Tetrahedron Letters 50 (2009) 5210–5214
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Silica-bonded N-propyl sulfamic acid as an efficient catalyst
for the formylation and acetylation of alcohols and amines under
heterogeneous conditions
Khodabakhsh Niknam *, Dariush Saberi
Department of Chemistry, Faculty of Sciences, Persian Gulf University, Bushehr 75169, Iran
a r t i c l e
i n f o
Article history:
Received 27 April 2009
Revised 23 June 2009
Accepted 29 June 2009
Available online 22 July 2009
Keywords:
Formylation
Silica-bonded N-propyl sulfamic acid
Ethyl formate
Alcohols
Acetylation
Amines
Acetic anhydride
a b s t r a c t
A simple and efficient procedure for the preparation of silica-bonded N-propyl sulfamic acid (SBNPSA) by
the reaction of 3-aminopropylsilica (1) and chlorosulfonic acid in chloroform is described. This solid acid
is employed as a new catalyst for the formylation of alcohols and amines with ethyl formate under mild
and heterogeneous conditions at room temperature with good to excellent yields. Also, SBNPSA catalyzed
acetylation of various alcohols and amines with acetic anhydride at room temperature.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
In recent years, the search for environmentally benign chemical
processes or methodologies has received much attention.1 Heterogenization of homogeneous catalysts has been an interesting area
of research from an industrial point of view; this combines the
advantages of homogeneous catalysts (high activity, selectivity,
etc.) with the engineering advantages of heterogeneous catalysts
(easy catalyst separation, long catalytic life, easy catalyst regeneration, thermal stability, and recyclability).2 Application of solid
acids in organic transformations is important, because, solid acids
have many advantages such as simplicity in handling, decreased
reactor and plant corrosion problems, and more environmentally
safe disposal.3–7
Protection of functional groups plays an important role in the
synthesis of complex organic molecules such as natural products.
Formylation and acetylation have found increasing attention as
highly familiar protocols for the protection of functional groups
in this context.
O-Formylation could be the method of choice for protecting an
alcoholic group in a complex synthetic sequence because deformy-
* Corresponding author. Tel.: +98 771 4541494; fax: +98 771 4545188.
E-mail addresses: niknam@pgu.ac.ir, khniknam@gmail.com (K. Niknam).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.06.140
lation can be affected selectively in the presence of acetate or other
ester protecting groups. Further, if the alcoholic group is planned
to be oxidized later in the synthetic scheme, the formylated alcoholic group need not be deprotected and direct oxidation under
Oppenauer conditions can be realized.8–11 Although various formylating agents have been reported previously, there are still serious
limitations for the preparation of formates due to the drastic reaction conditions, the use of uncommon reagents, formation of undesirable or toxic by-products, the application of expensive catalysts
for preparation of formylating agents, and thermal instability of
the reagents.12 To the best of our knowledge, one of the most common formylating agents is formic acid which is corrosive and toxic.
