Organic sponge photocatalysis

Article abstract of DOI:10.1039/c6gc03558b

The successfully developed polydimethylsiloxane (PDMS, a green and highly transparent polymer material) sponge photocatalyst can catalyze cross-dehydrogenative coupling (CDC) of tertiary amines and various nucleophiles with high efficiency and reusability under visible light irradiation. Through an easy-to-build continuous flow reactor, the sponge photocatalytic reaction can be facilely scaled up.

Full text of DOI:10.1039/c6gc03558b

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Polydimethylsiloxane organic sponge photocatalyst: high
efficiency and reusability for visible light mediated cross-
dehydrogenative coupling reactions†
Received 00th January 20xx,
Accepted 00th January 20xx
Xingrong Li,‡ Yaoyao Li,‡ Yuxing Huang, Teng Zhang, Yizhen Liu, Bo Yang, Chuanxin He, Xuechang
Zhou and Junmin Zhang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The successfully developed polydimethylsiloxane (PDMS, a green
and highly transparent polymer material) sponge photocatalyst
can catalyze cross-dehydrogenative coupling (CDC) of tertiary
amines and various nucleophiles with high efficiency and
reusability under visible light irradiation. Through an easy-to-build
continuous flow reactor, the sponge photocatalytic reaction can
be facilely scaled up.
Si
O
OH
OH
O
O
OH
air plasma
silanization
PDMS sponge
O
Visible-light photocatalysis,1,2 which only needs sunlight or
household lamps for irradiation, has attracted great interest in
recent years based on the purpose of developing novel, green
and sustainable synthetic methods under mild reaction
conditions. Both transition metal and organic dye based
photocatalysts have been successfully applied in various
organic transformations, such as Ru, Ir complexes, as well as
Rose Bengal, eosin Y, etc.1,2 Since most of the reactions were
carried out in homogeneous systems, therefore, it is difficult to
recycle the catalysts for reusing. Clearly, a straightforward
solution for this challenge is the immobilization of
photocatalysts onto insoluble solid-state materials, resulting in
a heterogeneous catalyst. In this regard, several types of
materials, such as silicas,3 polymers,4 metal-organic
frameworks (MOFs)5 and graphene6 have been used as
support for this purpose. However conventional filtration or
centrifugation is required for the recycle of these kinds of
immobilized catalysts as they are generally small solid
particles. The extra separation process greatly limited their
reusability and practical application in industry.
free radical
polymerization
O
(METAC)
N
Cl
Cl
n O
Cl
Cl
Cl
Si
O
n O
CO2Na
I
O
O
O
I
Si
O
O
N
O
O
NaO
O
O
I
I
N
Cl
Rose bengal (RB)
- NaCl
PDMS-RB sponge
Scheme 1 Preparation of PDMS-RB sponge photocatalyst.
catalyst could be very easily separated for reusing from the
reaction system. To achieve this purpose, the material of
sponge support should be highly transparent to let the light
smoothly
penetrate
for
the
photocatalysis.
Polydimethylsiloxane (PDMS) is a green and highly transparent
polymer material which can be used for the fabrication of
contact lenses.8
Therefore we chose PDMS for making the sponge support.
Using a conventional sugar-templating method, PDMS sponge
can be easily prepared.9 By anchoring organic photocatalyst on
this polymer sponge, we assuming this novel heterogeneous
catalyst can gain advantages in separation comparing to the
reported immobilized organic photocatalysts. Instead of
conventional filtration or centrifugation, we can separate the
catalyst from the reaction system by simply grabbing the
PDMS sponge out of the reaction mixtures. A challenge for
immobilizing organic photocatalyst is the chemical inertness of
PDMS material. In this study, we used the technology of
surface modification (air plasma treatment) for introducing
hydroxyl group on the surface of PDMS and consequently
In order to make immobilized photocatalyst more
convenient for separation and more practical for industrial
application, we envisaged that sponge as catalyst carrier would
be a good candidate,7 as the ideal catalytic process will be
sucking the reactants into the sponge catalyst and squeezing
out the products after the reaction, and obviously the sponge
College of Chemistry and Environmental Engineering, Shenzhen University,
Shenzhen, 518060, P. R. China. E-mail: zhangjm@szu.edu.cn
† Electronic Supplementary Information (ESI) available: Detailed experimental
procedures and characterization data. See DOI: 10.1039/x0xx00000x
‡
These authors contributed equally.
