Dihydrobenzofuran production from catalytic tandem Claisen rearrangement-intramolecular hydroaryloxylation of allyl phenyl ethers in subcritical water

Article abstract of DOI:10.1039/c4ra04689g

We herein report a mild method for the preparation of dihydrobenzofurans through hydrothermal catalytic tandem Claisen rearrangement-intramolecular hydroaryloxylation of allyl phenyl ethers. This reaction provides a new method for constructing dihydrobenzofurans, a process that is potentially applicable to natural product synthesis. SBA-15, TS-1, HZSM-5 were chosen as catalysts in a hydrothermal reaction medium between 200 and 320 °C. HZSM-5 catalyst showed the highest catalytic activity, and the effects of molar ratio of allyl phenyl ether-water, time, pressure, temperature and catalyst on the Claisen hydroaryloxylation in hydrothermal condition were investigated. The latter two process variables had the greatest influence on the product yields and distribution. A series of allyl phenyl ether derivatives were also treated in hydrothermal condition with HZSM-5 catalyst to offer high yield of corresponding dihydrobenzofurans.

Full text of DOI:10.1039/c4ra04689g

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Dihydrobenzofuran production from catalytic
tandem Claisen rearrangement–intramolecular
hydroaryloxylation of allyl phenyl ethers in
subcritical water†
Cite this: RSC Adv., 2014, 4, 29527
*
*
Ligang Luo, Chunze Liu, Zhiqiang Hou, Yuanyuan Wang and Liyi Dai
We herein report a mild method for the preparation of dihydrobenzofurans through hydrothermal catalytic
tandem Claisen rearrangement–intramolecular hydroaryloxylation of allyl phenyl ethers. This reaction
provides a new method for constructing dihydrobenzofurans, a process that is potentially applicable to
natural product synthesis. SBA-15, TS-1, HZSM-5 were chosen as catalysts in a hydrothermal reaction
medium between 200 and 320 ꢀC. HZSM-5 catalyst showed the highest catalytic activity, and the effects
of molar ratio of allyl phenyl ether–water, time, pressure, temperature and catalyst on the Claisen
hydroaryloxylation in hydrothermal condition were investigated. The latter two process variables had the
greatest influence on the product yields and distribution. A series of allyl phenyl ether derivatives were
also treated in hydrothermal condition with HZSM-5 catalyst to offer high yield of corresponding
dihydrobenzofurans.
Received 19th May 2014
Accepted 19th June 2014
DOI: 10.1039/c4ra04689g
www.rsc.org/advances
reagent (DMF, CH3CN, methallyl chloride),12 catalyst recy-
cling,13 and moderate targeted product yield14 also accompa-
1. Introduction
nied with this method. Therefore, increasing attentions are
being shied to the method development of preparing dihy-
drobenzofuran and its derivatives in a more efficient and envi-
ronmental-friendly way.
Dihydrobenzofuran is the key structure moiety of many highly
biologically active materials, constructing pharmaceuticals
(furadan), lignans,1 and other biologically natural compounds
(pterocarpans).2 Pterocarpans have a 2,3-dihydrobenzofuran
skeleton, which could respond to fungi infections and have
biological activities against HIV, central nervous system (CNS)
injury and malaria.3 Hence, many effective methods for the
construction of dihydrobenzofurans have been proposed
during the past several decades.4 Traditional methods such as
oxidative coupling reaction,5 Lewis acids catalyzed Schmidt
reaction,6 radical-based cyclizations,7 Pummerer reactions,8
directed a-metalations,9 and transition metal-mediated reac-
tions,10 have been developed for the synthesis of dihy-
drobenzofuran and its derivatives. While the above methods
provide the desired heterocycles, few can be universally applied
and several produce a variety of side products. Recently, a new
and milder method for the preparation of dihydrobenzofurans
was proposed, which consists of two tandem steps: Claisen
rearrangement and intramolecular hydroalkoxylation as shown
in Scheme 1.11 Catalysts such as transition-metal complexes
(Cu(OTf)2), Lewis acid, Brønsted acid have been prepared to
accelerate the reaction rate.12 However, drawbacks such as toxic
Water above its boiling point and below its critical point is
usually dened as subcritical water (SBW) which exhibits
properties that are very different from that of water at ambient
temperature. SBW was employed as reaction medium in organic
and inorganic syntheses, biomass conversion, biopolymer
degradation, and aqueous waste stream remediation due to its
abundance, low cost, and ease of product isolation.15 The
physicochemical characteristics of water are greatly changeable
only by varying the pressure or temperature.16 Moreover, SBW
behaves like many organic solvents so that most of small
organic compounds are completely miscible with it. Grieco
et al.17 for the rst time demonstrated the benet of water for
the rearrangement of allyl vinyl ethers. An et al.18 suggested that
acid facilitated the hydroaryloxylation reaction in SBW
Department of Chemistry, East China Normal University, 200062 Shanghai, China.
