Efficient and convenient procedure for protection of hydroxyl groups to the THP, THF and TMS ethers and oxidation of these ethers to their aldehydes or ketones in [BPy]FeCl4 as a low cost room temperature ionic liquid

Article abstract of DOI:10.1515/znb-2006-0313

Alcohols were converted to the corresponding THP, THF or TMS ethers in high to excellent yields in 1-n-butylpyridinium chloroferrate media as a stable and low cost room temperature ionic liquid. In addition, oxidation of these ethers to their aldehydes or ketones without any overoxidation reactions in this ionic liquid was also performed.

Full text of DOI:10.1515/znb-2006-0313

Efficient and Convenient Procedure for Protection of Hydroxyl Groups
to the THP, THF and TMS Ethers and Oxidation of these Ethers to their
Aldehydes or Ketones in [BPy]FeCl4 as a Low Cost Room Temperature
Ionic Liquid
Ahmad R. Khosropour, Mohammad M. Khodaei, and Sattar Ghaderi
Department of Chemistry, Faculty of Science, Razi University, Kermanshah 67149, Iran
Reprint requests to Dr. A. R. Khosropour. Fax: +98-831-427-4559. E-mail: arkhosropour@razi.ac.ir
Z. Naturforsch. 61b, 326 – 330 (2006); received October 4, 2005
Alcohols were converted to the corresponding THP, THF or TMS ethers in high to excellent yields
in 1-n-butylpyridinium chloroferrate media as a stable and low cost room temperature ionic liquid. In
addition, oxidation of these ethers to their aldehydes or ketones without any overoxidation reactions
in this ionic liquid was also performed.
Key words: Tetrahydropyranylation, Tetrahydrofuranylation, Trimethylsilylation,
n-Butypyridinium Tetrachloroferrate
Introduction
pounds. Although these methods are suitable for many
synthetic conditions, but the practical application of
these methods may suffer from one or more disadvan-
tages such as the use of expensive or less easily avail-
able regents, vigorous reaction conditions, unsatisfac-
tory yields, prolonged standing, and tedious manipu-
lations in isolation of the pure products or the use of
toxic volatile solvents. Therefore, due to the impor-
tance of these compounds a need still exists for versa-
tile, simple, inexpensive and environmentally friendly
processes whereby the protecting groups may be ob-
tained under milder conditions.
Protection of functional groups and one-pot con-
version to the next functional groups play the crit-
ical role in successful synthesis of multifunctional
complex molecules. Among them, tetrahydropyranyl
(THP), tetrahydrofuranyl (THF) and trimethylsilyl
(TMS) ethers are the most versatile protecting forms
of the hydroxyl groups, due to the reasonable sta-
bility to the nonacidic media widely utilized in or-
ganic synthesis [1 – 3]. On the other hand, oxidation
of these ethers to their aldehydes or ketones is an
important transformation in organic chemistry [4, 5].
A variety of reagents have been developed for the
preparation of these compounds which include mainly
Lewis acids, as well as other miscellaneous cata-
Results and Discussion
Room temperature ionic liquids (RTILs) could be
lysts. In this context, Lewis acids such as LiPF4 [6], suitable and environmentally safer replacements for
CuSO4 · 5H2O/CH3CN [7], SO3H-SiO2/CH2Cl2 [8], the volatile, toxic and flammable organic solvents cur-
PdCl2(CH3CN)2 [9], In(OTf)3/CH2Cl2 [10], H6P2 rently used in synthetic and catalytic reactions [20, 21].
