccharide-responsive, and ($96%), indicating a high monomer conversion. These micro-
CO2-sensitive components being integrated into a single object, gels can be reproduced from batch to batch.
these organoboron acid catalysts should present prospects for
The PBA@PM could be well dispersed in many organic
in situ structural variations to tune catalysis by bubbling with solvents, such as dichloromethane (CH2Cl2), ethanol, and their
N2/CO2 gas.
mixtures, or even in the presence of some water. For instance, in
an immiscible mixture of CH2Cl2 and water at a volume ratio of
9 : 1 (aer adding PBA@PM, the mixture would not be strati-
ed, but this may occur if more water was added), or a ternary
miscible solvent of CH2Cl2, ethanol and water at a volume ratio
of 7 : 2 : 1, PBA@PM (the concentration of boronic acids in the
nal dispersion was ca. 1.2 ꢂ 10ꢁ5 mol Lꢁ1) showed very good
stability. No sediment was observed even aer 6 months of
storage at room temperature in a N2 atmosphere, but there was
only a marginal change on the size distribution measured by
2. Results and discussion
Typically, these PBA-containing polymer microgels were
prepared (Scheme 1) according to
method,23,24 which involves rst, the synthesis of vinyl mono-
mers (3,5-diacrylamidophenyl)boronic acid (1; Fig. S1 in ESI†)
and N,N0-(anthracene-9,10-diylbis(methylene))diacrylamide (2;
Fig. S2 in ESI†), followed by free-radical polymerization of the
a reported literature
two
vinyl
monomers
using
2,20-azobis(2-
methylpropionamidine)dihydrochloride (AAPH) as an initiator
in a N2 atmosphere at 70.0 ꢀC. The choice of these two vinyl
monomers for microgel synthesis is specically considered to
ensure negligible change in the average hydrodynamic diameter
(hDhi; Fig. S3 in ESI†) of the ready-prepared PBA@PM in
solvents upon heating/cooling.18–24 The feeding molar ratio of 1
and 2 was set to a moderate value of ca. 1 : 4.86 to ensure the
formation of bis-boronate complexes upon adding glucose in
a N2 atmosphere (see below).23,24 TEM images of microgels
indicate the formation of a sphere-like morphology (Fig. 1a–c).
The distribution of the boronic acid inside the microgels was
studied by electron energy loss spectroscopy (EELS; Fig. 1d),
which showed that the line scan curves of normalized EELS
intensities of carbon, nitrogen and boron elements displayed
a similar prole, indicating that those elements and thus PBA
groups of 1 and anthracene rings of 2 are homogeneously
distributed throughout the whole microgels. IR spectral
comparison between 1, 2, and the microgels (Fig. 2) conrmed
the incorporation of PBA groups of 1 and anthracene rings of 2
into the microgels, wherein the characteristic bands of B–O
Fig. 1 Typical (a–c) TEM images of the PBA@PM. (d) EELS spectra of
stretching vibration at ca. 1384 cmꢁ1 and O–H stretching the PBA@PM along the line shown in the TEM image in (c).
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centered at ca. ꢁ7.9 ppm, which is due to the fast exchange
averaging of the two boronic acid species, i.e., tetrahedral and
trigonal species; in contrast, the 11B NMR spectrum of 1 dis-
played a peak at ca. 13.9 ppm, which is consistent with the
preponderance of trigonal species.32 Interestingly, if bubbling
with 30 min (the same below) of CO2 gas, the solution pH
decreased from ca. 7.1 to ca. 5.8; meanwhile, a remarkable
increase in the chemical shi for the 11B NMR peak of the
PBA@PM (ca. 10.4 ppm) toward for 1 (ca. 14.1 ppm) was
observed (Fig. 3), accompanied with an increase in the zeta
potential from ca. ꢁ13.0 mV to ca. ꢁ2.6 mV (Fig. S6 in ESI†) and
a decrease in the hDhi from ca. 136.1 nm to ca. 66.1 nm (Fig. S3
and S7 in ESI†), deriving from the reduction in the ionization
degree of PBA groups along with the decrease in the solution
pH.18–20,33 All 11B NMR, solution pH value, surface zeta potential
and size changes are completely reversible upon bubbling once
again with 30 min (the same below) of N2 gas to expel CO2.
Similar changes upon bubbling with N2/CO2 gas were observed
on the PBA@PM dispersed in other solvents containing water.
These results can not only provide additional proofs for the
successful incorporation of both 1 and 2 into the microgels, but
also foreshadow a novel class of organoboron acid catalysts that
may allow tuning catalysis by simply bubbling with N2/CO2 gas.
To be an efficient catalyst, the PBA@PM should be so
permeable to both the reactants and products that mass-
transfer limitations can be avoided to maximize the chemical
reaction rates. High porosity of the PBA@PM is then crucial for
the catalysis application. Glucose-responsive swelling–deswel-
ling phase transition behaviors, which were quantitatively
measured by DLS that is a powerful tool to study volume phase
transition behavior in situ of polymer gel particles in solu-
tions,18–26 do conrm the highly porous nature of the PBA@PM.
Fig. 4 shows that hDhi decreases by ca. 1.64 times when the
solution glucose concentration ([Glu]) increases from 0.0 mM to
5.0 mM upon bubbling with N2 gas, verifying the deswelling
behavior of the PBA@PM upon addition of glucose. This
deswelling behavior of the PBA@PM upon addition of glucose
can be interpreted as the occurrence of additional cross-linking
Fig. 2 Typical IR spectrum of the PBA@PM. IR spectra of 1 and 2 are
shown for comparison.
dynamic light scattering (DLS) (Fig. S4 in ESI†), implying that
both hDhi and dispersibility of the PBA@PM remained nearly
unchanged, and that the degradation/aggregation was negli-
gible. The good dispersibility of the PBA@PM in the presence of
water may be due to the partial ionization of PBA groups (Fig. S5
in ESI†) derived from the localization of PBA groups adjacent to
the anthracene rings.23,24 It is reported that anthracenes could
be used as electron acceptors due to their strong electron
affinities.29 This should favor the interaction of anthracene
rings with neighboring PBA groups via Nd+/Bdꢁ interac-
tions,30,31 and possibly p/Bdꢁ interactions as well,23,24 both of
which could promote the dissociation equilibrium of boronic
acids moving to right, resulting in a decrease in pKa (ca. 7.5 for
PBA groups in the PBA@PM, much lower than ca. 8.8 for 1).23,24
Indeed, the 11B NMR spectrum (Fig. 3) of the PBA@PM upon
bubbling with N2 gas displayed a single broad peak that
Fig. 3 Typical 11B NMR spectra of the PBA@PM dispersed in CD2Cl2/ Fig. 4 Glucose-dependent hDhi of the PBA@PM dispersed in CH2Cl2/
D2O (9 : 1 in volume) upon bubbling with N2 or CO2 gas. Results of 1 water (9 : 1 in volume) upon bubbling with N2 or CO2 gas, measured at
are shown for comparison.
