2,6-Dicarboxypyridinium fluorochromate: A mild and efficient reagent for oxidative deprotection of trimethylsilyl ethers to their corresponding carbonyl compounds

Article abstract of DOI:10.1080/104265090920822

Deprotection of trimethylsilyl ethers to their parent aldehydes and ketones in high yields has been carried out using 2,6-dicarboxypyridinium fluorochromate under mild conditions. Copyright Taylor & Francis Inc.

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Phosphorus, Sulfur, and Silicon
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2,6-Dicarboxypyridinium
Fluorochromate: A Mild and
Efficient Reagent for Oxidative
Deprotection of Trimethylsilyl
Ethers to Their Corresponding
Carbonyl Compounds
Rahman Hosseinzadeh a , Mahmood Tajbakhsh a
,
Farhad Ramzanian-Lehmali a & Marzieh Sadatshahabi
a
a University of Mazandaran, Chemistry Department ,
Babolsar, Iran
Published online: 22 Aug 2006.
To cite this article: Rahman Hosseinzadeh , Mahmood Tajbakhsh , Farhad Ramzanian-
Lehmali & Marzieh Sadatshahabi (2005) 2,6-Dicarboxypyridinium Fluorochromate: A
Mild and Efficient Reagent for Oxidative Deprotection of Trimethylsilyl Ethers to Their
Corresponding Carbonyl Compounds, Phosphorus, Sulfur, and Silicon and the Related
Elements, 180:10, 2279-2283, DOI: 10.1080/104265090920822
To link to this article: http://dx.doi.org/10.1080/104265090920822
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Phosphorus, Sulfur, and Silicon, 180:2279–2283, 2005
Copyright © Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/104265090920822
2,6-Dicarboxypyridinium Fluorochromate: A Mild
and Efficient Reagent for Oxidative Deprotection
of Trimethylsilyl Ethers to Their Corresponding
Carbonyl Compounds
Rahman Hosseinzadeh
Mahmood Tajbakhsh
Farhad Ramzanian-Lehmali
Marzieh Sadatshahabi
University of Mazandaran, Chemistry Department, Babolsar, Iran
Deprotection of trimethylsilyl ethers to their parent aldehydes and ketones in high
yields has been carried out using 2,6-dicarboxypyridinium fluorochromate under
mild conditions.
Keywords 2,6-Dicarboxypyridinium fluorochromate; deprotection; oxidative; trime-
thylsilyl ethers
INTRODUCTION
The protection of certain functional groups and the deprotection of the
protected drivatives constitute important processes in synthetic organic
chemistry in polyfunctional molecules, including the total synthesis of
natural products. Thus, a large number of protective groups have been
developed along with numerous methods for their removal.1−5
Conversion of the hydroxy function to trimethylsilyl ether is one of
the most useful and convenient methods for the protection of this func-
tional group.6−8 Direct oxidation of trimethylsilyl ethers to their corre-
sponding carbonyl compounds has found considerable attention during
recent years.9−10
However, some of the reported methods show limitations such as
the use of expensive reagents, a long reaction time, low yields of the
Received September 14, 2004; accepted October 14, 2004.
We are thankful to the Mazandaran Universities Research Council for the partial
support of this work.
Address correspondence to Mahmood Tajbakhsh, Department of Chemistry, School
of Sciences, University of Mazandaran, P.O. Box 453, Babolsar, Iran. E-mail: tajbaksh@
umz.ac
2279

2280
R. Hosseinzadeh et al.
products, and tedious work-up. Therfore, the introduction of new meth-
ods and inexpensive reagents for such functional group transformation
is still in demand.
RESULTS AND DISCUSSION
As a part of our program related to developing new oxidation meth-
ods, we wish to report here a new, simple, and general procedure
that can be used for the oxidative deprotection of trimethylsilyl ethers
to their corresponding carbonyl compounds under nonaqueous condi-
tions at room temperature.11 2,6-dicarboxypyridinium fluorochromate
(2,6-DCPFC) is a new, rapid, and efficient reagent for this purpose
(Scheme 1).
SCHEME 1
This reagent is easily and cheaply prepared from pyridine 2,6-
dicarboxylic acid, 40% aq. hydrofluoric acid, and chromium (VI) ox-
ide in a molar ratio of 1:1.5:1. This compound is nonhygroscopic and
an air-stable reagent that can be stored in a polythene bag for longer
periods without decomposition. It is soluble in polar solvents such
as acetonitrile and acetone; slightly soluble in THF, chloroform, and
dichloromethane; and insoluble in benzene, n-hexane, and carbon tetra-
chloride. The chromium content of the reagent was determined by
atomic absorption. Also, elemental analysis (C, H, N) were performed
and the experimental and calculated results are in a very good agree-
ment.
The effect of solvent in the oxidation reaction was evaluated by car-
rying out the oxidation in a series of solvents with varying polarity.
Deprotection of benzaldehyde trimethylsilyl ether with a 1:1 ratio was
carried out in dichloromethane, chloroform, tetrahydrofuran, n-hexane,
and acetonitrile at room temperature. The results are presented in
Table I.
A wide variety of trimethylsilyl ethers such as benzylic, allylic, and
aliphatic have been converted to their corresponding carbonyl com-
pounds using this reagent at room temperature (Table II).
It is noteworthy that, unlike other oxidative hydrolytic meth-
ods, the major drawback of overoxidation of the resulting aldehydes

