Organoselenium chemistry 3064009 200900002 2008-03-25T21:39:03Z V8rik 195918 /* Selenoxide oxidations */ Riley oxidation '''Organoselenium compounds''' are [[chemical compound]]s containing [[carbon]] to [[selenium]] [[chemical bond]]s. '''Organoselenium chemistry''' is the corresponding science exploring their properties and reactivity.<ref>A. Krief, L. Hevesi, ''Organoselenium Chemistry I. Functional Group Transformations''., Springer, Berlin, '''1988''' ISBN 0-387-18629-8</ref> <ref>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</ref> <ref>Paulmier, C. ''Selenium Reagents and Intermediates in Organic Synthesis''; Baldwin, J. E., Ed.; Pergamon Books Ltd.: New York, '''1986''' ISBN 0-08-032484-3</ref> Selenium belongs with oxygen and sulfur to the [[group 16 element]]s 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 [[kilojoule|kJ]]/[[mole (unit)|mol]] for the C–Se bond and 272 kJ/mol for the C–S bond) and the [[bond length]]s 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 XH<sub>2</sub> are 16 for oxygen, 7 for sulfur and 3.8 for selenium. In contrast to [[sulfoxide]]s the corresponding selenoxides are unstable in the presence of β-protons and this property is utilized in many [[organic reaction]]s of selenium, notably in selenoxide oxidations and in selenoxide eliminations. The first organoselenium compound ever isolated was diethylselenide in 1836.<ref>Lwig, C. J. Pogg. Ann. '''1836''', 37, 552</ref> ==Selenium compounds== '''Selenols''' RSeH are the selenium equivalents of [[alcohol]]s and [[thiol]]s. These compounds are toxic and generally have an unpleasant smell. [[benzeneselenol|Phenylselenol]] or selenaphenol, PhSeH, is more acidic (pKa 5.9) than thiophenol (pKa 6.5) and also oxidizes more readily to the [[diphenyldiselenide|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 [[peroxide]]s and [[disulfide]]s 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 [[ether]]s and [[thioether]]s and the organic counterparts of inorganic [[Selenide]]s. These compounds are ambiphilic on account of the identical [[electronegativity|electronegativities]] of carbon and selenium and can react as a [[nucleophile]] or an [[electrophile]]. They react as nucleophiles with [[alkyl halide]]s R'-X to trivalent '''selenonium salts''' R'RRSe<sub>+</sub>X<sup>-</sup> but as electrophiles with [[organolithium]] reagents R'Li to the [[ate complex]] R'RRSe<sub>-</sub>Li<sup>+</sup> 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 [[sulfoxide]]s which can be further oxidized to '''selenones''' R-SeO<sub>2</sub>-R, [[sulfone]]s with sulfur replaced again by selenium. ==Vinylic selenides== '''Vinylic selenides''' play an important role in the synthesis of [[organoselenium compound]]s, especially in the development of many convenient methods for the [[stereoselective]] preparation of functionalized [[alkenes]].<ref>Comasseto, J. V.; Ling, L. W.; Petragnani, N.; Stefani, H. A. Synthesis 1997, 373-403. (b) Zeni, G.; Stracke, M. P.; Lissner, E.; Braga, A. L. Synlett 2003, 12, 1880-1882.</ref> 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]]. <ref>(a) Comasseto, J. V. J. Organomet. Chem. 1983, 253, 131-181. (b) Zeni, G.; Stracke, M. P.; Nogueira,C. W.; Braga,A. L.; Menezes,P. H.; Stefani H. A. Organic Letters 2004, 6, 1135-1138. (c) Dabdoub, M. J.; Dabdoub, V.B.; Pereira, M. A. Tetrahedron Lett. 2001, 42, 1595-1597. (d) Dabdoub,M. J.; Baroni, A. C. M.; Lenardão, E. J.; Gianeti, T. R.; Hurtado, G. R.Tetrahedron 2001, 57, 4271-4276. (e) Dabdoub, M. J.; Cassol, T. M.;Batista, A. C. F. Tetrahedron Lett. 1996, 37, 9005-9008. (f) Barros, O. S. D.; Lang, E. S.; de Oliveira, C. A. F.; Peppe, C.; Zeni, G.Tetrahedron Lett. 2002, 43, 7921-7923. </ref> 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. <ref>Comasseto, J. V.; Ferreira, J. T. B. J. Organomet. Chem. 1981, 216,287-294.