Epoxides reaction number

There have been a number of computational studies of the epoxidation reaction. These studies have generally found that the hydrogen-bonded peroxy acid is approximately perpendicular to the axis of the double bond, giving a spiro structure.75 Figure 12.8 shows TS structures and Ea values based on B3LYP/6-31G computations. The Ea trend is as expected for an electrophilic process OCH3 < CH3 CH = CH2 < H < CN. Similar trends were found in MP4/6-31G and QCISD/6-31G computations. 

Schreiber s model has also proved to be a general approach to a series of oxygenated metabolites of arachidonic acid, such as lipoxin A and lipoxin B.50 The family of linear oxygenated metabolites of arachidonic acid has been implicated in immediate hypersensitivity reactions, inflammation, and a number of other health problems. Among these metabolites, several compounds, such as lipoxin A, lipoxin B, 5,6-diHETE, and 14,15-diHETE possess 1-substituted (/ )-1 -alken-3.4-diol 84 as a common substructural moiety. Therefore, the car-binol 83 is an ideal substrate for generating compound 84 by applying Sharpless epoxidation reaction.50... 

It is realised that both ruthenium and the substituted porphyrins are expensive catalyst components for industrial applications. Both turnover frequencies and turnover numbers are modest. Nevertheless it remains an interesting option to use dioxygen directly in epoxidation reactions. 

To avoid the inherent limitations of a kinetic resolution process, the reaction was extended to desymmetrization of prochiral meso epoxides. A number of cyclic di-methylidene epoxides were synthesized and subjected to treatment with Et2Zn in the presence of Cu(OTf)2 and ligands 42 or 43. As in the case mentioned above, ligand 42 was superior in terms of selectivity. Cydohexane derivative 46 gave the ring-opened product with a 97% ee and in a 90% isolated yield, with a y/a ratio of 98 2 (Scheme 8.28). The other substrates investigated produced sigmficantly lower ees of between 66% and 85%. 

RuClj(Hcbx)(cbx) (Hcbx=A-2 -chlorophenyl-2-pyridine-carboxamide). This red-brown material is made from the ligand and RuClj, and its X-ray crystal structure determined (Fig. 1.36). The system RuCl2(Hcbx)(cbx)/Oj/M( butyraldehyde/DCE epoxidised a number of cyclic alkenes efficiently at room temperatures (Table 3.1). Addition of the radical trap 2,6-di-ferf-butyl-4-methylphenol stopped epoxidation reactions altogether, suggesting that a mechanism involving radicals is involved [801],... 

In subsequent research, it turned out that two-state reactivity can also provide a concept for the understanding of oxidation reactions way beyond the scope of gas-phase ion chemistry and can actually resolve a number of existing mechanistic puzzles. In enzymatic oxidations involving cytochrome P450, for example, changes in spin multiplicity appear to act as a kind of mechanistic distributor for product formation [27-29], and in the case of manganese-catalyzed epoxidation reactions, two-state scenarios have been put forward to account for the experimentally observed stereoselectivities [30-32], Two-state reactivity is not restricted to oxidation reactions, and similar scenarios have been proposed for a number of other experimentally studied reactions of 3d metal compounds [33-37]. Moreover, two-state scenarios have recently also been involved in the chemistry of main group elements [38]. The concept of two-state reactivity developed from the four-atomic system FeO /H2... 

Direct Oxidation with Stoichiometric Oxidants. Discovered by Prilezhaev in 1909,211 the typical epoxidation reaction of alkenes is their oxidation with organic peracids. Of the large number of different peroxycarboxylic acids used in... 

The number of catalytic cycles (v) in propylene epoxidation reaction with hydrogen peroxide... 

High-valent oxo-complexes, isolated or in situ-generated, interact most often with electron-rich n -systems 1 or suitable C-H bonds with low bond dissociation energy (BDE) in substrates 3 (Fig. 2). These reactions may occur concerted via transition states 1A or 3A leading to epoxides 2 or alcohols 4. On the other hand, a number of epoxidation reactions, such as the Jacobsen-Katsuki epoxidation, is known to proceed by a stepwise pathway via transition state IB to radical intermediate 1C [39]. Similarly, hydrocarbon oxidation to 4 can proceed by a hydrogen abstraction/S ... 

Epoxides are a very versatile class of compounds and the interest in catalytic epoxidation reactions is very high.70,71 They are the key raw materials in the syntheses of a wide variety of chemicals. A number of compounds have been shown to be catalytically active, but the regular laboratory reagents for epoxidations are generally methyl trioxorhenium(VII)72-81 and the Jacobsen-Katsuki-catalysts82-94 which can even introduce chirality. They are also theoretically well investigated95-106 and are described below. 

