A photoswitchable organocatalyst based on a catalyst-imprinted polymer containing azobenzene

Article abstract of DOI:10.1039/c5ra10343f

An l-proline-catalyzed aldol reaction was photo-controlled using an l-proline-imprinted polymer containing azobenzene. Upon UV irradiation, azobenzene chromophores underwent trans → cis isomerization, thereby releasing l-proline to catalyze the aldol reac

Full text of DOI:10.1039/c5ra10343f

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A photoswitchable organocatalyst based on a
catalyst-imprinted polymer containing
azobenzene†
Cite this: RSC Adv., 2015, 5, 62539
Received 1st June 2015
Accepted 15th July 2015
a
a
a
a
Hua-dong Liu, An-xun Zheng, Cheng-bin Gong,* Xue-bing Ma, Michael Hon-Wah
b
c
ac
Lam, Cheuk-fai Chow* and Qian Tang*
DOI: 10.1039/c5ra10343f
www.rsc.org/advances
An L-proline-catalyzed aldol reaction was photo-controlled using an
Molecular imprinting is a technology for the preparation of
L-proline-imprinted polymer containing azobenzene. Upon UV irra- molecularly imprinted polymer (MIP) which possesses specic
diation, azobenzene chromophores underwent trans / cis isomeri- recognition sites with memory of the shape, size, and func-
22–25
zation, thereby releasing L-proline to catalyze the aldol reaction.
tionality of templates and its structural analogs.
The uptake
and release of the template in response to a light stimulus can
be induced by incorporating photoresponsive azobenzene into
Gated catalytic systems can translate an incoming stimulus into
a chemical signal. As such, these systems have been extensively
26–28
29
MIP matrices.
By utilizing substrate analogues, transition
30
31
state analogues, and product analogues as the templates,
MIP has been applied in catalysis successfully. However, to the
best of our knowledge, the report on MIP in catalysis utilizing
catalyst or catalyst analogues as the template is scarce. In the
present study, a photoswitchable organocatalyst based on MIP
containing azobenzene is developed using a catalyst as the
template (Fig. 1). In the “OFF state”, the catalyst is bound to
azobenzene functional monomer (in a trans form); therefore,
the catalyst cannot promote a reaction. Upon UV irradiation,
trans / cis isomerization occurs and triggers the “ON state”; in
the ON state, the catalyst is released and the reaction is
1
2,3
4
investigated. Different stimuli, such as anions, light, redox,
5
6
pH, and acid–base compounds, have been exploited to control
these catalytic systems. Among these stimuli, light likely
exhibits the most remarkable effect on the control of these
systems because light irradiation is nondestructive; light irra-
diation can also be controlled remotely, spatially, and tempo-
7
rally to help inhibit side effects. In these photoresponsive
8
systems, photochromic moieties, including spiropyran,
9–11
7,12–19
dithienylethene,
azobenzene,
overcrowded alkenes and
20
retinal-based switches are commonly incorporated into cata-
lyst systems; the reactivity of a photoswitchable catalyst can be
gated via photo-irradiation at different wavelengths. Azo-
benzene and its derivatives have been widely studied because of
their unique properties, such as good stability, good photome-
promoted. For instance, aldol reaction is catalyzed by
32,33
L-proline;
in this catalytic reaction, MIP is fabricated using
L-proline as the template and catalyst, ethyleneglycol dimetha-
crylate as the cross-linker, azo-bis-isobutyronitrile as the initi-
ator, and 4-[(4-methacryloyloxy)phenylazo]-benzenesulfonic
2
1
chanical properties,
isomerization.
and fast and reversible photo-
In these reported methods, functional units
are tethered to azobenzene, and catalytic activities are modied
7,12–18
34
acid as the functional monomer (see ESI† for the detailed
information of the fabrication and characterization of MIP).
The uptake and release of L-proline are likely photoswitchable.
Photoresponsive structural changes in L-proline-MIP were
evaluated through spectral studies (Fig. 2). MIP exhibits a typical
photoisomerization behavior of an azobenzene; the MIP solution
7,12–14
by either changing the steric around the functional units
modulating the relative disposition of two cooperative func-
or
15–18
tional units.
