Leaving group effect thioethers

However, the reaction requires only a general acid catalyst rather than the specific acid catalyst H+, and the corresponding reactions of the soft thioether may be better mediated by softer Lewis acids such as Cu+, Ag+, Hg2+, Pd2+, Pt2+ or Au3+. In many cases the aqua-ted metal ion is the most convenient Lewis acid, but in the case of some metals, particularly the second and third row transition metal ions, the aqua ions are not isolable and other complexes (particularly those with chloride ligands) are equally effective. The role of these softer metal ions as Lewis acids is two-fold. Firstly, the sulfur is co-ordinated to the metal, which increases the polarisation of the C-S bond and enhances the electrophilic character of the carbon, and, secondly, the thiol (or thiolate) leaving group is stabilised by co-ordination (Fig. 4-39). 

The coordination chemistry of thioethers has been reviewed. They are generally poor ligands and highly labile, and so can be used as leaving groups for metal complex syntheses. For example, [Au(tht)2]+ (54 tht = tetrahydrothiophene) is an effective starting material... 

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