Ligand precursors

The method of Ito et at. 50) as applied by Bakos et at. 12) to the reduction of acctylacetone to either ( —)-(2R,4J )- or ( + H25,45)-2,4-pentanediol will serve to illustrate how a chiral heterogeneous catalyst has been used to prepare a chiral homogeneous ligand precursor. 

Depending on the coordinative properties of the anion and on the degree of the cation s reactivity, the ionic liquid can be regarded as an innocent solvent, as a ligand (or ligand precursor), as a co-catalyst, or as the catalyst itself... 

Ionic liquid as solvent and ligand/ligand precursor... 

Both the cation and the anion of an ionic liquid can act as a ligand or ligand precursor for a transition metal complex dissolved in the ionic liquid. 

With respect to the ionic liquid s cation the situation is quite different, since catalytic reactions with anionic transition metal complexes are not yet very common in ionic liquids. However, an imidazolium moiety as an ionic liquid cation can act as a ligand precursor for the dissolved transition metal. Its transformation into a lig-... 

Figures 2,3,5, and 6 show anionic compounds 5 to 22 and 25 to 30, which have been described in the literature since 1997. In these adducts, as in examples 1 to 4, the P(VI) derivatives have carbon or oxygen atoms in the immediate proximity of the central (pseudo-)octahedral atom. This is probably due to the accessibility of the ligand precursors, the ease of their manipulation and, more importantly, to the sheer strength of the resulting P-C and P-0 bonds. They all present tris(bidendate) structures in which the three chelating rings can be identical (Fig. 2 and most of Fig. 6) or of two different types (Fig. 3). The ligands can be monooxo (Fig. 6) or dioxo (Fig. 2 and Fig. 3). These differences in composition have, of course, consequences for the making of the derivatives.

Fig. 2.23 Chiral NHC ligand precursors and complexes used in the asymmetric alkylation of conjugated enones...

A tridentate SNO ligand coordinates with a bipyridine to give ((2,2 -bipyridine)[2-(2-mercapto-phenyl)iminophenoxy])zinc. The monomeric complex has a distorted trigonal bipyramidal geometry with a phenolate and a thiolate coordinated. The synthesis was carried out electrochemically from anodic zinc in the presence of a disulfide-linked ligand precursor.867... 

Scheme 6.2 Heck reactions of aryl chlorides involving air-stable phosphonium salts as ligand precursors.

II. Synthesis and Characterization of Ligand Precursors and Low-Valent Metal Complexes for Small-Molecule Activation... 

Scheme 3.49. Anionic ligand precursors for the preparation of chiral heteroatomcuprates.

A very convenient method for the selective preparation of aNHC complexes is to block the imidazolium ligand precursors by substitution at C2 with alkyl or aryl groups [29, 30]. Following this procedure, the coordination of 1,2,3-trimethylimid-azolium iodide to [Cp IrCl2]2 afforded Cp Ir(aNHC), as shown in Scheme 3.14 [31]. 

The access to NHC complexes is mainly based on three routes the in situ deprotonation of ligand precursors, the complexation of the free, preisolated NHCs, and the cleavage of electron-rich olefins (Scheme 5). A variety of other methods, mainly of importance in special cases, will be presented at the end of this chapter. 

Elimination of Small Molecules from Neutral Ligand Precursors... 

In cases where the free NHC cannot be synthesized the complex formation has to be accomplished in situ from a ligand precursor, e.g., the imidazolium salt in the case of imidazolin-2-ylidenes. By this method, it is often possible to prepare complexes which do not have the maximum number of NHC ligands attached to the metal center. 

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