À1
in DMSO (0.15 mg mL ) irradiated at 365 nm for 45 min
a
Key Laboratory of Applied Chemistry of Chongqing Municipality, College of Chemistry
and Chemical Engineering, Southwest University, Chongqing, China. E-mail:
gongcbtq@swu.edu.cn; qiantang@swu.edu.cn
undergoes trans / cis isomerization (Fig. 2a). By contrast, the
same MIP solution subsequently irradiated at 440 nm for 8 min
undergoes cis / trans isomerization (Fig. 2b). The rate constants
of trans / cis and cis / trans isomerization of the azobenzene
b
Department of Biology and Chemistry, City University of Hong Kong, Hong Kong,
China
À3
c
chromophores of the MIP material are (2.23 Æ 0.12) Â 10 and
Department of Science and Environmental Studies, The Hong Kong Institute of
À3 À1
35
Education, Hong Kong, China. E-mail: cfchow@ied.edu.hk
(10.7 Æ 1.18) Â 10
s , respectively. This result indicates that
†
Electronic supplementary information (ESI) available: Details of experimental azobenzene chromophore of MIP undergoes reversible photo-
and characterization of MIP and products in aldol reaction. See DOI:
0.1039/c5ra10343f
isomerization between trans and cis forms.
1
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Fig. 3 Photo-regulated release and uptake of L-proline by MIP and
CMIP in DMSO.
Fig. 3 shows the changes in the binding capacities of the MIP
and control MIP (CMIP) to L-proline under repeated photo-
switching conditions. MIP irradiated at 365 nm releases the
bound L-proline into the solution as a consequence of trans /
cis photoisomerization; the percentage of unbound L-proline
increases from approximately 73% to approximately 93%. By
contrast, MIP subsequently irradiated at 440 nm undergoes cis
Fig. 1 Concept of photoswitchable catalyst based on photo-induced
reversible azobenzene isomerization.
/
trans photoisomerization; as a result, MIP binds to L-proline.
The percentage of unbound L-proline decreases from approxi-
mately 93% to approximately 74%. The release and uptake
amounts of L-proline are similar to those of the previous cycles
when photoswitching cycles are repeated. This result demon-
strates the photocontrolled uptake and release of L-proline.
However, under the same conditions, CMIP demonstrates
unremarkable photocontrolled release and uptake of L-proline.
The percentage of unbound L-proline only changes from
approximately 97% to approximately 96% and from 96% to
96.5% upon irradiation at 365 nm and 440 nm, respectively,
demonstrating that CMIP is not photocontrolled. The above
results are similar to some previous reports using conventional
2
6,27,36
MIP.
The bound L-proline could be increased if surface
28,37
molecularly imprinted polymer is used.
L-Proline-catalyzed aldol reaction between 4-nitro-
benzaldehyde and acetone in DMSO is selected as a standard
reaction to evaluate the photoswitchable catalyst activity of
3
8–40
MIP.
L-Proline (30 mol%) in DMSO/acetone (9 : 1) catalyzes
ꢀ
the reaction of p-nitrobenzaldehyde with acetone at 20 C for
6
h, yielding 68% aldol product 1, which is comparable to that
33
obtained in a previous report (Scheme 1).
The amount of L-proline-MIP effect on the yield of aldol
reaction was investigated (Table 2 in ESI†), and 40 mol% of
L-proline with respect to 4-nitrobenzaldehyde performs best.
L-Proline-MIP (40 mol% of L-proline with respect to 4-
À1
Fig. 2 UV-Vis spectra and spectral changes of MIP (0.15 mg mL ) in
DMSO irradiated (a) at 365 nm for 1.5, 3, 5, 8, 12, 18, 25, and 45 min and
(
b) at 440 nm for 10, 30, 60, 120, 240, and 480 s. Insets: kinetics of MIP
photoisomerization.
Scheme 1 L-Proline catalyzed aldol reaction.
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Fig. 4 Photoswitchable catalyst activity of L-proline-imprinted MIP in
aldol reaction.
Fig. 5 Photocontrolled reusability of the catalyst.
