Zinc zirconium phosphate as an efficient catalyst for chemoselective synthesis of 1,1-diacetates under solvent-free conditions

Article abstract of DOI:10.1007/s12039-015-0955-2

In the present study, a mild, rapid, and efficient method for the protection of aldehydes with acetic anhydride (AA) in the presence of zinc zirconium phosphate (ZPZn) as a nano catalyst, at room temperature is reported. Selective conversion of aldehydes was observed in the presence of ketones. Under these conditions, different aldehydes bearing electron-withdrawing and electron-donating substituents were reacted with AA and the corresponding 1,1-diacetates (acylals) were obtained in high to excellent yields. The steric and electronic properties of the different substrates had a significant influence on the reaction conditions. Also, the deprotection of 1,1-diacetates has been achieved using this catalyst in water. This nanocatalyst was characterized by several physico-chemical techniques. It was recovered easily from the reaction mixture, regenerated and reused at least 7 times without significant loss in catalytic activity. This protocol has the advantages of easy availability, stability, reusability of the eco-friendliness, chemoselectivity, simple experimental and work-up procedure, solvent-free conditions and usage of only a stoichiometric amount of AA.

Full text of DOI:10.1007/s12039-015-0955-2

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0955-2
Zinc zirconium phosphate as an efficient catalyst for chemoselective
synthesis of 1,1-diacetates under solvent-free conditions
ABDOL R HAJIPOURa,b,∗ and HIRBOD KARIMIa,c
aPharmaceutical Research Laboratory, Department of Chemistry, Isfahan University of Technology,
Isfahan 84156, IR, Iran
bDepartment of Neuroscience, University of Wisconsin, Medical School, Madison, WI 53706-1532, USA
cYoung Researchers and Elite Club, Shahreza Branch, Islamic Azad University, Shahreza, Iran
e-mail: haji@cc.iut.ac.ir
MS received 13 April 2015; revised 5 August 2015; accepted 13 August 2015
Abstract. In the present study, a mild, rapid, and efficient method for the protection of aldehydes with acetic
anhydride (AA) in the presence of zinc zirconium phosphate (ZPZn) as a nano catalyst, at room temperature is
reported. Selective conversion of aldehydes was observed in the presence of ketones. Under these conditions,
different aldehydes bearing electron-withdrawing and electron-donating substituents were reacted with AA and
the corresponding 1,1-diacetates (acylals) were obtained in high to excellent yields. The steric and electronic
properties of the different substrates had a significant influence on the reaction conditions. Also, the deprotec-
tion of 1,1-diacetates has been achieved using this catalyst in water. This nanocatalyst was characterized by
several physico-chemical techniques. It was recovered easily from the reaction mixture, regenerated and reused
at least 7 times without significant loss in catalytic activity. This protocol has the advantages of easy availabil-
ity, stability, reusability of the eco-friendliness, chemoselectivity, simple experimental and work-up procedure,
solvent-free conditions and usage of only a stoichiometric amount of AA.
Keywords. Zinc zirconium phosphate; nanoparticles; heterogeneous catalysis; diacetate; solvent-free
synthesis.
1. Introduction
their involvement in various natural products synthe-
sis.3,4 Several reagents and catalysts have been reported
for the synthesis of acylals from aldehydes using acetic
anhydride (AA), such as SiPW-8,6 SiO2/B(SO4H)3,7
S-CKT,8 SuSA,9 SBA-15-Ph-PrSO3H,10 Zr(HSO4)4,11
PEG-SO3H,12 Sulphated Zirconia,13 SO42−/SnO2,14
ZSM-5-SO3H,15 PS/TiCl4,16 Schiff base complex of
The electrophilic nature of carbonyl groups is a dom-
inant feature of their extensive chemistry. Selective
protection and deprotection of aromatic or aliphatic
carbonyl groups are essential steps in modern organic
chemistry.1 The protection of aldehydes, as acetals,
acylals, oxathioacetals, or dithioacetals, is a common Cr(III),17 Solid sulfuric acid,18 SBSSA,19 CPTS–HOAc,20
H2NSO3H,21 ZrCl4,22 Zeolite Y,23 [bmpy]HSO4,24 [Hmim]
practice for manipulation of other functional groups
during multisteps synthesis. Geminal diacetates (acylals)
HSO4,25 [morH]HSO4,26 SiO2-SO3H,27 P2O5/Al2O3,28
RuCl3.H2O,29 H6P2W18O62.24H2O,30 zirconium sul-
are one of the essential carbonyl protecting groups due
fophenyl phosphonate,31 La(NO3)3·6H2O,32 (NH4)3
to their stability under neutral, basic and acidic condi-
PW12O40,33 PBBS,34 SbCl3,35 Cu3/2PMo12O40/SiO2,36
tions. Furthermore, they can be easily converted into
Si–[SbSipim][PF6],37 DOWEX(R)50WX4,38[bmim][Fe
parent aldehydes, which are frequently used as protect-
Cl4],39 Mo/TiO2–ZrO2,40 Cyanuric chloride,41 CuSO4·
ing groups for aldehydes.1,2 Besides, the acylals func-
