Microwave thermolysis: A rapid and selective method for the cleavage of THP ethers using CrO3 supported onto zeolite in solventless system

Article abstract of DOI:10.1080/10426500214895

Oxidative deprotection of tetrahydropyranyl ethers (THP ethers) using chromium trioxide supported onto zeolite under microwave irradiation in solvent free condition is described.

Full text of DOI:10.1080/10426500214895

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Phosphorus, Sulfur, and Silicon and the Related
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Microwave Thermolysis: A Rapid and Selective Method
for the Cleavage of THP Ethers Using CrO 3 Supported
onto Zeolite in Solventless System
Majid M. Heravi a , Hossein A. Oskooie a , Ladan Sangsefidi a , Mitra Ghassemzadeh b
Koroush Tabar-Hydar b
&
a Azzahra University , Vanak, Tehran, Iran
b Chemistry and Chemical Engineering Research Center of Iran , Tehran, Iran
Published online: 27 Oct 2010.
To cite this article: Majid M. Heravi , Hossein A. Oskooie , Ladan Sangsefidi , Mitra Ghassemzadeh & Koroush Tabar-
Hydar (2002) Microwave Thermolysis: A Rapid and Selective Method for the Cleavage of THP Ethers Using CrO 3 Supported
onto Zeolite in Solventless System, Phosphorus, Sulfur, and Silicon and the Related Elements, 177:12, 2843-2846, DOI:
10.1080/10426500214895
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Phosphorus, Sulfur and Silicon, 2002, Vol. 177:2843–2846
C
Copyright 2002 Taylor & Francis
1042-6507/02 $12.00 + .00
DOI: 10.1080/10426500290146884
MICROWAVE THERMOLYSIS: A RAPID
AND SELECTIVE METHOD FOR THE CLEAVAGE
OF THP ETHERS USING CrO3 SUPPORTED ONTO
ZEOLITE IN SOLVENTLESS SYSTEM
Majid M. Heravi,a Hossein A. Oskooie,a Ladan Sangsefidi,a
Mitra Ghassemzadeh,b and Koroush Tabar-Hydarb
Azzahra University, Vanak, Tehran, Irana and Chemistry and
Chemical Engineering Research Center of Iran, Tehran, Iranb
(Received April 16, 2002; accepted May 14, 2002)
Oxidative deprotection of tetrahydropyranyl ethers (THP ethers) using
chromium trioxide supported onto zeolite under microwave irradiation
in solvent free condition is described.
Keywords: Chromium trioxide; microwave thermolysis; solventless
system; THP ethers; zeolite
An oxidant that is efficient in transferring oxygen, selective in its
reactivity, mild toward oxidized product, possesses catalytic activity,
and is commercially available would unquestionably be the synthetic
chemist’s delight. Chromium-based reagents have been extensively
used in organic synthesis.1 The oxidation of primary and secondary
alcohols to carbonyl compounds is not frequently accomplished by the
use of chromium(VI) reagents in quantities ranging from stoichiometric
to a large excess.2 Since the chromium residues are environmentally
hazardous and have the potential danger (ignition or explosion) in
term of product isolation and vast disposal,3 it would be advantageous
to develop oxidizing methods using chromium trioxide along with solid
supports.
Introduction of reagents on solid supports has opened an attractive
avenue in organic synthesis in view of selectivity and associated ease
of manipulation.4 It applied especially to chromium compounds.5 Mi-
crowave heating and its application in organic chemistry for reactions
is currently being developed successfully6 and in recent years there
Address correspondence to M. M. Heravi, Department of Chemistry, School of Sci-
ences, Azzahra University, Vanak, Tehran, Iran. E-mail: mmheravi@azzahra.ac.ir
2843

