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... 

Scheme 4.5. Trapping of a vinyl zircono-cene with an aryl selenenyl halide.

Organotellurium(II) compounds can also contain a three-center, four-electron bond as shown for 39 to 42 in Fig. 19. Typically, these molecules contain an odd number ligands around the central atom and an electronegative atom helps to stabilize a tellurenyl halide or selenenyl halide bond through chelation to form a four, five, or six membered ring. " Such molecules are described as lO-Te-3 and lO-Se-3... 

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... 

Trifluoromethaneseleninyl chloride (3) can be prepared by the reaction of trifluoromethane-sclcnenyl chloride with ozone whereas nitrogen dioxide transforms the initially formed acid choride to the anhydride (CF3Se0)20.316 Trifluoromethaneselenenic acid chloride and bromide are oxidized with hypofluorous acid in acetonitrile to the corresponding seleninyl halides 3 and 4, respectively.204 When an excess of hypofluorous acid is employed, the selenenyl halides and trifluoromethaneseleninic acid (5) are oxidized to the anhydride 6.304... 

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... 

Selenenyl halides are relatively stable, though moisture sensitive, compounds that are generally prepared by the reactions shown in Scheme 7 and behave as electrophihc selenium species. " They react with ketones and aldehydes via their enols or enolates to afford a-seleno derivatives (e.g. (17) in equation 11). Similar a-selenenylations of /3-dicarbonyl compounds, esters, and lactones can be performed, although the latter two types of compounds require prior formation of their enolates. Moreover, the a-selenenylation of anions stabilized by nitrile, nifro, sulfone, or various types of phosphorus substituents has also been reported (equation 12). In many such cases, the selenenylation step is followed by oxidation to the selenoxide and spontaneous syn elimination to provide a convenient method for the preparation of the corresponding a ,/3-unsaturated compound (e.g. 18 in equation 11). Enones react with benzeneselenenyl chloride (PhSeCl) and pyridine to afford a-phenylselenoenones (equation 13). 

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). 

Selenenamides (23) are obtained by the substitution of selenenyl halides with amines or by the metathesis of the former compounds with Af-silylamines. N-(Phenylseleno)phthalimide (24) is similarly obtained using potassium phthalimide (Scheme 10). These compounds can be isolated but are prone to hydrolyze when exposed to moisture. Selenenamides react with aldehydes or jS-dicarbonyl compounds to afford a-seleno derivatives (as in the process shown in equation 11), and add to activated double and triple bonds, as in the example in equation (19). The imide (24) is a useful alternative to PhSeCl in various selenenylation reactions, and to ArSeCN in the conversion of alcohols and carboxylic acids to selenides and selenoesters (8), as shown in Scheme 3. 

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. 

Several types of electrophilic selenium species have been immobilized on solid supports, for more convenient application to organic synthesis. These include selenenyl halides, " selenocyanates, and selenosulfonates. ... 

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. 

The 1,2-addition of selenenyl halides is generally highly stereoselective affording tram addition products. Thus, cyclohexene gives fru/K-adducts 1 on treatment with phenylselenenyl chloride... 

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. 

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