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connecting of polymer brush to successfully solve the the corresponding PDMS-RB sponge catalyst, which was
problem.10 Herein we reported the ionic immobilization of analyzed by scanning electron microscopy (SEM) and
Rose Bengal (RB) onto post modified PDMS sponge by ionic quantitative energy-dispersive X-ray spectroscopy (EDS). The
change (Scheme 1). The obtained supported catalyst (PDMS- morphology of PDMS-RB shown by SEM implies a porous,
RB sponge) has been proven to be an excellent visible-light interconnected three-dimensional framework (Fig. 1a). EDS
photocatalyst in cross-dehydrogenative coupling (CDC) was employed for quantitative analysis. The results shown in
reactions. The sponge photocatalyst can be easily recovered Fig. 1b suggests that the elements C, N, O, Si, Cl and I are
and demonstrates high efficiency in recycling experiments.
homogeneously spread on the surface of PDMS-RB sponge.
1
As illustrated in Scheme 1, after air plasma treatment, PDMS The amount of RB on PDMS sponge was determined by H
sponges prepared by using a conventional sugar-templating NMR using 1,3,5-trimethoxybenzene as internal standard (for
method were first functionalized with vinyltrimethoxysilane details, see SI).
(VTMS) by a silanization process, followed by a typical free
radical polymerization step with [2-(methacryloyloxy) ethyl- attractive efficient and green strategy by making carbon-
trimethylammonium-chloride] (METAC). The as-made carbon bond directly from two different C-H bonds under
Cross-dehydrogenative coupling (CDC) reaction11,12 is an
PMETAC-PDMS sponges were then mixed with Rose Bengal oxidative conditions. To test the catalytic activity of obtained
(RB) disodium salt in EtOH/H2O (1:1) for ionic change to afford PDMS-RB sponge photocatalyst, the CDC reaction between N-
phenyl tetrahydroisoquinoline and nitromethane was chose as
model reaction for the investigation. As described in former
reports, RB could generate the excited species RB* under
visible light irradiation. RB* together with O2 in the air can
convert the tertiary amine substrate into iminium ion, which is
vulnerable to the nucleophilic attack by nucleophiles.12 Initially
different solvents were screened and the results are shown in
Table 1 Optimization of reaction conditions for the PDMS-
RB sponge catalyzed CDC reaction.[a]
Entry
1
Solvent
CH3CN
DMF
Volume (mL)
Yield (%)[b]
4
4
4
4
4
4
4
2
2
2
2
2
42
2
45
3
DMSO
DCM
39
4
35
5
THF
trace
6
Toluene
EtOH
18
7
68
8
EtOH
97 (93[c])/0[d]/0[e]
9
CH3OH
i-PrOH
t-BuOH
H2O
84
10
11
12
75
39
67/73[f]
[a] Reaction conditions: tertiary amine (0.1 mmol),
nitromethane (1.0 mmol), 2 mol% PDMS-RB with different
solvents, the reaction was performed in air at room
temperature with 12 W green LED light belt for 24 hours. [b]
Yield determined by H NMR with 1,3,5-trimethoxybenzene
as internal standard. [c] Isolated yield. [d] Without light. [e]
Without catalyst. [f] H2O and EtOH mixture (V/V=1/1).
1
Fig. 1 (a) SEM image of PDMS-RB sponge and (b)
corresponding quantitative EDS mapping of C, N, O, Si, Cl,
I.
2 | J. Name., 2012, 00, 1-3
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Table 3 PDMS-RB sponge catalyzed dehydrogentavie-
Mannich reaction of tertiary amines and acetone.[a]
Table 2 PDMS-RB sponge catalyzed CDC reaction of
tertiary amines and nitroalkanes.[a]
[a] Reaction conditions: tertiary amine (0.1 mmol),
nitromethane (1.0 mmol), 2 mol% PDMS-RB with EtOH as
solvent, the reaction was performed at room temperature
with 12 W green LED light belt for 24 hours. The yields are
isolated yields.
[a] Reaction conditions: tertiary amine (0.1 mmol), 2
mol% PDMS-RB, and 30 mol% pyrrolidine/TFA in 2 mL
acetone, the reaction was performed at room
temperature with 12 W green LED light belt for 24 hours.
