Fundamental ligand product

Three features of chelation chemistry are fundamental to most of the appHcations of the chelating agents. The first and probably the most extensively used feature is the control of free metal ion concentration by means of the binding—dissociation equiUbria. The second, often called the preparative feature, is that in which the special properties of the chelate itself provide the basis of the appHcation. The third feature comprises displacement reactions metal by other metal ions, chelant by chelant, and chelant by other ligands or ions. An appHcation may be termed defensive if an undesirable property in a process or product is mitigated, or aggressive if a new and beneficial property is induced. 

A fundamental point in both molecular and surface chemistry concerns the involvement of donor atoms (from the ancillary ligand or the surface) in acid/base reactions.77 The reaction of H+ (from PyHCl) and Me+ (from MeOTf) with the anionic 1-metallacyclopropene 161 (Scheme 29) has been investigated. Although protonation gave back the starting material as the only product observed in solution ( h NMR), the reaction with MeOTf led to the neutral 1-metallacyclopropene, 162,... 

Dialkylzinc derivatives are inert towards conjugated enones (e.g. 181) in hydrocarbon or ethereal solvents. The discovery that a conjugate addition can be promoted by Cu(I) salts in the presence of suitable ligands L (e.g. sulphonamide 182) opened a new route to zinc enolates (e.g. 183), and hence to the development of three-component protocols, such as the tandem 1,4-addition/aldol addition process outlined in equation 92186. If the addition of the aldehyde is carried out at —78 °C, the single adduct 184 is formed, among four possible diastereomeric products. The presence of sulphonamide is fundamental in terms of reaction kinetics its role is supposed to be in binding both Cu(I) and Zn(II) and forming a mixed metal cluster compound which acts as the true 1,4-addition catalyst. 

As we considered above, one of the fundamental problems associated with the preparation of macrocyclic ligands is concerned with the orientation of reactive sites such that they give intramolecular (cyclic) rather than intermolecular (acyclic) products. This is associated with the conformation of the reactants and the reactive sites, and so we might expect that judicious location of donor atoms might allow for metal ion control over such a cyclisation process. This is known as a template synthesis, and the metal ion may be viewed as a template about which the macrocyclic product is formed. This methodology was first developed in the 1960s, and has been very widely investigated since that time. At the present, template reactions usually prove to be the method of choice for the synthesis of many macrocyclic complexes (with the possible exceptions of those of crown ethers and tetraazaalkanes). When the reactions are successful, they provide an extremely convenient method of synthesis. 

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