Polystyrene-supported TiCl4 as a novel, efficient and reusable polymeric Lewis acid catalyst for the chemoselective synthesis and deprotection of 1,1-diacetates under eco-friendly conditions

Article abstract of DOI:10.1016/j.crci.2013.01.012

Copolymer beads of styrene and divinylbenzene (5-7%) were synthesized and combined with titanium tetrachloride in CS₂ to form a stable complex. The PS/TiCl₄ complex was used as a mild and efficient polymer-supported Lewis acid catalyst for the preparation of 1,1-diacetates from various types of aldehydes under heterogeneous conditions at room temperature. Deprotection of the resulting 1,1-diacetates has also been achieved using the same catalyst in methanol. This new protocol has the advantages of easy availability, stability, reusability of the eco-friendly catalyst, high to excellent yields, chemoselectivity, simple experimental and work-up procedure. Moreover, this polymeric catalyst could be recovered easily and reused several times without significant loss in activity.

Full text of DOI:10.1016/j.crci.2013.01.012

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Full paper/Memoire
Polystyrene-supported TiCl4 as a novel, efficient and reusable polymeric
Lewis acid catalyst for the chemoselective synthesis and deprotection of
1,1-diacetates under eco-friendly conditions
*
Ali Rahmatpour , Sara Mohammadian
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:
Copolymer beads of styrene and divinylbenzene (5–7%) were synthesized and
combined with titanium tetrachloride in CS2 to form a stable complex. The PS/TiCl4
complex was used as a mild and efficient polymer-supported Lewis acid catalyst for the
preparation of 1,1-diacetates from various types of aldehydes under heterogeneous
conditions at room temperature. Deprotection of the resulting 1,1-diacetates has also
been achieved using the same catalyst in methanol. This new protocol has the
advantages of easy availability, stability, reusability of the eco-friendly catalyst, high to
excellent yields, chemoselectivity, simple experimental and work-up procedure.
Moreover, this polymeric catalyst could be recovered easily and reused several times
without significant loss in activity.
Received 22 August 2012
Accepted after revision 15 January 2013
Available online xxx
Keywords:
Polymer-supported catalyst
1,1-diacetate
Chemoselectivity
Heterogeneous Lewis acid catalyst
Aldehydes
´
ß 2013 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Protection
1. Introduction
InCl3 [9], LiBF4 [10], PCl3 [11], NBS [12], I2 [13], TMSCl–NaI
[14], FeCl3 [15], Cu(OTf)2 [16], Bi(OTf)3 [17], Bi(NO3)3ꢀ5H2O
The selective protection and deprotection of carbonyl
groups are essential steps in synthetic organic chemistry
[1]. The protection of aldehydes, such as acetals, acylals,
oxathioacetals or dithioacetals, is common practice for
manipulation of other functional groups during multi-
steps synthesis. 1,1-Diacetates (acylals) are appropriate
candidates to this aim due to their stability under both
neutral and basic media and under acidic conditions and
also their easy conversion into parent aldehydes [2].
Moreover, the acylal functionality can be converted into
other useful functional groups by reaction with appropri-
ate nucleophiles [3,4] and they are used as cross-linking
agents for cellulose in cotton [5]. Over the years, several
types of the catalysts have been used for the preparation of
1,1-diacetates from aldehydes and acetic anhydride, such
as sulfuric acid [6], triflic acid [7], CH3SO3H [7], LiOTf [8],
[18], Al(HSO4)3 [19], tetrabutylammonium tribromide
(TBATB) [20], Wells–Dawson acid (H6P2W18O62ꢀ24H2O)
[21], zeolite-Y(b) [22], P2O5/Al2O3 [23], SSA [24], mont-
morillonite clay [25], PS/Al(OTf)3 [26], expansive graphite
[27], sulfated zirconia [28] and Brønsted acidic ionic
liquids under ultrasonic irradiation [29]. Although these
methods are suitable for many synthetic conditions,
many of these are associated with several drawbacks,
which include prolonged reaction time and low yields,
harsh reaction conditions, use of moisture sensitive and
costly catalysts, use of excess acetic anhydride, poor
selectivity and use of unrecyclable catalysts, which
eventually result in the generation of a large amount
of environmentally hazardous waste materials. In addi-
tion, a few of the above-mentioned catalysts are claimed
to give protection as well as deprotection. Therefore,
there is a scope to develop an alternative method
for the protection of aldehydes as 1,1-diacetates and
their deprotection using catalysts that could be superior
or equivalent to the existing ones with regard to
* Corresponding author.
