Diradical versus concerted mechanisms for the dihydroxylation of protoanemonin by OsO<inf>4</inf> and OsO<inf>4</inf>-NH<inf>3</inf> - The effect of the base in the reaction

The dihydroxylation of a set of monosubstituted olefins [ethene (1), propene (2), styrene (3)] and 5-methylene-2(5H)-furanone [protoanemonin (4)] by osmium tetroxide have been studied by means of DFT (B3LYP) calculations. The effect on the reaction mechanism of the coordination of a base (NH3) to OsO4 (5) to form OsO4-NH3 (6) has been investigated. These calculations show that the activation barriers for the [3+2] concerted mechanisms are lowered by the presence of an amine coordinating the osmium tetroxide, in agreement with experiment. For the dihydroxylation of protoanemonin (an olefin having a stepwise diradical mechanism in the Diels-Alder reaction) both the concerted and the diradical mechanisms have been evaluated. These studies indicate that the dihydroxylation of olefins proceeds through a concerted mechanism, although in the dihydroxylation of protoanemonin the diradical mechanism may play a significant role. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.