SbCl3 as a highly efficient catalyst for the acetylation of alcohols, phenols, and amines under solvent-free conditions

Article abstract of DOI:10.1080/00397910801928640

Antimony trichloride has been found to be an efficient and expedient catalyst for the acylation of alcohols, phenols, amines, and sugars with acetic anhydride in high yields and in a short reaction time under solvent-free conditions at room temperature. Also, racemization of chiral alcohols and epimerization of sugars were not observed in any of the substrates. Copyright Taylor & Francis Group, LLC.

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SbCl3 as a Highly Efficient Catalyst for the
Acetylation of Alcohols, Phenols, and Amines
under Solvent‐Free Conditions
Dr Asish K. Bhattacharya a , Mamadou A. Diallo a & Krishna N. Ganesh a
a Combi Chem‐Bio Resource Centre and Division of Organic Chemistry ,
National Chemical Laboratory , Pune, Maharashtra, India
Published online: 24 Apr 2008.
To cite this article: Dr Asish K. Bhattacharya , Mamadou A. Diallo & Krishna N. Ganesh (2008) SbCl3 as a
Highly Efficient Catalyst for the Acetylation of Alcohols, Phenols, and Amines under Solvent‐Free Conditions,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
38:10, 1518-1526, DOI: 10.1080/00397910801928640
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Synthetic Communicationsw, 38: 1518–1526, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910801928640
SbCl3 as a Highly Efficient Catalyst for the
Acetylation of Alcohols, Phenols, and
Amines under Solvent-Free Conditions
Asish K. Bhattacharya, Mamadou A. Diallo, and Krishna N.
Ganesh
Combi Chem-Bio Resorce Centre and Division of Organic Chemistry,
National Chemical Laboratory, Pune, India
Abstract: Antimony trichloride has been found to be an efficient and expedient catalyst
for the acylation of alcohols, phenols, amines, and sugars with acetic anhydride in high
yields and in a short reaction time under solvent-free conditions at room temperature.
Also, racemization of chiral alcohols and epimerization of sugars were not observed in
any of the substrates.
Keywords: Acetic anhydride, acetylation, antimony trichloride, solvent-free conditions
INTRODUCTION
Strategies concerning functional group protection are central theme to the target
molecule synthesis. The protection of alcohols, phenols, and amines as acetates
is one of the most useful and versatile transformations in organic, medicinal,
carbohydrate, and multistep synthesis of many natural products. In addition,
the protection of the hydroxyl group as acetate is preferred because of its ease
of introduction, stability under mild acidic reaction conditions, and ease of
removal under mild alkaline hydrolysis.[1] Acylation of alcohols, phenols, or
amines is generally carried out with acid anhydrides[2] or acetyl chloride[3] in
the presence of bases such as triethylamine or pyridine. Also,
Received in the U.S.A September 19, 2007
Address correspondence to Dr. Asish K. Bhattacharya, Combi Chem-Bio Resource
Centre and Division of Organic Chemistry, National Chemical Laboratory, Dr. Homi
Bhabha Road, Pune 411 008, Maharashtra, India. E-mail: ak.bhattacharya@ncl.res.in
1518

