Zeolite-supported chromium(VI) oxide: A mild, efficient, and inexpensive reagent for oxidative deprotection of trimethylsilyl ethers under microwave irradiation

Article abstract of DOI:10.1080/10426500490463853

Primary and secondary trimethylsilyl ethers are efficiently converted, to the corresponding carbonyl compounds using HZSM-5 zeolite-supported CrO ₃ under microwave irradiation in solventless system.

Full text of DOI:10.1080/10426500490463853

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Phosphorus, Sulfur, and Silicon and the Related
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ZEOLITE-SUPPORTED CHROMIUM(VI) OXIDE: A MILD,
EFFICIENT, AND INEXPENSIVE REAGENT FOR OXIDATIVE
DEPROTECTION OF TRIMETHYLSILYL ETHERS UNDER
MICROWAVE IRRADIATION
Majid M. Heravi a , Fereshteh Hydarzadeh a , Yahya Farhangi a & Mitra Ghassemzadeh b
a Azzahra University , Vanak, Tehran, Iran
b Chemistry & Chemical Engineering Research Center of Iran , Tehran, Iran
Published online: 16 Aug 2010.
To cite this article: Majid M. Heravi , Fereshteh Hydarzadeh , Yahya Farhangi & Mitra Ghassemzadeh (2004) ZEOLITE-
SUPPORTED CHROMIUM(VI) OXIDE: A MILD, EFFICIENT, AND INEXPENSIVE REAGENT FOR OXIDATIVE DEPROTECTION OF
TRIMETHYLSILYL ETHERS UNDER MICROWAVE IRRADIATION, Phosphorus, Sulfur, and Silicon and the Related Elements, 179:8,
1473-1475, DOI: 10.1080/10426500490463853
To link to this article: http://dx.doi.org/10.1080/10426500490463853
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Phosphorus, Sulfur, and Silicon, 179:1473–1475, 2004
ꢀC
Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490463853
ZEOLITE-SUPPORTED CHROMIUM(VI) OXIDE:
A MILD, EFFICIENT, AND INEXPENSIVE
REAGENT FOR OXIDATIVE DEPROTECTION
OF TRIMETHYLSILYL ETHERS UNDER
MICROWAVE IRRADIATION
Majid M. Heravi,a Fereshteh Hydarzadeh,a Yahya Farhangi,a
and Mitra Ghassemzadehb
Azzahra University, Vanak, Tehran, Iran;a and Chemistry &
Chemical Engineering Research Center of Iran, Tehran, Iranb
(Received November 27, 2003; accepted March 2, 2004)
Primary and secondary trimethylsilyl ethers are effeciently converted
to the corresponding carbonyl compounds using HZSM-5 zeolite-
supported CrO3 under microwave irradiation in solventless system.
Keywords: Carbonyl compound; microwave irradiation; oxidative
deprotection; silyl ethers; solventless system
INTRODUCTION
Trimethylsilyl group is one of the most widely used protection groups in
organic synthesis and is often used to prepare silyl ethers as versatile
derivatives of alcohols and phenols.1
Direct oxidation of trimethylsilyl ethers to the corresponding car-
bonyl compounds has attracted considerable attention in recent years,2
but the introduction of new methods, inexpensive reagents, and envi-
ronmentally friendly conditions for such functional group transforma-
tion is still in demand.
Chromium-based reagents have been widely used in organic
synthesis.3 However, due to its inherent toxicity and potential danger
in the handling of its complexes, difficulties in terms of product isola-
tion and waste disposal, chemists are reluctant to use it. However, the
concept of utilizing a reagent adsorbed on inert inorganic supports has
circumvented some of these problems. Therefore, reagents impregnated
Address correspondence to Majid M. Heravi, Department of Chemistry, School of Sci-
ences, Azzahra University, Vanak, Tehran, Iran. E-mail: mmheravi@azzahra.ac.ir
1473

