Olefins electrophilically activated

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled --orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin ji -orbital (back donation), leads to an electrophilic activation of the alkene9. 

As shown in Scheme 4-6, the reaction proceeds via a Ti(IV) mixed-ligand complex A bearing allyl alkoxide and TBHP anions as ligands. The alkyl peroxide is electrophilically activated by bidentate coordination to the Ti(IV) center. Oxygen transfer to the olefinic bond occurs to provide the complex B, in which Ti(IV) is coordinated by epoxy alkoxide and r-butoxide. In complex B,... 

Once the N—H bond has been oxidatively added to the Ir(I) complex (in the context of the CCM cycle, vide supra), the resultant Ir(III) intermediate is a Lewis acid that is thought to coordinate the olefin. A synergistic effect between the coordinated electrophilically activated olefin and the highly nucleophilic nature of the amido function is believed to facilitate the C—N bond formation within the coordination sphere of the Ir center (see 56). Alkyl-amino-Ir(III) complexes, such as the key intermediate 24 of the CCM system (as described in Section 6.2.1) are of paramount importance to better understand Ir-catalyzed hydroaminations. Complex... 

These results can be interpreted in terms of protosolvation of the nitronium ion. While the monocationic nitronium ion is a sufficiently polarizible electrophile to react with strong nucleophiles such as olefins and activated arenes, it is generally not reactive enough to react with weak nucleophiles including methane. Partial or complete protonation of the nitronium oxygen then leads to the superelectrophilic species 8. The... 

The electrophilic activation of a C—C multiple bond as a result of coordination to an electron-deficient metal ion is fundamental to much of organometallic chemistry, both conceptually and in synthetic applications (11). The Wacker process, a classic example of an efficient catalytic oxidation, is an important industrial reaction, used for the conversion of ethylene into acetaldehyde. The catalytic reaction begins with the coordination of ethylene to a Pd(ll) center, leading to activation of the ethylene moiety. The key step is the reaction of the metal-olefin complex with a nucleophile to give substituted metal-alkyl species (12). The integration of this reaction into a productive catalytic cycle requires the eventual cleavage of the newly generated M—C bond. 

Additions to nonactivated olefins and dienes are important reactions in organic synthesis [1]. Although cycloadditions may be used for additions to double bonds, the most common way to achieve such reactions is to activate the olefins with an electrophilic reagent. Electrophilic activation of the olefin or diene followed by a nucleophilic attack at one of the sp carbon atoms leads to a 1,2- or 1,4-addition. More recently, transition metals have been employed for the electrophilic activation of the double bond [2]. In particular, palladium(II) salts are known to activate carbon-carbon double bonds toward nucleophilic attack [3] and this is the basis for the Wacker process for industrial oxidation of ethylene to acetaldehyde [41. In this process, the key step is the nucleophilic attack by water on a (jt-ethylene)palladium complex. 

Since palladium(II) is electrophilic, olefins are activated toward nucleophiles by coordination to palladium(II) species. The attack of nucleophiles occurs at the more substituted vinylic carbon from the anti side of palladium to give alkylpalladium complexes (eq (92)) [123]. 

Conjugate additions of organometaiiic reagents to electrophilically activated olefins constitute one of the versatile methodologies for forming carbon-carbon bonds [1]. The products are the corresponding P-substituted carbonyl com-... 

Heterolytic activation of H2 on electrophilic complexes is important in catalytic hydrogenation and dates back 70 years.More recently, electrophilic activation of H2 has been found to be valuable in ionic hydrogenation where a mixture of an organometallic hydride, for example, CpMoH(CO)3, and a strong acid, for example, HO3SCF3, reduces sterically hindered olefins to alkanes via protonation to carbocations followed by hydride transfer from the metal hydride. It is likely that an acidic H2 complex is involved in the proton-transfer step (Scheme 7). 

In 2004, we reported the Cobalt-catalyzed hydrohydrazination of olefins with di-ferf-butyl azodicarboxylate (5) and phenylsilane (Scheme 4.1). Our approach was based on a stepwise introduction of a hydride and an electrophilic nitrogen source, instead of the more classical approach based on electrophilic activation of the olefin followed by addition of a hydrazine nucleophile. This solution to override the inherently low reactivity of aUcenes was first introduced by Mukaiyama for the related Cobalt-catalyzed hydroperoxidation reaction. The introduction of new Cobalt-catalyst 4 was the key for an efficient hydrohydrazination reaction, as the Cobalt-complexes with acetylacetonate-derived ligands used by Mukaiyama promoted direct reduction of the azodicarboxylate. 

The MBH/aza-MBH reactions have seen tremendous growth in terms of three components, that is, the activated olefins, electrophiles, and catalysts or catalytic systems during the past 40 years. Recently, several research papers descibed applications of novel activated olefins, electrophiles, or catalysts in MBH/aza-MBH reactions. 

The fact that olefins are susceptible to electrophilic attack has led to the development of a wide range of methods in which electrophilic activation of the olefin is followed by intramolecular nucleophilic attack. In particular, halolactonizations and -etherifications have enjoyed widespread popularity in stereoselective synthesis. A range of other asymmetric electrophilic olefin cyclizations are also discussed in this section [38, 39]. 

Scheme 2 shows an alternative inner-sphere mechanism, first involving the initial oxidative addition of NuH to the metal followed by olefin insertion into the M-Nu bond. The resulting M-C bond is cleaved by a C-H reductive elimination or by protonolysis (Scheme 2).While this mechanism is generally preferred for more electron-rich metals such as rhodium and iridium, several studies suggest that platinum and palladium-catalyzed additions of N-H or O-H nucleophiles more likely run by the outer-sphere electrophilic activation mechanism shown in Scheme 1.

The six-membered rings are assumed to be formed via 7t-complexation of the triple bond by the electrophilic metal complex, followed by a nucleophilic attack of the olefin moiety on the activated alkyne (Fig. 10.14) and a subsequent H-migration step. The electrophilic activations of enynes by 7r-complexation are common cycloisomerization pathways especially for platinum and gold catalysts. Other examples are presented in Sects. 10.4 and 10.5 of this book. 

Aryl and vinylic bromides and iodides react with the least substituted and most electrophilic carbon atoms of activated olefins, e.g., styrenes, allylic alcohols, a,p-unsaturated esters and nitriles. 

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