E-mail address: rahmatpoura@ripi.ir (A. Rahmatpour).
1631-0748/$ – see front matter ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.01.012
Please cite this article in press as: Rahmatpour A, Mohammadian S. Polystyrene-supported TiCl4 as a novel, efficient
and reusable polymeric Lewis acid catalyst for the chemoselective synthesis and deprotection of 1,1-diacetates under
eco-friendly conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.012

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A. Rahmatpour, S. Mohammadian / C. R. Chimie xxx (2013) xxx–xxx
selectivity, handling, reusability, toxicity, and environmental
compatibility.
2. Experimental
The utility of polymer-supported catalysts is now well-
recognized because of their ease of work-up and of
separation of products and catalysts, from the economical
point of view, and in application to industrial processes,
etc. [30,31]. In general, catalysts are immobilized on
polymers via coordinate or covalent bonds. A large number
of polymer-supported Lewis acid catalysts have been
prepared by immobilization of the catalysts on polymer via
coordination or covalent bonds [32,33]. Such heteroge-
neous supported Lewis acids or polymeric Lewis acid
catalysts are usually as active and selective as their
homogeneous or solution-phase counterparts while hav-
ing the distinguishing characteristics of being easily
separable from the reaction mixture, recyclability, easier
handling, non-toxicity, enhanced stability, and improved
selectivity in various organic reactions. Cross-linked
polymers with specific properties are widely used as
catalyst supports as they are inert, nontoxic, and nonvola-
tile and offer the advantageous features of heterogeneous
catalysis, such as thermal stability, selectivity, and
recyclability [34,35].
Titanium tetrachloride (TiCl4), as a powerful Lewis acid,
has found many applications in organic synthesis and
industry [36,37]. However, this compound is a liquid and
highly aggressive material that creates clouds of HCl when
it is exposed to air and moisture. Therefore, its handling
needs serious precautions and strict anhydrous conditions
and stoichiometric amounts or more are needed in order to
achieve good yields. Furthermore, tedious work-up proce-
dure after reaction to hydrolyze excess catalyst produces
large amounts of acidic waste water.
Polystyrene is one of the most widely studied
heterogeneous and polymeric supports due to its environ-
mental stability and hydrophobic nature which protects
water-sensitive Lewis acids from hydrolysis by atmo-
spheric moisture until it is suspended in an appropriate
solvent where it can be used in a chemical reaction [38].
Neckers et al., first, reported polystyrene-supported AlCl3
as a catalyst in Friedel–Crafts reaction and then was used
for some organic transformations [39]. Although the use of
immobilized catalysts is of continuing interest, few
examples are known for polymer-supported Lewis acids
[40]. In continuation of our recent works on the prepara-
tion and use of polymeric Lewis acid catalysts in organic
transformations [41–45], herein, we use the reported
strategy to support TiCl4 on cross-linked polystyrene, and
the polymer catalyst obtained (PS/TiCl4) is used as an
effective as well as highly chemoselective heterogeneous
catalyst for the synthesis of 1,1-diacetates from structur-
ally diverse aldehydes and the subsequent deprotection of
the obtained 1,1-diacetates with methanol (Scheme 1).