E-mail: yywang@chem.ecnu.edu.cn; lydai@chem.ecnu.edu.cn; Fax: +86-021-
62602447; Tel: +86-021-62233037
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra04689g
Scheme
1 The tandem Claisen rearrangement–intermolecular
hydroalkoxylation of allyl phenyl ethers.
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condition. To date, however, no study on the production of
In a typical run, APE and water was directly loaded into the
dihydrobenzofuran from the tandem Claisen rearrangement reactor with or without catalyst. Reactions were carried out by
and intermolecular hydroaryloxylation and its derivatives under placing the loaded reactors in an isothermal molten-salts bath
hydrothermal conditions in the presence of potential hetero- preheated to the desired temperature which was controlled by
geneous catalysts was reported.
using an Omega type temperature controller. Molten-salts bath
Herein we report a new dihydrobenzofuran ring-forming was controlled to 180–380 ꢁ 3 ꢀC.27 The preheating time is
process via tandem Claisen rearrangement–intermolecular between 1 and 3 min within the temperature range examined.
hydroaryloxylation. Reactions of allyl phenyl etheꢀ r were carried Aer the desired holding time, the reactors were removed out of
out in SBW at a temperature range of 200–320 C in the pres- the molten-salts bath and placed into cold water bath. Aer
ence of three different heterogeneous zeolite catalysts (SBA-15, reaching room temperature, the reactors were taken out of the
TS-1, and HZSM-5). These three zeolite catalysts have meso- water and dried. The reactors were opened and most of the
porous pore, high specic surface area, and uniform size and aqueous-phase products including catalysts were transferred
shape of the pores over micrometer length scales.19 High into a beaker. The dumped reactor was washed several times
thermal stability and the advantage of tuning the pore size with additional 10–20 ml dichloromethane (DCM) to ensure
make these materials interesting for use as supports and cata- that all contents were recovered. All the collected contents were
lysts for various heterogeneously catalyzed reactions.19,20 subjected to ltration for the liquid and catalyst separation.
Furthermore, the application of zeolites, as catalysts, also relies Product molar yields were calculated as the moles of products
on their intrinsic acidity for processes such as cracking, isom- formed by per mole of all initially loaded into the reactor.
erization and alkylation.21 In this present study, we examine the
Duplicated independent runs were conducted under iden-
effect of reaction time, temperature, catalyst type and allyl tical conditions to determine the uncertainties in the experi-
phenyl ether to water molar ratio on the yields and selectivity of mental results. Results reported herein represent the mean
liquid products, aiming to maximize the desired product yield. values for the two independent trials.
Next, a series of allyl phenyl ether derivatives were treated under
hydrothermal condition in the presence of HZSM-5 catalyst.
Finally, a reaction network was proposed based on the experi-
mental results.
2.3. Analysis of products
Products quantication was performed in a Gas-Chromato-
graph (6890N, Agilent) equipped with an autosampler, auto-
injector, mass spectrometric detector, and an Agilent HP-5
nonpolar capillary column (50 m ꢂ 200 mm ꢂ 0.33 mm). A
volume of 2 ml was injected for each sample, and the inlet
2. Experimental section
2.1. Materials and chemicals
ꢀ
temperature and split ratio were 300 C and 3 : 1, respectively.
The detector temperature was set at 250 ꢀC. The column was
initially held at 60 ꢀC. The temperature was ramped to 250 ꢀC at
15 ꢀC minꢃ1 and held isothermally for 2 minutes, giving a total
runtime of about 15 min. Helium served as the carrier gas (1 ml
minꢃ1). The compositions of compounds were obtained from
electronic integration measurements using ame ionization
detection. The sample also was identied by GC (6890-5973,
Agilent), which was operated at 70Ev (condition:EI) under the
same conditions as GC-MS analysis. Quantication of products
was carried out on an Agilent Technologies 7890 gas chro-
matograph with a ame ionization detector (GC-FID) at the
same conditions as described above.
