Discussions of the mechanism of the oxygen transfer to the double bond have led to controversy. Depending on the substrate and additives, the formation of side products with trans stereochemistry points to a radical mechanism, whereas alkyl-substituted olefins stereoselectively give only cis products via a concerted mechanism. The suggested formation of manganaoxetanes receives support from calculations on a theoretical level, and from experiments reported by Katsuki using derivatives of the Jacobsen catalyst.