Polystyrene-supported GaCl3: A new, highly efficient and recyclable heterogeneous Lewis acid catalyst for tetrahydropyranylation of alcohols and phenols

Article abstract of DOI:10.1016/j.poly.2012.06.063

A new, simple and highly chemoselective method for tetrahydropyranylation of alcohols and phenols with 3,4-dihydro-2H-pyran (DHP) in the presence of polystyrene-supported gallium trichloride (PS/GaCl₃) as a highly active and reusable heterogeneous Lewis acid catalyst at room temperature is presented.

Full text of DOI:10.1016/j.poly.2012.06.063

Polyhedron 44 (2012) 66–71
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Polystyrene-supported GaCl3: A new, highly efficient and recyclable
heterogeneous Lewis acid catalyst for tetrahydropyranylation
of alcohols and phenols
⇑
Ali Rahmatpour
Polymer Science and Technology Division, Research Institute of Petroleum Industry (RIPI), 14665-1137 Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A new, simple and highly chemoselective method for tetrahydropyranylation of alcohols and phenols
with 3,4-dihydro-2H-pyran (DHP) in the presence of polystyrene-supported gallium trichloride (PS/
GaCl3) as a highly active and reusable heterogeneous Lewis acid catalyst at room temperature is pre-
sented. In this catalytic system, primary, secondary and tertiary alcohols, as well as phenols, were con-
verted to the corresponding tetrahydropyranyl (THP) ethers with short reaction times and high yields.
The heterogenized catalyst is of high reusability and stability in the pyranylation reactions and was
recovered several times with negligible loss in its activity and with negligible catalyst leaching, and also
there is no need for regeneration. The method also shows good chemoselectivity for mono-tetrahydro-
pyranylation of symmetrical diols.
Received 7 March 2012
Accepted 11 June 2012
Available online 3 July 2012
Keywords:
Polymer-supported Lewis acid catalyst
Alcohol
Tetrahydropyranylation
Gallium trichloride
Phenol
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
stability and hydrophobic nature which protects water-sensitive
Lewis acids from hydrolysis by atmospheric moisture until it is
Replacement of conventional, toxic and polluting Bronsted and
Lewis acid catalysts with eco-friendly reusable heterogeneous cat-
alysts is an area of current interest. Nowadays, studies focused on
the development of highly efficient, selective and safe Lewis acid-
based promoting systems are a topic for numerous researchers. In
this context, immobilized Lewis acid catalysts are receiving a great
deal of attention due to environmental, practical and economical
concerns [1]. Of particular relevance are the polymer-supported
metal catalysts that, due to the fine tuning of their physico-chem-
ical properties, are emerging as alternative promoting agents for
several challenging organic transformations [2]. Immobilization
of a catalyst on a solid support improves the stability, hygroscopic
properties, handling and reusability of the catalyst, all factors
which are important in industry [3]. A large number of polymer
supported Lewis acid catalysts have been prepared by immobiliza-
tion of the catalysts on polymers via coordination or covalent
bonds [4]. Such polymeric catalysts are usually as active and selec-
tive as their homogeneous or solution-phase counterparts, while
having the distinguishing characteristics of being easily separable
from the reaction mixture, recyclable, easier to handle, non-toxic
and having enhanced stability and improved selectivity in various
organic reactions. Polystyrene is one of the most widely studied
heterogeneous and polymeric supports due to its environmental
suspended in an appropriate solvent where it can be used in a
chemical reaction [5]. It is well known that gallium trichloride is
a strong Lewis acid and an important catalyst in organic transfor-
mations. However, it easily hydrolyzes in air, so that its use, stor-
age and separation from reaction mixtures are inconvenient and
difficult. Polystyrene-supported gallium chloride, PS/GaCl3, which
is a tightly bound and stable complex between anhydrous GaCl3
and polystyrene–divinylbenzene copolymer beads, was described
for the first time by Ruicheng et al. [6]. The use of the PS/GaCl3
complex catalyst has several advantages over a conventional Lewis
acid catalyst, such as its ease of handling (as a bench top catalyst),
recyclability and tunable Lewis acidity.
