Phenylselenenyl halides

Nitroselenenylation of alkenes141-149 is performed in a mixture of acetonitrile/tetrahydro-furan (compared to the nitromercuration reaction, this allows application to alkenes that may be insoluble in water) by the addition of phenylselenenyl halides, mercury(ii) chloride, and silver nitrite. Mercury chloride is necessary to significantly or completely suppress the formation of -hydroxy selenides due to the ambident character of the nitrite anion. 

Pyrrolizidines 1 were obtained with high stereocontrol (100% tram addition) by intramolecular cyclization of l-aza-4-cyclooctene with phenylsulfenyl or phenylselenenyl halides. Analogous results were also obtained by the mercury-induced cyclization performed with mercury(II) chloride in tetrahydrofuran113. 

Heterocyclization of 2-(allylthio, propargylthio)thieno[2,3-<7]pyrimidin-4-ones 202 with phenylselenenyl halides yielded thieno[3/,2, 5,6]pyrimido[2,l-Z>][l,4,3]thiaselena-zinium halides 203 (2002UKZ43, 2002UKZ111). 

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... 

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... 

An alternative procedure for the preparation of /J-alkoxy selenides involves treatment of cyclic or acyclic alkenes with phenylselenenyl cyanide in the appropriate alcohol in the presence of catalytic amounts of copper(ri) or nickel(ll) halides. The reaction using methanol as the alcohol with cyclic alkenes is tram stereoselective giving /1-methoxy selenides 23 usually in high yield34. 

When cyclohexene is allowed to react with phenylselenenyl cyanide in a mixture of tetrahydro-furan and water in the presence of a copper(ll) or nickel(II) halide, /ra s-2-(phenylseleno)cyclo-hexanol (27) is formed as a single diastereomer34a. 

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. 

The anion (147), a synthon of methyl a-lithioacrylate, has been prepared but unfortunately, owing to its chelated structure, it is rather unreactive and viable yields have been obtained only in reactions with primary allylic halides (c/. 3, 112). Sensitive enol pyruvates (149) can be obtained in ca. 30% yield from the corresponding methyl a-hydroxypropionate derivatives (148) by sequential enolate formation, addition of phenylselenenyl bromide, and oxidative elimination using hydrogen peroxide. ... 

---