Distortion, Interaction, and Conceptual DFT Perspectives of MO4-Alkene (M = Os, Re, Tc, Mn) Cycloadditions

The reaction pathways (including the transition states) of ethylene addition to osmium tetroxide (OsO4, and amine ligated), rhenate (ReO4-), technetate (TcO4-), and permanganate (MnO4-) have been studied by qualitative and quantitative analyses. Distortion/interaction and absolutely localized energy decomposition analyses provide new insights into why the (3 + 2) pathway is highly preferred over the (2 + 2) pathway, the origin of rate enhancement from ligated base, and reactivity differences between OsO4, ReO4-, TcO4-, and MnO4-. The (2 + 2) transition state has a much larger barrier than the (3 + 2) transition state because (1) the Os-O bond is stretched significantly resulting in a larger distortion energy (Delta E-d double dagger) value and (2) the transition state interaction energy (Delta E-i double dagger) is destabilizing due to large exchange repulsions overwhelming stabilizing charge-transfer terms. Base ligation lowers osmium tetroxide and ethylene distortion energies due to the ground-state O-Os-O angle being predistorted from 110 degrees to 103 degrees. Because MO4 distortion energies are comparable, reactivity differences between OsO4, ReO4-, TcO4-, and MnO4- is shown to be a function of ethylene to MO4 charge-transfer. This interaction also dictates the position of the transition state along the reaction coordinate and corresponds to the onset of a stabilizing Delta E-i double dagger value. The conceptual DFT hardness profile and hardness response show that the Q + 2) reaction pathway may be classified as an allowed pathway while the (2 + 2) reaction coordinate is best designated as forbidden.