Ligand-free systems

In this chapter, we will focus only on the latest reports, first dealing with a ligand-free system and later with multidentate N,N- and N,0-ligand systems. As a general rule of thumb, non-heme iron catalysts work best if they have exchangeable ligands [28]. 

Perhaps more surprisingly, [Pd(OAc)2] and PdCl2 could be used in just the same way and, since no phosphine was added, constituted a ligand-free system. For [Pd(OAc)2], adding extra acetate, in the form of NaOAc, led to an increase in the rate, but a decrease in selectivity, resulting in the formation of 5% of the cis-cinnamate. With PdCl2 catalyst decomposition was evident during the reaction. 

FIG. 7 Effect of pH on the uptake of nickel by a commercial activated carbon (F-400, Calgon Carbon Coip.) comparison of ligand-free. system (O) to that in which the Ni/ EDTA molar ratio is 1/10 ( ). (From Ref. 174.)... 

The first investigations in the framework of hydroformylation by Otto Roelen and successors were based on ligand-free systems. Of course, such catalysts are not constituted by the naked metal. They host, besides H and CO, diolefins, carboxylates, or halogenides, which are replaced either in the beginning or throughout the hydroformylation mechanism by solvent molecules, substrates, or reagents. Such unmodified catalysts are still in use and give excellent results in numerous applications. 

Ligand-free systems have also been developed. For example, in 2010, Yong and Teo [78] reported the cross-coupling of alkyl amines and aryl iodides under ligand-free copper(I) iodide-catalyzed conditions. The reaction scope was quite good, and very good yields could be obtained, using both iodoarenes and bromoarenes and primary and secondary amines. This protocol in fact is an excellent... 

In 2013, Sawant and coworkers [106] reported a ligand-free system using a Cu-Mn bimetallic catalyst. The catalyst was used at a loading of 25 wt% in water at room temperature, affording secondary amines in very good yields, in about 3-4h. 

The data in Figure 3.5 show that concentration profiles of some particles have the maximum. They are determined by interaction of two processes - diffusion and a chemical reaction. This state is impossible in the ligand-free systems. 

SnL " increases up to i/i 0.2. Such an effect results from the respective shifts in chemical equilibria and is impossible for simple ligand-free systems. A sharp change in [SnLH ]j is observed at the certain ili. It becomes most obvious when presenting the data in semilogarithmic coordinates (inset in Figure 8.28). The surface concentration of free Sn " " ions varies in the same manner. The data obtained makes it possible to construct reversible voltammograms. As in the case of real system, simulated curves contain prewaves whose height varies with pH in the similar manner. 

CH3peH provides the first example of a ligand-free system undergoing reductive elimination. Ozin and McCaffrey have observed that it takes part in a photoreversible reaction as in equation (94) ... 

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