10.0 ꢀC and at a scattering angle of 45ꢀ.
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junctions due to the formation of 1 : 2 (i.e., glucose–bis- electrophiles.3,4 Stemmed from these key points reported
boronate) complexes, exceeding over the contribution of previously, and motived from our observations on structural
glucose binding on the ionization of PBA groups.23,24 This is also variations in the PBA@PM upon bubbling with N2/CO2 gas, our
supported by the inverse, swelling behavior of the PBA@PM initial venture into organoboron acid catalysis of the PBA@PM
upon addition of fructose, galactose or mannose (Fig. S8 in (the concentration of boronic acids in the reaction system was
ESI†), which are the natural stereoisomers of glucose, but that ca. 1.2 ꢂ 10ꢁ5 mol Lꢁ1) was commenced by performing a cata-
can only form 1 : 1 (i.e., glucose–boronate) rather than 1 : 2 lytic aza-Michael addition of hydroxamic acid 4 (2.5 ꢂ
complexes.34,35 Upon the dissociation of the complexes by 10ꢁ3 mol Lꢁ1) to quinone imine ketal 3 (1.0 ꢂ 10ꢁ4 mol Lꢁ1
;
removing glucose via dialysis,23,24 the PBA@PM could swell Fig. S13 in ESI†) to give nal product 6 (Fig. S14 in ESI†) in
reversibly, with hDhi nearly recovered to the original values CH2Cl2/water (9 : 1 in volume; 100.0 mL) at 10.0 C (Fig. 5a),11
ꢀ
(Fig. S9 in ESI†). A facile but different way to realize the swelling wherein the quinone imine ketal can be activated by hydrogen
of the PBA@PM was achieved by bubbling with CO2 gas (Fig. 4), bonding with boronic acids and attacked by the nucleophilically
or others containing an appropriate amount of CO2 (Fig. S10 in activated hydroxamic acid,11 which would not progress without
ESI†). The distinct difference in those phase-transition proles any organoboron acid catalysts under otherwise identical reac-
upon bubbling with N2/CO2 gas was derived from the structural tion conditions (Fig. S15 in ESI†). The glucose (if any) that was
variations in the microgels (Scheme 2). Upon neutralization of added into the catalytic system is stable, which could be sepa-
alkaline boronate anions aer bubbling with CO2 gas,25 the rated from the products by chromatography and conrmed
dissociation of the complexes and, thus, a decreased degree of simply by spectroscopy (data not shown). Since the uncyclized
ionization should reduce both the polymer water solubility and adduct 5 that may derive from initial aza-Michael addition re-
Donnan potential,18–20,33 resulting in a decrease in hDhi to ca. ported previously11 was not isolated in our cases, and the
66.1 nm in our experimental [Glu] window of 0.0–5.0 mM concentration of 4 exceeds over 25-times than that of 3, the
(Fig. 4), which can be readily recovered upon the subsequent reaction overall may be simplied as pseudo rst order.35,36
purging with N2 gas to remove CO2.26,27 In the control experi- Fig. 5b–e shows that this is the case for the solution temperature
ments adding fructose, galactose or mannose, the neutraliza- range of 10.0–20.0 ꢀC, as measured by monitoring the yield of 6
tion would also occur upon bubbling with CO2 gas (Fig. S11 in (Fig. S16 in ESI†). Linear relationships between ln(C3,0/(C3,0
ꢁ
ESI†), with hDhi also dropping to ca. 66.1 nm, and recover by C6,t)) and t are obtained at the low conversions (#60%),37 where
purging with N2 gas (Fig. S12 in ESI†).
C3,0 is the starting concentration of 3, and C6,t is the concen-
It has been documented that trigonal boronic acids can tration of 6 at the reaction time t. For present purpose it suffices
provide electrophilic activation of an alcohol or a carboxylic acid to discuss the initial rates of the reaction.37 The apparent
for a desired nucleophilic substituent or addition; with the reaction rate constant k for the reaction catalyzed by the
formation of tetrahedral neutral or even anionic boronate PBA@PM upon bubbling with N2 gas at 10.0 ꢀC thereby is
anions, boronic acids can facilitate the enolization of carbonyl estimated to be ca. 4.90 ꢂ 10ꢁ4 minꢁ1 in the absence of glucose.
compounds, or nucleophilic activation of hydroxy groups, As is expected, the reaction can be accelerated in the presence of
wherein the reactivity is enabled by an increase in the electronic glucose over the entire range of 0.0 mM < [Glu] # 5.0 mM,
density of the oxygen atoms, which, in turn, translates into an whereas instead of a simple linear relationship, the change of
increase in their nucleophilicity towards potential the k with the [Glu] can be roughly divided into two regions
Scheme 2 In situ structural variations in the PBA@PM upon bubbling with N2/CO2 gas, in the absence or in the presence of glucose.
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Fig. 5 (a) The catalytic aza-Michael addition as a model for evaluating the accessibility and the catalytic performance of the PBA@PM. (b–e)
Influence of temperature on k, measured in the reaction mixture in the absence (b and c; [Glu] ¼ 0.0 mM) or in the presence (d and e; [Glu] ¼ 1.5
mM) of glucose, upon bubbling with N2 (b and d) or CO2 (c and e) gas, where lines are first-order kinetic fits.
