Oxidative deprotection of trimethylsilyl ethers with hexamethylenetetramine-bromine supported onto silica gel under microwave irradiation in solventless system

Article abstract of DOI:10.1080/10426500490494697

Hexamethylenetetramine-bromine supported onto silica gel rapidly converts trimethylsilyl ethers to their corresponding carbonyl compounds under microwave irradiation in solventless system.

Full text of DOI:10.1080/10426500490494697

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OXIDATIVE DEPROTECTION OF
TRIMETHYLSILYL ETHERS WITH
HEXAMETHYLENETETRAMINEBROMI
SUPPORTED ONTO SILICA
GEL UNDER MICROWAVE
IRRADIATION IN SOLVENTLESS
SYSTEM
a
Mahboobeh Behnam Dehkordi , Majid M. Heravi
b
a
a
,
Parviz Khosrofar , Ahmad Ziafat & Mitra
c
Ghassemzadeh
a
Department of Chemistry, Ferdowsi University of
Mashhad , Mashhad, Iran
b
Department of Chemistry, School of Sciences,
Azzahra University , Tehran, Iran
c
Chemistry & Chemical Engineering Research Center
of Iran , Tehran, Iran
Published online: 16 Aug 2010.
To cite this article: Mahboobeh Behnam Dehkordi , Majid M. Heravi , Parviz
Khosrofar , Ahmad Ziafat & Mitra Ghassemzadeh (2004) OXIDATIVE DEPROTECTION
OF TRIMETHYLSILYL ETHERS WITH HEXAMETHYLENETETRAMINEBROMINE SUPPORTED
ONTO SILICA GEL UNDER MICROWAVE IRRADIATION IN SOLVENTLESS SYSTEM,
Phosphorus, Sulfur, and Silicon and the Related Elements, 179:12, 2595-2598, DOI:
10.1080/10426500490494697
To link to this article: http://dx.doi.org/10.1080/10426500490494697

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Phosphorus, Sulfur, and Silicon, 179:2595–2598, 2004
Copyright ꢀC Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490494697
OXIDATIVE DEPROTECTION OF TRIMETHYLSILYL
ETHERS WITH HEXAMETHYLENETETRAMINE-
BROMINE SUPPORTED ONTO SILICA GEL
UNDER MICROWAVE IRRADIATION
IN SOLVENTLESS SYSTEM
a
b
Mahboobeh Behnam Dehkordi, Majid M. Heravi,
a
a
c
Parviz Khosrofar, Ahmad Ziafat, and Mitra Ghassemzadeh
Department of Chemistry, Ferdowsi University of Mashhad,
a
Mashhad, Iran; Department of Chemistry, School of Sciences,
b
Azzahra University, Vanak, Tehran, Iran; and Chemistry &
c
Chemical Engineering Research Center of Iran, Tehran, Iran
(
Received May 13, 2004; accepted May 13, 2004)
Hexamethylenetetramine-bromine supported onto silica gel rapidly
converts trimethylsilyl ethers to their corresponding carbonyl com-
pounds under microwave irradiation in solventless system.
Keywords: Deprotection; hexamethylenetetramine-bromine; micro-
wave irradiation; oxidation
INTRODUCTION
The protection of certain functional groups and the deprotection of
the protected derivatives constitute important processes in the mul-
tistage synthetic chemistry of polyfunctional molecules, including the
1
total synthesis of natural products. One of the most useful and conve-
nient methods for protection of hydroxy groups is their transformation
to trialkylsilyl ethers. Direct oxidation of trimethylsilyl ethers to the
corresponding aldehydes and ketones is of great relevance to organic
synthesis.
Although various methods have been reported for such a trans-
2
formation, some of them show limitations such as the requirement
Partial financial support from Research Council of Ferdowsi University Mashhad is
gratefully acknowledged.
Address correspondence to Majid M. Heravi, Department of Chemistry, School of
Sciences, Azzahra University, Vanak, Tehran, Iran. E-mail: mmheravi@azzahra.ac.ir
2
595

