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

Article abstract of DOI:10.1016/j.crci.2012.08.005

A new, simple and highly chemoselective method for both acetylation and benzoylation of alcohols and phenols with acetic anhydride in the presence of polystyrene-supported gallium trichloride (PS/GaCl₃) as a highly active and reusable heterogeneous Lewis acid catalyst is presented. In this catalytic system, primary, secondary and tertiary alcohols as well as phenols were converted to the corresponding acetates and benzoates with high yields. The heterogenized catalyst is of high reusability and stability in the acetylation reactions and was recovered several times with negligible loss in its activity or a negligible catalyst leaching, and also there is no need for regeneration. Remarkably, a selective mono-acetylation of symmetrical diols can be achieved chemoselectively by employing the same catalyst.

Full text of DOI:10.1016/j.crci.2012.08.005

C. R. Chimie 15 (2012) 1048–1054
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Full paper/Memoire
Polystyrene-supported GaCl3: A new, highly efficient and recyclable
heterogeneous Lewis acid catalyst for acetylation and benzoylation 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 both acetylation and benzoylation of
alcohols and phenols with acetic anhydride in the presence of polystyrene-supported
gallium trichloride (PS/GaCl3) as a highly active and reusable heterogeneous Lewis acid
catalyst is presented. In this catalytic system, primary, secondary and tertiary alcohols as
well as phenols were converted to the corresponding acetates and benzoates with high
yields. The heterogenized catalyst is of high reusability and stability in the acetylation
reactions and was recovered several times with negligible loss in its activity or a negligible
catalyst leaching, and also there is no need for regeneration. Remarkably, a selective
mono-acetylation of symmetrical diols can be achieved chemoselectively by employing
the same catalyst.
Received 5 March 2012
Accepted after revision 13 August 2012
Available online 29 September 2012
Keywords:
Polymer- supported catalyst
Alcohol
Acetylation
Heterogeneous Lewis acid catalyst
Phenol
´
ß 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
separable from the reaction mixture, recyclability, easier
handling, non-toxicity, enhanced stability, and improved
Conventional Lewis acids are very effective catalysts for
a wide variety of organic reactions [1], but one of the major
problems associated with these homogeneous catalysts is
the recovery of catalyst from the reaction medium. In
recent years, there have been intense efforts to develop
methods recovering and reusing the homogeneous cata-
lysts [2–6]. Immobilization of catalysts on a solid support
improves the available active sites, stability, hygroscopic
properties, handling, and reusability of catalysts which
factors are important in industry [7]. Therefore, use of
supported and reusable catalysts in organic transforma-
tions has economical and environmental benefits. A large
number of polymer-supported Lewis acid catalysts have
been prepared by immobilization of the catalysts on
polymer via coordination or covalent bonds [8]. Such
polymeric catalysts are usually as active and selective as
their homogeneous or solution-phase counterparts while
having the distinguishing characteristics of being easily
selectivity in various organic reactions. Polystyrene is one
of the most widely studied heterogeneous and polymeric
supports due to its environmental stability and hydropho-
bic nature which protects water-sensitive Lewis acids from
hydrolysis by atmospheric moisture until it is suspended in
an appropriate solvent where it can be used in a chemical
reaction [4]. It is well known that gallium trichloride is a
strong Lewis acid and an important catalyst in organic
transformations. However, it easily hydrolyzes in air, so
that its use, storage, and separation from the reaction
mixture are inconvenient and difficult. Polystyrene-sup-
ported gallium chloride, PS/GaCl3, which is a tightly bound
and stable complex between anhydrous GaCl3 and
polystyrene-divinylbenzene copolymer beads, has been
described for the first time by Ruicheng et al. [9]. The use of
PS/GaCl3 complex catalyst has several advantages over
conventional Lewis acid catalyst such as its cost-effective-
ness, ease of handling, recyclability, and tunable Lewis
acidity.
Acylation of alcohols and phenols is of enormous
interest in organic synthesis as it provides a useful and
Email address: rahmatpoura@ripi.ir.
