Mechanism into selective oxidation of cinnamaldehyde using β-cyclodextrin polymer as phase-transfer catalyst

Article abstract of DOI:10.1016/j.tet.2012.04.092

Selective oxidation of cinnamaldehyde to benzaldehyde in the presence of bicarbonate and hydrogen peroxide and catalyzed by β-cyclodextrin polymer (β-CDP) was investigated. β-CDP was utilized as the phase-transfer catalyst in this study. The immobilized β

Full text of DOI:10.1016/j.tet.2012.04.092

Tetrahedron 68 (2012) 5912e5919
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journal homepage: www.elsevier.com/locate/tet
Mechanism into selective oxidation of cinnamaldehyde using
polymer as phase-transfer catalyst
b-cyclodextrin
Zujin Yang, Hui Zeng, Xiantai Zhou, Hongbing Ji *
School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective oxidation of cinnamaldehyde to benzaldehyde in the presence of bicarbonate and hydrogen
peroxide and catalyzed by -cyclodextrin polymer ( -CDP) was investigated. -CDP was utilized as the
phase-transfer catalyst in this study. The immobilized -cyclodextrin possessing a hydrophobic host
cavity was found to be efficient as mass-transfer promoters for the oxidation of cinnamaldehyde.
Benzaldehyde could be obtained in 63% yield under optimal reaction conditions. The results indicate the
Received 10 January 2012
Received in revised form 15 April 2012
Accepted 24 April 2012
b
b
b
b
Available online 2 May 2012
complex formed between cinnamaldehyde and
b-CDP via the intermolecular weak interactions, e.g.,
Keywords:
Selective oxidation
hydrogen bond was the reason why the conversion and selectivity could be significantly promoted. Fi-
nally, the reusability of the catalyst was also evaluated. Its catalytic activity remained unchanged even
after recycled for six times, which suggests it is an efficient catalyst for oxidation of cinnamaldehyde.
Ó 2012 Elsevier Ltd. All rights reserved.
b-Cyclodextrin polymer
Phase-transfer catalyst
Cinnamaldehyde
Kinetics
Mechanism
1. Introduction
increase the solubility in water.7,8 The most common and well-
studied CDs is
b
-CD. For example, recently, alkaline hydrolysis of
-CD was
Benzaldehyde, the second largest perfume in the world, is also
cinnamaldehyde to benzaldehyde in the presence of
b
1,2
9
an important pharmaceutical intermediate.
With more care
developed. Besides, in order to speed up reaction, oxidant sodium
hypochlorite was adopted to promote the oxidative cleavage of
about food quality, the demand for natural benzaldehyde is in-
creasing quickly.3 Natural benzaldehyde is mostly from the seeds
and kernels of bitter almonds, apricots, peaches, plums, cherries,
and cherry laurel. Because of its limited supply and high price,
synthetic benzaldehyde from natural cinnamon oil under mild
conditions, which contains more than 80% of cinnamaldehyde, has
10
cinnamaldehyde to benzaldehyde, and the reaction rate from
oxidation was superior over that from hydrolysis. However, the
main disadvantages of using b-CD as phase-transfer catalyst, es-
pecially in commercial applications, is the need to separate the
catalyst from the reaction mixture, large amount of solvent should
be used, making these processes less benign. Therefore, immobi-
4
been used as a substitute. However, the main drawback of such
reactions is the phase transfer limitation leading to very low re-
action rates because cinnamaldehyde is poorly soluble in water.
Phase-transfer catalysis (PTC) has been applied in organic synthesis
lized
b-CD onto polymer as a solideliquid phase-transfer catalyst is
gathering much attention due to its recyclability, easy recovery, and
cost-effectiveness.
In the present work, in order to make the process cleaner, the
substitute of oxidant hydrogen peroxide for sodium hypochlorite,
and the substitute of b-CDP for b-CD were introduced to synthesize
benzaldehyde from natural cinnamaldehyde (Scheme 1). A differ-
ent plausible mechanism of phase transfer catalytic reaction was
also given.
11
5
to circumvent this crucial problem of two-phase reactions. The
advantages of PTC technique are enhancement of the reaction rate,
mild conditions, high selectivity of the main product, high con-
version of the reactant, and suitability for industrial production.