W18O62· 24H2O (Wells-Dawson acid) [11], TPP· These solvents possess a number of interesting proper-
HBr/[bmim]PF6 [12], and K5CoW12O40 · 3H2O [13] ties, especially their lack of vapor pressure, a widely
for tetrahydropyranylation, peroxy-λ 3-iodane/ CCl4 accessible temperature range with lack of flammability
[14] for tetrahydrofuranylation, I2 [15], α-Zr(O3 and ease of reuse. Most RTILs are composed of 1,3- di-
PCH3)12(O3PC6H4SO3H)0.8 [16] and LiClO4 [17] alkylimidazolium or 1-n-alkylpyridinium cations and
for trimethylsilylation of hydroxyl groups have been anions such as AlCl4−, BF4−, CF3SO3−, PF6−, and
used. In contrast, only a limited number of reagents (CF3SO2)2N− that cause some of the more impor-
such as Fe(NO3)3-montmorillonite K-10 [18] and tant reactions including alkylation [22], Friedel-Crafts
(CH2=CHCH2PPh3)2S2O8 [19] have been reported for reactions [23], Diels-Alder reactions [24], Suzuki re-
the oxidation of these ethers to their carbonyl com- actions [25], Michael addition [26], etc. But despite
c
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A. R. Khosropour et al. · Formation of TMS, THP and THF Ethers and Oxidation of these Ethers
327
Table 1. Comparison of the effect of catalystsa in the de- Table 2. Protection of alcohols and phenols to the THP and
hydropyranylation of benzyl alcohol in the presence of THF ethers in the presence of CAN-[BPy]FeCl4.
b
[BPy]FeCl4
.
Entry Product (ROY)a
Y
Time (min) Yield(%)b
Entry
Catalyst
Time (h)
4
2
1
1
0.5
1
0.3
1
Yield (%)c
1
C6H5CH2OY
THP
THP
THP
THP
THP
THP
THP
THP
THP
THP
THP
THP
THP
THP
18
25
19
27
29
27
28
40
32
30
30
36
21
30
33
35
23
30
33
38
36
40
30
40
42
40
40
36
45
30
35
33
40
45
30
23
28
30
32
40
96
93
94
92
90
93
94
86
87
90
88
90
91
92
89
93
90
90
85
90
89
92
91
88
89
80
72
74
75
95
93
91
92
90
91
95
80
75
77
80
1
2
3
4
5
6
7
8
–
25
40
63
80
82
67
96
16
60
2
3
4
5
6
7
8
9
2-ClC6H4CH2OY
4-ClC6H4CH2OY
2,4-Cl2C6H3CH2OY
2-BrC6H4CH2OY
4-BrC6H4CH2OY
4-FC6H4CH2OY
2-NO2C6H4CH2OY
4-NO2C6H4CH2OY
furfuryl-OY
Montmorrilonite K-10d
BiCl3
Bi(NO3)3 ·5H2O
Ce(NO3)3 ·xH2O
CeCl3 ·7H2O
CAN
(NH4)6Mo7O24
d
9
a
H2SO4/SiO2
1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
b
c
d
thiophene-2-CH2 OY
4-PhC6H4CH2OY
4-CH3C6H4CH2OY
2,4-(CH3)2C6H3CH2OY
4-(CH3)2CHC6H4CH2OY THP
4-(CH3)3CC6H4CH2OY
4-CH3OC6H4CH2OY
10 mol-%; 50 mol-%; isolated yields; 100% wt.
THP
THP
Scheme 1.
2,4-(CH3O)2C6H3 CH2OY THP
their wide range of synthetic application of TMS, THP
and THF ethers, only little investigation has been per-
formed for synthesis of these ethers in RTIL [12].
However, no progress has been made in using RTILs
for one-pot interconversion of these ethers to their
aldehydes or ketones. Thus the developmentof “green”
and inexpensive synthetic methods towards these pro-
tecting forms constitutes an active area of investiga-
tion in organic chemistry. As part of our program
aimed at developing new selective and environmen-
tally friendly methodologies for the preparation of fine
chemicals, especially research on ionic liquids [27–
29], we found that 1-n-butyl pyridinium chloroferrate
[30] ([BPy]FeCl4) as a low cost RTIL, unlike to many
other ionic liquids (e. g. [BPy]AlCl4), is relatively sta-
ble, easy to handle and insensitive to air and moisture.
Herein, we wish to disclose a simple, efficient and
convenient protocol for the protection of hydroxyl
groups and subsequent interconversionof them to alde-
hydes or ketones in [BPy]FeCl4 media. Initially, we
have studied tetrahydropyranylation and tetrahydro-
furanylation of alcohols and phenols in the presence
of a catalytic amount of cerium(IV) ammonium ni-
trate (CAN) immobilized in [BPy]FeCl4 as a RTIL
(Scheme 1).