2.1. Materials and instruments
All chemical reagents were purchased from Fluka and
Merck chemical companies and were used without further
purification. Cross-linked polystyrene (5–7% divinylben-
zene, mesh size: 16–60) was prepared via suspension
polymerization as reported in the literature [41]. PS/AlCl3
was prepared as reported previously [41]. The products
were characterized by comparing the physical data with
those of known samples or by their spectral data. 1H and
13C NMR spectra were recorded on a Bruker DPX-250
Advance spectrometer at 250.13 MHz. FT-IR spectra of the
samples were recorded from 400 to 4000 cmꢁ1 using a
Unicam Matteson 1000 spectrophotometer. UV spectra
were taken using a Pharmacia Biotech Ultraspec 3000
model 80-2106-20 spectrometer. The capacity of the
catalyst was determined by the 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. All yields refer to
isolated products.
2.2. Preparation of PS/TiCl4
Anhydrous TiCl4 (4 g) was added to polystyrene (5–7%
divinylbenzene, mesh size: 16–60, 10 g) in carbon disulfide
(30 mL) as the reaction medium. The mixture was stirred
using a magnetic stirrer under reflux condition for 2 h,
cooled, and then water (50 mL) was cautiously added to
hydrolyze the excess TiCl4. The mixture was stirred until
the brown red color disappeared, and the polymer became
light yellow. The polymer beads were collected by
filtration and washed with water (300 mL) and then with
Table 1
Reaction conditions optimizationa in the acetylation of 4-methylbenzal-
dehyde with Ac2O catalyzed by PS/TiCl4 at room temperature.
Entry
Solvent
Catalyst (mol %)
Time (min)
Yieldb (%)
1
n-Hexane
EtOAc
10
10
10
10
–
30
30
30
30
90
60
45
40
40
30
40
15
40
28
51
14
62
NR
48
76
93
93
NR
52
NR
51
2
3
C2H5OH
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
4
5c
6
1
7
5
8
10
15
–
9
10d
11e
12f
13g
–
10
10
NR: no reaction.
a
Reaction conditions: 4-methylbenzaldehyde (1 mmol), Ac2O
(1.2 mmol), solvent (3 mL).
PS/TiCl , CH Cl , r.t.
4
2 2
b
Ac O
2
RCH(OAc)
2
CHO
Isolated yield.
R
+
c
PS/TiC , CH OH r.t.
No catalyst.
4
3
,
d
R: Alkyl / Aryl
PS was used as catalyst.
2a-2p
e
The toluene–TiCl4 complex was used as a catalyst.
1
f
Catalyst was filtered after 15 min.
g
Scheme 1.
S/AlCl3 (0.15 mmol, 0.47 mmol AlCl3/g) was used as a catalyst.
Please cite this article in press as: Rahmatpour A, Mohammadian S. Polystyrene-supported TiCl4 as a novel, efficient
and reusable polymeric Lewis acid catalyst for the chemoselective synthesis and deprotection of 1,1-diacetates under
eco-friendly conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.012

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3
Table 2
PS/TiCl4 catalyzed protectiona of aldehydes as 1,1-diacetates and their deprotection.
Entry
1
R (1)
Product (2)
Protection
Time (min)
40
Deprotection
Time (min)
55
Yieldb (%)
93
Yieldb (%)
88
C6H5
CH(OAc)
2
2a
2
4-Me–C6H4
40
93
50
90
CH(OAc)
2
2b
Me
3
4
3-MeO–C6H4
4-Cl–C6H4
40
40
94
94
70
50
87
86
CH(OAc)
MeO
2
2c
CH(OAc)
2
2d
Cl
Cl
5
3-Cl–C6H4
45
75
94
84
50
80
85
85
CH(OAc)
2
2e
6c
4-O2N–C6H4
CH(OAc)
CH(OAc)
2
2f
O N
2
7c
3-O2N–C6H4
4-Br–C6H4
60
40
85
88
75
50
83
85
O N
2
2
2g
8
CH(OAc)
2
2h
Br
9
C6H5CH5CH–
40
40
89
87
50
50
86
85
CH(OAc)
2
2i
10
2j
CH(OAc)
2
O
O
11
40
86
45
84
2k
CH(OAc)
2
S
S
12
13
14
C6H13
C3H7
CH3(CH2)4CH2CH(OAc)2 2l
CH3(CH2)2CH(OAc)2 2m
40
40
40
87
88
90
50
50
75
86
85
84
C6H5CH2
CH(OAc)
2
2n
15d
16
4-HO–C6H4
50
60
88
91
80
90
85
87
CH(OAc)
2
2o
AcO
O
O
O
CHO
CH(OAc)
2
2p
O
O
17
NR
90
0
–
Please cite this article in press as: Rahmatpour A, Mohammadian S. Polystyrene-supported TiCl4 as a novel, efficient
and reusable polymeric Lewis acid catalyst for the chemoselective synthesis and deprotection of 1,1-diacetates under
eco-friendly conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.012

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Table 2 (Continued )
Entry
R (1)
Product (2)
NR
Protection
Time (min)
90
Deprotection
Time (min)
–
Yieldb (%)
0
Yieldb (%)
18
COCH
3
19
NR
90
0
–
O
NR: no reaction.
