Mimoun epoxidation mechanism

Mimoun, H., Mechanism of olefins epoxidation, Angcw. Chem., 94, 751, 1982. 

At odds with other similar peroxo metal complexes, V0(02)pic(H20)2, 36, performs non-selective epoxidation reactions. On this occasion Mimoun proposed a mechanism where a homolytic rupture of one metal-peroxo oxygen bond produces the active oxidant (Scheme 14). When aromatic substrates are allowed to react with 36, hydroxylation reaction takes place by way of the same active species as indicated in Scheme 15. 

Mimoun proposed a mechanism that is general for both stoichiometric epoxidations with peroxo complexes and for catalytic systems employing alkyl hydroperoxides.292-294 It involves an alkylperoxidic species with the alkene complexed through the metal ... 

The metallacycle mechanism can also be considered a concerted mechanism. It is analogous to the one proposed for metal peroxo complexes and is based on the assumed formation of a cyclic intermediate that includes the peroxo group, the reactant molecule, and the metal ion (Mimoun, 1982, 1987 Huybrechts et al., 1992). For alkene epoxidation, the sequence of events would be represented as follows [Eq. (31)] ... 

Epoxidation. Mimoun et at. report that the M0O5 HMPT complex reacts with olefins to form epoxides in high yield. Alkyl substituents on the double bond increase the rate of epoxidation. Aprotic solvents also enhance the rate highest rates arc observed in methylene chloride. The reaction is very slow in DMF or THF. The epoxidation is stcreospecific with retention of configuration of the olefin. Thus dj-bulene-2 is convened into cu-2,3-epoxybulane and iran. -bulene-2 into /ran5-2,3-epoxybutane. The French chemists proposed the mechanism shown in scheme I. 

An alternative mechanism for oxygen transfer was proposed by Mimoun [21-25]. In this mechanism (eq. (11)) initial coordination of the olefin to the metal is followed by its rate-limiting insertion into the metal-oxygen bond giving a pseudocyclic dioxometallocyclopentane (Structure 3). The latter decomposes to the epoxide and the metal alkoxide. 

Key Words Ethylene oxide, Propylene oxide. Epoxybutene, Market, Isoamylene oxide. Cyclohexene oxide. Styrene oxide, Norbornene oxide. Epichlorohydrin, Epoxy resins, Carbamazepine, Terpenes, Limonene, a-Pinene, Fatty acid epoxides, Allyl epoxides, Sharpless epoxidation. Turnover frequency, Space time yield. Hydrogen peroxide, Polyoxometallates, Phase-transfer reagents, Methyltrioxorhenium (MTO), Fluorinated acetone, Alkylmetaborate esters. Alumina, Iminium salts, Porphyrins, Jacobsen-Katsuki oxidation, Salen, Peroxoacetic acid, P450 BM-3, Escherichia coli, lodosylbenzene, Oxometallacycle, DFT, Lewis acid mechanism, Metalladioxolane, Mimoun complex, Sheldon complex, Michaelis-Menten, Schiff bases. Redox mechanism. Oxygen-rebound mechanism, Spiro structure. 2008 Elsevier B.V. 

The mechanism of epoxidation of propylene by ferf-butylhydroperoxide on V(V) complexes has been thoroughly investigated by Mimoun [488] and bears many similarities to epoxidation by Mo(VI) complexes. Notable conclusions are the following (a) The reaction is highly stereoselective, cis olefins give cis epoxides and trans olefins give trans epoxides, (b) The reactivity of olefins increases with... 

H. Mimoun, M. Mignard, P. Brechot, L. Saussine, Selective epoxidation of olefins by oxo[N-(2-oxidophenyl)salicylidenaminato]vanadium(V) alkylperoxides. On the mechanism of the Halcon epoxidation process, J. Am. Chem. Soc. 108 (1986) 3711. 

The formation of an intermediate organometaUacycle, a metalla-2,3-dioxo-lane, is a central part of the mechanism proposed in the pioneering work of Mimoun and co-workers (1970) for the olefin epoxidation with Mo diperoxo complexes (Fig. 4). The initial step of the suggested mechanism is the coordination of the olefin with the d° metal. The second step is the formation of the organometaUacycle upon cycloinsertion of the coordinated olefin into a metal-oxygen bond of the metal diperoxo complex. Note that metalla-2,3-dioxolanes of late transition metals are known [73,74]. The final step is the cycloreversion of the organometaUacycle, yielding the oxirane. 

The two seminal contributions of Mimoun and Sharpless laboratories led to a controversy on the reaction mechanism that was lasting longer than for two decades [82] and expanded to the olefin epoxidation with other metal peroxo complexes, in particular those of rhenium. Kinetic studies of Al-Ajlouni and Espenson [83,84] on the MTO-catalyzed olefin epoxidation with H2O2 revealed the importance of both mono- and diperoxo species in the catalytic process as well as substituent effects on reaction rates, but the molecular mechanism remained uncertain. 

A completely different mechanism of epoxidation has obtained some support in the recent Uterature. This mechanism originated with the observation by Mimoun, Seree de Roch, and Sajus [95] that the covalent molybdenum VI peroxo complexes, [MoO(02)2Lx], reacted with olefins to form epoxides in high yield, equation (253). These Mo(VI) peroxo complexes are readily prepared from Mo03, the neutral ligand and aqueous hydrogen peroxide [398-400]. 

Figure 24.1 The Mimoun and Sharpless mechanisms for olefin epoxidation.

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