Current Epoxidation Reactions

The functionalisation of hydrocarbon feedstocks is a key value-creating step in the chemical industry. Selective oxidation reactions, such as the epoxidation of alkenes, are a substantial basis of fine chemical production [98]. For example, propylene oxide is used industrially in the production of polyurethane and propylene glycol [99], ethylene oxide (EO) is used to produce ethylene glycol and various ethoxylates [100], while cyclohexene oxide is used as an alicyclic chemical intermediate in the production of pesticides, pharmaceuticals, perfumes and dyes [101] or as a monomer in photoreactive polymerisation [102]. 

However, due to the strong nucleophilic character of O, its application in fine chemical production can be problematic, though examples using active and selective catalysts with gas-phase reactants exist such as Union Carbide/Dow Chemical Company s METEOR process for oxidation of ethylene to EO over silver [108], 

Reactions with can lead to over-oxidation of the desired product and 

Oxidation reactions with produce water as a by-product and are inherently more atom efficient reactions than those where traditional stoichiometric counterparts are used [115], thus allowing for a more environmentally benign oxidation. It is less nucleophilic than and is therefore less likely to lead to over-oxidation of the desired product, when also used with a selective catalyst. 

Since the development of titanium silicate (TS-1) materials as catalysts in the 1980s, heterogeneous Ti-catalysed oxidation reactions utilising aqueous have produced many effective and versatile reactions such as olefin epoxidation [116-118], alcohol oxidation [ 116,119,120] and phenol hydroxylation [121, 122], which adhere more closely to the principles of green chemistry. 

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Several significant reviews have appeared in recent years which contributed greatly to current knowledge of epoxide reaction mechanisms. Among them may be cited excellent discussions by Winetein and Henderson,am Kliel, 0 and Parker and Isaacs. Older articles of an encyclopedic nature include those of Bodforss,1 Meerwein,11 and Tiffeneau.1117 The well-known review by StreifcwieeerlMa may be consulted for a broader treatment of nucleophilic displacement reactions in general. 

The catalyst indicated extremely high resistance to the effects of oxidants and their intermediates and relatively high temperature, e.g. it preserves the ability to epoxidize during the whole time of the biomimic tests. Owing to the mentioned properties of the mimic, kinetic regularities of current monooxygenase reaction were studied in a broad range of reaction parameters with reproducible results. 

In recent years the discovery of novel methods of asymmetric synthesis has greatly increased the ability of organic chemists to synthesize optically active sugars. For example, the asymmetric epoxidation reaction discovered by Katsuki and Sharpless [142] was recently used as the key step in a synthesis of D-oleandrose 118 from divinyl carbinol 119 by Hatakeyama et al. [143]. An alternative approach to asymmetric synthesis of oleandrose was taken by Danishefsky et al. [144,32] in their synthesis of avermectin which is the first, and currently the only, reported total synthesis of an avermectin. The key step of this synthesis was a cyclocondensation reaction of optically active diene 121 with acetaldehyde catalyzed by the optically active Lewis acid (-h)-Eu(hfc)3 [145]. The resulting chiral pyrone was then elaborated to methyl-L-oleandroside 113. This was further converted to the disaccharide glycal 122 by a 4 step sequence in which glycoside formation was accomplished by iV-iodosuccinimide mediated addition of the alcohol to a glycal followed by tributyltin hydride... 

In current epoxidation processes, chlorine, hydroperoxides, and peracids are the most commonly used oxidants 1-2). Organic and inorganic compounds are coproduced in the reaction, which need to be recycled or disposed off. 

Work in the mid-1970s demonstrated that the vitamin K-dependent step in prothrombin synthesis was the conversion of glutamyl residues to y-carboxyglutamyl residues. Subsequent studies more cleady defined the role of vitamin K in this conversion and have led to the current theory that the vitamin K-dependent carboxylation reaction is essentially a two-step process which first involves generation of a carbanion at the y-position of the glutamyl (Gla) residue. This event is coupled with the epoxidation of the reduced form of vitamin K and in a subsequent step, the carbanion is carboxylated (77—80). Studies have provided thermochemical confirmation for the mechanism of vitamin K and have shown the oxidation of vitamin KH2 (15) can produce a base of sufficient strength to deprotonate the y-position of the glutamate (81—83). 

The most important chemical reaction of chi orohydrin s is dehydrochloriaation to produce epoxides. In the case of propylene oxide. The Dow Chemical Company is the only manufacturer ia the United States that still uses the chlorohydrin technology. In 1990 the U.S. propylene oxide production capacity was hsted as 1.43 x 10 t/yr, shared almost equally by Dow and Arco Chemical Co., which uses a process based on hydroperoxide iatermediates (69,70). More recentiy, Dow Europe SA, aimounced a decision to expand its propylene oxide capacity by 160,000 metric tons per year at the Stade, Germany site. This represents about a 40% iacrease over the current capacity (71). 

Maltol. Otsuka Chemical Co. in Japan has operated several electroorganic processes on a small commercial scale. It has used plate and frame and aimular cells at currents in the range of 4500—6000 A (133). The process for the synthesis of maltol [118-71 -8], a food additive and flavor enhancer, starts from furfural [98-01-1] (see Food additives Flavors and spices). The electrochemical step is the oxidation of a-methylfurfural to give a cycHc acetal. The remaining reaction sequence is acid-catalyzed ring expansion, epoxidation with hydrogen peroxide, and then acid-catalyzed rearrangement to yield maltol, ie ... 

The most common method of epoxidation is the reaction of olefins with per-acids. For over twenty years, perbenzoic acid and monoperphthalic acid have been the most frequently used reagents. Recently, m-chloroperbenzoic acid has proved to be an equally efficient reagent which is commercially available (Aldrich Chemicals). The general electrophilic addition mechanism of the peracid-olefin reaction is currently believed to involve either an intra-molecularly bonded spiro species (1) or a 1,3-dipolar adduct of a carbonyl oxide, cf. (2). The electrophilic addition reaction is sensitive to steric effects. 

The AE reaction has been applied to a large number of diverse allylic alcohols. Illustration of the synthetic utility of substrates with a primary alcohol is presented by substitution pattern on the olefin and will follow the format used in previous reviews by Sharpless but with more current examples. Epoxidation of substrates bearing a chiral secondary alcohol is presented in the context of a kinetic resolution or a match versus mismatch with the chiral ligand. Epoxidation of substrates bearing a tertiary alcohol is not presented, as this class of substrate reacts extremely slowly. 

Semiconducting devices, switches and miniaturised v.h.f. circuits are all particularly sensitive to the slightest reaction on critical surfaces, and in devices calling for the highest levels of reliability even the most inert of the phenolic, epoxide and silicone resins are not considered to be fully acceptablecorrosion of electronic assemblies may often be enhanced by migration of ions to sensitive areas under applied potentials, and by local heating effects associated with current flows. 

In the present study the dimer (salen)CoAlX3 showed enhanced activity and enantioselectivity. The catalyst can be synthesized easily by readily commercially available precatalyst Co(salen) in both enantiomeric forms. Potentially, the catalyst may be used on an industrial scale and could be recycled. Currently we are looking for the applicability of the catalyst to asymmetric reaction of terminal and meso epoxides with other nucleophiles and related electrophile-nucleophile reactions. 

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