a
Reaction conditions: aldehyde (1 mmol), Ac2O (1.2 mmol), PS/TiCl4 (0.1 mmol) at room temperature.
The products were characterized from their spectra (IR, 1H NMR) and comparison with authentic samples.
1.5 equiv of acetic anhydride was used.
b
c
d
3 equiv of acetic anhydride was used.
ether (30 mL) and chloroform (30 mL). The catalyst was
2.5. Catalyst recovery and reuse
dried in a vacuum oven overnight at 50 8C before use. The
chlorine content of PS/TiCl4 was 10.32% analyzed by the
Mohr titration method [46] and the loading capacity of
TiCl4 on the polymeric catalyst or the amount of TiCl4
complexed with polystyrene, was calculated to be
0.726 mmol/g [47].
The reusability of the catalyst was checked in the
multiple acylation reaction of 4-methylbenzaldehyde with
acetic anhydride. At the end of each reaction, the catalyst
was recovered from the reaction mixture by addition of
methylene chloride, simple filtration and drying in a
vacuum oven, and then reused. The recycled catalyst was
used for further runs with fresh 4-methylbenzaldehyde,
Ac2O and CH2Cl2.
2.3. General experimental procedure for the synthesis of 1,1-
diacetates (2)
A mixtureof aldehyde (1, 1 mmol), freshly distilled acetic
anhydride (1.2–1.5 mmol), and PS/TiCl4 (0.1 mmol) in
methylene chloride (3 mL) was stirred at ambient tempera-
ture for an appropriate time (Table 2). The progress of the
reaction was monitored by thin layer chromatography (TLC)
and gas chromatography (GC). After completion of the
reaction, the mixture was diluted with methylene chloride
(10 mL) and filtered to separate the catalyst, and washed
with methylene chloride. The filtrate was washed with
water (10 mL) and the organic phase was dried over
anhydrous Na2SO4, and concentrated on a rotary evaporator
under reduced pressure to afford the pure 1,1-diacetate or
chromatographed on silica gel [Merck, 100–200 mesh,
petroleum ether (60–80 8C)-ethyl acetate] where necessary.
After each reaction, the spent polymeric catalyst from
different experiments was combined, washed with ether
and dried overnight in a vacuum oven and reused.
2.6. The spectral data for selected compounds
(Table 2, entry 2, 2b): IR (KBr)
1367, 1241, 1206, 1068, 1006, 959, 816 cmꢁ1
(250 MHz, CDCl3): = 7.62 (s, 1H, CH(OAc)2), 7.42 (d,
y: 3033, 2360, 1771,
.
1H NMR
d
J = 8.4 Hz, 2H, Ar-H), 7.22 (d, J = 8.4 Hz, 2H, Ar-H), 2.4 (s, 3H,
CH3), 2.15 (s, 6H, 2 ꢂ COCH3) ppm.
(Table 2, entry 2, 2d): IR (KBr) y: 3090, 2990, 2920 1760,
1738, 1592, 1490, 1376, 1242, 1200, 1084, 1060, 1010, 993,
970, 938, 910, 840, 820, 608, 540 cmꢁ1. 1H NMR (250 MHz,
CDCl3):
7.34 (d, J = 7.2 Hz, 2H), 2.11 (s, 6H, 2 ꢂ COCH3) ppm.
