Mechanistic aspects of the bismuth-catalysed oxidation of epoxides to α-diketones

The mechanism of the direct oxidation of 1,2-disubstituted epoxides to alpha-diketones catalysed by Bi-III derivatives was examined. The oxidation was carried out in DMSO under dioxygen. Experimental evidence for the various steps of the reaction was obtained from spectroscopic data, isolation of intermediate species, reactions with Bi-0, reaction under dinitrogen rather than under dioxygen, measurement of the oxygen consumption, cyclic voltammetry, and oxidations performed in the presence of methyl phenyl sulfoxide. The direct epoxide-to-diketone oxidation involves two main reaction steps: a first DMSO-mediated oxidative ring-opening of the oxirane ring, and a second Bi-catalysed oxidation to the alpha-diketone by dioxygen. A catalytic cycle is proposed combining the different results. A new alternative preparation of Bi(OTf)(3) by oxidative dissolution of Bi-0 is also described. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.