Polyvinylpolypyrrolidoniume tribromide as new and metal-free catalyst for the formylation and trimethylsilylation of hydroxyl group

Article abstract of DOI:10.1016/j.cclet.2011.05.020

Trimethylsilylation of alcohols was achieved using 1,1,1,3,3,3- hexamethyldisilazane (HMDS) as silylating agent, in the presence of polyvinylpolypyrrolidoniume tribromide in acetonitrile at room temperature. Also a variety of alcohols were converted into alkyl formates by ethyl formate and a catalytic amount of polyvinylpolypyrrolidoniume tribromide under solvent free conditions at room temperature.

Full text of DOI:10.1016/j.cclet.2011.05.020

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 1207–1210
www.elsevier.com/locate/cclet
Polyvinylpolypyrrolidoniume tribromide as new
and metal-free catalyst for the formylation and
trimethylsilylation of hydroxyl group
*
Arash Ghorbani-Choghamarani , Hamid Goudarziafshar, Parisa Zamani
Department of Chemistry, Faculty of Science, Ilam University, P.O. Box 69315516 Ilam, Iran
Received 22 February 2011
Available online 18 July 2011
Abstract
Trimethylsilylation of alcohols was achieved using 1,1,1,3,3,3-hexamethyldisilazane (HMDS) as silylating agent, in the
presence of polyvinylpolypyrrolidoniume tribromide in acetonitrile at room temperature. Also a variety of alcohols were converted
into alkyl formates by ethyl formate and a catalytic amount of polyvinylpolypyrrolidoniume tribromide under solvent free
conditions at room temperature.
#
2011 Arash Ghorbani-Choghamarani. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Polyvinylpolypyrrolidone; Polyvinylpolypyrrolidoniume tribromide; Formylation; Silylation; Ethyl format; 1,1,1,3,3,3-Hexamethyl-
disilazane (HMDS)
Finding molecules, which are able to catalyze the reaction between others, is an important contribution of
molecular chemists to increase the efficiency of chemical reactions whereby our daily life based on consumption of
chemicals is shifted closer to an ecologically and economically tolerable equilibrium with our environment [1]. The
development of heterogeneous catalysts for fine chemicals synthesis has become a major area of research mainly
because the reactions are carried out under mild conditions and the organic products are easily isolated from the
reaction media. The protection–deprotection steps of active protic functional groups are frequently required in
multistep synthesis to prevent their interference while modifying other functional groups in the same molecule [2].
Trimethylsilylation of organic compounds including labile hydrogen atoms find increasing use in analytical and in
preparative organic chemistry [3]. Silyl ethers are one of the most popular protecting groups of hydroxyl moiety in
synthetic organic chemistry and a various types of silyl ethers have been reported in the last decade [4–6]. Also O-
formylation might be the method of choice for protecting a hydroxyl group in a complex synthetic sequence because
deformylation can be occurred selectively in the presence of acetate or other ester groups.
1
,1,1,3,3,3-Hexamethyldisilazane (HMDS) is one of the most common silylating agent for silylation of hydroxyl
group [7–10,6]. Also there are several reports on the formylation of alcohols by ethyl formate as cheap and non-toxic
formylating reagent [11–14]. Their handling does not require special precautions, and the workup is not time-
*
Corresponding author.
E-mail addresses: arashghch58@yahoo.com, a.ghorbani@mail.ilam.ac.ir (A. Ghorbani-Choghamarani).
1001-8417/$ – see front matter # 2011 Arash Ghorbani-Choghamarani. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.05.020

