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 t he 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 po tential for Re with a large valence basis set. The alternative two-step rea ction path via [2 + 2] addition yielding an oxetane intermediate and subseq uent 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 C lReO3 and CpReO3 have barriers clearly lower than that of OsO4. However, th e activation energies for the [2 + 2] addition remain in all cases higher t han the barriers for the [3 + 2] reaction, although the differences between the barrier heights is much less for LReO3 than for OsO4. The activation e nergies for rearrangement of the oxetane intermediate to the dioxylate are very high for all LReO3 species, which rules out that the suggested two-ste p mechanism for alkene extrusion from Re(V) dioxylates takes place. Approxi mate 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 dioxyl ates, because the carbon atoms of the metallaoxetanes have a zwitterionic c haracter in the [2 + 2] transition state and in the product. The Cp ligand shows unique properties as "stereoelectronic mediator" by adopting differen t bonding modes with the metal in CpReO3-C2H4 isomers. This leads to energi es for the activation barriers and reaction products which are not obvious when free CpReO3 becomes analyzed.