Transition Metal-Mediated Processes

Transition metal complexes are widely used in aromatic chemistry and organic synthesis in general. They are of particular value because com-plexation of an organic molecule to a metal centre often modifies its reactivity. The metal can subsequently be removed. After discussing the ways by which these complexes react, we will discuss their use in the synthesis of aromatic compounds. 

The chemistry of organometallic transition metal o-complexes can largely be explained by the operation of fundamental processes such as oxidative addition, reductive elimination and P-elimination. 

The process is termed oxidative addition because the oxidation state of the metai increases by two (as the groups A and B are normally more electronegative than the metal) and the coordination number also increases by two. These reactions may be illustrated using standard arrow-pushing diagrams, but here the arrows convey no real mechanistic meaning. 

In general, the process is easy for coordinatively unsaturated metal species, in particular 16-electron (d and d ) metals [Ni(0), Pd(0)], since these can attain a stable 18-electron configuration as a consequence. Additionally, two metal oxidation states must be sufficiently stable. For example, tetrakis(triphenylphosphine)palladium inserts readily at 80 °C into the C-Br bond of bromobenzene to give the organometallic complex PhPd(PPh3)4Br. 

In the context of organic synthesis, the process is mainly confined to the insertion of metals [mainly Pd(0) but to a lesser extent Ni(0)] into a C-X bond, where X is a leaving group such as a halide or triflate (CF3SO3). The order of reactivity is I Br CF3SO3 Cl F. Reactions of iodides and bromides can often be carried out selectively in the presence of chloride and fluoride substituents. 

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Abstract A literature overview, up to the end of 2004, of the most important microwave-assisted transition-metal-mediated processes used for the decoration and construction of heterocycles is presented. The emphasis of the chapter lies in the use of palladium-assisted reactions but examples of copper- and nickel-mediated processes are also incorporated. 

Several transition metal mediated processes rely on the participation of Lewis acids, among these, the Nicholas reaction figures prominently. After treatment with BF3-Et20, Co2(CO)6 complexed propargylic alcohols provide the carbocation, which can be trapped intramolecularly with various nucleophiles including epoxides (eq 55), or via hydride transfer from an appropriately poised benzyl moiety (eq 56) or aromatic rings. In the latter case, access to the relatively small [7] metacyclophane derivatives was possible. 

In this article, a brief and general review of catenanes in terms of their nomenclature, self-assembly, and a few pertinent properties will be presented. Examples of the concepts discussed were limited to representative transition metal-mediated processes. 

Most radical reactions can be conducted under rruld conditions. In contrast to ionic reactions and many transition metal-mediated processes, most of the functional groups are tolerated under radical conditions. Moreover, radical reactions can be performed in various solvents even water is tolerated as a reaction medium. These facts, among others, make radical processes highly useful for arylations. Radical arylations can be performed using SsNl-type reactions, by homolytic aromatic substitutions, and by reactions of aryl radicals with various radical acceptors. In this chapter we first focus on SR Ttype reactions [1], and later concentrate on homolytic aromatic substitutions. Unfortunately, due to limitations of space, we cannot provide a comprehensive overview on this topic hence, for further information the reader is referred to some excellent reviews on this issue [2]. 

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