The protection of alcohols and phenols are fundamental and
useful transformations in organic synthesis. Among the many
protecting groups for hydroxyls, tetrahydropyranyl ether (THP) is
a very attractive choice for the protection of the alcohol and phenol
hydroxyl group, and is used with high frequency due to its low cost,
easy installation as well as its stability and compatibility [7]. This
behavior is possible under different reaction conditions and with
several reagents, such as metal hydrides and Grignard reagents. A
variety of catalysts, including protonic and Lewis acid catalysts such
as p-toluenesulfonic acid (p-TSA) [8], acidic montmorillonite K-10
clay [9], bis[trimethylsilyl] sulfate [10], Ru(CH3)3(triphos)(OTf)2
[11], (CH3)3SiI [12], CuCl2 [13], DDQ [14], heteropolyacids [15], alu-
mina impregnated with ZnCl2 [16], silica chloride [17], silica-based
sulfonic acid [18], ZrCl4 [19], I2 [20], ionic liquids [21], polyaniline
⇑
Tel.: +98 21 44739518; fax: +98 21 44739517.
E-mail address: rahmatpoura@ripi.ir
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.06.063

A. Rahmatpour / Polyhedron 44 (2012) 66–71
67
(30 mL) and chloroform (30 mL). The catalyst was dried in a vac-
uum oven overnight at 50 °C before use. The chlorine content of
PS/GaCl3 was 4.17%, as analyzed by the Mohr titration method
[39], and the loading capacity of GaCl3 on the polymeric catalyst
or the amount of GaCl3 complexed with polystyrene was calculated
to be 0.391 mmol/g.1
PS/GaCl3
CH2Cl2, r.t.
R
OH +
RO
O
O
R: Alkyl / Aryl
Scheme 1. Tetrahydropyranylation of alcohols and phenols with DHP catalyzed by
PS/GaCl3.
2.2. General experimental procedure for tetrahydropyranylation of
alcohols and phenols with DHP catalyzed by the PS/GaCl3 complex
sulfate salt [22], LiOTf [23], La(NO3)3Á6H2O [24], Amberlyst [25],
Nafion-H [12], zeolites [26], tetrabutyl ammonium tribromide
(Bu4N+Br3À) [27], In(OTf)3 [28], [Sn(TPP)(OTf)2] [29], PdCl2(CH3CN)2
[30], CuSO4Á5H2O [31], SiO2/AlCl3 [32], VO(OAc)2 [33], Bi(NO3)3Á
5H2O [34], BF3ÁEt2O [35] and [V(TPP)(OTf)2] [36], have been
developed for the tetrahydropyranylation of hydroxy functions in
alcohols and phenols. Although a large number of procedures for
this conversion are available, many suffer from limitations, like long
reaction times, harsh reaction conditions, poor selectivity, the
occurrence of side reactions (for example the formation of
polymeric by-products of dihydropyran (DHP) and isomerization),
toxic reagents, intolerance of other functional groups and a high
catalyst to substrate ratio. In addition, some of these catalysts are
not recyclable and require work-up of the reaction mixture. Thus,
there is still demand for the introduction of mild, selective and
environmentally benign methods, especially using recyclable
heterogeneous catalysts for this transformation.
To a solution of alcohol or phenol (1 mmol) and DHP (1.1–
1.3 mmol) in methylene chloride (5 mL) was added PS/GaCl3
(0.1 mmol), and the resulting mixture was stirred at room temper-
ature. The progress of the reaction was monitored by TLC and GC.
After completion of the reaction, the catalyst was filtered off and
washed with methylene chloride (2 Â 10 mL) and the filtrate was
concentrated on a rotary evaporator under reduced pressure to af-
ford the crude product. Further purification was achieved by col-
umn chromatography on silica gel (Merck, 100–200 mesh,
hexane–EtOAC, 8:2). The spent polymeric catalyst from different
experiments was combined, washed with ether and dried over-
night in a vacuum oven and reused. All of the THP-ether products
are known and gave satisfactory physical and spectral data com-
pared with those of authentic samples.
2.3. Catalyst recovery and reuse
In continuation of our recent works on the use of polymeric
Lewis acid catalysts in organic transformations [37,38], herein
the preparation and investigation of the catalytic activity of poly-
styrene-supported gallium chloride as a stable, highly active and
reusable heterogeneous catalyst in the tetrahydropyranylation of
alcohols and phenols with 3,4-dihydro-2H-pyran at room temper-
ature is reported (Scheme 1).
