Cerium polyoxometalate as a reusable catalyst for acetylation and formylation of alcohols

Article abstract of DOI:10.1007/s00706-004-0196-4

Efficient esterification of primary and sterically-hindered secondary or tertiary alcohols with acetic anhydride was achieved in the presence of ammonium decatungestocerate(IV) icosahydrate, (NH₄)₈[CeW ₁₀O₃₆]·OH₂O, as catalyst in high yields. Primary and secondary alcohols were also converted to their corresponding acetates and formates with acetic acid and ethyl formate in the presence of this catalyst. Easy work-up, non-toxicity, reusability, and stability of the catalyst are noteworthy advantages of this method. Springer-Verlag 2004.

Full text of DOI:10.1007/s00706-004-0196-4

Monatshefte f€ur Chemie 135, 1257–1263 (2004)
DOI 10.1007/s00706-004-0196-4
Cerium Polyoxometalate
as a Reusable Catalyst for Acetylation
and Formylation of Alcohols
Valiollah Mirkhani1;ꢀ, Shahram Tangestaninejad1, Majid Moghadam2,
Bahram Yadollahi3, and Ladan Alipanah1
1
Chemistry Department, Isfahan University, Isfahan 81746-73441, Iran
2
Chemistry Department, Yasouj University, Yasouj 75914-353, Iran
3
Institute for Advanced Studies in Basic Sciences, Gava-Zang, Zanjan 45138-79368, Iran
Received January 7, 2004; accepted (revised) February 2, 2004
Published online June 30, 2004 # Springer-Verlag 2004
Summary. Efficient esterification of primary and sterically-hindered secondary or tertiary alcohols
with acetic anhydride was achieved in the presence of ammonium decatungestocerate(IV) icosahy-
drate, (NH4)8[CeW10O36]ꢁ20H2O, as catalyst in high yields. Primary and secondary alcohols were also
converted to their corresponding acetates and formates with acetic acid and ethyl formate in the
presence of this catalyst. Easy work-up, non-toxicity, reusability, and stability of the catalyst are
noteworthy advantages of this method.
Keywords. Cerium polyoxometalate; Acetylation; Formylation; Alcohol.
Introduction
Developing efficient and mild methods for the protection of hydroxyl groups of
alcohols is of importance in synthetic organic chemistry. One of the most common
methods for the protection of these compounds is the formation of acetyl derivatives
[1].
The protection of such functional groups is often necessary during the
course of various transformations in a synthesis sequence, in particular in the
construction of polyfunctional molecules such as nucleosides, carbohydrates,
steroids, and natural products. A number of methods and reagents such as
4-(dimethylamino)pyridine, 4-pyrrolidinopyridine [2], N,N,N0,N0-tetramethyl-
ethylenediamine [3], tributylphosphine [4], iodine [5], p-toluenesulfonic acid [6],
alumina [7], zinc chloride [8], cobalt chloride [9], montmorillonite K-10 and
KSF [10], zeolite HSZ-360 [11], zirconium sulfophenyl phosphonate [12], scan-
ꢀ
Corresponding author. E-mail: mirkhani@sci.ui.ac.ir

