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
Dv. Deubel et G. Frenking, 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, J AM CHEM S, 121(10), 1999, pp. 2021-2031
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.