Tetrahydropyranylation of alcohols and phenols using polystyrene supported lewis acids as catalysts

Article abstract of DOI:10.1002/cjoc.201200713

Polystyrene supported TiCl₄ (Ps-TiCl₄) and polystyrene supported FeCl₃(Ps-FeCl₃) were prepared by coordinating Lewis acids with polystyrene. The catalysts were characterized by TGA, BET, SEM, IR and pyridine-adsorbed IR. The loading of Ps-TiCl₄ and Ps-FeCl₃ were 0.35 and 0.3 mmol·g^-1 respectively. Both catalysts were found to be efficient for the tetrahydropyranylation and detetrahydropyranylation of various alcohols and phenols in different solvents. Two catalysts can be recovered and reused for five times with good activity. Polystyrene supported TiCl₄ (Ps-TiCl₄) and polystyrene supported FeCl₃(Ps-FeCl ₃) were prepared by coordinating Lewis acids with polystyrene. The catalysts were characterized by TGA, BET, SEM, IR and pyridine-adsorbed IR. The loading of Ps-TiCl₄ and Ps-FeCl₃ were 0.35 and 0.3 mmol·g^-1 respectively. Both catalysts were found to be efficient for the tetrahydropyranylation and detetrahydropyranylation of various alcohols and phenols in different solvents. Two catalysts can be recovered and reused for five times with good activity. Copyright

Full text of DOI:10.1002/cjoc.201200713

FULL PAPER
DOI: 10.1002/cjoc.201200713
Tetrahydropyranylation of Alcohols and Phenols Using Poly-
styrene Supported Lewis Acids as Catalysts
Zhang, Yi(张毅)
Dou, Qianqian(窦倩倩)
Liu, Yuan(刘员)
Dai, Liyan*(戴立言)
Wang, Xiaozhong(王晓钟)
Chen, Yingqi(陈英奇)
Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China
Polystyrene supported TiCl4 (Ps-TiCl4) and polystyrene supported FeCl3 (Ps-FeCl3) were prepared by coordi-
nating Lewis acids with polystyrene. The catalysts were characterized by TGA, BET, SEM, IR and pyri-
dine-adsorbed IR. The loading of Ps-TiCl4 and Ps-FeCl3 were 0.35 and 0.3 mmol•g－ respectively. Both catalysts
1
were found to be efficient for the tetrahydropyranylation and detetrahydropyranylation of various alcohols and
phenols in different solvents. Two catalysts can be recovered and reused for five times with good activity.
Keywords tetrahydropyranylation/detetrahydropyranylation, Ps-TiCl4, Ps-FeCl3, polystyrene supported, Lewis
acids
Introduction
ied, while the research on supporting of TiCl4 and FeCl3
is relative rare. In view of the wide applications and
tunable reactivities of them in organic reactions, we be-
gan the study of the preparation of Ps-TiCl4 and Ps-
FeCl3 and their physical and chemical properties.
In the context of green chemistry, polymer supported
Lewis acids are a class of important atom-economic and
environmentally benign catalytic systems that have been
developed and applied successfully in a variety of
chemical reactions.[1] Generally, polymer-supported
Lewis acids are prepared by immobilization of Lewis
acids on solid supports such as polymeric microspheres
or inorganic materials via covalent bonds or coordina-
tion. Heterogeneous supported Lewis acids active as its
homogeneous counterpart have several distinguishing
advantages like easy workup and separation from the
reaction system, recyclability, nontoxicity, enhanced
stability, etc.[2]
TiCl4 and FeCl3 are important Lewis acids that show
powerful catalytic activity in a series of organic reac-
tions.[3] Both of them are water-sensitive, TiCl4 will
even lose its activity in moist environments because it
reacts with water easily. Thus, strict anhydrous condi-
tions and stoichiometric amounts or more are needed in
order to achieve good yields. Furthermore, tedious
workup procedure after reaction to hydrolyze excess
catalyst produces large amounts of acidic waste water.
