Transition-metal catalyzed oxidations .7. Zirconium-catalyzed oxidation of primary and secondary alcohols with hydroperoxides

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 Angstrom molecular sieves is presented. Secondary alcohols-if not severely sterically hindered-are 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.