Catalysts/catalysis ligand substitution

These discussions will embrace homogeneous solutions of polymer-metal complexes. Of course one of the important advantages offered by the use of a polymer ligand, especially a crosslinked polymer ligand, in catalysis is the insolubilization of the attached complexes the insolubility of the polymer catalyst makes it very easy to separate from the other components of the reaction mixture. Several polymer-metal complexes have been used for this purpose, although such applications are not covered in this article. The aim here is (1) to characterize polymer-metal complexes and their behavior in such simple but important elementary reactions as complex formation, ligand substitution, and electron transfer, and (2) to describe their catalytic activity. 

Many catalysts are metals, metal oxides or other simple salts, or metal complexes. For example, formation of platinum(IV) complexes involving ligand substitution is an extremely slow process, due to the kinetic inertness of this oxidation state. However, the addition of small amounts of a platinum(II) complex to the reaction mixture leads to excellent catalysis of the reaction, assigned to mixed oxidation state bridged intermediates that promote ligand transfer. 

There has been a review of the amination of azoles by copper-catalysed, acid-promoted, and metal-free processes. It has been shown that substitution of bromide by azide ions in meio-bromoporphyrins may be achieved without a catalyst. New ligands derived from quinoxaline have been found for the efficient catalysis by copper... 

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

Cyclodiphosphazanes(III) 27 shown in Scheme 16 undergo oxidation reactions to give the cyclodiphosphazanes(V) of type 28. These are prospective ligands in catalysis since these ligands due to lack of phosphorus lone-pairs are less susceptible to the destructive cycloreversion of the ligands. Hence they could prevent catalyst deactivation in the process. When treated with trimethyl aluminum the cyclodiphosphazanes form symmetrically substituted bimetallic species of type 29 [90]. Characterization by single-crystal X-ray studies show... 

Trost and Hachiya [140] studied asymmetric molybdenum-catalyzed alkylations. Interestingly, they noticed that the regioselectivity of this transformation performed with a non-symmetric allylic substrate varied according to the nature of the metal Pd-catalyzed substitutions on aryl-substituted allyl systems led to attack at the less substituted carbon, whereas molybdenum catalysis afforded the more substituted product. They prepared the bis(pyridylamide) ligand 105 (Scheme 55) and synthesized the corresponding Mo-complex from (C2H5 - CN)3Mo(CO)3. With such a catalyst, the allylic... 

---