Hydrophobic polystyrene which protects water-sensitive
Lewis acids from hydrolysis is used extensively as sup-
port in the field of heterogeneous immobilized Lewis
acids. For example, Neckers et al.[4] first reported poly-
styrene supported AlCl3 as a catalyst in Friedel-Crafts
reaction,[5] and its applications in synthesis of pyrroles,[6]
and preparation of dihydropyrimidinones.[7] Till now,
the immobilization of AlCl3 has been extensively stud-
Protection and deprotection of the hydroxy group in
multistep organic synthesis in the manufacture of phar-
maceuticals and other natural products are inevitable.
Amongst many protocols used for the protection of al-
cohols and phenols, tetrahydropyranylation is one of the
most frequently used method due to its easy introduc-
tion and removal, low cost, and stability of tetrahydro-
pyranyl ethers (THP ethers) towards acylating and al-
kylating reagents, metal hydrides, redox reagents, Grig-
nard reagents and catalytic hydrogenation. A large
number of reagents have been developed as catalysts for
tetrahydropyranylation, such as Al(OTf)3,[8] PPTS,[9]
sulfonic acid functionalized carbon catalyst,[10] modified
zeolites,[11] PdCl2(CH3CN)2,[12] benzene sulfonamide
(BNBBS),[13] CuSO4•5H2O,[14] Bi(OTf)3•4H2O,[15]
LiPF6,[16] polyaniline salt,[17] dialkylimidazolium tetra-
chloroaluminates,[18] AlCl3·6H2O,[19] ceric ammonium
nitrate,[20]
vanadyl
(IV)
acetate,[21]
lithium
perchlorate,[22] TaCl5-Silica gel,[23] and polystyrene
supported Lewis acids,[24] etc.
However, some of these methods have several dis-
advantages including long reaction time, harsh reaction
conditions, involving moisture sensitive reagents and
tedious experimental procedures. Thus, it is still neces-
sary to develop a simple and efficient method for tetra-
hydropyranylation/detetrahydropyranylation. In the
*
E-mail: dailiyan@zju.edu.cn
Received July 12, 2012; accepted August 28, 2012.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201200713 or from the author.
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Zhang et al.
FULL PAPER
[25]
Typical procedure for preparation of Ps-TiCl4
present work, we report the preparation and characteri-
zation of Ps-TiCl4 and Ps-FeCl3 as well as their use as
catalysts for tetrahydropyranylation of alcohols and
phenols with 3,4-dihydro-2H-pyran and the subsequent
detetrahydropyranylation of the obtained THP ethers
with methanol (Scheme 1).
Excessive anhydrous TiCl4 was added to polystyrene
(8% divinylbenzene, 3.5 g) in carbon disulfide (25 mL).
The mixture was stirred under reflux for 2.5—3 h,
cooled to room temperature and then poured into ice
water (50 mL) to destroy excess TiCl4. The mixture was
stirred until the brown-red color disappeared and the
catalyst became light yellow. It was then filtered, and
the polymer beads were collected, washed with water
(350 mL), then with ether, acetone and ether, and dried
to constant weight under vacuum at 40 ℃.
Scheme 1 Tetrahydropyranylation of alcohols and phenols and
depyranylation catalyzed by Ps-Lewis acid
O
+
O
Ps-Lewis acid/CH2Cl2
R
O
O
+
R
R
H
[26]
O
Typical procedure for preparation of Ps-FeCl3
Anhydrous FeCl3 (4.5 g) was added to polystyrene
(8% divinylbenzene, 3.0 g) in chloroform (30 mL). The
mixture was stirred under reflux for 2 h and cooled to
room temperature. The polymer beads were then filtered
and washed with chloroform (100 mL) and ethyl ether
(100 mL) and dried in vacuum overnight at 40 ℃.
O
O
O
Ps-Lewis acid/MeOH
O
R
H
R=alkyl/aryl
Experimental
Typical procedure for tetrahydropyranylation
To a solution of 1-butanol (10 mmol) and 3,4-dihy-
dro-2H-pyran (DHP 12 mmol) in methylene chloride
(10 mL), Ps-TiCl4 or Ps-FeCl3 (1.5 mmol) was added,
and the reaction mixture was stirred at room tempera-
ture, after the completion of the reaction as monitored
by TLC, the mixture was filtered. Ps-TiCl4 or Ps-FeCl3
was washed with methylene chloride to remove any
physisorbed reagents, dried and used in next run under
the same reaction conditions. The filtrate was concen-
trated under reduced pressure, and then it was purified
through a short basic alumina column to give the almost
pure product. All the obtained tetrahydropyranyl ethers
were characterized by 1H NMR spectroscopy.
