Rapid and efficient acetylation of alcohols and phenols with acetic anhydride catalyzed by electron-deficient tin(IV) porphyrin

Article abstract of DOI:10.1016/j.molcata.2004.05.004

Rapid and efficient esterification of alcohols and phenols with acetic anhydride was achieved in the presence of tin(IV) tetraphenylporphyrinato trifluoromethanesulfonate, Sn^IV(tpp)(OTf)₂, as a catalyst. The remarkably high catalytic activity of Sn^IV(tpp)(OTf)₂ can be used to assist the acetylation of not only primary alcohols but also sterically-hindered secondary and tertiary alcohols with acetic anhydride. This catalyst can also catalyze the acetylation of phenols with acetic anhydride.

Full text of DOI:10.1016/j.molcata.2004.05.004

Journal of Molecular Catalysis A: Chemical 219 (2004) 73–78
Rapid and efficient acetylation of alcohols and phenols with acetic
anhydride catalyzed by electron-deficient tin(IV) porphyrin
Majid Moghadama,∗, Shahram Tangestaninejadb, Valiollah Mirkhanib,
Iraj Mohammadpour-Baltorkb, Reza Shaibania
a
Chemistry Department, Yasouj University, Yasouj 75914-353, Iran
b
Chemistry Department, Isfahan University, Isfahan 81746-73441, Iran
Received 14 December 2003; received in revised form 26 April 2004; accepted 6 May 2004
Available online 5 June 2004
Abstract
Rapid and efficient esterification of alcohols and phenols with acetic anhydride was achieved in the presence of tin(IV) tetraphenylpor-
phyrinato trifluoromethanesulfonate, SnIV(tpp)(OTf)2, as a catalyst. The remarkably high catalytic activity of SnIV(tpp)(OTf)2 can be used
to assist the acetylation of not only primary alcohols but also sterically-hindered secondary and tertiary alcohols with acetic anhydride. This
catalyst can also catalyze the acetylation of phenols with acetic anhydride.
© 2004 Elsevier B.V. All rights reserved.
1. Introduction
[In(OTf)3] [17], magnesium bromide [18], bismuth(III)
salts [19], ferric perchlorate adsorbed on silica-gel [20] and
tin(IV) tetraphenylporphyrin perchlorate [21]. However,
some of the reported methods for the acetylation of alcohols
suffer from one or more of the following disadvantages such
as high temperature and drastic reaction conditions, forma-
tion of undesirable or toxic byproducts, expensive reagents,
hygroscopicity and thermal instability of the reagents, long
reaction times, low yields of the desire products and bulk
requirements of solid bed. Therefore, introduction of new
methods and catalysts for the preparation of esters is still
in demand.
The successful use of metalloporphyrins as mild Lewis
acid catalysts [21,22] prompted us to explore the poten-
tial of these complexes for catalysts for the conversion of
alcohols and phenols to their corresponding esters. Here,
we wish to report a simple, rapid and efficient method for
the acetylation of alcohols and phenols with acetic anhy-
dride catalyzed by tin(IV) tetraphenylporphyrinato trifluo-
romethanesulfonate, SnIV(tpp)(OTf)2, at room temperature
(Scheme 1).
Developing efficient and mild methods for the pro-
tection of hydroxyl group of alcohols and phenols is of
great 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 syn-
thetic sequence, especially in the construction of poly-
functional molecules such as nucleosides, carbohydrates,
steroids and natural products. A number of procedures
are available for the preparation of acetyl derivatives, in-
cluding homogeneous and heterogeneous reagents such as
4-(dimethylamino)pyridine and 4-pyrolidinopyridine [2],
N,N,Nꢀ,Nꢀ-tetramethylethylenediamine [3], tributylphos-
phine [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], scandium trifluoromethanesulfonate
[Sc(OTf)3] [13], TaCl5 [14], trimethylsilyl trifluoromethane-
sulfonate (TMSOTf) [15], copper trifluoromethansul-
fonate [Cu(OTf)2] [16], indium trifluoromethanesulfonate
∗
Corresponding author. Tel.: +98 913 3108960;
fax: +98 741 2223048.
E-mail address: moghadamm@mail.yu.ac.ir (M. Moghadam).
Scheme 1.
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.05.004

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M. Moghadam et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 73–78
Table 1
Acetylation of alcohols with Ac2O catalyzed by SnIV(tpp)(OTf)2 at room temperature
Entry
Substrate
Product
Time (min)
1
Yield (%)a
95
1
2
3
1
1
94
95
4
5
1
1
93
92
6
7
1
1
93
95
8
9
1
1
1
92
90
89
10
11
1
92
13
14
1
1
91
90
15
5
95
16
17
18
1
1
1
88
89
87
19
1
90

M. Moghadam et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 73–78
75
Table1 (Continued )
Entry
Substrate
Product
Time (min)
Yield (%)a
93
20
1
21
22
23
1
1
1
90
90
88
24
25
1
90
95
15
a
Isolated yields.
Table 2
Acetylation of phenols with Ac2O catalyzed by SnIV(tpp)(OTf)2 at room temperature
Entry
Substrate
Product
Time (min)
1
Yield (%)a
94
1
2
3
1
1
92
91
4
1
93
5
6
1
1
94
90
7
1
91

