Efficient role of ionic liquid (bmim)HSO4 as novel catalyst for monotetrahydropyranylation of diols and tetrahydropyranylation of alcohols

Article abstract of DOI:10.1080/00397910600770839

A simple procedure for the monotetrahydropyranylation of diols and alcohols, in excellent to moderate yields, has been reported using a catalytic amount of the acidic ionic liquid, 1-butyl-3-methylimidazolium hydrogensulphate (bmim)HSO₄ under microwave and ultrasonic irradiation. Results have been compared with those obtained by reactions in the absence of these energies, and effectiveness of the recycled ionic liquid has also been studied. Copyright Taylor & Francis Group, LLC.

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Efficient Role of Ionic Liquid
(
bmim)HSO as Novel Catalyst for
4
Monotetrahydropyranylation of Diols and
Tetrahydropyranylation of Alcohols
a
a
a
b
Jasvinder Singh , Neeraj Gupta , Goverdhan L. Kad & Jasamrit Kaur
a
Department of Chemistry and Centre for Advanced Studies in Chemistry,
Panjab University, Chandigarh, India
b
GGDSD College, Chandigarh, India
Version of record first published: 16 Feb 2007.
To cite this article: Jasvinder Singh, Neeraj Gupta, Goverdhan L. Kad & Jasamrit Kaur (2006): Efficient
Role of Ionic Liquid (bmim)HSO as Novel Catalyst for Monotetrahydropyranylation of Diols and
4
Tetrahydropyranylation of Alcohols, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 36:19, 2893-2900
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Synthetic Communications , 36: 2893–2900, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600770839
Efficient Role of Ionic Liquid (bmim)HSO4
as Novel Catalyst for
Monotetrahydropyranylation of Diols and
Tetrahydropyranylation of Alcohols
Jasvinder Singh, Neeraj Gupta, and Goverdhan L. Kad
Department of Chemistry and Centre for Advanced Studies in Chemistry,
Panjab University, Chandigarh, India
Jasamrit Kaur
GGDSD College, Chandigarh, India
Abstract: A simple procedure for the monotetrahydropyranylation of diols and
alcohols, in excellent to moderate yields, has been reported using a catalytic amount
of the acidic ionic liquid, 1-butyl-3-methylimidazolium hydrogensulphate
(
bmim)HSO4 under microwave and ultrasonic irradiation. Results have been
compared with those obtained by reactions in the absence of these energies, and
effectiveness of the recycled ionic liquid has also been studied.
Keywords: Ionic liquid, microwave irradiation, monotetrahydropyranylation,
selective, solvent-free, tetrahydropyranylation, ultrasound
[1]
Room-temperature ionic liquids have gained great impetus as attractive,
[2]
promising, and green media or catalysts for various organic syntheses. This
popularity arises because of their unique properties such as negligible vapor
pressure, thermal stability, recyclability, immiscibility with nonpolar organic
solvents, catalytic efficiency, and use in biphasic systems. In fact, their high
polarity makes them excellent media to facilitate microwave-assisted organic
[3]
reactions.
Received in India February 28, 2006
Address correspondence to Jasvinder Singh, Department of Chemistry and Centre
for Advanced Studies in Chemistry, Panjab University, Chandigarh 160014, India.
E-mail: jsbrar_pu@yahoo.com
2
893