(Table 2, entry, 2l): IR (Neat) : 2930, 2863, 1762 (CO),
1465, 1378, 1250, 1214, 1112, 1015, 968 cmꢁ1 1H NMR
(250 MHz, CDCl3): = 0.98 (t, 3H, J = 6.8 Hz, CH3), 1.22–1.40
d = 7.60 (s, 1H, CH(OAc)2), 7.40 (d, J = 7.2 Hz, 2H),
y
.
d
(m, 8H, CH2), 1.66–1.80 (m, 2H, CH2), 2.07 (s, 6H,
2 ꢂ COCH3), 6.77 (t, 1H, CH(OAc)2) ppm.
(Table 2, entry 16, 2p): m.p. 131-132 8C; IR(KBr)
y:
2.4. General experimental procedure for the deprotection of
3085, 2987, 2940, 1761, 1645, 1625, 1612, 1470, 1417,
1192, 1073, 933 cmꢁ1 1H NMR (250 MHz, CDCl3)
= 8.27
1,1-diacetates (2)
;
d
(d, J = 9 Hz, 1H), 8.24 (m, 1H, Ar-H), 7.61 (s, 1H), 7.47-7.52
(m, 2H, Ar-H), 2.16 (s, 6H, 2 ꢂ COCH3).
A mixture of 1,1-diacetate (2, 1 mmol) and PS/TiCl4
(0.1 mmol) in methanol (3 mL) was stirred vigorously at
ambient temperature for an appropriate time (Table 2).
The course of the reaction was monitored by TLC and GC.
After completion of the reaction, the reaction mixture was
diluted with Et2O (10 mL) and filtered to separate the
catalyst. The filtrate was washed with water to remove
excess acetic anhydride and dried over anhydrous Na2SO4.
The solvent was evaporated on a rotary evaporator under
reduced pressure. The resultant product was passed
through a short column of silica gel to afford the pure
product. The spent polymeric catalyst from different
experiments was combined, washed with ether and dried
overnight in a vacuum oven and reused.
3. Results and discussion
3.1. Preparation of PS/TiCl4 complex
First, cross-linked polystyrene (5–7% divinylbenzene,
mesh size: 16–60) was prepared via suspension polymeri-
zation as previously reported in the literature [41]. Then,
PS/TiCl4 was prepared by addition of anhydrous TiCl4 to
polystyrene (5–7% 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 0.726 mmol
Please cite this article in press as: Rahmatpour A, Mohammadian S. Polystyrene-supported TiCl4 as a novel, efficient
and reusable polymeric Lewis acid catalyst for the chemoselective synthesis and deprotection of 1,1-diacetates under
eco-friendly conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.012

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5
TiCl4/g of complex beads catalyst [46]. The data obtained
by these two techniques showed, within experimental
error, that the catalyzing species are in the form of TiCl4
supported on the polymeric support. The UV spectrum of
the solution of PS/TiCl4 complex in CS2 showed a new
strong band at 390–491 nm, which is due to the formation
accomplish this protection, which indicated that the TiCl4
was the real active site in the supported catalyst, 138 mg
(10 mol%) of PS/TiCl4 per 1 mmol of aldehyde were
optimum in terms of reaction time and isolated yield.
The key role played by the Lewis acidity of the heteroge-
neous catalyst PS/TiCl4 was proved by employing the
polystyrene beads (Table 1, entry 10) and the TiCl4–
toluene complex (Table 1, entry 11) 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 52% (Table 1, entry 11). Also, PS/TiCl4 was found
to be a more effective catalyst than PS/AlCl3 for protection
of 4-methylbenzaldehyde under identical conditions
(Table 1, entry 13).