Cyclodextrins (abbreviated as CDs) as phase-transfer catalysts
have been introduced into some organic reactions in aqueous so-
6
lution. They are cyclic oligosaccharides composed of six, seven or
eight glucopyranose units linked by glycosidic bonds (named as
, and -CD, respectively). It is known that CDs can form inclusion
complexes with apolar guest compounds in aqueous solution to
a,
2
2
. Results and discussion
b
g
.1. Catalytic activity investigation
Generally speaking, it is difficult to oxidize organic components
using the water-soluble oxidant agent. Therefore, it is essential to
*
Corresponding author. Tel.: þ86 20 84113658; fax: þ86 20 84113654;
e-mail address: jihb@mail.sysu.edu.cn (H. Ji).
0
040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.04.092

Z. Yang et al. / Tetrahedron 68 (2012) 5912e5919
5913
affecting the conversion of cinnamaldehyde. Fig. 2 showed the effect
of the agitation speed on the conversion of cinnamaldehyde. Below
600 rpm, the conversion was highly affected by the agitation speed
in which the mass transfer played a key role during reaction process.
A further increase in the agitation showed no significant increase in
the conversion of cinnamaldehyde, indicating the mass transfer is
no longer a key step. The value of the apparent rate constant Kapp
under various agitation speeds is depicted as in Table 2.
Scheme 1. Oxidation of cinnamaldehyde to benzaldehyde catalyzed by b-CDP.
investigate the role of these oxidizing agents and phase-transfer
catalysts to promote the reaction. In the present research, -CDP
was utilized as phase-transfer catalyst. As shown in Fig. 1, high
conversion of cinnamaldehyde was observed in the presence of
b
1.0
0
2
4
6
8
rpm
00 rpm
00 rpm
00 rpm
00 rpm
H
2
O
2
, bicarbonate, and
b
-CDP. The conversion is sharply decreased
0.8
0.6
0.4
0.2
0.0
by the deficiency of any one of the components. Hydrogen peroxide
is an environmentally-friendly oxidant for water is the sole
byproduct.1
2,13
1000 rpm
However, hydrogen peroxide is a rather weak oxi-
14,15
dant in the absence of activators. According to previous reports,
a key aspect of such reactions is that hydrogen peroxide and bi-
carbonate combine in an equilibrium process to produce perox-
ymonocarbonate (Eq. 1).
HCO3 þ H O ¼ HCO4ꢀ1 þ H O
ꢀ
1
(1)
2
2
2
H O
2
2
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
NaHCO3
H O + NaHCO
3
0
50
100
150
T/min
200
250
2
2
H O + NaHCO +β-CDP
2
2
3
Fig. 2. Effect of agitation speed on the cinnamaldehyde: cinnamaldehyde (1 mmol), b-
ꢁ
3 2 2
CDP (2 g), NaHCO (1.5 mmol), H O (2 mL, 30% (w/w)), temperature (30 C).
Table 2
Apparent rate constants under various agitation speeds
Agitation speed (rpm)
0
200
400
600
800
1000
3.57
3
ꢀ1
K
appꢂ10 (min
)
0.413
1.54
2.44
3.29
3.34
2
.3. Effect of
b
-CDP
0
20
40
60
T/min
80
100
120
b
-CD loading of catalysts significantly affected their catalytic
performance. Effect of -CDP amount on the oxidation of cinna-
b
Fig. 1. Effect of the oxidizing agent and phase-transfer catalyst on the cinnamaldehyde:
maldehyde was investigated by varying its amount, as shown in
Fig. 3. The results indicated that the conversion of cinnamaldehyde
cinnamaldehyde (1 mmol),
temperature (40 C), 800 rpm.
b
-CDP (2 g), NaHCO
3
(1.5 mmol), H
2
O
2
(2 mL, 30% (w/w)),
ꢁ
increased with an increase of the amount of
a slight enhancement of yield for benzaldehyde. Maximal conver-
sion (99%) was obtained when using 2 g of -CDP, and the corre-
sponding yield of benzaldehyde was 63%. A further increase in the
amount of -CDP did not increase the conversion of cinnamalde-
b-CDP, coupling with
Peroxymonocarbonate is an active oxidant, with structure
ꢀ
b
HOOCO2 , which efficiently promoted the epoxidation of cinna-
maldehyde, and which was then further oxidized to benzaldehyde.
