A new procedure for the oxidation (dehydrogenation) of primary and
secondary alcohols employing
Zr(O-t-Bu)4 or
Zr(O-n-Pr)4/tert-butyl
hydroperoxide/3 Å molecular sieves is presented.
Secondary
alcoholsif not severely sterically hinderedare usually converted
quantitatively to the corresponding ketones. Esters or acids can be byproducts in the reaction
of primary alcohols. However,
the aldehydes are obtained in good yield by lowering the reaction
temperature, decreasing the
amount of TBHP or replacing TBHP by cumene hydroperoxide (CHP), and/or
exchanging the catalyst
Zr(O-t-Bu)4 by
Zr(O-n-Pr)4 or silica gel-supported
Zr(OR)
x
. A remarkable selectivity of
equatorial
alcohol groups (e.g., 11 and 13) is observed in
contrast to chromium(VI)-based oxidations.
Strongly
chelating substrates such as furfuryl alcohol (18) or
1,2-diol 25 that prevent hydride transfer in
the six-membered transition state A are not
converted.
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