OXIDATIONS OF CYCLIC BETA-DIKETONES CATALYZED BY METHYLRHENIUM TRIOXIDE

Methylrhenium trioxide (CH3ReO3 or MTO) catalyzes the oxidation of bet a-diketones by hydrogen peroxide. The kinetics of the initial oxidatio n step have been investigated in CH3CN/H2O (1:1 v/v) at 25 degrees C f or a group of cyclic beta-diketones. The initial oxidation step featur es the enol form, the majority species, as the reactant. Its rate resp onds to substituents in the ''normal'' manner: electron-donating group s accelerate the reaction. We suggest that the double bond of the enol attacks a peroxo oxygen of a peroxorhenium complex A CH3Re(O)(2)(O-2) or B = CH3Re(O)(O-2)(2)(H2O). This reaction affords a 2-hydroxy-1,3-d icarbonyl intermediate, which in some instances was detected by H-1 NM R. This hydroxy intermediate is susceptible to cleavage via a Baeyer-V illiger oxidation to yield carboxylic acids as final products. In cont rast to the first reaction, this step may feature the peroxorhenium co mplexes A and B as nucleophiles rather than their customary electrophi lic behavior; perhaps the trend is reversed by substrate binding to rh enium. Time profiles for the different stages of the reaction were als o determined. The mechanistic aspects of these multistep catalytic oxi dations are discussed in terms of the electronic nature of the activat ed rhenium-bound peroxo ligands.