The reusability of the catalyst was checked in the multiple tet-
rahydropyranylation of benzyl alcohol with DHP. At the end of
each reaction, the solvent was evaporated, ether (Et2O, 10 mL)
was added and the catalyst was filtered. The recovered catalyst
was used with fresh benzyl alcohol, DHP and CH2Cl2.
2. Experimental
3. Results and discussion
All chemical reagents were obtained from Fluka or Merck
chemical companies and were used without further purification.
Cross-linked polystyrene (8% divinylbenzene, grain size range:
0.25–0.68 mm) was prepared via suspension polymerization, as
reported in the literature [37]. PS/AlCl3 was prepared as reported
previously [37]. 1H and 13C NMR spectra were recorded on a Bruker
DPX-250 Avance spectrometer at 250.13 MHz. FT-IR spectra of the
samples were recorded from 400 to 4000 cmÀ1 on a Unicam
Matteson 1000 spectrophotometer. UV spectra were taken using
a Pharmacia Biotech Ultraspec 3000 model 80-2106-20 spectrom-
eter. Gas chromatography experiments (GC) were performed with
a Shimadzu GC-16A instrument using a 2 m column packed with
silicon DC-200 or carbowax 20 M. The capacity of the catalyst
was determined by the gravimetric method (Mohr titration meth-
od) and the atomic absorption technique using a Philips atomic
absorption instrument. Reaction monitoring and purity determina-
tion of the products were accomplished by TLC on silica gel
polygram SILG/UV254 plates.
3.1. Preparation of the PS/GaCl3 complex
First, the cross-linked polystyrene (8% divinylbenzene, grain
size range: 0.25–0.68 mm) was prepared via suspension polymer-
ization, as reported in the literature [37]. Then, PS/GaCl3 was pre-
pared by addition of anhydrous gallium chloride to polystyrene (8%
divinylbenzene) in carbon disulfide under reflux conditions. The
loading capacity of the polymeric catalyst, obtained by the gravi-
metric method and checked by the atomic absorption technique,
was 0.391 mmol GaCl3/g of complex beads catalyst [39]. The data
obtained by these two techniques showed, within experimental er-
ror, that the catalyzing species are in the form of GaCl3 supported
on the polymeric support. The UV spectrum of a solution of the PS–
GaCl3 complex in CS2 showed a new strong band at 470 nm, which
is due to the formation of a stable
p ? p type coordination complex
between the benzene rings in the polystyrene carrier with gallium
trichloride. The FT IR spectrum of PS/GaCl3 showed new absorption
peaks due to the C–C stretching vibration and the C–H bending
vibration of the benzene ring at 1500–1560 and 400–800 cmÀ1
,
2.1. Preparation of polystyrene-supported gallium trichloride (PS/
GaCl3)
by which complex formation was demonstrated. The structure of
the PS–GaCl3 complex is similar to that of the PS–AlCl3 complex,
as suggested by Neckers et al. [40], because the Lewis acid GaCl3
is complexed with the benzene rings of the polystyrene and GaCl3
is stabilized due to the decreased mobility of the benzene rings,
hindered by the long polystyrene chain. The PS–GaCl3 complex cat-
alyst is a non-hygroscopic, water tolerant and especially stable
Anhydrous GaCl3 (4 g) was added to polystyrene (8% divinyl-
benzene, grain size range: 0.25–0.68 mm, 8 g) in carbon disulfide
(30 mL) as the reaction medium. The mixture was stirred using a
magnetic stirrer under reflux conditions for 1 h, cooled, and then
water (50 mL) was cautiously added to hydrolyze the excess GaCl3.
The mixture was stirred until the bright red color disappeared and
the polymer became yellow. The polymer beads were collected by
filtration and washed with water (300 mL), and then with ether
1
1.0 g of solid catalyst (PS/GaCl3) was decomposed by burning with Na metal,
extracted with 10 mL of water and filtered. The chlorine content of the filtrate was
determined by the Mohr titration method [39].

