Redox mechanism epoxidation

The occurred activated complex may induce formation of either epoxide or allyl alcohol. However, under current experimental conditions the biomimic possesses exclusive epoxi-dizing ability, and the catalytic cycle is completed at this stage. The number of cycles per perFTPhPFe(III)OH mole under optimal process conditions is 80 per hour [82], This example of the mimetic catalysis indicates the unity of acidic-basic and redox mechanisms typical of the enzyme catalysis. 

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. 

HOMOGENEOUS EPOXIDATION BY LATE TRANSITION METALS (REDOX MECHANISM)... 

Late transition metal ions that can accommodate a two-electron rise in their oxidation state, like Cr(III), Mn(III), and Fe(III), and likely Ru(I), operate by a redox mechanism of epoxidation. They receive an oxygen atom from a TO to form an oxene species (MO) which then transfers the oxygen to an olefin by the intermediacy of a metallacycle, or a radical or cation species. Interestingly, these systems are not inhibited by water or alcohol as are the Lewis acid metals. 

In the previous sections, epoxidation was accompanied by a net oxidation of the substrate. We would like to conclude our discussion of various mechanisms of epoxide formation by presenting one example in which the substrate does not undergo a change in redox state. 

The reactions of aldehydes at 313 K [69] or 323 K [70] in CoAlPO-5 in the presence of oxygen results in formation of an oxidant capable of converting olefins to epoxides and ketones to lactones (Fig. 23). This reaction is a zeolite-catalyzed variant of metal [71-73] and non-metal-catalyzed oxidations [73,74], which utilize a sacrificial aldehyde. Jarboe and Beak [75] have suggested that these reactions proceed via the intermediacy of an acyl radical that is converted either to an acyl peroxy radical or peroxy acid which acts as the oxygen-transfer agent. Although the detailed intrazeolite mechanism has not been elucidated a similar type IIaRH reaction is likely to be operative in the interior of the redox catalysts. The catalytically active sites have been demonstrated to be framework-substituted Co° or Mn ions [70]. In addition, a sufficient pore size to allow access to these centers by the aldehyde is required for oxidation [70]. 

Some bismuth(III) carboxylates catalyze the oxidative C-C bond cleavage of epoxides into carboxylic acids in a DMSO-O2 system (Scheme 14.109) [229], In the initial stage DMSO transfers an oxygen atom to epoxides activated by Bi(OCOR)3, producing a-hydroxy ketones [230]. a-Hydroxy ketones can be converted into carboxylic acids by O2 under bismufh(III) mandelate catalysis (Scheme 14.110). In the presence of catalytic amounts of Bi and Cu(OTf)2, 1,2-disubstituted epoxides are oxidized to a-diketones by a combination of O2 and DMSO (Scheme 14.111) [231]. The mechanism presumably involves two catalytic cycles fhe first oxidative ringopening to a-hydroxy ketones is catalyzed by Cu(OTf)2-DMSO, and the second oxidation to a-diketones is achieved by the Bi /Bi redox cycle under the action of O2. 

The group of Busch also presented evidence that the Mn(n) complex of a cross-bridged cyclam ligand, 4,ll-dimethyl-l,4,8,ll-tetraazabicyclo[6.6.2]hexadecanc, denoted as Mn"(Me2EBC)CL, forms a Mn(IV) adduct with lodosylbenzene which is a new active intermediate in epoxidation reactions [600,609,610]. These examples with Mn, and in the previous section with Fe, are instances of late transition metals catalyzing epoxidation reactions by both the redox and the Lewis acid mechanisms (see Chapter 3). 

The above mechanism suggested that the use of olefin activators other than paUadium(lI), which are not capable of promoting the p-hydride elimination, may lead to other types of olefin oxidization products, such as epoxides. Since thallium(in) is a known oxidant for olefin epoxidation, it was therefore postulated that replacement of the palladium(II) activator by T1(III) benzoate in the C0-NO2/NO redox system would lead to the accomplishment of olefin epoxidation [118]. 

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