Silica triflate as an efficient reagent for the chemoselective formylation of alcohols

Article abstract of DOI:10.1080/10426500601160710

Silica triflate, as a new and stable silica-based reagent, is prepared by a reaction of silica gel with trifluoromethane sulfonyl chloride at room temperature. This reagent can be used for the efficient and selective formylation of alcohols in the presence of phenols in a relatively short reaction time and high yields under heterogeneous reaction conditions.

Full text of DOI:10.1080/10426500601160710

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Phosphorus, Sulfur, and Silicon
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Silica Triflate as an
Efficient Reagent for the
Chemoselective Formylation of
Alcohols
Farhad Shirini a , Katayoun Marjani b , Hossein
Taherpour Nahzomi a c & Mohammad Ali Zolfigol d
a Department of Chemistry , Guilan University ,
Rasht, Iran
b College of Chemistry , Teacher Training
University , Tehran, Iran
c Teacher Training University , Iran
d Department of Chemistry , Bu-Ali Sina University ,
Hamadan, Iran
Published online: 29 Aug 2007.
To cite this article: Farhad Shirini , Katayoun Marjani , Hossein Taherpour Nahzomi
& Mohammad Ali Zolfigol (2007) Silica Triflate as an Efficient Reagent for the
Chemoselective Formylation of Alcohols, Phosphorus, Sulfur, and Silicon and the
Related Elements, 182:6, 1245-1251, DOI: 10.1080/10426500601160710
To link to this article: http://dx.doi.org/10.1080/10426500601160710
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Phosphorus, Sulfur, and Silicon, 182:1245–1251, 2007
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500601160710
Silica Triflate as an Efficient Reagent
for the Chemoselective Formylation of Alcohols
Farhad Shirini
Department of Chemistry, Guilan University, Rasht, Iran
Katayoun Marjani
College of Chemistry, Teacher Training University, Tehran, Iran
Hossein Taherpour Nahzomi
Department of Chemistry, Guilan University, Rasht, Iran; and
Teacher Training University, Iran
Mohammad Ali Zolfigol
Department of Chemistry, Bu-Ali Sina University, Hamadan, Iran
Silica triflate, as a new and stable silica-based reagent, is prepared by a reaction of
silica gel with trifluoromethane sulfonyl chloride at room temperature. This reagent
can be used for the efficient and selective formylation of alcohols in the presence of
phenols in a relatively short reaction time and high yields under heterogeneous
reaction conditions.
Keywords Formylation; heterogeneous reaction conditions; silica triflate; silica gel; tri-
fluoromethane sulfonyl chloride
Because of the importance of formate esters as useful synthetic reagents
and intermediates,1 formylation of the hydroxyl group is considered
one of the most widely used transformations in organic chemistry.2,3
Despite these uses and considerable potential, the formyl protect-
ing group has been rather overlooked. This is partly due to the fact
that efficient formylation procedures under mild conditions are not
available. The direct esterification of alcohols with carboxylic acids
Received October 12, 2006; accepted October 25, 2006.
Partial support of this work by the Guilan and Teacher Training Universities Research
Councils is acknowledged. F. Shirini is thankful to Guilan Science and Technology Park
(GSTP) for its continuous support. F. Shirini is also indebted to Dr. M. Mottaghitalab for
his sincere assistance.
Address correspondence to Farhad Shirini, Department of Chemistry, Guilan Univer-
sity, Rasht, Guilan, 41335-1914, Iran. E-mail: shirini@guilan.ac.ir
1245

