Substitution at the Dicoordinate Sulfur Atom

Reactions of this type are numerous and important for biological systems [164-166]. An ion-molecular reaction of mixed sulfur dihalogenide with the fluoride ion was studied by the CNDO/2 method in order to determine the optimal trajectory for an approach of the nucleophile to the dicoordinate sulfur atom [167]  

The T-shaped structure LVI is strongly stabilized with respect to the separate reactants and the reaction leading to its formation along the paths 1 and 3 proceeds without the activation energy (see Fig. 5.12). Although the CNDO/2 method overestimates the stability of anion complexes (see Sect. 2.3.2.1), there are reasons to believe that the type LVI T-structures are genuine intermediates rather than transition states in the reactions of the 8 2 type. The same conclusion follows also from the results of ab initio (DZ basis set) calculations [169] on the reaction of nucleophilic cleavage of the disulfide bond  

The calculated reaction path 1 and the type of the intermediate structures LVI correspond to an inversion of configuration at the dicoordinate sulfur atom (in the two-dimensional space—Ref. [172]). In actual fact, this result can be observed in reactions of this type of the compounds with axial chirality, namely, the derivatives of amino sulfenyl chlorides LX. 

Reactions of degenerate racemization of compounds LX associated with rotation about the S—N bond are considerably accelerated upon addition of the chloride ion. This acceleration can be accounted for by the switching on of a supplementary mechanism of configuration inversion at the sulfur atom, i.e., the reaction of nucleophilic substitution in Eq. (5.18)—see Ref. [173]. 

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