5H2O,42 H3PW12O40,43 H2SO4-silica,44 Supported
tionality can be converted into other useful functional
POM,45 and P2O5/SiO2.46,47 Although some of these
groups by reaction with appropriate nucleophiles which
methods have convenient protocols with good to high
are also useful intermediates in industries, such as cross
yields, some of these methods suffer at least from
linking agent for cellulose in cotton or used as stain-
one of the following disadvantages: reaction under
bleaching agents.3–5 In addition, the preparation of
oxidizing conditions, use of harmful organic solvents,
1,1-diacetates from the corresponding aldehydes, in the
long reaction times, use of excess AA, unrecyclable
presence of ketones, can be very important due to
catalysts, high cost, and high toxicity. Also, very few
reports are applicable to both the synthesis as well as
deprotection of 1,1-diacetates.14,15,22,31,48
∗For correspondence

Abdol R Hajipour and Hirbod Karimi
with minor modifications.50,60–62 ZP was prepared
according to the following procedure. ZrOCl2·8H2O
(5 g) was heated at reflux in a solution of H3PO4 (50 mL,
12 mol/L) for 24 h. The resulting mixture was cooled
to ambient temperature to give a suspension, which was
filtered, and the filter cake was then washed with a solu-
tion of H3PO4 (0.1 mol/L) until the filtrate was free of
chloride ions. In order to determine the presence of any
chlorine ino in the filtrate, the silver nitrate test was
used. The filter cake was then washed several times with
distilled water until the pH of the filtrate was neutral.
The solid was then collected and dried in an oven at
110◦C for 24 h.50 ZPZn was prepared through an ion-
exchange reaction.60–62 Briefly, ZP (3 g) was dispersed
in deionized water (50 mL) at 50◦C, and the result-
ing suspension was treated with a solution of Zn(OAc)2
(100 mL, 0.1 mol/L) in water (excess amount of Zn2+).
This mixture was then heated at reflux for 4 d. It is note-
worthy that the acetate ion performed effectively as a
base to keep the hydrogen ion concentration in solu-
tion sufficiently low to achieve high loadings of the
catalyst.72 A complete exchange between the cations
and the hydrogen ions of the P–OH groups could not
be achieved in less than 3 d or at temperatures below
80◦C.61 The resulting slurry was filtered hot to give a
light white solid, which was washed with distilled water
until no Zn2+ ions could be detected in the filtrate (i.e.,
until the filtrate was colorless). The solid product was
then dried at 110◦C for 8 h before being calcined at
600◦C for 4 h to give the final product, ZPZn, as a white
solid (scheme 1).
α−Zirconium phosphate (ZP) is one of the most
important compounds in inorganic chemistry, and the
layered structure of this compound has been used in
different fields.49–51 The layered structure of ZP con-
sists of zirconium ions in a semiplanar arrangement,
located slightly above and below the mean plane, while
each Zr4+ ion is connected through the oxygen atoms
of phosphate groups from above and below. Three of
the four oxygen atoms in the phosphate groups are
bonded to three different zirconium atoms. The fourth
oxygen atom of the phosphate groups that bonds to a
proton, the free –OH group, is pointing into the inter-
layer region.50,51 ZP behaves as a unique ion exchanger
because of its exceptionally poor aqueous solubility,
high thermal stability, resistance to radiation and abra-
sive properties.52,53 The H+ of the P–OH moiety in ZP
can be exchanged for various other ions, which results
in the enlargement of the interlayer distance.54–56 Sev-
eral studies pertaining to the successful exchange of the
H+ of the P–OH group in ZP with various divalent and
trivalent cations have been reported in the litera-
ture.57–62 It has also been reported that ZP possesses
excellent selectivity towards Pb2+, Zn2+, and Fe3+ as an
ion exchanger.63–65 Furthermore, ZP has been reported
to exhibit antibacterial activity when it was loaded with
Cu2+, Zn2+, or Ce3+.60–62 Several reports have also
appeared in the literature concerning the catalytic activ-
ities of ion exchanged materials of this type, includ-
ing the use of copper zirconium phosphate (ZPCu) as
catalyst in the acetylation of alcohols and phenols and
the use of zirconium phosphate-ferric chloride complex
and potassium iron zirconium phosphate as catalyst in
Friedel-Crafts reaction.66–72
2.2 Catalyst characterization
To the best of our knowledge, there is no report avail-
able in the literature for using ZPZn as catalyst for
the preparation of 1,1-diacetates from carbonyl com-
pounds. Therefore, in continuation of our reports using
various catalysts for the synthesis of 1,1-diacetates from
aldehydes,25–29 we report a convenient, recyclable and
chemoselective procedure for conversion of aldehydes
to the corresponding acylals in the presence of AA.
