Sigmatropic rearrangements Boron trifluoride etherate

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. 

E)-a,p-Unsaturated aldehydes. The anion of 2-alkenyl N,N-diethyldithio-carbamates (2) reacts with (1) at the a-carbon atom to form a product (a) that undergoes facile [3,3] sigmatropic rearrangement to form (3). The products are hydrolyzed to aldehydes (4) by red mercuric oxide and boron trifluoride etherate (3,136). ... 

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