1258
V. Mirkhani et al.
dium trifluoromethanesulfonate [Sc(OTf )3] [13], TaCl5 [14], trimethylsilyl tri-
fluoromethanesulfonate [15], copper trifluoromethanesulfonate [16], indium tri-
fluoromethanesulfonate [17], magnesium bromide [18], bismuth(III) salts [19],
ferric perchlorate adsorbed on silica-gel [20], and tin(IV) tetraphenylporphyrin
perchlorate [21] have been reported for the acetylation of alcohols. Formylation
is also a very important process and several catalysts, such as bismuth(III) salts
[22], Cu(NO3)2ꢁ3H2O [23], and Ce(OTf )4 [24] have been used for this trans-
formation. Due to the instability of the anhydride and the acid chloride of
formic acid, formylation of alcohols by ethyl formate is an important synthetic
reaction.
However, some of the reported methods for the acetylation and formylation of
alcohols suffer from one or more of the following disadvantages, such as high
temperature and drastic reaction conditions, formation of undesirable or toxic
byproducts, expensive reagents, hygroscopicity and thermal instability of the
reagents, long reaction times, low yields of the desired products and bulk require-
ment of solid bed. Therefore, introduction of new methods and catalysts for the
preparation of esters is still in demand.
In recent years there has been a tremendous upsurge of interest in various
chemical transformations performed under heterogeneous catalysis [25]. More-
over, using inexpensive and none corrosive heterogeneous catalysts, chemical trans-
formations occur with better efficiency, higher purity of products, and easier
work-up, with evident economic and ecological advantages especially for industrial
processes.
Polyoxometalates have been proven to be good catalysts in various oxidations.
They are applied in bulk or in supported forms, and both homogenous and hetero-
geneous catalysis are possible. Due to their acidic and redox properties, heteropoly
compounds are useful and versatile catalysts in a number of transformations [26].
In continuation of our research on the application of polyoxometalates in organic
synthesis [27–32], we wish to report a simple, efficient, and selective method for
the acetylation of alcohols with acetic anhydride and acetic acid (Scheme 1) and
formylation of alcohols with ethyl formate (Scheme 2) using (NH4)8
[CeW10O36]ꢁ20H2O as a heterogeneous, reusable, and stable catalyst at room tem-
perature or under reflux conditions.
Scheme 1
Scheme 2

Cerium Polyoxometalate
1259
Results and Discussion
Acetylation of Alcohols with Acetic Anhydride and Acetic Acid
Catalyzed by (NH4)8[CeW10O36]ꢁ20H2O
Initially, benzyl alcohol was chosen as a model substrate for the acylation reac-
tions. The reaction of this alcohol with acetic anhydride (2 cm3) in the presence of
0.04 molar equivalents of (NH4)8[CeW10O36]ꢁ20H2O at room temperature was
completed in 30 min and the corresponding acetate was obtained in 96% yield.
In the absence of (NH4)8[CeW10O36]ꢁ20H2O as catalyst, Ac2O was much less
efficient for the conversion of alcohols to esters (5–12%).
As shown in Table 1 a series of primary alcohols (including aliphatic and
aromatic alcohols) and secondary alcohols were acetylated with Ac2O in the pre-
sence of 0.04 mol% of (NH4)8[CeW10O36]ꢁ20H2O at room temperature or under
reflux conditions. Tertiary alcohols such as 1-adamantanol and triphenylmethanol
were also acetylated in good yields (entries 17 and 18). In contrast with the data
reported with other heterogeneous catalysts such as montmorillonite clay [10],
Table 1. Acetylation of alcohols with Ac2O and HOAc catalyzed by (NH4)8[CeW10O36]ꢁ20H2O at
refluxa
Entry
Alcohol
Productb
Acetic
Acetic acid
anhydride
Time
Yield
=%c
Time
Yield
=%c
=min
=h
1
2
PhCH2OH
PhCH2OAc
30
5
96
100
96
90
97
80
100
97
99
99
97
92
88
98
80
98
8
8
6
6
8
8
4
5
8
8
4
6
6
6
4
6
90
87
94
78
36
82
90
95
91
93
96
40
30
60
33
50
4-ClC6H4CH2OH
4-CH3OC6H4CH2OH
3-CH3OC6H4CH2OH
2-NO2C6H4CH2OH
PhC6H4CH2OH
PhCH2CH2CH2OH
n-Pentanol
4-ClC6H4CH2OAc
4-CH3OC6H4CH2OAc
3-CH3OC6H4CH2OAc
2-NO2C6H4CH2OAc
PhC6H4CH2OAc
PhCH2CH2CH2OAc
n-Pentyl acetate
3
15
30
90
90
15
15
15
15
15
90
90
15
90
90
4
5
6
7
8
9
n-Heptanol
n-Heptyl acetate
n-Octyl acetate
10
11
12
13
14
15
16
n-Octanol
2-Ethylhexanol
Cyclohexanol
2-Ethylhexyl acetate
Cyclohexyl acetate
Menthyl acetate
Menthol
PhCH(OH)CH3
ꢀ-Tetralol
Phenyl-2-pyridylmethanol
PhCH(OAc)CH3
ꢀ-Tetralyl acetate
Phenyl-2-pyridylmethyl
acetate
17
18
1-Adamantanol
1-Adamantyl acetate
Triphenylmethyl acetate
180
180
92
72
8
8
0
0
Triphenylmethanol
a
In entries 1 and 3 alcohols reacted in acetylation with Ac2O at room temperature; b all products were
identified by comparison of their physical and spectral data with those of authentic samples; c isolated
yields

