Immediate and high yielding solvent-free oxidation of silyl and pyranyl ethers to their correspounding carbonyl compounds with zinc dichromate trihydrate (ZnCr2O7·3H2O)

Article abstract of DOI:10.1080/10426500490463628

Zinc dichromate trihydrate (ZnCr₂O₇·3H ₂O) is an efficient reagent for the immediate deprotection of aromatic and aliphatic silyl and pyranyl ethers into their corresponding carbonyl compounds in high yields at room temperature under solvent-free conditions.

Full text of DOI:10.1080/10426500490463628

Phosphorus, Sulfur, and Silicon, 179:1417–1422, 2004
Copyright ꢀC Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490463628
IMMEDIATE AND HIGH YIELDING SOLVENT-FREE
OXIDATION OF SILYL AND PYRANYL ETHERS
TO THEIR CORRESPOUNDING CARBONYL
COMPOUNDS WITH ZINC DICHROMATE
TRIHYDRATE (ZnCr O ·3H O)
2
7
2
Habib Firouzabadi, Hassan Hassani, and Mostafa Gholizadeh
Shiraz University, Shiraz, Iran
(
Received October 14, 2003; accepted November 25, 2003)
Zinc dichromate trihydrate (ZnCr O ·3H O) is an efficient reagent for
2
7
2
the immediate deprotection of aromatic and aliphatic silyl and pyranyl
ethers into their corresponding carbonyl compounds in high yields at
room temperature under solvent-free conditions.
Keywords: Carbonyl compounds; deprotection; oxidation; pyranyl
ethers; silyl ethers; solvent-free; zinc dichromate trihydrate
Transformation of hydroxy functional groups into their silyl and
pyranyl ethers is a common way for their protection during the to-
1
–3
tal synthesis of complex organic molecules.
Direct oxidation of silyl
and pyranyl ethers into their carbonyl compounds is a useful trans-
formation during the process of a multistep synthesis of organic com-
pounds. This transformation has been under attention and varieties of
methods have been reported by investigators. The reported methods in-
4
clude; Tris[trinitratocerium(IV)] paraperiodate ([(NO3)3Ce]3·H2IO6),
5
Fe(NO3)3·1.5N2O4, and Cu(NO3)2·N2O4, silver and sodium bromate in
6
the presence of aluminum chloride, 2,3-Dichloro-5,6-dicyanoquinone
7
,8
9
(
(
DDQ),
cerric ammonium nitrate (CAN), N-bromosuccinimide
1
0
11
NBC), potassium permanganate in the presence of Lewis acids,
+
3
−
12
nitrosonium tetrafluoroborate (NO BF4 ),
IV)] chromate [(NO3)3Ce]2CrO4,
acetone),
bis[trinitrato cerium
Jones reagent (CrO3/H2SO4/
zinc chlorochromate nonahydrate (ZCCNH), bis(tri-
phenylsilyl)chromate ((Ph3SiO)2CrO2), pyridinium fluorochromate
1
(
1
4–16
17
18
The authors are thankful to the Shiraz University Research Council for the support
of this work.
Address correspondence to Habib Firouzabadi, Department of Chemistry, College of
Sciences, Shiraz University, Shiraz 71454, Iran. E-mail: firouzabadi@chem.susc.ac.ir
1
417

1
418
H. Firouzabadi et al.
20
(
PFC),19 trimethylsilyl chlorochromate [ClCrO2(OSiMe3)], Collins
2
1
22,23
reagent (CrO3·2Py),
phridinum chlorochromate (PCC),
3-
2
4
carboxypyridinium chlorochromate (CPCC),
alumina supported
chromium(VI) oxide,25 and pyridinium dichromate(PDC).
20
Some of the reported procedures for the oxidation of silyl and pyranyl
ethers suffer at least from one of the following disadvantages: us-
4
,13
14–16
ing expensive reagents,
high acidity of the media,
long reac-
and difficult
1
8,19
21–24
tion times,
dangerous procedure for preparation,
1
8
procedures. Therefore, efficient and one-pot direct oxidation of silyl
and pyranyl ethers under mild and solvent-free conditions can be con-
sidered as a useful procedure for further manipulation of the hydroxy
functionality in organic synthesis. In this article, we report a new effi-
cient method for deprotection of various types of silyl and pyranyl ethers
into their corresponding carbonyl compounds under solvent-free condi-
2
6
tions using zinc dichromate trihydrate (ZDC) as an easily prepared,
stable, and inexpensive reagent (Scheme 1, Tables I–III).
SCHEME 1
Primary and secondary benzylic trimethylsilyl ethers were oxidized
easily to the corresponding aldehydes and ketons in good to excellent
yields (Table I, entries 1–12). Cinnamyl trimethylsilyl ether was oxi-
dized to cinnamylaldehyde in 88% yield by this method (Table I, en-
try 9). Saturated primary and secondary trimethylsilyl ethers were
converted into the corresponding carbonyl compound in good yields
(
Table I, entries 13–16).
We also studied oxidative deprotection of tert-butyldimethylsilyl
ethers of similar alcohols with ZDC under solvent-free conditions. We
found that this reagent was also very effective for this purpose and con-
verted tert-butyldimethylsilyl ethers effectively to their carbonyl com-
pounds in 80–94% yields under solvent-free conditions and at room
temperature (Scheme 1, Table II). Primary and secondary benzylic
tert-butyldimethylsilyl ethers were oxidized easily to the correspond-
ing aldehydes and ketones in good to excellent yields (Table II, entries
1–12). Cinnamyl tert-butyldimethylsilyl ether was transformed into