The reaction was easily carried out at room temperature
under solvent-free condition with short reaction times
for a wide range of aldehydes.
The chemical composition of the ZPZn catalyst was
evaluated at different stages of the reaction (i.e., before
and after the catalytic reaction) by ICP-OES using an
Optima 7300 V ICP-OES spectrometer (PerkinElmer)
(table 1). The samples were ground into a fine powder and
analyzed by XRD on a Philips X’pert X-ray diffractome-
ter. The specific surface areas of the samples were deter-
mined from their N2 adsorption-desorption isotherms
2. Experimental
2.1 Catalyst synthesis
All the reagents and solvents used in the current study
were purchased from the Merck Chemical Company
and used without further purification. The catalyst was
prepared according to previously published procedures,
Scheme 1. Procedure for the preparation of ZPZn.

Chemoselective synthesis of 1,1-diacetates
Table 1. Elemental contents (%) of ZPZn and physical properties of the
catalysts before and after reaction.
Total acidity
Entry Sample
Zn
–
O
Zr
P
BET (m2/g) (mmol NH3/g)
1
2
3
4
5
ZP
65.3 12.4 22.3
118.2
102.4
102.1
96.4
2.5
1.6
1.52
1.05
0.78
ZPZn
12.1 54.1 12.3 21.5
12 54.3 12.2 21.5
8.7 59.2 12.1 20.0
4.7 63.5 12 19.8
ZPZn a
ZPZn b
ZPZn c
86.4
a After the first cycle.
b
After the 6th cycle.
c After the 7th cycle.
Table 2. Conversion of benzaldehyde to its correspond-
ing diacetate in different solvents and under solvent-free
conditions in the presence of ZPZn.
using the Brunauer-Emmett-Teller (BET) method on a
Quantachrome ChemBET 3000 instrument. Each sample
was degassed at 400◦C for 2 h before being analyzed to
remove any adsorbed species from their surfaces. The
BET surface areas of the materials were estimated from
their N2 adsorption-desorption isotherms. The surface
morphologies of the ZP and ZPZn materials were stud-
ied by SEM on a Philips XL scabbing electron micro-
scope (Philips). TEM images of ZPZn were obtained on
a CENTRA 100 TEM system (Zeiss).
Entry
Solvent a
Yield (%)b
Time (min)
1
2
3
4
5
6
Diethylether
Cyclohexane
Dichloromethane
Acetonitrile
51
38
56
67
55
90
90
120
90
120
120
15
Ethylacetate
Solvent-freec
a
The reaction was carried out in 5 ml of solvents at reflux
conditions.
The yields refer to isolated pure products.
2.3 Typical procedure for preparation of
1,1-diacetates
b
c The reaction was carried out at r.t.
Typically, in a 25 mL round bottom flask, equipped
with a magnetic stirrer, substrate (5 mmol) and AA
(10 mmol) and catalyst (1 mol%) were transferred. The
reaction mixture was stirred at room temperature for the
specified time under solvent-free conditions. Samples
were collected periodically and analyzed by gas chro-
matography (GC). After completion of the reaction, the
mixture was diluted with Et2O and the catalyst was
recovered by centrifuge. The organic layer was washed
with 10% NaHCO3 solution and then dried over anhy-
drous CaCl2. The solvent was evaporated under reduced
pressure to give the corresponding pure 1,1-diacetate.
The results and general experimental procedure are
summarized in tables 2 and 3.
dried over anhydrous CaCl2. The solvent was evapo-
rated under reduced pressure to give the corresponding
aldehyde.
2.5 Recyclability studies of catalyst
To examine the recyclability of the catalyst, the used
ZPZn was recovered from the reaction media and re-
used. For recycling, after the first use, the catalyst
was separated from the reaction mixture by centrifuga-
tion, washed sequentially with ethanol and water before
being dried at 110◦C for 2 h, and then activated at 450◦C
for 2 h.