General
Polystyrene (8% divinylbenzene) was purchased
from Nanjing Microspheres Hi-Efficiency Isolation
Carrier Co., Ltd. It was washed with chloroform, ace-
tone, water and methanol, and then dried to constant
weight under vacuum before use. Chemicals used were
analytical reagent and purchased from Sinopharm
Chemical Reagent Co. Ltd. 1H NMR spectra were re-
corded in CDCl3 using TMS as an internal standard on a
Bruker spectrophotometer at 500 MHz.
Characterization
Thermal analysis tests were made in SDT Q600 TA
equipment. The samples were heated from 20 ℃ up to
800 ℃ at a rate of 10 ℃•min-1 in air atmosphere with
flow rate of 120 mL•min－1 without a reference material.
The amount used for this experiment was 10 mg for
each sample. The surface properties such as BET spe-
cific surface area, total pore volume, and mean pore
diameter were determined from N2 adsorption isotherms
at 77 K with a Micrometrics Tristar3020. Specific sur-
face areas and pore size distribution were calculated
with the BET equation and the BJH method respectively.
The total pore volume was taken from the desorption
branch of the isotherm at p/p0＝0.95, assuming com-
plete pore saturation. SEM images were recorded on a
scanning electron microscope (SEM, S-4800) at an op-
erating voltage of 3.0 kV.
The polystyrene, Ps-TiCl4 and Ps-FeCl3 were dehy-
drated in an Infrared (IR) cell at 100 ℃ for 2 h under
vacuum before pyridine adsorption. Pyridine was led
into the IR cell at 100 ℃. Pyridine adsorption IR spec-
tra of polystyrene, Ps-TiCl4 and Ps-FeCl3 were meas-
ured at room temperature after desorption at 100 ℃ for
2 h using a Bruker Tensor 27.
2-(Allyloxy)tetrahydro-2H-pyran Colorless liq-
uid; 1H NMR (CDCl3, 500 MHz) δ: 5.95—5.91 (m, 1H),
2
3
2
5.30 (dd, J＝1.5, J＝17 Hz, 1H), 5.17 (dd, J＝1.5,
3J＝10.5 Hz, 1H), 4.65 (t, J＝3.5, 1H), 4.24 (dd, 2J＝5,
3J＝13 Hz, 1H), 3.98 (dd, J＝6, J＝13 Hz, 1H), 3.87
(q, J＝5.5 Hz, 1H), 3.51 (t, J＝5 Hz, 1H), 1.86—1.84
(m, 1H), 1.736—1.730 (m, 1H), 1.64—1.51 (m, 4H).
2
3
Typical procedure for deprotection of tetrahydro-
pyranyl ethers
To a solution of THP ether (10 mmol) in methanol
(15 mL), Ps-TiCl4 or Ps-FeCl3 (1.5 mmol) was added,
and the reaction mixture was stirred at room tempera-
ture or under reflux, after the completion of the reaction
as monitored by TLC, the mixture was filtered, and the
filtrate was concentrated under reduced pressure to give
the corresponding alcohol.
Results and Discussion
Two supported Lewis acids (Ps-TiCl4, Ps-FeCl3)
were prepared through immobilizing anhydrous Lewis
acids on polystyrene (8% divinylbenzene) in carbon
disulfide or chloroform under reflux. Although free
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Tetrahydropyranylation of Alcohols and Phenols Using Polystyrene Supported Lewis Acids as Catalysts
TiCl4 and FeCl3 were moisture sensitive and corrosive,
polystyrene supported TiCl4 and FeCl3 were stable, wa-
ter-resistant and could be stored in air for a few months
without appreciable loss of activity. The supported
catalysts were characterized by TGA, BET, IR, pyri-
dine-adsorbed IR and SEM.
the surface area and pore volume, but the reduction of
pore diameter, and these results were explained by
SEM.