The N-alkylation reaction represents a bifurcation of the normal alkene epoxidation reaction cycle and, therefore, N-alkylation is a suicide event that leads to catalytic inhibition in the native system. With synthetic tetraarylporphyrins that mimic the N-alkylation reaction, the use of halogen-substituted catalysts that are stable toward oxidative degradation (26, 27) provide the most useful model systems because the heme model remains intact for a significantly greater number of turnovers than the partition number. The partition number is the ratio of epoxidation cycles to N-alkylation cycles, i.e., N-alkyl porphyrins are formed before the heme is oxidatively destroyed. 

The use of amines to catalyze (asymmetric) epoxidation reactions was introduced by Aggarwal and co-workers in 2000 <2000JA8317>. (Y)-2-(Diphenylmethyl)pyrrolidine 69 was examined in the asymmetric epoxidation of a number of cycloalkenes (Table 6). Subsequently it was found that more consistent and reproducible results were obtained when... 

Figure 14. (a) Structure of a zeolite-entrapped perfluorinated ruthenium phthalocyanine (RuFi6Pc) complex used in epoxidation reactions, (b) Comparison of turnover numbers for cyclohexane oxidation using ruthenium phthalocyanine (RuPc), RuFi Pc and zeolite X-entrapped RuFi Pc. 

This ylide chemistry is treated in a number of monographs and reviews. The formation and properties of 77 and 78 were first dealt with by Corey et al. Reactive ylides are prepared from dialkyl sulfide and dialkyl sulfoxide in a non-aqueous medium. The solvent for the epoxidation reaction may be an aqueous basic solution. No side-reactions occur in a two-phase system or under the more recent phase-transfer conditions. ... 

Compounds with the oxirane rings are special cases which, beside this general methodology, are also prepared by a number of other methods (direct epoxidation, reaction with sulfur ylides, Darzens reaction, etc.) [1]. 

Figure 9.4 illustrates the effect of Au loading of the 13.2 wt.% Ag/7-AI2O3 catalyst on the ethylene epoxidation reaction by plotting Au loading versus the normalized turnover number (the turnover number ratio of Au-Ag catalyst to Ag catalyst). The normalized turnover number of ethylene oxide increased slightly with... 

The introduction of microporous and mesoporous supports with well-dispersed Ti has allowed for some detailed kinetic studies into the PO and water generation mechanisms primarily as a result of the outstanding stability of catalysts prepared from these materials. Prior to the synthesis of stable catalysts, a number of reaction mechanisms were proposed based on observed trends in reactivity, DPT calculations [63,64,76,78], and analogs to liquid-phase epoxidation reactions over Ti-based catalysts [14,15,89] rather than kinetic analysis. The first proposed mechanisms were constructed for the Au/Ti02 and Au/Ti02/Si02 system. 

In many cases, the addition of Lewis bases capable of coordinating to the metal center during epoxidation catalysis has been found to have a beneficial effect on catalyst turnover rate and number as well as epoxide yield. Commonly used additives include pyridine, imidazole, and pyridine N-oxide derivatives. The proposed roles of N-oxide derivatives in [Cr(salen)] -catalyzed and [Mn(salen)] -catalyzed epoxidation reactions include activation of the intermediate metal-0X0 complex [15,50], dissociation of umeactive p-oxo dimer complexes to reactive monomeric species [19,25], and/or solubilization of the active oxidant in bi-phasic reaction media [51]. 

A number of excellent reviews with comprehensive coverage on the literature of biooxidations have appeared in journals and books l 8). In this chapter we will only try to highlight some of these biotransformation reactions, in particular hydroxyla-tion of non-activated carbon atoms and double-bond epoxidation reactions. 

The turn of the millenium will see the 20th anniversary of the seminal discovery of the asymmetric epoxidation [1, 2] of ally lie alcohols catalysed by titanium(IV) isopropoxide and tartrate esters. The utility of this transformation largely results from the regio- and stereocontrol possible in subsequent nucleophilic ring opening reactions of the derived epoxy alcohols. Thus, a sequence of asymmetric epoxidation, epoxide opening and further functionalisation leads to a diverse array of molecules in enantiomerically pure form. In comparision, asymmetric epoxidation of unfunctionalised alkenes [3] has yet to match the enantioselectivities which the Ti-tartrate system can deliver with allylic alcohols. The recent discovery of other asymmetric epoxidation reactions [4] suggests that a number of practical options may eventually become available. 

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