Fig. 6 (a) Influence of the [Glu] on the k of aza-Michael addition catalyzed by the PBA@PM at 10.0 ꢀC. (b) The ln k–1000/T plots, where the solid
lines are linear fitting of the plots.
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Table 1 Apparent activation parameters for the aza-Michael addition
catalyzed by the PBA@PM
centrifugation and reused directly for the next cycle. In the more
than ten times repeated bubbling with N2 or CO2 gas, no
signicant erosion of the catalytic activity (the k can return to
>92% of original value; Fig. S19 in ESI†) was observed, due to
the stability and reversibility of microgels upon bubbling/
removing N2/CO2 gas (Fig. 4, S3, S4, S7 and S20 in ESI†). Addi-
tional recycling experiments also revealed the robustness of the
PBA@PM as organoboron acid catalysts (Fig. S21 in ESI†). The
slight decrease in the activities aer ten cycles might be due to
the occupation of the catalytic site in the PBA@PM by reactants
or products, which is conrmed by the nearly unchanged
results if the recycled PBA@PM was puried before reuse
(Fig. S22 in ESI†).
This N2/CO2-modulated catalysis of boronic acids in micro-
gels is observed throughout our experimental temperature
window of 10.0–20.0 ꢀC. In a further analysis, both the apparent
activation energy (Ea) and the pre-exponential factor (A) can be
determined (Table 1) on the basis of the Arrhenius equation and
the linear tting of ln k–Tꢁ1 plots (Fig. 6b).37 While the inter-
pretation of the apparent activation parameters is complicated
by the fact that these represent the combination of the diffusion
and the catalysis processes, for the reaction catalyzed by the
PBA@PM upon bubbling with N2 gas, the observation of the
smaller Ea (theoretically indicating a lower energy required to
reach transition state of reactants, thereby a lower energy
required to initiate the reaction) but smaller A (theoretically
indicating a smaller total number of collisions per minute)
when adding glucose is consistent with a model between
diffusional control of the reaction and control by the reaction at
the catalytic sites.37,40 In agreement with the observation on the
inuence of the [Glu] on k (Fig. 6a) discussed above that could
be associated with the counterbalance of the two effects, the
much denser polymer network (to some extent) of the collapsed
microgels imposes a higher diffusional resistance,38–40 and the
Bubbling with N2 gas
Bubbling with CO2 gas
A (minꢁ1 Ea (kJ molꢁ1
)
[Glu] (mM) A (minꢁ1
)
Ea (kJ molꢁ1
)
)
0.0
1.5
1.3 ꢂ 1016 101.3
9.1 ꢂ 1015 85.7
1.3 ꢂ 1014 109.3
1.2 ꢂ 1014 109.4
(Fig. 6a and S17 in ESI†): at low [Glu] of # ca. 1.5 mM, the k
increases with the [Glu], but the k decreases gradually when
[Glu] $ ca. 1.5 mM. This non-linear relationship can be
assigned to the formation of glucose–bis-boronate complexes
(Fig. 4), consequently the counterbalance of the deswelling of
microgels (rendering an inhibiting effect) and the increased
amount of boronate anions (rendering a promoting effect
here):3,4,38–40 at low [Glu]s that the microgels are in relatively
solvophilic and swollen state, the diffusion of chemicals into/
out the microgels is slightly or even negligibly affected by the
deswelling of microgels, meanwhile the formation of glucose–
bis-boronate complexes provides more boronate anions with an
increase in the [Glu], leading to an increase in k; when the
microgels collapse to some extent with further increase in the
[Glu], the cumulative decrease in the porosity would signi-
cantly hinder the diffusion (Fig. S18 in ESI†) and surpasses the
promoting effect, resulting in a decrease in the k. Nevertheless,
in the cases both in the absence and in the presence of glucose,
the overall rate for the reaction catalyzed by the PBA@PM upon
bubbling with N2 gas is clearly larger than that upon bubbling
with CO2 gas. The reaction can be signicantly decelerated
upon bubbling with CO2 gas, with k decreasing to almost the
same value of (7.85 ꢃ 0.13) ꢂ 10ꢁ5 minꢁ1. Aer full conversion
of the starting 3, the PBA@PM can be simply separated by
Fig. 7 (a) A model aldol reaction of hydroxyacetone and aldehyde catalyzed by the PBA@PM. (b and c) Time trace of the yield of aldol adduct 7,
measured in the reaction mixture in the absence (b; [Glu] ¼ 0.0 mM) or in the presence (c; [Glu] ¼ 1.5 mM) of glucose, upon bubbling with N2
(-,,) or CO2 (C,B) gas, and at 10.0 ꢀC, where CHA,0 is the starting concentration of hydroxyacetone, C7,t is the concentration of 7 at the
reaction time t, and lines are first-order kinetic fits.
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Fig. 8 (a) A model amidation reaction of carboxylic acid and amine catalyzed by the PBA@PM. (b and c) Time trace of the yield of amide product
8, measured in the reaction mixture in the absence (b; [Glu] ¼ 0.0 mM) or in the presence (c; [Glu] ¼ 1.5 mM) of glucose, upon bubbling with N2
(-,,) or CO2 (C,B) gas, and at 10.0 ꢀC, where CCA,0 is the starting concentration of carboxylic acid, C8,t is the concentration of 8 at the reaction
time t, and lines are first-order kinetic fits.
formation of more boronate anions that is prior to trigonal aza-Michael addition, we tried to extend the use of the PBA@PM
boronic acids for facilitating the nucleophilic activation should (the concentration of boronic acids in reaction systems was ca.
