Potassium dodecatangestocobaltate trihydrate (K5CoW12O40·3H2O): A mild and efficient catalyst for deprotection of dioxolanes and trimethylsilyl ethers

Article abstract of DOI:10.1016/S0040-4039(01)01379-X

Mild and easy deprotection of benzylic dioxolanes to carbonyl compounds (in refluxing dry acetone) and trimethylsilyl ethers to alcohols in acetonitrile at ambient temperature has been carried out in excellent yields under K₅CoW₁₂O₄₀·3H₂O (0.01 molar equiv.) catalysis.

Full text of DOI:10.1016/S0040-4039(01)01379-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6771–6774
Potassium dodecatangestocobaltate trihydrate
(K5CoW12O40·3H2O): a mild and efficient catalyst for
deprotection of dioxolanes and trimethylsilyl ethers
Mohammad H. Habibi,* Shahram Tangestaninejad, Iraj Mohammadpoor-Baltork,
Valiollah Mirkhani and Bahram Yadollahi
Chemistry Department, Isfahan University, Isfahan 81745-117, Iran
Received 27 April 2001; revised 16 July 2001; accepted 27 July 2001
Abstract—Mild and easy deprotection of benzylic dioxolanes to carbonyl compounds (in refluxing dry acetone) and trimethylsilyl
ethers to alcohols in acetonitrile at ambient temperature has been carried out in excellent yields under K5CoW12O40·3H2O (0.01
molar equiv.) catalysis. © 2001 Elsevier Science Ltd. All rights reserved.
There are many published procedures for the cleavage
of acetals and ketals into carbonyl compounds,1 but the
development of new improved methods continues to
attract attention.2 Generally, this deprotection is car-
ried out by acid-catalyzed aqueous hydrolysis.3 How-
ever, very often this method is incompatible with the
presence of some other functional group in the
molecule, e.g. a protected hydroxyl group. Several
nonacidic methods for cleavage of acetals, such as wet
silica gel4 or lithium tetrafluoroborate in wet
acetonitrile5 have been employed. A few nonaqueous
methods utilizing phosphorous triiodide and diphos-
phorous tetraiodide,6 boron trifluoride–iodide ion,7 tita-
nium (IV) chloride,8 iodotrimethylsilane,9 chlorotri-
methylsilane–samarium trichloride,10 chlorotrimethylsi-
lane–sodium iodide11 and cerium(III) chloride12 have
been reported for deprotection of acetals to carbonyl
compounds. Many of these procedures suffer from lack
of selectivity, unsatisfactory yields, cost or toxicity of
the reagents, or necessity of anhydrous conditions.
These limitations prompted us to investigate further a
new catalyst, which is able to carry out the cleavage of
acetals and ketals with excellent yields.
been achieved by using aqueous HF–MeCN,13 KF–
Crown ether,14 BF3·Et2O–CHCl3,15 N-bromo succin-
imide,16 CF3SO3SiMe3–CH2Cl2,17 NH4F–MeOH,18
SiF4–MeCN,19 aqueous acid,20 tetrabutylammonium
fluoride (TBAF),21 K2CO3,22 Lewis acids23 and neutral
alumina.24 However, while fluoride ions are very basic,
especially under anhydrous conditions, and therefore
cannot be used for base-sensitive molecules,25,26 other
procedures are not acceptable for acid-sensitive systems
and, consequently, the development of alternative
methods of deprotection under neutral conditions
would be desirable.
Heteropoly compounds provide a good basis for the
design of mixed oxide catalysts and they have high
capability for practical uses. This is because the rela-
tionships among the following four levels of informa-
tion can be established on a molecular basis: (i)
catalytic performance; (ii) chemical and physical prop-
erties; (iii) molecular and bulk composition and struc-
ture; and (iv) the method of synthesis of catalysts in
which these relationships are most useful for the design
of practical catalysts. The catalytic function of hetero-
poly compounds has attracted much attention and they
are used in solution as well as in the solid state. The
The conversion of alcohols into trimethylsilyl ethers is a
widely used method of protection in organic synthesis.1
Selective removal of the trimethylsilyl (TMS) group has
Keywords: catalysis; polyoxometalate; deprotection; dioxolane;
trimethylsilyl ether.
* Corresponding author: Tel.: +98-311-793-2707; fax: +98-311-6689-
732; e-mail: habibi@sci.ui.ac.ir
Scheme 1.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01379-X