The effect of the different components on the apparent rate con-
stant, kapp, is depicted as in Table 1.
b
hyde and the yield of benzaldehyde significantly. The reason is that
the limited amount of peroxymonocarbonate. However, in the
same reaction conditions, similar data could be obtained using 1 g
Table 1
b
-CD as phase-transfer catalyst. The results demonstrate that
is crucial for the oxidation of cinnamaldehyde. It should be noted
that the immobilized -CDP has realized its easy recyclability from
the reaction mixture without loss of its activity.
b-CD
Apparent rate constants under different catalytic systems
Components
H
2
O
2
NaHCO
3
H
2
O
2
þNaHCO
3
H
2
O
2
þNaHCO
3
þb-CDP
b
3
ꢀ1
)
K
appꢂ10 (min
0.282 1.61
10.73
25.11
2 2
2.4. Effect of amount of H O
2
.2. Effect of varying stirring speed
The amount of H
hyde oxidation, as shown in Table 3. The conversion rose with in-
creasing the amount of H from 0 mL to 5 mL. 99% conversion of
cinnamaldehyde and 63% yield of benzaldehyde were obtained
when the amount of H reached 4 mL.
2 2
O also influences the degree of cinnamalde-
Effect of varying stirring on the conversion of cinnamaldehyde
using -CDP as phase-transfer catalyst was investigated in the range
of 0e1000 rpm. It should be noted that the mass transfer of the
b
2 2
O
component between two phases is also an important factor in
2 2
O

5914
Z. Yang et al. / Tetrahedron 68 (2012) 5912e5919
mentioned that lower temperature was especially favorable for
keeping the natural essence of benzaldehyde. Thus, the optimal
100
9
temperature for oxidation of cinnamaldehyde to benzaldehyde was
ꢁ
7
0 C.
80
60
40
20
0
2
.6. Effect of amount of bicarbonate
Effect of sodium bicarbonate amount on the oxidation of cin-
namaldehyde was investigated and the results were summarized in
Table 5. The amount of NaHCO plays an important role for the
oxidation of cinnamaldehyde by activating H . In the absence of
NaHCO , the conversion of cinnamaldehyde and yield of benzal-
dehyde were only 6% and 4%, respectively. The conversion of cin-
namaldehyde significantly increased to 99% and 63% yield of
3
conversion of cinnamaldehyde (%)
yield of benzaldehyde (%)
2 2
O
3
benzaldehyde was obtained when the amount of NaHCO
1.5 mmol.
3
reached
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Amount of β−CDP
2.7. Kinetic studies
Fig. 3. Effect of amount of
b
-CDP on the oxidation of cinnamaldehyde and yield of
The oxidation rate of cinnamaldehyde in the presence of
b
-CDP
benzaldehyde: 1 mmol cinnamaldehyde, 4 mL H
2
O
2
(w/w), 1.5 mmol NaHCO , 25 mL
3
ꢁ
2
H O, 2.5 h, 70 C, 600 rpm.
was represented in Fig. 4. Linear relationship between ln(C /C
) and
0
t
Table 3
Effect of the different amount of H
2
O
2
on the oxidation of cinnamaldehydea
Conversion of cinnamaldehyde (%)
Entry
Amount of H
2
O
2
(mL)
Yield of benzaldehyde (%)
Yield of epoxide (%)
Yield of phenylacetaldehyde (%)
1
2
3
4
5
6
0
1
2
15
74
89
91
99
99
7
43
52
60
63
63
0
20
13
8
3
2
0
5
8
9
12
14
2þ1
2þ2
2þ2þ1
a
ꢁ
Reaction conditions: cinnamaldehyde (1 mmol), b-CDP (2 g), NaHCO (1.5 mmol), H O (25 mL), 70 C, 2.5 h, 600 rpm.
3
2
2
.5. Effect of temperature
time was observed, indicating the reactions followed pseudo-first-
order kinetics (Eq. 2)
Effect of reaction temperature on the oxidation of cinnamalde-
C0
hyde was studied, as shown in Table 4. The conversion increased
with increasing in reaction temperature. The yield of benzaldehyde
ln
¼ kt
(2)
C
t
ꢁ
rose to 63% when the temperature increased to 70 C. When the
ꢁ
temperature exceeded 70 C, the yield decreased due to de-
where C
0
, C
t
are the initial concentration of cinnamaldehyde and
composition of hydrogen peroxide. In addition, it should be
the concentration at t. t and k are reaction time (min) and the globe
Table 4
a
Effect of temperature on the oxidation of cinnamaldehyde
ꢁ
Entry
Temperature ( C)
Conversion of cinnamaldehyde (%)
Yield of benzaldehyde (%)
Yield of epoxide (%)
Yield of phenylacetaldehyde (%)
1
2
3
4
5
40
50
60
70
80
74
84
95
99
99
23
37
55
63
58
40
28
13
3
3
7
10
12
15
1
a
2 2 3 2
Reaction conditions: cinnamaldehyde (1 mmol), b-CDP (2 g), 4 mL H O (w/w), NaHCO (1.5 mmol), H O (25 mL), 2.5 h, 600 rpm.
