Ligands reaction

About a decade after the discovery of the asymmetric epoxidation described in Chapter 14.2, another exciting discovery was reported from the laboratories of Sharpless, namely the asymmetric dihydroxylation of alkenes using osmium tetroxide. Osmium tetroxide in water by itself will slowly convert alkenes into 1,2-diols, but as discovered by Criegee [15] and pointed out by Sharpless, an amine ligand accelerates the reaction (Ligand-Accelerated Catalysis [16]), and if the amine is chiral an enantioselectivity may be brought about. 

As an alternative, the use of soluble supported ligands has emerged. This approach is advantageous in a sense that most characteristics of the original homogeneous AD are maintained. After the reaction, ligand recovery is easily accomplished by precipitation upon addition of an appropriate solvent followed by filtration. 

In contrast, those cluster compounds with an 18-electron environment at the central heterometal do not undergo nucleophilic addition reactions. Ligand substitution reactions at both the central heterometal and the peripheral gold atoms are observed to occur for both 16- and 18-electron cluster compounds, as in the reactions shown below ... 

Despite the fact that a coordination site is not formally required for reductive elimination reactions, ligand dissociation from the metal center apparently facilitates reductive elimination. In the iridium examples described in Table 11, halide dissociation is important in other examples, phosphine ligand dissociation is important. 

Pulse radiolysis has been used to study the transient formation and decomposition of cobalt-alkyl bonds in aqueous solution in the same manner as it has been used for chromium alkyls. And as for chromium alkyls, bond homolysis is a major decomposition pathway (28). For bond formation reactions, pulse radiolysis shows that they are assisted by increases in pressure. This feature results from the homolysis having a larger activation volume than the bond formation reaction, resulting in a significantly negative overall reaction volume for the process (29). In general for all of these metal-alkyl bond homolysis reactions of the aquo complexes, steric hindrance facilitates the reaction. Ligand effects also play a role, but the factors involved are more subtle. 

The reactions, summarized in Table 1, show that reductive elimination from Ir " results in formation of different types of bonds. These reductive elimination reactions occur under very similar conditions, indicating that the nature of R and R does not significantly affect the rateh However, the coupling of two sp carbon centers does not occur. Theoretical studies have suggested that the directionality of the sp hybrid inhibits bond formation. Although a coordination site is not required for reductive elimination reactions, ligand dissociation from the metal center apparently facilitates reductive elimination. In the iridium examples, described in Table 1, halide dissociation is important, in other examples, phosphine ligand dissociation is important. 

A property common to the two cycles of Chart V is that, in the first reaction, ligand 5 replaces 4. In the second, ligand 4 replaces 3. In the third, ligand... 

Surface chemical reaction ligand replacement, electron and group transfer reactions, addition or elimination reactions. 

Chan and coworkers applied catalysts based on (R) P Phos (16, Table 6.4, entries 3 and 4) [118], BINAPO (17 and 18, entries 5 and 6) [119], and atropisomeric (13, entry 7) [ 120] ligands to the reaction. Ligand 16 was particularly interesting because its polar nature allowed the catalyst to be immobilized and recycled. After using [Ir(COD)Cl[2/ I6/I2 to catalyze the hydrogenation of 2 methylquinoline in biphasic mixtures of hexane and dimethyl poly (ethylene glycol) (dmpeg), the authors decanted off the... 

A third type of derivatives of iodonium salts, the perfluoroalkyl phenyliodonium (dfluoromethane sulfonates (FITS), showed an enhanced reactivity compared to the chloride and the tetrafluoroborate salts. In a series of papers, Umemoto et al have extensively studied the synthesis of these compounds and their reactivity towards a wide range of more or less powerful nucleophiles. 206 Different mechanistic pathways have been demonstrated to occur in these reactions. Ligand coupling appears to be involved in a number of nucleophilic displacements implying various types of carbanions, thiols and other nucleophilic substrates. 

Another important group of ligands for metal-catalysed reactions are based on oxazolines derived from amino acids. Evans30 has used the bis-oxazoline or box ligands in Cu(II)-catalysed Diels-Alder reactions. Ligand 133 is derived from the unnatural amino-acid /< r/-leucine and, complexed with Cu(OTf)2, catalyses Diels-Alder reactions of the same dienophile 125 with various dienes. The best counterions are triflate (OTf) and SbF6. 

Keywords Hypervalency, Sulfurane, Persulfonium salt, Persulfurane, Ligand exchange reaction, Ligand coupling reaction, Pseudorotation, Apical bond, Equatorial bond, Trigonal bipyrami-dal, Octahedral, Three-center four-electron bond... 

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