Meanwhile, acid-sensitive functional groups may be affected and
side reactions may be increased in the formylation reaction by
using formic acid. Among formylating agents ethyl formate offers
several advantages such as easy work-up, availability of the reagent, and relatively low cost. Some of the recently reported methods include formylation with ethyl formate in the presence of
different reagents or catalysts, namely metal triflates such as
Ce(OTf)4 ,13 In(OTf)3,14 Bi(III) salts,15 heteropoly acids,16 silphos
[PCl3 n(SiO2)n],10 silica sulfuric acid and Al(HSO4)3,17 cerium polyoxometalate,18 silica triflate,19 and TiCl3(OTf).20
Acetylation of alcohols, phenols, and thiols is usually carried out by
using acid anhydrides or acyl chlorides in the presence of protic
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K. Niknam, D. Saberi / Tetrahedron Letters 50 (2009) 5210–5214
acids,21 various Lewis acid catalysts, or basic reagents such as
4-(dimethylamino)pyridine DMAP,22,23 tributylphosphine,24 and
La(NO3)36H2O.25 Performing the reaction in the presence of acidic
catalysts has become more popular and some of the recently reported
methods include applying metal triflates such as cerium triflate,26
gadolinium triflate,27 aluminum triflate,28 scandium triflate,29,30 and
Sc(Tf2)3,31 metal halides such as TaCl5,32 InCl3,33 and ZrOCl28H2O,34
and solid acid catalysts such as heteropoly acids,35 Mg(NTf2)2,36 aluminum-supported MoO3,37 cation-exchanged montmorillonite K-10
clay,38 sulfamic acid,39 NaHSO4SiO2,40 phosphomolybdic acid,41
polymer-supported gadolinium triflate,42 polystyrene-bound tetrafluorophenylbis(triflyl)-methane,43 BiFeO3,44 heterogeneous cobalt(II) salen,45 and [TMBSA][HSO4] ionic liquid.46
2. Results and discussion
In the continuation of our studies on the applications of solid
acid catalysts in chemical transformations,47–50 herein, we wish
to describe the preparation of new silica-bonded N-propyl sulfamic
acid (SBNPSA) as illustrated in Scheme 1, and its use as a catalyst
for the formylation and acetylation of alcohols. Elemental analysis
showed the S content to be 9.29%. Typically a loading of ca.
0.35 mmol/g was obtained. When SBNPSA was placed in aqueous
NaCl solution, the pH of the solution dropped almost instantaneously to 1.87, as ion exchange occurred between the protons
and sodium ions (proton exchange capacity: 0.34 mmol/g of
SBNPSA).
2.1. Formylation
As we mentioned in the introduction, owing to the easy deprotection of the formyl group, costs, and the work-up, formylation is
regarded as an attractive option for the protection of different
functional groups. Therefore, we investigated the formylation of
alcohols with ethyl formate in the presence of SBNPSA as a catalyst. First, the transesterification reaction of benzyl alcohol
(1 mmol) with ethyl formate (3 mL) was chosen as a model reaction. As shown in Table 1, in the absence of catalyst benzyl alcohol
remains intact even after 24 h. However, in the presence of sulfamic acid (1 mmol), benzyl formate was afforded in only 45% yield
after 6 h at room temperature (Table 1, entry 2). Then, we examined this reaction in the presence of different catalyst loadings of
SBNPSA at room temperature (Table 1). The optimal amount of
SBNPSA was 0.1 g (3.4 mol %) per 1 mmol of alcohols and 3 mL of
ethyl formate at room temperature.
The optimized conditions (0.1 g of SBNPSA, and 3 mL of ethyl
formate) were then used in the formylation of various alcohols under mild, nearly neutral and heterogeneous conditions (Table 2 and
Scheme 2).
As shown in Table 2, primary alcohols reacted with ethyl
formate faster than secondary and tertiary alcohols in the presence
Table 1
Influence of the amounts of SBNPSA on the formylation of benzyl alcohol with ethyl
formate at room temperaturea
O
O
OH
+ H
SBNPSA
SiO 2
(MeO) 3Si
NH2
OH
Toluene, reflux, 18 h
SiO2
OH
OH
O
Entry
Catalyst
The amount of catalyst (g)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
None
Sulfamic acid
SBNPSA
SBNPSA
SBNPSA
SBNPSA
SBNPSA
None
1 mmol
0.01
0.015
0.05
0.1
0.15
24 h
360
360
240
100
45
45
0
45
32
51
72
91
92
a
The reaction conditions: alcohol (1 mmol) and ethyl formate (3 mL) at room
temperature and under neat conditions.
b
Isolated yield.