1246
F. Shirini et al.
is a reversible process that needs to eliminate water from the reac-
tion mixture to drive the process to completion; thus the method is
generally avoided. The acid halide or anhydride procedure is unsuit-
able for formylation because formyl halides and anhydride are un-
stable. Several formylation procedures have been reported over the
years. These are based on formylation with formic acid in the pres-
ence of a dehydrating agent (Ac2O,4 DCC,5 and 1,1’-oxalyldiimidazole6),
transesterification with methyl/ethyl formate (catalyzed by silica-gel
supported metal sulphate)7 Ce(OTf)4,8 Cu(NO3)2.3H2O,9 PPh3/CBr4,10
KCoW12O40.3H2O,11 TMSOTf,12 Cu(OTf)2,13 Sc(OTf)3,14 In(OTf)3,15
Bi(OTf)3,16 Chloral,17 Al(HSO4)3,18 and Mg(HSO4)2.19 Most of these
methods, however, use uncommon and in some cases moisture sensi-
tive or thermally unstable reagents that need to be prepared before
use, in some cases, by multistep procedures employimg expensive cata-
lysts. Many of these methods also suffer from some of the following lim-
itations: elevated temperatures, long reaction times, inert atmosphere,
separation of the spent or deformylated reagent, acidic reaction condi-
tions or work-up, side reactions, and moderate yields.
In recent years our research group, as many others, focused on the
development of the application of silica gel and its new derivatives in
organic chemistry. Along these studies we have reported that these
reagents are able to be used efficiently in many organic reactions.20
In continuation of these studies we have found that trifluoromethane
sulfonyl chloride reacts with silica gel, in an easy, clean, and immediate
reaction, to give silica triflate (I) and HCl gas as the only products
(Scheme 1).
SCHEME 1
We anticipated that silica triflate, as a new triflate derivative, would
be able to accelerate the formylation of alcohols as the other reported
triflates14,15 (Scheme 2).
A variety of alcohols including primary, secondary, tertiary, and ben-
zylic ones were converted to their formate esters in a single step by
mixing the alcohol, formic acid, and silica triflate in refluxing hexane in
good to high yields (Table I). The formylation of allylic alcohols was not
clean and gave a complex mixture of products along with the unreacted

Silica Triflate as an Efficient Reagent
1247
SCHEME 2
alcohol, so this method is not suitable for the formylation of these kinds
of alcohols (Table I). Phenols remained unreacted under these condi-
tions, indicating that the reaction could be selective for the formylation
of alcohols in the presence of phenols. The selectivity was demonstrated
by competitive reactions, in which equimolar mixtures of an alcohol and
a phenol were subjected to formylation, whereby the alcohols were com-
pletely formylated and the phenols were mostly recovered unchanged
(Table II).
We have found that silica triflate is a reusable catalyst, and even
after five runs for the formylation of alcohols, the catalytic activity of the
reagent was almost the same as the freshly used catalyst. The stability
of the composition of the reagent was demonstrated by its IR spectra,
which were the same before and after the reactions.
It should be noted that the formylation reaction did not proceed using
silica gel and trifluoromethane sulfonyl chloride alone or by the sepa-
rate addition of these reagents to the reaction mixture (Table 1). These
results indicate that SiOTf is the reagent responsible for the catalytic
activity observed.
In order to show the efficiency of the present method, Table III
compares some of the results with some of those reported in the
literature.11,19
In conclusion this article reports the synthesis of a new triflate
derivative based on silica gel and its applications in the efficient formy-
lation of alcohols. Mild reaction conditions, simplicity of the proce-
dure, short reaction times, good to high yields of the products, het-
erogeneous reaction conditions, and general applicability for primary
through tertiary alcohols make this method a useful addition to the ex-
isting methodologies. Work on the effect of silica triflate on other types
of organic reactions is currently ongoing in our laboratory.
EXPERIMENTAL
General
Chemicals were purchased from Fluka, Merck (Darmstadt, Germany),
and Aldrich Chemical Companies. Products were characterized by their

1248
F. Shirini et al.
TABLE I Formylation of Alcoholsa
Substrate (mmol): Time Yieldc
Substrate
Product
Catalyst (mg) b
(min)
(%)
2-ClC6H4CH2OH
4-ClC6H4CH2OH
4-BrC6H4CH2OH
2-NO2C6H4CH2OH
3-NO2C6H4CH2OH
4-NO2C6H4CH2OH
2-Pyridyl methanol
3-Pyridyl methanol
2-ClC6H4CH2OCOH
4-ClC6H4CH2OCOH
4-BrC6H4CH2OCOH
2-NO2C6H4CH2OCOH
3-NO2C6H4CH2OCOH
4-NO2C6H4CH2OCOH
2-Pyridyl methyl formate
3-Pyridyl methyl formate
1:60
1:60
10
10
8
60
60
60
5
95
90
92
80
85
80
90
93
95
90
95
1:120
1:120
1:120
1:120
1:60
1:60
1:60
1:60
1:60
4
C6H5CH2CH2CH2OH C6H5CH2CH2CH2OCOH
C6H5CH(CH3)CH2OH C6H5CH(CH3)CH2OCOH
C6H5CH2CH(OH)CH3 C6H5CH2CH(OCOH)CH3
10
12
11
1:90
1:90
8
90
87
11
1:90
1:90
1:90
1:120
1:60
5
8
3
20
30
90
95
90
(-)-Menthol
(CH3)3COH
(-)-Menthyl formate
(CH3)3COCOH
d
C6H5CH=CHCH2OH C6H5CH=CHCH2OCOH
—
C6H5OH
C6H5OCOH
0 e
1:60
1:60
40
15
0 e
0e
2-Naphthol
2-Naphthyl formate
1:60
1:60
1:60
1:60
20
15
15
15
0e
4-ClC6H4CH2OH
2-NO2C6H4CH2OH
C6H5CH2CH2CH2OH C6H5CH2CH2CH2OCOH
4-ClC6H4CH2OCOH
2-NO2C6H4CH2OCOH
0 e, f
0 e,g
0 e,h
aProducts were characterized by their physical constants, in comparison with
authentic samples and IR and NMR spectroscopy.
bReaction is performed in the presence of 1.2 mmol of formic acid.
cIsolated yield.
dMixture of products.
eThe starting material was recovered intact.
f Reaction was performed in the presence of silica gel.
g
Reaction was performed in the presence of trifluoromethane sulfonyl chloride.
h Reaction was performed in the presence of a mixture of trifluoromethane sulfonyl
chloride and silica gel.

