Introduction
<P>Heteropolyoxometalates as stable inorganic porphyrins have attracted
much attention in the last two decades. The versatility of these
catalysts has been demonstrated in various organic transformations
due to their weak super-acidicity, redox properties, low toxicity,
stability, inexpensiveness, water tolerance and reusability.
[
¹]
Potassium dodecatungstocobaltate
(PDTC) is apparently a perfect outer-sphere one-electron oxidant,
due to the presence of a sheath of chemically inert oxygen atoms,
which protects the central ion from undesired inner-sphere substitution reactions.
[
²]
For this reason, electron
transfer with PDTC typically leads to selective reactions and clean
chemistry.
</P><P>Habibi et al. introduced this reagent for the acetylation, formylation,
tetrahydropyranylation of alcohols, and detetrahydropyranylation
of ethers.
[
³]
Further PDTC
was successfully used in various organic transformations
including esterification and transesterification, regeneration of
carbonyl compounds from oximes, and synthesis of substituted 3,4-dihydropyrimidin-2(1
H)-ones, quinolines, dihydroquinolines,
pyridines, imidazoles and xanthenes.</P><P>For the preparation of PDTC, cobaltous
acetate and sodium tungstate
were initially treated with acetic acid and water at pH ˜7
to give sodium tungstodicobalt(II)ate. The sodium salt was then
converted into the potassium salt by treatment with potassium chloride.
Finally, the cobalt(II) complex was oxidized to the cobalt(III)
complex by potassium persulfate in sulfuric acid. Crystallization
in MeOH yielded PDTC as light blue solid.
[
²]
[
5]
</P>