Table 5
Effect of the different amount of NaHCO
on the oxidation of cinnamaldehydea
Conversion of cinnamaldehyde (%)
3
Entry
Amount of NaHCO
3
(mmol)
Yield of benzaldehyde (%)
Yield of epoxide (%)
Yield of phenylacetaldehyde (%)
1
2
3
4
5
0
0.5
1
1.5
2
6
60
83
99
99
4
39
48
63
62
0
15
11
3
0
4
7
12
12
2
a
ꢁ
Reaction conditions: cinnamaldehyde (1 mmol),
b
-CDP (2 g), 4 mL H
2
O
2
(w/w), H
2
O (25 mL), 70 C, 2.5 h, 600 rpm.

Z. Yang et al. / Tetrahedron 68 (2012) 5912e5919
5915
Table 7
2
2
1
1
0
0
.5
.0
.5
.0
.5
.0
The rate orders, rate constants, and correlation coefficient at different reaction
temperature in the absence of -CDP
b
3
3
3
3
03 K
13 K
23 K
33 K
T (K)
n
k (min 1
ꢀ
)
Correlation coefficient (R)
303
313
1
1
1
1
0.00382
0.00961
0.01628
0.02610
0.9992
0.9974
0.9978
0.9983
323
333
Arrhenius plots of ln k and 1/T were drawn in Fig. 6. The plots are
of good linearity and the activation energies E in the presence and
absence of -CDP were calculated as 44.83 kJ mol
a
ꢀ
1
b
and
-CDP could decrease
-CDP is an efficient phase-
ꢀ
1
5
3.04 kJ mol , respectively. It is obvious that
the energy barrier of the oxidation, thus
b
b
transfer catalyst for the oxidation of cinnamaldehyde to natural
benzaldehyde.
0
20
40
T/min
60
80
Fig. 4. Pseudo-first-order plots of oxidation reaction of cinnamaldehyde in the pres-
ence of -CDP by H eNaHCO system at different reaction temperatures.
b
2
O
2
3
-3.5
reaction apparent rate (minꢀ1), respectively. The slope of ln (C
versus t was used to determine k.
0
t
/C )
Ea=44.83 KJ/mol
-
4.0
4.5
The parameters k (rate constant, minꢀ1), n (rate orders), and R
correlation coefficient) of oxidation process were shown in Table 6.
(
The rate constant of pseudo-first-order kinetics increased with the
-
rise of reaction temperature, and was found to increase from
ꢀ1
ꢀ1
0
.00619 min to 0.0313 min with the temperature increased
Ea=53.04 KJ/mol
from 303 K to 333 K.
-5.0
-5.5
Table 6
The rate orders, rate constants, and correlation coefficient at different reaction
temperature in the presence of
b-CDP
T (K)
n
k (minꢀ1)
Correlation coefficient (R)
3.00
3.05
3.10
3.15
3.20
3.25
3.30
303
313
323
333
1
1
1
1
0.00619
0.01334
0.02072
0.03130
0.9977
0.9982
0.9992
0.9982
-1
1
000/T( K )
Fig. 6. Arrhenius plots of the rates constants for cinnamaldehyde oxidation ( : in the
presence of -CDP; -: in the absence of -CDP).
b
b
The fitting results of first-order kinetic model also indicate the
oxidation of cinnamaldehyde to benzaldehyde in the absence of
2.8. Reusability of the catalyst
b
-
CDP followed pseudo-first-order kinetic model (Fig. 5). The corre-
In order to further evaluate the catalytic performance, reuse ex-
periments of the catalyst were carried out. The insoluble pre-
cipitation (b-CDP) could be easily recycled by centrifugation. The
obtained precipitation was washed with ethanol and deionized
water. After drying, the catalyst was subjected to the next run under
the same conditions. As shown in Fig. 7, the results indicate that the
sponding parameters were shown in Table 7. The rate constant
ꢀ
1
ꢀ1
increased from 0.00382 min to 0.0261 min with the temper-
ature increased from 303 K to 333 K.