2,6-DCPFC: A Mild and Efficient Reagent
2281
TABLE I Oxidative Deprotection of Benzaldehyde Trimethylsilyl
Ether in Different Solvents Using 2,6-DCPFC at Room Temperature
Reaction period
Entry
Solvent
(min)
Yield (%)a
1
2
3
4
5
CH3CN
CHCl3
CH2Cl2
n-C6H12
THF
5
90
90
90
90
99
70
74
15
35
aYields refer to isolated products.
is not observed under the reaction conditions. Interestingly the
α, β-unsaturated derivatives underwent deprotection very efficiently
without affecting the olefinic bond and the reaction is essen-
tially chemoselective (Table II, entry 14). Furthermore, functional
groups such as chloro, methoxy, nitro, and alkyl also were inert to
this reagent and no byproduct formation was observed (Table II,
entries 3–10).
In order to show the efficiency of this method, we have compared
some of the results with relevant results reported in the literature9,10
TABLE II Oxidative Deprotection of Trimethylsilyl Ethers With
2,6-DCPFC in CH3CN at Room Temperature
Entry
Substrate
Time (min)
Producta
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
C6H5CH2OSiMe3
(C6H5)2CHOSiMe3
5
5
6
6
7
6
5
6
12
7
C6H5CHO
(C6H5)2CO
99
100
98
97
95
96
96
96
92
95
86
82
94
93
o-ClC6H4CH2OSiMe3
p-ClC6H4CH2OSiMe3
m-MeOC6H4CH2OSiMe3
p-MeOC6H4CH2OSiMe3
o-MeOC6HCH2OSiMe3
p-MeC6H4CH2OSiMe3
p-NO2C6H4CH2OSiMe3
C6H5CH(p-CIC6H4)OSiMe3
C6H5CH2CH2CH2OSiMe3
CH3 (CH2)7OSiMe3
o-ClC6H4CHO
p-ClC6H4CHO
m-MeOC6H4CHO
p-MeOC6H4CHO
o-MeOC6H4CHO
p-MeC6H4CHO
p-NO2C6H4CHO
C6H5C(p-CIC6H4)O
C6H5CH2CH2CHO
CH3 (CH2)6CHO
α-tetralon
14
30
11
7
α-tetralol trimethylsilyl ether
C6H5CH=CHCH2OSiMe3
Cinnamaldehyde
aAll of the products were characterized by comparing them with known compounds
and their IR and 1H NMR showed physical and spectral data in accordance with their
expected structure and with authentic samples.
bYields refer to isolated products.

2282
R. Hosseinzadeh et al.
TABLE III Comparison of Some of the Results Obtained by the
Oxidative Deprotection of Trimethylsilyl Ethers With 2,6-DCPFCa
With Some of Those Obtained by QFC9,b and BTPPC10,c
Time
Entry
1
Reagent
Substrate
(min)
Conditions
Yield (%)
2,6-DCPFC
p-NO2C6H4CH2OSiMe3
(C6H5)2CHOSiMe3
p-NO2C6H4CH2OSiMe3
(C6H5)2CHOSiMe3
p-NO2C6H4CH2OSiMe3
(C6H5)2CHOSiMe3
12
5
90
70
420
18
CH3CN, r.t.d
92
100
89
93
90
2
3
QFC
CH3CN, r.t.
BTPPC
CH3CN, reflux
95
a2,6-Dicarboxypyridinium fluorochromate.
bQuinolinium fluorochromate.
cBenzyltriphenylphosphonium chlorate.
dRoom temperature.
(Table III). As indicated in Table III, 2,6-DCPFC compared to the other
reagents performs this transformation in a higher yield, shorter reac-
tion times, and milder conditions.
In conclusion, 2,6-dicarboxypyridinium fluorochromate is a mild, ef-
ficient, and inexpensive reagent for the one-pot oxidative deprotection
of trimethylsilyl ethers to their corresponding carbonyl compounds in
nonaqueous conditions.
EXPERIMENTAL
Products were isolated and their physical data were compared with
those of known samples. Solvent was freshly distilled. Trimethylsilyl
ethers were prepared according to the described procedure.12
Preparation of 2,6-DCPFC
To a solution of 2 g (0.02 mol) of CrO3 in 1.0 mL of water in a polyethy-
lene beaker, 1.5 mL (0.03 mol) of 40% HF was added with stirring.
An orange-red solution was obtained. The reaction mixture was then
cooled in an ice bath (−5◦C) and 3.342 g (0.02 mol) of pyridine and
2,6-dicarboxylic acid were added portion-wise with stirring. When an
orange crystalline compound separated out, this was filtered under vac-
uum using a polyethylene funnel, washed with petroleum ether (3 ×
10 mL), rapidly dried in a vacuum desicator, and finally stored in a
polyethylene bag. The yield of 2,6-DCPFC was 5.28 g (92%).

2,6-DCPFC: A Mild and Efficient Reagent
2283
General Procedure for the Oxidative Deprotection
of Trimethylsilyl Ethers with 2,6-DCPFC
In a round-bottomed flask (50 mL) equipped with a magnetic stirrer, a
mixture of the substrate (1 mmol) in CH3CN (5 mL) and 2,6-DCPFC
(1 mmol) was placed. The reaction mixture was stirred at room tem-
perature for the specified time (Table II). The progress of the reaction
was monitored by TLC. After the reaction was completed, the mixture
was filtered and the solid material was washed with diethyl ether (2 ×
10 mL). The combined filtrate was evaporated under reduced pressure
to afford an almost pure carbonyl compound in 82–100%. If necessary,
the products were further purified on a silica-gel plate or silica-gel col-
umn with an appropriate eluent.
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