Table 2
Formylation of alcohols and amines with ethyl formate in the presence of SBNPSA at
room temperaturea
Entry
Substrate
Time (min)
Yieldsb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Benzyl alcohol
4-Chlorobenzyl alcohol
2-Chlorobenzyl alcohol
4-Bromobenzyl alcohol
4-Methoxybenzyl alcohol
4-t-Butylbenzyl alcohol
4-Nitrobenzyl alcohol
n-Octanol
2-Phenylethanol
3-Phenylpropanol
1-Phenylethanol
Indanol
Cyclohexanol
1-Phenyl-2-methyl-2-propanol
Phenol
p-Cresol
Aniline
Aniline
4-Nitroaniline
4-Methylaniline
4-Bromoaniline
Benzylamine
4-Methoxybenzylamine
45, 45,c 50,c 50c
80
90
80
30
40
120
70
40
45
40, 40,c 50,c 120c
70
120
24 h
180
180
40, 40,c 45,c 45c
24 he
8 h (24 h)e
40 (24 h)e
45 (24 h)e
5 (180)e
5 (180)e
91, 90,c 88,c 89c
93
89
90
94
92
35
80
91
89
91, 90,c 90,c 88c
87
78
30
—d
—d
95, 93,c 95,c 90c
80e
40 (—)d,e
85 (80)e
85 (77)e
90 (89)e
90 (88)e
a
The reaction conditions: substrate (1 mmol), ethyl formate (3 mL), and catalyst
0.1 g (3.4 mol %) at room temperature and under neat conditions.
b
Isolated yield.
c
The reaction was accomplished with reused catalyst.
d
No reaction.
e
The reaction was accomplished without catalyst.
R
NH2
Si
O
1
X
H
+ H
O
SBNPSA
R
OEt
X
H
Solvent-free, rt
X = O, NH
R= primary, secondary, and tertiary alkyl
Scheme 2. Conversion of alcohols and amines into corresponding formate esters
and formamides with ethyl formate in the presence of SBNPSA.
O
SiO2
O
H
Solvent-free/ rt
O
O
O
OEt
1) ClSO3H/ CHCl3 /4 h
Si
O
NH-SO3 H
2) Washing with EtOH
2
Scheme 1. Preparation of silica-bonded N-propyl sulfamic acid (2).
of SBNPSA at room temperature. Electron-donating substituents
on the aromatic ring of benzyl alcohols, such as para-methoxybenzyl alcohol, reacted faster than benzyl alcohol (Table 2, entry 5),
while formylation of para-nitrobenzyl alcohol was sluggish and
the corresponding ester was isolated in only 35% yield after
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K. Niknam, D. Saberi / Tetrahedron Letters 50 (2009) 5210–5214
120 min at room temperature (Table 2, entry 7). Our investigation
clarified that benzyl amines and aniline derivatives reacted with
ethyl formate in short reaction times and very good to excellent
yields at room temperature. However, anilines with electron-withdrawing groups such as NO2 required longer reaction times and
were low yielding (Table 2, entry 18). It is important to note that
amines reacted with ethyl formate without any catalyst in long reaction times and only near 90% conversion (Table 2, entries 18–23).
The described catalytic formylating systems are chemoselective
so that formylation of alcohols occurred selectively in the presence
of phenols. In addition, the selective formylation of primary or
secondary alcohols in the presence of tertiary alcohols was investigated. This method was shown to be highly selective for the primary alcohols such as benzyl alcohol and 2-phenylethanol. The
primary alcohols were completely converted to the corresponding
formate ester, while tertiary alcohols were left untouched (Scheme
3). Excellent chemoselectivity was also observed for the conversion
of secondary alcohols in the presence of tertiary alcohols such as
1-phenyl-2-methyl-2-propanol. No selectivity was observed between primary and secondary alcohols such as 2-phenylethanol
and 1-phenylethanol (Scheme 3).
The possibility of recycling the catalyst was examined using the
reaction of benzyl alcohol, 1-phenylethanol, and aniline with ethyl
formate under optimized conditions. Upon completion, the reaction mixture was filtered and the solid was washed with diethyl
ether, and the recycled catalyst was saved for next reaction. The
recycled catalyst could be reused thrice without any treatment.