1474
M. M. Heravi et al.
on mineral solid supports have been recently employed by several in-
vestigators and laboratories,4 including our own.5
We have recently reported on the fast microwave-assisted zeolite
HZSM-5 catalyzed oxidation of alcohols with chromium trioxide under
solvent-free conditions.6 Oxidative deprotection of trimethylsilyl ethers
has also been one of the main topics in our laboratory.2,7 Armed with
these experiences, we now report on the use of HZSM-5 zeolite along
with chromium trioxide, which oxidatively deprotects trimethylsilyl
ethers to afford the corresponding carbonyl compounds.
HZSM-5 zeolite was prepared following a previously described
procedure.8 The SiO2/Al2O3 ratio (53:1) was determined by X-ray flu-
orescence prior to the experiments, and the zeolite was activated by
heating at 400◦C for 8 h. Two equivalents of CrO3 and a weight equiva-
lent of HZSM-5 zeolite per mole of benzyltrimethylsilyl ether were used
and exposed to microwave irradiation for 2 min, which led to the for-
mation of benzaldehyde almost quantitatively. No trace of benzoic acid
was observed, showing that no over-oxidation occurs. To establish the
generality of the method, a variety of trimethylsilyl ethers were reacted
under the same conditions specified to yield the corresponding carbonyl
compounds in excellent yields (Table I).
Among the various supports, such as silica gel, alumina, montmo-
rillonite K-10, HY zeolite, NaY zeolite, and HZSM-5 zeolite, the latter
gave the best results regarding yield, reaction time, and selectivity.
It is notable that in the absence of solid support the reaction under
microwave irradiation is sluggish and a considerable amount of start-
ing material, along with deprotected alcohol and carbonyl compound,
even upon extended reaction time, are obtained. It is also noteworthy
to mention that at least a weight equivalent of HZSM-5 zeolite is re-
quired for satisfactory conversion under microwave irradiation in the
solventless system. As far as the reaction catalytic ability of different
TABLE I Oxidative Deprotection of Trimethylsilyl Ethers with HZSM-5
Zeolite
Entry
Substrate
Reaction time min
Product
Yield
1
2
3
4
5
6
7
8
9
PhCH2OSiMe3
4-MeO-C6H4OSiMe3
1.5
1.5
1.6
1.6
2
1.5
1.8
2
PhCHO
4-MeO-C6H4CHO
2-NO2-3-OMe-C6H3CHO 96
2-NO2-5-OMe-C6H3CHO 99
4-NO2-C6H4CHO
OCHCH2CHO
CH3(CH2)14CHO
Cyclohexanone
CH3(CH2)6CHO
91
82
2-NO2-3-OMe-C6H3OSiMe3
2-NO2-5-OMe-C6H3OSiMe3
4-NO2-C6H4CH2OSiMe3
Me3SiOCH2CH2CH2OSiMe3
CH3(CH2)15OSiMe3
96
99
83
89
93
c-C6H11OSiMe3
CH3(CH2)6CH2OSiMe3
1.5

Deprotection of Silyl Ethers
1475
ratio of zeolite/CrO3 for various substrates is concerned, the optimized
ratio is 1.2 equivalent of HZSM-5 zeolite:
EXPERIMENTAL
Trimethylsilyl ethers were prepared according to a known procedure.8
All oxidation products were identified by comparison of their physical
data with those given in the literature. Yields refer to isolated products.
Although we did not observe any accident, use of the microwave oven
in an efficient hood is highly recommended.
Oxidative Deprotection of Trimethylsilyl
Ether. General Procedure
Chromium trioxide (2 mmol) and 1.5 equivalent of HZSM-5 zeolite
were crushed together in a mortar to form an intimate mixture. A neat
trimethylsilyl either was added to this mixture. The resulting mixture
was mixed thoroughly using a spatula. The mixture was placed in a
household microwave oven and irradiated for an indicated time. The
crude product was directly subjected to column chromatography us-
ing hexane:EtOAc 8:2 as eluent, affording the corresponding carbonyl
compound.
REFERENCES
[1] a) T. W. Green and P. G. M. Wutz, Protective Groups in Organic Synthesis, 2nd ed.
(Wiley, New York, 1991), p. 68; b) C. B. Rees, Protective Groups in Organic Chemistry
(Plenum Press, London, 1973), Chap. 3.
[2] M. M. Heravi, D. Ajami, and M. Ghassemzadeh, Synthesis, 3, 393 (1999) and refer-
ences cited therein.
[3] A. J. Fatiadi, In Organic Synthesis by Oxidation with Metal Compounds, edited by
W. J. Mijis, C. R. H. I. de Jonge, (Plenum Press, New York, 1986), pp. 119–260.
[4] R. S. Varma and H. M. Meshram, Tetrahedron Lett., 38, 5127 (1997).
[5] a) M. M. Heravi, D. Ajami, M. M. Mojtahedi, and M. Ghassemzadeh, Tetrahedron
Lett., 40, 561 (1999); b) M. M. Heravi, D. Ajami, K. Aghapoor, and M. Ghassemzadeh,
Chem. Commun., 833 (1999).
[6] M. M. Heravi, D. Ajami, K. Tabar-Hydar, and M. Ghassemzadeh, J. Chem. Res., 334
(1999) and references cited therein.
[7] M. M. Heravi, D. Ajami, and K. Tabar-Hydar, Monatsh. Chem., 130, 337 (1999).
[8] G. Maity and S. C. Roy, Synth. Commun., 8, 1667 (1993).