Introduction of a New Ionic Liquid Catalyst for the Trimethylsilyl and Tetrahydropyranyl Protection of Alcohols

Article abstract of DOI:10.1080/10426507.2015.1034311

1,1'-Disulfo-[2,2'-bipyridine]-1,1'-hydrogen sulfate, BiPy(SO₃H)₂(HSO₄)₂, is prepared and identified as a new ionic liquid. This reagent was used for the promotion of the chemoselective trimethylsilyl and tetrahydropyranyl protection of alcohols. All reactions were performed under mild reaction conditions in high to excellent yields. Ease of the preparation of the heterogeneous catalyst, simplicity and easy work-up procedure, high reaction rates, and recyclability and reusability of the catalyst are the main advantages of this method.

Full text of DOI:10.1080/10426507.2015.1034311

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Introduction of a new ionic liquid catalyst for the
trimethylsilyl and tetrahydropyranyl protection of
alcohols
a
a
a
a
Farhad Shirini , Masoumeh Abedini , Nosratollah Mahmoodi , Mohammad Biglari &
a
Mohaddeseh Safarpoor Nikoo Langrudi
a
Department of Chemistry, College of Science, University of Guilan, Rasht 41335, Iran
Accepted author version posted online: 13 May 2015.
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To cite this article: Farhad Shirini, Masoumeh Abedini, Nosratollah Mahmoodi, Mohammad Biglari & Mohaddeseh Safarpoor
Nikoo Langrudi (2015): Introduction of a new ionic liquid catalyst for the trimethylsilyl and tetrahydropyranyl protection of
alcohols, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI: 10.1080/10426507.2015.1034311
To link to this article: http://dx.doi.org/10.1080/10426507.2015.1034311
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INTRODUCTION OF A NEW IONIC LIQUID CATALYST FOR THE
TRIMETHYLSILYL AND TETRAHYDROPYRANYL PROTECTION OF ALCOHOLS
Farhad Shirini,* Masoumeh Abedini, Nosratollah Mahmoodi, Mohammad Biglari, Mohaddeseh
Safarpoor Nikoo Langrudi
Department of Chemistry, College of Science, University of Guilan, Rasht 41335,
Iran. E-Mail: shirini@guilan.ac.ir; Fax: +98 131 3220066
Abstract: 1,1'-Disulfo-[2,2'-bipyridine]-1,1'-hydrogen sulfate, BiPy(SO
3
H)
2
(HSO
4
2
) , is prepared
and identified as a new ionic liquid. This reagent was used for the promotion of the
chemoselective trimethylsilyl (TMS) and tetrahydropyranyl (THP) protection of alcohols. All
reactions were performed under mild reaction conditions in high to excellent yields. Ease of the
preparation of the heterogeneous catalyst, simplicity and easy work-up procedure, high reaction
rates and recyclability and reusability of the catalyst are the main advantages of this method.
HSO4 SO H
SO H
3
Cl
N
3
N
N
2
ClSO H
2 H SO
2 4
3
Dry CH Cl2
Ice bath, 2 h
2
Dry CH Cl2
2
N
N
Ice bath, 4 h
N
HO S HSO
HO3S Cl
3
4
1
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BiPy(SO H) (HSO ) ( 10 mg)
3
2
4 2
ROH
ROH
ROTMS
ROTHP
HMDS (0.75 mmol)
CH CN, r.t.
3
BiPy(SO H) (HSO ) ( 10 mg)
3
2
4 2
DHP (1.4 mmol)
CH Cl , r.t.
2
2
Keywords: BiPy(SO
3
H)
2
(HSO
4
)
2
; Ionic liquid; Trimethylsilylation; Tetrahydropyranylation;
Chemoselective reactions.
2
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Introduction
Protection of the hydroxyl group during a multi-step synthetic process as an important method,
[1]
has attracted the attention of many organic chemists. Between the several methods available
for the protection of this functional group, trimethylsilylation and tetrahydropyranylation are
selected as the most important ones. Although appropriate commercially available reagents (such
as HMDS and DHP) exist for each types of the protections, the poor silylating and
tetrahydropyranylating power of these reagents is considered as the main drawback for the
application of these materials. To overcome this restriction, several catalysts have been reported
[2-11]
[12-22]
for the preparation of TMS
and THP
ethers. Although these procedures are an
[
11,19]
improvement, most of them suffer from disadvantages such as long reaction times,
forceful
low selectivity,
Therefore, introduction of efficient
[22]
[7, 16, 18, 19, 21, 22]
reaction conditions,
use of large amounts of the catalyst,
[2, 4, 5]
tedious work-up and use of toxic or expensive reagents.
and economical catalysts that solve these drawbacks is still under condensation.
