Transition-metal organic compounds, alkyl

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). 

The study of the reactivity of transition metal hydride compounds towards unsaturated organic molecules, mainly olefines and alkynes, has been traditionally centered in monohydrides. In general, the reactions lead to the insertion products, alkyl or alkenyl, which have a limited chemistry. Furthermore, the generation of subsequent carbon-carbon or carbon-heteroatom bonds requires the presence of other active ligands, such as halogen or carbon monoxide, in the coordination sphere of the metallic center of the alkyl or alkenyl intermediates. 

The sol-gel process is based on the hydrolysis and condensation of molecular precursors [11]. The most versatile precursors are the metal organic compounds, metal alkoxides, M(OR) . The transition metal is represented by M, n is the valence of the metal, and R is an alkyl group such as methyl, ethyl, propyl, etc. 

Although transition-metal-free C-alkylation reactions have only recently been developed [236-238], existing transition-metal-catalyzed C-alkylation methods involving the transfer hydrogenation of an alcohol have been enhanced considerably though ligand modification, reaction rate acceleration, and the development of reactions that run under base-free conditions. Furthermore, these methods may be used for the activation of unactivated compounds, such as methanol, acetonitrile, acetamide, and esters, which are all important chemical feedstock in organic synthesis. 

Peroxyl radicals with a strong oxidative effect along with ROOH are continuously generated in oxidized organic compounds. They rapidly react with ion-reducing agents such as transition metal cations. Hydroxyl radicals react with transition metal ions in an aqueous solution extremely rapidly. Alkyl radicals are oxidized by transition metal ions in the higher valence state. The rate constants of these reactions are collected in Table 10.5. 

This chapter mainly treats transition metal-catalyzed direct functionalization of carbon-hydrogen bonds in organic compounds. This methodology is emphasized by focusing on important functionalizations for synthetic use. The contents reviewed here are as follows (i) alkylation of C-H bonds, (ii) alkenylation of C-H bonds, (iii) arylation of C-H bonds, (iv) carbonylation of C-H bonds, (v) hydroxylation and the related reactions, and (vi) other reactions and applications. 

In this chapter we will restrict our discussion of organometallic compounds to the alkyl and aryl compounds of magnesium and lithium, and the sodium and potassium salts of 1-alkynes. These substances normally are derived directly or indirectly from organohalogen compounds and are used very widely in organic synthesis. Organometallic compounds of transition metals and of boron are discussed in Chapters 11 and 31. 

---