TS-1, SBA-15, HZSM-5 were prepared strictly based on the
procedure described in ref. 22 and 23. Four different types of
catalysts were prepared and denoted as TS-1, SBA-15, HZSM-
5(Si : Al ¼ 22 : 1) and HZSM-5(Si : Al ¼ 200 : 1). The compounds
including allyl phenyl ether(APE), allyl p-tolyl ether, allyl o-tolyl
ether, allyl m-tolyl ether, 1-(allyloxy)naphthalene, allyl phenyl
ethers with NO2, NH2, Cl, Br, C(CH3)3 and OCH3 in para posi-
tion and S, methallyl and crotyl system, were synthesized from
the corresponding phenols with allyl bromide according to the
modied method.24–26
All other chemicals, such as solvents, were obtained from
Sigma-Aldrich in high purity ($99%). Freshly double-distilled
water was used throughout the experiments. All solutions were
prepared with double-distilled water. The reactors, which were
welded by stainless-steel Swagelok type tube tting, were used
in most experiments. The assembled reactors had an internal
volume of 17.5 ml. The body of the reactor consisted of 1-inch
port connector sealed with two 1-inch caps at both ends.
2.4. Characterization of catalysts
X-ray diffraction patterns (XRD) of the catalysts were performed
on a Bruker D8 diffractometer with Cu Ka radiation from 0.5 to
90ꢀ with a scan speed 1ꢀ minꢃ1. X-ray photoelectron spectros-
copy (XPS) measurements were obtained on a PHI-5500 spec-
trometer with Al Ka X-ray radiation as the X-ray source for
excitation.
2.2. Procedure
Temperature programmed desorption (TPD) was conducted
Before used in reactions, all reactors were rstly washed with by Chembet 3000 chemical adsorbed instrument from Quan-
acetone and air dried, and then loaded with 10 ml of duplicate tachrom. Before adsorption, the sample (0.1 g) was dried in
distilled water and heated to 380 ꢀC for 8 h to expose the reactor owing He (99.99%, 30 ml minꢃ1) at 500 ꢀC for 30 min. The
walls to a hydrothermal environment. These reactors were then adsorption of catalyst was performed by passing a gas mixture,
cleaned with acetone and dried by air, waiting for use.
containing NH3 with N2 as balance gas, through the sample bed
29528 | RSC Adv., 2014, 4, 29527–29533
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ꢃ1
ꢀ
22
Table 1 Tentative identities and area % of major peaks in total ion
chromatograms
at 100 C for 2 h with a total ow rate of 100 ml min
. TPD
measurements were carried out from 100 to 650 ꢀC in a heating
rate of 10 C minꢃ1, with He as the carrying gas. The effluent
ꢀ
Peak no. Compound Name
Structure
Area/% RT/min
stream was monitored continuously with a thermal conductivity
detector (TCD) to determine the rate of ammonia desorption.
1
2
Phenol
8.64
1.63
4.80
3. Results and discussion
3.1. Products identication
2-Allylphenol
10.42
Fig. 1 shows the total ion chromatogram from the tandem
Claisen rearrangement–intramolecular hydroaryloxylation (CR–
IH) of APE in SBW at 260 ꢀC for 20 min. The peaks in Fig. 1 were
identied by using a National Institute of Standards and
Technology (NIST) mass spectral database with the GC-MS.
Table 1 provides the detailed information about the compound
identication and peak area % of the major components in the
reaction products. Table 1 shows that 2-methyl-2,3-dihy-
drobenzofuran (peak 4 in Fig. 1) is by far the most abundant
products from catalytic the CR–IH of APE in SBW. 2-Allylphenol
(peak 2 in Fig. 1) is an abundant intermediate product (peak 3 in
Fig. 1) could be resulted from migration of the double bond in
product 2 with the aromatic ring,28 which was less stable likely
and would undergo thermal cyclization to form product 3 and 2-
methylbenzofuran (peak 6 in Fig. 1). 3-(2-Hydroxyphenyl)-2-
propanol (peak 7 in Fig. 1) was formed by the hydration of the
allylic double bond of product 2. Obviously, those compounds
could be decomposed into phenol (peak 1 in Fig. 1) in SBW.