2
894
J. Singh et al.
The prevalence of alcoholic groups in many multifunctional natural and
pharmaceutical products makes their protection an essential prerequisite for
many synthetic endeavors. Of the many protective reagents available for such
[4]
a conversion, 3,4-dihydro-2H-pyran (DHP) has proven to be very efficient,
more so because the resultant tetrahydropyranyl (THP) ether is easily
prepared, inexpensive, easy to deprotect, and stable in strong basic media.
Several methods are available for the protection of alcohols as THP
[
5]
ethers; very few, however, report the selective monoprotection of 1,n
diols as THP ethers. More recently, acidic ion exchange resins/metallic
[6]
sulphate supported on silica gel and microwave energy in combination
[
7]
with iodine have been used to carry out the monoprotection of different
diols. Ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate
[8]
[(bmim)PF ] has also been reported in conjunction with indium trichloride
6
for the protection of alcoholic group. Most of the methods require long
reaction times, tedious reaction setup, difficult separation, and a large
amount of solid support, leading to waste-disposal problems.
The acidic ionic liquid 1-butyl-3-methylimidazolium hydrogensulphate
[9]
(
extent even though it holds great potential because of its Bronsted
bmim)HSO has yet not been explored in organic reactions to a great
4
[
10]
acidity
and highly polar nature, which augments absorption of
microwave energy. In continuation of our endeavor for the utilization of
greener technologies, such as microwave (MW) and ultrasound (US), for
[
11]
assisting organic synthesis
and ongoing work on the use of (bmim)HSO4
[
12]
in organic reactions, we report herein its use for the tetrahydropyranylation
of different alcohols and selective monotetrahydropyranylation of 1,n diols
under solvent-free or biphasic conditions using microwave and ultrasonic
energies. In all the reactions carried out, the separation of the ionic liquid
was very effective and convenient by simply extracting the reaction mixture
with diethyl ether or separating the hexane layer; the residue thus obtained
contained the ionic liquid.
Protection of alcohols as THP ethers (Scheme 1) was carried by mixing
equimolar amounts of alcohol, DHP, and a catalytic amount of ionic liquid
[
12]
(
which was easily prepared under microwave irradiation ) followed by
exposure to microwave radiations (160 W) for 1.5 to 2 min, ultrasound
energy for 5 to 7 min, or stirring at room temperature up to 60 min to yield
the pure ethers in excellent to quantitative yields without the need for
1
further purification. The products were characterized by H NMR
Scheme 1.

Ionic Liquid (bmim)HSO4
2895
spectroscopy, which showed the absence of unreacted alcohol. Results
obtained are given in Table 1.
When diols were subjected to similar reaction conditions (i.e., mixing
equimolar amounts of alcohol, DHP, and a catalytic amount of ionic liquid)
Table 1. Tetrahydropyranylation of different alcohols using (bmim)HSO
Microwave Ultrasound
4
Stirring
Sr.
no.
Time Yield Time Yield Time Yield
(%)
Reactant
(min)
(%)
(min)
(%)
(min)
1
2
1.5
1.5
98
89
5
5
97
81
25
45
92
78
3
4
5
1.5
2
84
94
93
5
6
7
82
92
92
45
60
60
80
90
90
2
6
7
2
2
96
88
6
6
96
86
60
60
82
89
8
9
2
75
6
71
60
62
2
2
86
90
6
6
85
92
60
60
74
74
1
0
1
1
1
2
2
90
98
6
5
90
98
45
45
90
95
1.5
1
1
3
4
2
2
89
76
6
6
90
74
60
60
89
71
1
1
5
6
1.5
1.5
92
89
6
5
92
90
60
45
90
90
1
Note: All the compounds were characterized on the basis of H NMR and IR spectral
1
data, which were found to be consistent with that reported in literature. [ H NMR and
IR data of product 1 is reported in Ref. 16(i)].