1631-0748/$ – see front matter ß 2012 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2012.08.005

A. Rahmatpour / C. R. Chimie 15 (2012) 1048–1054
1049
PS/GaCl
efficient protection protocol in a multistep synthetic process
[10]. Moreover, this reaction has biological significance
because of the presence of alcoholic and phenolic hydroxyl
groups in a variety of biologically active compounds that
necessitates the manipulation of the chemical reactivity of
these functional groups during the synthesis of multifunc-
tional synthetic targets possessing one or more of these
groups. Acylation is usually carried out by treatment of an
alcohol or phenol with acetyl chloride or acetic anhydride in
the presence of an acid or a base catalyst in a suitable organic
solvent, although acetic anhydride is the most commonly
used, as it is less toxic. However, the catalysts traditionally
used are bases or acids, which have many disadvantages in
the acetylation process. For example, some of the base
catalysts are toxic, flammable, and possess offensive odors
[11,12]. In addition, some of the traditional protic acid
catalysts are not entirely satisfactory in terms of the stability
of reactants or products under the reaction conditions and
the time-consuming work-up procedures. A variety of
procedures using homogeneous or heterogeneous catalysts
such as p-TSA [13], CoCl2 [14], alumina [15], montmorillonie
K-10and KSF [16], zeolite HSZ-360 [17], MgBr2 [18], Bi(TFA)3
[19], TaCl5 [20], NH2SO3H [21], La(NO3)3.6H2O [22], 3-
nitrobenzene boronic acid [23], TMSOTf [24], Cp2Ti(O-
SO2C8F17)2 [25], Cp2ZrCl2 [26], ZrOCl2.8H2O [27], Mg(ClO4)2
[28], Zn(ClO4)2.6H2O [29], HBF4-SiO2 [30], Cu(BF4)2 [31],
HClO4-SiO2 [32], electron-deficient tin (IV) porphyrins [33–
35], alumina-supported MoO3 [36], zirconium sulfophenyl
phosphonate [37], o-benzene disulfonimide [38], sacchar-
insulfonic acid [39], cerium polyoxometallate [40], BiO(-
ClO4)2 [41], cobalt(II) and Mn(III) salen complexes [42],
Gd(OTf)3 [43], InCl3 [44], Ph3P+CH2COCH3Brꢀ [45], [Mn(III)(-
haacac)Cl] complex [46], and LiOTf [47] have been reported
for acetylation of alcohols and phenols with acetic anhy-
dride. However, most of the catalysts suffer from limitations
such as long reaction times, harsh reaction condition or
tedious work-up, use of expensive, moisture sensitive and
toxic catalysts, regulatory constraints, the occurrence of side
reactions, intolerance to other functional groups, poor
selectivity, and high catalyst to substrate ratio. Apart from
these difficulties, most above methods do not satisfy the
requirements of green synthesis due to the inability to
recovery and reuse of the catalyst. As a result, new kinds of
catalysts, especially using recyclable heterogeneous cata-
lysts or mild, selective and environmentally benign meth-
odologies for these transformations are still in demand.
In continuation of our recent works on the use of
polymeric Lewis acid catalysts in organic transformations
[48,49], herein we report the preparation and investigation
of catalytic activity of polystyrene-supported gallium
chloride as a stable, highly active and reusable heteroge-
neous catalyst in the acetylation and benzoylation of
alcohols and phenols with acetic and benzoic anhydride,
(Ac2O), (PhCO)2O, respectively (Scheme 1).
3
’
’
R CO R
+ (R CO) O
R
OH
2
2
CH Cl r.t or Reflux
2,
2
R : Alkyl ,Aryl
’
: Me, Ph
R
Scheme 1. Acetylation and benzoylation of alcohols and phenols with
Ac2O and (PhCO)2O catalyzed by PS/GaCl3.
polymerization as reported in the literature [48]. PS/AlCl3
was also prepared as previously reported [49]. 1H and 13
C
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 cmꢀ1 on a Unicam
Matteson 1000 spectrophotometer. UV spectra were taken
using a Pharmacia Biotech Ultraspec 3000 model 80-2106-
20 spectrometer. 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 deter-
mined by the gravimetric method (Mohr titration method)
and atomic absorption technique using a Philips atomic
absorption instrument. Reaction monitoring and purity
determination of the products were accomplished by TLC
on silica gel polygram SILG/UV254 plates.
