Radical addition reactions selenenylation

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. 

In this chapter, we discuss reactions that either add adjacent (vicinal) groups to a carbon-carbon double bond (addition) or remove two adjacent groups to form a new double bond (elimination). The discussion focuses on addition reactions that proceed by electrophilic polar (heterolytic) mechanisms. In subsequent chapters we discuss addition reactions that proceed by radical (homolytic), nucleophilic, and concerted mechanisms. The electrophiles discussed include protic acids, halogens, sulfenyl and selenenyl reagents, epoxidation reagents, and mercuric and related metal cations, as well as diborane and alkylboranes. We emphasize the relationship between the regio-and stereoselectivity of addition reactions and the reaction mechanism. 

Aryl selenides have also proven to be excellent reagents in group transfer reactions. Photolysis of selenides in an inert solvent such as benzene can initiate chain reactions. Various substituted radicals can be generated in this manner by using a-selenenyl derivatives of esters, nitriles, malonates, P-ketoesters, a-methoxyesters, and phosphonates. The resulting radicals undergo addition to alkenes to generate y-seleno derivatives. 

An efficient method for palladium(II) catalytic desulfitative conjugate addition of arylsulfinic acids with a, -unsaturated carbonyl compound has been developed. Experimental evidences for the key reacting intermediates including aryl Pd(II) sulfinic intermediate, aryl Pd(II), and C=0-Pd complexes were provided. The mechanism of the dehydrosulfenylation of 2-arylsulfinyl esters for furnishing enoates has been determined to be a homolytic process. The interception of the radical intermediate using a nitroxyl radical and mass spectrometry techniques were useful for identification of the intermediates involved in the dehydrosulfenylation of 2-arylsulfinyl esters. These data indicated that a radical-mediated process is operative. Tetrabutylammonium iodide has been found to promote deselenylation reaction of ) -chloro- and -oxyselenides to afford alkenes efficiently with formation of selenenyl iodides. A catalytic version of the transformation has also been developed. 

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