Table 1 Physical and chemical properties of support and cata-
lyst
The thermal stability of Ps, Ps-TiCl4 and Ps-FeCl3
was measured by TGA analysis (Figure 1). Little weight
loss (less than 1.1% of total weight) was observed over
the range from 20 to 350 ℃ for Ps which indicated that
Ps was chemically stable. 2.17% and 7.74% of initial
weight loss which was due to the removal of adsorbed
water were observed for Ps-TiCl4 and Ps-FeCl3 respec-
tively, then the biggest weight loss of Ps, Ps-TiCl4 and
Ps-FeCl3 were observed from 400 to 500 ℃ which was
attributed to the decomposition of samples. As was
shown in the DTA curves, the endothermic peaks of Ps,
Ps-TiCl4 and Ps-FeCl3 were observed at 421, 431 and
430 ℃, respectively. It is clear that the thermal stability
of Ps was increased after the immobilization.
Pore
Catalyst
Surface area/ Pore volume/
Sample
diameter/ loading a/
(m2•g-－
)
(mL•g－
)
1
1
(mmol•g－
0
)
1
nm
Polystyrene
Ps-TiCl4
13.35
2.65
0.13
0.01
0.16
38.9
17.6
26.0
0.35
Ps-FeCl3
28.40
0.30
a
The capacity of the supported catalysts based on its chloride
content.[27]
SEM images of Ps, Ps-TiCl4, and Ps-FeCl3 were
shown in Figure 2. From the image of Ps, the morphol-
ogy of Ps was smooth ball with many pores on the sur-
face, and it was shown that the surface of Ps-TiCl4 was
dispersed with small amount of TiO2 particles, which
was due to the hydrolysis of TiCl4 in the preparation
process. In the image of Ps-FeCl3, lots of small irregular
FeCl3 molecules were adsorbed on the surface of the
ball which led to the increase in surface area and pore
volume of Ps-FeCl3 compared with Ps.
Figure 1 TGA thermograms for Ps, Ps-TiCl4 and Ps-FeCl3.
BET surface area, pore volume and pore diameter of
polystyrene and Ps-Lewis acids were measured after
drying in vacuum at 90 ℃ for 3 h in order to verify the
introduction of Lewis acids onto polystyrene. In the
case of polystyrene (Table 1), the surface area was
－
1
found to be 13.35 m2•g , the pore volume was 0.13
mL•g－ and pore diameter was 38.9 nm, therefore, it
1
was classified as a mesoporous material. For Ps-TiCl4,
－
1
the surface area was 2.65 m2•g , the pore volume was
0.01 mL•g－1 and pore diameter was 17.6 nm, indicating
that immobilization of TiCl4 on polystyrene resulted in a
reduction of surface area, pore volume, and pore di-
ameter. This was because that the difference in specific
density between TiCl4 and polystyrene had an effect on
the specific surface area, and the immobilization
blocked pores of polystyrene. Interestingly, immobiliza-
tion of FeCl3 on polystyrene resulted in an increase of
Figure 2 SEM images of Ps (a), Ps-TiCl4 (b), Ps-FeCl3 (c).
The infrared spectra of polystyrene, Ps-TiCl4 and
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Zhang et al.
FULL PAPER
Ps-FeCl3 were shown in Figure 3. The peak at 3026
was selected to determine the optimal reaction condi-
tions. Various solvents such as toluene, cyclohexane,
n-hexane, acetonitrile, water, methylene chloride were
tried for the reaction, and methylene chloride was
proved to be the best one. It was because the polysty-
rene swelled in methylene chloride, so the substrates
could smoothly access to the catalytic centers wrapped
in the pores of polystyrene. 80—180 min of reaction
time was needed for good yields, while only 50 min was
needed for the tetrahydropyranylation catalyzed by an-
hydrous TiCl4 because the acidity of the supported TiCl4
was weaker than that of anhydrous TiCl4. Moreover,
mesoporous pores and channels of polystyrene were
unfavorable for the molecular mass transfer process.