2844
M. M. Heravi et al.
has been a tremendous interest in combining this technique with
dry conditions7 where the absence of solvent reduces the risk of haz-
ardous explosion. The protection and selective deprotection of func-
tional groups is of great significance in multistep organic synthesis.8
The large repertoire of protecting groups available for the hydroxyl
functionality8,9 exemplifies its sensitivity. The protection of hydroxy
compounds as THP ethers is common practice in organic synthesis.10
Although literature enumerates quite a number of methods for the
oxidative deprotection of THP ethers to carbonyl compounds,11 careful
inspection of the reaction procedures reveals some drawbacks. There-
fore, introducing a mild, selective, and inexpensive reagent for such
functional group transformation still is in much demand.
The oxidation of alcohols to the carbonyl compounds using chromium
trioxide supported onto HZSM-5 zeolite has been reported.12 In this
communication, we are pleased to describe our results for the oxidative
deprotection of THP ethers by the above reagent in a solventless system
under microwave irradiation.
HZSM-5 zeolite was prepared following a previously described
procedure.13 Two equivalents of CrO3 and a weight equivalent of
HZSM-5 zeolite per mmol of benzyl THP ether were mixed throughly
and exposed to microwave irradiation for 90 s, which led to the forma-
tion of benzaldehyde almost quantitatively. Similar treatment of other
THP ethers gave the corresponding carbonyl compounds in a short time
in excellent yields. Table I shows some of the aldehydes and ketones,
which are oxidatively obtained under microwave irradiation. Overoxi-
dation was not observed with this supported reagent. Cinnamyl THP
ether was successfully converted to cinnamylaldehyde in high yield
showing that the carbon-carbon double bonds are not prone to cleavage
using this method.
TABLE I Oxidative Deprotection of THP Ethers with Chromium
Trioxide-HZSM-5 Zeolite Under Microwave Irradiation in Solvent
Free Conditions
Entry
Substrate
Time (sec) Molar ratioa
Yield (%)
1
2
3
4
5
6
7
8
C6H5CH2OTHP
90
120
60
120
120
90
1:2
1:2.5
1:2
1:2.5
1:2.5
1:2
95
97
92
93
95
98
98
95
2-NO2-5-Me-C6H3CH2OTHP
4-NO2-C6H4CH2OTHP
2-NO2-3-Me-C6H3CH2OTHP
4-Me-C6H4CH2OTHP
Ph2CHOTHP
c-C6H11OTHP
PhCh CH CH2OTHP
60
60
1:2
1:2
aSubstrate: CrO3.

Microwave Thermolysis
2845
It is noteworthy that in the absence of zeolite oxidative deprotection
of THP ethers do not occur.
The use of different ratios of substrate/reagent was investigated and
it was found that 1:2.5 is suitable for the successful oxidative cleav-
age. Furthermore, it was noticed that the reactions remain incomplete
when they are done in a solvent (CH2Cl2), even with a higher ratio of
reagent (1:5) and longer reaction time (2 h). However, the reaction pro-
ceeds smoothly and efficiently in a very short time under microwave
irradiation in solvent free conditions in a few minutes.
In summary oxidative deprotection of THP ethers using CrO3-
HZSM-5 zeolite under microwave irradiation in a solventless system
has advantages over previously reported methods and provides a facile,
useful, and inexpensive addition to the existing methodologies. The ad-
vantages of this procedure are mild reaction condition, excellent yields,
short reaction time, a solvent less system, and minimization of side
products.
R
R
CrO /HZSM-5 zeolite
3
e
→
e
%
OTHP
O
%
0
0
R
R
EXPERIMENTAL SECTION
All products are known compounds and are characterized by compar-
ison of their physical and spectroscopic data with those of authentic
samples. THP ethers are synthesized via reported procedure.10a GC
analysis was performed with a Fisson 8000a gas chromatograph with
a flame ionization detector using a column 10% carbowax 20M on a
chromsorb W/AW, 1.8 M 6 mm.
Oxidative Deprotection of THP Ethers
by Zeolite-HZSM-5-CrO3
General Procedure
Chromium trioxide (2 mmol) and an indicated equivalent of HZSM-5
zeolite were crushed together in a mortar so as to form an intimate
mixture. A neat THP ether (1 mmol) was added to this mixture. The
resulting mixture was mixed throughly using a spatula. This mix-
ture was placed in a household microwave oven for an indicated time.
The progress of reaction was monitored by TLC using petroleum ether-
ethyl acetate. The solvent was evaporated to afford the corresponding
carbonyl compound (Table I). (Caution: Although this procedure worked

2846
M. M. Heravi et al.
safely in our hands, it is recommended to use a microwave oven in an
efficient and standard hood.)
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