Chemo- And periselectivity in the addition of [OsO₂(CH ₂)₂] to ethylene: A theoretical study

Quantum chemical calculations by using density functional theory at the B3LYP level have been carried out to elucidate the reaction course for the addition of ethylene to [OsO₂-(CH₂)₂] (1). The calculations predict that the kinetically most favorable reaction proceeds with an activation barrier of 8.1 kcal mol^-1 via [3+2] addition across the O=Os=CH₂ moiety. This reaction is -42.4 kcal mol^-1 exothermic. Alternatively, the [3+2] addition to the H₂C=Os=CH ₂ fragment of 1 leads to the most stable addition product 4 (-72.7 kcal mol^-1), yet this process has a higher activation barrier (13.0 kcal mol^-1). The [3+2] addition to the O= Os=O fragment yielding 2 is kinetically (27.5 kcal mol^-1) and thermodynamically (-7.0 kcal mol^-1) the least favorable [3+2] reaction. The formal [2+2] addition to the Os=O and Os=CH₂ double bonds proceeds by initial rearrangement of 1 to the metallaoxirane 1a. The rearrangement 1→1a and the following [2+2] additions have significantly higher activation barriers (>30 kcal mol^-1) than the [3+2] reactions. Another isomer of 1 is the dioxoosmacyclopropane 1b, which is 56.2 kcal mol^-1 lower in energy than 1. The activation barrier for the 1→1b isomerization is 15.7 kcal mol^-1. The calculations predict that there are no energetically favorable addition reactions of ethylene with 1b. The isomeric form 1c containing a peroxo group is too high in energy to be relevant for the reaction course. The accuracy of the B3LYP results is corroborated by high level post-HF CCSD(T) calculations for a subset of species.