New applications of solid silica chloride (-SiO2Cl): Efficient oxidation of cyclic thioacetals, TMS, TBDMS, and THP ethers to their carbonyl compounds by solid silica chloride/KMnO4 system

Article abstract of DOI:10.1080/10426500490262531

Solid silica chloride/KMnO₄ system oxidizes effectively aromatic and aliphatic cyclic thioacetals (1,3-dithiolanes, 1,3-dithianes) and silyl and pyranyl ethers into their corresponding carbonyl compounds in dry CH₃CN at room temperature in high yields. Over-oxidation of aldehydes to carboxylic acids has not been observed by this system. In the absence of silica chloride, KMnO₄ in dry CH₃CN is an ineffective reagent for these oxidations.

Full text of DOI:10.1080/10426500490262531

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NEW APPLICATIONS OF SOLID SILICA CHLORIDE
(─SiO2Cl): EFFICIENT OXIDATION OF CYCLIC
THIOACETALS, TMS, TBDMS, AND THP ETHERS TO THEIR
CARBONYL COMPOUNDS BY SOLID SILICA CHLORIDE/
KMnO4 SYSTEM
Habib Firouzabadi a , Hassan Hazarkhani a & Hassan Hassani a
a Shiraz University , Shiraz, Iran
Published online: 11 Aug 2010.
To cite this article: Habib Firouzabadi , Hassan Hazarkhani & Hassan Hassani (2004) NEW APPLICATIONS OF SOLID SILICA
CHLORIDE (─SiO2Cl): EFFICIENT OXIDATION OF CYCLIC THIOACETALS, TMS, TBDMS, AND THP ETHERS TO THEIR CARBONYL
COMPOUNDS BY SOLID SILICA CHLORIDE/KMnO4 SYSTEM, Phosphorus, Sulfur, and Silicon and the Related Elements, 179:2,
403-409, DOI: 10.1080/10426500490262531
To link to this article: http://dx.doi.org/10.1080/10426500490262531
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Phosphorus, Sulfur, and Silicon, 179:403–409, 2004
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Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490262531
NEW APPLICATIONS OF SOLID SILICA CHLORIDE
( SiO2Cl): EFFICIENT OXIDATION OF CYCLIC
THIOACETALS, TMS, TBDMS, AND THP ETHERS TO
THEIR CARBONYL COMPOUNDS BY SOLID SILICA
CHLORIDE/KMnO4 SYSTEM
Habib Firouzabadi, Hassan Hazarkhani, and Hassan Hassani
Shiraz University, Shiraz, Iran
(Received June 26, 2003; in final form August 12, 2003)
Solid silica chloride/KMnO4 system oxidizes effectively aromatic and
aliphatic cyclic thioacetals (1,3-dithiolanes, 1,3-dithianes) and silyl
and pyranyl ethers into their corresponding carbonyl compounds in dry
CH3CN at room temperature in high yields. Over-oxidation of aldehydes
to carboxylic acids has not been observed by this system. In the absence
of silica chloride, KMnO4 in dry CH3CN is an ineffective reagent for
these oxidations.
Keywords: Carbonyl compounds; oxidations; potassium perman-
ganate; silica chloride; silyl ethers; tetrahydropyranyl ethers;
thioacetals
The use of permanganate as an oxidant in organic chemistry goes back
to Kekule’s and Werner’s era. It was used for the cis hydroxylation of
carbon-carbon double bonds. Since then this oxidant has been used ex-
tensively for varieties of oxidation reactions. Permanganate is one of
the most cited oxidant in chemical reactions and in addition to its uses
in laboratories it is used industrially for the preparation of organic
compounds. The major drawback of using permanganate, especially for
large-scale operations, is the formation of manganese dioxide as a solid
waste coproduct in the process of the reaction. Recently, a chemical pro-
cess has been invented by which this solid waste can be easily recycled
to produce permanganate. Today, by this recycling process, perman-
ganate is considered as a green and versatile laboratory reagent and
an industrial oxidant.1 Potassium permanganate also has been used
The authors thank Shiraz University Research Council for the partial support of this
work.
Address correspondence to H. Firouzabadi, Department of Chemistry, College of Sci-
ences, Shiraz University, Shiraz 71454, Iran. E-mail: Firouzabadi@chem.susc.ac.ir
403

404
H. Firouzabadi et al.
for some functional group transformations in the presence of Lewis
acids2 such as ferric chloride, zinc chloride, and boron trifluoride in
polar solvents.
Recently, we have looked at the preparation of high capacity silica
chloride and explored its synthetic utility as a catalyst and also as a
reagent in organic reactions.3 The advantages of using silica chloride
as an alternative to TMSCl, thionyl chloride, oxalyl chloride, etc. are:
a) its very low vapor pressure, which is not harmful to the respiratory
system and to the eyes; b) its lower moisture sensitivity; c) its lack of cor-
rosion to damage reaction vessels and apparatus; d) its easy handling
for the large scale operations; and e) its easy isolation from the reac-
tion mixture. With these significant practical aspects of silica chloride
and KMnO4 in mind, now we report that a combination of solid silica
chloride and KMnO4 provides an effective oxidizing system for the mild
and highly selective oxidation of cyclic thioacetals(1,3-dithiolanes and
1,3-dithiones) and silyl and tetrahydropyranyl ethers into their car-
bonyl compounds in dry CH3CN at room temperature in high yields
(Schemes 1 and 2). By this oxidation system, cyclic thioacetals (1,3-
dithiolanes and 1,3-dithianes) derived from aromatic aldehydes and
ketones were cleanly converted into their aldehydes and ketones in ex-
cellent yields at room temperature (Table I, entries 1–20).
SCHEME 1
SCHEME 2

