Epoxidation reactions involved

The most intriguing epoxide reaction involving participation of remote bonding electrons is the m s-annular hydride shift observed when the pjS.ii -epoxide (i) was treated with perchloric acid or hydrogen fluoride in non-polar solvents [37 3 ] The A 3.4).structure (3) of the product is reliably established [38], and separate experiments have confirmed... 

Mechanism of the Epoxidation Reaction - Involvement of Adsorbed Oxygen Species. Herzog has reported that the use of nitrous oxide rather than oxygen as the oxidant for ethylene epoxidation results in a considerably reduced selectivity. Since nitrous oxide decomposition leads primarily to adsorbed atomic oxygen, Herzog concluded that molecular oxygen species are required for the partial oxidation reaction. More recently this experiment has been repeated in a closed recirculatory system the same result was obtained. When the reaction was carried out in the presence of both 02 and N2 0 the ethylene oxide contained 0 exclusively while was incorporated into the carbon dioxide. 

A buffer is often added to epoxidation reactions involving peroxyacids. The byproduct of the reaction is a carboxylic acid, as mentioned several times above. If trifluoroperoxyacetic acid (CF3CO3H) is used as the... 

As we ve just seen nucleophilic ring opening of ethylene oxide yields 2 substituted derivatives of ethanol Those reactions involved nucleophilic attack on the carbon of the ring under neutral or basic conditions Other nucleophilic ring openings of epoxides like wise give 2 substituted derivatives of ethanol but either involve an acid as a reactant or occur under conditions of acid catalysis... 

Potassium Amides. The strong, extremely soluble, stable, and nonnucleophilic potassium amide base (42), potassium hexamethyldisilazane [40949-94-8] (KHMDS), KN [Si(CH2]2, pX = 28, has been developed and commercialized. KHMDS, ideal for regio/stereospecific deprotonation and enolization reactions for less acidic compounds, is available in both THF and toluene solutions. It has demonstrated benefits for reactions involving kinetic enolates (43), alkylation and acylation (44), Wittig reaction (45), epoxidation (46), Ireland-Claison rearrangement (47,48), isomerization (49,50), Darzen reaction (51), Dieckmann condensation (52), cyclization (53), chain and ring expansion (54,55), and elimination (56). 

Further dechlorination may occur with the formation of substituted diphenyhnethanes. If enough aluminum metal is present, the Friedel-Crafts reactions involved may generate considerable heat and smoke and substantial amounts of hydrogen chloride, which reacts with more aluminum metal, rapidly forming AlCl. The addition of an epoxide inhibits the initiation of this reaction by consuming HCl. Alkali, alkaline-earth, magnesium, and zinc metals also present a potential reactivity hazard with chlorinated solvents such as methylene chloride. 

Other reactions involving the hydrogen atom of the hydroxyl group in ethyl alcohol include the opening of epoxide rings to form hydroxy ethers. 

The Jacobsen-Katsuki epoxidation reaction is an efficient and highly selective method for the preparation of a wide variety of structurally and electronically diverse chiral epoxides from olefins. The reaction involves the use of a catalytic amount of a chiral Mn(III)salen complex 1 (salen refers to ligands composed of the N,N -ethylenebis(salicylideneaminato) core), a stoichiometric amount of a terminal oxidant, and the substrate olefin 2 in the appropriate solvent (Scheme 1.4.1). The reaction protocol is straightforward and does not require any special handling techniques. 

Thomson MOV/ Click Organic interactive o use a web-based palette to predict products from a variety of reactions involving ethers and epoxides. 

Epoxides bearing electron-withdrawing groups have been most commonly synthesized by the Darzens reaction. The Darzens reaction involves the initial addition of an ct-halo enolate 40 to the carbonyl compound 41, followed by ring-closure of the alkoxide 42 (Scheme 1.17). Several approaches for inducing asymmetry into this reaction - the use of chiral auxiliaries, reagents, or catalysts - have emerged. 

The chemistry of a-metalated epoxides and aziridines (the a prefix will from now on not be included but should be assumed) has been reviewed previously [1], but in this chapter it is our intention to focus on those reactions involving them that are useful in synthesis, rather than just of pedagogical interest. Beginning with metalated epoxides, since the greater amount of work has involved them, we intend to present carefully chosen examples of their behavior that delineate the diverse nature of their chemistry. We will then move on to metalated aziridines, the chemistry of which, it will become apparent, closely mirrors that of their epoxide cousins. 

The overall hydroxylation or epoxidation reaction catalyzed by cytochrome P450s involves the insertion of one oxygen atom, derived from molecular oxygen, into a C-H bond or into the Jt-system of an olefin, with the concomitant reduction of the... 

The results obtained in reactions involving the two first examples showed a reduced catalytic activity compared to the homogeneous catalyst, a situation that may be due to diffusion problems. Enantioselectivity was similar or slightly lower than in solution, with 80% ee [21] and 58% ee [22] in the epox-idation of ds-/l-methylstyrene with NaOCl providing the best results. Only in the last example was an improvement in enantioselectivity reported from 51% to 91% ee in the epoxidation of a-methylstyrene. Recovery of the catalyst was only considered in one case [21] and a significant decrease in enantioselectivity was observed on reuse. 

The principle of active-site-directed inactivation of glycosidases by gly-con-related epoxides can be extended to compounds having an exocyclic oxirane ring, either directly attached to the six-membered ring (32) or at some distance (33,34). Studies with -o-glucosidase from sweet almonds and intestinal sucrase-isomaltase revealed that, in spite of the higher intrinsic reactivity of these epoxides, this shift of the position of the epoxide function causes a 10- to 30-fold decrease of kj(max)/Ki, an effect which probably reflects the limited flexibility of the catalytic groups involved in the epoxide reaction. 

The scope of reactions involving hydrogen peroxide and PTC is large, and some idea of the versatility can be found from Table 4.2. A relatively new combined oxidation/phase transfer catalyst for alkene epoxidation is based on MeRe03 in conjunction with 4-substituted pyridines (e.g. 4-methoxy pyridine), the resulting complex accomplishing both catalytic roles. 

An illustration of the plethora of reactions that may occur is afforded by the transformation of caryophyllene oxide by Botrytis cinerea. Although most of the reactions were hydrox-ylations or epoxidations, two involved transannular reactions (a) between the C4-epoxide oxygen and Cy and (b) between the C4-epoxide and C13 with formation of a caryolane (Figure 7.47) (Duran et al. 1999). 

Entry 8 involves a migration initiated by epoxide ring opening. This reaction involves migration of a vinyl substituent. Entry 9 is a stereospecific migration of the aryl group. The DiBAlH both promotes the rearrangement and reduces the product aldehyde. 

It has been demonstrated that ionic intermediates are not involved in the epoxidation reaction. The reaction rate is not very sensitive to solvent polarity.71 Stereospecific syn addition is consistently observed. The oxidation is therefore believed to be a concerted process. A representation of the transition structure is shown below. 

A more synthetically reliable version of this reaction involves epoxidation of silyl enol ethers. Epoxidation of the silyl enol ethers followed by aqueous workup gives a-hydroxyketones and a-hydroxyaldehydes.144... 

An explosion was experienced dining work up of an epoxide opening reaction involving acidified sodium azide in a dichloromethane/dimethyl sulfoxide solvent. The author ascribes this to diazidomethane formation from dichloromethane [1]. A second report of an analoguous accident, also attributed to diazidomethane, almost certainly involved hydrogen azide for the cold traps of a vacuum pump on a rotary evaporator were involved this implies an explosive more volatile than dichloromethane. It is recommended that halogenated solvents be not used for azide reactions [2]. 

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