Ionic liquids (ILs) are compounds with unique properties such as non-flammability,
negligible vapor, reasonable thermal and chemical stability and the ability to use many times
without considerable decrease in their activity. Because of these important characteristics, use of
ILs in organic reactions is under considerable attention of many organic chemists. These types
[
23-27]
of compounds were successfully used in the acceleration of various organic reactions.
In the past decade a considerable part of our research program is focused on the development
of new methods and use of new reagents for the protection of the hydroxyl groups in organic
[
28-31]
compounds.
3
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Recently, preparation and application of ionic liquids in organic reactions became an
[
32-35]
important part of our ongoing research program.
studies, we wish to report the preparation and characterization of 1,1'-disulfo-[2,2'-bipyridine]-
,1'-hydrogen sulfate, BiPy(SO H) (HSO , as an efficient reusable ionic liquid and its
Herein and in continuation of these
1
3
2
)
4 2
applicability in the promotion of the chemoselective trimethylsilylation and
tetrahydropyranylation of alcohols. All reactions are performed under mild reaction conditions in
high to excellent yields.
After preparation and characterization, we were interested to investigate the efficiency of the
prepared reagent in the promotion of the reactions which need the acidic catalysts to speed-up
which of them the protection reactions are the most important ones.
Initially, and after preparation and characterization, we decided to explore the synthesis of
silyl ethers, using BiPy(SO
chlorobenzyl alcohol with HMDS (0.7 mmol)) was carried out using different polar and nonpolar
solvents (CH Cl , n-hexane, EtOH and CH CN) and also solvent-free conditions at room
temperature in the presence of various amounts of the catalyst. The best results were obtained
when 10 mg (1.95 mol %) of the ionic liquid was used in CH CN (Table 1, entry 3).
3
H)
2
(HSO
4
)
2
as the catalyst. The model reaction (silylation of 4-
2
2
3
3
After optimization of the reaction conditions and in order to show the general applicability of
this method, different types of alcohols were subjected to the same reaction under the determined
conditions (Table 2, Scheme 1). The obtained results clearly show that different types of
aromatic, aliphatic and allylic alcohols were silylated in the presence of catalytic amounts of this
ionic liquid in high to excellent yields in very short times (Table 2, entries 1-22).
4
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Under the selected conditions hindered tertiary alcohols, amines and thiols remain unchanged
so the selective silylation of alcohols in the presence of these compounds can be carried out
successfully (Table 2, entries 23, 27, 28).
After the successful use of BiPy(SO
3
H)
2
(HSO
4
2
) in the trimethylsilylation reactions , we
decided to study the preparation of tetrahydropyranyl ethers in the presence of this reagent.
In order to optimize the reaction conditions the protection of 4-chlorobenzyl alcohol with
DHP (1.4 mmol) in different solvents and / or in the absence of solvent in the presence of
different amounts of BiPy(SO
0 mg (1.95 mol%) of the catalyst was used in CH
After optimization of the reaction conditions and in order to show the applicability of the
3
H)
2
(HSO
4
)
2
was studied and the best results were obtained when
1
2
Cl at room temperature (Scheme 1).
2
proposed method, different types of alcohols were subjected to the same reaction and the results
are tabulated in Table 3.
Obviously, the present method is quite general and very effective for the
tetrahydropyranylation of different types of alcohols but it seems that the rates of the reaction of
alcohols with electron-withdrawing groups or sterically hindered ones were slower than the other
types of alcohols (Table 3, entries 6, 7, 20).
In order to show the efficiency of the present method, we have compared some of our results
obtained from the silylation and tetrahydropyranylation of alcohols catalyzed by this reagent
with the other results reported in the literature (Table 4).
5
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It should be noted that, although some of the compared methods are completed in shorter
times in the presence of lower amounts of the catalysts (Table 4, entries 1, 2), preparation of
them is difficult with respect to BiPy(SO
BiPy](HSO Cl , as the starting material of BiPy(SO
same reactions, it is not reusable at all (Table 4, entries 9, 21).
