Selenosulfonates addition reactions

In the same manner as described before, arenesulfonyl thiocyanates are able to show self-addition to conjugated systems yielding sulfones243,244. More important, however, is that reactions of selenosulfonates with unsaturated systems as well as with nucleophilic carbon have been proved. 

Selenium-containing molecules have also been used as precursors for radical seleno group transfer reactions. This is a very powerful method for radical additions to alkenes and alkynes it is especially interesting from an atom economy point of view since all atoms remain in the product molecule. The free-radical addition of selenosulfonates 146 can be initiated either photochemically or thermally using AIBN. The addition of 146 not only to alkynes 147,255-257 km also to alkenes258-261 or allenes,261 has been reported and the products such as 148 are versatile building blocks for subsequent reactions (Scheme 39). For example, vinyl selenides 148 can be easily transformed into allenes. 

The selenosulfonates (26) comprise another class of selenenyl pseudohalides. They are stable, crystalline compounds available from the reaction of selenenyl halides with sulftnate salts (Scheme 10) or more conveniently from the oxidation of either sulfonohydrazides (ArS02NHNH2) or sulftnic acids (ArS02H) with benzeneseleninic acid (27) (equations 21 and 22). Selenosulfonates add to alkenes via an electrophilic mechanism catalyzed by boron trifluoride etherate, or via a radical mechanism initiated thermally or photolytically. The two reaction modes produce complementary regioselectivity, but only the electrophilic processes are stereospecific (anti). Similar radical additions to acetylenes and allenes have been reported, with the regio- and stereochemistry as shown in Scheme 11. When these selenosulfonation reactions are used in conjunction with subsequent selenoxide eliminations or [2,3] sigmatropic rearrangements, they provide access to a variety of unsaturated sulfone products. 

Numerous studies have been devoted to the addition of RSO2X to olefins. The propagation steps for these chain processes are shown in Scheme 13. The main factors controlling the reactivity of sulfonyl radicals are polar effects and the reversibility of the addition. Sulfonyl radicals exhibit an electrophilic character with respect to addition to olefins [98]. Evidence for the reversibility came early from the observation of the isomerization of cis- and rran -2-butenes during their copolymerization with sulfur dioxide and their Cu(I)-mediated reaction with benzenesulfonyl chloride [99]. Therefore, the chain transfer of alky] adduct radica]s has to compete effectively with -elimination of sulfonyl radicals (see Scheme 13). Selenosulfonates, sulfonyl halides, and sulfonyl cyanides are the most popular substrates that meet this property. 

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