2.0
1.6
1.2
0.8
0.4
0.0
Conversion of cinnamaldehyde (%)
Yield of benzaldehyde (%)
3
03 K
313 K
1
00
3
3
23 K
33 K
80
6
4
2
0
0
0
0
0
20
40
T/min
60
80
Fresh First Scecond Third Fourth Fifth Sixth
Fig. 5. Pseudo-first-order plots of oxidation reaction of cinnamaldehyde in the absence
of -CDP by H eNaHCO system at different reaction temperatures.
Fig. 7.
(2 g), H
b
-CDP recyclability data. Reaction conditions: cinnamaldehyde (1 mmol),
b
-CDP
ꢁ
b
2
O
2
3
2
O
2
(4 mL, 30% (w/w)), NaHCO
3
2
(1.5 mmol), H O (25 mL), 70 C, 2.5 h, 600 rpm.

5916
Z. Yang et al. / Tetrahedron 68 (2012) 5912e5919
activity of the reused catalyst remained unchanged for six times. This
phenomenon demonstrates that the catalyst can be efficiently re-
covered and recycled. It possesses potential application in industry.
glucopyranoside unit in the isolated b-CDP can interact with cin-
namaldehyde or benzaldehyde to form stable interactions by in-
termolecular weak interactions, e.g., van der Waals interaction,
electrostatic interaction, and hydrogen bond interaction etc., re-
spectively. Hydrogen bond interactions, e.g., OeH/O has been
2
.9. Promotion mechanisms of hydrogen bond
18,19
considered. As previously reported references,
the cut-off cri-
teria of hydrogen bond interaction is the O/O distance from
2
.9.1. XPS analysis of inclusion complex. XPS was carried out to
ꢀ
ꢁ
OeH/O hydrogen bond (ꢃ3.2 A), and the bond angle (ꢄ90 ). The
evaluate the surface elements and the ratio of elements, as shown
in Fig. 8. C and O were found on the surface of the inclusion com-
plexes, and mole ratio of C/O was 3.0, higher than the ratio of C/O of
ꢀ
bond lengths of OeH/O (a) and (b) are in turn 2.354 and 2.785 A,
ꢁ
and the corresponding bond angles are in turn 113.587 and
ꢁ
1
07.840 , which demonstrates that the hydrogen bond between
b-CDP, 1.7, which means that some carbon has been incorporated
cinnamaldehyde or benzaldehyde and the isolated
b-CDP plays
into the complex.
(a)
(b)
O1s
2
50000
00000
50000
00000
250000
200000
150000
100000
50000
O1s
C1s
2
1
1
C1s
50000
0
0
1000
800
600
400
200
0
1000
800
600
400
200
0
Binding Energy (eV)
Binding Energy (eV)
Fig. 8. XPS graph of b-CDP (a) and its inclusion complexes with cinnamaldehyde (b).
The C1s spectra of
namaldehyde are shown in Fig. 9(a) and (b). The position of these
peaks are determined to be 248.8, 286.6, and 288.4 eV, arising from
b
-CDP before and after inclusion with cin-
a crucial role in the reaction processes. Hydrogen bond, which
makes more cinnamaldehyde insert into the hydrophobic cavity of
b-CD, effectively expands the interface between the organic and
16,17
C1(CeC), C2 (CeO), and C3 (OeCeO and/or C]O), respectively.
aqueous and significantly promotes the reaction rate of cinna-
2
0,21
A relative increase and a significant decrease of the XPS peaks
corresponding to the C1 atoms and the C2 atoms were observed in
Fig. 9(a) and (b). The experimental results indicated that the con-
centration of C1 atoms and C2 atoms increased from 28.96% to
maldehyde to benzaldehyde. According to the previous reports,
the difference of the two binding energies between cinnamalde-
hyde and benzaldehyde with the isolated -CDP is the base for
selective separation of the different aldehyde via -CD inclusion in
-CDP. On the other hand, the binding energy for the glucopyr-
b
b
7
1.70% and decreased from 61.78% to 23.33%, respectively. From the
b
above discussion, the weak interaction should exist in the -CDP/
cinnamaldehyde inclusion complex, which significantly changed
b
anoside unit/cinnamaldehyde (ꢀ57.05 kJ/mol) was negative, much
lower than that of the glucopyranoside unit/benzaldehyde
the chemical environment of carbon in the inclusion complex.