2.4 Procedure for the deprotection of 1,1 diacetates
to aldehyde
3. Results and discussion
A solution of 1,1-diacetate (5 mmol) in water (2 mL)
and ZPZn (1 mol%) was introduced into a round bottom
3.1 Catalyst characterization
flask equipped with a magnetic stirrer and condenser The ICP-OES analyses of ZP and ZPZn are shown
at 80◦C. The reaction was monitored by GC. After in table 1. The results obtained in the current study
the reaction, the mixture was diluted with Et2O and for ZPZn were compared with those reported.60–62 Our
the catalyst was recovered by centrifuge. The organic results revealed that there was a negligible leach of
layer was washed with water. The organic extracts zinc ions into the reaction media after the reaction (i.e.,
were combined and washed with 10% NaHCO3 and following the first use of the catalyst).

Abdol R Hajipour and Hirbod Karimi
Table 3. Preparation of acylals in the presence of ZPZn under solvent-free conditions at r.t.
M.p. (◦C) or B.p. (◦C)
Entry
Substrate
Time (min)
Yeild (%)a
Found
Reaported [ref]
1
2
3
4
5
6
7
8
C6H5CHO
4-Me-C6H4CHO
2-Me-C6H4CHO
2-MeO-C6H4CHO
3-MeO-C6H4CHO
4-MeO-C6H4CHO
3,4-di-MeO-C6H4CHO
2,5-di-MeO-C6H4CHO
4-OH-C6H4CHO b
2-OH-C6H4CHOb
2-(CH3)2CH-C6H4CHO
2-(CH3)3C-C6H4CHO
2-Cl-C6H4CHO
3-Cl-C6H4CHO
2,6-di-Cl-C6H4CHO
4-Cl-C6H4CHO
4-Br-C6H4CHO
2-Br-C6H4CHO
2-NO2-C6H4CHO
4-NO2-C6H4CHO
2-NC-C6H4CHO
4-NC-C6H4CHO
2-F-C6H4CHO
15
30
45
45
20
30
45
45
120
180
75
120
15
10
20
10
10
15
20
10
10
10
10
10
180
30
10
45
45
20
45
180
180
180
180
90
90
85
85
89
84
84
83
85
82
83
74
95
95
85
91
94
91
88
95
96
96
90
95
–
43–45
80–82
64–66
68–80
Oil
44–456–10
81–826–10
64-66 913,81,82
73–746–10
Oil6–10
65–66
70–72
107–108
90–92
101–102
58–60
68–70
51–52
64–65
89–90
81–82
93–95
80–82
91–93
124–126
74–76
100–101
24–26
50–52
–
52–53
84–86
Oil
Oil
Oil
Oil
–
–
–
64–656–10
72–746–10
1106–10
89–906–10
101–1036–10
–
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
–
52–536–10
64–656–10
88–906–10
82–836–10
92–956–10
8083
90–918,9,18,27
125–1276–10
76–7711
100–1026–10
25–26 883
50–528,24,28
–
4-F-C6H4CHO
4-(N,N-di-MeN)-C6H4CHO
Furfural
Cinnamaldehyde
Hexanal
70
92
77
77
71
71
–
–
–
–
52–536–10
84–856–10
Oil6–10
2-butenal
Oil6–10
C6H4CH2CH2CHO
Isobutyraldehyde
Cyclohexanone
Oil6–10
Oil6–10
–
–
–
–
C6H5COCH3
4-NO2C6H4COCH3
4-NO2-C6H4CHO c
–
a All products were identified by their M.p., IR, GC–MS and 1H NMR spectra with authentic samples.
b AA (15 mmol).
c The reactions was performed in the absence of ZPZn at 60◦C.
It is well known that the ionic radii of Zn2+ (0.74 Å) and
hydrated Zn2+ (2.16 Å) are smaller than the basal spac-
ing of ZP (7.5 Å).73,74 The crystal structure of ZPZn was
affected by the exchange of zinc ions. When the zinc
ions were introduced into the ZP, the crystallinity grad-
ually decreased. Its XRD pattern shows a layered mate-
rial, slightly disordered, that, however, clearly shows
the harmonics of the first d002 reflection. These results
therefore indicated that Zn2+ ions had inserted into the
interlayer of ZP and increased the basal spacing of the
modified ZP after the exchange.60–62 Taken together,
Figure 1 shows the powder XRD patterns of the ZP
and ZPZn materials. The results show some character-
istic reflections in the 2θ range of 5◦ – 40◦. The diffrac-
tion peak in ZP at 2θ ∼ 12◦ was assigned to a d002
basal spacing of 7.5 Å between the planes, which was
consistent with the patterns previously reported for ZP
and its derivatives with a hexagonal crystal system.50
It shows that the d-spacing of the (002) plane of ZPZn
had increased, which indicated that the Zn2+ ions had
intercalated into the interlayer of ZP and increased the
d002 basal interlamellar spacing of ZP from 7.5 to 8.7 Å.

