Transition metal catalysts olefin epoxidation

The compehtion of one-electron pathways is sometimes detectable in the epoxidations catalyzed by transition metal catalysts [67]. However, in the epoxidahon of unhindered olefins on TS-1, the typical radical products are below the detection limits. Their presence could no longer be neglected when the rate of epoxidation is so slow as to become comparable to that of homolytic side reactions, for example with bulky olefins (see also Section 18.11). It is possible that, within these limits only, the epoxide is produced in part through the addition of a radical peroxy intermediate to the double bond [68, 69]. Even so, a homolytic pathway has again been proposed as a generally vahd epoxidation mechanism [7]. 

A new variant of the Sharpless epoxidation is a one-pot procedure. It is known that on photochemical oxidation of olefins in the presence of tetraphenylporphine the corresponding allyl hydroperoxides are formed. The latter were partly reduced into allylic alcohols, which were epoxidized by the remaining hydroperoxide in the presence of transition metal catalysts. By addition of l-(+)-DET to the photochemical oxidation of 2,3,3-trimethyl but-l-ene, Adam et al. [16] succeded in preparing the (S)-epoxy alcohol in a good yield, while the epoxidation under standard conditions delivers only a smaller amount of product (Scheme 10). 

NFPA Health 3, Flammability 4, Reactivity 3 Uses Catalyst for Ziegler-Natta polymerization of olefins and dienes catalyst for epoxidation in organic synthesis synthesis of cyclopropanes and ketocarbenes in preservation of archival papers reactant in prod, of transition metal catalysts Manuf./Distrib. ABCR http //www.abcr.de, Acros Org. http //www.acros.be, Akzo Nobel http //www.akzonobel.com, Aldrich http //www.sigma-aldrich.com, FIuka http //www.sigma-aldrich. com Difenacoum CAS 56073-07-5... 

The highest stereoselectivities are reached with disubstituted (Z) -alkenes (with ee-values up to 99%) as well as trisubstituted alkenes, whereas mono-substituted olefins are poorer substrates. Concerning the mechanism of the Jacobsen epoxidation (see Ref. [21]), asymmetric epoxidation of non-activated olefins can be performed with numerous other transition-metal catalysts [22]. 

Asymmetric manganese-salen-catalyzed epoxidation of unfunctionalized olefins was reported by Jacobsen et al. [74] in 1990, which allowed the enantioselective epoxidation of unfunctionalized olefins. In particular, the high enantioselectivities obtained for Jacobsen epoxidation on cis-olefins, nicely complement the Sharpless epoxidation and dihydroxylation protocols, which give reduced enantioselectivities for these substrates. The Sharpless and Jacobsen procedures are frequently used asymmetric oxidative reactions in API synthesis. More recently, organocatalytic procedures such as Shi epoxidations [75] were also employed to avoid toxic transition metal catalysts. 

In 1990, Jacobsen and subsequently Katsuki independently communicated that chiral Mn(III)salen complexes are effective catalysts for the enantioselective epoxidation of unfunctionalized olefins. For the first time, high enantioselectivities were attainable for the epoxidation of unfunctionalized olefins using a readily available and inexpensive chiral catalyst. In addition, the reaction was one of the first transition metal-catalyzed... 

The phenomenon that early transition metals in combination with alkyl hydroperoxides could participate in olefin epoxidation was discovered in the early 1970s [30, 31]. While m-CPBA was known to oxidize more reactive isolated olefins, it was discovered that allylic alcohols were oxidized to the corresponding epoxides at the same rate or even faster than a simple double bond when Vv or MoVI catalysts were employed in the reaction [Eq. (2)] [30]. 

A chiral diphosphine ligand was bound to silica via carbamate links and was used for enantioselective hydrogenation.178 The activity of the neutral catalyst decreased when the loading was increased. It clearly indicates the formation of catalytically inactive chlorine-bridged dimers. At the same time, the cationic diphosphine-Rh catalysts had no tendency to interact with each other (site isolation).179 New cross-linked chiral transition-metal-complexing polymers were used for the chemo- and enantioselective epoxidation of olefins.180... 

Zinc compounds have recently been used as pre-catalysts for the polymerization of lactides and the co-polymerization of epoxides and carbon dioxide (see Sections 2.06.8-2.06.12). The active catalysts in these reactions are not organozinc compounds, but their protonolyzed products. A few well-defined organozinc compounds, however, have been used as co-catalysts and chain-transfer reagents in the transition metal-catalyzed polymerization of olefins. 

Many transition-metal complexes have been widely studied in their application as catalysts in alkene epoxidation. Nickel is unique in the respect that its simple soluble salts such as Ni(N03)2 6H20 are completely ineffective in the catalytic epoxidation of alkenes, whereas soluble manganese, iron, cobalt, or copper salts in acetonitrile catalyze the epoxidation of stilbene or substituted alkenes with iodosylbenzene as oxidant. However, the Ni(II) complexes of tetraaza macrocycles as well as other chelating ligands dramatically enhance the reactivity of epoxidation of olefins (90, 91). 

Co2 +-Substitution at the addenda atoms gives catalysts for the epoxidation of olefins in the presence of aldehyde [293). PWM-Co is the most active among the mono-transition-metal-substituted polyanions the order of activity is PWn-Co > -Mn 2= -Fe 2= -Cu > -Ni. Here, PWll(M + )0(379", ) (M = Co2 +, Cu2+, Fe3 +, Ni2 +, Mn2 + ) is denoted by PWn M. The same order was observed for the oxidation of isobutyraldehyde, suggesting that the oxidation of aldehyde to give peracid is an important step in the reaction. It has been reported that substitution of V5+ for Mo6+ in PMo O3 gives a good catalyst for epoxidation and the Baeyer-Villiger reaction [294). 

As mentioned previously, the role of the metal is not specific. Even transition metal free layered double hydroxides are suitable catalysts for the olefin epoxidation with O2 and a sacrificial aldehyde (205). Leaching of the metal from the solid catalyst is a serious problem since organic acids that are potential metal ligands accumulate during the reaction. Leaching occurs, for example, with the polybenzimidazole-supported Ni2+ catalyst (199). 

An appreciable number of monographs and reviews deal with the meth-Q rsi,275,329-333 jjjg j j.gg gj jpunt of experimental work that has been performed provides a possibility for establishing favorable conditions of epoxidation with regard to the roles of the catalyst, the organic hydroperoxide, the structure of the olefin, and the medium. Simitar to the hydrogen peroxide-transition-metal complex reaction, this is an electrophilic reaction (Eq. 30). ... 

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