Acid-base bifunctional amphiphilic organic nanotubes as a catalyst for one-pot cascade reactions in water

Article abstract of DOI:10.1039/c7nj04209d

A novel acid-base bifunctional amphiphilic organic nanotube (acid-base-nanotube) containing 1-(2-(prop-2-yn-1-yloxy)ethyl)-1H-imidazole (PEI) groups as basic sites on the wall and benzenesulfonic acid (BSA) groups as acidic sites on the corona of nanotubes was successfully synthesized by a combination of molecular-template core-shell bottlebrush copolymers and click reaction. Owing to the hydrophobic cavity microenvironments, a hydrophilic corona, and site-isolation features of organic tubular structures, the resultant acid-base-nanotube showed high catalytic performance for one-pot deacetalization-Knoevenagel cascade reactions in water.

Full text of DOI:10.1039/c7nj04209d

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Acid-Base Bifunctional Amphiphilic Organic Nanotubes as Catalyst
for One-pot Cascade Reactions in Water
Received 00th January 20xx,
Accepted 00th January 20xx
Linfeng Xiongb, Hui Zhanga, Zidong Hea, Tianqi Wanga, Yang Xua, Minghong Zhoua and Kun Huanga*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel acid-base bifunctional amphiphilic organic nanotube a one-pot multistep22, 23. More recently, our group also
(Acid-Base-Nanotube) containing 1-(2-(prop-2-yn-1-yloxy)ethyl)- reported the synthesis of the SO3H- and NH2- bifunctionalized
1H- imidazole (PEI) groups as basic sites on the wall and microporous organic nanotube networks as the heterogeneous
benzenesulfonic acid (BSA) groups as acidic sites on the corona of catalysts for one-pot reactions24. Although these attempts lead
nanotubes was successfully synthesized by a combination of to encouraging results, the design and fabrication of the novel
molecular-template core-shell bottlebrush copolymers and “click acid-base bifunctional catalyst with high catalytic activity, easy
reaction”. Owing to hydrophobic cavity microenvironments, recyclable and environmental friendly features are still issues
hydrophilic corona and site-isolation feature of organic tubular that need to be addressed.
structure, the resultant Acid-Base-Nanotube showed
a
high
Molecular bottlebrush polymers are single graft copolymer
catalytic performance for one-pot deacetalization-Knoevenagel molecules with a well-defined cylindrical shape which is a
cascade reactions in water.
result of steric hindrance between the polymeric side chains
forcing the backbone to adopt nearly extended
a
conformation25-27. Applications of bottlebrush polymers in
catalyst28-30, drug delivery31, 32, stimuli-responsive coating33, 34
had shown effective performance. Recently, we described a
new strategy to synthesize the BSA or PEI single-functionalized
Bifunctional catalysts have received much attention due to
their potential application in efficiently catalyzing a variety of
reactions, such as one-pot cascade reactions1-6
, transfer
hydrogenation reactions7, nucleophilic addition reactions8, CO2
10
conversion9,
and so on. Compared to the traditional
organic nanotube catalysts based
on single-molecular
catalysts, the bifunctional catalysts showed more advantages
in solving environment concerns because of step-saving, low-
cost and decrease the number of purifications, which
represents the development trend of modern organic
synthesis11-13. As a typical example, the bifunctional catalysts
with acid and base sites were comprehensively reported and
many systems have been used to immobilize the incompatible
templating core-shell bottlebrush copolymers, in which the
site-isolated acid and base catalysts showed excellent catalytic
performance for a tandem reaction in one-pot29. The organic
nanotubes have many outstanding advantages than other
systems, such as superior dimensional control (including length
and pore diameter), tunable chemical compositions, open-
ended tubular structure and an accessible cavity for loading
multifarious functional molecules35-38
.
acid and base regents to avoid their direct contact, such as
15
linear polymers14,
,
micelles16, ionic liquids17, porous
Herein, we report a simpler route to synthesize acid-base
bifunctional amphipathic organic nanotubes (Acid-Base-
Nanotube) as a single homogeneous catalyst by combination
molecular-template core-shell bottlebrush copolymers and
“click reaction”. Through the rational designed strategy, the
benzenesulfonic acid (BSA) groups as acidic sites and imidazole
(PEI) groups as basic sites could be successfully introduced on
the corona and the wall of nanotubes, respectively. Our work
showed that, owing to hydrophobic cavity microenvironments,
hydrophilic corona and site-isolation feature of organic tubular
structures, the resultant Acid-Base-Nanotubes are stable and
robust, and also present excellent catalytic performance for
one-pot cascade reactions, especially in an environmental
friendly water solution.
materials18, 19, or particle-stabilized emulsions20. For example,
Tan and co-workers reported that the incompatible acid and
base catalysts can be combined in hyper-crosslinked polymers
to catalyze cascade reactions in one-pot21. Li and co-workers
also reported that the graphene oxide could be functionalized
as a bifunctional carbocatalyst with remarkable acceleration in
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technique have efficient reinitiation and the formation of well-
defined copolymers. Except for the final precursor polymer, all
the copolymers had low polydispersity (Mw/Mn<1.2). The
molecular weight of P(GM-g-LA-g-VBC/BS-g-PVBS/NIPAAM)
was too enormous (2.0 × 104 kg mol-1) to be analyzed by the
SEC test.