No observation of any appreciable loss in the catalytic activity of
SBNPSA was observed (Table 2, entries 1, 11, and 17).
2.2. Acetylation
Acetylation of alcohols is usually achieved with acetic anhydride. Acetyl is the most common protecting group in view of being
stable under acidic conditions, and being easily removable by mild
alkaline hydrolysis. The poor nucleophilic properties of hydroxyl
compounds necessitate activation of the anhydride. Silica-bonded
N-propyl sulfamic acid has proved to be efficient catalyst for this
purpose at room temperature. The optimal amount of the SBNPSA
was 0.1 g (3.4 mol %) per 1 mmol of benzyl alcohol and 1 mL of acetic anhydride. As illustrated in Table 3, a range of benzylic, aliphatic primary, secondary, and tertiary alcohols were acetylated
with acetic anhydride in the presence of SBNPSA in short reaction
times at room temperature (Scheme 4).
Acetylation of ortho- and para-nitrobenzyl alcohols was fast and
the corresponding esters were isolated in high yields (89% and 91%
yields) after 60 min, respectively (Table 3, entries 6 and 7). Phenols
were also, acetylated in short reaction times with acetic anhydride
in the presence of SBNPSA (Table 3, entries 14–17). It is important
to note that according to the previous report39 most of these acylation reactions proceeded at 62–80 °C in the presence of sulfamic
acid. Amines were acetylated with acetic anhydride in the presence
of SBNPSA or without any catalyst in short reaction times in excellent yields (Table 3, entries 18–24).
OCHO
OH
100%
SBNPSA (0.1 g)
1
ethyl formate (3 mL)
rt, 40 min
OH
0%
OCHO
OCHO
OH
2
100%
SBNPSA (0.1 g)
ethyl formate (3 mL)
rt, 45 min
OH
0%
OCHO
OCHO
OH
100%
SBNPSA (0.1 g)
3
ethyl formate (3 mL)
rt, 40 min
OH
0%
OCHO
OCHO
100%
OH
SBNPSA (0.1 g)
4
OH
ethyl formate (3 mL)
rt, 70 min
OCHO
100%
Scheme 3. Reaction conditions: each substrate (1 mmol), ethyl formate 3 mL, and catalyst SBNPSA (3.4 mol %) at room temperature (the yields referred to GC yield).
5213
K. Niknam, D. Saberi / Tetrahedron Letters 50 (2009) 5210–5214
Table 3
Acetylation of alcohols, phenols, and amines in the presence of SBNPSA with acetic anhydride at room temperaturea
Entry
Substrate
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
a
b
c
d
Yieldsb (%)
Time (min)
c
c
c
c
c
c
20, 20, 20, 25, 25, 28, 30, 30
30
20
25
30
60
60
25
30, 30,c 30,c 30,c 45c
30
45
20
260
360
340
350
350
10, 10,c 10,c 10,c 10,c
5d
20 (15)d
10 (5)d
10 (5)d
15 (5)d
15 (5)d
Benzyl alcohol
4-Chlorobenzyl alcohol
4-Methoxybenzyl alcohol
4-t-Butylbenzyl alcohol
3-Phenoxybenzyl alcohol
4-Nitrobenzyl alcohol
2-Nitrobenzyl alcohol
2-Phenylethanol
1-Phenylethanol
Indanol
Cyclohexanol
Benzhydrol
Triphenylcarbinol
Phenol
p-Cresol
4-Bromophenol
2-Naphthol
Aniline
Aniline
4-Nitroaniline
4-Methylaniline
4-Bromoaniline
Benzylamine
4-Methoxybenzylamine
c
98, 94,c 96,c 95,c 91,c 92,c 90,c 90c
93
93
91
90
91
89
90
90, 90,c 90,c 88,c 88c
93
86
91
90
92
90
91
90
95, 93,c 92,c 93,c 91c
93d
90 (91)d
95 (93)d
91 (89)d
93 (90)d
88 (89)d
The reaction conditions: substrate (1 mmol), acetic anhydride (1 mL), and catalyst 0.1 g (3.4 mol%) at room temperature.