2
896
J. Singh et al.
Scheme 2.
followed by exposure to microwave/ultrasonic energies (Method 1), selectiv-
ity up to 75% of monoether was obtained.
However, when the reaction was conducted under biphasic conditions
(
(
Scheme 2), using hexane as the upper nonpolar layer, excellent selectivity
ꢀ90%) was achieved. Results have been tabulated (Table 2). Understand-
ably, diols have greater miscibility in the lower (ionic liquid) phase and on
conversion to monotetrahydropyranyl ether, pass into the hexane layer, mini-
mizing formation of ditetrahydropyranyl ether and at the same time facilitat-
ing the recovery of monotetrahydropyranyl ether from the hexane layer.
For the microwave-assisted reactions, different conditions were studied
for optimization of results. It was found that output power of microwaves of
more than 320 W resulted in charring of the reaction mixture whereas
irradiation at 320 W led to a decrease in the activity of the recycled ionic
liquid with no further increase in the yields. The power output of choice
was thus set at 160 W. In the case of reactions conducted ultrasonically and
by stirring, further increase in the time beyond that reported did not result
in any appreciable increase in the yield of the products.
In the case of 3,7-dimethyloctane-1,6-diol (entry 5, Table 2), solventless
conditions (Method 1) provided moderate yields and biphasic conditions
(
Method 2) gave good yields of monoether of the corresponding primary
hydroxyl group.
The effectiveness of recycled ionic liquid was studied in the experiments
relating to tetrahydropyranylation of 1-butanol. We were able to recycle the
ionic liquid once under microwave irradiation, seven times under ultrasonic
conditions, and at least nine times under conditions of stirring (Fig. 1) to
obtain minimum yields up to 50%.
It was observed that experiments under microwave irradiation gave the
best yields, but led to decomposition of the ionic liquid, resulting in only
50% yield of the protected alcohol from the first recycled crop of ionic
liquid. Under conditions of stirring at room temperature, the time taken to
carry out the reaction was much longer as compared with those assisted by
microwave or ultrasonic energies.
The reaction seems to progress via the normal acid-catalyzed pathway
brought about by the highly acidic nature of the ionic liquid.
In conclusion, a new method has been projected for the solvent-free
convenient and simple protection of alcohols as tetrahydropyranyl ethers
in excellent yields and the monoprotection of diols with high selectivity

Table 2. Monotetrahydropyranylation of different diols using acidic ionic liquid (bmim)HSO4
Microwave, yield (%) of ether(s) Ultrasound, yield (%) of ether(s)
Stirring, yield (%) of ether(s)
a
b
a
b
a
b
Method 1
Method 2
Method 1
Method 2
Mono
Method 1
Method 2
Sr. no.
Reactant
Mono
Di
Mono
Di
Mono
Di
Di
Mono
Di
Mono
Di
1
2
3
4
69
68
75
72
25
23
15
18
86
89
97
98
4
5
66
65
72
68
25
23
18
18
85
86
88
87
7
8
5
7
66
65
68
65
29
30
25
28
84
88
85
86
10
7
—
—
8
8
5
55
20
75
12
52
25
70
15
50
32
60
18
6
70
20
92
4
68
22
80
18
64
30
72
25
1
Note: All the compounds were characterized on the basis of H NMR and IR spectral data, which were found to be consistent with that reported in
1
literature. [ H NMR and IR data of product 3 is reported in Ref. 16(ii)].
a
Percentage yields of reactions executed via Method 1 after purification by column chromatography.
b
Percentage yields of reactions executed via Method 2 after purification by column chromatography.

2
898
J. Singh et al.
Figure 1. Comparison of yields from recycled lonic liquid.
via the use of recyclable and catalytic amount of ionic liquid (bmim)HSO4,
which does not require any additives. The notable features of the method
are its operational simplicity, ease of preparation of catalyst, lability with
which the ionic liquid can be removed from the reaction mixture without
contamination, and reduced reaction time and waste products.
General Procedure for Tetrahydropyranylation of Alcohols
Microwave Irradiation
A mixture of alcohol (3 mmol), DHP (3 mmol), and (bmim)HSO (0.1 g,
4
0
.42 mmol) was put in a 10-mL conical flask, covered with a watch glass,
and exposed to MW radiations (160 W) in a domestic microwave oven
LGMS-194A (800 W)] for the specified time. The mixture was cooled and
[
extracted with ether (2 Â 10 mL), and the combined ethereal layer was
passed through a short pad of silica. Evaporation of solvent under reduced
pressure afforded pure protected alcohol (Table 1).
Sonication
The previously mentioned reaction mixture was exposed to ultrasonic
waves using a Branson [B-2200 EI, (60 W, 47 kHz)] ultrasonic cleaning
bath for the requisite time followed by workup to yield the pure product
(Table 1).
Stirring
The reaction mixture previously mentioned was put in a round-bottomed
flask and stirred at room temperature. On completion of the reaction, as
monitored by TLC, the usual workup was carried out to yield the pure
product (Table 1).

