Organoselenium chemistry

Organoselenium compounds are chemical compounds containing carbon to selenium chemical bonds. Organoselenium chemistry is the corresponding science exploring their properties and reactivity. Selenium belongs with oxygen and sulfur to the group 16 elements and similarities in chemistry are to be expected.

Selenium can exist with oxidation state -2, +2, +4, +6. Se(II) is the dominant form in organoselenium chemistry. Down the column the bond strength becomes increasingly weaker (234 kJ/mol for the C–Se bond and 272 kJ/mol for the C–S bond) and the bond lengths longer (C–Se 198 pm, C–S 181 pm and C–O 141 pm). Selenium compounds are more nucleophilic than the corresponding sulfur compounds and also more acidic. The pKa values of XH2 are 16 for oxygen, 7 for sulfur and 3.8 for selenium. In contrast to sulfoxides the corresponding selenoxides are unstable in the presence of β-protons and this property is utilized in many organic reactions of selenium, notably in selenoxide oxidations and in selenoxide eliminations.

The first organoselenium compound ever isolated was diethylselenide in 1836.

Selenium compounds
Selenols RSeH are the selenium equivalents of alcohols and thiols. These compounds are toxic and generally have an unpleasant smell. Phenylselenol or selenaphenol, PhSeH, is more acidic (pKa 5.9) than thiophenol (pKa 6.5) and also oxidizes more readily to the diselenide. A procedure for the synthesis of selenaphenol starts from phenylmagnesium bromide and elemental selenium with aqueous acidic workup.

Diselenides R-Se-Se-R are the selenium equivalents of peroxides and disulfides and used in organic chemistry as starting compounds for selenols and selenenyl halides R-Se-Cl or R-Se-Br.

Selenides R-Se-R are the selenium equivalents of ethers and thioethers and the organic counterparts of inorganic Selenides. These compounds are ambiphilic on account of the identical electronegativities of carbon and selenium and can react as a nucleophile or an electrophile. They react as nucleophiles with alkyl halides R'-X to trivalent selenonium salts R'RRSe+X- but as electrophiles with organolithium reagents R'Li to the ate complex R'RRSe-Li+ in which the lone pair carbanion is stabilized by the unfilled selenium 4d orbital. This ate complex collapses back to a selenide but with exchange of ligands as in R'-se-Se.

Selenoxides R-Se=O-R are the selenium equivalents of sulfoxides which can be further oxidized to selenones R-SeO2-R, sulfones with sulfur replaced again by selenium.

Vinylic selenides
Vinylic selenides play an important role in the synthesis of organoselenium compounds, especially in the development of many convenient methods for the stereoselective preparation of functionalized alkenes. Although various methods are mentioned for the preparation of vinylic selenides, a more useful procedure has centered on the nucleophilic or electrophilic organoselenium addition to terminal or internal alkynes. For example, the nucleophilic addition of senophenol to alkynes affords, preferentially, the Z-vinylic selenides after longer reaction times at room temperature.The reaction is faster at a high temperature; however, the mixture of Z- and E-vinylic selenides was obtained in an almost 1:1 ratio. On the other hand, the adducts, depending on the nature of the substituents at the triple bond. Conversely, vinylic selenides can be prepared by palladium-catalyzed hydroselenation of alkynes to afford the Markownikov adduct in good yields. There are some limitations associated with the methodologies to prepare vinylic selenides illustrated above; procedures described employ diorganoyl diselenides or selenophenol as starting materials, which are volatile and unstable and have an unpleasant odor. Also, the preparation of these compounds is complex.

Selenoxide oxidations
Allylic oxidation is an organic oxidation converting an allylic methylene group into an allylic alcohol or a ketone. This chemical transformation is an important organic reaction. Selenium dioxide is one representative of a group of oxidizing agents that can bring about this reaction.

This type of reaction often involves free radicals but in some cases a pericyclic concerted reaction mechanism is found for selenium oxide oxidations. The first step is an ene reaction, transferring the allylic proton to the selenium oxide, and the second step is a [2,3] sigmatropic reaction.

Oxidations involving selenium dioxide are often carried out with catalytic amounts of the selenium compound and in presence of a sacrificial catalyst or co-oxidant such as hydrogen peroxide. Selenious acid (H2SeO3) and pyridinium chlorochromate are other oxidizing reagents.

The type of substrate can be extended to α-carbonyl compounds such as ketones converting them to diketones. This type of oxidation with selenium oxide is called Riley oxidation

Selenoxide eliminations
In presence of a β-proton, a selenide will give an elimination reaction after oxidation to an alkene and leaving a selenol. In the elimination reaction all five participating reaction centers a coplanar and therefore the reaction mode is syn. oxidizing agents are hydrogen peroxide, ozone or MCPBA. This reaction type is often used with ketones leading to enones.



The Grieco elimination is a similar selenoxide elimination with o-nitrophenylselenocyanate and tributylphosphine.

Deselenation
The three-membered seleniranes are related to the oxygen pendants oxiranes but unlike oxiranes they are kinetically unstable, extruding selenium directly (without oxidation) to form alkenes. This property has been utilized in synthetic organic chemistry.