Jacobsen-Katsuki epoxidation Mechanism

Jonsson, S., Odille Fabrice, G.J., Norrby, P.-O. and Warnmark, K. (2006) Modulation of the reactivity, stability and substrate- and enantioselectivity of an epoxidation catalyst by noncovalent dynamic attachment of a receptor functionality - aspects on the mechanism of the Jacobsen-Katsuki epoxidation applied to a supramolecular system. Org. Biomol. Chem., 4, 1927-1948 Jonsson, S., Odille Fabrice, G.J., Norrby, P.-O. and Warnmark, K. (2005) A dynamic supramolecular system exhibiting substrate selectivity in the catalytic epoxidation of olefins. Chem. Commun., 549-551. 

T. Linker, The Jacobsen-Katsuki epoxidation and its controversial mechanism, Angew. Chem. 1997, 109,2150-2152 Angew. Chem. Int. Ed. Engl. 1997, 36, 2060-2062. 

The first reports of a reaction of an amine with an aldehyde by Schiff [584] led to the establishment of a large class of ligands called Schiff bases. Among the most important of the Schiff bases are the tetradentate salen ligands (N,N -bis(salicy-laldehydo)ethylenediamine), which were studied extensively by Kochi and coworkers, who observed their high potential in chemoselective catalytic epoxidation reactions [585]. The best known method to epoxidize unfunctionalized olefins enantioselectively is the Jacobsen-Katsuki epoxidation reported independently by these researchers in 1990 [220,221]. In this method [515,586-589], optically active Mn salen) compounds are used as catalysts, with usually PhlO or NaOCl as the terminal oxygen sources, and with a O=Mn (salen) species as the active [590,591] oxidant [586-594]. Despite the undisputed synthetic value of this method, the mechanism by which the reaction occurs is still the subject of considerable research [514,586,591]. The subject has been covered in a recent extensive review [595], which also discusses the less-studied Cr (salen) complexes, which can display different, and thus useful selectivity [596]. Computational and H NMR studies have related observed epoxide enantioselectivities... 

FIGURE 1.28 Likely mechanism of the isomerization in the Jacobsen-Katsuki epoxidation of c/s-stilbene. 

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. 

In the proposed mechanism for the epoxidation of olefins such as styrene, the Mn(III) on nano magnesium oxide is complexed with a chiral ligand and subsequently oxidized by f-BuOOH to form a metal-oxo [Mn(IV) = O] (B) (Scheme 5.9), as indicated by XPS analysis.Mn(IV) is indeed found as an active species in Jacobsen-Katsuki catalytic epoxidations. Interaction of the olefin and (B),... 

The protocol developed by Jacobsen and Katsuki for the salen-Mn catalyzed asymmetric epoxidation of unfunctionalized alkenes continues to dominate the field. The mechanism of the oxygen transfer has not yet been fully elucidated, although recent molecular orbital calculations based on density functional theory suggest a radical intermediate (2), whose stability and lifetime dictate the degree of cis/trans isomerization during the epoxidation <00AG(E)589>. 

The mechanism of the J-K epoxidation is not fully understood, but most likely a manganese(V)-specles Is the reactive intermediate, which Is formed upon the oxidation of the Mn(lll)-salen complex. The enantioselectivity Is explained by either a top-on approach (Jacobsen) or by a side-on approach (Katsuki) of the olefin. The three major mechanistic pathways are shown below. The radical intermediate accounts for the formation of mixed epoxides when conjugated olefins are used as substrates. 

Mn(III) sits neatly in a tetracoordinate pocket in the ligand, and catalyses the epoxidation of simple alkenes by sodium hypochlorite, NaOCl, ordinary domestic bleach. Best results are obtained when the alkenes are cis (although an alternative range of ligands, developed by Tsutomu Katsuki, work well with trans alkenes), and one of the most significant applications of the Jacobsen epoxidation is with indene, which gives an epoxide in 84% ee with <1% of the catalyst. The mechanism of the reaction is complex and not fully understood, although it probably involves a Mn(V) oxo species and may involve radical intermediates. 

Imido and 0x0 compounds are intermediates in many of the transfers of oxygen atoms and nitrene units to olefins to form epoxides and aziridines, and they are intermediates in many of the insertions of oxygen atoms and nitrene units into the C-H bonds of hydrocarbons to form alcohols and amine derivatives. The enantioselective epoxidation of allylic alcohols (Scheme 13.22) " is the most widely used epoxida-tion process, and the discovery and development of this process was one of the sets of chemistry that led K. Barry Sharpless to receive the Nobel Prize in Chemistry in 2001. The mechanism of this process is not well established, despite the long time since its discovery and development. Nevertheless, most people accept that transfer of the oxygen atom occurs from a titanium-peroxo complex - rather than from an 0x0 complex. Jacobsen s and Katsuki s - manganese-salen catalysts for the enantioselective epoxidations of unfunctionalized olefins, which were based on Kochi s achiral chromium- and manganese-salen complexes, are a second set of... 

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