Oxidation of Alcohols by Hydrogen Peroxide, Catalyzed by Methyltrioxorhenium (MTO):  A Hydride Abstraction

Primary and secondary alcohols are oxidized using hydrogen peroxide as an oxygen donor and methyltrioxorhenium (MTO) as a catalyst. The methylrhenium di-peroxide, CH(3)Re(O)(eta(2)-O(2))(2)(H(2)O), was the dominant and reactive form of the catalyst. Representative rate constants k/L mol(-)(1) s(-)(1) are 1.02 x 10(-)(4) for 4-Me-alpha-methylbenzyl alcohol and 4.9 x 10(-)(5) for 4-Cl-alpha-methylbenzyl alcohol. There was a kinetic isotope effect of 3.2 for the alpha C-H bond. When sec-phenethyl alcohol was labeled with (18)O, 80% of the oxygen was retained in the ketone. Tests for the possible intervention of a free radical intermediate were carried out; the evidence was entirely negative. A mechanism featuring hydride abstraction is proposed, the first time for the H(2)O(2)/MTO system. Also, a cocatalytic set of reaction conditions has been developed on the synthetic scale, using bromide and MTO as cocatalysts, which cuts the reaction time from hours to minutes.