prefer the aza-Michael addition.3,4,11 More importantly, in 1.2 ꢂ 10ꢁ5 mol Lꢁ1) as organoboron acid catalysts to several
comparison with the apparent activation parameters obtained other chemical transformations in CH2Cl2/water (9 : 1 in
for the reaction upon bubbling with N2 gas, both the smaller Ea volume; 100.0 mL) at 10.0 ꢀC, including a model aldol reaction
and A values are obtained for the reaction upon bubbling with of hydroxyacetone (1.0 ꢂ 10ꢁ4 mol Lꢁ1) and aldehyde (2.5 ꢂ
CO2 gas; both the collapsed gel and trigonal boronic acids can 10ꢁ3 mol Lꢁ1) (Fig. 7 and S23 in ESI†), a model amidation
be envisaged to hinder the reaction to move. These results reaction of carboxylic acid (1.0 ꢂ 10ꢁ4 mol Lꢁ1) and amine (2.5
conrm the feasibility on tuning catalysis of boronic acids in ꢂ 10ꢁ3 mol Lꢁ1) (Fig. 8 and S24 in ESI†), and a model [4 + 2]
microgels by bubbling with N2/CO2 gas.
cycloaddition between acrylic acid (1.0 ꢂ 10ꢁ4 mol Lꢁ1) and
Encouraged by the above-mentioned observation on N2/CO2- diene (2.5 ꢂ 10ꢁ3 mol Lꢁ1) (Fig. 9 and S25 in ESI†). The exper-
modulated catalysis of boronic acids in microgels for the model imental results showed that for the model aldol reaction, the
Fig. 9 (a) A model [4 + 2] cycloaddition of acrylic acid and diene catalyzed by the PBA@PM. (b and c) Time trace of the yield of cycloadduct 9,
measured in the reaction mixture in the absence (b; [Glu] ¼ 0.0 mM) or in the presence (c; [Glu] ¼ 1.5 mM) of glucose, upon bubbling with N2
(-,,) or CO2 (C,B) gas, and at 10.0 ꢀC, where CAA,0 is the starting concentration of acrylic acid, C9,t is the concentration of 9 at the reaction
time t, and lines are first-order kinetic fits.
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Table 2 Comparison of results for the model boronic-acid-catalyzed reactions
Product
Catalyst
Temperature
Time
Yield
a
6
PBA@PM
PBA@PM
PBA@PM
PBA@PM
PBA@PM
Cata-ref. 11
PBA@PM
PBA@PM
cata-ref. 11
PBA@PM
PBA@PM
Cata-ref. 11
PBA@PM
PBA@PM
Cata-ref. 7
PBA@PM
PBA@PM
Cata-ref. 7
PBA@PM
PBA@PM
Cata-ref. 7
PBA@PM
PBA@PM
Cata-ref. 9
PBA@PM
PBA@PM
Cata-ref. 9
PBA@PM
PBA@PM
Cata-ref. 9
10.0 ꢀC
12.5 ꢀC
15.0 ꢀC
17.5 ꢀC
20.0 ꢀC
16 h
16 h
16 h
16 h
16 h
16 h
16 h
16 h
16 h
16 h
16 h
24 h
7 h
7 h
7 h
7 h
7 h
7 h
7 h
7 h
7 h
48 h
48 h
48 h
48 h
48 h
48 h
48 h
48 h
48 h
36% ; 9% b; 62% c; 9% d
This worke
This worke
This worke
This worke
This worke
Ref. 11f
a
52% ; 11% b; 74% c; 11% d
a
58% ; 15% b; 83% c; 15% d
a
69% ; 21% b; 91% c; 22% d
a
79% ; 27% b; 95% c; 30% d
ꢁ10 ꢀ
C
89%
a
6-a
6-b
7
10.0 ꢀC
20.0 ꢀC
37% ; 7% b; 61% c; 7% d
This worke
This worke
Ref. 11f
a
80% ; 25% b; 94% c; 25% d
ꢁ10 ꢀ
C
88%
a
10.0 ꢀC
20.0 ꢀC
39% ; 7% b; 67% c; 6% d
This worke
This worke
Ref. 11f
a
82% ; 23% b; 93% c; 24% d
ꢁ10 ꢀ
C
81%
a
10.0 ꢀC
20.0 ꢀC
R.T.
76% ; 41% b; 92% c; 40% d
This workg
This workg
Ref. 7h
a
95% ; 59% b; 97% c; 61% d
64%
a
7-a
7-b
8
10.0 ꢀC
20.0 ꢀC
R.T.
81% ; 45% b; 93% c; 47% d
This workg
This workg
Ref. 7h
a
99% ; 61% b; 99% c; 61% d
68%
a
10.0 ꢀC
20.0 ꢀC
R.T.
79% ; 39% b; 77% c; 38% d
This workg
This workg
Ref. 7h
a
96% ; 43% b; 94% c; 44% d
62%
a
10.0 ꢀC
20.0 ꢀC
R.T.
8% ; 75% b; 5% c; 77% d
This worki
This worki
Ref. 9j
9% ; 97% b; 7% c; 99% d
b
99%
b
8-a
8-b
10.0 ꢀC
20.0 ꢀC
R.T.
5% ; 64% b; 4% c; 65% d
This worki
This worki
Ref. 9j
6% ; 92% b; 6% c; 91% d
a
95%
a
10.0 ꢀC
20.0 ꢀC
R.T.
3% ; 49% b; 2% c; 55% d
This worki
This worki
Ref. 9j
6% ; 84% b; 5% c; 83% d
a
73%
a
Bubbling with N2 gas, [Glu] ¼ 0.0 mM. b Bubbling with CO2 gas, [Glu] ¼ 0.0 mM. c Bubbling with N2 gas, [Glu] ¼ 1.5 mM. d Bubbling with CO2 gas,
[Glu] ¼ 1.5 mM. Performed with hydroxamic acid (2.5 ꢂ 10ꢁ3 mol Lꢁ1) and quinone imine ketal (1.0 ꢂ 10ꢁ4 mol Lꢁ1) in CH2Cl2/water (9 : 1 in
e
volume; 100.0 mL), and the concentration of boronic acids in the reaction system of ca. 1.2 ꢂ 10ꢁ5 mol Lꢁ1
.