1208
A. Ghorbani-Choghamarani et al. / Chinese Chemical Letters 22 (2011) 1207–1210
H
H
C
H
C
H
C
H
C
H
C
2
2
2
C
N
HBr
N
Br2
N
O
OH
OH
n
n
n
Br-
Br3-
Scheme 1.
consuming, because the by-product of the reaction is ammonia and ethanol, which is simple to remove from the
reaction media. However, the low activity of these reagents is the main drawback to their applications. Therefore, an
appropriate catalyst should be used to activate these reagents.
In continuation of our ongoing efforts to develop new procedures for the protection and deprotection of organic
functional groups [15–21], we decided to use polyvinylpolypyrrolidoniume tribromide as new and efficient catalyst
for the preparation of trimethylsilyl ethers and alkyl formates.
Polyvinylpolypyrrolidoniume tribromide was prepared via reaction of polyvinylpolypyrrolidone with hydrobromic
acid (HBr); then combination of resulting salt with bromine (Br2).
Initially, in order to find appropriate conditions for the trimethylsilylation and formylation reactions, different
solvents were screened. After solvents screening, we found that solvent-free conditions is the best choice for the
preparation of alkyl formates, and acetonitrile is the appropriate solvent for the trimethylsilylation reaction.
Consequently, herein we disclosed a new catalytic protocol for the trimethylsilylation of primary, secondary and
tertiary alcohols to produce the corresponding trimethylsilyl ethers using 1,1,1,3,3,3-hexamethyldisilazane (HMDS)
and a catalytic amount of polyvinylpolypyrrolidoniume tribromide in acetonitrile, (as solvent), at room temperature
(Scheme 1 and Table 1).
Trimethylsilylation was carried out heterogeneously under mild and neutral conditions. Trimethylsilyl ethers easily
obtained by mixing 1 mmol of alcohols, 0.8 mmol of 1,1,1,3,3,3-hexamethyldisilazane and 0.02 g of
polyvinylpolypyrrolidoniume tribromide; then stirring of the resulting mixture at room temperature. After
completion, reaction was quenched with water and the pure product was easily obtained by evaporation of solvent.
Table 1
Trimethylsilylation of hydroxyl groups using hexamethyldisilazane (HMDS) in the presence of a catalytic amount of polyvinylpolypyrrolidoniume
a
tribromide in acetonitrile at room temperature.
b
Entry
Substrate
Product
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
2-Chlorobenzyl alcohol
4-Nitrobenzyl alcohol
3-Flourobenzyl alcohol
4-Bromobenzyl alcohol
4-Chlorobenzyl alcohol
4-iso-Propylbenzyl alcohol
4-tert-Butylbenzyl alcohol
Pentaflourobenzyl alcohol
2-Phenyl ethanol
2-Chlorobenzyl trimethylsilyl ether
4-Nitrobenzyl trimethylsilyl ether
3-Flourobenzyl trimethylsilyl ether
4-Bromobenzyl trimethylsilyl ether
4-Chlorobenzyl trimethylsilyl ether
4-iso-Propylbenzyl trimethylsilyl ether
4-tert-Butylbenzyl trimethylsilyl ether
Pentaflourobenzyl trimethylsilyl ether
2-Phenyl ethyl trimethylsilyl ether
2-Oxo-1,2-diphenylethyl trimethylsilyl ether
Cholesterol trimethylsilyl ether
5
60
8
99
90
98
94
90
99
99
92
90
82
99
96
98
85
94
60
91
80
95
98
97
15
5
5
10
11
2
1
1
1
1
1
1
1
1
1
1
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2-hydroxy-1,2-diphenylethanone
Cholesterol
2-Admantanol
10
25
5
2-Admantyl trimethylsilyl ether
1-Phenyl-1-propanol
2-Phenylpropan-1-ol
1-Heptanol
1-Phenylpropyl trimethylsilyl ether
2-Phenylpropyl trimethylsilyl ether
1-Heptyl trimethylsilyl ether
7
2
5
Pyridin-3-ylmethanol
Furan-2-ylmethanol
2-Methyl-1-phenyl-2-propanol
1-Admantanol
Pyridin-3-ylmethyl trimethylsilyl ether
Furan-2-ylmethyl trimethylsilyl ether
2-Methyl-1-phenyl-2-propyl trimethylsilyl ether
1-Admantyl trimethylsilyl ether
15
2
90
9
(4-Chlorophenyl)(phenyl)methanol
p-Cresol
(4-Chlorophenyl)(phenyl)methyl trimethylsilyl ether
p-Tolyl trimethylsilyl ether
85
2
a
Substrate/HMDS/catalyst = 1 mmol/0.8 mmol/0.02 g.
Isolated yield.
b