2.1. Preparation of polystyrene-supported gallium trichloride
(PS/GaCl3)
Anhydrous GaCl3 (4 g) was added to polystyrene (8%
divinylbenzene, mesh size: 20–75 mm, 8 g) in carbon
disulfide (30 mL) as the reaction medium. The mixture
was stirred using a magnetic stirrer under reflux condition
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 (30 mL) and chloroform (30 mL). The
catalyst was dried in a vacuum oven overnight at 50 8C
before use. The chlorine content of PS/AlCl3 was 4.17%
analyzed by the Mohr titration method [50] 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.
2.2. General experimental procedure for acetylation of
alcohols and phenols with Ac2O catalyzed by PS/GaCl3
complex
To a solution of alcohol or phenol (1 mmol) and Ac2O
(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 temperature. The progress of the reaction was
monitored by TLC and GLC. After completion of the
reaction, the catalyst was filtered off and washed with
methylene chloride (2 ꢁ 10 mL) and the filtrate concen-
trated on a rotary evaporator under reduced pressure to
afford the crude acetate product. Further purification was
achieved by column chromatography on silica gel (Merck,
100–200 mesh, hexane-EtOAC, 90/10, v/v). The spent
polymeric catalyst from different experiments was
2. Experimental
All chemical reagents were obtained from Fluka or
Merck chemical companies and were used without further
purification. Cross-linked polystyrene (5–7% divinylben-
zene, mesh size: 20–75 mm) was prepared via suspension

1050
A. Rahmatpour / C. R. Chimie 15 (2012) 1048–1054
combined, washed with ether and dried overnight in a
vacuum oven and reused.
polymeric catalyst is easy to prepare, stable in air for a long
time (over 2 years) without any change, easily recycled and
reused without appreciable loss of its activity.
2.3. General experimental procedure for benzoylation of
alcohols and phenols with (PhCO)2O catalyzed by PS/GaCl3
complex
3.2. Acetylation and benzoylation of alcohols and phenols
with Ac2O and (PhCO)2O catalyzed by PS/GaCl3
To a solution of alcohol or phenol (1 mmol) and
(PhCO)2O (1.2–1.5 mmol) in methylene chloride (10 mL)
was added PS/GaCl3 (0.1 mmol) and the resulting mixture
was stirred under reflux conditions for the appropriate
time according to Table 2. The progress of the reaction
followed by GLC or TLC. After completion of the reaction,
the catalyst was filtered off and washed with methylene
chloride (2 ꢁ 10 mL) and the filtrate concentrated on a
rotary evaporator under reduced pressure to afford the
crude benzoate product which was passed through a short
pad of silica gel (Merck, 100–200 mesh, hexane-EtOAC, 95/
5, v/v) to provide the pure benzoylated product.
In our initial evaluations, we used as the model reaction,
the reaction of benzyl alcohol (1 equiv.) with acetic
anhydride (1.1 equiv.). The choice of the solvent appeared
crucial in order to maximize yield. Among the various
solvents surveyed, the highest yield was obtained in CH2Cl2
(Table1, entry 7). The highestyield obtained byusingCH2Cl2
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. The optimum 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 the polystyrene
beads (Table 1, entry 9) and the GaCl3-benzene complex as
catalysts. In fact, while in the former case no reaction
occurred, which indicated that polystyrene itself did not
promote the reaction, in the latter, the desired product was
isolated in low yield 46% (Table 1, entry 10). Also, PS/GaCl3
was found to be a more effective catalyst than PS/AlCl3 in
terms of reaction time and the yield of the product for
acetylation of benzyl alcohol under identical conditions
(Table 1, entry12).Wethenturnedourattentiononwhether
the same catalyst could be employed for acetylation of
alcohols using acetic acid and ethyl acetate as acylating
agents. For this purpose, the model reaction was carried out
withaceticacidandethylacetateunderidenticalconditions,
and only 36% and 23% of the corresponding acetates were
produced, respectively.
2.4. Catalyst recovery and reuse
The reusability of the catalyst was checked in the
multiple acetylation of benzyl alcohol. 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, Ac2O and
CH2Cl2.