3
4
(E,Z)-2-Propenylphenol
4.51
5.31
6.12
2-Methyl-2,3-
dihydrobenzofuran
63.24
5
6
Allyl phenyl ether
4.20
3.23
7.17
7.48
2-Methylbenzofuran
3-(2-Hydroxyphenyl)-
2-propanol
7
4.76
7.65
3.2. Effects of temperature
Previous study28 suggested that temperature was the most
inuential parameter upon the products yield from the reaction
of APE in high temperature. Therefore, temperature was
selected as the rst parameter rather than other variables to
investigate its inuence on the conversion of APE and yield of
dominant product of 2-methyl-2,3-dihydrobenzofuran.
The reactions were conducted at a xed reaction time of 15
min with varying temperature from 200 to 320 ꢀC. Yield of
2-methyl-2,3-dihydrobenzofuran are plotted as a function of
temperature in Fig. 2. The yield of 2-methyl-2,3-dihy-
drobenzofuran increased with increasing temperature and hit
the highest value at 260 ꢀC. Thereaer, the yield of 2-methyl-2,3-
dihydrobenzofuran decreased due to its further reaction or the
promotion of side reactions at more severe temperatures. Fox
example, 2-methyl-2,3-dihydrobenzofuran could partially
decompose to 2-allylphenol and 2-propenylphenol from the ring
opening reaction at high temperature water.29
Fig. 3 shows the yield of selected product 4 and the corre-
sponding conversion of APE at four different temperatures
without catalysts. It shows that the conversion of APE is always
the lowest at 200 ꢀC. Increasing temperature improved the
conversion of APE and the yield of 2-methyl-2,3-dihy-
drobenzofuran. The yield of 2-methyl-2,3-dihydrobenzofuran is
very low (<5%) as the conversion of APE is less than 20%, sug-
gesting that formation of 4 was a multiple step reaction which
was difficult to occur at low temperatures.
In the CR–IH of APE, changes of APE–water molar ratio also
affected the yield of 2-methyl-2,3-dihydrobenzofuran. At
different APE–water molar ratios, the highest yield of 2-methyl-
2,3-dihydrobenzofuran was always observed at a temperature of
260 ꢀC. Fig. 2 shows that the yield of 2-methyl-2,3-dihy-
drobenzofuran increased with increasing the APE–water molar
ratio. The highest yield (65%) of 2-methyl-2,3-dihy-
ꢀ
drobenzofuran was achieved at 260 C.
3.3. Effect of time
Batch experiments in two different mediums, i.e., with and
without HZSM-5 catalyst were carried out with the reactor at
Fig. 1 Total ion chromatogram for the major products from the CR–
IH of APE in SBW at 260 ꢀC for 20 min.
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Fig. 4 CR–IH of APE at 260 ꢀC without (a) and with (b) HZSM-5(Si : Al
¼ 22 : 1), the yield of APE ( ), 2-methyl-2,3-dihydrobenzofuran ( ), 2-
allylphenol ( ), other products ( ) and selectivity of 2-methyl-2,3-
dihydrobenzofuran 4 ( ).
Fig. 2 Effect of temperature on the yield of 2-methyl-2,3-dihy-
drobenzofuran at different APE–water mole ratios of: 1 : 20( ),
1 : 40( ), 1 : 60( ), and 1 : 80( ) at a reaction time of 15 min.
260 ꢀC for a reaction time ranging from 5 to 30 min. The
conversion of APE, the contents of products and selectivity from
these two medium systems are displayed in Fig. 4a and b,
respectively.
At a reaction time of 15 min, addition of HZSM-5 increased
the conversion from 87% without catalyst to 95% in the pres-
ence of HZSM-5. Higher yield and selectivity of 2-methyl-2,3-
dihydrobenzofuran was also observed in the presence of HZSM-
5 relative to the uncatalyzed reaction. The yield of 2-allylphenol
and other products also decreased in the presence of catalyst.
These result suggested that addition of catalyst is necessary if
one expect to achieved higher yield and selectivity of 2,3-
dihydrobenzofuran.
Scheme 2 Possible reaction network for the reaction of APE under
hydrothermal condition.
Steps 1–3 are the main reaction paths of the CR–IH of APE.
Step 1 involves Claisen rearrangement to form 2-allylphenol (2).
