Density Functional Study of the [2+2]- and [2+3]-Cycloaddition Mechanisms for the Osmium-Catalyzed Dihydroxylation of Olefins

The postulated intermediates in the base-free and base-assisted addition of OsO4 to olefins have been optimized using density functional theory (DFT). Ammonia was chosen as the base and ethylene as the olefin. The corresponding transition states have been characterized fully. Further, the activation barriers have been computed at the nonlocal level, and special attention has been given to the two different mechanistic 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 kcal mol-1. The addition of a NH3 ligand to the osmium catalyst does not reduce significantly the [2+2] energy barrier. By contrast, it seems perfectly feasible that the dihydroxylation reaction proceeds through a [2+3] mechanism leading to the formation of a five-member ring intermediate as claimed by Corey. Such a process is found to be clearly exothermic and to involve a very small activation barrier of less than 2 kcal mol-1. A detailed analysis of the sequence describing exactly how the cycloaddition proceeds along the reaction path has also been performed by means of intrinsic reaction coordinate (IRC) calculations for the two studied mechanisms.