Oxidative Reactions of Alkenes

Palladium (II) compounds coordinate to alkenes to form 71-complexes. Roughly speaking, the decrease of alkene electron density caused by coordination to an electrophilic Pd(II) compound enables an attack by nucleophiles on the coordinated alkenes. The attack of a nucleophile with concomitant formation of a carbon-palladium e-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. Unlike the products of mercuration which are stable and isolable, palladation product 1 is usually unstable and undergoes rapid decomposition. Palladation is followed by two reactions. The elimination of H-Pd-X from 1 to form the vinyl compounds 2 is one path, resulting in nucleophilic substitution of the alkene. Displacement of the Pd in 1 by an other nucleophile effects nucleophilic addition of the alkene to give 3. Depending on the reactants and conditions, either nucleophilic substitution of the alkene or the nucleophilic addition to the alkene takes place [4,5]. 

The oxidative reactions of alkenes can be classified further based on the attacking species. 

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Typical nucleophiles known to react with coordinated alkenes are water, alcohols, carboxylic acids, ammonia, amines, enamines, and active methylene compounds 11.12]. The intramolecular version is particularly useful for syntheses of various heterocyclic compounds[l 3,14]. CO and aromatics also react with alkenes. The oxidation reactions of alkenes can be classified further based on these attacking species. Under certain conditions, especially in the presence of bases, the rr-alkene complex 4 is converted into the 7r-allylic complex 5. Various stoichiometric reactions of alkenes via 7r-allylic complex 5 are treated in Section 4. 

Oxidative reactions of alkenes with Pd(II) are treated in Chapter 3,. Section 2, catalytic reactions of alkenes with organic halides are discussed in this chapter. Section 1.1, and other catalytic reactions of alkenes are discussed in this section. 

Osmium-catalysed dihydroxylation of olefins is a powerful route towards enantioselective introduction of chiral centers into organic substrates [82]. Its importance is remarkable because of its common use in organic and natural product synthesis, due to its ability to introduce two vicinal functional groups into hydrocarbons with no functional groups [83]. Prof. Sharpless received the 2001 Nobel Prize in chemistry for his development of asymmetric catalytic oxidation reactions of alkenes, including his outstanding achievements in the osmium asymmetric dihydroxylation of olefins. 

Allylic oxidation. Reaction of alkenes with /-butyl hydroperoxide (90%) in the presence of catalytic amounts of Cr(CO)6 results in oxidation to a,p-enones in 25-100%... 

For practical purposes, most organic chemists mean by oxidation either addition of oxygen to the substrate (such as epoxidation of an alkene), removal of hydrogen (such as the conversion of an alcohol to an aldehyde or ketone), or removal of one electron (such as the conversion of phenoxide anion to the phenoxy radical). Examples of oxidation reactions of alkenes have been described in Chapter 5, including epoxidation, aziridination, dihydroxylation and Wacker oxidation. This chapter therefore concentrates on oxidations of hydrocarbons, alcohols and ketones. 

Peroxides of transition metals are themselves active intermediates in heterolytic and homolytic liquid-phase catalytic oxidation reactions of alkenes, aromatic hydrocarbons and alkanes. Heterolytic oxidations are characterized by a requirement for a free coordination volume near the transition metal atom. Homolytic oxidations proceed via M-O bond cleavage in peroxo complexes. 

Abstract The oxidative functionalization of olefins is an important reaction for organic synthesis as well as for the industrial production of bulk chemicals. Various processes have been explored, among them also metal-catalyzed methods using strong oxidants like osmium tetroxide. Especially, the asymmetric dihydroxylation of olefins by osmium(Vlll) complexes has proven to be a valuable reaction for the synthetic chemist. A large number of experimental studies had been conducted, but the mechanisms of the various osmium-catalyzed reactions remained a controversial issue. This changed when density functional theory calculations became available and computational studies helped to unravel the open mechanistic questions. This mini review will focus on recent mechanistic studies on osmium-mediated oxidation reactions of alkenes. 

We begin this section with a review of the definitions of oxidation and reduction. We then consider two common oxidation reactions of alkenes. 

Oxidative Reactions of Alkenes. Oxidative reactions of alkenes can be classified into two types oxidative substitution and oxidative addition, as shown in eq 1. Here X and Y represent nucleophiles such as HO, RO, RCO2, R2N and CO, as well as soft carbon nucleophiles such as active methylene corrpounds. 

Liu X, Chen W. Pyridazine-based N-heterocychc carbene complexes and ruthenium-catalyzed oxidation reaction of alkenes. Organometallics. 2012 31 6614-6622. 

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