Deprotection of Silyl and Pyranyl Ethers
1419
TABLE I Deprotection of Trimethylsilyl Ethers with ZDC under
Solvent-Free Conditions and at Room Temperature
Entry
R1
R2
Oxidant/substrate
Yielda,b (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
Ph
H
H
H
H
H
H
H
H
1/1
1.2/1
1/1
1.2/1
1.4/1
2/1
90c
92
92
93
90
89
95
88
88
95
94
92
88
90
85
85
4-ClC6H4
4-MeC6H4
4-BrC6H4
4-NO2C6H4
2-NO2C6H4
4-MeOC6H4
2-NH2C6H4
Cinnamyl
Ph
Ph
Ph
PhCh2Ch2
PhCh2
C7H15
1/1
1.5/1
1.2/1
1/1
1/1
1/1
1/1
1/1
3/1
3/1
H
1
1
1
1
1
1
1
Ph
Me
Et
H
H
H
C6H13
CH3
a
Isolated yields.
b
c
GC purity >96%.
Benzaldehyde was isolated as its 2,4-dinitrophenyl hydrazine derivative.
TABLE II Deprotection of tert-Butyldimethylsilyl Ethers with ZDC under
Solvent-Free Conditions and at Room Temperature
Entry
R1
R2
Oxidant/substrate
Yielda,b (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
Ph
H
H
H
H
H
H
H
H
2/1
2/1
2/1
2.5/1
3/1
4/1
2/1
2.5/1
2/1
4/1
4/1
90c
91
92
92
90
88
94
88
90
94
93
92
85
85
80
80
4-ClC6H4
4-MeC6H4
4-BrC6H4
4-NO2C6H4
2-NO2C6H4
4-MeOC6H4
2-NH2C6H4
Cinnamyl
Ph
H
1
1
1
1
1
1
1
Ph
Me
Et
H
H
H
Ph
Ph
4/1
4/1
5/1
5/1
PhCH2CH2
PhCH2
C7H15
C6H13
CH3
5/1
a
Isolated yields.
b
c
GC purity >96%.
Benzaldehyde was isolated as its 2,4-dinitrophenyl hydrazine derivative.

1
420
H. Firouzabadi et al.
cinnamylaldehyde in 90% yield (Table II, entry 9). Saturated primary
and secondary tert-butyldimethylsilyl ethers were also converted to the
corresponding carbonyl compounds in high yields under similar reac-
tion conditions (Table II, entries 13–16).
Oxidative deprotection of tetrahydropyranyl ethers of various alco-
hols with zinc dichromate trihydrate under solvent-free condition and
at room temperature were also investigated (Scheme 1, Table III). By
this procedure, various types of aromatic tetrahydropyranyl ethers with
electron-releasing and electron-withdrawing groups (Table III, entries
1–13) were immediately converted to their corresponding carbonyl com-
pounds (85–94%). Primary and secondary aliphatic THP-ethers also
were oxidized to the corresponding aldehydes and ketones in 80–85%
yields (Table III, entries 13–16).
In this study, we have introduced a new and useful applications
of ZnCr2O7·3H2O in organic synthesis. By using this reagent under
solvent-free and mild reaction conditions, we have presented immedi-
ate and high yielding conversions of TMS, TBDMS and THP ethers
into their aldehydes and ketones. Isolation of the products is easy,
clean and not a time consuming process. Quantitative preparation of
the reagent from cheap and available compounds; ZnCO3 and aqueous
solution CrO3 is also worthy of mention for the presented method.
TABLE III Deprotection of Tetrahydropyranyl Ethers with ZDC under
Solvent-Free Conditions and at Room Temperature
Entry
R1
R2
Oxidant/substrate
Yielda,b (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
Ph
H
H
H
H
H
H
H
H
2/1
2/1
2/1
2.5/1
3/1
4/1
2/1
2.5/1
2/1
4/1
4/1
90c
91
90
89
89
85
92
86
90
94
92
92
85
84
80
80
4-ClC6H4
4-MeC6H4
4-BrC6H4
4-NO2C6H4
2-NO2C6H4
4-MeOC6H4
2-NH2C6H4
Cinnamyl
Ph
H
1
1
1
1
1
1
1
Ph
Me
Et
H
H
H
Ph
Ph
4/1
4/1
5/1
5/1
PhCH2CH2
PhCH2
C7H15
C6H13
CH3
5/1
a
Isolated yields.
b
c
GC purity >96%.
Benzaldehyde was isolated as its 2,4-dinitrophenyl hydrazine derivative.

Deprotection of Silyl and Pyranyl Ethers
1421
EXPERIMENTAL
An immediate reaction occurred by mixing silyl or pyranyl ethers
(
1 mmol) with ZnCr2O7·3H2O (1–5 mmol) at room temperature. To
the resulting solid mixture, ether (20 mL) was added and after vig-
orous shaking the mixture was filtered through a silica gel pad (3 Cm
thick). The resulting filter cake was poured into ether (20 mL) and
was shaken vigorously and filtered. Evaporation of the combined fil-
trates resulted in the desired known carbonyl compounds in high yields
(
Tables I–III).
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