(ꢀ29.32 kJ/mol) in the isolated
b-CDP. It should be noted that dif-
(a)
(b)
C2
C3
C2
C1
C1
C3
282
284
286
288
290
292
280
282
284
286
288
290
292
294
Binding Energy (eV)
Binding Energy (eV)
Fig. 9. XPS of C1s (a) core-level spectra of b-CDP and C1s (b) core-level spectra of b-CDP/cinnamaldehyde inclusion complex.
2
.9.2. Theoretical results. Geometries and energies of the most
stable inclusion complexes of glucopyranoside unit with cinna-
maldehyde or benzaldehyde in the isolated -CDP optimized by the
program package DMol3 method were displayed in Fig. 10. The
negative binding energies further demonstrate that
ferent binding energies between cinnamaldehyde or benzaldehyde
with glucopyranoside unit in the -CDP can lead to different sta-
bilities of the inclusion complexes. From the point of view of en-
ergy, it can be concluded that cinnamaldehyde prefer to penetrate
the hydrophobic cavity rather than stay in the bulk of the aqueous
b
b

Z. Yang et al. / Tetrahedron 68 (2012) 5912e5919
5917
100
80
60
40
20
0
a
b
c
d
0
40
80
120
160
Reaction time / min
Fig. 11. Plots of conversion and yields of the products versus reaction time for the
ꢁ
cinnamaldehyde/
b-CDP/H
2
O
2
eNaHCO
3
system at 70 C; (a: conversion of cinna-
maldehyde; b: yield of epoxide; c: yield of benzaldehyde; d: yield of
phenylacetaldehyde).
be included by
mechanism as parent
can form the inclusion complex with the intermolecular hydrogen
bond OeH/O at the second rim of -CD (Fig. 12), and the hoste
guest interaction between cinnamaldehyde and parent -CD was
b-CD moiety immobilized on
b-CDP with the same
b
-CD. -CD in the -CDP and cinnamaldehyde
b
b
b
b
1
9
analyzed by H NMR and ROESY as previously reported results.
Secondly, due to nucleophilic attack of hydroxide ion from hydro-
lysis of sodium bicarbonate on the substrate, a minor benzaldehyde
is directly produced via hydrolysis, and the result was in accordance
9
with the previous reported data. Simultaneously, perox-
ꢀ
ymonocarbonate ðHCO Þ ,
a peroxide with a structure of
HOOCO2 , was formed from bicarbonate and hydrogen peroxide
and can epoxidize alkenes through the nucleophilic attack of
4
ꢀ
1
ꢀ
HOOCO2 species to the C]C double bond. The non-covalent in-
Fig. 10. Optimized structure of glucopyranoside unit/cinnamaldehyde (a), glucopyr-
anoside unit/benzaldehyde (b) in the isolated -CDP obtained by DMol3 method.
termolecular interaction between b-CD in b-CDP and cinnamalde-
b
hyde also promoted the nucleophilic oxidation. Afterward, large
amount of cinnamaldehyde was converted to its epoxide product
and a minor phenylacetaldehyde was produced, resulting from the
phase. The produced benzaldehyde tends to escape the cavities of
-CD. Therefore, -CDP can be used to enhance the mass transfer
significantly, that is why the ratio of cinnamaldehyde/ -CDP can be
much lower than previously reported data (1:1). Therefore mass
a b
nucleophilic attack of peroxymonocarbonate on C precedes C in
b
b
the C]C double bond. Then, the corresponding epoxide was fur-
ther oxidized to benzaldehyde by peroxymonocarbonate ion. Fi-
nally, the catalyst returned its initial state.
b
9
transfer of
selectivity.
b-CDP significantly improves the catalytic activity and
2.11. Large-scale experiment
2
.10. Plausible mechanism for the reaction
The plots of the conversion of cinnamaldehyde and the yields of
In order to evaluate the feasibility of applying this method in
a preparative scale, a large-scale experiment for the oxidation of
cinnamaldehyde to benzaldehyde was carried out using -CDP as
b
the products versus reaction time were shown in Fig. 11. The con-
version of cinnamaldehyde greatly increased by addition of
H O eNaHCO and extending the reaction time. The yield of
2 2 3
PTC in the presence of hydrogen peroxide and sodium bicarbonate
in water under the above optimum reaction conditions, as shown in
Scheme 2. The yield of purified benzaldehyde was up to 58%.