68
A. Rahmatpour / Polyhedron 44 (2012) 66–71
Table 1
species. In addition, this polymeric catalyst is easy to prepare, sta-
ble in air for a long time (over 2 years) without any change, easily
recycled and reusable without any appreciable loss of its activity.
Reaction condition optimization in the tetrahydropyranylation of benzyl alcohol with
DHP catalyzed by PS/GaCl3 at room temperature.a
Entry
Solvent
Catalyst (mol%)
Time (min)
Yieldb (%)
1
2
3
n-Hexane
THF
CH3Cl
CH3CN
H2O
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
10
10
10
10
10
–
0.1
1
5
25
25
25
25
25
60
50
50
25
50
25
25
25
25
10
25
17
25
39
56
NRd
NR
21
41
64
72
98
97
NR
36
40h
72
3.2. Tetrahydropyranylation of alcohols and phenols with DHP
catalyzed by PS/GaCl3
4
5
In our initial evaluations, we used as the model reaction, the
reaction of benzyl alcohol (1 equiv) with DHP (1.1 equiv). The
choice of the solvent appeared crucial in order to maximize the
yield. Among the various solvents surveyed, the highest yield
was obtained in CH2Cl2 (Table 1, entry 11). The highest yield ob-
tained by using CH2Cl2 as the solvent can be ascribed to the fruitful
swelling of the polymer network of the catalyst in this media,
allowing the metal particles located inside the polymer matrix to
effect the catalysis. No reaction occurred when H2O was used as
the solvent (Table 1, entry 5), because of the aggregation of the cat-
alyst caused by its hydrophobic nature, leading to inadequate ac-
cess of substrates to the active sites of the catalyst [41]. In
addition, we further studied the influence of the amount of PS/
GaCl3 on the reaction yields. In the presence of 10, 5, 1 and
0.1 mol% PS/GaCl3, the corresponding yields were 98%, 72%, 41%
and 21%, respectively (Table 1, entries 7, 8, 10 and 11). The opti-
mum molar ratio of the polymeric catalyst to hydroxyl compound
was found to be 0.1:1. The key role played by the Lewis acidity of
the heterogeneous catalyst PS/GaCl3 was proved by employing
6c
7
8
9
10
11
12
13e
14f
15g
16i
5
10
20
–
–
10
10
a
b
c
d
e
f
Reaction conditions: benzyl alcohol (1 mmol), DHP (1.1 mmol), solvent (5 mL).
Isolated yield.
No catalyst.
NR: no reaction.
PS was used as the catalyst.
The toluene–GaCl3 complex was used as the catalyst.
The catalyst was filtered after 10 min.
GC yield.
g
h
i
PS/AlCl3 was used as the catalyst.
Table 2
Tetrahydropyranylation of alcohols and phenols catalyzed by PS/GaCl3.a
Entry
ROH
Subst./DHP
Product (THP-ether)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
7
9
10
11
12
13
14
15
C6H5CH2OH
1:1.1
1:1.1
1:1.2
1:1.2
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.2
1:1.1
1:1.2
1:1.3
1:1.2
C6H5CH2OTHP
25
30
60
80
25
20
25
40
40
40
45
50
50
60
55
98
95
92
91
96
97
97
95
95
94
93
93
95
95
93
p-(Cl)C6H4CH2OH
p-(NO2)C6H4CH2OH
o-(NO2)C6H4CH2OH
p-(CH3)C6H4CH2OH
p-(OCH3)C6H4CH2OH
C6H5CH2CH2CH2OH
n-CH3(CH2)6CH2OH
n-CH3(CH2)5CH2OH
CH3(CH2)3CH(OH)CH3
(CH3)2CHOH
p-(Cl)C6H4CH2OTHP