Performed with hydroxamic acid
f
(1.3 ꢂ 10ꢁ1 mol Lꢁ1) and quinone imine ketal (0.1 mol Lꢁ1) in CH2Cl2 (1.0 mL); the concentration of boronic acids in the reaction system was
ca. 1.0 ꢂ 10ꢁ2 mol Lꢁ1, and o-nitrobenzoic acid (5.0 ꢂ 10ꢁ2 mol Lꢁ1) as a co-catalyst. Performed with hydroxyacetone (1.0 ꢂ 10ꢁ4 mol Lꢁ1
)
g
and aldehyde (2.5 ꢂ 10ꢁ3 mol Lꢁ1) in CH2Cl2/water (9 : 1 in volume; 100.0 mL), and the concentration of boronic acids in the reaction system
of ca. 1.2 ꢂ 10ꢁ5 mol Lꢁ1
.
Performed with hydroxyacetone (5.5 mol Lꢁ1) and aldehyde (5.5 ꢂ 10ꢁ1 mol Lꢁ1) in water (2.0 mL), and the
h
concentration of boronic acids in the reaction system of ca. 3.6 ꢂ 10ꢁ2 mol Lꢁ1
.
Performed with carboxylic acid (1.0 ꢂ 10ꢁ4 mol Lꢁ1) and
i
amine (2.5 ꢂ 10ꢁ3 mol Lꢁ1) in CH2Cl2/water (9 : 1 in volume; 100.0 mL), and the concentration of boronic acids in the reaction system of ca. 1.2
ꢂ 10ꢁ5 mol Lꢁ1
.
Performed with carboxylic acid (7.8 ꢂ 10ꢁ2 mol Lꢁ1) and amine (7.1 ꢂ 10ꢁ2 mol Lꢁ1) in CH2Cl2 (7.0 mL), the concentration of
j
boronic acids in the reaction system of ca. 7.1 ꢂ 10ꢁ3 mol Lꢁ1, and adding the activated 4A molecular sieves (1.0 g).
PBA@PM upon bubbling with N2 gas (k ¼ 2.77 ꢂ 10ꢁ3 minꢁ1 the model aza-Michael addition; in both these two reactions
and 4.72 ꢂ 10ꢁ3 minꢁ1 for the cases with [Glu] ¼ 0.0 mM and reported previously in the literature, organoboron acids are
[Glu] ¼ 1.5 mM, respectively) is much more active than that thought to induce nucleophilic activation of carbonyl
upon bubbling with CO2 gas (correspondingly, k ¼ 1.29 ꢂ compounds (herein 4 and hydroxyacetone).4,7,8,11 In contrast, for
10ꢁ3 minꢁ1 and 1.25 ꢂ 10ꢁ3 minꢁ1), similar to the results for the model amidation reaction and the model [4
+ 2]
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cycloaddition, organoboron acids are reported to induce elec- glucose, making the catalytic activity comparable or even
trophilic activation of carboxylic acids.4,9 It is found that the superior to that of the corresponding free boronic acid mono-
PBA@PM upon bubbling with CO2 gas could provide much mer 1 homogeneously dispersed in solutions and, more
faster rates (for amidation and cycloaddition, k ¼ (4.17 ꢃ 0.06) importantly, making the PBA@PM able to catalyze alternate
ꢂ 10ꢁ4 minꢁ1 and (3.56 ꢃ 0.33) ꢂ 10ꢁ4 minꢁ1, respectively) over reactions that may extend the usefulness. Based on the studies,
upon bubbling with N2 gas (correspondingly, k ¼ 2.61 ꢂ this enhanced catalytic function of the PBA@PM is plausibly
10ꢁ5 minꢁ1 at [Glu] ¼ 0.0 mM and 1.68 ꢂ 10ꢁ5 minꢁ1 at [Glu] ¼ achieved via in situ reversibly structural variations. With
1.5 mM, and k ¼ 4.89 ꢂ 10ꢁ5 minꢁ1 at [Glu] ¼ 0.0 mM and 2.63 signicant advances in the development of the stimuli-
ꢂ 10ꢁ5 minꢁ1 at [Glu] ¼ 1.5 mM). These are particularly exciting responsive polymer microgels containing boronic acids, we
results as they suggest the possibility of tuning catalysis of the anticipate that our results may underscore the vast potential of
boronic acids in microgels by bubbling with N2/CO2 gas, which developing high-performance organoboron acid catalysts based
can potentially be applied to other chemical transformations, on boronic-acid-containing polymer microgels in a wide range
and the ability of the PBA@PM to catalyze alternate reactions of elds.
that may differ in the functional group and/or the catalytic
mechanism involved.
In addition, the control experiments were conducted on the
four model reactions with free organoboron acids 1 under
4. Experimental section
4.1. Materials
otherwise the same reaction conditions, and k was estimated to
be (6.58 ꢃ 0.05) ꢂ 10ꢁ5 minꢁ1, (6.01 ꢃ 0.04) ꢂ 10ꢁ4 minꢁ1, (5.04
ꢃ 0.07) ꢂ 10ꢁ6 minꢁ1, and (2.58 ꢃ 0.21) ꢂ 10ꢁ5 minꢁ1 (Fig. S26
in ESI†), respectively, for those model aza-Michael addition,
aldol, amidation, and [4 + 2] cycloaddition reactions; however,
no signicant difference on k was observed upon bubbling with
N2 and CO2 gases, either in the absence or in the presence of 2,
due to the negligible change in PBA groups of 1 (Fig. 3). Clearly,
the corresponding free boronic acid monomer 1, more or less,
displayed smaller k values than the PBA@PM upon bubbling
with N2/CO2 gas, indicating a considerable high catalytic activity
of the boronic acids in microgels comparable or even superior
to that of the corresponding boronic acid monomer homoge-
neously dispersed in solutions. In particular, while the corre-
sponding free boronic acid monomer 1 might be barely thought
to be useful for the model aldol reaction, the PBA@PM can
catalyze all four model reactions by tuning catalysis via
bubbling with N2/CO2 gas, and in some cases also adding
glucose. Further in comparison with those organoboron acid
catalysts reported previously in the literature, the PBA@PM still
indicates considerable high catalytic activity, as summarized in
Table 2. Several reactants were also applied without difficulty to
give the desired products, upon properly adjusting the reaction
conditions by bubbling with N2/CO2 gas and in some cases
adding glucose, making the yields comparable or even superior
to the corresponding results with the reactions catalyzed by
organoboron acid catalysts reported previously. This enhanced
catalytic function of the PBA@PM may extend their usefulness
in synthesis.