C6H5CH=CHCH2OY
C6H5CH(CH3)OY
4-PhC6H4CH(CH3)OY
(C6H5)2CHOY
C6H5CH2CH(CH3)OY
α-tetralyl-OY
(-)-menthyl-OY
C6H5OY
2-CH3C6H4OY
4-C2H5C6H4OY
β-naphthyl-OY
C6H5CH2OY
2-BrC6H4CH2OY
4-(CH3)3CC6H4CH2OY
4-PhC6H4CH2OY
(C6H5)2CHOY
C6H5CH2CH(CH3)OY
thiophene-2-CH2 OY
C6H5OY
THP
THP
THP
THP
THP
THP
THP
THP
THP
THP
THP
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
2-CH3C6H4OY
4-C2H5C6H4OY
β-naphthyl-OY
40
a
The products were identified by comparison of their physical and
spectral data with those of authentic samples or reported in literature;
b
isolated yields.
ing THP or THF ethers in the presence of 10 mol-%
of CAN and 0.5 ml of the ionic liquid at ambient tem-
perature in high to excellent yields. The presence of
electron-donating and electron-withdrawing groups on
the aromatic ring did not make any obvious difference
in the reaction (Table 2, entries 1 – 9, 12 – 18, 26 – 34
and 37 – 40). Aliphatic as well as unsaturated (Table 2,
entry 19) or secondary alcohols (Table 2, 20 – 25, 34
and 35) gave the corresponding THP or THF ethers in
high yields under similar reaction conditions. Tertiary
alcohols such as t-butyl alcohol, however, due to their
In order to investigate the influence of the catalysts
in this reaction, dehydropyranylation of benzyl alco-
hol with DHP was carried out in the presence of dif-
ferent Lewis acids (Table 1). Among all the catalysts
tested, CAN shows more effective catalytic reactivity
in terms of yields and reaction times. As shown in Ta-
ble 2, a wide series of aliphatic and aromatic alcohols
reacted with DHP or DHF to afford the correspond-
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328
A. R. Khosropour et al. · Formation of TMS, THP and THF Ethers and Oxidation of these Ethers
Table 3. Protection of alcohols and phenols to the TMS ethers Table 4. Oxidation of THP, THF and TMS ethers to the cor-
in [BPy]FeCl4.
Entry Product
responding aldehydes or ketones in the presence of PCC-
[BPy]FeCl4.
Time Yield
(min) (%)
Entry
Substrate
Producta
Yield(%)b /
Time (min)
93 / 10
1
C6H5CH2OTMS
8
94
95
89
90
92
88
90
90
94
93
89
87
90
83
89
92
93
90
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
2-ClC6H4CH2OTMS
2,4-Cl2C6H3CH2OTMS
4-BrC6H4CH2OTMS
4-FC6H4CH2OTMS
4-PhC6H4CH2OTMS
4-CH3C6H4CH2OTMS
2,4-(CH3)2C6H3CH2OTMS
4-CH3OC6H4CH2OTMS
2,4-(CH3O)2C6H3 CH2OTMS
citronellyl-OTMS
α-tetralyl-OTMS
(-)-menthyl-OTMS
2-norbornyl-OTMS
2,6,6-trimethylbicyclo[3.1.1]heptan-2-OTMS 14
C6H5OTMS
2-CH3C6H4OTMS
4-BrC6H4OTMS
8
9
8
8
1
2
3
4
5
C6H5CH2OTMS
C6H5CHO
4-ClC6H4CH2OTMS
2,4-Cl2C6H3CH2OTMS
2-NO2C6H4CH2OTMS
4-NO2C6H4CH2OTMS
4-CH3C6H4CH2OTMS
4-ClC6H4CHO
2,4-Cl2C6H3CHO
2-NO2C6H4CHO
4-NO2C6H4CHO
4-CH3C6H4CHO
92 / 15
94 / 8
87 / 13c
90 / 15c
93 / 10
12
9
6
10
6
7
8
2,4-(CH3)2C6H3CH2OTMS 2,4-(CH3)2C6H3CHO 89 / 10
C6H5CH=CHCH2OTMS C6H5CH=CHCHO 85 / 10
12
14
17
13
18
9
furyl-CH2OTMS
(C6H5)2CHOTMS
2-norbornyl-OTMS
(-)-menthyl-OTMS
C6H5CH2OTHP
furfural
(C6H5)2CO
2-norbornanone
(-)-menthone
C6H5CHO
4-ClC6H4CHO
4-FC6H4CHO
2,4-Cl2C6H3CHO
2-NO2C6H4CHO
4-NO2C6H4CHO
4-CH3C6H4CHO
88 / 9
10
11
12
13
14
15
89 / 20c
c
84 / 25
87 / 20c
94 / 9
4-ClC6H4CH2OTHP
4-FC6H4CH2OTHP
91 / 11
89 / 10
92 / 10
88 / 20c
90 / 20c
90 / 10
8
7
9
16 2,4-Cl2C6H3CH2OTHP
17
18
19
2-NO2C6H4CH2OTHP
4-NO2C6H4CH2OTHP
4-CH3C6H4CH2OTHP
a Products were identified by comparison of their physical and spec-
tral data with those of authentic samples or reported in literature;
b
20 2,4-(CH3)2C6H3CH2OTHP 2,4-(CH3)2C6H3CHO 89 / 12
isolated yields.