The yields are isolated yields.
Table 1. Interestingly, when the reaction was performed in
CH3CN only moderate yield was obtained (Table 1, entry 1),
however excellent yields were reported with similar conditions conducted in pure ethanol. Therefore, we sticked to ethanol as
in homogeneous RB catalytic systems.13 When using DMF, the optimized solvent for further study.
DMSO, and DCM as solvent, moderate yields were also
With the optimized condition, we screened other substrates
obtained (Table 1, entries 2 to 4). The reaction failed when for this photocatalytic CDC reaction to demonstrate the broad
performed in THF, and afforded quite low yield in toluene usefulness of the sponge catalyst. As summarized in Table 2, a
(Table 1, entries 5 and 6). Finally we were delight to find out series of substituted tetrahydroquinoline derivatives were able
that EtOH, a greener solvent, performed best in the reaction to couple with nitroalkanes efficiently by affording the desired
with 68% NMR yield (Table 1, entry 7). However, the moderate
Table 4 PDMS-RB sponge catalyzed CDC of tertiary amines
and ethyl diazoacetate.[a]
yield was not satisfactory and we suspected the slow diffusion
of reactants into the sponge might be the reason of the poor
catalytic performance. Therefore we reduced the volume of
the reaction system from 4 mL to 2 mL to increase the
concentration of reactants, so that more reactants could get
into the pore of the sponge to have contact with the catalytic
site. Gratifyingly, we found out that the yield increased to 97%
after reducing the reaction system volume (Table 1, entry 8).
The performance of PDMS-RB is in CDC reaction between N-
phenyl tetrahydroisoquinoline and nitromethane in ethanol is
comparable with noble metal based photocatalysts, such as Ir
based catalytic systems.11 Both the visible light from the LED
belt and the catalyst are essential for the reaction as no
conversion was observed when the reaction was conducted
either in dark environment or without the catalyst (Table 1,
entry 8). Other alcohols as solvent were also tested, the results
showed a trend of primary alcohol > secondary alcohol >
tertiary alcohol (Table 1, entries 8-11) and EtOH was still the
optimal choice of solvents. To find
a more greener
[a] Reaction conditions: tertiary amine (0.1 mmol), ethyl
diazoacetate (1.0 mmol), 2 mol% PDMS-RB sponge in 1 mL
DCM, the reaction was performed at room temperature
under O2 with 12 W green LED light belt for 24 hours. The
yields are isolated yields.
replacement for the solvent, H2O and the mixture of H2O and
ethanol were tested in the reaction. The yields are good (Table
1, entry 12), but not comparable with the yield of the reaction
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Together with the aforementioned reactions, the overall
results indicate that PDMS-RB sponge has a wide range of
application in organic photocatalysis.
In the meantime, the recyclability of PDMS-RB sponge as a
heterogeneous photocatalyst was also investigated. The model
reaction of N-phenyl tetrahydroisoquinoline and nitromethane
was chose for the investigation. The catalyst can be convenient
removed from the reaction system by simple grabbing it out
from the solution. It can be reused 15 times without significant
loss of the catalytic activity, and the results are shown in Fig. 2.
After 15 cycles, a small yield reduction was observed. This
might due to the leaching of catalyst as it is anchored on the
support by non-covalent interaction. It should be pointed out
that the PDMS-RB sponge kept intact after 15 times recycling,
which shows the good stability of such catalyst in ethanol. This
can be attributed to the swelling property of PDMS in
ethanol.17
Fig. 2 Recycling experiments of PDMS-RB sponge catalyst
for the CDC reaction of N-phenyl tetrahydroisoquinoline
and nitromethane.
To demonstrate the possible industrial application of PDMS-
RB sponge catalyst, a continuous flow reactor was built up (Fig.
