|
Organoselenium compounds are chemical compounds containing carbon-to-selenium chemical bonds. Organoselenium chemistry is the corresponding science exploring their properties and reactivity.〔A. Krief, L. Hevesi, ''Organoselenium Chemistry I. Functional Group Transformations''., Springer, Berlin, 1988 ISBN 3-540-18629-8〕〔S. Patai, Z. Rappoport (Eds.), ''The Chemistry of Organic Selenium and Tellurium Compounds'', John. Wiley and Sons, Chichester, Vol. 1, 1986 ISBN 0-471-90425-2〕〔Paulmier, C. ''Selenium Reagents and Intermediates in Organic Synthesis''; Baldwin, J. E., Ed.; Pergamon Books Ltd.: New York, 1986 ISBN 0-08-032484-3〕 Selenium belongs with oxygen and sulfur to the group 16 elements or chalcogens, 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 group 16 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 p''K''a 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. Organoselenium compounds are found at trace levels in ambient waters, soils and sediments. The first organoselenium compound ever isolated was diethyl selenide in 1836.〔Lwig, C. J. Pogg. Ann. 1836, 37, 552〕 ==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 (pKa 5.9) than thiophenol (pKa 6.5) and also oxidizes more readily to the diselenide. Selenaphenol is prepared by reduction of diphenyldiselenide.〔Organic Syntheses, Coll. Vol. 3, p. 771 (1955); Vol. 24, p. 89 (1944) (Online Article ).〕 *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.〔Organic Syntheses, Coll. Vol. 6, p. 533 (1988); Vol. 59, p. 141 (1979) (Article )〕 *Selenyl 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.〔''Chemistry of hypervalent compounds'' (1999) Kin-ya Akiba ISBN 978-0-471-24019-8〕 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.〔(Link ''Developments in the chemistry of selenaheterocyclic compounds of practical importance in synthesis and medicinal biology'' ) Arkivoc 2006 (JE-1901MR) Jacek Młochowski, Krystian Kloc, Rafał Lisiak, Piotr Potaczek, and Halina Wójtowicz〕 *Selones (R2C=Se, sometimes called selenones) are the selenium analogues of ketones. They are rare due 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「organoselenium chemistry」の詳細全文を読む スポンサード リンク
|