Leaving group effect thiocyanates

Leaving-group effects are diminished in S 2 reactions, because the nucleophile assists in bond breaking. The mesylate/bromide ratio is compressed from 2 x 10 (Table 4.6) to only about 10 for azide ion in methanol, as shown in Table 4.8. In the aprotic dipolar solvent DMF, the leaving-group order is > Br > OjSCHj for both azide and thiocyanate anions. 

Moreover, product formation was not the same. Dibenzyl ethylsulfonium salt 72 gave only benzyl ethyl sulfide (73) and benzyl thiocyanate 74, but no dibenzyl sulfide or ethyl thiocyanate. By contrast, the cyclic sulfonium salt 75 gave a mixture containing chiefly (80%) dihydroisothianaphthene (76) with an equivalent amount of ethyl thiocyanate (77) and a minor amount (20%) of the sulfide-thiocyanate 78. This work represents the first direct experimental information on the rate of bimolecular nucleophilic substitution and relative orientation of the benzene ring and the leaving group and it demonstrates clearly the importance of stereoelectronic effects in these reactions. 

In these reactions the polarisation effect of the metal can activate an alkyl group towards nucleophilic attack. The incoming nucleophile is most commonly a halide or pseudohalide, and iodide or thiocyanate are particularly active in these processes. Numerous examples of such reactions are known, which are accelerated by co-ordination of the metal to oxygen. An example is shown in Fig. 4-46 in which the metal stabilises a phos-phonate leaving group. 

One early and successful route to better leaving groups involved the use of metal ions that are external to the complex cation under study to promote the loss of a halide or thiocyanate ion from the coordination sphere (158). This occurs by complexation of the added metal ion with the coordinated ligand, effectively generating a cationic or neutral leaving group that departs more rapidly (see Section III,B). Conducted in potentially coordinating solvents, this chemistry permits the syn-... 

Mercury(n) is also an effective catalyst for pseudohalide leaving groups, for example thiocyanate. In this connection, recently determined equilibrium data for complex formation between cobalt(ra)-thiocyanate complexes and mercury(n) are of relevance. ... 

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