Oxidation of Thioacetals
405
1,3-Dithiolanes and 1,3-dithianes derived from aliphatic carbonyl
compounds were oxidized smoothly to regenerate their corresponding
carbonyl compounds in good yields (Table I, entries 21–23). Oxidation of
cinnamyl 1,3-dithiolane and its corresponding 1,3-dithiane proceeded
TABLE I Oxidation of Cyclic Thioacetals (1,3-Dithianes and 1,3-Dithiolanes)
with Silica Chloride/KMnO4 System in Dry CH3CN to their Carbonyl
Compounds
Entry
R1
R2
X
Time (min.)
Yield (%)a,b,c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Ph
Ph
H
H
H
H
H
H
H
H
H
H
H
H
Ph
Ph
Me
Me
Me
Me
Et
S(CH2)2S
S(CH2)3S
S(CH2)2S
S(CH2)3S
S(CH2)2S
S(CH2)3S
S(CH2)2S
S(CH2)2S
S(CH2)2S
S(CH2)3S
S(CH2)2S
S(CH2)3S
S(CH2)2S
S(CH2)3S
S(CH2)2S
S(CH2)3S
S(CH2)2S
S(CH2)3S
S(CH2)2S
S(CH2)2S
15
20
25
20
20
20
25
35
15
20
15
17
5
85d
85d
87
90
91
90
90
92
91
89
85e
86e
90
91
85
92
92
90
92
90
4-Cl C6H4
4-Cl C6H4
4-CH3 C6H4
4-CH3 C6H4
4-Br C6H4
4-NO2 C6H4
4-CH3O C6H4
4-CH3O C6H4
Cinnamyl
Cinnamyl
Ph
Ph
5
4-Cl C6H4
4-Cl C6H4
Ph
Ph
Ph
10
10
5
5
7
5
21
22
S(CH2)2S
S(CH2)2S
15
10
81
88
23
S(CH2)2S
10
87
aYields refer to isolated products.
bGC purity > 96%.
cStructures were determined by IR, 1HNMR, 13CNMR, and m.p./b.p.
dBenzaldehyde was isolated as its 2,4-dinitrophenylhydrazine derivative.
eBenzaldehyde was isolated.

406
H. Firouzabadi et al.
smoothly with the cleavage of the carbon-carbon double bond to give
benzaldehyde in 88% yield (Table I, entries 11, 12). In the reactions we
have studied for this purpose, overoxidation of aldehydes to their corre-
sponding carboxylic acids was not observed. We observed that KMnO4
in the absence of silica chloride was not an effective oxidant for the
conversion of thioacetals to their corresponding carbonyl compounds.
We have found that the sequence of the addition of silica chloride and
KMnO4 is very crucial in the reactions. Solid silica chloride should be
added first to the reaction mixture and then the appropriate amounts
of solid KMnO4 should be added to the mixture to bring the reaction
to completion. Even though the detail of the mechanism of this oxida-
tion reaction is obscure to us but by considering the sequence of the
addition of silica chloride first, we can propose a pathway in which the
chlorine atom of silica chloride is attacked by the nucleophilic sulfur
atom of thioacetals to generate a sulfonium chloride intermediate that
undergoes immediate carbon-sulfur cleavage to produce a resonance
stabilized carbocationic species. This highly reactive intermediate can
easily react with KMnO4 to produce the desired carbonyl compound
(Scheme 2).
Conversion of silyl and tetrahydropyranyl ethers to their carbonyl
compounds under mild conditions is also of practical importance there-
fore we also studied these reactions by this new oxidation system
(Scheme 3).
SCHEME 3
First, oxidation of benzyltrimethylsilyl ether with the silica
chloride/KMnO4 system in dry CH3CN at room temperature was stud-
ied. The reaction proceeded smoothly and benzaldehyde was produced
in high yield without further oxidation to benzoic acid (Table II, en-
try 1). The sequence of the addition of silica chloride and KMnO4 was
also important in this reaction. Solid silica chloride should be added
first to the solution of the substrate in dry CH3CN and then addition
of KMnO4 brought the reaction to completion. We applied similar re-
action conditions for the oxidation of some other primary benzylic and

