Selenenyl pseudohalides

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. 

Alkenes react with many selenenyl electrophiles to form 1,2-adducts thus, selenenyl chlorides, bromides and pseudohalides afford a-chloro-, a-bromo-, or other a-functionalized selenides, respectively. When the addition of a selenenyl halide to an alkene is carried out in the presence of a nucleophile, in some cases the nucleophile is incorporated at the a position instead of the halide atom. 

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