p-(NO2)C6H4CH2OTHP
o-(NO2)C6H4CH2OTHP
p-(CH3)C6H4CH2OTHP
p-(OCH3)C6H4CH2OTHP
C6H5CH2CH2CH2OTHP
n-CH3(CH2)6CH2OTHP
n-CH3(CH2)5CH2OTHP
CH3(CH2)3CH(OTHP)CH3
(CH3)2CHOTHP
C6H5CH(CH3)CH2OH
C6H5CH(CH3)OH
C6H5C(CH3)2OH
C6H5CH(CH3)CH2OTHP
C6H5CH(CH3) OTHP
C6H5C(CH3)2OTHP
OTHP
CH2@CHCH2OTHP
OH
16
17
CH2@CHCH2OH
1:1.2
1:1.3
60
90
92
92
O
O
HO
PHTO
18
19
20
21
22
C6H5OH
1:1.3
1:1.2
1:1.2
1:1.2
1:1.3
C6H5OTHP
45
35
40
40
60
94
93
93
91
91
p-(OCH3)C6H4OH
p-(CH3)C6H4OH
p-(Cl)C6H4OH
p-(OCH3)C6H4OTHP
p-(CH3)C6H4OTHP
p-(Cl)C6H4OTHP
OH
OTHP
23
1:1.3
60
92
OH
OTHP
24
25
26
27
28
HOCH2(CH2)3CH2OAc
HOCH2(CH2)6CH2OBn
HOCH2(CH2)6CH2OBz
HOCH2(CH2)3CH2OTr
HOCH2(CH2)3CH2OTs
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
PHTOCH2(CH2)3CH2OAc
PHTOCH2(CH2)6CH2OBn
PHTOCH2(CH2)6CH2OBz
PHTOCH2(CH2)3CH2OTr
PHTOCH2(CH2)3CH2OTs
40
35
35
45
45
92
92
92
91
91
a
All reactions were carried out in CH2Cl2 (5 mL) in the presence of PS/GaCl3 (0.1 mmol) at room temperature.
Isolated yield.
b

A. Rahmatpour / Polyhedron 44 (2012) 66–71
69
Table 3
Selective tetrahydropyranylation of alcohols and phenols catalyzed by PS/GaCl3.a
Entry
Subst. 1
Subst. 2
Subst. 1/subst. 2/DHP
Time (min)
Yield (%)b
THP-ether 1
3
THP-ether 2
93
1
2
1:1:1.1
1:1:1.1
45
50
OH
CH3
OH
CH OH
2
2
93
CH OH
2
3
4
1:1:1.1
1:1:1.1
45
45
4
3
94
94
OH
CH OH
2
6
OH
CH3
OH
6
6
5
1:1:1.1
60
5
93
OH
OH
OH
OH
6
7
1:1:1.1
1:1:1.1
50
45
4
3
92
94
CH OH
2
OH
6
8
9
1:1:1.1
1:1:1.1
1:1:1.2
1:1:1.2
75
50
70
50
3
5
3
3
92
92
94
95
OH
OH
CH OH
2
OH
10
11
OH
OH
CH OH
OH
2
12
13
14
15
HOCH2CH2CH2OH
1:1.1
1:1.1
1:1.1
1:1.1
70
75
72
75
8c
7c
3c
3c
91c
91d
95d
96d
HOCH2(CH2)3CH2OH
HOCH2(CH2)4CH2OH
HOCH2(CH2)6CH2OH
a
b
c
All reactions were carried out in CH2Cl2 in the presence of PS/GaCl3 (0.1 mmol) at room temperature.
Yields based on GC and NMR.
Di THP-ether.
d
Mono THP-ether.
polystyrene beads (Table 1, entry 13) and the GaCl3–toluene com-
plex (Table 1, entry 14) as catalysts. In fact, while in the former
case no reaction occurred, which indicated that polystyrene itself
did not promote the reaction, in the later, the desired product
was isolated in a low yield of 36% (Table 1, entry 14). Also, PS/GaCl3
was found to be a more effective catalyst than PS/AlCl3 for tetrahy-
dropyranylation of benzyl alcohol under identical conditions (Ta-
ble 1, entry 16).
The lifetime and leaching of active sites into the reaction mix-
ture are important issues to consider when heterogeneous and me-
tal-supported catalysts are used, particularly for practical
applications of the reaction. Our preliminary investigations dem-
PS/GaCl3 is stable, no significant leaching of Lewis acid moieties is
operating under our reaction conditions and that the observed
catalysis is truly heterogeneous in nature.
Further, we carried out reactions between DHP and various
alcohols to explore the reaction scope of the PS/GaCl3-catalyzed
tetrahydropyranylation, and the results are summarized in Table 2.