The chemicals (3,5-diaminophenyl)boronic acid and 33 wt%
HBr acetic acid solution were purchased from 1ClickChemistry
Inc. (USA) and Meryer Co., Ltd (China), respectively. Acryloyl
chloride, anthracene, paraformaldehyde, hexamethylenetetra-
mine, ethylene bromohydrin, benzoyl chloride, 2-methyl-4-
nitrophenol, iodobenzene diacetate (PhI(OAc)2), triethylamine,
2,20-azobis(2-methylpropionamidine)dihydrochloride (AAPH),
sodium dodecyl sulfate (SDS), and other chemicals were
purchased from Aldrich. All chemicals were used directly as
received without further purication. The water used in all
experiments was of Millipore Milli-Q grade.
4.2. Synthesis of 1
(3,5-Diaminophenyl)boronic acid (1.000 g) was dissolved in
a 5.0 wt% NaOH solution (20.00 mL) in an ice bath. Aer
10 min, acryloyl chloride (2.15 mL) was added dropwise into the
above-mentioned solution, followed by stirring for 3 h at room
temperature. Aer the reaction, the pH value was adjusted to ca.
1 with a 2 M HCl solution. The yielded 1 was collected, and then
washed with water to yield a white solid. 1H NMR (500 MHz, in
DMSO-d6; Fig. S1†): d ¼ 10.10 (2H, NH), 8.22 (1H, ArH), 7.99
(2H, B(OH)2), 7.67 (2H, ArH), 6.47 (2H, CH]CH2), 6.25 (2H,
CH]CH2), 5.73 (2H, CH]CH2); 13C NMR (500 MHz, in DMSO-
d6; Fig. S1†): d ¼ 113.14, 120.90, 126.53, 132.05, 135.51, 138.42,
163.10; FT-MS (Fig. S1†): m/z 283.09 [M + Na+].
4.3. Synthesis of 2
First, anthracene (10.000 g) and paraformaldehyde (3.400 g)
were dissolved in glacial acetic acid (40.00 mL). Then, a 33 wt%
HBr acetic acid solution (55.00 mL) was added dropwise (which
3. Conclusion
ꢀ
took ca. 1 h). The mixture was heated to 80 C and allowed to
We have devised a class of organoboron acid catalysts PBA@PM react overnight. The yielded 9,10-bis(bromomethyl)anthracene
by immobilizing PBA in microgels. With those boronic-acid- was washed with water and further puried by recrystallization
catalyzed aza-Michael addition, aldol, amidation, and [4 + 2] in methylbenzene. Second, 9,10-bis(bromomethyl)anthracene
cycloaddition reactions in CH2Cl2/water (9 : 1 in volume) as the (1.000 g) and hexamethylenetetramine (1.000 g) were dissolved
model reactions to evaluate the accessibility and catalytic in chloroform (150.00 mL) and reuxed for 48 h. Aer cooling to
performance of the PBA@PM, we have demonstrated the room temperature, the precipitate was collected, dried in air,
feasibility of tuning catalysis of boronic acids in microgels by and added into ethanol (130.00 mL) and concentrated HCl
simply bubbling with N2/CO2 gas, and in some cases adding (25.00 mL), and reuxed for another 48 h. Aer cooling to ca.
3742 | RSC Adv., 2020, 10, 3734–3744
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ꢀ
0 C, the precipitate was collected, washed with cold ethanol, PBA@PM (the concentration of boronic acids in reaction
extracted with chloroform/water, and dried to get anthracene- systems was ca. 1.2 ꢂ 10ꢁ5 mol Lꢁ1) as an organoboron acid
ꢀ
9,10-diyldimethanamine.
Finally,
anthracene-9,10- catalyst in CH2Cl2/water (9 : 1 in volume; 100.0 mL) at 10.0 C.
diyldimethanamine (0.600 g) was rst dissolved in dichloro- The model aldol reaction of hydroxyacetone (1-hydroxypropan-
methane (40.00 mL) in a 100 mL round-bottomed ask, and 2-one; 1.0 ꢂ 10ꢁ4 mol Lꢁ1) and aldehyde (cinnamaldehyde;
then anhydrous triethylamine (1.06 mL) was added to the bottle 2.5 ꢂ 10ꢁ3 mol Lꢁ1) yielded aldol adduct 7, i.e., (E)-3,4-
in an ice-bath. Then, acryloyl chloride (0.690 g) was dissolved in dihydroxy-6-phenylhex-5-en-2-one (1H NMR (in CDCl3): d ¼
anhydrous dichloromethane, and added into the ask drop- 7.33–7.15 (5H), 6.62 (1H), 6.26 (1H), 4.61 (1H), 4.18 (1H), 3.74
wise. Aer that, the reaction was proceeded overnight at room (1H), 2.47 (1H), 2.25 (3H); 13C NMR (in CDCl3): d ¼ 207.5, 136.2,
temperature. The yielded 2 was collected, washed with water, 132.5, 128.5, 128.1, 127.6, 126.9, 80.2, 72.8, 26.2; Fig. S23†).7,42
and dried in the vacuum. 1H NMR (500 MHz, in DMSO-d6; The model amidation reaction of carboxylic acid (2-phenylacetic
Fig. S2†): d ¼ 8.56 (2H, NH), 8.48 (4H, ArH), 7.63 (2H, ArH), acid; 1.0 ꢂ 10ꢁ4 mol Lꢁ1) and amine (phenylmethanamine; 2.5
6.21 (4H, ArCH2), 5.59 (2H, CH]CH2), 5.40 (4H, CH]CH2); FT- ꢂ 10ꢁ3 mol Lꢁ1) yielded amide product 8, i.e., N-benzyl-2-
MS (Fig. S2†): m/z 367.14 [M + Na+].