3
H)
2
(HSO
4
)
2
. The other important point is that, although
[
3
)
2
2
3
H) (HSO 2, is an efficient catalyst for the
2
4
)
In continuation, we decided to study the catalytic activity of the recycled catalyst for the
synthesis of trimethylsilyl and tetrahydropyranyl ethers. We have found that
BiPy(SO
3
H)
2
(HSO
4
2
) is a very stable catalyst and can be recycled by filtration, washing with
diethyl ether and drying at 50 °C. For the reaction of 2-bromobenzyl alcohol with HMDS or
DHP, even after four cycles, the catalyst still has excellent yields (Figure S 1, Supplemental
Materials). It implies that BiPy(SO
3
H)
2
(HSO
4
2
) can be reused without appreciable loss of its
activity. It is very important to note that the FT-IR of the recovered catalyst was the same as the
freshly prepared one (Figure S 2, Supplemental Materials).
In conclusion, we have prepared, identified and applied BiPy(SO
3
H)
2
(HSO ) ionic liquid as a
4
highly efficient catalyst for the promotion of the acceleration of the chemoselective
trimethylsilylation and tetrahydropyranylation of alcohols under mild conditions. Ease of the
preparation and handling of the catalyst, simple procedure and easy work-up, high reaction rates,
excellent yields of the products and reusability of the catalyst are the most important advantages
of this method, which make this procedure a useful and attractive addition to the available
methods.
6
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EXPERIMENTAL
General: All chemicals were purchased from Merck, Aldrich and Fluka Chemical Companies
and used without further purification. Products were characterized by their physical constants
and comparison with authentic samples. Products were separated and purified by different
chromatographic techniques (TLC using silica gel SIL G/UV 254 plates) and were identified by
the comparison of their IR, NMR and refractive index with those reported for the authentic
samples. All yields refer to the isolated products.
Instrumentation: The IR spectra were recorded on a Perkin Elmer 781 Spectrophotometer. In
1
all the cases the H NMR spectra were recorded with Bruker Avance 400 and 500 MHz
instruments. All chemical shifts are quoted in parts per million (ppm) relative to TMS using
1
3
deuterated solvent. The C NMR data were collected on Bruker Avance 400 MHz instrument.
Melting points were recorded on a Büchi B-545 apparatus in open capillary tubes.
Preparation of BiPy(SO
reported method in the literature. Then H
to a round-bottomed flask (10 mL) containing [BiPy](HSO
3
H)
2
(HSO
4
)
2
: [BiPy](HSO
SO (0.5 mL, 10 mmol, 98 %) was added dropwise
Cl (3.17 g) in dry CH Cl (50 mL),
3
)
2
Cl was prepared according to the
2
[
36]
2
4
3
)
2
2
2
2
over a period of 2 min in an ice bath. After the addition was completed, the reaction mixture was
stirred for 4 h, and then stood for 3 min, and the solvent was decanted. The residue was washed
with dry diethylether (3 × 10 mL) and dried under vacuum to give BiPy(SO
white solid in 90% yield (Scheme 2).
3
H)
2
(HSO
4
2
) as a
Spectroscopic data for BiPy(SO
3
H)
2
(HSO
4
)
2
are as: M.p: 86-88 °C; MS : 64, 80, 98, 156, 510
−
1
1
m/z; FT-IR (KBr, cm )
max: 3500-2500, 1281, 1175, 882, 856. H NMR (400 MHz, DMSO-d
6
):
7
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= 7.92 (1H, ddd, J
1H, s), 8.80 (1H, s), 8.91 (1H, ddd, J
) δ = 147.8, 147.0, 143.4, 127.7, 124.2 ppm (Figures S 3 and S 4)
1
=8, J
2
= 5.2, J
3
=1.2), 8.46 (1H, td, J
1
=8, J =1.6), 8.70 (1H, d, J=8), 8.85
2
1
3
(
1
=5.2, J =1.6, J =0.8) ppm; C NMR (100 MHz, DMSO-
2
3
d
6
General procedure for the protection of alcohols: A mixture of the substrate (1 mmol),
hexamethyldisilazane (0.70 mmol) and / or 3,4-dihydro-2H-pyran (1.4 mmol) and
BiPy(SO
at room temperature. The progress of the reaction was monitored by TLC (n-hexane: EtOAc;
0:1) and/ or GC. After completion of the reaction, the mixture was filtered to separate the solid
catalyst. Then the solution was filtered through a silica gel pad and washed with CH CN (2 × 3
(2 × 3 mL). Evaporation of the solvent gave the desired products in high
3
H)
2
(HSO
4
)
2
(10 mg, 1.95 mol%) in CH
3
CN (3 mL) and / or CH
2
Cl
2
(3 mL) was stirred
1
3
mL) and / or CH
2
Cl
2
purity.