3. Results and discussion
3.1. Preparation of PS/GaCl3 complex
First, cross-linked polystyrene (5–7% divinylbenzene,
mesh size: 20–75 mm) was prepared via suspension
polymerization as reported in the literature [48]. Then,
PS/GaCl3 was prepared 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 gravimetric method
and checked by atomic absorption technique was
0.391 mmol GaCl3/g of complex beads catalyst [50]. The
data obtained by these two techniques showed, within
experimental error, that the catalyzing species are in the
form of GaCl3 supported on the polymeric support. The UV
spectrum of the solution of PS-GaCl3 complex in CS2
showed a new strong band at 470 nm, which is due to the
When using a heterogeneous and metal-supported
catalyst one of the most important issues is the possibility
Table 1
Reaction conditions optimization in the acetylation of benzyl alcohol with
Ac2O catalyzed by PS/GaCl3 at room temperature.a
Entry
Solvent
Catalyst (mol %)
Time (min)
Yieldb (%)
1
n-Hexane
THF
10
10
10
10
–
25
25
25
25
60
25
25
25
25
25
15
60
17
21
40
58
NR
60
97
97
NR
46
NR
76
2
3
CH3Cl
4
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
5c
6
formation of a stable
p
!p type coordination complex
5
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
to 1560 and 400 to 800 cmꢀ1, 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. [51], because the Lewis acid GaCl3 is
complexed with the benzene rings of the polystyrene and
the GaCl3 is stabilized due to the decreased mobility of the
benzene rings hindered by the long polystyrene chain. The
PS-GaCl3 complex catalyst is a non-hygroscopic, water-
tolerant, and especially stable species. In addition, this
7
10
20
–
8
9d
10e
11f
12g
–
10
10
NR: no reaction.
a
Reaction conditions: benzyl alcohol (1 mmol), Ac2O (1.1 mmol),
solvent (5 mL).
b
c
d
e
f
Isolated yield.
No catalyst.
PS was used as catalyst.
The toluene-GaCl3 complex was used as catalyst.
Catalyst was filtered after 15 min.
PS/AlCl3 was used as catalyst.
g

A. Rahmatpour / C. R. Chimie 15 (2012) 1048–1054
1051
Table 2
Acetylationa and benzoylationb of alcohols and phenols with Ac2O and (PhCO)2O catalyzed by PS/GaCl3.
Entry
Substrate
Ratioc
Productd
Time (min)
Yielde (%)
C6H5CH2OH
C6H5CH2OH
1:1.1
1:1.2
C6H5CH2OAc
25
45
97
90
1
2
C6H5CH2OCOPh
p-(Cl)C6H4CH2OH
p-(Cl)C6H4CH2OH
1:1.1
1:1.2
p-(Cl)C6H4CH2OAc
30
50
93
89
p-(Cl)C6H4CH2OCOPh
3
p-(NO2)C6H4CH2OH
p-(NO2)C6H4CH2OH
1:1.1
1:1.2
p-(NO2)C6H4CH2OAc
30
65
91
88
p-(NO2)C6H4CH2OCOPh
4
o-(NO2)C6H4CH2OH
o-(NO2)C6H4CH2OH
1:1.1
1:1.2
o-(NO2)C6H4CH2OAc
70
91
84
o-(NO2)C6H4CH2OCOPh
100
5
p-(CH3)C6H4CH2OH
p-(CH3)C6H4CH2OH
1:1.1
1:1.2
p-(CH3)C6H4CH2OAc
28
45
96
89
p-(CH3)C6H4CH2OCOPh
6
p-(OCH3)C6H4CH2OH
p-(OCH3)C6H4CH2OH
1:1.1
1:1.2
p-(OCH3)C6H4CH2OAc
27
