Disulfonium dication, addition

The generalized Woodward-Hoffmann rule suggests that a synchronous addition of disulfonium dications at the double C=C bond of alkenes would be a thermally forbidden process and so would be hardly probable. Simulation of the frontal attack by ethylene on l,4-dithioniabicyclo[2.2.0]hexane 115 gave no optimal structure of an intermediate complex. On the other hand in the lateral approach of the reactants, orbital factors favor attack of the double bond by one of the sulfonium sulfur atoms of the dication. This pattern corresponds to SN2-like substitution at sulfur atom as depicted in Figure 5. Using such a reactant orientation, the structure of intermediate jc-complex was successfully optimized. The distances between the reaction centers in the complex, that is, between the carbon atoms of the ethylene fragment and the nearest sulfur atom of the dication, are 2.74 and 2.96 A, respectively. 

Decomposition of highly reactive disulfonium dications in sufficiently nucleophilic medium is accompanied by formation of disulfides. Even hydrolysis of disulfonium dications generated from sterically rigid bis-sulfides often affords some disulfide as a by-product in addition to the monosulfoxide main product. For example, hydrolysis of dication 72 yields sulfoxide 74 and trace amounts of heterocycle 73 (Scheme 28).56... 

According to the stepwise electrophilic reaction mechanism, the differences in the stereochemistries of the products from the reactions of alkenes with cyclic 49 and acyclic 51 disulfonium dications can be explained by the larger rates of the intramolecular reactions. In the case of a cyclic dication, the carbocationic center in intermediate 94, which is formed as the result of initial attack by a S-S dication on a double C=C bond reacts with nucleophile intramolecularly, thus conserving the configuration of the substituents at the double bond. On the other hand, an acyclic dication undergoes transformation to two separate particles (95 and dimethylsulfide) with a consequent loss of stereoselectivity. Additional experiments with deuteretad alkenes confirm that reaction is not stereoselective, lending further support to the stepwise mechanism (Scheme 36).106... 

Stereochemistry of the addition of a cyclic disulfonium dication generated from 1,4-dithiane to alkenes has been shown to be non-stereospecific, presumably as a result of a stepwise mechanism.44... 

Acyclic S-S dications 51 and 52 also give 1,2-disulfonium salts 90 in reactions with olefins.79 Formation of a mixture of diastereomers is observed in the case of 1,2-disubstituted alkenes. Reaction with conjugated dienes leads exclusively to the product of 1,4-addition 89 as mixtures of E- and Z-products whose ratio depends on steric factors (Scheme 34).79... 

These dications react with alkenes to give 1,2-disulfonium salts, and with conjugated dienes to afford 1,4-adducts. Furthermore, while 1,4-disubstituted linear dienes yield complex mixtures of unidentified substances, 1,3-cyclohexadiene (96) produces a moderately stable salt 102 (equation 106). The formation of the kinetically controlled 1,2-addition product has never been observed. 

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