Microwave assisted deprotection of trimethylsilyl ethers under solvent-free conditions catalyzed by clay or a palladium complex

Article abstract of DOI:10.1007/PL00010296

A simple method for the cleavage of trimethylsilyl ethers under microwave irradiation and solvent-free conditions in the presence of montmorillonite K-10 or a palladium complex catalyst in almost quantitative yield is reported.

Full text of DOI:10.1007/PL00010296

Monatshefte f uÈ r Chemie 130, 1175±1178 (1999)
Microwave Assisted Deprotection of
Trimethylsilyl Ethers under Solvent-Free
Conditions Catalyzed by Clay
or a Palladium Complex
Mohammad M. Mojtahedi , Mohammad R. Saidi , Majid M. Heravi2,
1
1;Ã
and Mohammad Bolourtchian2
1
Department of Chemistry, Sharif University of Technology, Tehran, Iran
2
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
Summary. A simple method for the cleavage of trimethylsilyl ethers under microwave irradiation
and solvent-free conditions in the presence of montmorillonite K-10 or a palladium complex catalyst
in almost quantitative yield is reported.
Keywords. Trimethylsilyl ether; Deprotection; Montmorillonite; Microwave; Palladium.
Durch Ton oder einen Palladiumkomplex katalysierte mikrowellenunterst uÈ tzte Entsch uÈ tzung
von Trimethylsilylethern unter l oÈ sungsmittelfreien Bedingungen
Zusammenfassung. Eine einfache Methode zur Spaltung von Trimethylsilylethern unter
Mikrowelleneinwirkung und l oÈ sungsmittelfreien Bedingungen in Gegenwart von Montmorillonit
K-10 oder einem Palladiumkomplex mit nahezu quantitativen Ausbeuten wird beschrieben.
Introduction
The transformation of alcohols and phenols to the corresponding trimethylsilyl
ethers is a common way for the protecting of hydroxy groups. It has been widely
used in organic synthesis to control the stereoselectivity of reactions [1].
Trimethylsilyl ethers can be deprotected conveniently by numerous methods
[2±4]. While all these methods have merits, there are some limitations that can lead
to lower yields in some cases. Microwave heating and its application in organic
synthesis is currently under intensive investigation [5]. Although clay and alumina
assisted deprotection reactions have been utilized in organic solvents, such a
methodology has not been used for the deprotection of the trimethylsilyl group
under microwave irradiation. The fact that clays possess Al(III) and other
interlamellar cations suggests that these naturally occuring supports promote
organic transformations via acid catalysis [6±8].
à Corresponding author

1176
M. M. Mojtahedi et al.
Results and Discussion
Successful solid state reactions have been recently reported in absence of solvents
for protecting alcohols and phenols with hexamethyldisilazane [9]. We now report
a new, simple, and fast method for the desilylation of a variety of phenols and
Scheme 1
Table 1. Irradiation time and yields for the deprotection of phenols and alcohols catalyzed by
montmorillonite K-10 or a palladium complex
Starting material
K-10
min)
Yield
(%)
PdCl (PhCN)
2
Yield
(%)
2
(
(min)
1
1
.0
.0
100
100
3.0
100
100
3.0
1
5
.0
.0
98
3.0
5.0
99
100
100
5
3
.0
.0
98
99
5.0
5.0
98
98
3.0
100
5.0
100
1
1
1
.0
.0
.0
99
98
3.0
3.0
5.0
98
98
100
100
5
3
.0
.0
97
88

Deprotection of Trimethylsilyl Ethers
1177
alcohols under solvent-free conditions in almost quantitative yields using a
conventional microwave oven and montmorillonite K-10 clay as solid support. The
deprotection reaction of the trimethylsilyl group was also carried out with
PdCl (PhCN)2 as the catalyst under microwave irradiation. An interesting
2
phenomenon with this complex is that the reaction works best if a drop of water
is added to the reaction mixture before heating in the microwave oven.
Interestingly enough, without addition of water the desilylation reaction did not
occur (Scheme 1). The protected phenols and alcohols are listed in Table 1.
In conclusion, the present study shows that deprotection of the trimethylsilyl
group in alcohols and phenols can successfully be performed in dry media
catalyzed by commercially available montmorillonite K-10 or by PdCl (PhCN) in
2
2
very short time and in almost quantitative yields.
Experimental
Activated montmorillonite K-10 clay was prepared by heating it in a commercial microwave oven
900 W) for ®ve minutes just before use as a solid support. All compounds were identi®ed by means
(
of their NMR and IR spectra.
Desilylation of phenols or alcohols in the presence of montmorillonite K-10 clay
3
mmol of silyl ether was placed in a 5 cm beaker, and 0.1 g of activated montmorillonite K-10 clay
1
3
was added. The beaker was placed in a 50 cm te¯on container and irradiated in a microwave oven
900 W) for 1 to 5 min. The progress of the reaction was monitored by GLC. After completion of the
reaction, the product was extracted with ether or CH Cl , ®ltered, and the solvent was evaporated
(
2
2
under reduced pressure to yield the corresponding phenol or alcohol.
Desilylation of phenols or alcohols in the presence of PdCl (PhCN)
2
2
3
1
mmol of silyl ether was placed in a 5 cm beaker, and 0.01 g of PdCl (PhCN) (1 mol%) was added.
2 2
3
Then one drop of water was added to the reaction mixture. The beaker was placed in a 50 cm te¯on
container and irradiated in a microwave oven (900 W) for 3 to 5 min. The progress of the reaction
was monitored by GLC. After completion of the reaction, the product was extracted with ether or
CH Cl , ®ltered, and the solvent was evaporated under reduced pressure to yield the corresponding
2
2
phenol and alcohol.
Acknowledgements
We thank the Iranian Scienti®c Research Council for ®nancial support of this work.
References
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Mingos DMP, Baghurst DR (1990) Chem Soc Rev 20: 1

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M. M. Mojtahedi et al.: Deprotection of Trimethylsilyl Ethers
[
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Received September 28, 1998. Accepted (revised) December 1, 1998