SbCl3: Catalyst for Acetylation
1519
4-(dimethylamino)pyridine (DMAP)[4a] and 4-pyrrolidino-pyridine (PPY)[4b]
are being used to catalyze the acetylation of alcohols. Further, sometimes tribu-
tylphosphine (Bu3P)[5] is also employed as a less basic catalyst for acylation
reactions, particularly for the base-sensitive substrates. The other catalysts
used for this transformation are Sc(OTf)3,[6a] Sc(NTf2)3,[6b] CoCl2,[6c]
[6d]
NBCl5
In(OTf)3,[6e] TMSOTf,[6f,g] TaCl5,[6h] Cu(OTf)2,[6i,j] bismuth(III)
salts,[6k] Ce(OTf)3,[6l] LiOTf,[7a] zeolites,[7b] clays,[7c,d] nafion-H,[7e] yttria-
zirconia,[7f] LiClO4,[7g] Mg(ClO4)2,[7h] Mg(NTf2)2,[7i] InCl3,[7j] ionic
[7o]
liquids,[7k,l] La(NO3)3 6H2O,[7m] I2,[7n] and Cu(BF4)2 6H2O.
.
.
However, most of the reported methodologies suffer from various disad-
vantages, such as highly toxic[8] and hazardous catalyst [e.g., the LD50 (intra-
venous in rat) value of DMAP is 56 mg/kg and Bu3P is highly flammable[9]
(flash point of 37 8C) and also undergoes aerial oxidation], longer reaction
times, stringent reaction conditions, and use of harmful organic solvents and
excess acylating agents. Triflates are expensive and moisture sensitive, and
special efforts are required to prepare the catalyst [e.g., Bi(OTf)3,[6k] nafion-
H,[7e] and yttria-zirconia[7f]]. Also, metal-triflate-catalyzed acylation
reactions require an excess of acylating agents and are often carried out at
low temperatures (28 to 260 8C).[7o] Therefore, there is a need to develop
a mild, efficient, and environmentally benign acylation reaction.
RESULTS AND DISCUSSION
In pursuit of our continued interest in the development of solvent-free and
environmentally friendly synthetic protocols,[10] herein we report an efficient
and convenient method for the acetylation of alcohols, phenols, and amines
with acetic anhydride, in good to excellent yields, in the presence of SbCl3
at room temperature under solvent-free conditions (Scheme 1). As a Lewis
acid, SbCl3 has been extensively used as a catalyst in organic synthesis,
including Friedel–Crafts acylation,[11a] diazotization of nucleoside derivati-
ves,[11b] catalytic fluorination of thiocarbonyl compounds,[11c] and imino
Diels–Alder reaction.[11d] Recently, we reported[10c] SbCl3-catalyzed
synthesis of acylals from aldehydes under solvent-free conditions.
A wide variety of structurally diverse substrates were subjected to acety-
lation to establish the generality of the present reaction and to examine the
tolerance of the other functional groups present in the substrates (Table 1). The
acetylated products were characterized by spectral data (IR, NMR, and mass).
In almost all cases, the reactions are faster than those earlier reported methods,
Scheme 1. SbCl3-catalyzed acetylation of alcohols, phenols, and amines.

1520
A. K. Bhattacharya, M. A. Diallo, and K. N. Ganesh
Table 1. SbCl3-catalyzed acetylation of phenols, alcohols, amines, and sugarsa
Entry
1a
Substrates
Products
Yield (%)b Time (min)
96
6
1b
2
95
7
95c
15
3
4
CH3-(CH2)10-CH2OH CH3-(CH2)10-CH2OAc
98
3
6
98c
5
6
94d
95
15
4
7
8
96
94
3
6
9
90
99
10
25
10a
10b
96
30
11
12
90
99
18
15
(continued)

SbCl3: Catalyst for Acetylation
1521
Table 1. Continued
Entry
13
Substrates
Products
Yield (%)b Time (min)
95
99
10
10
14
15
16
93
83
45
50
17
18
99d
86e
30
105
19
20
92
60
95c
120
21
22
79c
99c
70
100
23
92d
50
aThe substrate was treated with Ac2O (2 mmol) in the presence of SbCl3 (10 mol%).
bIsolated yield of the corresponding acetylated product.
cAc2O (6 mmol) was used.
dAc2O (4 mmol) was used.
eAc2O (9 mmol) was used.

