Selenenyl halides, addition reactions with

Addition of a selenenyl halide to an alkyne takes a course analogous to that of the sulfenyl halide, as seen in the reaction of l,4-dichlorobut-2-yne with PhSeCl or PhSeBr (Scheme 19).28-62 Selenenyl halides also undergo facile addition to allenes with exclusive attack by selenium occurring at the central allenic carbon.63... 

There are various ways to generate and synthesize selenium electrophiles and some of these compounds are commercially available. The addition reaction can also be dependent on the counterion X of these reagents and several protocols have been developed to exchange the counterions. The most commonly used electrophile is the phenylselenyl electrophile and compounds like phenylselenenyl chloride 6 and phenylselenenyl bromide 7 are commercially available. They can also be easily generated from diphenyl diselenide 8 by treatment with sulfuryl chloride or elementary chlorine or bromine, respectively. Diselenides in general are very versatile precursors for selenium electrophiles. For addition reactions using external nucleophiles the use of selenenyl halides can lead to complications, because chloride or bromide ions can also act as nucleophiles and lead to undesired side-products. An... 

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). 

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. 

Generation of seleno carboxylates occurs instantaneously on treatment of selenenyl halides with carboxylic acids and, consequently, the phenylseleno-lactonization of unsaturated carboxylic acids by phenylselenenyl chloride may be formally considered as intramolecular addition of phenylselenenyl carboxylate to the C-C double bond. The reaction of 4-cyclohep-tene-l-carboxylic acid with phenylselenenyl chloride to give lactone 40 is a good example. 

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... 

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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