Diradical versus Concerted Mechanisms for the Dihydroxylation of Protoanemonin by OsO 4 and OsO 4 −NH 3 − 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(5 H )‐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 (NH 3 ) to OsO 4 ( 5 ) to form OsO 4 −NH 3 ( 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)