Chemoselective synthesis of 1,1-diacetates
which is characteristic of a mesoporous material.75 The
hysteresis loop (type H3) is associated with the occur-
rence of capillary condensation in the mesopores, which
indicates the presence of a mesoporous structure in the
ZPZn catalyst. The observed increase in adsorption at
the higher P/P0 value indicated the presence of larger
mesopores in the sample.61–66 The surface area of ZPZn
after the 7th run was found to be 86.4 m2/g.
these data indicated that ZPZn had been formed suc-
cessfully. The XRD pattern of the ZPZn catalyst after
the 7th run showed that the basal spacing of ZP was
about 9.5 Å, which was only a little larger than that of
the fresh ZPZn catalyst. This increase may have oc-
curred because of the presence of less Zn2+ on the sur-
face of ZP, and an increase in the number of water
molecules between the layers following the seventh run
(i.e., Zn2+ ions may have been washed off during the
regeneration of the catalyst, table 1). Our findings
showed good agreement with earlier reports.60–62
Figure 2 shows the N2 adsorption-desorption iso-
therm of ZPZn, as a representative example, in the rel-
ative pressure range (P/P0) of 0.1–1.0. The surface area
of ZPZn was determined to be 102.4 m2/g. The isotherm
for ZPZn shows three adsorption stages. The first of
these stages was observed at P/P0 < 0.46, whereas the
second stage was observed in the range of 0.46 < P/P0
< 0.95, and the third stage was observed at higher rela-
tive pressures (P/P0 > 0.95). The N2 adsorption-desorp-
tion isotherm of ZPZn exhibited a typical “type
IV” isotherm shape with a distinct hysteresis loop,
The nature of the acid sites of ZP and ZPZn was
studied by IR spectroscopy using pyridine as a probe
molecule. Pyridine adsorption was used to determine
the acidic sites using FTIR. Prior to the measurements,
20 mg of a catalyst was pressed in self-supporting
disc and activated in the IR cell attached to a vacuum
line at 350◦C for 4 h. The adsorption of pyridine was
performed at 150◦C for 30 min. The excess of probe
molecules was further evacuated at 150◦C for 0.5 h.
The adsorption–evacuation was repeated several times
until no change in the spectra was observed (figure 3).
The main bands observed over the samples are assigned
according to the literature data.76,77 Pyridine adsorbed
FTIR spectra of the ZPZn show the strong bands at
1632 and 1541 cm−1, indicating typical pyridinium
ion. The band at 1488 cm−1 is a combination band
between those at 1541 and 1444 cm−1, corresponding to
Brønsted and Lewis acid sites, respectively. The origin
of Brønsted acidity of the ZP is due to the presence of
P-OH groups,78,79 while the main acitidy of ZPZn is
due to its Lewis acid sites (Zn2+) (figure 3). Figure 3
shows the Py-FTIR spectrum of the catalyst after the 7th
run. It clearly indicates that the replacement of Zn2+ on
the surface of the catalyst with H+ during the catalyst
regeneration process has reduced the number of Lewis
acid sites (band at 1444 cm−1 was reduced). Conversely,
the amount of Brønsted acid sites was increased (band
at 1632 cm−1 was increased).
Total acidity of the samples was determined by tem-
perature-programmed desorption of ammonia (TPD-
NH3) with a Quantachrome ChemBET 3000. Before the
Figure 1. XRD patterns of powder ZP (down), ZPZn fresh
(middle) and after the 7th run (up).
Figure 3. Pyridinedesorbed FTIR spectra of ZPZn (Fresh
and after the 7th run) and ZrP.
Figure 2. N2 adsorption-desorption isotherm of ZPZn.

Abdol R Hajipour and Hirbod Karimi
adsorption of ammonia, the samples were pre-treated The surface morphology of the ZP and ZPZn was
in He at 250◦C for 30 min and then, 1 h at 350◦C and studied by SEM (SEM, Philips XL) (figure 5). The
cooled to 100◦C. Then ammonia was adsorbed on the SEM image of ZP (figure 5(a)) revealed the presence
samples for 1 h. The TPD-NH3 was carried out between of hexagonal plates with well-defined shapes and very
150 and 550◦C, at 10◦C/min, and analyzed by a ther- smooth surfaces. Figures 5(b) and (c) (different magni-
mal conductivity detector (TCD) for continuous mon- fication) show the SEM images of ZPZn. These images
itoring of the desorbed ammonia. TPD-NH3 provides revealed that the structure of ZPZn was less ordered
a quantitative estimation of the total number of acid than that of ZP, and that the ZPZn particles had aggre-
sites and the distribution of acid strengths. Because of gated to form both sheets and spheres of different
the strong basicity of NH3 gas, it was expected that all shapes and sizes.60,61,66
acid sites on the catalysts interact with NH3. The total
Figure 6 shows the TEM (CENTRA 100, Zeiss)
amount of NH3 desorbed after saturation permits the images of ZPZn. It shows that ZPZn catalyst retained
quantification of the number of acid sites on the sur- the original morphology of ZP (layered structure) and
face, while the position of the peak, desorption temper- that the particles were approximately 150 nm in size.
