Oxidation of Alkenes Epoxidation

FIGURE 8.7 Reduction of the carbon-carbon double bond in trans-crotonyl ACP, a step in the biosynthesis of fatty acids. One hydrogen (blue) is delivered from NADPH as a hydride ion, H the other hydrogen (red) is delivered by protonation of the anion intermediate with an acid, HA. As is often the case in biological reactions, the structure of the biochemical reagent, NADPH in this case, is relatively complex considering the apparent simplicity of the transformation itself. 

What product would you obtain from catalytic hydrogenation of the following alkenes  

Like the word reduction used in the previous section for addition of hydrogen to a double bond, the word oxidation has a slightly different meaning in organic chemistry than what you might have previously learned. In general chemistry, an oxidation is defined as the loss of one or more electrons by an atom. In organic chemistry, however, an oxidation is a reaction that results in a loss of 

In the laboratory, alkenes are oxidized to give epoxides on treatment with a peroxyacid, RCO3H, such as mefa-chloroperoxybenzoic acid. An epoxide, also called an oxirane, is a cyclic ether with an oxygen atom in a three-membered ring. For example  

Peroxyacids transfer an oxygen atom to the alkene with syn stereochemistry— both C-O bonds form on the same face of the double bond— through a one-step mechanism without intermediates. The oxygen atom farthest from the carbonyl group is the one transferred. 

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Three membered rings that contain oxygen are called epoxides At one time epox ides were named as oxides of alkenes Ethylene oxide and propylene oxide for exam pie are the common names of two industrially important epoxides... 

Overall the stereospecificity of this method is the same as that observed m per oxy acid oxidation of alkenes Substituents that are cis to each other m the alkene remain CIS m the epoxide This is because formation of the bromohydrm involves anti addition and the ensuing intramolecular nucleophilic substitution reaction takes place with mver Sion of configuration at the carbon that bears the halide leaving group... 

Most frequent are oxidations of alkenes that can be converted to a series of compounds such as epoxides, halohydnns and their esters, ozonides (1,2,4 tri-oxolanes), a-hydroxyketones, a-hydroxyketone fluorosulfonates, ot-diketones, and carboxylic acids and their denvatives... 

Peroxy acid oxidation of alkenes (Sections 6.18 and 16.9) Peroxy acids transfer oxygen to alkenes to yield epoxides. Stereospecific syn addition is observed. 

The oxidation of alkenes and allylic alcohols with the urea-EL202 adduct (UELP) as oxidant and methyltrioxorhenium (MTO) dissolved in [EMIM][BF4] as catalyst was described by Abu-Omar et al. [61]. Both MTO and UHP dissolved completely in the ionic liquid. Conversions were found to depend on the reactivity of the olefin and the solubility of the olefinic substrate in the reactive layer. In general, the reaction rates of the epoxidation reaction were found to be comparable to those obtained in classical solvents. 

The reaction is carried out using a titanium silicalite-1 (TS-1) zeolite catalyst [30, 122]. This type of catalyst is known to accelerate the selective oxidation of alcohols, epoxidation of alkenes and hydroxylation of aromatics. These reactions have importance for fine-chemical production. 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

As with several other functional groups considered earlier, epoxides are most commonly found in alkaloid metabolites rather than the original compound. These epoxides may arise via oxidation of alkenes or aromatic hydrocarbons. Epoxide hydrolase catalyzes hydrolysis of epoxides to the more hydrophilic diol. As seen in Scheme 6, this is usually a stereospecific reaction that always yields a... 

Diketones. The Sharpless procedure for oxidation of alkenes with NaIO catalyzed by Ru02 (11,462-463) is equally efficient for oxidation of alkynes to 1,2-diones. In fact, alkenes and alkynes react at a similar rate, but ether, epoxide, and ester groups are stable to the reagent. A 1-silylacetylene is oxidized to an acylsilane. Yields are moderate to high.1... 

In contrast, a number of alkene epoxides (10.3) are chemically quite stable, i.e., intrinsically less reactive than arene oxides. Examples of epoxide metabolites that have proven to be stable enough to be isolated in the absence of degrading enzymes include 1,2-epoxyoctane (10.4), 1,2-epoxycyclohex-ane (10.5), 1-phenyl-1,2-epoxy ethane (styrene oxide, 10.6), and cis- 1,2-diphenyl-1,2-epoxyethane (cfv-stilbene oxide, 10.7) [12], The same is true of alclofenac epoxide (10.8), hexobarbital epoxide (10.9), and a few other epoxides of bioactive compounds. 

The kinetics of the catalytic oxidation of cyclopentene to glutaraldehyde by aqueous hydrogen peroxide and tungstic acid have been studied and a compatible mechanism was proposed, which proceeds via cyclopentene oxide and /3-hydroxycyclopentenyl hydroperoxide. " Monosubstituted heteropolytungstate-catalysed oxidation of alkenes by t-butyl hydroperoxide, iodosobenzene, and dioxygen have been studied a radical mechanism was proved for the reaction of alkenes with t-BuOOH and O2, but alkene epoxidation by iodosobenzene proceeds via oxidant coordination to the catalyst and has a heterolytic mechanism. ... 

The tra x-[Ru (0)2(por)] complexes are active stoichiometric oxidants of alkenes and alkylaro-matics under ambient conditions. Unlike cationic macrocyclic dioxoruthenium I) complexes that give substantial C=C bond cleavage products, the oxidation of alkenes by [Ru (0)2(por)] affords epoxides in good yields.Stereoretentive epoxidation of trans- and cw-stilbenes by [Ru (0)2(L)1 (L = TPP and sterically bulky porphyrins) has been observed, whereas the reaction between [Ru (0)2(OEP)] and cix-stilbene gives a mixture of cis- and trani-stilbene oxides. Adamantane and methylcyclohexane are hydroxylated at the tertiary C—H positions. Using [Ru (0)2(i)4-por)], enantioselective epoxidation of alkenes can be achieved with ee up to 77%. In the oxidation of aromatic hydrocarbons such as ethylbenzenes, 2-ethylnaphthalene, indane, and tetrahydronaphthalene by [Ru (0)2(Z>4-por )], enantioselective hydroxylation of benzylic C—H bonds occurs resulting in enantioenriched alcohols with ee up to 76%. ... 

Abstract This chapter covers one of the most important areas of Ru-catalysed oxidative chemistry. First, alkene oxidations are covered in which the double bond is not cleaved (3.1) epoxidation, cis-dihydroxylation, ketohydroxylation and miscellaneous non-cleavage reactions follow. The second section (3.2) concerns reactions in which C=C bond cleavage does occur (oxidation of alkenes to aldehydes, ketones or carboxylic acids), followed by a short survey of other alkene cleavage oxidations. Section 3.3 covers arene oxidations, and finally, in section 3.4, the corresponding topics for aUcyne oxidations are considered, most being cleavage reactions. 

Oxidations of alkenes and alkynes have been reviewed, including mechanistic information in some cases. They include treatment of epoxidations [1-9], ketohydroxylations [7-9] and alkene cleavage [4, 6,10-14]. Oxidations of alkynes have been reviewed in [4, 12, 14, 15]. 

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