Bronsted acidic ionic liquids: Fast, mild, and efficient catalysts for solvent-free tetrahydropyranylation of alcohols

Article abstract of DOI:10.1081/SCC-200064995

Bronsted acidic ionic liquids as efficient and reusable catalysts for the protection of alcohols as tetrahydropyranyl (THP) ethers under solvent-free conditions at room temperature was investigated. Good to excellent yields were obtained at a faster rate over [BMIm][HSO₄] or [BMIm][H₂PO₄]. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1081/SCC-200064995

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Brønsted Acidic Ionic Liquids:
Fast, Mild, and Efficient
Catalysts for Solvent‐Free
Tetrahydropyranylation of
Alcohols
a
a
a
Zhiying Duan , Yanlong Gu & Youquan Deng
a
Center for Green Chemistry and Catalysis, Lanzhou
Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou, China
Published online: 16 Aug 2006.
To cite this article: Zhiying Duan , Yanlong Gu & Youquan Deng (2005) Brønsted
Acidic Ionic Liquids: Fast, Mild, and Efficient Catalysts for Solvent‐Free
Tetrahydropyranylation of Alcohols, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 35:14, 1939-1945,
DOI: 10.1081/SCC-200064995
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Synthetic Communications , 35: 1939–1945, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200064995
Brønsted Acidic Ionic Liquids: Fast, Mild,
and Efficient Catalysts for Solvent-Free
Tetrahydropyranylation of Alcohols
Zhiying Duan, Yanlong Gu, and Youquan Deng
Center for Green Chemistry and Catalysis, Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences, Lanzhou, China
Abstract: Brønsted acidic ionic liquids as efficient and reusable catalysts for the pro-
tection of alcohols as tetrahydropyranyl (THP) ethers under solvent-free conditions at
room temperature was investigated. Good to excellent yields were obtained at a faster
4 2 4
rate over [BMIm][HSO ] or [BMIm][H PO ].
Keywords: Alcohols, Brønsted acidic ionic liquids, tetrahydropyranylation
Protection of alcohols as tetrahydropyranyl (THP) ethers is useful in
organic synthesis mainly because of their stability and compatibility under
various reaction conditions and reagents. Apart from the commonly used
[
1]
protic and Lewis acids, other reagents such as iodotrimethylsilane,
[
2]
[3]
Ru(CH ) -(triphos)](OTf) ,
montmorillonite K-10,
zirconium sulfo-
[7]
phenyl phosphonate, LiClO4, polyaniline salts, vanadyl(IV) acetate,
3
3
2
[
4]
[5]
[6]
[
8–10]
[11]
[12]
zeolites,
In(OTf)3,
heteropoly acids,
[14]
and more recently SO H–SiO ,
3 2
[13]
[15]
Ps-AlCl3,
and PdCl (CH CN)
2
have also been reported
2
3
in the preparation of THP ethers. However, some have limitations such as
use of high reaction temperatures, tedious and time-consuming workup
Received in Poland March 10, 2005
Address correspondence to Youquan Deng, Center for Green Chemistry and
Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000, China. Fax: þ86-931-4968116; E-mail: ydeng@ns.lzb.ac.an
1
939

1940
Z. Duan, Y. Gu, and Y. Deng
procedures, longer reaction times, and nonreusability. Therefore, an efficient
and practical method for these transformations is desired.
Brønsted acidic ionic liquids, which are air and moisture stable, represent
one class of potential new acid materials and have been shown to be effective
[16–18]
catalysts for a wide range of acid-catalyzed reactions,
such as esterifica-
tion, Mannich reaction, and olefins oligomerization. More recently, it was
found that silica gel confined functional acidic ionic liquids are effective
[
19]
catalysts for deoximation
of aldehydes.
under mild conditions as well as acetalization
As a part of ongoing research on acidic ionic liquid in the
[20]
clean synthesis, we present herein Brønsted acidic ionic liquid as a highly
effective and reusable catalyst for tetrahydropyranylation of alcohols.
A series of Brønsted acidic ionic liquids (Figure 1) were synthesized by
[
17,19,21]
the methods in previous papers
and initially were tested in the tetrahy-
dropyranylation of n-hexylxyol as shown in Table 1. It was noted that ionic
liquid is miscible with alcohol and immiscible with the product; the ionic
liquid catalyst could be easily recovered by decantation. In case of
[BMIm][HSO ], the reaction went to complete conversion in 30 min with
very high selectivity to THP ether (99%) when n-hexylxyol is used as
4
probe. After the reaction was complete, [BMIm][HSO ] was recycled and
4
reused. In the third cycle the yield was comparable to that of fresh one. Satis-
factory results are obtained in the present of [BMIm][H PO ], in which the
activity of [BMIm][H PO ] is almost equal to that of [BMIm][HSO ].
2
4
2
4
4
[
HMIm]TSO also performed very well as a catalyst in this reaction, but
its activity seems to be slightly inferior as compared with that of
BMIm][HSO ] or [BMIm][H PO ] because the reaction time has to
[
4
2
4
prolonged to 4 h for the complete conversion of alcohol. [HMIm]BF and
4
[
AcMIm]Cl are capable of catalyzing the reaction with a yield of 71% and
8%, respectively. Attempts to perform the reaction with [HMIm]Tfa
1
Figure 1. Series of Brønsted acidic ionic liquids were synthesized.

