Mechanism of the olefin epoxidation catalyzed by molybdenum diperoxo complexes: Quantum-chemical calculations give an answer to a long-standing question
Dv. Deubel et al., Mechanism of the olefin epoxidation catalyzed by molybdenum diperoxo complexes: Quantum-chemical calculations give an answer to a long-standing question, J AM CHEM S, 122(41), 2000, pp. 10101-10108
Quantum-chemical calculations at the B3LYP level have been carried our to e
lucidate the reaction mechanism of the epoxidation of ethylene with the mol
ybdenum diperoxo complex MoO(O-2)(2)OPH3. All relevant transition states an
d intermediates which belong to the reaction pathways suggested by Mimoun a
nd by Sharpless were optimized. The calculations show that there is no reac
tion channel from the ethylene complex tb the putative metalla-2,3-dioxolan
e intermediate as suggested by Mimoun. There is a transition state for the
direct formation of the five-membered cyclic intermediate from ethylene and
the diperoxo complex. However, the subsequent extrusion of a C2H4O species
from the metalla-2,3-dioxolane does not yield the epoxide but acetaldehyde
. The calculations show that the reaction of MoO(O-2)(2)OPH3 with ethylene
can directly lead to the epoxide as suggested by Sharpless; The activation
energy for the latter process is 15.2 kcal/mol, which is lower than the bar
rier for the formation of the metalla-2,3-dioxolane (23.7 kcal/mol). Calcul
ations with the ligand OPMe3 instead of OPH3 show an even larger preference
of the pathway leading to the epoxide than the formation of the five-membe
red ring. The calculations strongly support the mechanism suggested by Shar
pless, while the Mimoun mechanism leads to carbonyl compounds as reaction p
roducts. Examination of the electronic structure of the transition state of
the epoxide formation with the Charge Decomposition Analysis shows that th
e reaction should be considered as nucleophilic attack of the olefin toward
the sigma* orbital of the peroxo bond.