Alkenes selenenyl halides

Terminal alkenes react with selenenyl halides with Markovnikov regioselectivity.64 However, the (J-selenyl halide addition products readily rearrange to the isomeric products.65... 

Terminal alkenes react with selenenyl halides with anti-Markownikoff regioselec-tivity.69 However, the /i-sclcncnyl halide addition product can readily rearrange to isomeric products70 ... 

Selenoxides are even more reactive than amine oxides toward [> elimination. In fact, many selenoxides react spontaneously when generated at room temperature. Synthetic procedures based on selenoxide eliminations usually involve synthesis of the corresponding selenide followed by oxidation and in situ elimination. We have already discussed examples of these procedures in Section 4.7, where the conversion of ketones and esters to their x,/J-unsatu rated derivatives was considered. Selenides can also be prepared by electrophilic addition of selenenyl halides and related compounds to alkenes (see Section... 

Other species of general stmcture RSeX, where X = a nonhalide leaving gronp, are also known and often show similar behavior to that of the selenenyl hahdes. " For example, benzeneselenenyl acetate (20), trifluoroacetate (21), and tosylate (22) can be generated in situ from the reactions of the selenenyl halides with silver acetate, trifluoroacetate, or tosylate, respectively (Scheme 10). The former two electrophiles react with enol acetates to produce a-seleno ketones and with alkenes and acetylenes to give 1,2-addition products, while the latter adds similarly to acetylenes. Examples are shown in equations (16) to (18). 

Selenocyanates produce selenols or diselenides upon either reduction (e g. with sodium borohydride) or hydrolysis (see Scheme 1). They undergo displacement of the cyanide ion by various nucleophiles and add to alkenes in a maimer similar to selenenyl halides (see equation 14), except that catalysis with Lewis acids is required in the case of unactivated alkenes. The selenocyanates are also popular reagents for the preparation of selenides from alcohols, and (8) from carboxylic acids, as indicated in Scheme 3. 

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. 

When selenenic acids are generated in the presence of alkenes, addition reactions similar to those of selenenyl halides occur. Oxidation and syn elimination of the adducts (42) provide a convenient synthesis of allylic alcohols (equation... 

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. 

Selenenyl chlorides add to alkenes, often via an Ade2 mechanism involving a bridged seleniranium ion intermediate " (19) (equation 14). These reactions are therefore highly stereospecific, resulting in anti addition. The regiochemistry of the process can be under either kinetic or thermodynamic control. In some cases, initial anti-Markovnikov products were observed at low temperature and Markovnikov adducts dominated after further equilibration. Analogous electrophilic additions to acetylenes and allenes (Scheme 9) have also been reported. When selenenyl halides react with alkenes in the presence of other nucleophiles such... 

Terminal alkenes react with selenenyl halides with anti-Markownikoff regioselec-... 

Sulphenyl and Selenenyl Halides.—The preparation of these compounds by halogenolysis of disulphides and diselenides, a particular example being the preparation of the stable 3-nitropyridine-2-sulphenyl chloride and bromide, has not proved suitable for sulphenyl fluorides. Evidence for the existence of PhSF as an intermediate in the conversion of diphenyl disulphide into thianthrene has been provided by F n.m.r. studies. Addition of SClj to alkenes is rarely used, since... 

References to addition reactions of sulphenyl halides with alkenes and alkynes are given in the Sulphides section. An incidental bonus from a study of the addition of PhSCl to adamantylideneadamantane is the formation of the 4e-chloro-derivative and PhSSPh, via an intermediate thiiranium salt. Sulphenyl and selenenyl chlorides have been used for the dehydration of aldoximes to nitriles. Whereas sulphinyl chlorides yield sulphinate esters with alkoxytri-methylsilanes, no reaction occurs with sulphenyl chlorides. ... 

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