ACKNOWLEDGEMENTS
The authors acknowledge financial support for this work from the Iran National Science
Foundation (INSF) (Grant number 93014856) and partial support of this work by the Research
Council of the University of Guilan.
8
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1
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Table 1 Reaction of 4-chlorobenzyl alcohol with HMDS (0.7 mmol) in different conditions at
room temperature.
a
BiPy(SO
3
H)
2
(HSO ) (mg)
4
Entry
Solvent
Time (min)
Yield (%)
b
1
2
3
4
5
6
7
10
10
10
10
10
5
Solvent-free
n-Hexane
30
40
50
50
98
0
0
80
97
c
c
CH
CH
EtOH
3
CN
Immediately
2
Cl
2
20
30
15
CH
CH
3
CN
CN
15
3
Immediately
a
b
c
GC yield.
1.95 mol%.
Immediately means < 2 min.
1
2
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Table 2 Silylation of alcohols in the presence of BiPy(SO
3
H)
2
(HSO
4
2
) .
a
b
c
Entry Product
Time (min)
Immediately
Immediately
Immediately
Immediately
Immediately
10
Yield (%)
89
1
2
3
4
5
6
7
8
9
PhCH
2-ClC
4-ClC
3,4-Cl
2-BrC
2
6
6
OSiMe
3
H
H
4
CH
CH
2
OSiMe
OSiMe
CH
OSiMe
CH
CH
3
95
89
95
95
92
93
93
90
88
90
94
93
90
90
93
4
2
3
2
C
6
H
3
2
OSiMe
3
6
H
4
CH
2
3
4-NO
2-NO
2
C
C
6
H
H
4
2
OSiMe
OSiMe
3
2
6
4
2
3
20
2-MeC
4-Me CHC
3-MeOC
4-MeOC
2-OHC
Ph CHOSiMe
6
H
4
CH
2
OSiMe
CH OSiMe
3
Immediately
Immediately
Immediately
3
Immediately
5
2
6
H
4
2
3
1
0
1
2
3
4
5
6
6
H
4
CH
CH
2
OSiMe
OSiMe
OSiMe
3
1
1
1
1
1
1
6
H
4
2
3
6
H
4
CH
2
3
2
3
PhCH(Me)CH
2
OSiMe
OSiMe
CH OSiMe
3
4
PhCH
PhCH
2
CH
CH
2
3
Immediately
Immediately
2
2
2
3
OSiMe3
17
4
92
OSiMe3
1
1
8
9
3
3
94
90
CH OSiMe3
2
OSiMe3
OSiMe3
2
2
0
1
10
3
91
94
2
2
2
3
C
6
H
5
CH=CHCH
2
OSiMe
OSiMe3
3
Immediately
Immediately
91
90
N
H
2
2
4
5
Ph
PhSSiMe
3
COSiMe
3
60
60
0
0
3
1
3
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2
2
6
7
PhNHSiMe
3
60
0
100
+
2
-ClPhCH OSiMe3
2
d
Immediately
+
Ph CHOSiMe3
3
d
0
4
-NO PhCH OSiMe
3
2
2
d
28
100
+
10
+
PhSSiMe3
d
0
a
Products were characterized by their physical constants and compared with the other results
reported in the literature ( Ref.: 8, 9).
b
Immediately means < 2 min.
c
Isolated yields.
d
GC yield.
1
4
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Table 3 Tetrahydropyranylation of alcohols in the presence of BiPy(SO
3
H)
2
(HSO
4
2
) .
