Electrophiles metal thiolate complexes

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-... 

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). 

One of the simplest and widely used methods of forming C-S bonds involves nucleophilic attack of a thiolate on a suitable C-centred electrophile such as an alkyl halide (Fig. 5-74). Co-ordinated thiolate ligands behave as nucleophiles in exactly the same manner, and the method has been extensively used for the preparation of thioethers and their metal complexes. The method has been particularly commonly utilised in the formation of macrocyclic ligands in templated syntheses (see Chapter 6). 

Many exotic electrophiles have been shown to react with co-ordinated thiolate for example new disulfide bonds may be formed by reaction with S2C12. The nickel(n) complex of a very unusual tetrasulfide macrocyclic ligand may be prepared by this method (Fig. 5-83). Notice that this reaction utilises the nickel complex of the N2S2 ligand prepared by a metal-directed reaction in Fig. 5-78. 

Thiols are susceptible to oxidation by peroxides, molecular oxygen, and other oxidizing processes (e.g., radical-catalyzed oxidation) (Fig. 67). Because thiols easily complex with transition metals, it is believed that most thiol autoxidation reactions are metal-catalyzed (108). Autoxidation of thiols is enhanced by deprotonation of the thiol to the thiolate anion. Thiol oxidation commonly leads to disulfides, although further autoxidation to the sulfinic and, ultimately, sulfonic acid can be accomplished under basic conditions. Disulfides can be reduced back to the thiol (e.g., upon addition of a reducing agent such as dithiothreitol). Thiols are nucleophilic and will readily react with available electrophilic sites. For a more thorough discussion, see Hovorka and Schoneich (108) and Luo et al. (200). 

Four oxidation states of palladium are encountered in organometallic chemistry see Palladium Inorganic Coordination Chemistry) In order of importance, they are Pd , Pd , Pd, and Pd . With the reduction of palladium from Pd to Pd , the metal changes its reactivity from electrophile to nucleophile. However, unlike main group nucleophiles such as thiolates or cyanide, Pd complexes react with both alkyl halides and aryl or vinyl halides. Reactions of Pd complexes with these latter sp halides generate new Pd aryl or vinyl bonds through the process of oxidative addition. 

The carbyne complexes [Mo(=CBu )(SAr)3] (SAr = TMT, TIPT) have been synthesized by adding 3 eq of Li[SAr] to [Mo(=CBu )Cl3(dme)] (dme = dimethoxyethane). The analogous W derivatives were made by a slightly modified route (32). The initial aim was to probe the acetylene metathesis catalytic properties of the complexes [M ( Bu KSArlg] (M = Mo, W SAr = TMT, TIPT). However, none of the complexes were active for metathesis, which was in contrast to the high activity of the analogous alkoxide compounds for metathesis. This was attributed to the stronger electron donation power of thiolate, which reduces the electrophilic nature of the metal center (32). 

A few final comments should be made on the insertions of substrates containing C-C multiple bonds into the bonds between a transition metal and an electronegative heteroatom. First, insertions of olefins into related thiolate and phosphide complexes are as rare as insertions into alkoxo and amido complexes. Reactions of acrylonitrile into the metal-phosphorus bonds of palladium- and platinum-phosphido complexes to give products from formal insertions have been observed, and one example is showm in Equation 9.90. However, these reactions are more likely to occur by direct attack of the phosphorus on the electrophilic carbon of acrylonitrile than by migratory insertion. Second, the insertions of alkynes into metal-oxygen or metal-nitrogen covalent bonds are rare, even though the C-C ir-bond in an alkyne is weaker than the ir-bond in an alkene. 

Interestingly, solid-state structure investigations on methyl thiosalicylate dialkylaluminum compounds uncovered close intermolecular S - C(ti) contacts (with an average S - C distance of 3.382 A significantly below the sum of the corresponding van der Waals radii [76, 77]) between the Al-S thiolate units and the ester component (28, Fig. 9) that can effectively compete with the putative sulfur-aluminum hypercoordinate bond (27, Fig. 9) [141]. The latter results provide the first evidence for the competition of intermolecular n Ti interactions, involving the thiolate sulfur atom and the electrophilic ester carbon atom, with the hypercoordinate bond in metal complexes it opens up an interesting area for further studies. 

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