Silica Triflate as an Efficient Reagent
1249
TABLE II Competitive Formylation of Alcohols and Phenols
Substrates
Phenol:Alcohol:Cat. (mg)a
Time (min)
10
Yield (%)b
1:1:60
0 + 90
1:1:150
10
0 + 95
C6H5OH
1:1:60
1:1:60
10
10
0 + 90
0 + 92
+
4-ClC6H4CH2 OH
C6H5OH
+
C6H5CH2CH2CH2 OH
aReaction is performed in the presence of 1.2 mmol of formic acid.
bIsolated yield.
physical constants, comparison with authentic samples, and IR and
NMR spectroscopy.
Preparation of Silica Triflate
A 500-mL suction flask, charged with 18.0 g silica gel (type 60,
15–40 µm), was equipped with a constant pressure-dropping funnel
containing trifluoromet-hane sulfonyl chloride (8.426 g, 0.05 mol) and
a gas inlet tube for conducting HCl gas over water. Trifluoromethane
sulfonyl chloride was added dropwise over a period of 30 min with slow
TABLE III Comparison of Some of the Results Obtained by
the Formylation of Alcohols Using Formic Acid in the
Presence of Silica Triflate, with Some of Those Reported for
KCoW12O40.3H2O[11]and Mg(HSO4)[219]
Time (min)/Yield (%)
Substrate
Silica Triflate
KCoW12O40.3H2O Mg(HSO4)2
(-)-Menthol
4-ClC6H4CH2 OH
C6H5CH2CH2CH2 OH
8/92
10/95
10/95
60/81
45/97
30/94
—
480/85
60/80

1250
F. Shirini et al.
stirring at r.t. Gas was evolved from the reaction vessel immediately. Af-
ter the addition was complete, the mixture was shaken for 30 min. The
reaction mixture was washed with 50 mL of dry CH2Cl2 and dried un-
der vacuum. Silica triflate was obtained as a white solid (20.4–20.6 g),
which was stored in a capped bottle. In order to determine the amount
of active triflate content of the reagent, the produced HCl from the
previously discussed reaction was titrated with 0.1 M aq NaOH. The
results from several runs showed the formation of 0.018–0.020 mol of
HCl; therefore, the amount of active triflate was determined to be 1
mmol per 1.04–1.08 g silica triflate; IR (KBr): 1255, 1230 (υS=O), 1130,
1010 (υC−F), 955, 820, 650, 530, 500 cm−1
.
General Procedure
A mixture of alcohol (1 mmol), formic acid (0.055 g, 1.2 mmol), and
silica triflate (0.06–0.12 g) in hexane (4 mL) was heated at reflux. The
progress of the reaction was monitored by GC or TLC. After completion
of the reaction, the mixture was filtered, and the solid residue was
washed with n-hexane (3 mL). The solvent was removed under reduced
pressure. Water (20 mL) was added to the residue, and the product was
extracted with CH2Cl2 (2 × 10 mL). The combined organic layer was
washed with saturated NaHCO3 solution (2 × 10 mL) and dried over
anhydrous MgSO4. Evaporation of the solvent under reduced pressure
gave almost pure products (Table I). Further purification was carried
out by column chromatography on silica gel to afford pure formate.
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