Selenium: Organoselenium Chemistry (2005)

Abstract Organoselenium compounds have been known since the nineteenth century and comprise a wide variety of structures that display a remarkable range of properties. Thus, selenols (RSeH) and their conjugate bases are powerful nucleophiles that react with, for example, alkyl halides and epoxides to afford selenides (RSeR). They are also strong reducing agents that, for example, convert NO and NN compounds into amines, and vicinal dihalides into alkenes. Selenols are readily oxidized to diselenides (RSeSeR) by exposure to oxygen. Selenides are generally stable compounds that can be alkylated to afford selenonium salts (R 3 Se + X − ); reduced to the corresponding hydrocarbons with, for example, nickel boride or tin hydrides; or oxidized to either the corresponding selenoxides (R 2 SeO) under mild conditions or selenones (R 2 SeO 2 ) under more forcing ones. Selenium‐stabilized anions can be alkylated or added to a variety of carbonyl compounds. Selenoxides containing β‐hydrogens undergo syn eliminations to produce alkenes, while allylic selenoxides undergo [2,3]sigmatropic rearrangements. Both processes occur under remarkably mild conditions. Selenones are excellent leaving groups and are therefore subject to facile nucleophilic substitution of the RSeO 2 − moiety. Diselenides are convenient starting materials for many other classes of organoselenium compounds because they are relatively stable when stored but can be reduced to selenols; oxidized to selenenic (RSeOH), seleninic (RSeO 2 H), or selenonic acids (RSeO 3 H); or subjected to halogenolysis to afford selenenyl halides (RSeX; X = halide). The latter compounds are convenient forms of electrophilic selenium that can be used to introduce alkyl or arylseleno groups into the α‐positions of carbonyl compounds via their enols or enolates. They afford 1,2‐addition products with alkenes and other unsaturated compounds. When nucleophiles stronger than the halide ion are present, the nucleophile adds instead of the halide, leading to cyclized products when the nucleophile is tethered to the alkene. Selenenyl pseudohalides, where the halide is replaced by, for example, cyanide, acetate, trifluoroacetate, or sulfonate are also useful electrophiles. Selenenic acids are usually unstable and disproportionate into mixtures of diselenides and seleninic acids. However, they are also electrophilic and can be trapped in situ by addition to alkenes. Seleninic acids are typically more stable, and the corresponding aryl derivatives serve as useful oxidizing agents toward a variety of organic functional groups. They can be employed in a catalytic role in, for instance, Baeyer–Villiger oxidations and epoxidations. Selones (R 2 CSe) are stable when they are protected by bulky substituents. They undergo a variety of cycloadditions, some of which may be used in the preparation of highly hindered alkenes by a twofold extrusion method. Selenoesters and selenoamides and related selenocarbonyl groups are also known. The selenium atom in unsymmetrical selenoxides is a stereocenter, making such compounds chiral. Many other chiral selenium compounds are known, in which the stereocenter resides in an attached auxiliary group. These compounds find application in a variety of enantioselective transformations. Certain selenium compounds, such as tetraselenafulvalene, are excellent donors in donor–acceptor complexes that act as organic conductors. Other types of selenium compounds function as biological antioxidants.