Result and discussion
The P(GM-g-LA-g-VBC/BS-g-PVBS/NIPAAM) precursor of
Acid-Base-Nanotube were successfully synthesized by
a
combination of living radical and ring-opening polymerizations
according to the previous reported procedure (Scheme S1)29.
In order to introduce the acidic sites and enhance the site-
isolated effect, phenyl 4-vinylbenzene sulfonate (PVBS) and N-
isopropyl-acrylamide (NIPAAM) were randomly grafted from
the P(GM-g-LA-g-VBC/BS) by a “grafting-from” method. The
number of PVBS (20) and PNIPAAM (300) were calculated by
comparing the characteristic signals of 1H NMR at 7.8 ppm
from PVBS and 4.0 ppm from PNIPAAM to the methylene of 4-
vinylbenzyl chloride (VBC) at 4.5 ppm (Fig. S1 and S2).
Subsequently, the P(GM-g-LA-g-VBC/BS-g-PVBS/NIPAAM)
precursors were transformed into the nanotube by
intramolecular crosslinking of 4-(3-butenyl)styrene (BS) using
Grubbs’ I as catalyst and following removal of PLA at basic
conditions (Scheme 1). At the same time, the 4-vinylbenzene
sulfonate (PVBS) units could be hydrolyzed to form sulfonic
acid sodium salt. The chlorine on VBC units were then
Fig. 1 TEM image of Acid-Base-Nanotube (Scale bar = 100 nm)
o
converted into azide groups (N3) in DMF at 50 C and an azide
FT-IR was applied to confirm the BSA and PEI attached onto
the corona shell and nanotube walls. In Fig. S4 (b), we clearly
observed that the PLA carbonyl group vibration at 1759 cm-1
disappeared and a new peak at 2098 cm-1 appeared, which
proved that the PLA in the core was removed and the azide
group was successfully formed on the nanotube walls. In Fig.
S4 (c), the peaks at 1008 cm-1 and 1128 cm-1 for the bending
vibrations of the para substituted sulfonated aromatic ring and
the peaks at 1036 cm-1 and 1175 cm-1 for the symmetric and
asymmetric stretching vibrations of the sulfonated groups are
observed, confirmed that the BSA group was successfully
attached on the shell35. In addition, the peak at 2098 cm-1 from
the azide group disappeared. This showed that the PEI can be
attached onto the wall of nanotubes through efficient copper
(I) catalyzed alkyne-azide cycloaddition (CuAAC) reaction. The
morphology of Acid-Base-Nanotube was also characterized by
functionalized nanotubes (N3-Nanotube) can be obtained.
Next, the basic sites 1-(2-(prop-2-yn-1-yloxy)ethyl)-1H-
imidazole (PEI) could be anchored into the wall of nanotubes
by reacting with the N3-Nanotube via copper-catalyzed azide-
alkyne cycloaddition (CuAAC) reaction. Finally, the acidic sites
were produced through an ion exchange reaction in an acidic
aqueous solution by dialysis using
a
semipermeable
membrane. The resulting acid-base bifunctional amphipathic
nanotubes with multisite catalytic active center and strong
organic molecular binding ability could be completely
dissolved in water. To characterize the molecular weight
distributions of the bottlebrush copolymers, size exclusion
chromatography (SEC) was utilized to monitor the synthesis
procedure. As shown in Fig. S3, the copolymers exhibited
monomodal molecular weight distributions, indicated the
grafting-from method and ring-opening polymerization
2 | J. Name., 2012, 00, 1-3
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transmission electron microscopy (TEM). Fig. 1 showed that that of dichloromethane. The above results indicated that the
the small channels running through almost every nanotube obtained Acid-Base-Nanotube possessed an amphiphilic
could be clearly observed. The length and pore diameter of the feature with a hydrophilic corona and an inner hydrophobic
Acid-Base-Nanotube were about 37 ± 4 nm and 4 ± 2 nm, cavity.
respectively. The above results are consistent with our
previous report. Moreover, the well-kept rigid morphology of
nanotubes will ensure the multisite catalytic active center for
highly efficient binding the small organic molecules in water.
Fig. 3 Fluorescence experiments with solvatochromic dye Nile
Red in MeOH (a = 642 nm), DCM (c = 598 nm), Ethyl Ether (d =
561 nm) and 1 mg/ml Acid-Base-Nanotube aqueous solution (b
= 604 nm) (λexc = 515 nm).
Fig. 2 Substrate capture experiment (A) Acid-Base-Nanotube
Table 1. One-pot Deacetalization-Knoevenagel Reactiona
captured Nile Red in water, (B) Poly(PTSA-co-NIPAAM)
captured Nile Red in water, (C) Ethyl ether extracted Nile Red
from Acid-Base-Nanotube captured in water.