As can be seen, all reactions proceeded very cleanly (checked by
GC) in good to excellent yields. A broad selection of alcohols,
including primary (benzylic and linear ones), secondary (including
aliphatic and aromatic alcohols), tertiary and allylic alcohols, were
converted to their corresponding THP-ethers successfully at room
temperature (Table 2). It is noteworthy that in the case of both ter-
tiary and allylic alcohols, which are acid-sensitive alcohols, no
elimination product or isomerization of C@C bonds was observed
and they were also converted to their corresponding THP-ethers
in high yields (Table 2, entries 14 and 16). Unlike strong acidic cat-
alysts that produce polymeric by-products from the dihydropyran
[7,42], no polymerization of the dihydropyran was detected when
PS/GaCl3 was used, probably due to its mild catalytic activity. We
were then interested in whether the same catalyst could be em-
ployed for the tetrahydropyranylation of phenolic compounds. By
following identical reaction procedures as described for the alco-
hols, various phenols (Table 2, entries 18–23) were smoothly con-
verted into the corresponding THP-ethers in good yields. As can be
seen, a phenol whose benzene ring is substituted with a strong
electron-donating group reacted quickly with an excellent yield
(Table 2, entries 19 and 20). We then turned our attention to
whether the same catalyst could be useful for tetrahydropyranyla-
onstrated that the PS/GaCl3 catalyst (a stable
p complex) is very
stable to air and moisture. To rule out the presence of concurrent
homogeneous catalysis, the model reaction was carried out in the
presence of the catalyst PS/GaCl3 until the conversion was 40%
(10 min), and at that point the catalyst was removed by filtration.
Further treatment of the filtrate under similar reaction conditions
did not proceed significantly (Table 1, entry 15). On the other hand,
after each run the filtrates were used for determination of catalyst
leaching (gallium content), which showed a negligible release of
GaCl3 by atomic absorption or ICP measurements. The capacity of
the catalyst after six uses was 0.38 mmol of GaCl3 per gram. Also,
the catalytic behavior of the separated liquid was tested by addi-
tion of fresh benzyl alcohol and Ac2O to the filtrates after each
run. Execution of the acetylation reaction under the same reaction
conditions, as with the catalyst, showed that the obtained results
were as same as the blank experiments. These findings suggest that

70
A. Rahmatpour / Polyhedron 44 (2012) 66–71
Table 4
Comparison of the results obtained for the tetrahydropyranylation of benzyl alcohol catalyzed by PS/GaCl3 with those obtained by some reported catalysts.
Entry
Catalyst (mol%)
Conditions
Time (min)
Yield (%)
Refs.
1
2
3
4
5
6
7
8
Bi(NO3)3Á5H2O (5)
CH2Cl2/RT
CH2Cl2/reflux
CH2Cl2/RT
Solvent-free/RT
Toluene/RT
CH2Cl2/RT
CH2Cl2/RT
CH2Cl2/RT
CHCl3/RT
CH2ClCH2Cl/reflux
CHCl3/RT
THF/RT
CH2Cl2/reflux
Et2O/RT
Solvent-free/RT
CH3CN/RT
Solvent-free/RT
Solvent-free/RT
CH2Cl2/RT
20
120
20
92
90
83
97
98
97
90
85
59
96
95
72
92
98
96
91
93
80
91
98
[34]
[43]
[13]
[44]
[45]
[46]
[47]
[27]
[22]
[23]
[33]
[30]
[19]
[48]
[21]
[31]
[24]
[49]
[50]
This work
H
14[NaP5W30O110] (0.1)
CuCl2 (15)
Fe(CH3SO3)2Á4H2O (2)
H6P2W18O62 (1)
AlCl3@PS (15)
60
120
0.7 h
150
60
480
150
60
60
180
90
240
40
150
16 h
30
NbCl5 (10 mol%)
À
n-Bu4N+Br3 (25 mol%)
9
Polyaniline sulfate salt (17 mol%)
LiOTf (60 mol%)
Vanadyl (IV) acetate (27 mol%)
PdCl2(CH3CN)2 (10 mol%)
ZrCl4 (5 mol%)
10
11
12
13
14
15
16
17
18
19
20
Fe(ClO4)3 (3)
Ionic liquid/pph3ÁHBr (10)
CuSO4Á5H2O (20)
La(NO3)3Á6H2O (10)
Ru(acac)3 (2)
Silicasulfuric acid (3.9)
PS/GaCl3 (10)
CH2Cl2/RT
25
of some reported catalysts in the literature, the results are summa-
rized in Table 4. The results have been compared with respect to
the reaction times, mol% of the catalyst used and yields. As demon-
strated in Table 4, PS/GaCl3 is an equally or more efficient catalyst
for the tetrahydropyranylation reaction in terms of yield and reac-
tion rate.
