Intermediates three-coordinate complexes

Another example of a Bill compound is offered by the ligand VAPOL ( vaulted bisphenanthrol), which combines with AICI3 to form a three-coordinate complex, with no dative interactions. This compound will catalyze Diels-Alder reactions [64]. A five-coordinate compound, with two dative carbonyl bonds attached to the aluminum, has been postulated as the active intermediate in the catalytic reaction. 

Thermolysis of these complexes at 9°C produced ethylene, cyclobutane, and butenes. The ratio of the gaseous products was found to be a function of the coordination number of the complex, or intermediate. Thus three coordinate complexes favoured butene formation, while four coordinate complexes favoured reductive elimination to form cyclobutane, and five coordinate complexes produced ethylene as shown in Scheme 25.83... 

With the help of DFT calculations the stability of the three-coordinate complexes g was recently confirmed, but no evidence has yet been found for the five-coordinate intermediates h [14,15]. Instead, Goo (Sen et al. disclosed the linear complex i as an alternative structure for the initial intermediate [15]. It offers a valid explanation for Jutand s findings and reinstates the original four-membered intermediates in the catalytic cycle, albeit starting from anionic precursors (Scheme 3, bottom). 

The addition of the third glutathione ligand to Hg(II) may result in the formation of a symmetric planar complex. All of the ligands in such a complex have the same probability of dissociating an [Hg(SR)3] complex would then provide a convenient mechanism for the rapid exchange of thiol between the bound and free forms. Since an associative exchange mechanism via a three-coordinate intermediate is expected, the existence of the three-coordinate complex provides a kinetic explanation for the observation that Hg(II)-thiolate complexes are more labile in cell extracts... 

Like the rates of reductive eliminations to form C-H and C-C bonds, the rates of reductive eliminations to form carbon-heteroatom bonds depend on the coordination number of the metal. The reductive elimination to form C-N bonds from Pd(0) has been shown to occur faster from three-coordinate complexes than from four-coordinate com-plexes." - The reaction of the triphenylphosphme complex in Equation 8.63 to form triarylamine and Pd(0) was conducted with varying concentrations of added ligand. The rate of the reaction was slower when conducted with higher concentrations of added PPhj. A detailed study of the dependence of the rate of reaction on the concentration of added PPhj revealed two pathways for reductive elimination of amine—one from a four-coordinate cis complex and one from a three-coordinate complex formed by dissociation of phosphine. Although the relative rates of these two pathways depend on the concentration of added ligand, reaction through the three-coordinate intermediate was the major pathway at the concentrations of free ligand that would be present in most reactions. 

Metal polysulfido complexes have attracted much interest not only from the viewpoint of fundamental chemistry but also because of their potential for applications. Various types of metal polysulfido complexes have been reported as shown in Fig. 1. The diversity of the structures results from the nature of sulfur atoms which can adopt a variety of coordination environments (mainly two- and three-coordination) and form catenated structures with various chain lengths. On the other hand, transition metal polysulfides have attracted interest as catalysts and intermediates in enzymatic processes and in catalytic reactions of industrial importance such as the desulfurization of oil and coal. In addition, there has been much interest in the use of metal polysulfido complexes as precursors for metal-sulfur clusters. The chemistry of metal polysulfido complexes has been studied extensively, and many reviews have been published [1-10]. 

This section reviews the developments in the chemistry of monoborane complexes of the transition metals especially borohydride and hydridoborate complexes. Although such complexes are not strictly metallaboranes in the sense that they are not cluster species, they are included here as they share many similarities with polyborane species of the transition metals such as three-center two-electron bonding. Additionally, as will be shown in Section 3.04.3.1 borohydride species can also be intermediates in the formation of larger M By clusters. In this chapter, three-coordinate monoborane species, which are best considered as cr-complexes between a transition metal and HBR2 or metal-boryl (M-B) species, are not considered. 

Perhaps the most controversial suggestion is that spontaneous isomerizations may proceed through a dissociative mechanism, invoking two three-coordinated intermediates (probably T-shaped). The labile intermediates may be cis-like (A) or trans-like (B) in structure. Applied to the monoorganoplatinum(II) complexes, PtL2XR, the scheme is ... 

The mechanism involving simple nitrogen-coordinated complexes also accounts for reactivities of certain sterically constrained systems. For instance, 3-(diethyamino)cyclohexene undergoes facile isomerization by the action of the BINAP-Rh catalyst (Scheme 18). The atomic arrangement of the substrate is ideal for the mechanism to involve a three-centered transition state for the C—H oxidative addition to produce the cyclometalated intermediate. The high reactivity of this cyclic substrate does not permit any other mechanisms that start from Rh-allylamine chelate complexes in which both the nitrogen and olefinic bond interact with the metallic center. On the other hand, fro/tt-3-(diethylamino)-4-isopropyl-l-methylcyclohexene is inert to the catalysis, because substantial I strain develops during the transition state of the C—H oxidative addition to Rh. 

These fundamental steps of the catalytic cycle have been confirmed by stoichiometric reactions starting from isolated stable complexes, and by DFT calculations [11], Although many aspects of the Heck olefination can be rationalized by this textbook mechanism , it provides no explanation of the pronounced influence that counter-ions of Pd(II) pre-catalysts or added salts have on catalytic activity [12], This led Amatore and Jutand to propose a slightly different reaction mechanism [13]. They revealed that the preformation of the catalytically active species from Pd(II) salts does not lead to neutral Pd(0)L2 species a instead, three-coordinate anionic Pd(0)-complexes g are formed (Scheme 3, top). They also observed that on the addition of aryl iodides la to such an intermediate g, a new species forms quantitatively within seconds and the solution remains free of iodide and acetate anions. It may then take several minutes before the expected stable, four-... 

Considering the mechanistic rationales of the transition metal-catalyzed enyne cycloisomerization, different catalytic pathways have been proposed, depending on the reaction conditions and the choice of metal catalyst [3-5, 45], Complexation of the transition metal to alkene or alkyne moieties can activate one or both of them. Depending on the manner of formation of the intermediates, three major mechanisms have been proposed. The simultaneous coordination of both unsaturated bonds to the transition metal led to the formation of metallacydes, which is the most common pathway in transition metal-catalyzed cycloisomerization reactions. Hydrometalation of the alkyne led to the corresponding vinylmetal species, which reacts in turn with olefins via carbometalation. The last possible pathway involves the formation of a Jt-allyl complex which could further react with the alkyne moiety. The Jt-allyl complex could be formed either with a functional group at the allylic position or via direct C-H activation. Here the three major pathways will be discussed in a generalized form to illustrate the mechanisms (Scheme 8). 

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