phenylacetamide (1H NMR (in CDCl3): d ¼ 7.33–7.15 (10H),
6.02 (1H), 4.37 (2H), 3.59 (2H); 13C NMR (in CDCl3): d ¼ 169.8,
137.2, 134.6, 129.3, 128.9, 128.5, 127.4, 127.3, 127.2, 43.6, 43.4;
Fig. S24†).9,43 The model [4 + 2] cycloaddition between acrylic
acid (2-bromoacrylic acid; 1.0 ꢂ 10ꢁ4 mol Lꢁ1) and diene (2,3-
dimethylbuta-1,3-diene; 2.5 ꢂ 10ꢁ3 mol Lꢁ1) yielded cyclo-
adduct 9, i.e., 1-bromo-3,4-dimethylcyclohex-3-ene-1-carboxylic
acid (1H NMR (in CDCl3): d ¼ 11.62 (1H), 2.85 (1H), 2.68 (1H),
2.27 (2H), 2.18 (2H), 1.63 (6H); 13C NMR (in CDCl3): d ¼ 177.3,
125.0, 122.7, 59.2, 43.0, 34.1, 30.3, 19.0, 18.6; Fig. S25†).9
The compounds 6-a,11 6-b,11 7-a,7 7-b,7 8-a9 and 8-b9 were
prepared using the corresponding procedure described above.
The gas bubbling (if any) was continued throughout the reac-
tion, and glucose (if any) was dissolved in the medium before
adding the reactants.
4.4. Synthesis of the PBA@PM
SDS (0.017 g), 1 (0.016 g) and 2 (0.103 g) were dissolved in water
(50.00 mL) in a 100 mL three-necked bottle, and stirred for
ꢀ
30 min at 70 C in a nitrogen atmosphere. AAPH (8.0 mg) was
dissolved in water (2.94 mL), and added immediately into the
bottle dropwise to initiate the polymerization. Aer proceeding
for 5 h, the product was puried by centrifugation (Thermo
Electron Co. SORVALL® RC-6 PLUS super speed centrifuge) and
redispersed in water, followed by 3 days of dialysis (Spectra/
Por® molecular porous membrane tubing, cutoff 12 000–
14 000) against water (replaced water every half day). The
concentration of the microgels can be determined by a weighing
method via drying a certain volume of the dispersion.
4.6. Characterization
4.5. Catalytic experiments
TEM images were obtained using a JEOL JEM-1400 trans-
mission electron microscope at an accelerating voltage of 100
kV. Microgel dispersions were dropped on a carbon-coated
copper grid, which was quickly put into liquid nitrogen and
freeze-dried using a freeze dryer before TEM measurements. IR
spectra were recorded using a Thermo Electron Corporation
Nicolet 380 Fourier transform infrared spectrometer. NMR
spectra were recorded using a Bruker AVIII 500 MHz solution-
state NMR spectrometer. The content of PBA groups in the
microgels was determined by mannitol-assisted UV-vis spec-
trophotometric titration, where mannitol was added to convert
boronic acids into a relatively strong monobasic acid which was
then titrated with 0.2 M NaOH. A change in UV-vis spectra of
PBA at 235 nm was used in UV spectrophotometric titration
using a Shimadzu UV-2550 UV-vis spectrometer equipped with
The aza-Michael addition of 4 to 3, where 3 was synthesized
according to a method reported in the literature (1H NMR (in
CDCl3): d ¼ 7.91 (2H), 7.56 (1H), 7.44 (2H), 6.42 (1H), 6.31 (1H),
6.19 (1H), 4.17 (4H), 1.92 (3H); 13C NMR (in CDCl3): d ¼ 180.3,
155.6, 150.9, 139.4, 133.4, 133.1, 129.6, 128.7, 125.0, 124.1,
100.2, 66.4, 16.9; Fig. S13†),11,41 was chosen as a model catalytic
reaction. In a typical run, 3 (1.0 ꢂ 10ꢁ4 mmol), 4 (2.5 ꢂ 10ꢁ3
mmol), and the CH2Cl2/water (9 : 1 in volume; 100.0 mL)
mixture were added into a three-necked round ask equipped
with a condenser and a magnetic stirrer, and then the temper-
ature was adjusted to 10.0 ꢀC by using a low-temperature
constant-temperature stirring reaction bath. Aer stirring for
30 min, the PBA@PM was added (the concentration of boronic
acids in the reaction system was ca. 1.2 ꢂ 10ꢁ5 mol Lꢁ1). This
model reaction yielded product 6, i.e., N-((1R,5S)-7-benzoyl-5-
methyl-6-oxa-7-azaspiro[bicyclo[3.2.1]octane-8,20-[1,3]dioxolan]-
3-en-3-yl)benzamide (1H NMR (in CDCl3): d ¼ 7.80–7.74 (4H),
7.66 (1H), 7.48–7.30 (6H), 6.52 (1H), 4.55 (1H), 4.13–4.03 (4H),
3.20 (1H), 3.01 (1H), 1.38 (3H); 13C NMR (in CDCl3): d ¼ 166.2,
165.6, 136.5, 134.6, 132.8, 131.8, 130.8, 129.2, 128.3, 127.7,
127.1, 112.1, 109.7, 81.6, 66.8, 65.7, 57.7, 34.5, 14.6; Fig. S14†),
with a selectivity of nearly 100%. To investigate the yield of
product 6, the samples (50.0 mL) were collected at different
reaction times and puried for NMR and other analyses.
Those boronic-acid-catalyzed aldol, amidation, and [4 + 2]
a temperature controller (ꢃ0.1 C).28 DLS was performed using
ꢀ
a standard laser light scattering spectrometer (BI-200SM)
equipped with a BI-9000AT digital autocorrelator (Brookhaven
Instruments, Inc.). A Mini-L30 diode laser (30 mW, 637 nm) was
used as the light source. All samples were passed through Mil-
lipore Millex-HV lters with a pore size of 0.80 mm to remove
dust before DLS measurements.
Conflicts of interest
cycloaddition reactions were also conducted using the There are no conicts to declare.
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21 Z. Wu, X. Zhang, H. Guo, C. Li and D. Yu, J. Mater. Chem.,
2012, 22, 22788.
