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.