1
7% in the OFF state (Fig. 5), demonstrating good fatigue
nitrobenzaldehyde), acetone (0.3 mL), DMSO (2.7 mL), and 4-
nitrobenzaldehyde (0.076 mg) were placed in a 3.0 mL quartz
cell (optical path length of 1.0 cm). Different measurements
were performed for “OFF state” and “ON state”. (1) In OFF state,
43
resistance.
The scope of L-proline-imprinted polymer-catalyzed aldol
reaction with various substituted benzaldehydes and ketones
was investigated (Table 1). Similar photoswitchable properties
were observed. Lower yield was obtained with 4-cyano-
ꢀ
the quartz cell was irradiated at 440 nm at 20 C for 6 h with
magnetic stirring. The mixture was treated with 3.0 mL of
saturated ammonium chloride aqueous solution and then
extracted with ethyl acetate (3 Â 5.0 mL). The organic layer was
benzaldehyde (entry 3) because the Hammett s -value of –CN
p
39
(0.70) was lower than that of –NO
2
(0.81). The results with 2-
substitued, 3-substituted (entries 4 and 5), and 4-substitued
nitrobenzaldehyde were consistent with previous reports.
Compared with cyclohexanone (entry 1), better result was
obtained with acetone, which was similar to two previous
reports.
4
dried with MgSO , and the organic solvent was removed
44,45
33
through rotary evaporation. The aldol reaction product was
dissolved in 2.0 mL of isopropyl alcohol and subjected to yield
analysis through HPLC. (2) In ON state, the quartz cell was
46,47
ꢀ
irradiated at 365 nm for 6 h at 20 C with magnetic stirring.
In summary, a conceptually new approach based on catalyst-
imprinted polymer containing azobenzene is described to
Aer completion of the reaction, similar treatment to that in
33
OFF state was adopted. L-Proline-CMIP was prepared in the
same manner as L-proline-MIP; however, CMIP was used
instead of MIP. MIP without the rebound L-proline was used as
background.
Table 1 Catalytic aldol reactions between ketones and aldehydes
catalyzed by L-proline-MIP
L-Proline-MIP can promote aldol reaction in response to a
light stimulus (Fig. 4). A yield of 67.8% was obtained in the “ON
Yield (%) Yield (%)
state” (irradiation at 365 nm) compared with a yield of 17.6% in Entry Aldehyde
the “OFF state” (irradiation at 440 nm). A yield of only 0.2% was
obtained for the background because of no presence of
L-proline in MIP. However, photoswitching had nearly no effect
for CMIP (the yield in the ON state was close to the yield in the
OFF state). In comparison with a yield of 0.2% for background, a
Ketones Product
ON state OFF state
1
44.3
79.1
22.1
29.2
yield of about 6.5% for L-proline-CMIP was probably caused by
2
unspecic adsorption of L-proline. These results show that the
designed system can induce the photocontrolled release of
L-proline to promote the corresponding reaction.
The catalyst can be easily recycled through irradiation at
40 nm aer a catalyzed reaction is completed. Furthermore,
3
4
54.3
47.4
37.1
24.8
4
cis / trans photoisomerization causes MIP-induced rebound of
the catalyst. Aer centrifugation is performed, catalyst-MIP is
obtained by discarding the supernatant; catalyst-MIP can also
be used in another batch of the same reaction. This photo-
controlled recycling procedure is simpler than previous
methods by which the recycled catalyst is collected and
5
52.1
8.2
41,42
washed.
In 20 batches of reactions, the yields of aldol reac-
tion range from 64% to 69% in the ON state and approximately
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design photoswitchable organocatalysts. The photoinduced 19 Responsive Materials and Methods, ed. A. Tiwari and H.
trans / cis isomerization of azobenzene switches the reaction Kobayashi, Scrivener Publishing LLC, 2014, pp. 27–58.
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promoted. The concept is successfully applied to perform RSC Adv., 2013, 3, 6241.
photocontrolled L-proline-catalyzed aldol reaction; indeed, the 21 Z. Mahimwalla, K. G. Yager, J. Mamiya, A. Shishido,
proposed concept is feasible. This concept can be further
extended to other reactions.
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