Acknowledgements
22 T. Zhou, Y. Guan and Y. J. Zhang, Polym. Chem., 2014, 5,
1782.
This work is supported by the National Natural Science Foun-
dation of China (21574107, 21774105, 21805164, 21274118, and
91227120), and the National Fund for Fostering Talents of Basic
Science (J1310024). The authors thank Xuezhen Lin for the help
in the catalytic tests.
23 M. M. Zhou, J. D. Xie, S. T. Yan, X. M. Jiang, T. Ye and
W. T. Wu, Macromolecules, 2014, 47, 6055.
24 Q. S. Wu, X. Du, A. P. Chang, X. M. Jiang, X. Y. Yan, X. Y. Cao,
Z. H. Farooqi and W. T. Wu, Polym. Chem., 2016, 7, 6500.
25 L. Liu, G. J. Ma, M. Zeng, W. Du and J. Y. Yuan, Chem.
Commun., 2018, 54, 12475.
References
¨
1 F. Jakle, Coord. Chem. Rev., 2006, 250, 1107.
`
26 D. H. Han, X. Tong, O. Boissiere and Y. Zhao, ACS Macro
´
2 E. Dimitrijevic and M. S. Taylor, ACS Catal., 2013, 3, 945.
Lett., 2012, 1, 57.
3 M. S. Taylor, Acc. Chem. Res., 2015, 48, 295.
4 D. G. Hall, Chem. Soc. Rev., 2019, 48, 3475.
5 Y. Mori, K. Manabe and S. Kobayashi, Angew. Chem., Int. Ed.,
2001, 40, 2815.
6 Y. Mori, J. Kobayashi, K. Manabe and S. Kobayashi,
Tetrahedron, 2002, 58, 8263.
`
27 D. H. Han, O. Boissiere, S. Kumar, X. Tong, L. Tremblay and
Y. Zhao, Macromolecules, 2012, 45, 7440.
28 S. Soundararajan, M. Badawi, C. M. Kohlrust and
J. H. Hageman, Anal. Biochem., 1989, 178, 125.
29 I. A. Fedorov, Y. N. Zhuravleva and V. P. Berveno, Phys. Chem.
Chem. Phys., 2011, 13, 5679.
7 K. Aelvoet, A. S. Batsanov, A. J. Blatch, C. Grosjean,
L. G. F. Patrick, C. A. Smethurst and A. Whiting, Angew.
Chem., Int. Ed., 2008, 47, 768.
30 I. Hisamitsu, K. Kataoka, T. Okano and Y. Sakurai, Pharm.
Res., 1997, 14, 289.
31 M. M. Muscatello, L. E. Stunja and S. A. Asher, Anal. Chem.,
2009, 81, 4978.
´
8 K. Arnold, B. Davies, D. Herault and A. Whiting, Angew.
Chem., Int. Ed., 2008, 47, 2673.
9 R. M. Al-Zoubi, O. Marion and D. G. Hall, Angew. Chem., Int.
Ed., 2008, 47, 2876.
10 D. Lee, S. G. Newman and M. S. Taylor, Org. Lett., 2009, 11,
5486.
32 J. E. Baldwin, T. D. W. Claridge, A. E. Derome, C. J. Schoeld
and B. D. Smith, Bioorg. Med. Chem. Lett., 1991, 1, 9.
33 J. Boeseken, Carbohydr. Chem., 1947, 4, 189.
34 J. P. Lorand and J. O. Edwards, J. Org. Chem., 1959, 24, 769.
35 K. Kondo, Y. Shiomi, M. Saisho, T. Harada and S. Shinkai,
Tetrahedron, 1992, 48, 8239.
´
11 T. Hashimoto, A. O. Galvez and K. Maruoka, J. Am. Chem.
Soc., 2015, 137, 16016.
12 N. Levi, A. M. Khenkin, B. Hailegnaw and R. Neumann, ACS
Sustainable Chem. Eng., 2016, 4, 5799.
13 J. W. Liu, H. Q. Yao and C. Wang, ACS Catal., 2018, 8, 9376.
14 R. Ye, J. Zhao, B. B. Wickemeyer, F. D. Toste and
G. A. Somorjai, Nat. Catal., 2018, 1, 318.
36 S. Lakhdar, M. Baidya and H. Mayr, Chem. Commun., 2012,
48, 4504.
37 K. J. Laidler, The World of Physical Chemistry, Oxford
University Press, 1993.
38 Y. Lu, Y. Mei, M. Drechsler and M. Ballauff, Angew. Chem.,
Int. Ed., 2006, 45, 813.
¨
15 E. M. Vogl, H. Groger and M. Shibasaki, Angew. Chem., Int.
´
39 S. Carregal-Romero, N. J. Buurma, J. Perez-Juste, L. M. Liz-
Ed., 1999, 38, 1570.
´
´
Marzan and P. Herves, Chem. Mater., 2010, 22, 3051.
40 R. Roaa, S. Angioletti-Uberti, Y. Lu, J. Dzubiella, F. Piazza
and M. Ballauff, Z. Phys. Chem., 2018, 232, 773.
41 T. Hashimoto, H. Nakatsu and K. Maruoka, Angew. Chem.,
Int. Ed., 2015, 54, 4617.
16 E. Wolf, E. Richmond and J. Moran, Chem. Sci., 2015, 6, 2501.
17 U. T. Bornscheuer and R. J. Kazlauskas, Angew. Chem., Int.
Ed., 2004, 43, 6032.
18 Y. J. Zhang, Y. Guan and S. Q. Zhou, Biomacromolecules,
2006, 7, 3196.
42 Y. Zhang and G. A. O'Doherty, Tetrahedron, 2005, 61, 6337.
43 W.-K. Chan, C.-M. Ho, M.-K. Wong and C.-M. Che, J. Am.
Chem. Soc., 2006, 128, 14796.
19 V. Lapeyre, I. Gosse, S. Chevreux and V. Ravaine,
Biomacromolecules, 2006, 7, 3356.
20 T. Hoare and R. Pelton, Macromolecules, 2007, 40, 670.
3744 | RSC Adv., 2020, 10, 3734–3744
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