A. Ghorbani-Choghamarani et al. / Chinese Chemical Letters 22 (2011) 1207–1210
1209
H
C
H
C
N
2
CH2OH
CH2OCHO
CH2OH
NO2
CH2OSiMe3
OH
R2
OH
(0.035 g)
OCHO
n
Br3-
HCOOEt (2 mL)
R1
R3
HMDS (0.8 mmol)
CH CN, rt, 5
R1
R3
Neeat, rt, 5
h
HCOOEt (2 mL), rt
2
R2
h
3
NO2
1
3
R = Alkyl or Aryl; R = H, Alkyl; Aryl R = H or Alky
6 %
73 %
Scheme 2.
Also different types of alkyl formates were prepared via reaction of a variety of alcohols with ethyl formate in the
presence of a catalytic amount of polyvinylpolypyrrolidoniume tribromide at room temperature under solvent-free
conditions (Scheme 2 and Table 2). To investigate the role of polyvinylpolypyrrolidoniume tribromide as catalyst in
described transformations, other reactions were designed in the absence of the catalyst (Scheme 2).
Formylation of alcohols heterogeneously performed under mild and solvent-free conditions. The results in Table 2
show that unhindered alcohols are more reactive than hindered one in the formylation reaction. And formylation and
silylation reactions without catalyst did not complete after 5 h.
In conclusion, the chemistry of polyvinylpolypyrrolidoniume tribromide has opened up a new methodological
option for the catalytic, versatile and efficient preparation of various types of trimethylsilyl ethers and alkyl formates.
The advantages of this procedure are the avoidance of metallic and acidic catalysts, mild and heterogeneous reaction
conditions, avoidance of corrosive and toxic reagents and operational simplicity.
1
. Experimental
Preparation of polyvinylpolypyrrolidoniume tribromide: In a 50 mL round-bottomed flask, 2 mL of HBr (47%) and
.80 g of polyvinylpolypyrrolidone was stirred for 1 h, then kept at 50 8C for 24 h to obtain dry polyvinyl-
3
polypyrrolidoniume bromide. In the next step 2.4 mL of Br was added to the resulting powder; this mixture stirred for
2
2
h and an orange crystalline solid, polyvinylpolypyrrolidoniume tribromide, was obtained quantitatively.
Table 2
O-Formylation of alcohols using ethyl formate in the presence of a catalytic amount of polyvinylpolypyrrolidoniume tribromide at room temperature
a
under solvent free conditions.
b
Entry
Substrate
Product
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
2-Chlorobenzyl alcohol
4-Nitrobenzyl alcohol
3-Flourobenzyl alcohol
4-Bromobenzyl alcohol
4-Chlorobenzyl alcohol
4-iso-Propylbenzyl alcohol
4-tert-Butylbenzyl alcohol
Pentaflourobenzyl alcohol
2-Phenyl ethanol
2-Chlorobenzyl formate
4-Nitrobenzyl formate
35
90
72
87
3-Flourobenzyl formate
4-Bromobenzyl formate
4-Chlorobenzyl formate
4-iso-Propylbenzyl formate
4-tert-Butylbenzyl formate
Pentaflourobenzyl formate
2-Phenyl ethyl formate
2-Oxo-1,2-diphenylethyl formate
Cholesterol formate
60
67
60
84
60
86
75
85
50
90
90
66
70
86
1
1
1
1
1
1
1
1
1
1
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2-hydroxy-1,2-diphenylethanone
Cholesterol
90
64
24 h
60
90
2-Admantanol
2-Admantyl formate
90
1-Phenyl-1-propanol
2-Phenylpropan-1-ol
1-Heptanol
1-Phenylpropyl formate
2-Phenylpropyl formate
1-Heptyl formate
55
94
60
99
40
93
Pyridin-3-ylmethanol
Furan-2-ylmethanol
2-Methyl-1-phenyl-2-propanol
1-Admantanol
Pyridin-3-ylmethyl formate
Furan-2-ylmethyl formate
2-Methyl-1-phenyl-2-propyl formate
1-Admantyl formate
120
40
No reaction
44
6 h
40
Trace
67
Sluggish
No reaction
(4-Chlorophenyl)(phenyl)methanol
p-Cresol
(4-Chlorophenyl)(phenyl)methyl formate
p-Tolyl format
60
6 h
a
Substrate/ethyl formate/catalyst = 1 mmol/2 mL/0.035 g.
Isolated yield.
b

1210
A. Ghorbani-Choghamarani et al. / Chinese Chemical Letters 22 (2011) 1207–1210
Polyvinylpolypyrrolidone: IR (KBr): V ¼ 3456; 2956; 1671; 1461; 1422; 1289; 1088 cm 1.
ꢀ
¯
ꢀ
1
¯
Polyvinylpolypyrrolidoniume bromide: IR (KBr): V ¼ 3776; 3698; 3391; 2923; 1601; 1288; 1089; 929; 607 cm .
Formylation of perfluorobenzyl alcohol using ethyl formate catalyzed by polyvinylpolypyrrolidoniume tribromide:
To a mixture of perfluorobenzyl alcohol, (1 mmol, 0.198 g) and ethyl formate (2 mL); polyvinylpolypyrrolidoniume
tribromide, (0.035 g) was added. The resulting mixture was stirred at room temperature for 90 min (the reaction
progress was monitored by TLC). After reaction completion, reaction mixture was passed on short column
chromatography (packed by silica gel) using dichloromethane as eluent to obtain perfluorobenzyl format in 66% yield
(
0.149 g).
Trimethylsilylation of cholesterol with HMDS catalyzed by polyvinylpolypyrrolidoniume tribromide: To a mixture
of cholesterol, (0.387 g, 1 mmol) and hexamethyldisilazane (0.129 g, 0.8 mmol) in CH CN (10 mL), polyvinyl-
3
polypyrrolidoniume tribromide (0.02 g) was added, and the mixture was stirred at room temperature for 35 min
(
Then organic phase dried over Na SO (3 g). Evaporation of the solvent gave pure trimethyl (cholesteroloxy)silane,
reaction progress monitored by TLC). Then reaction was quenched with water (10 mL) and 20 mL of CH Cl added.
2 2
2
4
(
0.454 g, 99%).
Acknowledgment
Financial support for this work by the Ilam University, Ilam, Iran is gratefully acknowledged.
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