Biological processes, transition metals

The red tetrathiomolybdate ion appears to be a principal participant in the biological Cu—Mo antagonism and is reactive toward other transition-metal ions to produce a wide variety of heteronuclear transition-metal sulfide complexes and clusters (13,14). For example, tetrathiomolybdate serves as a bidentate ligand for Co, forming Co(MoSTetrathiomolybdates and their mixed metal complexes are of interest as catalyst precursors for the hydrotreating of petroleum (qv) (15) and the hydroHquefaction of coal (see Coal conversion processes) (16). The intermediate forms MoOS Mo02S 2> MoO S have also been prepared (17). 

In this chapter, we survey the diversity of transition metals, beginning with an overview. Then we describe the stmcture and bonding in transition metal complexes. We describe metallurgy, the processes by which pure metals are extracted from mineral ores. The chapter ends with a presentation of some properties of transition metals and their biological roles. 

In the previous section, we have described some of the mechanisms that may lead to the fijrmation of lipid hydroperoxides or peroxyl radicals in lipids. If the peroxyl radical is formed, then this will lead to propagation if no chain-breaking antioxidants are present (Scheme 2.1). However, in many biological situations chain-breaking antioxidants are present, for example, in LDL, and these will terminate the peroxyl radical and are consumed in the process. This will concomitandy increase the size of the peroxide pool in the membrane or lipoprotein. Such peroxides may be metabolized by the glutathione peroxidases in a cellular environment but are probably more stable in the plasma comjxutment. In the next section, the promotion of lipid peroxidation if the lipid peroxides encounter a transition metal will be considered. 

N-Acyl-a-amino acids are important compounds in both chemistry and biology. They are easily obtained in a transition metal-catalyzed, three-component domino reaction of an aldehyde, an amide, and CO. Whereas cobalt was mainly used for this process, Beller and coworkers [159] have recently shown that palladium has a... 

Metal complexes, especially involving transition metals, are known for their role as catalysts in a broad variety of chemical processes including isomerization, oxidization, hydrogenation, and polymerization. Such catalytic reactions play an important role not only in many industrial processes, such as petroleum and polymer industries, but also in many biological systems, e.g., a variety of selective oxidation catalysts with heme (1) and nonheme (2) iron centers. The transition metals in these systems usually constitute a fundamental part of the catalyst, due to their... 

The Alder-ene reaction has traditionally been performed under thermal conditions—generally at temperatures in excess of 200 °C. Transition metal catalysis not only maintains the attractive atom-economical feature of the Alder-ene reaction, but also allows for regiocontrol and, in many cases, stereoselectivity. A multitude of transition metal complexes has shown the ability to catalyze the intramolecular Alder-ene reaction. Each possesses a unique reactivity that is reflected in the diversity of carbocyclic and heterocyclic products accessible via the transition metal-catalyzed intramolecular Alder-ene reaction. Presumably for these reasons, investigation of the thermal Alder-ene reaction seems to have stopped almost completely. For example, more than 40 papers pertaining to the transition metal-catalyzed intramolecular Alder-ene reaction have been published over the last decade. In the process of writing this review, we encountered only three recent examples of the thermal intramolecular Alder-ene reaction, two of which were applications to the synthesis of biologically relevant compounds (see Section 10.12.6). 

During the last three decades, peroxo compounds of early transition metals (TMs) in their highest oxidation state, like TiIV, Vv, MoVI, WV1, and Revn, attracted much interest due to their activity in oxygen transfer processes which are important for many chemical and biological applications. Olefin epoxidation is of particular significance since epoxides are key starting compounds for a large variety of chemicals and polymers [1]. Yet, details of the mechanism of olefin epoxidation by TM peroxides are still under discussion. 

Group VIII transition metal catalyzed hydrogen exchange reactions are important in providing useful information concerning the fundamental processes of bond rupture and bond formation on catalyst surfaces. The reactions are also a convenient source of deuterated and tritiated compounds for chemical and biological research (i). 

The electron density in transition metal complexes is of unusual interest. The chemistry of transition metal compounds is of relevance for catalysis, for solid-state properties, and for a large number of key biological processes. The importance of transition-metal-based materials needs no further mention after the discovery of the high-Tc superconducting cuprates, the properties of which depend critically on the electronic structure in the CuOz planes. 

Organic complexes of transition metal cations have been known for a long time and have been extensively studied. They are often very stable and their stereochemistry is strongly dependent on the nature of the central cation. Their biological role is that of oligoelements participating in specific processes. 

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