ature, indicates the strength of the catalyst, i.e., higher
These images also showed nanoparticles of different
temperature of desorption, stronger is the acid strength. sizes on the smooth surface of the ZP. The presence of
The TPD-NH3 curves of ZPZn are shown in figure 4 metallic crystal nanoparticles on the surface of ZP indi-
ZPZn desorbed ammonia in a wide range of temper- cated that the zinc deposited on the surface of the ZP
atures from 212 to 538◦C, which mostly correspond had agglomerated. Similar observations have also been
to the medium and strong acidic sites. The NH3 des- reported for copper, zinc, and cerium with ZP.61,62,72
orption peak at temperatures below 250◦C belongs to Figure 5(d) and 6(c,d) show the SEM and TEM images
the physisorption/chemisorptions of NH3 molecules on of the catalyst after several regenerations, respectively.
weak acidic sites. The peak at about 250-450◦C shows All these images showed that the sheets and particles
the existence of intermediate strength acidic sites and had conglomerated to a much greater extent following
finally the peak at 450-538◦C demonstrates the presence the 7th run because of the process used to regenerate the
of strong acidic sites on the surface of ZPZn. Figure 4 catalyst.
shows that the desorption of ammonia starts at almost
212◦C, centered at 317◦C. The NH3-TPD curves sub-
3.2 Synthesis of 1,1-diacetates and expected
mechanism
sequently decreased with further increase in temper-
ature and almost complete at 538◦C. This indicates
that ZPZn contains a considerable number of acid sites
which is attributed to the presence of Zn2+ groups on
the surface of zirconium phosphate layers and make
it suitable solid acid catalyst.66 The extent of desorp-
tions is found to be ca. 1.6 mmol NH3/g of catalyst. A
TPD experiment was carried out after the 7th cycle by
recovering the catalyst, in order to magnify the differ-
ence from the fresh catalysts (table 1).
In order to find the most appropriate reaction condi-
tions and evaluate the catalytic efficiency of ZPZn on
the protection of aldehydes to the corresponding 1,1-
diacetates, we tried to convert benzaldehyde (5 mmol)
to its corresponding acylal with ZPZn (1 mol%) and AA
(10 mmol) in various solvents and also under solvent-
free conditions. We observed that the yield of the reac-
tion under solvent-free condition is higher and the
reaction time is shorter as compared to the other meth-
ods, table 2.
Therefore, to establish the generality and scope of
the method, we employed the above conditions for con-
version of various aldehydes (aromatic, heterocyclic,
aliphatics and α, β-unsaturated aldehydes) to the cor-
responding diacetates, table 3. Both aromatic and ali-
phatic aldehydes react smoothly with AA to afford the
corresponding 1,1-diacetates in good to excellent yields
in short reaction times at room temperature (table 3,
entries 1-23), which are, in general, similar or higher
than those described in the literature.6–47 The reactions
did not proceed in the absence of ZPZn even under heat-
ing conditions (table 3, entry 27). Under these reaction
conditions, various functional groups (Me, isopropyl,
Figure 4. NH3-TPD profile of ZPZn.

Chemoselective synthesis of 1,1-diacetates
(a)
(b)
(c)
(d)
Figure 5. SEM images of regular morphology of prepared ZP (a), ZPZn fresh (b, c) and after the 7th run (d).
(a)
(b)
(c)
(d)
Figure 6. TEM images of regular morphology of prepared ZPZn fresh (a,b)
(different magnification) and after the 6th (c) and the 8th run (d).
t-butyl, OMe, OH, F, Cl, Br, CN, NO2) were tolerated. corresponding 1,1-diacetates in lower yields and longer
The electronic properties of the substituents on the aro- reaction times (table 3, entries 2-12). This may be due to
matic aldehydes have a major effect on the reaction the reduced electrophilicity of the aldehyde group as a
yield and time. The aldehydes with electron-withdraw- result of the electron-rich nature of the phenyl ring to
ing groups (NO2, CN, F, Cl and Br), afforded the cor- which the aldehyde is attached. Also, the reaction rate
responding diacetates in higher yields (table 3, entries was found to be dependent on steric crowding surround-
13–24).80 But, aldehydes bearing electron-donating ing the aldehyde group. Thus, the presence of substi-
groups (Me, isopropyl, t-butyl, and OMe) gave the tutes at the para or the meta position (table 3, entries

Abdol R Hajipour and Hirbod Karimi
2,5,6,9,14,16,17,20,22 and 24) made the reaction faster In addition to these results, we further studied the
with better yields than those with substitutes at the possibility of deprotection of resulting acylals in this
ortho position (table 3, entries 3,4,8,10,13,15,18,19,21 catalytic system by addition of water as a green solvent.
