Transition metal complexes fundamental processes

One of the most fundamental questions when dealing with the activation of dioxygen by transition metal complexes is whether the process is controlled kinetically by ligand substitution or by electron transfer. A model system that involved the binding of dioxygen to a macrocyclic hexamethylcyclam Co(II) complex to form the correspond-... 

The additions proceed regioselectively in favor of terminal boron adducts to produce (Z)-l-alkenylboron compounds through a syn addition of the X-B bond to 1-alkynes. The mechanism is fundamentally different from the uncatalyzed process and is postulated to proceed through the oxidative addition of the X-B bonds (X = H, RS, Y2B) to the transition-metal complex [M(0)] to form X-M-BYj species (28), followed by the migratory cis insertion... 

The hydrosilation of alkenes and alkynes is catalyzed by transition metal complexes, including several platinum species, and oxidative addition of the Si—H bond is a fundamental step in the process. 

A fundamental understanding of oxidation-reduction reactions is vital to the inorganic chemist in contexts ranging from energy transduction - chemical to electrical and the converse, in technical matters in corrosion processes and metallurgy, redox processes in environmental chemistry and metalloenzymes and metallo-proteins involved in electron transfer. Electron-transfer reactions of transition metal complexes are accompanied by a change in the oxidation state of the metal... 

The behavior of early transition metal alkyls toward CO is somewhat different from that of late transition metal alkyls. Very little applications using early transition metal complexes for carbonylation processes have been reported in contrast to the abundant examples of applications of late transition metal complexes to carbonylation of organic substrates. However, fundamental studies on the chemistry of early transition metal alkyls toward CO insertion provide us with important information regarding the mechanisms of catalytic carbonylation processes. Thus we deal here with the chemistry of CO insertion into early transition metal alkyls and into late transition metal alkyls separately. 

Especially noteworthy is the field of asymmetric catalysis. Asymmetric catalytic reactions with transition metal complexes are used advantageously for hydrogenation, cyclization, codimerization, alkylation, epoxidation, hydroformylation, hydroesterification, hydrosilylation, hydrocyanation, and isomerization. In many cases, even higher regio- and stereoselectivities are required. Fundamental investigations of the mechanism of chirality transfer are also of interest. New chiral ligands that are suitable for catalytic processes are needed. 

The activation of molecular oxygen by transition metal complexes is of fundamental interest for chemistry, biology, and medicine. As it is well known from biochemistry, iron is crucial in the transportation, storage, and activation of oxygen, and with respect to these processes only copper is of comparable relevance. 

Alkyl halides are usually considered to be less suitable for double carbonylation because of the possibility of the direct reaction of alkyl halides with nucleophiles and of instability of alkyl-transition metal complexes involved in the catalytic process. However, allylic halides were found amenable to double carbonylation promoted by zerovalent palladium complex. It is well known that allylic halides undergo ready oxidative addition with a Pd(0) species to produce Tj -allylpalladium halide complexes. Thus, it was reasoned that the double carbonylation process might be realized if CO insertion into the aUyl-palladium bond proceeds before attack of amine on the 17 -allylpaUadium halide takes place. On the basis of fundamental studies on the behavior of i7 -allylpalladium halide complexes with CO and secondary amines, double carbonylation processes of substituted aUyl halides to give a-keto amides in high yields have recently been achieved (Eqs. 15 and... 

Compared to other fundamental organometallic reactions involving transition metal complexes (e.g. oxidative addition), transmetalation reactions have been much less studied to the point that have even been considered simply as ligand substitutions. Recently, however, as a consequence of its participation in relevant catalytic processes (e.g. C-C cross-coupling reactions), they have started being more studied and, at present, are recognized as a differentiated process. A prove of its increasingly importance is the number of citations per year in scientific journals for transmetalation as topic (Fig. 1.7). 

An elementary step to cleave C-C bonds is a reverse process of a C-C bond forming process. Oxidative addition of a C-C bond to a low-valent transition metal complex is a reverse process of reductive elimination, which occurs with a high-valent diorganometal, forming a C-C bond. P-Carbon elimination is a reverse process of insertion of an unsaturated bond into a carbon-metal bond, that is, carbometallation, or 1,2-addition of an organometal across a double bond. Such fundamental reactions are described along with typical examples. Besides this chapter, there are some excellent reviews on C-C bond cleavage available [1]. 

Other language has been used to describe this process. Indeed many authors have intermittently used other names/abbrevia-tions for reactions utilizing the same components to clarify specific aspects of the reaction. " This multiplicity of nomenclature may have aeated confusion as to the fundamental similarity, or indeed identical nature of the reactions being discussed. A recent recommendation by lUPAC clarifies this position by recommending that specific reversible-deactivation radical polymerizations (RDRPs) should adopt terminology consistent with that in lUPAC documents, specifically that the controlled RDRP procedures in which the deactivation of the radicals involves catalyzed reversible atom transfer or reversible group transfer usually, though not exclusively, by transition metal complexes be named atom transfer radical polymerization, ATRP. °... 

Obviously, with the development of the first catalytic reactions in ionic liquids, the general research focus turned away from basic studies of metal complexes dissolved in ionic liquids. Today there is a clear lack of fundamental understanding of many catalytic processes in ionic liquids on a molecular level. Much more fundamental work is undoubtedly needed and should be encouraged in order to speed up the future development of transition metal catalysis in ionic liquids. 

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