</ref> On the other hand, the adducts, depending on the nature of the [[substituent]]s 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 [[allyl|allylic]] [[methylene]] group into an [[allyl alcohol|allylic alcohol]] or a [[ketone]]. This chemical transformation is an important [[organic reaction]]. [[Selenium dioxide]] is one representative of a group of [[oxidizing agent]]s that can bring about this reaction. [[Image:SeleniumdioxideOxidation.gif|center|Scheme 1. Selenium dioxide oxidation]] This type of reaction often involves [[radical (chemistry)|free radicals]] but in some cases a [[pericyclic]] [[concerted reaction|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]] (H<sub>2</sub>SeO<sub>3</sub>) and [[pyridinium chlorochromate]] are other oxidizing reagents. The type of substrate can be extended to α-carbonyl compounds such as [[ketone]]s converting them to diketones. This type of oxidation with selenium oxide is called '''Riley oxidation''' <ref>''255. Selenium dioxide, a new oxidising agent. Part I. Its reaction with aldehydes and ketones'' Harry Lister Riley, John Frederick Morley and Norman Alfred Child Friend, [[J. Chem. Soc.]] '''1932''', 1875 {{DOI|10.1039/JR9320001875}}</ref> ==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 elimination|syn]]. oxidizing agents are [[hydrogen peroxide]], [[ozone]] or [[MCPBA]]. This reaction type is often used with [[ketone]]s leading to [[enone]]s. [[Image:SelenoxideElimination.png|center|600px|Scheme 2. Selenoxide elimination of carbonyl compounds]] The [[Grieco elimination]] is a similar selenoxide elimination with o-nitrophenylselenocyanate and tributylphosphine. ===Deselenation=== The three-membered '''seleniranes''' are related to the oxygen pendants [[epoxide|oxirane]]s but unlike oxiranes they are kinetically unstable, extruding selenium directly (without oxidation) to form [[alkene]]s. This property has been utilized in synthetic organic chemistry <ref>''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 [http://www.arkat-usa.org/home.aspx?VIEW=MANUSCRIPT&MSID=1901 Link]</ref>. ==See also== * The chemistry of carbon bonded to other elements in the periodic table: {{ChemicalBondsToCarbon}} == External links== * [[cycloheptatriene]] oxidation to [[tropone]] with selenium dioxide and [[potassium dihydrogen phosphate]] in [[organic Syntheses]] Coll. Vol. 9, p.396 (1998); Vol. 71, p.181 (1993) [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv9p0396 Online article] * β-[[Pinene]] oxidation with selenium dioxide and hydrogen peroxide in [[organic Syntheses]] Coll. Vol. 6, p.946 (1988); Vol. 56, p.25 (1977). [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0946 Online article] *[[Cyclohexanone]] oxidation to 1,2-Cyclohexanedione with selenium dioxide in [[Organic Syntheses]], Coll. Vol. 4, p.229 (1963); Vol. 32, p.35 (1952). [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv4p0229 Online article] * [[Paraldehyde]] oxidation to [[glyoxal]] with selenius acid in [[Organic Syntheses]], Coll. Vol. 3, p.438(1955); Vol. 24, p.61 (1944). [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv3p0438 Online Article] * Acetylcyclohexanone elimination with benzeneselenylchloride and [[sodium hydride]] in [[Organic Syntheses]] Coll. Vol. 6, p.23 (1988); Vol. 59, p.58 (1979) [http://www.orgsynth.org/orgsyn/prep.asp?prep=cv6p0023 Online Article] * Selenophenol from [[phenylmagnesium bromide]] and elemental selenium in [[Organic Syntheses]], Coll. Vol. 3, p.771 (1955); Vol. 24, p.89 (1944) [http://www.orgsynth.org/orgsyn/prep.asp?prep=cv3p0771 Online Article]. Trivial note: as a precaution against selenium toxicity the authors suggest ''to work with them on alternate days''. * Diphenyldiselenide and phenylselenenyl bromide from [[bromobenzene]], magnesium, selenium, bromine and chloride in [[Organic Syntheses]], Coll. Vol. 6, p.533 (1988); Vol. 59, p.141 (1979) [http://www.orgsynth.org/orgsyn/prep.asp?prep=CV6P0533 Article]. ==References== <div class="references-small"><references/></div> [[Category:selenium compounds]] [[ja:有機セレン化合物]]