2
596
M. B. Dehkordi et al.
of aqueous reaction conditions, use of expensive reagents, long reaction
times, tedious workup, and low yield of products. Therefore, presen-
tation of new methods and inexpensive and environmentally friendly
reagents for such functional group transformation is still in much de-
mand. Hexamethylenetetramine-bromine is an inexpensive reagent,
3
which has been reported as an oxidizing agent. We have recently suc-
cessfully used this oxidant in deoximation and desemicarbonization
4
reactions.
In recent years, the organic reactions that are assisted by
5
6
microwave, especially under solventless systems, have attracted at-
tention because of their enhanced selectivity, mild reaction conditions,
and associated ease of manipulation.
In continuation of our ongoing efforts in microwave-assisted
7
reactions, herein we wish to report a simple oxidative deprotection
of trimethylsilyl ethers with hexamethylenetetramine-bromine sup-
ported onto silica gel in solventless system, which is accelerated by
microwave irradiations.
In the absence of solid support, the attempted oxidative depro-
tection of silyl ethers failed with hexamethylenetetramine-bromine
under thermal as well as microwave irradiation even after pro-
longed reaction periods. In view of established beneficial effects of
the reagents on minerals,8 we used silica gel as a support for
hexamethylenetetramine-bromine. Hexamethylenetetramine-bromine
was mixed intimately with an equivalent weight of silica gel. When this
supported reagent was mixed thoroughly with benzylsilyl ether using
a spatula in a beaker and placed in microwave oven the reaction was
completed in 2 min as confirmed by thin layer chromatography (TLC;
hexane:ethyl acetate, 8:2: see Table I Entry 2).
To establish generality of the method, trimethylsilyl ethers of various
alcohols were used under the above condition.
TABLE I Oxidative Deprotection of Trimethylsilyl Ethers with
Hexamethylene-tetramine-bromine Supported onto Silica Gel under
Microwave Irradiation
Time
(s)
Yielda
(%)
Entry
Substrate
PhCH2OSiMe3
Cyclohexyltrimethylsilyl ether
n-CH3(CH2)6CH2OSiMe3
CH3(CH2)5CH2OSiMe3
Product
PhCHO
Cyclohexanone
CH3(CH2)6CHO
CH3(CH2)5CHO
Menthone
1
2
3
4
5
6
120
120
120
150
180
150
93
88
89
72
71
67
(−)-Menthylsilyl ether
Cholestryltrimethylsilyl ether
Cholostenone
aYields refer to isolated product.

Deprotection of Silylethers
2597
In conclusion, we have developed a solid-state method for
facile direct oxidative deprotection of trimethylsilyl ethers using
hexamethylenetetramine-bromine supported onto silica gel under mi-
crowave irradiation in solventless system.
The reagent could be readily prepared by adding bromine to a chloro-
form solution of commercially available amine. This homogenous non-
hygroscopic solid is very stable at room temperature and is not affected
by ordinary exposure to light, air, or water, and it has no offensive odor
of bromine or amine (Scheme 1).
SCHEME 1
EXPERIMENTAL
All compounds are known and characterized with their physical data
by comparison with those of authentic samples. In view of the hazard
associated with an oxidant and evolution of gases during the reaction,
caution is recommended for its use at elevated temperature. We suggest
that the microwave oven be operated carefully and for a shorter time
due to possibility of a high localized temperature.
Oxidative Deprotection of Trimethylsilyl Ethers:
General Procedure
Hexamethylenetetramine-bromide supported on silica gel (60 mesh,
2
mmol) was mixed thoroughly with the appropriate trimethylsilyl
ether (1 mmol) in a beaker. The beaker was placed in a microwave
oven and irradiated for specific time (Table I). The progress of reaction
was monitored by TLC. The crude CH2Cl2 (5 ml) was added and filtered
off. The filtrate was evaporated to dryness to afford the corresponding
carbonyl compound (Table I).
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