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.
Zauche
Timothy H.
TH
Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 50011.
Espenson
James H.
JH