To further exam the organic molecular binding ability of the
Entry
Catalyst
Conversion
of A
Yield
of Bb
5%
Yield
of Cb
3%
Acid-Base-Nanotube in water, Nile Red (a hydrophobic
solvatochromic dye) was chosen as a model compound. As
shown in Fig. 2 A, the Nile Red molecules could be captured
smoothly by the Acid-Base-Nanotube, which might be related
to the inner hydrophobic environment of nanotubes. In order
to exclude the possibility of adsorption effect from the surface
of Acid-Base-Nanotube, poly(PTSA-co-NIPAAM) diblock
copolymers with a similar structure as the corona of Acid-Base-
Nanotube were synthesized and used to compare (Synthesis
procedure see supporting information and 1H NMR Spectrum
of P(PVBS-co-NIPAAM) precursors see Fig. S5). The adsorption
results showed that there is almost no adsorption for Nile Red
molecules for poly(PTSA-co-NIPAAM) (Fig. 2 B), which further
confirmed that the Nile Red molecules were indeed captured
by the voids of Acid-Base-Nanotube, and not adsorbed by the
outside corona. Especially, we found that the Nile Red
molecules captured in Acid-Base-Nanotube could be further
extracted by diethyl ether (Fig. 2 C), which also provided a
possible separation pathway for the next cascade reactions. To
further test the inner microenvironment of the Acid-Base-
Nanotube, we carried out the fluorescence experiments (Fig.
3). A strong emission at λmax = 604 nm was observed for the
Nile Red dyes when they were captured in Acid-Base-
Nanotube. This peak is much closed to the emission (λmax = 598
1
2
None
8%
Acid-Base-
Nanotube
100%
0
100%
3
4
Acid-Nanotube
Base-Nanotube
Acid-Base-
Nanotube + PTSA
Acid-Base-
100%
13%
92%
2%
8%
11%
5
6
95%
0
95%
0
0
0
Nanotube +HEIZ
a
Reaction conditions: A (0.13 mmol), D (1.3 mmol), H2O (2 ml),
about 10 mol% acid and 10 mol% base catalysts were used. The
reaction mixtures were runs for 24 h at RT. b Yields are based on GC
measurements (Fig. S6-S11).
Cooperative catalysis with acidic and basic sites is very
common in enzymatic catalytic process41,
42
. Reversible
dehydration condensation such as Knoevenagel condensation
in water is in principle a difficult transformation because the
large excess of water pushes the equilibrium in generating
ingredient43. Herein, the one-pot deacetalization-Knoevenagel
in water was selected as a model reaction in order to
demonstrate the concept of multisite catalyst and test the
organic molecular binding effect in water (Table 1). Reactions
with either free acid or base, and with Acid-Base-Nanotube
nm) of the Nile Red dyes in dichloromethane (Fig. 3 b and c)40
,
which suggested that the inner microenvironment of Acid-
Base-Nanotube has a remarkably low polarity comparable to
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catalysts were also performed as control experiments. The friendly and highly efficient nanomaterial.
deacetalization reaction is first catalyzed by the BSA groups on
the corona, followed by the Knoevenagel reaction catalyzed by
Acknowledgements
the PEI groups in the nanotube. As expected, the cascade
reaction can be efficiently catalyzed by Acid-Base-Nanotube
with high activity (100% conversion, entry 2). This indicated
that the acidic site on the corona and the basic site on the wall
of the Acid-Base-Nanotube maintained highly catalytic activity.
Meanwhile, we found that the single Acid-Nanotube can only
catalyze the first step reaction while the single Base-Nanotube
can catalyze the cascade reactions with little conversion (entry
3 and 4). These data implied the single Acid-nanotube or Base-
nanotube almost cannot catalyze the deacetalization-
Knoevenagel reactions in one pot. Moreover, no cascade
reactions occurred when the sequence was carried out only in
the presence of small-molecular catalysts N-(2-Hydroxyethyl)-
imidazole (HEIZ) or p-toluenesulfonic acid (PTSA) with Acid-
Base-Nanotube, respectively. As predicted, the acidic small
molecular catalysts PTSA could freely diffuse into the Acid-
Base-Nanotube and deactivate the basic groups. Similarly, HEIZ
reacted with the acidic groups on the corona rapidly (entry 5
and 6). These experimental results can be interpreted in terms
of the acid-base neutralization and the well mass transport
effect of the Aicd-Base-Nanotube. Furthermore, the Acid-Base-
Nanotube can catalyze the reaction 6 times without obvious
catalytic decrease (Fig. 4). All the results suggested that the
This work was financially supported by the National Natural
Science Foundation of China (51273066, 21574042).
Supported by large instruments Open Foundation of East
China Normal University.
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GraphicalAbstract
A novel acid-base bifunctional amphiphilic organic nanotubes is
synthesized and used for one-pot deacetalization-Knoevenagel cascade
reactions in water.