Silica triflate as a new, mild and efficient catalyst for tetrahydropyranylation of alcohols and deprotection of tetrahydropyranyl ethers

Article abstract of DOI:10.1080/10426500701407839

Silica triflate, a new stable derivative of silica gel, is easily prepared by the reaction of silica gel with trifluoromethanesulfonyl chloride. Silica triflate acts as a highly effective and reusable catalyst for tetrahydropyranylation of alcohols and deprotection of tetrahydropyranyl ethers under mild and completely heterogeneous reaction conditions.

Full text of DOI:10.1080/10426500701407839

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Phosphorus, Sulfur, and Silicon
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Silica Triflate as a New, Mild
and Efficient Catalyst for
Tetrahydropyranylation of
Alcohols and Deprotection of
Tetrahydropyranyl Ethers
F. Shirini a , K. Marjani b & H. Taherpour Nahzomi a b
a Department of Chemistry, College of Science ,
Guilan University , Rasht, Iran
b Faculty of Chemistry , Teacher Training University ,
Tehran, Iran
Published online: 16 Jul 2007.
To cite this article: F. Shirini , K. Marjani & H. Taherpour Nahzomi (2007) Silica
Triflate as a New, Mild and Efficient Catalyst for Tetrahydropyranylation of Alcohols
and Deprotection of Tetrahydropyranyl Ethers, Phosphorus, Sulfur, and Silicon and the
Related Elements, 182:9, 2235-2240, DOI: 10.1080/10426500701407839
To link to this article: http://dx.doi.org/10.1080/10426500701407839
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Phosphorus, Sulfur, and Silicon, 182:2235–2240, 2007
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701407839
Silica Triflate as a New, Mild and Efficient Catalyst for
Tetrahydropyranylation of Alcohols and Deprotection
of Tetrahydropyranyl Ethers
F. Shirini
Department of Chemistry, College of Science, Guilan University, Rasht,
Iran
K. Marjani
Faculty of Chemistry, Teacher Training University, Tehran, Iran
H. Taherpour Nahzomi
Department of Chemistry, College of Science, Guilan University, Rasht,
Iran and Faculty of Chemistry, Teacher Training University, Tehran,
Iran
Silica triflate, a new stable derivative of silica gel, is easily prepared by the reaction
of silica gel with trifluoromethanesulfonyl chloride. Silica triflate acts as a highly
effective and reusable catalyst for tetrahydropyranylation of alcohols and deprotec-
tion of tetrahydropyranyl ethers under mild and completely heterogeneous reaction
conditions.
Keywords Alcohols; silica gel; silica triflate; tetrahydropyranylation; trifluoromethane-
sulfonyl chloride
INTRODUCTION
Due to the remarkable stability of tetrahydropyranyl ethers towards a
variety of reaction conditions such as strongly basic media, Grignard
reagents, oxidizing and reducing agents, alkylation and acylation re-
actions, and because of their easy removal under mild acidic con-
ditions, tetrahydropyranylation is considered as one of the most
Received October 2, 2006; accepted March 30, 2007.
Partial support of this work by the Guilan and Teacher Training Universities Research
Council is acknowledged. F. Shirini is thankful to Guilan Science and Technology Park
(GSTP) for its continous support. F. Shirini is also indebted to Dr. M. Mottaghitalab for
his sincere assistance.
Address correspondence to F. Shirini, Department of Chemistry, College of Science,
Guilan University, Rasht, Iran. E-mail: shirini@guilan.ac.ir
2235

2236
F. Shirini et al.
important methods for the protection of alcoholic hydroxyl groups. A
variety of reagents have been used for tetrahydropyranylation and
detetrahydropyranylation, e. g., ZrCl4,1 I2,2 LiBr,3 PdCl2(CH3CN)2,4
Wells-Dawson heteropolyacid,5 Fe(ClO4)3,6 sulfuric acid supported
on silica gel,7 potassium dodecatungstocobaltate trihydrate,8 DDQ,9
PPh3Br2,10 ion exchange resins,11 BF3.OEt2,12 acetyltriphenylphospho-
nium bromide,13 and electrogenerated acid.14 More recently, Sc(OTf)3,15
18
19
[Ru(CH3)3(triphos)](OTf)2,16 LiOTf,17 In(OTf)3 and Bi(OTf)3 have
also been employed for this purpose. Although these methods are sat-
isfactory for many molecules, some have limitations, such as the use
of expensive and toxic reagents, high temperature, long reaction times,
incompatibility with other acid-sensitive functional groups, and non-
reusability. Therefore, there is a need to develop an alternative method
for tetrahydropyranylation as well as detetrahydropyranylation under
mild reaction conditions.
In continuation of our studies on the preparation of new deriva-
tives of silica gel and their applications in functional group
transformations,20−22 we have found that trifluoromethanesulfonyl
chloride reacts with silica gel to give silica triflate (I) as a white stable
powder. It is interesting to note, that the reaction is easy and clean
without any work-up procedure because HCl gas is evolved from the
reaction vessel immediately (Scheme 1).
SCHEME 1
We anticipated that silica triflate (I) would be more efficient for run-
ning reactions under mild and heterogeneous conditions as compared
to most of the reported triflates. Therefore, we were interested in us-
ing this reagent for the promotion of tetrahydropyranylation of alcohols
and deprotection of tetrahydropyranyl ethers (Scheme 2).
SCHEME 2
Optimization of the reaction conditions is carried out by the tetrahy-
dropyranylation of 2-bromobenzyl alcohol with 3,4-dihydro-2H-pyran
(DHP) in the presence of silica triflate. The best result was achieved by