Brønsted Acidic Ionic Liquids
1941
Table 1. Tetrahydropyranylation of n-hexylxyol over different Brønsted acidic ionic
a
liquids
Time
(h)
Yield
b
(%)
Time
(h)
Yield
b
(%)
Catalyst
Catalyst
[
[
[
[
HMIm]BF
BMIm][H PO ]
4
4
71
96
97
0
[BMIm][HSO
[(CH ) SO HMIm]OTf
4
]
0.5
0.1
4
96
—
18
0.5
4
2
4
2 4
3
HMIm]TSO
HMIm]Tfa
[AcMIm]Cl
[BMIm][HSO
c
4
4
]
0.5
94
a
Reaction conditions: n-hexylxyol: 15 mmol, 3,4-dihydro-2H-pyran: 18 mmol;
catalyst: 3 mmol, 208C.
b
Isolated yields, n-hexylxyol alcohol tetrahydropyranyl ether, colouless oil, bp (8C)/torr:
[8]
[23]
1
28–130/25 (Lit. 130/25 , 105–110/22 ).
c
Three times reused.
and [(CH ) SO HMIm]OTf have completely failed; the former has no activity
3
2
4
and the latter is unstable with the substrates. In view of the foregoing,
[
BMIm][HSO ] was chosen for the experiment.
4
A wide variety of hydroxylated compounds are readily converted to the
corresponding THP ethers using [BMIm][HSO ] as catalyst via this
4
procedure and the results are listed in Table 2. The reactions are reasonably
fast, even with a bulky molecule such as benzhydrol. It is important to note
that an acid-sensitive alcohol such as tert-butanol and sec-butanol undergo
protection as tetrahydropyranyl ether without the formation of a dehydration
product. The [BMIm][HSO ] system proved to be very active toward other
4
substrates such as primary, secondary, benzylic, allylic, and acetylenic
alcohols, leading to high yields even at room temperature. The low acid
strength of the catalyst and mild reaction conditions have not affected
double or triple bonds during the reaction of allylic and acetylenic alcohols.
Furfuryl alcohol and tetrahydrofurfuryl alcohol in which the ether bonds are
readily cleaved under acidic conditions received exclusive selectivities.
In conclusion, our protocol provides a simple and effective method for
tetrahydropyranylation of alcohols using Brønsted acidic ionic liquids
as catalysts. It is also worthy to note that the ionic liquid catalyst could
be easily recovered and reused. Enhanced reaction rates, easy workup,
and mild and solvent-free conditions are also the features of this catalytic
system.
EXPERIMENTAL
Melting points were taken on a hot-plate microscope apparatus and were not
1
corrected. H NMR was measured on a Bruker 300-MHz spectrometer in

1942
Z. Duan, Y. Gu, and Y. Deng
a
4
Table 2. Tetrahydropyranylation of alcohols catalyzed by [BMIm][HSO ]
Time
(min)
Yield
b
(%)
Alcohols
Bp (8C)/torr
109/100
Bp (8C)/torr
[
8]
3
3
0
0
96
92
58–59/10
[
3]
62/33
65/38
[
8]
5
95
90
125/10
125/10
[
8]
3
0
54–56/10
55/10
[
8]
3
1
0
0
77
98
150–155/25
122–128/4
152–153/25
[
8]
130/5
[
8]
1
5
5
0
97
94
95
75/22
80/25
78/33
76–77/25
[
24]
67/10
[
26]
3
75.5/43
[
25]
3
0
90
57 (mp)
55–57
(mp)
[
2]
1
3
5
0
94
91
105/5
80/1
105/4
1
[
8]
05/5
[
23]
71–80/1
[
22]
3
3
0
0
86
95
129/1
125–130/1
[
3,24]
100–102/4
100/4
a
Reaction conditions: alcohol, 15 mmol; 3, 4-dihydro-2H-pyran, 18 mmol; catalyst,
mmol; temperature, 208C.
Yields refer to pure isolated products based on alcohol.
3
b

