Selenosulfonates alkenes

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. 

Selenosulfonates (see also Section 3.6.2.3) add to alkenes when catalyzed by Lewis acids. These adducts are transformed via oxidative elimination to vinyl sulfones in good overall yields. Similar to other methods only trans addition is observed. Mixtures of regioisomers are often produced with unsyminetri-cal unactivated alkenes. All other alkenes (i.e. styrene and acrylonitrile) provide Maikovnikov additions. Radical processes yield anti-Markovnikov adducts. Thus, either regioisomeric vinyl sulfone may be prepared from almost any activated alkene by simply varying the mode of addition. ... 

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. 

Arenesulfonyl selenides are known for a long time to be excellent radical precursors. They have been widely used for selenosulfonation of alkenes and related compounds. Reviews on this subject have already appeared [2, 89] and only selected examples with high synthetic potential will be given here. 

Selenosulfonation of Alkenes, AUenes, and Dienes. The 1,2-additions of the selenosulfonate to unsaturated substrates are... 

When one of the alkene units of the diene is replaced by an allyl sulfone moiety, cyclization may be initiated with a smaller than stoichiometric amount of the selenosulfonate, since displacement of a sulfonyl radical from the allyl sulfone group propagates the chain reaction (eq 18). A related procedure that leads to the cyclization of sulfonyl-suhstituted indoles is shown in eq 19. ... 

Radical Cyclizations. Several examples of radical cycliza-tions have been reported during the selenosulfonation of cyclic or acyclic dienes. When the tether linking the two alkene moieties includes a heteroatom, the procedure affords the corresponding heterocycles. Illustrative examples are shown in eqs I4 i7.3,22d,22b,22f jjj gq quatemization of the nitrogen atom with a Lewis acid affects the yield and stereochemistry of the radical cyclization. Similar cychzations occur when unconjugated enynes or bisallenes are subjected to selenosulfonation. 

---