3). A gram scale reaction of the model reaction between N-
phenyl tetrahydroisoquinoline and nitromethane was tested
and 88% of isolated yield was obtained after 48 h (the catalyst
loading is 1.4 mol%). This setup is quite convenient to build -
one only needs a peristaltic pump, a glass column and LED belt
to build such reactor (for details, see the SI). Therefore a
typical organic laboratory can afford such setup and perform
big scale synthesis using PDMS-RB sponge catalyst.
coupling products in good to excellent yields, which were
comparable to literature reports.13 The electronic effect is not
prominent for the substitution groups on N-phenyl ring as the
yields for electron rich and electron poor N-phenyl ring were
quite close (Table 2, 3b-3d). The lower yields for nitropropane
and nitroethane can be attributed to the steric effect of the
nucleophiles (Table 2, 3e and 3f).14
The dehydrogenative-Mannich reaction is possibly more
superior than the conventional Mannich reactions for saving
the extra step of preparation of the imine, as the reactive
iminium intermediate is formed in situ for this method.15 RB
was reported to be an effective catalyst for this kind of
reaction under visible-light irradiation.13 To verify that the
heterogeneous PDMS-RB sponge catalyst still can work
effectively in this kind of reaction, several substituted
tetrahydroquinoline derivatives were chose for the reaction
and good to excellent yields were obtained as shown in Table
3. In this type of reaction, electron deficient
tetrahydroquinoline derivatives gave the best results (Table 3,
entries 5d and 5e).
In conclusion, by adopting the technology of surface
modification and polymer brushes synthesis of polymer
material science into organic photocatalysis, we have
successfully developed a facile and efficient protocol for
preparing a novel organic sponge photocatalyst by anchoring
rose bengal dye on PDMS sponge. This new sponge
photocatalyst reveals excellent photocatalytic activity in cross-
dehydrogenative coupling reactions under visible light
irradiation. The sponge photocatalyst could be readily recycled
and reused without significant loss of catalytic activity for
fifteen cycles. The photocatalytic reaction could be facilely
scaled up through an easy-to-build continuous flow reactor.
These results demonstrate that PDMS sponge is a promising
support for the immobilization of organic photocatalysts. The
study of immobilizing other photocatalysts (including
multifunctional photocatalysts) on PDMS sponge and the
related more interesting transformations is currently
undergoing in our lab and will be reported in due course.
To further explore the applications of PDMS-RB sponge
catalyst in organic synthesis reactions, a series CDC reaction of
tertiary amines with ethyl diazoacetate were performed.16 The
good to excellent yields were obtained as shown in Table 4.
Acknowledgements
We thank the generous financial supports from National
Natural Science Foundation of China (21402124, 21404072 and
21605104), Training Foundation for Outstanding Young
Teachers in Higher Education Institutions of Guangdong
(YQ2015145), Shenzhen Science and Technology Foundation
(JCYJ20150324141711702,
JCYJ20150324140036849
and
Fig. 3 Schematic illustration of the continuous flow
reactor and the picture of the setup, C stands for a glass
column filled with PDMS-RB sponge catalyst.
JCYJ20140418182819116), and the Natural Science Foundation
of SZU (827000038, 201446, 201447 and 201519). We also
appreciate the thoughtful suggestions from the referees.
4 | J. Name., 2012, 00, 1-3
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7
Sponge is a material commonly used in daily life, but its
exploration in catalysis has been rare. This concept of
‘Organic Sponge Catalysis’ is inspired by the very impressive
work ‘Organotextile Catalysis’ from the List group, see: J. W.
Lee, T. Mayer-Gall, K. Opwis, C. E. Song, J. S. Gutmann and B.
List, Science, 2013, 341, 1225.
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Green Chemistry
Page 6 of 6
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DOI: 10.1039/C6GC03558B
COMMUNICATION
Journal Name
Table of Contents
Cl
O
Organic Sponge
Photocatalysis
Cl
Cl
Cl
CO2Na
I
I
N
NaO
O
I
Ph
Ph
Green LEDs
3a
I
NO2
O
N
N
Si
O
Ph
PDMS-RB
sponge
1a
H
O
O
5a
O
O
H
(2 mol%)
+
Nu
N
2
PDMS-RB
sponge
N
up to 98% yield
Ph
after 15 cycles, 85% yield
EtO2C
N2
7a
by a flow reactor, 1g scale, 88% yield
Organic sponge photocatalysis: green and highly transparent catalyst carrier
(polydimethylsiloxane), pure organic photocatalyst, visible light source, high reusability of
catalyst, easy to scale up by a flow reactor and clean cross-dehydrogenative coupling
reactions; the ideal catalytic process is sucking the reactants into the sponge photocatalyst
and squeezing out the products after the reaction under visible light irradiation.
6 | J. Name., 2012, 00, 1-3
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