Brønsted Acidic Ionic Liquids
1943
CDCl with TMS as internal standard. All yields refer to isolated products
3
after purification. All the tetrahydropyranyl ethers prepared are known
compounds and were characterized by comparison of their physical and
[3,8,22–26]
spectral data with those reported in the literature
samples.
or authentic
General Procedure
Typical procedure of tetrahydropyranylation: Acidic ionic liquid (3 mmol)
was added dropwise to a stirred mixture of an alcohol (15 mmol) and 3,
4
-dihydro-2H-pyran (18 mmol) in a reaction flask at room temperature
208C). The progress of the reaction was monitored by Hewlett-Packard
890/5793 GC/MS. After completion of the reaction, the products are immis-
(
6
cible with ionic liquid and the catalyst was recovered from the mixture by
decantation. The upper liquid was dried over anhydrous Na SO and then
2
4
evaporated to give an almost pure product. Further purification was
achieved through a short column of silica gel (20 cm in length, 0.5 cm in
diameter) with petroleum ether to obtain the pure tetrahydropyranyl ether.
Benzyl alcohol tetrahydropyranyl ether: Colorless oil (2.72 g, yield 94%).
H NMR (CDCl , 300 MHz, dppm): 1.55–1.79 (m, 6H), 3.56–3.62 (m, 1H),
1
3
3
.92–3.99 (m, 1H), 4.53 (d, J ¼ 12.0 Hz, 1H), 4.75 (t, J ¼ 3.3 Hz, 1H), 4.82
d, J ¼ 12.0 Hz, 1H), 7.35–7.42 (m, 5H).
-Phenylethanol tetrahydropyranyl ether: Colorless oil (2.80 g, yield
(
2
1
9
1%). H NMR (CDCl , 300 MHz, dppm): 1.52–1.85 (m, 6H), 2.78–2.85
3
(
m, 2H), 3.56–3.62 (m, 1H), 3.89–3.95 (m, 1H), 4.02–4.06 (m, 1H), 4.20–
.25 (m, 1H), 4.78 (t, J ¼ 3.3 Hz, 1H), 7.20-7.28 (m, 5H).
4
Cyclohexanol tetrahydropyranyl ether: Colorless oil (2.11 g, yield 77%).
H NMR (CDCl , 300 MHz, dppm): 1.15–2.01 (m, 16H), 3.48–3.53 (m, 1H),
1
3
3
.50–3.56 (m, 1H), 3.90–3.96 (m, 1H), 4.72 (t, J ¼ 3.3 Hz, 1H).
Allyl alcohol tetrahydropyranyl ether: Colorless oil (2.04 g, yield 97%).
H NMR (CDCl , 300 MHz, dppm): 1.55–1.98 (m, 6H), 3.48–3.54 (m, 1H)
1
3
d, 3.86–3.92 (m, 1H), 3.98–4.05 (m, 1H), 4.20–4.27 (m, 1H), 4.70
(
Cinnamyl alcohol tetrahydropyranyl ether: Yellowish oil (2.81 g, yield
t, J ¼ 3.3 Hz, 1H), 5.18–5.30 (m, 2H), 5.88–5.97 (m, 1H).
1
6%). H NMR (CDCl , 300 MHz, dppm): 1.45–1.85 (m, 6H), 3.52–3.58
8
3
(
m, 1H), 3.88–3.95 (m, 1H), 4.28–4.37 (m, 2H), 4.76 (t, J ¼ 3.3 Hz, 1H),
.20–6.31 (m, 1H), 6.64 (d, J ¼ 14.4 Hz, 1H), 7.26–7.45 (m, 5H).
6
ACKNOWLEDGMENTS
This work was financially supported by National Natural Science Foundation
of China (No. 20225309).

1944
Z. Duan, Y. Gu, and Y. Deng
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