Organoselenium Chemistry - Structural Classification of Organoselenium Compounds

Structural Classification of Organoselenium Compounds

• Selenols (RSeH) are the selenium equivalents of alcohols and thiols. These compounds are relatively unstable and generally have an unpleasant smell. Phenylselenol (also called selenaphenol or PhSeH) is more acidic (pK_a 5.9) than thiophenol (pK_a 6.5) and also oxidizes more readily to the diselenide. Selenaphenol is prepared by reduction of diphenyldiselenide.
• Diselenides (R-Se-Se-R) are the selenium equivalents of peroxides and disulfides. They are useful shelf-stable precursors to more reactive organoselenium reagents such as selenols and selenenyl halides. Best known in organic chemistry is diphenyldiselenide, prepared from phenylmagnesium bromide and selenium followed by oxidation of the product PhSeMgBr.
• Selenenyl halides (R-Se-Cl, R-Se-Br) are prepared by halogenation of diselenides. Bromination of diphenyldiselenide gives phenylselenyl bromide (PhSeBr). These compounds are sources of "PhSe+".
• Selenides (R-Se-R), also called selenoethers, are the selenium equivalents of ethers and thioethers. These are the most prevalent organoselenium compounds. Symmetrical selenides are usually prepared by alkylation of alkali metal selenide salts, e.g. sodium selenide. Unsymmetrical selenides are prepared by alkylation of selenoates. These compounds are typically react as a nucleophiles, e.g. with alkyl halides (R'-X) to give selenonium salts R'RRSe+X-. Divalent selenium can also interact with soft heteroatoms to form hypervalent selenium centers. They also react in some circumstances as electrophiles, e.g. with organolithium reagents (R'Li) to the ate complex R'RRSe_-Li+.
• Selenoxides (R-Se(O)-R) are the selenium equivalents of sulfoxides. They can be further oxidized to selenones R-Se(O)₂R, the selenium analogues of sulfones.
• Selenenic acids (RSe-OH) are intermediates in the oxidation of selenols. They occur in some selenoenzymes, such as glutathione peroxidase.
• Seleninic acids (RSe(O)OH) are analogues of sulfinic acids.
• Perseleninic acids (RSe(O)OOH) catalyse epoxidation reactions and Baeyer–Villiger oxidations.
• Selenuranes are hypervalent organoselenium compounds, formally derived from the tetrahalides such as SeCl₄. Examples are of the type ArSeCl₃. The chlorides are obtained by chlorination of the selenenyl chloride.
• Seleniranes are three-membered rings (parent: C₂H₄Se) related to thiiranes but, unlike thiiranes, seleniranes are kinetically unstable, extruding selenium directly (without oxidation) to form alkenes. This property has been utilized in synthetic organic chemistry.
• Selones (R₂C=Se, sometimes called selenones) are the selenium analogues of ketones. They are rare because to their tendency to oligomerize. Diselenobenzoquinone is stable as a metal complex Selenourea is an example of a stable compound containing a C=Se bond.