The amount of Ps-Lewis acid had influence on the yield,
15 mol% of catalyst was sufficient to give 1-butanol
tetrahydropyranyl ether in 94% yield, however, in-
creasing the amount of catalyst did not improve the
yields, and the reaction did not occurred without cata-
lysts or only with polystyrene. To find out whether the
reaction took place in the solid matrix of Ps-TiCl4 and
Ps-FeCl3 or catalyzed by free TiCl4 and FeCl3 released
from the support, Ps-TiCl4 and Ps-FeCl3 were stirred in
methylene chloride for 120 min, and filtered off, then
reactants were added to the filtrates and stirred for 120
min, and no products appeared. Encouraged by the re-
sults, we investigated different kinds of alcohols (pri-
mary, secondary, tertiary, allylic and benzylic) and
phenols using Ps-TiCl4 and Ps-FeCl3 as catalysts to give
the corresponding THP ethers at room temperature or
under reflux (for Ps-FeCl3) in good to excellent yields
(Table 2). As shown in Table 2, the order of catalytic
activity of the two catalysts was Ps-TiCl4＞Ps-FeCl3.
This was because the acidity of TiCl4 was stronger than
that of FeCl3.
cm－ was related to C—H stretching vibration of ben-
1
zene ring, and the peak at 2920 and 2850 cm－1 belonged
to C—H stretching vibration of CH2. Comparing the IR
spectra of supported catalysts and polystyrene, it is
shown that the main difference lies in 1450—1500 and
－
1
2900—3100 cm . Those regions of IR spectra corre-
sponded to C＝C and C—H stretching vibration of aro-
matic ring. These data proved that the complex was
formed between the Lewis acids and polystyrene.[25]
Figure 3 FTIR spectra of Ps, Ps-TiCl4 and Ps-FeCl3.
The nature of the surface acidic sites in solid was
defined by Bronsted acidic site and Lewis acidic site.
FT-IR spectroscopy of adsorbed pyridine is a useful
technique widely used for distinguishing of Lewis and
Bronsted acid sites. The adsorbed pyridine tends to cou-
ple with protonic (Bronsted) or aprotic (Lewis) acidic
centers through the nitrogen lone-pair electrons and thus
could be detected by monitoring their ring vibrations.
The FT-IR spectra of pyridine adsorbed on two
Ps-Lewis acids samples in Figure 4 showed characteris-
As to the reaction process, it could be rationalized
based on the theoretical and experimental approaches
reported in the literature. The mechanism pathway may
proceed via the coordination of Ps-TiCl4 to the oxygen
atom of alcohol generating intermediate complex A.
This intermediate may be in equilibrium with chelate B
(in which the Ps-TiCl4 is coordinated with both the
oxygen atom of the alcohol as well as the C＝C double
bond of DHP) and complex C (in which only the C＝C
double bond is coordinated with the Ps-TiCl4) (Figure
5).[29] Finally, these transition states lead to the forma-
tion of THP ethers.
tic bands at 1450 cm － which was attributed to
1
L-sites,[28] however, no band was monitored on Ps sam-
ple. It was proved again that the Lewis acids were suc-
cessfully immobilized on polystyrene.
Interestingly, we found that Ps-TiCl4 and Ps-FeCl3
could be used for depyranylation of THP ether further.
For example, the Benzyl alcohol tetrahydropyranyl eth-
er was depyranylated in methanol within 95 min in 94%
yield with 15 mol% of Ps-TiCl4. Similarly various THP
ethers were converted to corresponding alcohols and
phenols within 95—150 min in 81%—94% yields with
Ps-TiCl4 or Ps-FeCl3, respectively, which was shown in
Table 3. The recycling property which was a key factor
in application was studied further, and it was found that
Ps-FeCl3 and Ps-TiCl4 could be reused for five runs
Figure 4 Pyridine-adsorbed FTIR of Ps, Ps-TiCl4 and Ps-FeCl3.
In our initial research, a model reaction of 1-butanol
(10 mmol) and 3,4-dihydro-2H-pyran (DHP 12 mmol)
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Tetrahydropyranylation of Alcohols and Phenols Using Polystyrene Supported Lewis Acids as Catalysts
Table 2 Tetrahydropyranylation of alcohols and phenols using
Ps-TiCl4 and Ps-FeCl3
Ps-TiCl4
a
O
+
R
H
O
Entry
Product
Catalyst Time/min Yield/%
R
H
B
O
O
O
A
B
A
B
A
B
A
B
A
115
120
110
120
140
160
120
125
120
93
78
93
78
91
79
94
80
93
O
R
H
Cl4Ti
1
O
Cl4Ti
O
TiCl4
A
O
O
C
2
3
4
TiCl4
O
O
O
O
O
R
R O
H
Ps-TiCl4
O
O
Figure 5 Proposed mechanism for tetrahydropyranylation of
alcohol catalyzed by Ps-TiCl4.