21 4-CH3OC6H4CH2OTHP 4-CH3OC6H4CHO
91 / 8
92 / 14
92 / 14
88 / 20
89 / 13
90 / 21c
80 / 20c
82 / 25c
87 / 28c
90 / 20
22
2,4-(CH3O)2C6H3-
CH2OTHP
2,4-(CH3O)2-
C6H3CHO
Scheme 2.
23
24
25
26
27
4-PhC6H4CH2OTHP
furyl-CH2OTHP
(C6H5)2CHOTHP
Norbornyl-2-OTHP
(-)-menthyl-OTHP
4-PhC6H4CHO
furfural
(C6H5)2CO
2-norbornanone
(-)-menthone
high sterically hindrance, remained unaffected even
when the reaction mixtures were stirred at room tem-
perature for one day, and the starting materials were
quantitatively recovered. The reaction conditions are
mild enough not to induce any isomerization for conju-
gated alcohols or damage to moieties such as methoxy,
which often undergoes cleavage in strongly acidic re-
action media. Side product formation was not observed
in the reactions we studied. Another advantage of this
promoter system is its recyclability. We found that af-
ter wash◦ing the reaction mixture with Et2O and dried
it at 80 C, CAN immobilized in [BPy]FeCl4 can be
reused in five runs without any loss of activity.
28 C6H5CH2CH(CH3)OTHP C6H5CH2COCH3
29
30
C6H5CH(CH3)OTHP
α-tetralyl-OTHP
C6H5COCH3
α-tetralone
2,4-Cl2C6H3CHO
4-CH3C6H4CHO
c
91 / 22
92 / 20
90 / 25
31 2,4-Cl2C6H3CH2OTHF
32
4-CH3C6H4CH2OTHF
33 4-CH3OC6H4CH2OTHF 4-CH3OC6H4CHO 94 / 18
34
35
4-PhC6H4CH2OTHF
(C6H5)2CHOTHF
4-PhC6H4CHO
(C6H5)2CO
89 / 30
90 / 32c
92 / 35c
36 C6H5CH2CH(CH3)OTHF C6H5CH2COCH3
37 α-tetralyl-OTHF α-tetralone
c
90 / 25
a Products were identified by comparison of their physical and spec-
b
c
tral data with those of authentic samples; isolated yields; in the
presence of of 2 mmol of PCC.
In continuation, we also decided to study the
trimethylsilylation of hydroxyl groups in [BPy]FeCl4
as a RTIL. We undertook an optimization of the re-
action conditions found that hydroxyl groups were
converted to TMS ethers quantitatively in the ab-
sence of any catalyst when a mixture of alcohols and
hexamethyldisilazane (HMDS) was stirred at r. t. in
[BPy]FeCl4 media (Scheme 2). The results are summa-
rized in Table 3. They show the general applicability of
the method for the conversion of a wide range of sub-
stituted and structurally diverse hydroxyl groups such
as primary (Table 3, entries 1 – 11), secondary (Table 3,
Scheme 3.
or phenols (Table 3, entries 16 – 18) to synthesize the
corresponding TMS ethers. Under these moderate con-
ditions, the yields were significantly improved to 83 –
95% and the reaction time was reduced dramatically
(6 – 18 min). On the other hand, reusability of the RTIL
in this reaction was also observed.