and 23). Moreover, aldehydes with hindered alkyl sub- Indeed, deprotection of phenylmethylene diacetate to
stitutes, 2-isopropylbenzaldehyde and 2-(tert-butyl)- the benzaldehyde was performed by treatment of acy-
benzaldehyde, produced the corresponding acylals in lals in water at 80◦C. By this procedure, related acylal
longer reaction times and lower yields (table 3, entries has been completely transformed into benzaldehyde in
11,12).
The acid-sensitive compounds such as furfural and
a short reaction time (scheme 3).
On the basis of literature studies, a plausible catalytic
cinnamaldehyde were also protected as 1,1-diacetates cycle for the regeneration of ZPZn has been proposed.
in good yields without any side products (table 3, entries As outlined in scheme 4, the possible mechanism of this
26 and 27). We investigated the reaction of 2-hydroxy- reaction may be involvement of either intermolecular
benzaldehyde and 4-hydroxybenzaldehyde under above or intramolecular transfer of the second acetate group
conditions (table 3, entries 9 and 10); it should be men- after the initial attack by AA. We suggest that ZPZn, as
tioned that the phenolic group was also protected as a Lewis acid, increases the electrophilicity of the car-
acetate in hydroxyl containing aromatic aldehyde under bonyl group on the aldehyde. Then, AA attacks com-
such conditions (3 equiv. of Ac2O). 4-(dimethylamino) plex (I) to produce the final 1,1-diacetate.7,9,17,25,27,29
benzaldehyde failed to give 1,1-diacetate under the
We were interested in studying the reusability of
same conditions which may be due to the electron dona- the catalyst because of economic and environmental
tion of dimethylamino group (table 3, entry 25). The aspects. Hence, the reaction of benzaldehyde with AA
explanation for this result may be due to the strong elec- was chosen as a model reaction in the presence of
tron donating properties of the dimethylamino group regenerated ZPZn under the optimum reaction condi-
which will reduce the reactivity.21 Moreover, the pro- tions and the results are summarized in table 4. Used
tocol could also equally work with aliphatic aldehydes ZPZn gave a similar yield of product as the fresh cata-
(table 3, entries 28-31). Because of aldol condensation lyst till the 7th cycle. The lowered activity of the used
as a competitive reaction, the yields of corresponding catalyst sample confirms the deactivation of catalyst
acylals of aliphatic aldehydes were low. Ketones proved during the reaction.
completely resistant to acylal synthesis with AA under
these reaction conditions.21 Cyclohexanone, acetophe-
none, and 4-nitroacetophenone were checked for the
reactivity. No diacetate formation was observed for
these compounds, neither under room temperature nor
reflux conditions (table 3, entries 32-35). Encouraged
by this result, we suggest that the chemoselective pro-
tection of aldehydes in the presence of ketones can be
achieved by this method (scheme 2). We found that ben-
zaldehyde was converted to the related gem-diacetate
while the acetophenone remained unaffected.
Scheme 3. Cleavage of acylal to benzaldehyde in water
catalyzed by ZPZn.
Scheme 2. Competitive acylal formation of aldehydes in
the presence of ZPZn.
Scheme 4. Plausible mechanism.

Chemoselective synthesis of 1,1-diacetates
The regenerated catalyst was characterized for its from the zirconium phosphate surface during the cat-
chemical composition by elemental analysis. The ele- alyst regeneration), which caused a reduction of the
mental composition of the catalyst remained almost product yield down to 68%. We believe that the cat-
unchanged upto its 7th run, although the amount of alytic activity of ZPZn is due to its Lewis acid sites
zinc in the catalyst was reduced by almost 60% com- (scheme 4) and when these active sites were reduced,
pared with the first run (table 1). As it can be seen from the catalytic activity was reduced as well. Also, another
this table, no significant change in composition of the reason for less activity of ZPZn after the 7th run is prob-
catalyst was observed after the first use. There was a ably because of catalyst agglomeration after several
negligible leach of Zn2+ into the reaction media after thermal regenerations (figure 6).
the reaction (table 1 entry 2,3). Also, as it is presented
In order to compare the catalytic potentiality of ZPZn
in table 4, the activity of the catalyst almost remained nanoparticles with some recently reported procedures in
intact till the 7th run. But, after the 7th run, the decom- the literature, we have shown the results of the synthesis
position of the catalyst was changed (Zn2+ was washed of acylal from benzaldehyde in the presence of various
Table 4. Catalyst re-usage under the optimum reaction conditions for 1,1-diacetate synthesis.
