Template ligands

The preparation of BINAP reported in 1980 has marked a landmark in asymmetric catalysis and has illustrated the peculiar stereorecognitive properties inherent with the axially chiral 1,1 -binaphthalene framework. Since then, a great deal of work has been devoted to the preparation of binaphthalene-templated ligands of related design. These efforts have resulted in the... 

Whitney, A. M. Ladame, S. Balasubramanian, S. Templated ligand assembly by using G-quadruplex DNA and dynamic covalent chemistry. Angew. Chem. Int. Ed. 2004,43, 1143-1146. 

New Supramolecular Approaches in Transition Metal Catalysis Template-Ligand Assisted Catalyst Encapsulation, Self-Assembled Ligands and Supramolecular Catalyst Immobilization... 

Figure 8.4 Modeled structure of an encapsulated transition metal catalyst, consisting of zinc(ii)TP) (gray), tris(meta-pyridyl)phosphine (3) (white) and M = [HRh(CO)3] (dark-gray). The different pyridylphosphine template-ligands used for assembly processes are also shown.

Figure 8.5 Molecular crystal structure of [4]2 [zinc ii)TPP]5 found in the solid state. The front view shows that the phosphine (black) of the template-ligand in this complex is still rather accessible.

A monophosphine complex is formed when 3 is mixed with three equivalents of a zinc(ii)salphen complex and half an equivalent of Rh(acac)(CO)2 (acac = acetyl acetonate), whereas the assembly based on template 4 and the zinc(n)salphen complexes forms a bis-phosphine rhodium species. In the latter case, the bisphosphine rhodium complex is completely encapsulated by six salphen building blocks. This difference in mono- versus diphosphine ligation to the Rh -center and, to a lesser extent, the difference in electronic features (and thus donating properties of the phosphine) between template ligands 3 and 4, can be used to induce a different catalytic behavior. 

Helical-Wheel Diagram of Template-Ligand Interactions... 

As mentioned above, reactions of this type have been widely used in the synthesis of macrocyclic ligands. Indeed, some of the earliest examples of templated ligand synthesis involve thiolate alkylations. Many of the most important uses of metal thiolate complexes in these syntheses utilise the reduced nucleophilicity of a co-ordinated thiolate ligand. The lower reactivity results in increased selectivity and more controllable reactions. This is exemplified in the formation of an A -donor ligand by the condensation of biacetyl with the nickel(n) complex of 2-aminoethanethiol (Fig. 5-78). The electrophilic carbonyl reacts specifically with the co-ordinated amine, to give a complex of a new diimine ligand. The beauty of this reaction is that the free ligand cannot be prepared in a metal-free reac-... 

C-type template ligands such as cyclooctatetraenyl [27] and porphyrin [181] have been shown to be very effective for stabilizing mononuclear complexes. In particular, porphyrin-type moieties are rather robust and less moisture-sensitive. 

Quadridentate ligands such as (237) are prepared by what must be one of the first (1944) uses of metal ion template ligand synthesis (Figure 5). ... 

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