a
b
c
Entry Product
Time (min)
4
Yield (%)
94
1
2
3
4
5
6
7
8
9
PhCH OTHP
2
2-ClC
4-ClC
6
H
H
4
CH
CH
2
OTHP
OTHPTHP
Immediately
Immediately
93
93
92
95
85
93
92
90
91
98
94
89
90
89
94
6
4
2
3,4-Cl
2-BrC
2
C
6
H
3
CH
2
OTHP
3
6
H
4
CH OTHP
2
Immediately
4-NO
2-NO
2
C
6
H
H
4
CH
CH
2
OTHP
OTHP
18
25
3
2
C
6
4
2
2-MeC
4-Me CHC
3-MeOC
4-MeOC
2-OHC
Ph CHOTHP
PhCH(Me)CH
6
H
4
CH
2
OTHP
CH OTHP
2
6
H
4
2
Immediately
1
0
1
2
3
4
5
6
6
H
4
CH
CH
2
OTHP
OTHP
7
1
1
1
1
1
1
6
H
4
2
Immediately
6
H
4
CH
2
OTHP
8
3
3
2
2
OTHP
OTHP
CH OTHP
PhCH
PhCH
2
CH
CH
2
Immediately
Immediately
2
2
2
OTHP
Immediately
17
91
OTHP
1
1
8
9
5
5
94
91
CH OTHP
2
OTHP
2
2
2
0
1
2
13
6
93
94
90
OTHP
C
6
H
5
CH=CHCH
2
OTHP
4
a
Products were characterized by their physical constants and compared with the other results
reported in the literature ( Ref.: 18, 21).
b
Immediately means < 2 min.
1
5
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c
Isolated yields.
1
6
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Table 4 Comparison of some of the results of the silylation and tetrahydropyranylation of benzyl
alcohols catalyzed by BiPy(SO
catalysts.
3
H)
2
(HSO
4
2
) with those obtained by some of the reported
a
[Ref.]
Entry Catalyst (mol %)
Time (min)
Yield (%)
100
100
98
IV
1
2
3
4
5
6
7
8
9
Ti (salophen)(OTf)
2
(1)
]@CMP (1)
Immediately
Immediately
4
5
IV
[Sn (TNH
2
PP)(OTf)
(2)
SANM (7.65)
[bmim][BF4] (1 mL)
[Msim]HSO ( 2)
2
[Dsim]HSO
4
Immediately
29
28
7
5
95
30
96
4
1
96
8
[py][Tfa] (0.2)
1
100
100
98
9
[SipIm]Cl (0.13)
1
9
[BiPy](HSO
3
2
)
2
Cl
2
(1.3)
(1.95)
Immediately
This
work
10
BiPy(SO H)
3
(HSO
4
)
2
Immediately
98
This
work
1
1
1
1
1
1
1
1
1
2
2
1
2
3
4
5
6
7
8
9
0
1
Solid silica-based sulfonic acid (2)
15
92
97
100
98
85
90
94
90
98
92
95
39
12
13
15
16
18
19
20
21
22
IV
[Sn (TNH
2
PP)(OTf)
2
]@CMP (1)
4
3
IV
[V (TPP)(OTf)
SANM (3.06)
2
] (1)
5
5
[C
[morH]HSO
[BMIm][HSO
[Et N(CH SO
[Hmim]HSO
4
mim][InCl
(10)
] (20)
H][OTs] (2)
4
] (25)
4
3
4
15
3
3
2
)
4
3
4
(25)
(14)
2
(bmim)HSO
[BiPy](HSO
4
1.5
4
3
)
2
Cl
2
(2.57)
(1.95)
This
work
2
2
BiPy(SO
3
H)
2
(HSO
4
)
2
4
94
This
work
a
Immediately means < 2 min.
1
7
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Table 5 Calculation of the Hammett acidity function (H
0
) for BiPy(SO
3
H)
2
(HSO
4 2
) .
+
Entry
1
2
Catalyst
—
BiPy(SO
A
2.2261
0.478
max
[I]
100
21.5084
s
%
[IH ]
0
s
%
H
0
—
0.4278
3
H)
2
(HSO
4
)
2
78.4916
Condition for UV–visible spectrum measurement: solvent: CCl
4
, indicator: 4- nitroaniline (pK
−
4
(
I)aq= 0.99), 1.44×10 mol/L (10 mL); Catalyst: BiPy(SO
3
H)
2
(HSO
4
2
) (10 mg), 25 °C.
1
8
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HMDS (0.75 mmol)
ROTMS
ROTHP
CH CN, r.t.
3
BiPy(SO H) (HSO )
4 2
3
2
ROH
(
10 mg, 1.95 mol%)
DHP (1.4 mmol)
CH Cl , r.t.
2
2
SCHEME 1
1
9
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HSO4 SO H
SO H
3
Cl
N
3
N
N
2
ClSO H
2 H SO
2 4
3
Dry CH Cl2
Ice bath, 2 h
2
Dry CH Cl2
2
N
N
N
Ice bath, 4 h
HO S HSO
HO3S Cl
3
4
Scheme 2
2
0
ACCEPTED MANUSCRIPT