Then two parallel reaction pathways are available for 2-methyl-
2,3-dihydrobenzofuran (4) in this hydrothermal environment:
undergoes C–O bond ring closure to form 4 (step 2) or isomer-
ization to form (E,Z)-2-propenylphenol (3) (step 4) then hydro-
alkoxylation (step 3). Apart for the main reactions, there exist
some side reactions. 2 could proceed in Markovnikov fashion
with addition of water (step 5) into 3-(2-hydroxyphenyl)-2-
propanol (7).30 On the other hand, 7 could also eliminate water
to give 3 in step 11. Substrate 3 was also cyclized to furnish
benzofuran (6) in step 6. Those compounds could decompose
thorough steps 7–10 into phenol (5).
3.4. Reaction paths
Based on the products distribution from the reaction of APE
under hydrothermal conditions, we propose a possible reaction
network of APE in Scheme 2 which can provide possible method
on how to suppress by-products formation and how to improve
the yield of dihydrobenzofuran through process optimization
and catalyst screening study.
3.5. Effect of catalyst
3.5.1. Catalyst activity. Table 2 shows the Claisen rear-
rangement–intermolecular hydroaryloxylation of using SBA-15,
TS-1, HZSM-5(Si : Al ¼ 22 : 1) and HZSM-5(Si : Al ¼ 200 : 1) as
ꢀ
catalysts in hydrothermal condition under 260 C. 10 mg cata-
lysts were employed into each reaction, respectively. The acidity
of catalysts was also determined by NH3-TPD experiment and
the NH3-TPD patterns were shown in Fig. 5. In Table 2, the
conversion using SBA-15 as the catalyst is almost the same as
the conversion without catalyst, which indicates that the SBA-
15's catalytic activity could be negligible in the selective reac-
tion. The conversion TS-1 and different ratio of Si : Al catalysts
of HZSM-5 could get range from 94–96%. Nevertheless, the
selectivity only reach 82% with TS-1 compared with 95% HZSM-
Fig. 3 Yield of 2-methyl-2,3-dihydrobenzofuran and conversion of
APE under different temperatures.
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Table 2 Different catalysts on the tandem Claisen hydroaryloxylation
of APE
can accelerate the tandem Claisen hydroaryloxylation reaction
more effectively than HZSM-5 with weaker acidity (high Si/Al
ratio) in subcritical water condition process.
Yield
Selectivity Yield of other
Entrya
Conversion/% of 4/% of 4/%
products/%
3.5.2. Recycling of catalyst. It is important to point out that
all of the catalysts were separated from the products through
ltration and could be easily reused. Fig. 6 shows the XRD
patterns of the samples before and aer the reaction. As shown
in Fig. 6, the structure did not change aer the reactions; the
crystal structure of HZSM-5 was maintained aer the reactions.
The results of the repeated use of the catalyst HZSM-5 (Si : Al
¼ 22 : 1) for the Claisen hydroaryloxylation of APE were
summarized in Table 3. The use of the recovered catalyst also
produced an excellent yield (run 6, 88%). The NH3-TPD curve for
the sixth recovered catalyst was shown in Fig. 5, which was
similar to the fresh HZSM-5 catalyst. These results clearly
indicate that the catalyst is low deactivation in hydrothermal
condition and can be used repeatedly without loss of activity.
Without
catalyst
87
65
73
23
SBA-15
TS-1
HZSM-5
(Si/Al ¼ 200)
HZSM-5
(Si/Al ¼ 22)
90
94
94
65
78
85
72
82
90
20
12
2
96
91
95
1–2
a
General reaction conditions: 260 ꢀC, APE–water ¼ 1 : 40, 15 min, 10
mg catalyst.
3.6. CR–IH of different substrates with catalyst
CR–IH reactions with a wide range of APE derivatives were
tested. Table 4 shows the results. High yields are obtained from
the reactions of allyl p-tolyl ether and allyl o-tolyl ether (95% and
91%, entries 1–2). The APE with meta position of CH3 was also
employed in the Claisen hydroaryloxylation. 2,6-dimethyl-2,3-
dihydrobenzofuran and 2,5-dimethyl-2,3-dihydrobenzofuran
were obtained as two major products, with 63% and 30%
reaction yield, respectively (entry 3). The reactions of APE
bearing electron-withdrawing groups such as NO2, OCH3, Cl
and Br at the para position, respectively, with proceeded
Fig. 5 NH3-TPD curves of different zeolites, HZSM-5(Si : Al ¼ 200 : 1)
( ), HZSM-5(Si : Al ¼ 22 : 1) ( ), TS-1 ( ), SBA-15 ( ), sixth recovered
HZSM-5(Si : Al ¼ 22 : 1) ( ).