Substratea
Fresh
run 1
run 2
run 3
run 4
run 5
run 6
run 7
84
run 8
68
C6H5CHO
90
90
90
89
88
88
85
a Reaction conditions: C6H5CHO (5 mmol), AA (10 mmol), catalyst (1 mol%), r.t. and 15 min.
Table 5. Comparison of the effect of catalysts for 1,1-diacetate synthesis from benzaldehyde.
Catalysta
Ac2O (mmol)
Time (min)
Yield %
Ref.
7
SiO2/B(SO4H)3
S-CKT
SuSA
SBA-15-Ph-PrSO3H
Zr(HSO4)4
PEG-SO3H
Sulphated Zirconiab
SO24−/SnO2
ZSM-5-SO3H
PS/TiClc4
Schiff base complex of Cr (III)
Solid sulfuric acid
SBSSA
2
5
2
1.2
3
10
2.5
2
1
1.2
3
15
15
4
3
3
3
3
2
2
4
2.5
6
4
5
12
5
5
15
5
15
1
40
15
5
97
96
98
100
90
89
99
97
97
93
96
83
84
92
90
90
90
97
90
85
90
87
76
90
98
90
85
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
32
33
4
CPTS–HOAc
H2NSO3H
ZrCl4
42
60
30
180
5
Zeolite Y
[bmpy]HSOd4
[Hmim]HSO4
[morH]HSO4
SiO2-SO3H
P2O5/Al2O3
RuCl3.H2O
La(NO3)3.6H2O
(NH4)3PW12O40
ZPZn
25
3
25
45
10
90
90
15
20
2.2
2
2
This work
This work
ZP
3
a Reaction conditions: r.t., solvent-free.
b Reaction temperature: 0◦C.
c CH2Cl2 as solvent.
d Ultrasonic irradiation, 30◦C.

Abdol R Hajipour and Hirbod Karimi
10. Zareyee D, Moosavi S M and Alaminezhad A 2013 J.
catalysts with respect to the amounts of AA, reaction
time and the yield of the products (table 5). The results
show that, ZPZn is an equally competitive or more effi-
cient catalyst for this reaction with regard to reaction
conditions and yield.
The better efficacy of ZPZn nanoparticles in solvent-
free synthesis of 1,1-diacetate from aldehydes might be
due to the presence of Lewis acid sites (Zn2+) on the
surface of the catalyst. Moreover, this procedure offers
advantages over some of the methodologies in terms of
efficiency, deprotection, as well as protection, reusabil-
ity of the catalyst and we found that ZPZn is a selective
catalyst in solvent-free conditions, which thus, makes it
environmentally more acceptable.
Mol. Catal. A: Chem. 378 227
11. Mirjalili B F, Zolfigol M A, Bamoniri A and Sheikhan
N 2006 J. Chin. Chem. Soc. 53 955
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1414
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4. Conclusions
In summary, in this paper, we have reported a mild,
solvent-free and efficient protocol for the protection of
aldehydes for their conversion to the corresponding 1,1-
diacetates. ZPZn was used for this reaction at room
temperature. This method is selective for the prepara-
tion of 1,1-diacetates from aldehydes in the presence
of ketones. Also, 1,1-diacetates can be conveniently
deprotected by using ZPZn in water. Other advantages
of this catalyst are good to excellent yields and reusabil-
ity of the catalyst. Further applications of this catalyst to
other transformations are currently under investigation.
20. Wang M, Song Z, Gong H and Jiang H 2008 Synth.
Commun. 38 961
21. Jin T S, Sun G, Li Y W and Li T S 2002 Green Chem. 4
255
22. Smitha G and Reddy C S 2003 Tetrahedron 59 9571
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1998 Tetrahedron Lett. 39 7587
24. Borikar S P and Daniel T 2011 Ultrason. Sonochem. 18
928
25. Hajipour A R, Khazdooz L and Ruoho A E 2008 Catal.
Commun. 9 89
26. Hajipour A R, Nasreesfahani Z and Ruoho A E 2008
Org. Prep. Proced. Int. 40 385
27. Hajipour A R, Zarei A, Khazdooz L, Mirjalili B B
F, Sheikhan N, Zahmatkesh S and Ruoho A E 2005
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29. Sheikhan N, Mirjalili B F, Hajipour A and Bamoniri A
2008 Acta Chim. Slov. 55 209
Acknowledgments
We gratefully acknowledge the funding support re-
ceived for this project from the Isfahan University of
Technology (IUT), IR Iran.
30. Romanelli G P, Thomas H J, Baronetti G T and Autino
J C 2003 Tetrahedron Lett. 44 1301
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