Transition metal catalyzed mechanistic investigations

Thus, the transition metal catalyzed arylation reactions of amines and alcohols would constitute powerful tools for synthetic chemists. We have been developing practical procedures for the palladium-catalyzed arylation of amines and alcohols with aryl halides or sulfonates. During the course of our investigations [9] as well as those of John Hartwig and co-workers [10] the substrate scope of these transformations has been incrementally expanded. With each cycle of this catalyst improvement process, advances in mechanistic understanding and ligand design have also been made. 

Within the field of Au-catalyzed hydroamination, many mechanistic questions remain unaddressed. The role of proton and counterion in these reactions is often unclear and the iruiocence of the Ag(I) halide abstracting agents commonly used has also been called into question [309]. Thus, mechanistic investigations into late-transition-metal-catalyzed hydroamination reactions have much to offer the synthetic community in designing improved catalyst systems. 

The 1,3-dipolar cycloaddition (1,3-DC) is the reaction of a dipolarophile with a 1,3-dipolar compound to form a five-membered ring, which is a kind of MBFT. The earliest 1,3-DC reactions were described in the late nineteenth century to the early twentieth century, following the discovery of 1,3-dipoles. Mechanistic investigations and synthetic applications were established by Rolf Huisgen in the 1960s [2], Now, the chemistry of the 1,3-DC reaction has thus evolved for more than 100 years, and a variety of different 1,3-dipoles have been discovered, which has significantly advanced the development of the 1,3-DC reactions. After several decades of development, transition-metal-catalyzed, stereoselective 1,3-DC has become one of the most useful synthetic routes to the synthesis of the five-membered heterocycles. 

Chiral phosphine based transition metal complexes are nsed as a powerful tool for asymmetric synthesis (3). A fundamental mechanistic nnderstanding is required for rhodium and mthenium catalyzed reactions. The starting point of those investigations was the clear and detailed stractnral description of the isolated pre catalyst system. 

The most prominent feature of homogeneous transition metal catalysts are the high selectivities that can be achieved. Homogeneously catalyzed reactions are controlled mainly by kinetics and less by material transport, because diffusion of the reactants to the catalyst can occur more readily. Due to the well-defined reaction site, the mechanism of homogeneous catalysis is relatively well understood. Mechanistic investigations can readily be carried out under reaction conditions by means of spectroscopic methods (Fig. 1-3). In contrast, processes occurring in heterogeneous catalysis are often obscure. 

This chapter provides an overview of the current state of the understanding of the mechanism of hydrogenations of double bonds catalyzed by the most commonly used transition metals, rhodium, iridium, and ruthenium. The focus of the review will be on recent computational studies, but older computational work and experimental investigations will be discussed in context. Where appropriate, open questions and mechanistic controversies will be addressed. 

Of course, these schemes indicate only that the overall reactions may be classified as nucleophilic 1,3-substitutions and, in the last case, as electrophilic 1,3-substitutions. The reactions often proceed only in the presence of catalytic or stoichiometric amounts of transition metal salts, while in their absence 1,1--substitutions or other processes are observed. The 1,1-substitutions are also catalyzed by salts of transition metals, and it is not yet well understood, which factors Influence the 1,1 to 1,3-ratio. In a number of 1,3-substitutions there is clear evidence for an addition-elimination course. At present the mechanistic aspects of the substitutions are under active investigation (see, for instance. Ref. 85-93). In the next section v/e give a short survey of 1,3-substitutions that have synthetic value. 

J.P. Collman et al. - Disodium Tetracarbonylferrate - A Transition Metal Analog of a Grignard Reagent, Acc. Chem. Res. 8, 342, 1975 Lewis Acid Catalyzed [RFe(CO)4] Alkyl Migration Reactions. A Mechanistic Investigation,... 

Since the first reports of Pd(II)-catalyzed hydroselenation and hydrothiolation 1992, considerable investigations have accumulated experimental evidence for the mechanism, in particular for Type I mechanism. Each step of Type I mechanism, structures of active catalysts, the reaction of alkynes with the active catalysts, and the protonolysis of the resulting vinyl metal complexes, has been verified for Pd, Ni, Zr, Ln, and An-catalyzed hydrochalcogenations by isolation of intermediates, isotope-labeled experiments, and kinetic studies. With regard to Type II mechanism, while the initial oxidative addition of REH (E = S, Se) to a low-valent transition metal catalyst (metal = Pd and Pt) has been verified by direct (for Pt) or indirect (for Pd) experimental evidence, the following steps of alkyne insertion to chalcogenolate-hydrido complex and reductive elimination of resultant vinyl metal complexes leave room for further mechanistic investigations to obtain direct evidence. On the other hand, a hybrid mechanism of Type I and Type II has been clarified for the hydrothiolation with Rh(I) complexes. 

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