5(Si : Al ¼ 22 : 1). Simultaneously, only trace amounts of by-
products (<2%) was detected in the reaction with HZSM-5
catalyst (Si : Al ¼ 22 : 1), exhibiting the high activity.
Due to the framework of crystalline microporous alumino-
silicates in zeolites, the charge-compensating protons of the
oxygen anions bridging between Al and Si give rise to strong
Brønsted acidity.31 Zeolites usually offer good selectivity owing
to the shape selective characteristics. They have been broadly
applied in industry, i.e., product shape, reactant shape, and
restricted transition state shape selectivity. In order to investi-
gate the relationship between zeolites and selectivity, the acidity
of zeolites should be investigated. As shown in Fig. 5, there are
no evident peaks in the TPD prole of pure silica SBA-15
material, indicating that SBA-15 material has no acid site.
HZSM-5 with different Si/Al ratios shoꢀws two desorption peaks,
one focused at the range of 150–250 C and the other at 350–
450 ꢀC, representing the Brønsted acid sites.32 Only one
desorption peak was observed in the NH3-TPD prole of TS-1,
which is very weak compared with that of HZSM-5. To the
different ratios of Si : Al in HZSM-5 catalysts, the higher Si/Al
ratio has weaker acid sites corresponding to weaker acidity.34
Compared the results in Table 2 and Fig. 5, we can get the
conclusion that HZSM-5 with stronger acidity (low Si/Al ratio)
Fig. 6 XRD patterns of original HZSM-5, fourth recovered HZSM-5
and sixth recovered HZSM-5 catalysts.
Table
3 Recycled use of the HZSM-5 catalyst in using Claisen
hydroaryloxylation of APE
Run
Yielda (%)
Run
Yield (%)
1
2
3
91
89
90
4
5
6
88
89
88
a
Reaction condition: 260 ꢀC, APE–water ¼ 1 : 40, 10 min, 10 mg
catalyst.
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Table HZSM-5 catalyzed CR–IH of APE under hydrothermal Table 4 (Contd.)
Paper
4
conditions
Entrya
Substrate
Product
Yield
87%
Entrya
Substrate
Product
Yield
93%
13
1
a
Reaction conditions: 260 ꢀC, 15 min, 10 mg HZSM-5 (Si : Al ¼ 22 : 1)
b
(entry 1–9 and 11–13). 280 ꢀC, 20 min, 10 mg HZSM-5 (Si : Al ¼
22 : 1) (entry 10).
2
3
91%
smoothly to furnish in good yields (87–95%, entries 4–7). Even
APE bearing C(CH3)3 or NH2, as electron-donating group, at the
para position, the reaction also gives excellent yield (90% and
91%, entries 8–9). Then, 1-(allyloxy)naphthalene is treated in
the subcritical water to offer the desired yield (83%, entry 10),
which was higher yield and less time than reported.11 Finally,
methallyl phenyl ether, 3-methyl-allyloxybenzene and allyl-
(phenyl)sulfane are conducted in the hydrothermal condition,
which acquire excellent yields(85%, 90% and 87%, entries 11–
13). The presence of S and dimethyl systems in the products is
very useful for further synthetic elaborations, which could be
synthesized to pharmaceuticals for injury, antioxidant, and
hepatopathy.33 All of the products are analyzed by 1H NMR and
their data were consistent with those reported in the literature.34
A 61%, B 26%
4
5
6
7
8
9
95%
87%
93%
91%
91%
90%
4. Conclusion
In summary, the current investigations have demonstrated that
Claisen rearrangement–intermolecular hydroaryloxylation was
successfully treated in the hydrothermal condition between
ꢀ
200–320 C. The presence of temperature and HZSM-5 catalyst
signicantly affected the product yields. The reaction can have a
lot of by-products in high temperature. Then, the moderate
temperature examined (260 ꢀC) appears to be the best for
producing dihydrobenzofuran in excellent yield. The higher
acidity of HZSM-5 with low ratio of Si/Al can effectively improve
the selectivity. A range of allyl phenyl ether derivatives exhibit
desired yield (83–95%) via subcritical water. These results
clearly indicate that the mild procedure is easy to set up and
allows the transformation of various substituted allyl phenol
ethers in the hydrothermal condition with HZSM-5 catalyst.
10b
11
82%
85%
90%
Acknowledgements
The authors wish to express their gratitude for the nancial
support received from the National Natural Science Foundation
of China (no. 21073064, 21003049).
12
Notes and references
`
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