Oxidation deprotection of trimethylsilyl ethers to carbonyl compounds by NaBrO3-NH4Cl reagent in aqueous acetonitrile

Article abstract of DOI:10.1081/SCC-100103266

Primary and secondary benzylic and secondary alkyl trimethlysilyl ethers are efficiently converted into their carbonyl compounds by NaBrO₃-NH₄Cl in aqueous acetonitrile. Primary alkyl silyl ethers give only the corresponding alcohols without further oxidation with NaBrO₃-NH₄Cl.

Full text of DOI:10.1081/SCC-100103266

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OXIDATION DEPROTECTION
OF TRIMETHYLSILYL ETHERS
TO CARBONYL COMPOUNDS
BY NaBrO3-NH4Cl REAGENT IN
AQUEOUS ACETONITRILE
Ahmad Shaabani a & Ali-Reza Karimi a
a Chemistry Department, Shahid Beheshti
University, P.O. Box 19396 – 4716, Tehran, Iran
Published online: 16 Aug 2006.
To cite this article: Ahmad Shaabani & Ali-Reza Karimi (2001) OXIDATION
DEPROTECTION OF TRIMETHYLSILYL ETHERS TO CARBONYL COMPOUNDS BY NaBrO3-
NH4Cl REAGENT IN AQUEOUS ACETONITRILE, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry,
31:5, 759-765, DOI: 10.1081/SCC-100103266
To link to this article: http://dx.doi.org/10.1081/SCC-100103266
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SYNTHETIC COMMUNICATIONS, 31(5), 759–765 (2001)
OXIDATION DEPROTECTION OF
TRIMETHYLSILYL ETHERS TO
CARBONYL COMPOUNDS BY
NaBrO3-NH4Cl REAGENT IN AQUEOUS
ACETONITRILE
Ahmad Shaabani* and Ali-Reza Karimi
Chemistry Department, Shahid Beheshti University,
P.O. Box 19396 – 4716, Tehran, Iran
ABSTRACT
Primary and secondary benzylic and secondary alkyl
trimethlysilyl ethers are efficiently converted into their carbo-
nyl compounds by NaBrO3-NH4Cl in aqueous acetonitrile.
Primary alkyl silyl ethers give only the corresponding alcohols
without further oxidation with NaBrO3-NH4Cl.
Protection of functional groups is of vital importance in the total
synthesis of complex organic compounds.1 The alcohol function is one of
the most important groups in organic molecules and its controlled manip-
ulation during the synthesis is of great value to synthetic organic
chemists. The transformation of alcohols into corresponding silyl ethers
is a very common way to protect hydroxy groups.1,2 Such a protection
can be used to prevent their oxidation, but sometimes it is necessary to
achieve the transformation of the silyl ether to the corresponding carbonyl
compound.
* Corresponding author.
759
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

760
SHAABANI AND KARIMI
A great number of oxidative deprotection agents can effect the con-
version of a silyl ether to a carbonyl compound,2,3 and a synthetic chemist is
faced with an amazing choice of methods for this reaction. However, the
susceptibility of aldehydes to further oxidation narrows the choice of reagent
for the oxidative deprotection of silyl ethers to aldehydes in good yields.
The discovery of new reagents for the transformation of silyl ethers
into carbonyl compounds under mild condition with a variety of silyl ethers
is of primary importance in synthetic organic chemistry. Conversion of silyl
ethers by Jones reagent (CrO3, H2SO4/acetone),4 CrO3/Py,5 PCC,6
PFC,7 PDC,6b (Ph3SiO)2CrO2/t-BuO2H,8 Ph3SiOH/CrO3/t-BuO2H,9
DMSO based reagents,10 NBS,11 DDQ,12 Ph3CBF4,13 Ce(IV)/NaBrO3,14
AgBrO3/AlCl315 and NaBrO3/AlCl315 have been reported. Most of the reac-
tions mentioned above suffer more or less serious drawbacks such as: dry
media and nonaqueous reaction conditions, poor availability of the
reagents, long reaction times and low yields of products, and sometimes
they involve a tedious work up. Recently, we have found that the mixture
NaBrO3-NH4Cl is an efficient oxidant for the oxidation of alcohols16 and
sulfides.17 Herein, we report our subsequent studies on oxidative deprotec-
tion of benzylic and secondary silyl ethers to the corresponding aldehydes
and ketones and conversion of primary alkyl silyl ethers only into corres-
ponding alcohols without further oxidations using NaBrO3-NH4Cl. We
have found that this method of oxidative deprotection is very convenient
for conversion of silyl ethers into carbonyl compounds, because of simplicity
and use of mild reaction conditions.
Furthermore, NH4Cl and NaBrO3 are both cheap and easily available
compounds relative to most other oxidizing deprotecting agents that have
so far been employed. As shown in Table 1, a wide variety of benzylic
and secondary alcohols could be easily converted into the corresponding
carbonyl compounds. However, saturated primary alcohols (Table 1, entries
11–13) were only converted to corresponding alcohols.
In conclusion, NaBrO3 in the presence of NH4Cl provides a new and
efficient reagent which promises to be economical for the one-pot oxidation
deprotection of benzylic and secondary trimethylsilyl ethers to their corre-
sponding carbonyl compounds in high yields and also conversion of primary
alkyl silyl ethers to the corresponding alcohols under mild reaction
conditions.
EXPERIMENTAL
All products are known compounds and were identified by compar-
ison of their physical and spectral data with those of authentic samples.

TRIMETHYLSILYL ETHERS
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SHAABANI AND KARIMI

TRIMETHYLSILYL ETHERS
763

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SHAABANI AND KARIMI
Melting points were determined in open capillaries using an oil-bath and are
uncorrected. IR spectra were recorded as neat films or as KBr pellets on a
Shimadzu 470 spectrometer. 1H NMR spectra were recorded at 90 MHz on
a JEOL EX-90 instrument with CDCl3 as solvent and Me4Si as an internal
standard. Yields reported refer to isolated products or 2,4-dinitrophenyl-
hydrazone derivatives (2,4-DNP)18,19 of the carbonyl compounds. Trimethyl
silyl ethers were prepared according to published procedures.20
General Procedure
Trimethylsilyl ether (5 mmol) was added to a mixture of NaBrO3
(0.755 g, 5 mmol) and NH4Cl (0.400 g, 7.5 mmol) in aqueous acetonitrile
(CH3CN-H2O ¼ 7:3, v:v; 10 ml). The mixture was stirred at 80ꢀC for 15 to
50 minutes. When the reaction was complete, the resulting solution was
extracted with methylene dichloride (20 ml). The combined organic layers
were dried over MgSO4. After filtration, the solution was evaporated on a
rotary evaporator and the resulting crude material was purified on a silica
gel column with appropriate eluent.
ACKNOWLEDGMENT
Financial support by the Research Council of Shahid Beheshti
University is acknowledged.
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Received in the U.K. November 11, 1999