Nucleophilic attack on coordinated olefins

Reactions illustrating nucleophilic attack on coordinated olefins and allyls are shown in Eqs 15.124 and 15.125. 

A number of other nucleophiles are capable of nucleophilic attack on coordinated olefins. For example, when ethylene is oxidized by PdCl2 in alcoholic solvents, the corresponding vinyl ethers,... 

Many additional routes to metal-alkyl complexes other than transmetallation and alkylation are discussed in later chapters of this text. For example, metal-alkyl complexes are generated by insertion of an olefin into a metal-hydride or or metal-hydrocarbyl species. Such insertion reactions are discussed in Chapter 9, but an example of the synthesis of a zirconium alkyl by olefin insertion into a zirconium hydride is shown in Equation 3.9. Metal-alkyl complexes are also generated by nucleophilic attack on coordinated olefins (Equation 3.10) or carbene ligands (Equation 3.11). These reactions are presented in detail in Chapter 11. 

Some of the classic studies of nucleophilic, attack on coordinated olefins were conducted with iron(II) species. Rosenblum reported the reactions of (ti -cyclopentadienyl) iron-olefin complexes with a wide range of carbanion and enamine nucleophiles. These reactions produce stable o-alkyliron complexes (Equation 11.22). The stereochemistry is cleanly trans. However, the regioselectivity of reactions of complexes of imsymmetrical olefins depended on the nucleophile. 

Hydroaminations occurring by nucleophilic attack on ir-ligands are the oldest class of hydroamination and are discussed first. A mechanism for the hydroamination of alk-enes and alkynes catalyzed by palladium(II) complexes is shown in Scheme 16.16. By this pathway, coordination of the alkene or alkyne through the -ir-system occurs to generate a cationic or electron-poor, neutral metal-olefin or metal-alkyne complex. Nucleophilic attack of the amine on the coordinated olefin or alkyne then occurs. Nucleophilic attack on coordinated olefins and alkynes is presented in detail in Chapter 11. As noted in Chapter 11, this nucleophilic attack occurs at the internal position of an alkene or alkyne. 

The same type of behavior can be found for nucleophilic attack of coordinated olefins and arenes. The C=C group of (cod)PdCl2 is attacked only by basic MeOH, while [(cod)Pd(P/ 3)Cl] is rapidly attacked even in neutral MeOH. " Hydride addition to coordinated pi-ligands tends to be controlled by the charge on the metal, for example ... 

Transition metal alkyne complexes also react with nucleophiles, in this case to generate CT-vinyl complexes. There are fewer stable alkyne complexes of higher oxidation state or cationic metals than olefin complexes. Because these types of alkyne complexes are most susceptible to nucleophilic attack, less information is available on tfiis reaction than on nucleophilic attack on coordinated alkenes. Nevertheless, reactions of several cationic alkyne complexes with nucleophiles have been reported, and a few examples are presented here. 

The C-0 and C-N bond in the product of the oxidations with oxygen and nitrogen donors forms by either nucleophilic attack on the coordinated olefin or by insertion of the olefin into a palladium-oxygen or palladium-nitrogen bond. More detailed descriptions of the nucleophilic attack on coordinated ligands were provided in Chapter 11 and a more detailed description of migratory insertions was provided in Chapter 9. These reactions are discussed in the context of the effect of additives on the stereochemistry of the catalytic processes in several earlier sections on the Wacker process. Henry conducted the same stereochemical study for reactions of alcohols with the resolved allylic alcohol in Scheme 16.24 as was conducted for reactions of water. The results of these experiments were similar to those on the reactions of water. ... 

The hydroamination of olefins has been shown to occur by the sequence of oxidative addition, migratory insertion, and reductive elimination in only one case. Because amines are nucleophilic, pathways are available for the additions of amines to olefins and alkynes that are unavailable for the additions of HCN, silanes, and boranes. For example, hydroaminations catalyzed by late transition metals are thought to occur in many cases by nucleophilic attack on coordinated alkenes and alkynes or by nucleophilic attack on ir-allyl, iT-benzyl, or TT-arene complexes. Hydroaminations catalyzed by lanthanide and actinide complexes occur by insertion of an olefin into a metal-amide bond. Finally, hydroamination catalyzed by dP group 4 metals have been shown to occur through imido complexes. In this case, a [2+2] cycloaddition forms the C-N bond, and protonolysis of the resulting metallacycle releases the organic product. 

Nucleophilic attack on coordinated unsaturated hydrocarbons is one of the fundamental and particularly well studied reactions in Organometallic Chemistry. The addition of carbonylme-tallates instead of common nucleophiles provides a directed synthesis of hydrocarbon bridged complexes. Carbonylmetallates (particularly Re(CO)5", Os(CO)4 ") add to 7C-bonded olefin, acetylene, allyl, diene, trimethylenemethane, dienyl, benzene, triene and cycloheptatrienyl ligands in cationic complexes and give hydrocarbon bridged bi- and trimetallic, homo- or he-teronuclear complexes [1]. 