5
6
7
Table
Ps-TiCl4 and Ps-FeCl3
3
Detetrahydropyranylation of THP ethers using
B
125
79
O
O
3
a
A
B
A
125
145
120
91
79
93
Entry
1
Product
Catalyst Time/min Yield/%
O
O
A
B
A
B
A
B
A
B
A
B
A
B
110
120
95
94
87
94
87
94
85
93
84
93
85
90
81
OH
OH
B
135
80
O
O
O
4
2
3
4
5
6
115
110
120
125
135
120
125
135
150
O
O
A
B
A
B
A
B
A
B
A
135
145
125
135
125
135
140
150
80
92
79
93
80
93
80
90
79
95
8
9
OH
O
O
O
6
7
8
OH
OH
10
11
O
OH
a All reactions catalyzed by Ps-TiCl4 were carried out in methanol
at room temperature, all reactions catalyzed by Ps-FeCl3 were
carried out in methanol under reflux condition; A＝Ps-TiCl4, B＝
Ps-FeCl3.
12
O
O
B
100
81
O
O
A
B
A
B
A
100
115
95
87
76
89
78
85
13
14
with good catalytic activity. The yield of each run was
94%, 94%, 92%, 87%, 85% for Ps-TiCl4; for Ps-FeCl3,
it was 81%, 81%, 80%, 76%, 75%. Further comparison
of Ps-TiCl4 and Ps-FeCl3 with various reported catalysts
in tetrahydropyranylation was made, as shown in Table
4, better yields, easy separation and low cost were the
obvious advantages when using them as catalysts.
O
O
110
150
O
O
O
O
15
B
180
72
Conclusions
A
B
135
145
91
79
In the present methodology, Ps-TiCl4 and Ps-FeCl3
as effective catalysts for tetrahydropyranylation and
depyranylation of alcohols and phenols was studied.
TGA analysis showed that polystyrene was chemically
stable and its stability was increased after immobiliza-
tion with TiCl4 or FeCl3. Pore analysis, SEM images as
well as FT-IR spectroscopy indicated that TiCl4 and
FeCl3 were successfully supported on polystyrene.
16
O
a
All reactions catalyzed by Ps-TiCl4 were carried out in CH2Cl2
at room temperature except for the phenols (Entries 13—15),
which were carried out under reflux condition; all reactions cata-
lyzed by Ps-FeCl3 were carried out in CH2Cl2 under reflux condi-
tion; A＝Ps-TiCl4, B＝Ps-FeCl3.
Chin. J. Chem. 2012, 30, 2567—2572
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Table 4 Comparisons of Ps-TiCl4 and Ps-FeCl3 with various
catalysts in tetrahydropyranylationa
Entry Catalyst
Mol/%
15
Time/min Yield/%
1
2
3
4
5
6
7
8
Ps-TiCl4
80
100
42
95
Ps-FeCl3
15
81
Ps-AlCl3
15
97[24a]
97[24c]
96[10]
91[14]
72[12]
59[17]
Ps-tin porphyrin
Carbon catalyst
CuSO4•5H2O
PdCl2(CH3CN)2
polyaniline salt
1
10b
4
120
40
20
10
60
17
480
a Substrate: benzyl alcohol and DHP. b Mass percentage.
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Experimental results suggested that tetrahydropyranyla-
tion occurred in the solid matrix of Ps-TiCl4 and
Ps-FeCl3 rather than catalyzed by free TiCl4 and FeCl3
released from the support. Mild reaction conditions,
good to excellent yields, easy preparation and low cost
were the obvious advantages of the present approach,
and these two supported catalysts can be recovered and
reused for five times with good activity.
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Acknowledgement
The authors would like to thank Dr. Linshen Chen
for TGA, Dr. Jingke Zhu for infrared spectra and Dr.
Zhengbao Wang for pyridine-adsorbed IR. This work
was supported by the NSFC (No. 21176213), and Zhe-
jiang Key Innovation Team of Green Pharmaceutical
Technology (No. 2010R50043).
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