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M. H. Habibi et al. / Tetrahedron Letters 42 (2001) 6771–6774
Table 1. Deprotection of 1,3-dioxolanes using
K5CoW12O40·3H2Oa
reason why heteropoly catalysts are attractive is their
variety and high potential as catalysts.27–29 Polyox-
ometalates have proved to be good catalysts for various
oxidations. They are applied in bulk or supported
forms, and both homogeneous and heterogeneous
catalysis are possible. Due to their acidic and redox
properties, heteropoly compounds (heteropoly acids
and salts) are useful and versatile catalysts in a number
of transformations.30,31 In continuation of our investi-
gation on application of polyoxometalates in organic
synthesis,32 we wish to describe here that the deprotec-
tion of benzylic dioxolanes, which remains the most
usual protecting group for the ketone functionality, can
be effectively realized under mild conditions by a cheap
and easily prepared cobalt heteropoly compound
(K5CoW12O40·3H2O) in acetone (Scheme 1).
The para-chlorobenzaldehyde 1,3-dioxolane (Table 1,
entry 2) is transformed to the corresponding carbonyl
compound after 10 minutes by treatment with 0.01
molar equivalents of K5CoW12O40·3H2O (95% yield).
The deprotection of para-chlorobenzaldehyde 1,3-diox-
olane were studied in different solvents and acetone
turned out to be the best solvent for the deprotection of
1,3-dioxolanes. As shown in Table 1, in the presence of
dodecatangestocobaltate (K5CoW12O40·3H2O), a num-
ber of benzylic dioxolanes were heated in refluxing
acetone for 0.1–6 h to provide the corresponding car-
bonyl compounds in excellent yields. The structures of
the carbonyl compounds were confirmed by compari-
son of their boiling points or melting points, Rf values
on TLC and spectroscopic data with authentic samples.
This procedure appears to be quite efficient for acetals
and ketals. In conclusion, we provide an efficient and
practical procedure for cleavage of dioxolanes to
acetals and ketals.
Also, we have found that cleavage of trimethylsilyl
ethers occurs efficiently in excellent yields at ambient
temperature using K5CoW12O40·3H2O as the catalyst.
This reaction takes place when 0.01 molar equivalents
of K5CoW12O40·3H2O is mixed with the trimethylsilyl
ether in acetonitrile at ambient temperature (Scheme 2).
As shown in Table 2, when trimethylsilyl ethers were
allowed to react at room temperature with
K5CoW12O40·3H2O in MeCN, cleavage of the ethers
occurred and the corresponding alcohols were obtained
readily in 3–45 min with excellent yields (96–100%).
Phenolic trimethylsilyl ethers are resistant toward
deprotection with this catalyst and only 5% of product
was obtained from the reaction mixture (Table 2, entry
18).
In addition, we found that cobalt polyoxometalate,
K5CoW12O40·3H2O, can be reused several times without
loss of activity, simply by filtering the catalyst, washing
with acetone, drying and immediately reusing.
Preparation of the catalyst: The synthesis of potassium
dodecatangestocobaltate trihydrate
3H2O) starts with the preparation of sodium tangesto-
(K5CoW12O40·
Scheme 2.

M. H. Habibi et al. / Tetrahedron Letters 42 (2001) 6771–6774
6773
Table 2. Deprotection of silylethers catalyzed by
K5CoW12O40·3H2Oa
dicobalt(II)ate from cobaltous acetate (2.5 g, 0.01 mol)
and sodium tangestate (19.8 g, 0.06 mol) in acetic acid
and water at pH 6.5 to 7.5. The sodium salt is then
converted into the potassium salt by treatment with
potassium chloride (13 g). Finally, the cobalt(II) com-
plex is oxidized to the cobalt(III) complex by potassium
persulfate (10 g) in 40 mL of 2 M H2SO4. The crystals
of K5CoW12O40·20H2O were dried at 200°C after
recrystallization with methanol, potassium dode-
catangestocobaltate trihydrate (K5CoW12O40·3H2O)
was obtained. K5Co(III)W12O40·3H2O as catalyst was
completely identified using chemical analysis, IR spec-
tra, magnetic susceptibility and other spectral data and
compared with reported physical and spectral data.33–36
General procedure for deprotection of 1,3-dioxolanes
with K5CoW12O40·3H2O: Dodecatangestocobaltate
(K5CoW12O40·3H2O) (32 mg, 0.01 mmol) was added to
a stirred solution of para-chlorobenzaldehyde 1,3-diox-
olane (174.5 mg, 1 mmol) in acetone (5 mL) and the
resulting mixture was refluxed for 10 min. (entry 2,
Table 1). After cooling, the catalyst was removed by
filtration through a layer of silica gel and washed with
diethyl ether (20 mL). The solvent was evaporated
under reduced pressure and the residue was further
purified by column chromatography on silica gel (elu-
ent: ether–petroleum ether, 1:1) to give para-chloroben-
zaldehyde (yield 95%).
General procedure for deprotection of tetramethylsilyl
ethers with K5CoW12O40·3H2O: To
a mixture of
C6H5CH2OSiMe3 (1 mmol, 183 mg) and acetonitrile (5
mL) was added K5CoW12O40·3H2O (32 mg, 0.01
mmol). The mixture was stirred for 3 min. at room
temperature (followed by TLC or GC), filtered through
a short column of silica gel, washed with small amount
of CHCl3 and purified by column chromatography to
afford the benzyl alcohol in excellent yields (100%).
Acknowledgements
The authors are thankful to Isfahan University
Research Council for financial support of this work.
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