benzaldehyde increased to 63% at 150 min. On the other hand, the
yield of epoxide almost simultaneously reached to 68% at 5 min,
afterward gradually decreased to 2% at 180 min. And the yield of
phenylacetaldehyde was obtained with the yield of 12% at 150 min
and did not significantly increase with extending the time. During
the reaction process, peroxymonocarbonate is an active oxidant. It
can efficiently promote epoxide to benzaldehyde in the system. The
above results suggest that epoxide is an intermediate in the
reaction.
9
Comparing with other hydrolysis of cinnamaldehyde, this process
realizes clean synthesis of natural benzaldehyde and provides
a good yield for benzaldehyde under mild conditions, which is
especially important for practical application in industry.
3. Conclusion
The catalytic oxidation of cinnamaldehyde was successfully
Based on the above experimental data, together with XPS and
calculation results, a plausible reaction mechanism has been pro-
carried out by using hydrogen peroxide, bicarbonate, and b-CDP as
PTC in a two-phase organic/aqueous solution medium. The PTC
catalyst was found to be recyclable and efficient for selective oxi-
dation of cinnamaldehyde to benzaldehyde and 99% conversion of
posed for
b
-CDP catalyzed-oxidation of cinnamaldehyde by
. Firstly, in our study, cinnamaldehyde is assumed to
H
2
O
2
eNaHCO
3

5918
Z. Yang et al. / Tetrahedron 68 (2012) 5912e5919
2 2 3
Fig. 12. The plausible mechanism of the oxidation of cinnamaldehyde catalyzed by b-CDP in the presence of H O and NaHCO .
insoluble polymers were poured into water. The resultant product
was filtrated, and further washed with acetone in a Soxhlet ex-
ꢁ
tractor for 24 h. After drying in vacuum oven at 80 C for 12 h, the
polymer product was crushed and granulated to 160e250
diameter. -CD content of -CDP was determined to be 50%
according to previously reported method.
mm in
b
b
2
3
Scheme 2. Large-scale oxidation of cinnamaldehyde to benzaldehyde using
PTC in the presence of H and NaHCO
b-CDP as
2
O
2
3
.
4.3. Preparation of inclusion complex of cinnamaldehyde
b
-CDP (1.0 g) and 1 mmol cinnamaldehyde were dispersed in
ꢁ
2
5 mL deionized water. The mixture was stirred at 60 C for 1 h, and
cinnamaldehyde and 63% yield of benzaldehyde could be obtained
within 2.5 h. The preparation of these catalysts is simple, and the
recovery and utilization of used catalyst is convenient. This aqueous
oxidation of cinnamaldehyde is clean, safe, and reproducible, with
mild reaction conditions. It is beneficial for keeping the natural
essence of benzaldehyde. These advantages of the catalyst made it
useful for synthesis of natural benzaldehyde in industrial process.
then centrifuged. The resultant solid was washed with distilled
ꢁ
water to remove free substrate, dried under vacuum at 50 C for
1
2 h, and stored in desiccator before use.
4.4. Characterization of catalyst and the inclusion complex
XPS analysis was performed on a Vacuum Generator ESCALAB
2
50 spectrometer using an Al K
and a hemispherical analyzer. A fixed analyzer pass energy of
50 eV was used for survey scans, and high-resolution scans of
a monochromatic X-ray source
4
4
. Experimental method and calculation model
.1. Materials and reagents
1
core-level regions were recorded using a 20 eV pass energy. All
core-level spectra were referenced to the C1s neutral carbon
peak at 284.5 eV and obtained at a take-off 90 to the sample
b-CD (>99%) was purchased from Shanghai Boao Biotechnology,
ꢁ
China. Epichlorohydrin (EPI, analytical grade, Tianjin Damao
Chemical Reagent Factory, China), cinnamaldehyde (>99%) were
obtained from Sinopharmacy Chemical Reagent, China. All other
reagents and solvents are of analytical grade and used without
further purification unless indicated.
surface.