1260
V. Mirkhani et al.
zeolite HSZ-360 [11], and zirconium sulfophenyl phosphonate [12], these results
clearly indicate that (NH4)8[CeW10O36]ꢁ20H2O is an excellent catalyst for the
acetylation of tertiary alcohols.
We extended the scope of this process by performing the reaction in acetic acid.
The results, which are summarized in Table 1, show that (NH4)8[CeW10O36]ꢁ
20H2O could catalyze the acetylation of primary and secondary alcohols in re-
fluxing acetic acid with good yields. Tertiary alcohols, such as 1-adamantanol and
triphenylmethanol (entries 17 and 18), remained unchanged under this reaction
condition.
Formylation of Alcohols with Ethyl Formate Catalyzed
by (NH4)8[CeW10O36]ꢁ20H2O
The formylation of alcohols in refluxing ethyl formate was also investigated in the
presence of (NH4)8[CeW10O36]ꢁ20H2O. The experimental results show that pri-
mary and secondary alcohols are converted to their corresponding formates in the
presence of 0.04 mol% of the catalyst in good yields (Table 2), whereas under this
condition tertiary alcohols such as 1-adamantanol and triphenylmethanol (entries
17 and 18) remained unchanged in the reaction mixture. It is worth mentioning that
cerium polyoxometalate (NH4)8[CeW10O36]ꢁ20H2O can be used as a recyclable
Table 2. Formylation of alcohols with ethyl formate catalyzed by (NH4)8[CeW10O36]ꢁ20H2O at
reflux
Entry
Alcohol
Producta
Time=h
Yield=%b
1
2
PhCH2OH
PhCH2OCOH
5
7
3
9
5
5
5
5
6
6
6
5
7
5
5
5
60
58
95
85
50
73
86
85
84
83
70
44
35
47
11
40
4-ClC6H4CH2OH
4-CH3OC6H4CH2OH
3-CH3OC6H4CH2OH
2-NO2C6H4CH2OH
PhC6H4CH2OH
PhCH2CH2CH2OH
n-Pentanol
4-ClC6H4CH2OCOH
4-CH3OC6H4CH2OCOH
3-CH3OC6H4CH2OCOH
2-NO2C6H4CH2OCOH
PhC6H4CH2OCOH
PhCH2CH2CH2OCOH
n-Pentyl formate
3
4
5
6
7
8
9
n-Heptanol
n-Heptyl formate
10
11
12
13
14
15
16
n-Octanol
n-Octyl formate
2-Ethylhexanol
Cyclohexanol
2-Ethylhexyl formate
Cyclohexyl formate
Menthyl formate
Menthol
PhCH(OH)CH3
ꢀ-Tetralol
Phenyl-2-pyridylmethanol
PhCH(OCOH)CH3
ꢀ-Tetralyl formate
Phenyl-2-pyridylmethyl
formate
17
18
1-Adamantanol
1-Adamantyl formate
Triphenylmethyl formate
5
5
0
0
Triphenylmethanol
a
All products were identified by comparison of their physical and spectral data with those of authentic
b
samples; isolated yields

