Oxidations of cyclic beta-diketones catalyzed by methylrhenium trioxide

Methylrhenium trioxide (CH3ReO3 or MTO) catalyzes the oxidation of beta-diketones by hydrogen peroxide. The kinetics of the initial oxidation step have been investigated in CH3CN/H2O (1:1 v/v) at 25 degrees C for a group of cyclic beta-diketones. The initial oxidation step features the enol form, the majority species, as the reactant. Its rate responds to substituents in the ''normal'' manner: electron-donating groups 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-dicarbonyl intermediate, which in some instances was detected by H-1 NMR. This hydroxy intermediate is susceptible to cleavage via a Baeyer-Villiger oxidation to yield carboxylic acids as final products. In contrast to the first reaction, this step may feature the peroxorhenium complexes A and B as nucleophiles rather than their customary electrophilic behavior; perhaps the trend is reversed by substrate binding to rhenium. Time profiles for the different stages of the reaction were also determined. The mechanistic aspects of these multistep catalytic oxidations are discussed in terms of the electronic nature of the activated rhenium-bound peroxo ligands.