50
94
90
p-(OCH3)C6H4CH2OCOPh
7
n-CH3(CH2)6CH2OH
n-CH3(CH2)6CH2OH
1:1.1
1:1.2
n-CH3(CH2)6CH2OAc
40
55
95
89
n-CH3(CH2)6CH2OCOPh
8
n-CH3(CH2)5CH2OH
n-CH3(CH2)5CH2OH
1:1.1
1:1.2
n-CH3(CH2)5CH2OAc
40
55
94
88
n-CH3(CH2)5CH2OCOPh
9
CH3(CH2)3CH(OH)CH3
CH3(CH2)3CH(OH)CH3
1:1.1
1:1.2
CH3(CH2)3CH(OAc)CH3
40
50
93
86
CH3(CH2)3CH(OCOPh)CH3
10
11
12
13
14
(CH3)2CHOH
(CH3)2CHOH
1:1.1
1:1.2
(CH3)2CHOAc
40
50
92
87
(CH3)2CHOCOPh
C6H5CH(CH3)CH2OH
C6H5CH(CH3)CH2OH
1:1.1
1:1.2
C6H5CH(CH3)CH2OAc
55
75
92
88
C6H5CH(CH3)CH2OCOPh
C6H5CH(CH3) OH
C6H5CH(CH3) OH
1:1.2
1:1.2
C6H5CH(CH3) OAc
55
80
94
85
C6H5CH(CH3) OCOPh
C6H5C(CH3)2 OH
C6H5C(CH3)2 OH
1:1.3
1:1.4
C6H5C(CH3)2 OAc
70
90
94
86
C6H5C(CH3)2 OCOPh
1:1.2
1:1.3
65
85
92
87
OAc
OH
OH
OCOPh
15
16
CH2 = CHCH2OH
CH2 = CHCH2OH
1:1.2
1:1.3
CH2 = CHCH2OAc
90
90
90
86
CH2 = CHCH2OCOPh
1:1.3
90
92
HO
HO
AcO
1:1.5
100
83
PhCOO
17
18
19
20
21
C6H5OH
C6H5OH
1:1.3
1:1.4
C6H5OAc
50
80
94
86
C6H5OCOPh
p-(OCH3)C6H4OH
p-(OCH3)C6H4OH
1:1.2
1:1.3
p-(OCH3)C6H4OAc
45
50
94
89
p-(OCH3)C6H4OCOPh
p-(CH3)C6H4OH
p-(CH3)C6H4OH
1:1.2
1:1.3
p-(CH3)C6H4OAc
45
50
93
88
p-(CH3)C6H4OCOPh
p- (CHO)C6H4OH
p- (CHO)C6H4OH
1:1.2
1:1.3
p-(CHO)C6H4OAc
65
91
82
p-(CHO)C6H4OCOPh
120
1:1.3
70
91
OH
OAc
1:1.4
100
82
OH
OCOPh

1052
A. Rahmatpour / C. R. Chimie 15 (2012) 1048–1054
Table 2 (Continued )
Entry
22
Substrate
Ratioc
1:1.3
Productd
Time (min)
70
Yielde (%)
92
OH
OH
OAc
1:1.4
100
81
OCOPh
23
24
25
26
HOCH2(CH2)3CH2OAc
HOCH2(CH2)6CH2OBn
HOCH2(CH2)6CH2OBz
HOCH2(CH2)3CH2OTs
1:1.1
1:1.1
1:1.1
1:1.1
AcOCH2(CH2)3CH2OAc
AcOCH2(CH2)6CH2OBn
AcOCH2(CH2)6CH2OBz
AcOCH2(CH2)3CH2OTs
45
40
40
45
92
92
92
91
a
All acetylation reactions were carried out in CH2Cl2 (5 mL) in the presence of PS/GaCl3 (0.1 mmol) at room temperature.
All benzoylation reactions were carried out in refluxing CH2Cl2 (5 mL) in the presence of PS/GaCl3 (0.1 mmol).
The mole ratio of (substrate/Ac2O and substrate/(PhCO)2O).
b
c
d
e
All the acetate and benzoate products were identified by comparison of their physical and spectral data with those of authentic samples.
Isolated yield.
of leaching of the reactive center into the reaction mixture,
particularly for practical applications of the reaction. Our
preliminary investigations demonstrated that PS/GaCl3
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 addition of fresh benzyl
alcohol and Ac2O to the filtrates after each run. Execution
of the acetylation reaction under the same reaction
conditions, as with catalyst, showed that the obtained
results were as same as blank experiments. These findings
confirmed that PS/GaCl3 is stable and no significant
leaching of Lewis acid moieties is operating under our
reaction conditions and that the observed catalysis is truly
heterogeneous in nature.