Allyl complexes have contributed significantly to the development of the organometallic chemistry of nickel and the applications of nickel complexes in organic synthesis, for example, nucleophilic attack on coordinated allyl ligands. In addition, allylnickel complexes have been identified as key intermediates in the oligomerization and cyclization of olefins and dienes. For example, the Ni(0)-catalyzed hydrocyanation of butadiene to adiponitrile, the main component of a major commercial process for the production of nylon, involves Ni (7r-allyl) intermediates. Moreover, the 77-rearrangements of allylnickel species have helped explain the facile isomerization of olefins in the presence of nickel complexes. The Ni-catalyzed homoallylation of carbonyl compounds with 1,3-dienes also involves Ni(7r-allyl) complexes this subject has been reviewed recently. New applications include the cleavage of G-G bonds in the deallylation of malonates, the preparation of cyclopentenones by carbonylative cycloaddi-... 

The Ptm-Ptm bond in the alkyl complexes exhibits a unique character in that the Pt atom acts both as Ptn and PtIV or the intermediate through electron localization and delocalization along the Pt-Pt axis (1) coordination of olefin is a Pt11 character, since no olefin-Pt17 complex is known and (2) the very easy and rapid nucleophilic attack on the coordinated alkyl ce-carbon atoms is a PtIV character. Alkyl-Ptn complexes do not easily undergo nucleophilic attack unless the com-... 

Palladium complexes are generally superior catalysts for oxidation reactions, whereas other noble metals are more active for other reactions, e.g., rhodium for hydroformylation. All of these reactions seemingly involve activation of the olefin substrate by rr-complex formation with the noble metal catalyst.513 The oxidation reactions discussed in the following generally depend on nucleophilic attack on the coordinated olefins (or other hydrocarbons) to effect oxidation of the substrate. 

Note that this latter mechanism requires nucleophilic attack on a coordinated olefin which is more electrophilic owing to the charge on the complex. Thus, the rate laws derived from mechanisms incorporating (15) or (16) differ in the order of solvent, which illustrates the potential pitfalls and... 

The mechanism of this oxidation has been studied in great detail. The results are summarized in Scheme 9. The olefin is activated toward nucleophilic attack by coordination to Pd(II), as in complex 6, which is converted to the aqua complex 7. Displacement of 2 Cl ions is supported by the inverse second order dependence of the rate on the chloride ion concentration. Two versions consistent with most of the... 

The insertion of olefins into the metal-oxygen bonds of isolated alkoxo, phenoxo, or hydroxo compounds has been observed directly in a few cases. As will be noted in Chapter 11, a hydroxy or alkoxyalkyl group can be formed by insertion of an olefin into a metal-oxygen bond or by attack of hydroxide or an alkoxide on a coordinated olefin. Many studies described in Chapter 11 imply that this type of compound is formed by nucleophilic attack on a coordinatively saturated olefin complex, and this reaction has been proposed as the C-0 bond-forming step during oxidations of olefins catalyzed by palladium complexes. However, Henry provided some of the first evidence that the C-0 bond forms by insertion of olefins into metal-hydroxo and -alkoxo complexes under certain reaction conditions. ... 

Hegedus developed an olefin amination process that generates indoles by intramolecular nucleophilic attack of substituted amines on allylarenes (Equation 11.27). In this process, an N-tosyl arylamine attacks the coordinated olefin of an allyl group in the ortho position of the aniline, and p-hydrogen elimination generates a tautomer of indole and a reduced palladium complex. This reduced complex is then re-oxidized by quinone to regenerate the starting palladium(II), and the product tautomerizes to the final indole product. This cycle was one of the earliest catalytic olefin aminations. 

Nucleophilic attack of stabilized carbon nucleophiles on coordinated olefins is also known. Hegedus developed the alkylation of olefins shown in Equation 11.31. The (olefin)palladium(II) chloride complexes did not react with malonate nucleophiles, but the triethylamine adduct does react with this carbon nucleophile to provide the alkylation product. This reaction has recently been incorporated into a catalytic alkylation of olefins by Widenhoefer. - Intramolecular reaction of the 1,3-dicarbonyl compounds with pendant olefins in the presence of (GHjCNl PdCl occurs to generate cyclic products containing a new C-C bond (Equation 11.32). Some intermolecular reactions with ethylene and propylene have also been developed by this group. Deuterium labeling studies (Equation 11.32) have shown that the addition occurs by external attack on the coordinated olefin. ... 

The oxidations of olefins with many oxygen nucleophiles other than water have also been reported. These reactions include the s5mthesis of vinylic and allylic ethers from reactions of olefins with alcohols and phenols, and vinylic and allylic esters from reactions of olefins with carboxylic acids. These reactions have been conducted with both monoenes and 1,3-dienes. Both intermolecular and intramolecular versions of each of these processes have been developed. Some discussion of these reactions was included in Chapter 11 because of their connection to the nucleophilic attack of oxygen nucleophiles on coordinated olefins and dienes. 

---