In the subsequent investigation, the oxidation reac-
entries 12 – 14) and tertiary alcohols (Table 3, entry 15) tion of these protecting forms of hydroxyl groups was
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A. R. Khosropour et al. · Formation of TMS, THP and THF Ethers and Oxidation of these Ethers
329
also examined. We found that one-pot conversion of [BPy]FeCl4 (0.5 ml)and CAN (5.48 mg, 0.01 mmol), alcohol
THP, THF or TMS ethers to their aldehydes or ke- (1 mmol) and DHP or DHF (1.2 mmol) were added. The re-
tones could be performed efficiently using pyridinium action mixture stirred magnetically at room temperature for
chlorochromate (PCC) in [BPy]FeCl4 media at moder- the appropriate time as shown in Table 2. The reaction was
ate temperature (Scheme 3). To the best of our knowl- followed by TLC or GLC. When the reaction was completed,
edge no report is available in the literature to carry out the mixture was washed with Et2O (3 × 5 ml). The crude
this transformation in RTIL. All the experimental re- products were separated by flash column chromatography
sults are summarized in Table 4.
on silica gel (60 – 120 mesh) using n-heptane/ethyl acetate
The method reported herein is fast and does not in- (3:1) as eluent. The pure products were prepared in 72 – 96%
volve any special reaction conditions. A wide range of yields.
acyclic, alicyclic, and substituted benzylic, phenolic or
General procedure for trimethylsilylation of alcohols: A
heterocyclic THP, THF or TMS ethers could be con- mixture of alcohol (1 mmol) and HMDS (96.6 mg, 0.6 mmol)
verted to their corresponding aldehydes or ketones in in [BPy]FeCl4 (0.5 ml) was stirred at room temperature for
high to excellent yields, and no overoxidation prod- the required time (Table 3). After completion of the reac-
ucts were observed under the reaction conditions (Ta- tion, the reaction mixture was washed with Et2O (3×5 ml).
ble 4, entries 1 – 37). In addition, sterically hindered The organic layer was evaporated under reduced pressure to
secondary alcohols (Table 4, entries 10 – 12, 25– 27 produce the crude product, which was purified by flash col-
and 35) or acid-sensitive primary substrates like cin-
namyl (Table 4, entry 8) or furfuryl (Table 4, entries 9
and 24) ethers were transformed to the corresponding
aldehydes or ketones using the same conditions.
In conclusion, we have established an efficient and
low cost method for tetrahydropyranylation, tetrahy-
drofuranylation and trimethylsilylation of alcohols or
phenols, as well as oxidation of these ethers to their
aldehydes or ketones in [BPy]FeCl4 as an inexpen-
sive and moisture tolerant room temperature ionic liq-
uid. The transformations according to the methods de-
scribed here are fast and occurred in high to excellent
yields at moderate temperature; therefore, they could
be highly useful especially in the total synthesis of nat-
ural products.
umn chromatography over silica gel (60 – 120 mesh) using
n-heptane/ethyl acetate (3:1) as solvent. The pure products
were prepared in 83 – 95% yields.
A typical protocol for the oxidation of benzyl trimethylsil-
yl ether (Table 4, entry 1): A mixture of benzyl trimethylsi-
lyl ether (180 mg, 1 mmol) and PCC (215.5 mg, 1 mmol)
in [BPy]FeCl4 (0.5 ml) was stirred at 50 ◦C for 10 min.
The reaction was monitored by TLC. After the reaction was
complete, H2O (10 ml) was added and the reaction solution
was extracted with Et2O (3×10 ml). The combined organic
phase dried over anhydrous Na2SO4 and concentrated under
reduced pressure. The products were purified by flash col-
umn chromatography over silica gel (60 – 120 mesh) using
n-heptane/ethyl acetate (5:1) to afford benzaldehyde in 93%
yield.
Experimental Section
Acknowledgements
General experimental procedure for tetrahydrofuranyla-
tion and tetrahydropyranylation of alcohols: To a mixture of
We are thankful to Isfahan University and Razi University
Research Councils for partial support of this work.
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