catalyst (a stable
p complex) is very stable to air and
moisture. To rule out the contribution of concurrent
homogeneous catalysis in the results shown in Table 2, the
catalyst was allowed to react with dry methylene chloride
for 15 min under the operated reaction conditions and then
filtered off. We observed that the filtrate (the catalyst-free
liquid) showed no catalytic activity in acetylation of benzyl
alcohol under the operated reaction condition (Table 1,
entry 11). 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 spectroscopy or ICP measurement. The
Further, we carried out the reactions between Ac2O and
various alcohols to explore the reaction scope of PS/GaCl3-
catalyzed acetylation, the results are summarized in Table
2. As can be seen, all reactions proceeded very cleanly
Table 3
Selective acetylationa and benzoylationb of alcohols and phenols catalyzed by PS/GaCl3.
Entry
1
Subst. 1
Subst. 2
Subst. 1/Subst. 2/(Ac2O) or (PhCO)2O
Time (min)
50
Yield (%)c
Ester 2
93
Ester 1
3
1:1.1
OH
CH OH
2
CH3
2
3
1:1:1.1 (1.2)d
1:1:1.1 (1.2)
50 (75)e
50 (85)
4 (7)f
4 (8)
94 (91)f
93 (90)
OH
CH OH
2
6
OH
OH
OH
CH3
6
4
1:1:1.1 (1.2)
65
5
92
OH
OH
6
5
6
1:1:1.1 (1.2)
1:1:1.1
60 (85)
55
4 (8)
3
92 (89)
93
CH OH
2
OH
OH
OH
6
7
8
1:1:1.1
90
4
92
OH
OH
1:1:1.2 (1.4)
1:1:1.2 (1.4)
85 (100)
60 (85)
4 (9)
3 (6)
92 (87)
94 (90)
OH
9
CH OH
OH
2
a
All acetylation reactions were carried out in CH2Cl2 (5 mL) in the presence of PS/GaCl3 (0.1 mmol) at room temperature.
All benzoylation reactions were carried out in refluxing CH2Cl2 (10 mL) in the presence of PS/GaCl3 (0.1 mmol).
Yields based on GC and NMR.
,e,fThe molar ratios, times and yields in the parentheses refer to benzoylation reactions.
b
c
d

A. Rahmatpour / C. R. Chimie 15 (2012) 1048–1054
1053
(checked by GC) in good to excellent yields. A broad
O
H C
3
selection of alcohols, including primary (benzylic and
linear ones), secondary (including aliphatic and aromatic
alcohols), tertiary and allylic alcohols, were converted to
their corresponding acetates successfully at room temper-
ature (Table 2). It is noteworthy that in the case of both
tertiary and allylic alcohols, which are acid-sensitive
alcohols, no elimination product or migration of C = C
bonds (rearrangement) was observed when PS/GaCl3 was
used, probably due to its mild catalytic activity and they
were also converted to their corresponding acetates in high
yields (Table 2, entries 13, 15, 16). Protection of phenolic
hydroxyl group was also achieved under the same reaction
conditions described for alcohols and the corresponding
acetates were produced in high yields (Table 2). As can be
seen, a phenol whose benzene ring substituted with a
strong electron-donating group, reacted quickly with
excellent yield (Table 2, entries 18, 19). We then turned
our attention on whether the same catalyst could be useful
for acetylation of substrates containing other protecting
groups. We observed that various protected alcohols
(Table 2, entries 23–26) containing protecting groups such
as acetyl, benzyl, benzoyl and tosyl were smoothly
converted into the corresponding acetates without affect-
ing the other protecting groups.
In order to extend the scope of the catalyst further, the
benzoylation of alcohols and phenols with benzoic
anhydride, a less reactive anhydride, was also investigated.
The results of this investigation are shown in Table 2.
Treatment of a series of alcohols and phenols with benzoic
anhydride in the presence catalytic amount of PS/GaCl3
provided the corresponding benzoates in good yields
(Table 2, entries 1–22).