When using a supported catalyst, one of the most
important issues is the possibility of leaching of the
reactive center into the reaction mixture. To rule out the
contribution of homogeneous catalysis (free TiCl4 released
from the support) in the results shown in Table 1, the
catalyst was stirred in methylene chloride for 15 min
under operated temperature and filtered off (Table 1, entry
12). We observed that when the reactants were added to
the filtrates and stirred for 1 h, no reaction took place. This
result indicates that any TiCl4 species that leached into the
reaction mixture is not an active homogeneous catalyst
(the release of catalyst is negligible) and that the observed
catalysis is truly heterogeneous in nature.
of a stable
p
!p type coordination complex between the
benzene rings in the polystyrene carrier with TiCl4. The FT-
IR spectrum of PS/TiCl4 showed new absorption peaks due
to the C–C and C–H stretching vibrations, and the C–H
bending vibration of the benzene ring at 1450–1500,
2950–3100 and 400–800 cmꢁ1, whose complex formation
between Lewis acid TiCl4 and polystyrene was demon-
strated. The structure of the PS/TiCl4 complex is similar to
that of the PS/AlCl3 and PS/GaCl3 complex, as suggested by
Neckers et al. [39] and our prior works [41–44], because
the Lewis acid TiCl4 is complexed with the benzene rings of
the polystyrene and the TiCl4 is stabilized due to the
decreased mobility of the benzene rings hindered by the
long polystyrene chain. The PS/TiCl4 complex catalyst is a
non-hygroscopic, water tolerant, and especially stable
species. In addition, this polymeric catalyst is easy to
prepare, stable in air for a long time (over 1 year) without
any change, easily recycled and reused without appreci-
able loss of its activity.
3.2. Synthesis and deprotection of 1,1-diacetates
In order to find the most appropriate reaction condi-
tions and evaluate the catalytic efficiency of PS/TiCl4
catalyst on the protection of aldehydes as the correspond-
ing 1,1-diacetates, initially the reaction of 4-methylben-
zaldehyde with acetic anhydride was chosen as a model
reaction. The swelling property of cross-linked resin (PS) in
organic solvents is an important factor for effective solid-
phase reactions [48]. To select the best solvent, this
transformation was studied in various organic solvents,
such as n-hexane, ethyl acetate, ethanol, acetonitrile,
methylene chloride. Among them, the highest yield was
obtained in CH2Cl2 (Table 1, entry 8) and the reaction in
CH2Cl2 proceeds faster than in other organic solvents in the
same volume of solvent. The highest yield obtained 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. In protic solvents,
such as ethanol, this protection reaction proceeded in
longer reaction time and with very poor yield, which may
be related to the instability of acetic anhydride in protic
solvents (Table 1, entry 3). In addition, we further studied
the influence of the amount of PS/TiCl4 on the reaction
yields. In the presence of 15, 10, 5, and 1 mol% PS/TiCl4, the
corresponding yields were 93, 93, 76, and 48%, respectively
(Table 1, entries 6–9). The optimum molar ratios of the
polymeric catalyst to aldehyde and 4-methylbenzaldehyde
to acetic anhydride were found to be 0.1:1 and 1:1.2,
respectively. Blank experiment in the absence of catalyst
showed that the reaction did not take place, even after
12 h. The results show clearly that PS/TiCl4 is an effective
catalyst for this transformation, although a lower catalyst
loading (13.8 mg, 1 mol% of PS/TiCl4) could be used to
To establish the generality, a variety of aromatic and
aliphatic aldehydes were subjected to 1,1-diacetate
formation 2a-2p in high yields under the catalytic
influence of PS/TiCl4 at ambient temperature (Table 2).
Various functional groups were tolerated under the
present conditions, for example Me, Cl, Br, OMe, NO2.