Table 5
Reusability of aPS/GaCl3 in the pyranylation of benzylalcohol with DHP.b
Run
THP-etherc (%)
Time (min)
1
2
3
4
5
6
98
98
97
96
95
94
25
25
25
25
25
25
3.3. Catalyst reusability
a
Finally, the reusability of this catalyst was also tested in the
multiple tetrahydropyranylation of benzyl alcohol with DHP under
the same reaction conditions described in the general procedure.
At the end of each of the repeated reactions, the catalyst was fil-
tered, washed exhaustively with methylene chloride and then
diethyl ether, and dried before using with fresh benzyl alcohol
and DHP. The catalyst was consecutively reused six times with
negligible loss in its activity and there is no need for regeneration
(Table 5).
The capacity of the catalyst after six uses was 0.38 mmol GaCl3 per gram.
Reaction conditions: benzyl alcohol (1 mmol), DHP (1.1 mmol), PS/GaCl3
b
(0.1 mmol), CH2Cl2 (5 mL).
c
Isolated yield.
tion of substrates containing other protecting groups. We observed
that various protected alcohols (Table 2, entries 24–28) containing
protecting groups such as acetyl, benzyl, benzoyl, trityl and tosyl
were smoothly converted into the corresponding THP-ethers with-
out affecting the other protecting groups.
Since this catalyst was highly active in the tetrahydropyranyla-
tion of alcohols and phenols, we decided to examine the chemose-
lectivity of the present method. A set of competitive reactions was
allowed between primary alcohols and secondary or tertiary alco-
hols, or phenol with DHP in the presence of PS/GaCl3 (Table 3). The
results indicated that PS/GaCl3 was able to discriminate between
different types of alcohols and or phenols from each other, a trans-
formation that is difficult to accomplish via conventional methods
(Table 3, entries 1–11). Also, we found that symmetrical diols in
the presence of a limited amount of DHP were protected as their
mono THP-ethers, selectively (Table 3, entries 12–15). It is worth
mentioning that diols with a higher number of carbon atoms
showed more selectivity towards mono THP-ether formation (Ta-
ble 3, entries 14 and 15). Although the reason for the observed che-
moselectivity is definitely unclear, it is possibly due to factors such
as the mild catalytic activity of PS/GaCl3, its pore size and its
hydrophobic nature or the more lipophilic microenvironment of
the metal particles in the polymer-supported catalyst, which af-
fected the extent of penetration (that is the access to the active
sites) of different hydroxyl compounds in the swollen beads [5].
The present chemoselectivities could be suitable for the protection
of alcohols and phenols in the syntheses of complex molecules by
multistep processes.
4. Conclusion
In conclusion, in this paper a simple, mild, efficient and highly
chemoselective method for tetrahrdropyranylation of alcohols
and phenols using the stable and heterogeneous polystyrene-sup-
ported gallium trichloride is reported. The short reaction times,
high to excellent yields, low cost, easy preparation and handling
of the polymeric Lewis acid catalyst are the advantages of the pres-
ent method. In addition, the use of water-tolerant PS/GaCl3 has re-
sulted in a reduction in unwanted and hazardous waste, and a
minimum amount of product contamination with metal that is
produced during conventional homogeneous processes. Most
importantly, the work-up is reduced to a mere filtration and evap-
oration of the solvent. Finally, this catalytic system showed good
catalytic activity in these reactions and this polymeric catalyst
can be recovered unchanged and used again at least six times with
negligible loss in its activity. Other applications of the PS/GaCl3
complex are currently under investigation and will be reported
in due course.
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3815.
In order to show the efficiency and applicability of the present
method, the catalytic activity of PS/GaCl3 was compared to that

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