DENSITY-FUNCTIONAL STUDY OF THE [2-CYCLOADDITION AND [2+3]-CYCLOADDITION MECHANISMS FOR THE OSMIUM-CATALYZED DIHYDROXYLATION OF OLEFINS(2])

The postulated intermediates in the base-free and base-assisted additi on of OsO4 to olefins have been optimized using density functional the ory (DFT). Ammonia was chosen as the base and ethylene as the olefin. The corresponding transition states have been characterized fully. Fur ther, the activation barriers have been computed at the nonlocal level , and special attention has been given to the two different mechanisti c hypothesis proposed for this reaction. In particular, the hypothesis by Sharpless of a [2+2]-cycloaddition pathway involving the formation of a four-member ring as an intermediate has been ruled out since the corresponding activation barrier was calculated to be as high as 39 k cal mol(-l). The addition of a NH3 ligand to the osmium catalyst does not reduce significantly the [2+2] energy barrier. By contrast, it see ms perfectly feasible that the dihydroxylation reaction proceeds throu gh a [2+3] mechanism leading to the formation of st five-member ring i ntermediate as claimed by Corey. Such a process is found to be clearly exothermic and to involve a very small activation barrier of less tha n 2 kcal mol(-1). A detailed analysis of the sequence describing exact ly how the cycloaddition proceeds along the reaction path has also bee n performed by means of intrinsic reaction coordinate (IRC) calculatio ns for the two studied mechanisms.