Are there metal oxides that prefer a [2 + 2] addition over a [3 + 2] addition to olefins? Theoretical study of the reaction mechanism of LReO₃ addition (L = O^-, Cl, Cp) to ethylene

The activation barriers and reaction energies for the [3 + 2] addition of the transition-metal oxides OsO₄ and LReO₃ (L = O^-, Cl, Cp) to ethylene have been investigated at the B3LYP level of theory, using an effective core potential for Re with a large valence basis set. The alternative two-step reaction path via [2 + 2] addition yielding an oxetane intermediate and subsequent rearrangement to the dioxylate has also been studied. It is found that the rhenium oxides LReO₃ have activation energies for the [3 + 2] addition significantly higher than that of OsO₄, whereas the [2 + 2] additions of ClReO₃ and CpReO₃ have barriers clearly lower than that of OSO₄. However, the activation energies for the [2 + 2] addition remain in all cases higher than the barriers for the [3 + 2] reaction, although the differences between the barrier heights is much less for LReO₃ than for OSO₄. The activation energies for rearrangement of the oxetane intermediate to the dioxylate are very high for all LReO₃ species, which rules out that the suggested two-step mechanism for alkene extrusion from Re(V) dioxylates takes place. Approximate calculations show that this conclusion is valid also for Cp*ReO₃. The calculations suggest the possibility that (1,2)pushpull- substituted olefins yield metallaoxetanes in metal oxide addition reactions rather than dioxylates, because the carbon atoms of the metallaoxetanes have a zwitterionic character in the [2 + 2] transition state and in the product. The Cp ligand shows unique properties as 'stereoelectronic mediator' by adopting different bonding modes with the metal in CpReO₃-C₂H₄ isomers. This leads to energies for the activation barriers and reaction products which are not obvious when free CpReO₃ becomes analyzed.