In order to show that whether the presented method is
potentially applicable for chemoselective conversion of
alcohols and phenols to the corresponding acetates or
benzoates, a set of competitive reactions was allowed
between a primary alcohols and a secondary or tertiary
alcohols, or phenol with Ac2O and (PhCO)2O in the
presence of PS/GaCl3 (Table 3). The results indicated that
PS/GaCl3 was able to discriminate between different types
O
H C
3
O
O
H C
3
O
H C
3
1
O
GaCl
O
3
GaCl
3
O
+
C CH
RO
C OH
H C
3
3
ROH
Scheme 2. The proposed catalytic mechanism for the formation of
acetates.
of alcohols and or phenols from each other, a transforma-
tion that is difficult to accomplish via conventional
methods (Table 3, entries 1–9).
The definite mechanism of the reaction is unclear.
However, a plausible mechanism for the formation of
acetates in the presence of PS/GaCl3 as a catalyst is shown
in Scheme 2. The acetic anhydride is first activated by
Ga(III) as a Lewis acid to afford 1. The hydroxyl compound
attacks 1 which in turn converts to the final product and
simultaneously releases the catalyst for the next catalytic
cycle.
In order to show the efficiency and applicability of the
present method, the catalytic activity of PS/GaCl3 was
compared with that of some reported catalysts in the
literature, the results are summarized in Table 4. The
results have been compared with respect to the reaction
times, mol-% of the catalyst used, and yields. As
demonstrated in Table 4, PS/GaCl3 is an equally or more
efficient catalyst for this acetylation reaction in terms of
yield and reaction rate.
Table 4
Comparison of the results obtained for the acetylation of benzyl alcohol catalyzed by PS/GaCl3 with those obtained by some reported catalysts.
Entry
Catalyst (mol %)
Conditions
Time (min)
Yield (%)
Ref.
1
BiCl3 (10)
CH3CN, reflux
CH3CN, reflux
CH3CN, RT
35
98
96
98
90
84
90
97
99
96
93
96
93
94
85
96
97
97
97
[19]
[19]
[14]
[16]
[17]
[39]
[43]
[42]
[31]
[26]
[20]
[42]
[30]
[44]
[45]
[46]
[47]
Table 2
2
Bi(TFA)3 (5)
60
3
CoCl2 (0.5)
240
60
4
Montmorillonite KSF (20 mg)
ZeoliteHSZ-360 (20 mg)
Saccharinsulfonic acid (13 mg)
Gd(OTf)3 (0.1)
Solvent-free, RT
Solvent-free, 60 8C
CH2Cl2/reflux
CH3CN, RT
5
60
6
120
30
7
8
Cobalt(II) salen complex (1)
Cu(BF4)2.XH2O
Solvent-free, 50 8C
Solvent-free, RT
Solvent-free, RT
CH2Cl2, RT
45
9
60
10
11
12
13
14
15
16
17
18
Cp2ZrCl2 (1)
10 h
120
6 h
60
TaCl5 (10)
Mn(III) salen complex (7)
HBF4-SiO2 (1)
Toluene, 80 8C
Solvent-free, RT
Solvent-free, RT
Solvent-free, RT
CH3NO2, RT
InCl3 (0.1)
30
Ph3P+CH2COCH3Brꢀ (5)
[Mn(III)(haacac)Cl] complex
LiOTf (20)
35
5 h
12 h
25
Solvent-free, RT
CH2Cl2, RT
PS/GaCl3 (10)

1054
A. Rahmatpour / C. R. Chimie 15 (2012) 1048–1054
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Reusability of aPS/GaCl3 in the acetylation of benzyl alcohol with Ac2O.b
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Run
Benzyl acetate (%)c
Time (min)
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1
2
3
4
5
6
97
97
96
96
95
94
25
25
25
25
25
25
a
The capacity of the catalyst after six uses was 0.38 mmol GaCl3 per
gram.
b
Reaction conditions: benzyl alcohol (1 mmol), Ac2O (1.1 mmol), PS/
GaCl3 (0.1 mmol), CH2Cl2 (5 mL)
c
Isolated yield.
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In conclusion, in this article a simple, mild, efficient and
highly chemoselective method for both acetylation and
benzoylation of alcohols and phenols using stable
and heterogeneous polystyrene-supported 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 present method. In addition, the use of water-tolerant
PS/GaCl3 has resulted in a reduction in the unwanted and
hazardous waste and 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 evapora-
tion of the solvent. Finally, this catalytic system showed a
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.
The other applications of PS/GaCl3 is currently under
investigation and will be reported in due course.
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