Epoxide loss

The alkene substrate is first dispersed in the reactor with the carboxylic acid in the presence of a solvent if necessary. Addition of hydrogen peroxide forms the peracid which facilitates epoxidation, and re-generates the carboxylic acid for further reaction. In consequence only low levels of carboxylic acid are required (0.2-0.3 mol per mol of double bond). This also has the advantage of reducing epoxide loss to acid-catalysed ring-opening. The application of two phases with or without the presence of solvent also improves the efficiency of the epoxidation. Peracids most suited to epoxidation in situ are performic and peracetic acid. 

Solid complexes of defined stoichiometry have been prepared for all the lithium halides with HMPA. For LiBr, both [LiBr(HMPA)2] and [LiBr(HMPA)4] have been obtained as solids of defined m.p., but the 1 1 complex, the kinetically active species for epoxide rearrangement, has not been isolated. The rate of epoxide loss and solubility of LiBr increased proportionately with added solubilizer (HMPA), to a maximum rate at a 1 1 ratio of addend LiBr. Additional HMPA beyond this ratio caused the rate to decrease even though all the LiBr remained in solution. At an addend LiBr ratio of 2 1, the reaction effectively ceased. These observations allow the conclusions that [LiBr(HMPA)2] is more stable than the reactive 1 1 complex in benzene, and that only the latter is kinetically competent. 

A large number of methods have been used to prepare perfluoroepoxides (5). AH of these methods must contend with the great chemical reactivity of the epoxide product, especially with subsequent ionic and thermal reactions which result in the loss of the desired epoxide. 

The methylated maleic acid adduct of phthalic anhydride, known as methyl nadic anhydride VI, is somewhat more useful. Heat distortion temperatures as high as 202°C have been quoted whilst cured systems, with bis-phenol epoxides, have very good heat stability as measured by weight loss over a period of time at elevated temperatures. The other advantage of this hardener is that it is a liquid easily incorporated into the resin. About 80 phr are used but curing cycles are rather long. A typical schedule is 16 hours at 120°C and 1 hour at 180°C. 

Jacobsen subsequently reported a practical and efficient method for promoting the highly enantioselective addition of TMSN3 to meso-epoxides (Scheme 7.3) [4]. The chiral (salen)Cl-Cl catalyst 2 is available commercially and is bench-stable. Other practical advantages of the system include the mild reaction conditions, tolerance of some Lewis basic functional groups, catalyst recyclability (up to 10 times at 1 mol% with no loss in activity or enantioselectivity), and amenability to use under solvent-free conditions. Song later demonstrated that the reaction could be performed in room temperature ionic liquids, such as l-butyl-3-methylimidazo-lium salts. Extraction of the product mixture with hexane allowed catalyst recycling and product isolation without recourse to distillation (Scheme 7.4) [5]. 

Vinylepoxides are known to be unstable on silica, and separation of epoxides 7 and 8 was possible only at the expense of decreased yield. To avoid this loss, we separated the isomers at a later stage in the synthesis. 

Epoxides such as ethylene oxide and higher olefin oxides may be produced by the catalytic oxidation of olefins in gas-liquid-particle operations of the slurry type (S7). The finely divided catalyst (for example, silver oxide on silica gel carrier) is suspended in a chemically inactive liquid, such as dibutyl-phthalate. The liquid functions as a heat sink and a heat-transfer medium, as in the three-phase Fischer-Tropsch processes. It is claimed that the process, because of the superior heat-transfer properties of the slurry reactor, may be operated at high olefin concentrations in the gaseous process stream without loss with respect to yield and selectivity, and that propylene oxide and higher... 

To avoid loss of the volatile epoxide, removal of the dichloromethane on a rotary evaporator is not recommended. 

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

The mesoporous character of MCM-41 overcomes the size limitations imposed by the use of zeolites and it is possible to prepare the complex by refluxing the chiral ligand in the presence of Mn +-exchanged Al-MCM-41 [34-36]. However, this method only gives 10% of Mn in the form of the complex, as shown by elemental analysis, and good results are only possible due to the very low catalytic activity of the uncomplexed Mn sites. The immobihzed catalyst was used in the epoxidation of (Z)-stilbene with iodosylbenzene and this led to a mixture of cis (meso) and trans (chiral) epoxides. Enantioselectivity in the trans epoxides was up to 70%, which is close to the value obtained in solution (78% ee). However, this value was much lower when (E)-stilbene was used (25% ee). As occurred with other immobilized catalysts, reuse of the catalyst led to a significant loss in activity and, to a greater extent, in enantioselectivity. 

The same type of porphyrin-Ru complex was immobilized by coordina-tive adsorption on aminopropylsilicas (Fig. 26) as either amorphous or crystalline supports [79]. Mesoporous crystalline MCM-48 was the best support, as shown by the improved results obtained in the epoxidation of styrene with 2,6-dichloropyridine N-oxide (TON > 13 000 and 74% ee). The versatility of this catalyst was demonstrated in the intramolecular cyclopropanation of frans-cinnamyl diazoacetate. TON was ten times higher than that obtained in solution and 85% ee was observed. The solid was recycled and reused, although partial loss of selectivity occurred. 

The result is not totally surprising, because hydride ion shifts are known in many methylations. Thus, it was proposed that the methyl carbinol is formed by the sequence methylation of a double bond - hydride shift - formation of terminal methylene - epoxidation - opening of the epoxide to aldehyde - reduction to carbinol (Scheme 6). The pathway can explain well the loss of two original hydrogens in methionine methyl group. 

The degradation of vinyl chloride and ethene has been examined in Mycobacterium sp. strain JS 60 (Coleman and Spain 2003) and in Nocardioides sp. strain JS614 (Mattes et al. 2005). For both substrates, the initially formed epoxides underwent reaction with reduced coenzyme M and, after dehydrogenation and formation of the coenzyme A esters, reductive loss of coenzyme M acetate resulted in the production of 5-acetyl-coenzyme A. The reductive fission is formally analogous to that in the glutathione-mediated reaction. 

Chemo-enzymatic epoxidation of unsaturated fatty acids with aqueous H2O2 has been conducted with considerable success and here we have a remarkable situation that undesirable ring opening of the epoxide does not occur. Excellent activity and stability has been realized with Novozym 435, a Candida antartica lipase B immobilized on polyacryl. This enzyme is readily separable, can be used several times without loss of activity, and has a turnover of more than 2,00,000 moles of products per mole of catalyst (Bierman et al., 2000). 

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