Deactivated aromatic aldehydes, bearing electron-with-
drawing substituents, such as nitrobenzaldehyde (Table 2,
entries 6,7) also produced good yields under the selected
conditions but need a longer time. It is noteworthy to
mention that the acid-sensitive substrates like cinnamal-
dehyde (Table 2, entry 9), furan-2-carbaldehyde (Table 2,
entry 10), and thiophene-2-carbaldehyde (Table 2, entry
11) gave the expected acylals in high yields without any
by-product formation. Moreover, the protocol could also
equally work with aliphatic aldehydes (Table 2, entries 12-
14). We also investigated the reaction of 4-hydroxyben-
zaldehyde (Table 2, entry 15) under the above-mentioned
conditions and observed that both the carbonyl and
phenolic group were acylated. Furthermore, when alde-
hyde and ketone groups are present in the same molecule,
only the aldehyde diacetate was obtained, the ketone
moiety remained intact. When this reaction was extended
to 3-formyl benzopyran-(4H)-4-one, its formyl diacetate
was obtained in excellent yield (Table 2, entry 16). Several
aliphatic and aromatic ketones (Table 2, entries 17–19),
including cyclohexanone, acetophenone and 2-heptanone
were not reactive under the described experimental
conditions, even after 1.5 h under reflux conditions.
The difference in reactivity of aldehydes and ketones for
1,1-diacetate formation encouraged us to test and extend
the present protocol for chemoselective synthesis of 1,1-
diacetate of an aldehyde in the presence of ketone. We
Please cite this article in press as: Rahmatpour A, Mohammadian S. Polystyrene-supported TiCl4 as a novel, efficient
and reusable polymeric Lewis acid catalyst for the chemoselective synthesis and deprotection of 1,1-diacetates under
eco-friendly conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.012

G Model
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A. Rahmatpour, S. Mohammadian / C. R. Chimie xxx (2013) xxx–xxx
Table 3
Competitive acylal formation of aldehydes using Ac2O in the presence of PS/TiCl4 at room temperature.
Entry Substrates
1
Product (selectivity %)a
Ac Oð1:2mmolÞ;PS=TiCl ð0:1mmolÞ
2
4
ꢁ!
Me
CHO
COCH
CH(OAc)
2
3
CH Cl ;40min
2
2
C(OAc)
2
(1 mmol)
(1 mmol)
(100%)
(0%)
Ac Oð1:2mmolÞ;PS=TiCl ð0:1mmolÞ
2
4
2
ꢁ!
2
CHO
Me
COCH
CH(OAc)
2
3
CH Cl ;40min
2
C(OAc)
2
Me
(1 mmol)
Me
(100%)
Me
(1 mmol)
Me
(0%)
Ac Oð1:2mmolÞ;PS=TiCl ð0:1mmolÞ
2
4
3
4
ꢁ!
2
Me
CH(OAc)
COCH
CHO
2
3
CH Cl ;40min
2
C(OAc)
2
Cl
Cl
Cl
(100%)
(1 mmol)
(1 mmol)
Cl
(0%)
Ac Oð1:2mmolÞ;PS=TiCl ð0:1mmolÞ
2
4
ꢁ!
O
Me
C(OAc)
OAc
H
COCH
3
CH Cl ;40min
2
2
OAc
2
H
(1 mmol)
(1 mmol)
(100%)
(0%)
Ac Oð1:2mmolÞ;PS=TiCl ð0:1mmolÞ
2
4
5
ꢁ!
2
CHO
CH(OAc)
CHO
CH(OAc)
2
2
CH Cl ;40min
2
Me
(96 %)
O N
2
(1 mmol)
O N
2
(0 %)
Me
(1 mmol)
a
Conversion (GC).
studied the competitive acylation reactions of aromatic
aldehydes in the presence of ketones using PS/TiCl4. Under
these conditions, exclusive acylation of the aldehyde
functions was observed. The chemoselective acylations
of benzaldehyde, 4-methylbenzaldehyde, 4-chlorobenzal-
dehyde and hexanal in the presence of acetophenone, 4-
methylacetophenone, 4-chloroacetophenone and aceto-
phenone are shown in Table 3 (entries 1–4). In addition,
the acylation of 4-methylbenzaldehyde versus 4-nitro-
benzaldehyde in the presence of 1.2 mmol of acetic
anhydride has been investigated (entry 5). This reaction
also proceeded with high selectivity using this catalytic
system and showed the importance of the electronic effect
upon this reaction.