4
.5. Evaluation of catalyst
Cinnamaldehyde (1 mmol) was dissolved in 25 mL of deionized
ꢁ
4
.2. Preparation of
b
-CDP
water at 70 C and 2.0 g
b
-CDP was added into a 150 mL three-
necked flask fitted with a reflux condenser and magnetic stirrer.
ꢁ
A typical procedure for preparing
b
-CD polymer was described
-CD, 4 g NaOH, and 4 mL water was stirred at
-CD was dissolved completely. And 15 mL of
The mixture was heated to 70 C in an oil bath with electric heater.
as:2 a mixture of 5 g
2
2
b
b
Then, 1.5 mmol sodium bicarbonate dissolved in 4 mL 30% (w/w)
ꢁ
5 C for 20 min till
2
H O
2
(35 mmol) was slowly dropped in. The resulting system was
ꢁ
EPI was slowly added dropwise into the mixture. The temperature
was monitored during polymerization and kept at 65 C under
vigorous stirring (200 rpm). After stirring for about 1e2 h, pre-
cipitate could be observed, and the viscosity of the solution was
also increased. The solution was mixed with acetone, and the
stirred with magnetic stirrer at 70 C for 2.5 h, 600 rpm. The
mixture was extracted by ethyl acetate and then centrifuged. The
extracted liquid mixture was analyzed by GCeMS with naphtha-
lene as an internal standard. The reproducibility for all the data was
within 5%.
ꢁ

Z. Yang et al. / Tetrahedron 68 (2012) 5912e5919
5919
4
.6. Kinetics of cinnamaldehyde oxidation
The computations were carried out using the program package
2
4e26
DMol3 method
in Materials Studio (version 4.0) of Accelrys
The reactor was a 150 mL three-necked flask fitted with a reflux
Inc. All calculations were performed using the local density ap-
proximation (LDA) in the Perden-Wang (PWC) form at the DND
basis set level.
condenser and magnetic stirrer. Known quantities of cinnamalde-
hyde (1 mmol) was completely dissolved in 100 mL of deionized
water. The solution was put into the reactor, which was submerged
into a well controlled temperature oil bath. The temperature of
The binding energy (BE) could be expressed as (Eq. 3):
BE ¼ Ecomplex ꢀ Eguest ꢀ Ehost
(3)
mixture was kept at 303 K (or 313 K, 323 K, and 333 K). Then 2.0 g
CDP was added to the solution while being stirred for 30 min.
Measured quantities of H (2 mL, 30% (w/w)) together with
.1 mmol sodium bicarbonate was then introduced into the reactor
b-
E
complex was the total energy of inclusion complex, Eguest was the
sum of total energy of guests, and Ehost was the total energy of host.
The isolated -CDP was used as host, and cinnamaldehyde or
2 2
O
1
b
to start the reaction. Reaction mixture (0.5 mL) was sampled in
a 1 mL pipette at 20-min intervals. The sample was diluted to
00 mL with deionized water and analyzed by UVevis at 290 nm.
Finally, the UVevis absorbance data were converted into the
product concentration. All the kinetic experiments were carried out
for three times and good reproducibility was found with the error
percentage within 5%.
benzaldehyde was selected as model guests. The more negative the
binding energy is, the more thermodynamically favorable is the
inclusion complex.
1
4.8. Large scale reaction experiment
The large-scale reaction experiments were performed under the
4
.7. Computational methods
optimum reaction conditions as: 10 mmol cinnamaldehyde was
dissolved in 250 mL of deionized water.
500 mL three-necked flask. The mixture was heated to 70 C in an
b
-CDP (20 g) was placed in
ꢁ
The inclusion complex was evaluated by quantum chemistry
calculation, for it could provide information of binding energy and
structure of the inclusion complex. However, DFT methods are
prohibitively expensive in treating such large systems. In order to
rationalize the mechanism of interaction, it was assumed that the
binding sites in the polymer are glucopyranoside unit of
isolated -CDP molecule (Fig. 13), and the global properties of these
sites are not significantly different from that of glucopyranoside
unit in the isolated -CDP molecule.
2 2
oil bath, and then a solution of 40 mL of 30% (w/w) H O
(350 mmol) and sodium bicarbonate (15 mmol) was slowly drop-
ped into the mixture, while being stirred for 2.5 h. After reaction,
the solution was extracted by ethyl acetate. The extracted liquid
mixture was analyzed by GCeMS with naphthalene as an internal
standard.
b-CD in the
b
b
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