Chemistry of Thioether Macrocyclic Complexes

Virtually all types of metal ions have been complexed with macrocyclic ligands.2-7 Complexes of transition metal ions have been studied extensively with tetraaza macrocycles (Chapter 21.2). Porphyrin and porphyrin-related complexes are of course notoriously present in biological systems and have been receiving considerable investigative attention (Chapter 22).8 Macrocyclic ligands derived from the Schiffbase and template-assisted condensation reactions of Curtis and Busch also figure prominantly with transition metal ions.6,7 The chemistry of these ions has been more recently expanded into the realm of polyaza, polynucleating and polycyclic systems.9 Transition metal complexes with thioether and phosphorus donor macrocycles are also known.2... 

For several reasons we further restrict consideration to those compounds that comprise solely thioethers as donor groups. First, complexes of such compounds most clearly evince the electronic consequences of thioether coordination. Second, homogeneity of donor group simplifies analysis of conformation. Such studies provide the trends and general principles for interpretation of the coordination chemistry of mixed donor macrocycles. Third, the surge of research effort in all-thioether macrocycles has yielded sufficient information to permit detailed comparisons between complexes of closely related Ugands inclusion of mixed-donor macrocycles contributes little to such discussion. 

In addition to the success just described with the well-known named processes, a number of other macrocyclization reactions mediated by palladium complexes have been reported. The first of these approaches exploits the established chemistry of palladium Jt-allyl complexes for use in activation towards reaction with nucleophiles. This reaction was employed by Harran et al. as a critical step in the construction of a series of macrocycles such as 116 via 115) designed to significantly reduce the peptidic character of known active peptides (Scheme 11.14). " The approach tolerates a variety of functionality, including alcohols, amides, thioethers and selected heteroaromatics, and was also successfully conducted on solid support. 

The first of these considerations, the potential parallel between thioethers and phosphines, suggests that thioethers might have extensive and industrially useful coordination chemistry. This possibility spurred earlier efforts to examine the complexes of macrocyclic thioethers [5, 6]. 

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