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The scission of the sulfur-sulfur bonds in a variety of compounds can be dramatically catalyzed by the cooperative efforts of an electrophile and a nucleophile. Although there are significant variations in the detailed mechanisms, in each instance the electrophilic part of the catalysis is in effect associated with the conversion of one sulfur into a better leaving group, while the nucleophilic part is associated with the displacement of this group by attack at the other sulfur. In some cases, as in the catalyzed hydrolysis of aryl sulfinyl sulfones, the mechanism involved represents a classic example of the "push-pull" type mechanism advocated for nucleophilic displacement reactions by Swain and Scott. Besides such concomitant electrophilic and nucleophilic catalysis, one also finds some systems in which catalysis by a nucleophile only is observed, and others where only electrophilic assistance is important. Data on nucleophilic catalysis of S-S bond scission, either for cases with or without accompanying electrophilic catalysis, can be used to provide quantitative information on the relative reactivity of a series of nucleophiles toward sulfur atoms in different oxidation states.

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Benzeneseleninic acid reacts rapidly at 0 °C with aromatic sulfinic acids according to the stoichiometry of eq 3 to form Se-phenyl areneselenosulfonates, PhSeS02Ar (2), in high yield. In contrast to thiosulfonates, PhSSOjAr, areneselenosulfonates are extremely photosensitive and undergo quite rapid photodecomposition. The principal products of this photodecomposition are the sulfonic anhydride, ArS020S02Ar, and diphenyl diselenide. In the presence of added alkenes the facile photodissociation of 2 can be used to initiate a free-radical chain reaction that results in the addition of 2 to the alkene to form ß-phenylseleno sulfones in good yield. The ß-phenylseleno sulfones can be converted to synthetically useful ,ß-unsaturated sulfones by oxidation of the ß-phenylseleno group to the corresponding selenoxide and subsequent elimination of PhSeOH. Photoaddition of 2 to 2,5norbornadiene gives a 5-(phenylseleno)-exo-3-nortricyclyl aryl sulfone as the almost exclusive product, while photoaddition to 1,5-cyclooctadiene gives a mixture of approximately equal amounts of the 1,2-adduct, 5-(phenylseleno)-6-arenesulfonyl-l-cyclooctene, and the product of transannular addition, 6-(phenylseleno)-exo-2-arenesulfonyl-cis-bicyclo[3.3.0]octane. Besides their extraordinary ease of photodissociation, compounds 2 also react extremely readily with nucleophiles: Nu~+ PhSeS02Ar -PhSeNu + ArS02~. Kinetic studies show that the reactivity of PhSeS02Ar with cyanide ion in such a reaction is 70000 times larger than the reactivity of the corresponding thiosulfonate, PhSS02Ar.