Zirconium hydride complexes insertion reactions

Concomitant with continued olefin insertion into the metal-carbon bond of the titanium-aluminum complex, alkyl exchange and hydrogen-transfer reactions are observed. Whereas the normal reduction mechanism for transition-metal-organic complexes is initiated by release of olefins with formation of hydride followed by hydride transfer (184, 185) to an alkyl group, in the case of some titanium and zirconium compounds a reverse reaction takes place. By the release of ethane, a dimetalloalkane is formed. In a second step, ethylene from the dimetalloalkane is evolved, and two reduced metal atoms remain (119). 

Addition of carbon monoxide and water to an alkene, i.e. hydrocarboxylation, is catalyzed by a variety of transition metal complexes, including [Ni(CO)4], [Co2(CO)s] and [HaPtClg]. Unfortunately this reaction usually leads to mixtures of products due to both metal-catalyzed alkene isomerization and the occurrence of Irath Markownikov and anti-Markownikov addition of the metal hydride intermediate to the alkene. The commercially available zirconium hydride [(C5Hs)2Zr(H)Cl] can be used as a stoichiometric reagent for conversion of alkenes to carboxylic acids under mild conditions (equation 23). In this case the reaction with linear alkenes gives exclusively terminal alkyl complexes even if the alkene double bond is internal. Insertion of CO followed by oxidative hydrolysis then leads to linear carboxylic acids in very good yield. 

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. 

Specific examples of alkene insertions are the reactions of cationic hydrides [Cp2ZrH(L)]+ to give metal alkyls which may either be stabilized by agostic interactions, as in (21-XLII), or free of such interactions (21-XLIII), depending on L.180 The insertion of isobutene into M—C bonds of electron-deficient zirconium complexes was found to be reversible.109... 

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