In order to show the efficiency and applicability of the
present method, the catalytic activity of PS/TiCl4 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, catalyst loading, and yields. The results show that
Table 4
Comparison of the efficiency of PS/TICl4 with some other catalysts for the acylation of 4-methylbenzaldehyde.
Entry
Catalyst (mol %)
Conditions
Time
Yield (%)
Reference
1
InCl3
CH2Cl2, r.t
1 h
96
93
92
79
84
87
93
92
94
87
98
95
94
94
93
[9]
2
NBS (10 mol %)
Solvent free, r.t
Solvent free, 40 8C
CH3CN, r.t
9 h
[12]
[10]
[18]
[21b]
[23]
[22]
[21a]
[26]
[17]
[16]
[8]
3
LiBF4 (10)
24 h
5 h
4
Bi(NO3)3ꢀ5H2O (10 mol %)
H3PW12O40/MCM-41 (20 wt %)
P2O5/Al2O3 (15, 0.1 g)
ß-zeolite (50 wt %)
H6P2W18O62ꢀ24H2O (45 mg)
PS/Al(OTf)3 (10)
5
Solvent free, 60 8C
Solvent free, r.t
Solvent free, 60 8C
Solvent free, r.t
CH2Cl2, r.t
2 h
6
45 min
2 h
7
8
30 min
1 h
9
10
11
12
13
14
15
Bi(OTf)3ꢀxH2O (31 mg)
Cu(OTf)2 (2.5)
CH3CN, r.t
3.5 h
2 h
CH2Cl2, r.t
LiOTf (20 mol %)
Solvent free, r.t
CH3CN, 0 8C
Solvent free, r.t
CH2Cl2, r.t
12 h
5 h
Sulfated zirconia (ZrO2/SO42ꢁ, 25 mg)
TBATB (10)
[28]
[20]
This work
2.5 h
40 min
PS/TiCl4 (10)
r.t: room temperature.
Please cite this article in press as: Rahmatpour A, Mohammadian S. Polystyrene-supported TiCl4 as a novel, efficient
and reusable polymeric Lewis acid catalyst for the chemoselective synthesis and deprotection of 1,1-diacetates under
eco-friendly conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.012

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A. Rahmatpour, S. Mohammadian / C. R. Chimie xxx (2013) xxx–xxx
7
Table 5
yields, ease of preparation and handling of the catalyst,
cost efficiency and stability and effective reusability of the
catalyst are noteworthy advantages of this method.
Studies on the use of the catalyst for other functional
group transformations are ongoing in our laboratory.
Reusability of aPS/TiCl4 in the acylation of 4-methylbenzaldehyde with
Ac2Ob.
Run
Product (%)c
Time (min)
1
2
3
4
5
93
92
89
88
85
40
40
40
45
45
Acknowledgments
a
The authorsaregratefultoTPC for providingpolystyrene.
The capacity of the catalyst after five uses was 0.633 mmol, PS/TiCl4
per gram.
b
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Reaction conditions: 4-methylbenzaldehyde (1 mmol), Ac2O
(1.2 mmol), PS/TiCl4 (0.1 mmol), CH2Cl2 (3 mL), r.t.
c
Isolated yield.
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aldehydes and deprotection of the obtained 1,1-diacetates
using polystyrene-supported TiCl4 as a novel heteroge-
neous polymeric Lewis acid catalyst. The mild reaction
conditions and simplicity of the procedure, excellent
Please cite this article in press as: Rahmatpour A, Mohammadian S. Polystyrene-supported TiCl4 as a novel, efficient
and reusable polymeric Lewis acid catalyst for the chemoselective synthesis and deprotection of 1,1-diacetates under
eco-friendly conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.012

G Model
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Please cite this article in press as: Rahmatpour A, Mohammadian S. Polystyrene-supported TiCl4 as a novel, efficient
and reusable polymeric Lewis acid catalyst for the chemoselective synthesis and deprotection of 1,1-diacetates under
eco-friendly conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.012