Mechanism of the Olefin Epoxidation Catalyzed by Molybdenum Diperoxo Complexes:  Quantum-Chemical Calculations Give an Answer to a Long-Standing Question

Abstract

Quantum-chemical calculations at the B3LYP level have been carried out to elucidate the reaction mechanism of the epoxidation of ethylene with the molybdenum diperoxo complex MoO(O2)2OPH3. All relevant transition states and intermediates which belong to the reaction pathways suggested by Mimoun and by Sharpless were optimized. The calculations show that there is no reaction channel from the ethylene complex to the putative metalla-2,3-dioxolane 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(O2)2OPH3 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 barrier for the formation of the metalla-2,3-dioxolane (23.7 kcal/mol). Calculations 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-membered ring. The calculations strongly support the mechanism suggested by Sharpless, while the Mimoun mechanism leads to carbonyl compounds as reaction products. Examination of the electronic structure of the transition state of the epoxide formation with the Charge Decomposition Analysis shows that the reaction should be considered as nucleophilic attack of the olefin toward the σ* orbital of the peroxo bond.

Stereoselective Tandem Epoxidation–Alcoholysis/Hydrolysis of Glycals with Molybdenum Catalysts

“…[27] The theoretically predicted energies of the transition states and the calculated intrinsic reaction coordinates show that the oxygen transfer takes place in a single-step reaction via a direct attack of the alkene to the peroxo group as proposed by Sharpless, [28] thus ruling out Mimouns metalla-2,3-dioxolane model. [26] We initially evaluated the epoxidation of unprotected glucal (1) under similar conditions to those previously explored, [23] using two molybdenum catalysts in polar solvents such as water and methanol and using different oxidizing reagents.…”

Diperoxo Complexes of Vanadium(V) Containing Mannich Base Ligands

“…[3][4][5][6][7][8][9][10] They are widely used in stoichiometric as well as catalytic oxidation in organic and biochemistry, 11 for example, in the oxidation of thioanisole, 12,13 methylbenzenes, 14 tertiary amines, alkenes, alcohols, 15,16 bromide 17 and also in olefin epoxidations. [18][19][20][21][22] There has been a continuous upsurge in interest in peroxo compounds of vanadium since it has been demonstrated that vanadate and peroxovanadates are capable of inhibiting the hydrolysis of phosphoproteins [23][24][25][26] and exhibit insulin-like properties. [27][28][29][30][31] The exact mechanisms by which peroxovanadates carry out these functions are yet to be fully understood.…”

Peroxo Complexes of Molybdenum(VI) Containing Mannich Base Ligands

“…1 They are widely used in stiochiometric as well as catalytic oxidation in organic and biochemistry, 2 for example in the oxidation of thioanisole, 3,4 methylbenzenes, 5 tertiaryamines, alkenes, alcohols, 6,7 bromide 8 and also in olefin epoxidations. [9][10][11][12][13] They also act as isomerisation catalysts for some allylic alcohols and have been applied to bleaching processes.…”