Ionic Liquid (bmim)HSO4
2899
General Procedure for Monotetrahydropyranylation of Diols
Method 1: Solvent-Free Conditions
Microwave irradiation. A mixture of 1,n diol (3 mmol), DHP (3 mmol), and
bmim)HSO (0.1 g, 0.42 mmol) in a 10-mL conical was covered with a
(
4
watch glass and irradiated at 160 W for 2 min. The mixture was cooled and
extracted with diethyl ether (2 Â 10 mL); combined organic extracts were
concentrated in vacuo and the resultant mixture of products was separated
via column chromatography. The diether was obtained on eluting with
hexane and monoether with 2% ethylacetate in hexane (Table 2).
Sonication. The reaction mixture was sonicated for 5 min and worked up
as before. Results are given in Table 2.
Stirring. The reaction mixture was stirred at room temperature for 1 h and
worked up as mentioned to yield mono and diethers (Table 2).
Method 2: Biphasic Conditions
Microwave irradiation. To a mixture of 1,n diol (3 mmol) and DHP (3 mmol)
in a 10-mL conical was added (bmim)HSO (0.2 g, 0.85 mmol) and hexane
4
(
1 mL). The conical was covered with a watch glass and irradiated at
60 W for the time specified. On cooling, the hexane layer was decanted,
1
the remaining ionic liquid was extracted with ether (2 Â 10 mL), and the
combined organic layer was evaporated in vacuo to yield the crude product,
which was purified by column chromatography using 2% ethyl acetate in
hexane as eluent to furnish the pure monoprotected alcohol in excellent
yield (Table 2).
Sonication. The reaction mixture was sonicated for the time specified and
worked up as before to yield (after column purification) the pure product
(Table 2).
Stirring. The mixture was stirred at room temperature and, on completion
of reaction, as monitored by TLC, the pure product was obtained after usual
workup and column purification (Table 2).
Recycling of Ionic Liquid
Residual ionic liquid obtained after the workup was stirred with diethyl ether
(10 mL) for 15 min, and the ethereal layer was decanted. The ionic liquid was
dried under reduced pressure and refrigerated under nitrogen for further use.
1
H NMR and IR Data of Selected Compounds
IR spectra were recorded on a Perkin-Elmer R X 1 FT-IR spectrophotometer,
1
and H NMR spectra were recorded on a FT JEOL 300-MHz spectrometer

2
900
J. Singh et al.
with CDCl and CCl as solvent. Chemical shift is reported as ppm downfield
3
4
from Me Si.
4
2-Butoxy-tetrahydro-pyran (product 1, Table 1)
1
H NMR: d 4.54 (s, 1H), 3.42–3.48 (m, 4H), 1.66–1.82 (m, 6H), 1.26–1.45
2
1
.
(
m, 4H), 0.98 (t, 3H). IR (neat) n: 2941, 2860, 1466, 1353, 1121, 1138 cm
6-(Tetrahydro-pyran-2-yloxy)-hexan-1-ol (product 3, Table-2)
1
H NMR: d 4.47 (s, 1H), 3.44–3.69 (m, 6H), 3.24 (s, 1H) D O exchangeable,
2
1
.61–1.78 (m, 6H), 1.32–1.49 (m, 8H). IR (neat) n: 3422, 2938, 2860, 1466,
2
1
1
353, 1121, 1138 cm
.
ACKNOWLEDGMENT
Authors are thankful to CSIR, DST, and UGC, New Delhi, India, for providing
financial assistance.
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