Quantum chemical study on ethylene addition to (O = )₂Os( = NH)₂ and (O = )₂Os( = NH)-cyclo-(-NHCH₂CH ₂HN-) as model complexes for the osmium-catalyzed aminohydroxylation of olefins

Quantum chemical calculations on the hybrid density functional (B3LYP) level of theory were carried out to elucidate the initial step of the catalytic cycle of the aminohydroxylation of olefins with osmium reagents. Various reaction pathways of the model systems (O = )₂Os( = NH)₂ (Os1) and (O = )₂Os(-NH)-cyclo-(-NHCH₂CH₂HN-) (Os2) and two of their isomers, (HO-)(O = )Os( = NH)( = N) (Os1′) and (O = )Os(=OH)( = N)-cyclo-(=NHCH₂CH₂HN-) (Os2′), with ethylene have been considered. Os1, Os1′, and Os2 prefer a [3+2] cycloaddition kinetically and thermodynamically, whereas Os2′ is predicted to favor a [1+2] pathway due to kinetic reasons. Thermodynamically, Os2′ prefers a consecutive rearrangement after the [1+2] addition. The isomer Os1′ is lower in energy than Os1, but the kinetically and thermodynamically most favorable reaction is the [3+2]_NH,NH ethylene addition to the latter isomer, which according to our calculations takes place without a barrier.