1522
A. K. Bhattacharya, M. A. Diallo, and K. N. Ganesh
and the yields are quite high. Also, noracemizationor epimerization to any extent
were observed in the acetylated products ofsubstrates having stereogenic centers.
To increase the scope of the present method, a large-scale acylation of menthol
and phenol was carried out (Table 1, entries 1b and 10b). The reaction
proceeded smoothly, and the acetylated products were obtained in 97 and 96%,
respectively.
`
The wide applicability and scope of the present methodology vis-a-vis
earlier reported methods is delineated in Table 2. The comparison of the
Table 2. Comparison of SbCl3 with other catalysts for the acetylation of somesubstrates
Time
Yield (%) Condition (min)
Substrate
Catalyst (%)a
Cu(BF4)2 Â H2O (1 mol)[7o]
BiCl3 (10 mmol)[6k]
Bi(TFA)3 (5 mmol)[6k]
I2 (10 mmol)[7n]
95
94
93
95
94
98
rt
reflux
rt
30
20
25
8
rt
[7m]
.
La(NO3)3 6H2O (10 mol)
rt
15
6
SbCl3 (10 mmol)
Cu(BF4)2 Â H2O (1 mol)[7o]
rt
97
97
90
99
rt
30
50
BiCl3 (10 mmol)[6k]
reflux
reflux
rt
Bi(TFA)3 (5 mmol)[6k]
SbCl3 (10 mmol)
120
25
Cu(BF4)2 Â H2O (1 mol)[7o]
96
98
96
93
98
rt
rt
rt
rt
rt
60
35
60
18
18
BiCl3 (10 mmol)[6k]
Bi(TFA)3 (5 mmol)[6k]
.
La(NO3)3 6H2O (10 mol)
[7m]
SbCl3 (10 mmol)
Cu(BF4)2 Â H2O (1 mol)[7o]
93
98
98
99
rt
rt
rt
rt
30
60
60
15
BiCl3 (10 mmol)[6k]
Bi(TFA)3 (5 mmol)[6k]
SbCl3 (10 mmol)
Cu(BF4)2 Â H2O (1 mol)[7o]
95
95
85
92
99
rt
15
90
BiCl3 (10 mmol)[6k]
reflux
reflux
80 8C
rt
Bi(TFA)3 (5 mmol)[6k]
Mg(NTf2)2 (1 mol)[7i]
SbCl3 (10 mmol)
120
30
30
Cu(BF4)2 Â H2O (1 mol)[7o]
BiCl3 (10 mmol)[6k]
Bi(TFA)3 (5 mmol)[6k]
I2 (10 mmol)[7n]
94
97
94
90
rt
30
35
reflux
reflux
rt
120
30
[7m]
.
La(NO3)3 6H2O (10 mol)
96
90
99
rt
rt
rt
15
15
12
Mg(NTf2)2 (1 mol)[7i]
SbCl3 (10 mmol)
aReferences for the earlier reported methods.

SbCl3: Catalyst for Acetylation
1523
data has been carried out with respect to yield, reaction time, and catalyst load
in some chosen model substrates. It is pertinent to mention here that menthol,
phenol, benzyl alcohol, 2-naphthol, 2-phenyl ethanol, and 4-methoxyphenol
(Table 2) furnished the respective acetylated products in higher yields and
shorter reaction times.
CONCLUSION
In summary, antimony trichloride is a cost-effective and highly efficient Lewis
acid catalyst for the acetylation of alcohols, phenols, and amines. The notable
features of this procedure are mild reaction conditions, cleaner reaction
profiles, improved yields, enhanced reaction rates, retention of chirality, and
simplicity of operation, which makes it a useful process for the acetylation
of alcohols, phenols, and amines. The highly catalytic nature of antimony tri-
chloride and its wide applicability should make this protocol an attractive
alternative over existing methods.
EXPERIMENTAL
General Procedure for Acylation
A mixture of the alcohol or phenol (1 mmol), acetic anhydride (2 mmol), and
SbCl3 (10 mol%) was stirred at room temperature for the length of time
indicated in Table 1. After completion of the reaction (thin-layer chromato-
graphy, TLC), the reaction mixture was diluted with water (30 mL) and
extracted with dichloromethane (3 Â 30 mL). The combined dichloromethane
extracts were washed successively with 10% sodium bicarbonate and H2O,
dried (Na2SO4), and concentrated under reduced pressure. In all the cases,
the product obtained after the usual workup gave satisfactory spectral data
without any further purification. All the products were characterized by com-
parison of their spectral data and physical properties with those of authentic
samples or literature data.
ACKNOWLEDGMENT
A. K. B. is grateful to Dr. S. Sivaram, director, NCL, Pune, India, for financial
support (MLP008626) and to Dr. Ganesh Pandey, head, Division of Organic
Chemistry, and Dr. M. K. Gurjar, former head, Division of Organic
Chemistry, for their constant encouragement and support. M. A. D. is
grateful to Department of Biotechnology, Government of India-Third World
Academy of Sciences (DBT-TWAS), Italy for the award of a DBT-TWAS
Postgraduate Fellowship.

1524
A. K. Bhattacharya, M. A. Diallo, and K. N. Ganesh
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