Ethylene addition to OsO₃(CH₂) - A theoretical study

Quantum chemical calculations at the B3LYP/TZVP level of theory have been carried out for the initial steps of the addition reaction of ethylene to OsO₃(CH₂). The calculations predict that there are two reaction channels with low activation barriers. The kinetically and thermodynamically most favored reaction is the [3+2]_{O, C} addition which has a barrier of only 2.3 kcal mol^-1. The [3+2]_{O, O} addition has a slightly higher barrier of 6.5 kcal mol^-1. Four other reactions of OsO₃(CH₂) with C₂H₄ have significantly larger activation barriers. The addition of ethylene to one oxo group with concomitant migration of one hydrogen atom from ethylene to the methylene ligand yields thermodynamically stable products but the activation energies for the reactions are 16.7 and 20.9 kcal mol^-1. Even higher barriers are calculated for the [2+2] addition to the Os{double bond, long}O bond (32.6 kcal mol^-1) and for the addition to the oxygen atom yielding an oxiran complex (41.2 kcal mol^-1). The activation barriers for the rearrangement to the bisoxoosmaoxirane isomer (36.3 kcal mol^-1) and for the addition reactions of the latter with C₂H₄ are also quite high. The most favorable reactions of the cyclic isomer are the slightly exothermic [2+2] addition across the Os{double bond, long}O bond which has an activation barrier of 46.6 kcal mol^-1 and the [3+2]_{O, O} addition which is an endothermic process with an activation barrier of 44.3 kcal mol^-1.