Oxidation of Thioacetals
407
TABLE II Oxidation of TMS, TBDMS, and THP Ethers with the Silica
Chloride/KMnO4 System in Dry CH3CN to their Carbonyl Compounds
Time (min.)
Yield (%)a,b,c
Entry
R1
R2
THP
TMS
TBDMS
THP
TMS
TBDMS
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Ph
H
H
H
H
H
H
H
Ph
Me
Et
H
2
7
5
7
10
5
5
5
5
7
5
5
30
35
35
45
120
30
30
20
25
27
88d
90
91
90
88
91
85e
90
92
90
87
90
70
75
88
91
89
91
89
90
82e
92
92
91
85
90
71
70
85
88
90
91
85
90
83e
92
91
90
87
75
70
60
4-Cl C6H4
4-CH3 C6H4
4-Br C6H4
2-NO2 C6H4
4-CH3O C6H4
Cinnamyl
Ph
Ph
Ph
Ph(CH2)2
Ph(CH2)
C7H15
15
17
25
10
15
20
20
20
70
10
120
120
10
10
30
30
100
20
150
140
H
H
Me
C6H13
aYields refer to isolated products.
bGC purity > 96%.
cStructures were determined by IR, 1HNMR, 13CNMR, m.p./b.p.
dBenzaldehyde was isolated as its 2,4-dinitrophenylhydrazine derivative.
eBenzaldehyde was isolated.
aliphatic silyl ethers to their corresponding aldehydes in good to excel-
lent yields (Table II, entries 1–6). Oxidation of secondary benzylic and
aliphatic trimethylsilyl ethers progressed easily to their correspond-
ing ketones in high to excellent yields (Table II, entries 8–10). Cin-
namyl trimethylsilyl ether oxidation was accompanied by the oxidation
of carbon-carbon bond to afford benzaldehyde in 85% yield (Table II, en-
try 7). We also studied oxidative deprotection of tert-butyldimethylsilyl
ethers (TBDMS ethers), which are more stable than their trimethylsilyl
analogs by this oxidation system in dry CH3CN. (Scheme 3, Table II).
Primary and secondary benzylic TBDMS ethers also were oxidized eas-
ily to their corresponding aldehydes and ketones in good to excellent
yields (Table II, entries 1–10). The carbon-carbon double bond of cin-
namyl TBDMS ether was cleaved to give benzaldehyde in 82% yield
(Table III, entries 7). Primary and secondary aliphatic TBDMS ethers
were cleanly oxidized to their corresponding aldehydes and ketones by
this oxidation system (Table II, entries 11–14).
Finally, we have studied oxidation of tetrahydropyranyl ethers by
silica chloride and KMnO4 in dry CH3CN at room temperature un-
der similar reaction conditions used for the oxidation of silyl ethers.

408
H. Firouzabadi et al.
Our results show that THP ethers were oxidized effectively to their
carbonyl compounds. Various types of primary and secondary aro-
matic tetrahydropyranyl ethers substituted with electron-releasing and
electron-withdrawing groups (Table II, entries 1–11) were converted
easily to their corresponding carbonyl compounds in high yields. Oxi-
dation of cinnamyl tetrahydropyranyl ether also was accompanied with
the cleavage of the carbon-carbon double bond to give benzaldehyde in
83% yield (Table II, entries 7).
In conclusion, in this study we have introduced a new oxidizing sys-
tem in which KMnO4 as a green oxidant and easy handling solid silica
chloride has been used for the effective oxidation of cyclic thioacetals,
silyl, and pyranyl ethers to their carbonyl compounds in dry CH3CN at
room temperature. By this method, overoxidation of aldehydes to their
carboxylic acids has not been observed.
GENERAL
All the products are known and commercially available compounds.
Therefore, their physical data are not given. The products were purified
by column or plate chromatography on silica gel, when necessary, the
purity determination of the products was accomplished by GC on a
Shimadzu model GC-14A instrument or by TLC on silica gel polygram
SIL G/UV 254 plates. The IR spectra were recorded on a Perkin Elmer
781 spectrophotometer. The NMR spectra were recorded on a Bruker
advance DPX 250 MHz spectrometer.
Oxidatrion of Cyclic Thiacetals, Silyl, and Pyranyl Ethers
with the Silica Chloride/KMnO4 System
General Procedure
To a solution of the substrate (1 mmol) in dry CH3CN (5 mL) silica
chloride (0.5 g for silyl ethers, 0.6 g for pyranyl ethers, and 1 g for
cyclic thioacetals) was added and then KMnO4 (0.158 g, 1 mmol for silyl
ethers, 0.189 g, 1.2 mmol for pyranyl ethers, and 0.474 g, 3 mmol for
cyclic thioacetals); the resulting mixture was stirred at room tempera-
ture for the appropriate reaction time (Tables I–IV). The progress of the
reaction was monitored by TLC and GC. After completion of the reac-
tion, the solvent was evaporated under reduced pressure. The resulting
solid mixture was added to a silica gel pad (3 cm thick) and was washed
with a mixture of hexanes/EtOAc (5:1, 3 × 20 mL). The filtrates were
combined and the solvent was evaporated under reduced pressure to
afford the desired products in good to excellent yields (Table I–IV). The

Oxidation of Thioacetals
409
quantity of the aldehydes with high vapor pressures were determined
by their conversion into their 1,4-dinitrophenylhydrazone derivatives.
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