Isolated yield.
The reaction was accomplished with reused catalyst.
The reaction was accomplished without catalyst.
O
R
X
H + H3 C
O
O
SBNPSA
CH3
O
R
Solvent-free, rt
X
CH3
3-Aminopropylsilica 1 (AMPS) was prepared according to previously reported procedure.3
4.2. Catalyst preparation
X = O, NH
R= primary, secondary, and tertiary alkyl
Scheme 4. Acetylation of alcohols, phenols, and amines with acetic anhydride in
the presence of SBNPSA.
The recycling potential of the catalyst was investigated in the
reaction of benzyl alcohol, 1-phenylethanol, and aniline with acetic
anhydride at room temperature in the presence of 0.1 g of SBNPSA.
After completion of the reaction, diethyl ether was added and the
mixture was filtered to separate the catalyst. The recycled catalyst
was used in further runs. No decrease in catalytic activity of
SBNPSA was observed even after five runs (Table 3, entries 1, 9,
and 18).
4.2.1. Silica-bonded N-propyl sulfamic acid 2
To a mixture of 3-aminopropylsilica 1 (5 g) in chloroform
(20 mL) chlorosulfonic acid (1 g, 0.6 mL) was added dropwise at
0 °C over 2 h. After addition was complete, the mixture was stirred
for 2 h until HCl gas evolution stopped. Then, the mixture was filtered and washed with ethanol (30 mL) and dried at room temperature to obtain silica-bonded N-propyl sulfamic acid (2) as a cream
powder (5.13 g). Sulfur content of the samples determined by conventional elemental analysis was 9.29%.
4.3. General procedure for formylation of alcohols with ethyl
formate
In conclusion, chemoselectivity, the cheapness and availability
of the reagents, nearly neutral and heterogeneous conditions, easy
and clean work-up, and high yields make this method practical for
multi-step synthesis. We believe that the present methodology
could be an important addition to the existing methodologies.
Solid acid SBNPSA (0.1 g) was added to ethyl formate (3 mL)
and the mixture was stirred for several minutes. Then the substrate
(1 mmol) was added to the above-mentioned mixture and the suspension was stirred at room temperature for the specified time given in Table 2. The progress of the reaction was monitored by TLC
or GC. Upon completion of the reaction, the suspension was filtered
off and the solid was washed with diethyl ether (2 10 mL). The
organic phases were combined and passed through a short column
of silica gel to give the pure products.
4. Experimental
4.4. General procedure for acetylation with acetic anhydride
4.1. General
To a mixture of substrate (1 mmol) and acetic anhydride (1 mL),
was added SBNPSA (0.1 g) and the mixture was stirred at room
temperature for the specified time given in Table 3. The progress
of the reaction was monitored by TLC or GC. When the reaction
was complete, 15 mL of diethyl ether was added and the mixture
was filtered, the solid was washed with diethyl ether
(2 10 mL). The combined organic phases were washed with
3. Conclusion
Chemicals were purchased from Fluka, Merck, and Aldrich
Chemical Companies. The yields of all volatile formylated products
have been determined by GC. The formylated products were characterized by comparison of their spectral and physical data with
previously reported data or with the authentic samples.16–42
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K. Niknam, D. Saberi / Tetrahedron Letters 50 (2009) 5210–5214
10% solution of sodium hydrogen carbonate, and twice with water
(2 10 mL). The organic layer was dried over Na2SO4, filtered, and
concentrated to afford the crude product. If further purification
was needed it was passed through a short column of silica gel.
Acknowledgment
The research council of Persian Gulf University, Bushehr, Iran, is
gratefully acknowledged for the financial support provided for this
work.
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