Cerium Polyoxometalate
1261
catalyst for the acetylation of alcohols without loss of catalytic activity (see experi-
mental section).
In conclusion, we have demonstrated that (NH4)8[CeW10O36]ꢁ20H2O is an
efficient, reusable, and heterogeneous catalyst for the acetylation and formylation
of alcohols. The advantage of this system is that even hindered substrates can be
acetylated with acetic anhydride in high yields. In addition, short reaction times,
simple work-up, stability, non-toxicity, and ease of preparation of the catalyst make
this method a useful addition to the present methodologies.
Experimental
All products were characterized by comparison of their spectral and physical data with those of
1
authentic samples. H NMR spectra was recorded in CHCl3 as solvent on a Bruker AM 80 MHz
spectrometer using TMS as an internal standard. Infrared spectra were run on a Philips PU9716 or
Shimadzu IR-435 spectrophotometer. All GC analyses were performed on a Shimadzu GC-16A
instrument with a flame ionization detector using silicon DC-200 or Carbowax 20-M columns. The
catalyst was characterized by UV-vis, IR spectroscopy, magnetic susceptibility, thermal gravimetric
analysis (TGA), and elemental analysis. Chemicals were purchased from Fluka and Merck chemical
companies.
Preparation of the Catalyst
Ammonium decatungestocerate icosahydrate was prepared according to Peacok [33]. Na2WO4ꢁ2H2O
(25 g, 75mmol) was dissolved in 100 cm3 of ion-exchanged H2O, the pH of which was adjusted to 7.2
by addition of acetic acid. After heating to 90ꢂC with stirring, an aqueous solution containing
(NH4)4Ce(SO4)2ꢁ2H2O (4.75 g, 7.5 mmol) and 1 equiv of H2SO4 were added to make CeW10 form
in the solution. This solution was obtained after filtration and used as catalyst. The structure of the
catalyst was confirmed by IR, UV-vis, magnetic susceptibility, thermal gravimetric analysis (TGA),
and elemental analysis.
General Procedure for Acetylation of Alcohols with Acetic Anhydride Catalyzed
by (NH4)8[CeW10O36]ꢁ20H2O
In a round-bottom flask (25 cm3) equipped with a magnetic stirrer, a solution of 1 mmol of alcohol in
2 cm3 of Ac2O was prepared. To this solution 0.04 mmol of (NH4)8[CeW10O36]ꢁ20H2O were added
and the reaction mixture was stirred at room temperature or under reflux conditions. The reaction
progress was monitored by GLC. After completion of the reaction, the suspension was filtered and
washed with 10cm3 of CH2Cl2. Evaporation of the solvent followed by chromatography on a short
column of silica-gel gave the pure product.
General Procedure for Acetylation of Alcohols with Acetic Acid Catalyzed
by (NH4)8[CeW10O36]ꢁ20H2O
To a solution of 1 mmol of alcohol in 3 cm3 of acetic acid were added 0.04 mmol of catalyst and the
reaction mixture was stirred under reflux. The progress of the reaction was monitored by GLC. Acetic
acid was removed under reduced pressure and 20 cm3 of ether were added. The reaction mixture was
washed with 5% NaHCO3 solution, then with H2O, and dried (MgSO4). Evaporation of the solvent
followed by silica-gel chromatography provided the pure acetate.

1262
V. Mirkhani et al.
General Procedure for Formylation of Alcohols with Ethyl Formate Catalyzed
by (NH4)8[CeW10O36]ꢁ20H2O
Alcohol (1mmol) and 0.04 mmol of catalyst were mixed together in 3 cm3 of ethyl formate. The
reaction mixture was stirred under reflux. The progress of the reaction was monitored by GLC. The
solution was evaporated under reduced pressure and 20cm3 of ether were added. The reaction mixture
was washed with H2O and dried (Na2SO4). Evaporation of the solvent followed by chromatography on
a short column of silica-gel gave the pure formate.
Catalyst Reuse and Stability
The stability of (NH4)8[CeW10O36]ꢁ20H2O was studied in repeated acetylation reactions. The acetyla-
tion of 1-heptanol with Ac2O was chosen as a model substrate for studying of catalyst’s reuse and
stability. The reaction was carried out as described above. At the end of each reaction, the catalyst was
removed by filtration, washed with H2O and acetone, and reused. After the use of catalyst for four
consecutive cycles, the yield of 1-heptyl acetate produced from the reaction of 1-heptanol with acetic
anhydride was 99% and after six times was 86%.
Acknowledgment
The authors wish to thank Isfahan University Research Council for financial support.
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