Silica Triflate as Catalyst
2237
carring out the reaction with 1.2 equiv. of DHP in the presence of 0.03
g of silica triflate at room temperature for 3 min in n-hexane (Table I).
In order to extend the scope of this method, a variety of alcohols
(primary, secondary, tertiary, benzylic, and allylic alcohols) were
used applying the same reaction conditions (Table I). All reactions
were performed under mild and completely heterogeneous reaction
conditions in good to high yields. It is interesting, that dehydration
products of secondary and tertiary alcohols were not observed during
the course of the reaction (Table I). In addition, the double bond of
cinnamyl alcohol, as an example of an allylic alcohol, was not affected
using this method (Table I).
Deprotection of THP ethers can also be carried out by using simi-
lar amounts of silica triflate in refluxing methanol. The results of the
deprotection reactions are also summarized in Table I.
TABLE I Tetrahydropyranylation and
Detetrahydropyranylation of Alcohols in the Presence of Silica
Triflatea,b
Protection in n-hexane
Deprotection in methanol
Substrate
Time (min)
Yield (%)
Time (min)
Yield (%)
2-BrC6H4CH2 OH
2-ClC6H4CH2 OH
4-ClC6H4CH2 OH
2-NO2C6H4CH2 OH
3-NO2C6H4CH2 OH
4-NO2C6H4CH2 OH
2-MeC6H4CH2 OH
4-MeOC6H4CH2OH
4-PhCH2OC6H4CH2OH
Ph2CHOH
3
6
3
12
6
6
4
2
2
5
90
92
90
85
90
95
90
92
85
95
90
95
90
80
5
4
8
10
8
8
5
8
6
20
12
9
10
25
92
90
90
90
95
90
87
90
95
87
90
92
95
90
C6H5CH2CH2CH2OH
C6H5CH(Me)CH2OH
C6H5CH2CH(OH)CH3
1
3
4
21
CH
3
OH
CH(CH
)
2
3
19
95
20
93
OH
(CH3)3COH
C6H5CH CHCH2OH
2
3
90
90
8
3
90
90
aProducts were characterized by their physical constants, comparison with
authentic samples, and IR and NMR spectroscopy; b Isolated yield.

2238
F. Shirini et al.
TABLE II Comparison of Some of our Results, with Some of Those
Reported in the Literature1,6,17
(min)(Yield %)
Substrate
Product
Silica triflate
ZrCl4
Fe(ClO4)3
LiOTf
Ph2CHOH
C6H5CH
Ph2CHOTHP
C6H5CH
(5)(95)
(3)(90)
(180)(90)
(120)(98)
(150)(75)
—
(240)(86)
—
CHCH2OH
(CH3)3COH
CHCH2OTHP
(CH3)3COTHP
(2)(90)
(120)(87) (180)(92.5)
—
We have found that silica triflate is a reusable catalyst and even
after three runs for tetrahydropyranylation of alcohols with DHP and
deprotection of THP ethers, the catalytic activity of the reagent was
almost the same as that of the freshly used catalyst. No change in the
IR spectrum of the catalyst before and after the reaction was observed,
which demonstrates the stability of the reagent.
A comparison of some of the results with some of those reported in the
literature1,6,17 (Table II) shows the efficiency of the presented method.
Mildness of the reaction conditions, short reaction times, high effi-
ciency, reasonable yields, simple and clean work-up, reusability of the
catalyst, and heterogeneous reaction conditions are important advan-
tages of this new method.
EXPERIMENTAL
Chemicals were purchased from Fluka, Merck, and Aldrich chemical
companies. The purity determination of the products was accom-
plished by TLC on silica gel polygram SIL G/UV254 plates. The FT-IR
spectra were recorded on Schimadzu model 8300 instrument. The
1H NMR spectra were recorded on a Bruker Advance DPX 250 MHz
spectrometer.
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
trifluoromethanesulfonyl chloride (8.4 g, 0.05 mol), and a gas inlet tube
for conducting HCl gas over an adsorbing solution, i.e., water. Trifluo-
romethanesulfonyl chloride was added dropwise over a period of 30 min
and the reaction mixture was slowly stirred at room temperature for
further 30 min. The mixture was then heated to 60◦C with stirring for
1 h to remove all HCl and excess amounts of trifluoromethanesulfonyl

Silica Triflate as Catalyst
2239
chloride. The reaction mixture was washed with 50 mL of dry CH2Cl2
and dried under vacuum. Silica triflate was obtained as a white solid
(20.38–20.62 g), which was stored in a capped bottle; IR (KBr): 1255,
1230, 1130, 1010, 955, 820, 650, 530, 500 cm−1
.
Protection of Alcohols Catalyzed by Silica Triflate
A mixture of the respective alcohol (1 mmol), DHP (0.1008 g, 1.2 mmol)
and silica triflate (0.03 g) in n-hexane (4 mL) was stirred at room temper-
ature. The progress of the reaction was monitored by GC or TLC. After
completion of the reaction, the reaction mixture was filtered through a
silica-gel pad, and the solid residue was washed with n-hexane (5 mL).
Evaporation of the solvent gave the respective THP ethers in good to
high yields (Table I).
Deprotection of THP Ethers Catalyzed by Silica Triflate
A mixture of the respective THP ether (1 mmol) and silica triflate (0.03
g) in methanol (4 mL) was stirred at reflux conditions. 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
methanol (5 mL). Evaporation of the solvent under reduced pressure
gave almost pure alcohol (Table I). Further purification was carried out
by column chromatography on silica gel to afford the pure alcohol.
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