76
M. Moghadam et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 73–78
Table 2 (Continued )
Entry
Substrate
Product
Time (min)
Yield (%)a
93
8
9
1
1
90
10
11
1
1
93
91
12
1
95
13
14
1
1
93
91
15
15
85
a
Isolated yields.
Table 3
Competitive acetylation of alcohols, acetals and silyl ethers catalysed by SnIV(tpp)(OTf)2
Run
Substrate
Product
Time (min)
Yield (%)a
99
1
1
1
0
2
3
1
1
99
0
1
1
99
0

M. Moghadam et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 73–78
77
Table 3 (Continued )
Run
Substrate
Product
Time (min)
1
Yield (%)a
99
4
1
0
a
GC yields.
2. Results and discussions
etals and silyl ethers (Table 3). This may be considered as
a useful practical achievement in esterification reactions.
2.1. Acetylation of alcohols with acetic anhydride
catalyzed by SnIV (tpp)(OTf)2
3. Conclusion
Initially, benzyl alcohol (1 mmol) was chosen as a model
substrate for the acylation reaction. The reaction of this
alcohol with acetic anhydride (2 equivalent) in acetoni-
trile (2 mL) in the presence of 0.01 molar equivalent of
SnIV(tpp)(OTf)2 at room temperature was completed in
1 min and the corresponding acetate was obtained in 99%
yield. In the absence of SnIV(tpp)(CF3SO3)2, the reaction
was much less efficient.
As shown in Table 1, a series of primary alcohols (includ-
ing aliphatic and aromatic alcohols) and secondary alcohols
were acetylated with acetic anhydride in the presence of 0.01
molar equivalent of SnIV(tpp)(CF3SO3)2 at room tempera-
ture in 99% yields. Tertiary alcohols (such as 1-adamantanol,
t-butanol, and triphenylmethanol) were also acetylated in
high yields, without the formation of dehydration products
(entries 23–25).
Although metalloporphyrins are widely used as redox cat-
alysts, there have been few studies on their catalytic ac-
tivity as Lewis acids. In this report, we have demonstrated
that the tin(IV) tetraphenylporphyrinato trifluoromethane-
sulfonate, SnIV(tpp)(OTf)2, which is a stable Sn(IV) com-
pound, can be considered as a mild Lewis acid for efficient
and catalytic acetylation of alcohols and phenols. The ad-
vantage of this system is that even hindered substrates can
be acetylated with acetic anhydride in high yields at room
temperature. In addition, low reaction times, non-toxicity,
ease of preparation of the catalyst make this method a useful
addition to the methodologies.
4. Experimental
Chemicals were purchased from Fluka and Merck chem-
ical companies. H NMR spectra was recorded in CHCl3
2.2. Acetylation of phenols with acetic anhydride catalyzed
by SnIV (tpp)(OTf)2
1
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 analy-
ses were performed on a Shimadzu GC-16A instrument with
a flame ionization detector using silicon DC-200 or Car-
bowax 20-M columns. Tetraphenylporphyrin was prepared
and metallated according to the literature [23,24].
We extended the scope of this transformation by perform-
ing the reaction of phenols with acetic anhydride (2 equiva-
lent for each hydroxyl group) in acetonitrile (2 mL). At room
temperature and in the presence of 0.01 molar equivalent
of the catalyst, phenols were converted to their correspond-
ing acetates in excellent yields (Table 2). The acetylation of
polyhydroxy phenols was also investigated and the corre-
sponding polyacetates were obtained in 99% yields (Table 2,
entries 5–8).
4.1. Preparation of tin(IV) tetraphenyporphyrinato
trifluoromethanesulfonate, SnIV (tpp)(OTf)2
In comparison with the data reported with SnIV(tpp)
(ClO4)2 catalyst [21], these results clearly indicate that
SnIV(tpp)(CF3SO3)2 is a more powerful catalyst for the
acetylation of alcohols and phenols.
To a solution of Sn(tpp)Cl2 (1.03 g, 1 mmol) in 100 mL
of THF, at 55 ◦C, AgCF3SO3 (0.54 g, 2 mmol) was added.
The solution was stirred at 55 ◦C for 30 min. The AgCl pre-
cipitate was filtered through a 0.45 ␮M filter. The resulting
solution was evaporated moderately. SnIV(tpp)(OTf)2 was
then extracted with CH2Cl2. The SnIV(tpp)(OTf)2 crystals
was obtained by evaporation of solvent at room temperature.
In order to check the selectivity of the described method,
we have also investigated the competitive acetylation of al-
cohols with acetals and silyl ethers. The results showed that
alcohols were acetylated selectively in the presence of ac-

78
M. Moghadam et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 73–78
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4.2. General procedure for acetylation reactions
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In a round-bottom flask (25 mL) equipped with a mag-
netic stirrer, a solution of alcohol or phenol (1 mmol) in
Ac2O (2 equivalent for each OH group of alcohol or phe-
nol) and CH3CN (2 mL) was prepared. SnIV(tpp)(OTf)2
(0.010 g, 0.01 mmol) was added to this solution and the re-
action mixture was stirred at room temperature. The reaction
was monitored by GLC. After completion of the reaction,
the mixture was directly passed through a short column
of silica-gel (hexane:ethyl acetate = 1:1) to remove the
catalyst. The elute was evaporated under reduced pressure
and the remaining residue was purified by silica-gel plate
chromatography (eluted with CCl4:Et2O = 9:1) to afford
the corresponding ester without any elimination products.
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Acknowledgements
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The partial support of this work by Yasouj University
Research Council is gratefully acknowledged.
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