Organoselenium chemistry

Organoselenium chemistry

Organoselenium compounds are chemical compounds containing carbon-to-selenium chemical bonds. Organoselenium chemistry is the corresponding science exploring their properties and reactivity.[1][2][3][4] 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 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 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. Organoselenium compounds are found at trace levels in ambient waters, soils and sediments.[5]

The first organoselenium compound ever isolated was diethylselenide in 1836.[6]

Contents

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.[7]
  • 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.[8]
  • 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.[9] 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.
  • 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.[10] 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.[11]
  • Selones (R2C=Se, sometimes called selenones) are the selenium analogues of ketones. They are rare because to their tendency to oligomerize.[12] Diselenobenzoquinone is stable as a metal complex [13] Selenourea is an example of a stable compound containing a C=Se bond.

Organoselenium compounds in nature

Selenium is required for life, albeit only in small amounts. Selenocysteine is a selenol-containing amino acid that is encoded in a special manner by DNA. Selenomethionine is a selenide-containing amino acid that also occurs naturally, but is generated by post-transcriptional modification. Glutathione oxidase is an enzyme with a diselenide at its active site.

Organoselenium chemistry in organic synthesis

Organoselenium compounds are specialized but useful collection of reagents useful in organic synthesis, although they are generally excluded from processes useful to pharmaceuticals owing to regulatory issues. Their usefulness hinges on certain attributes, including (i) the weakness of the C-Se bond and (ii) the easy oxidation of divalent selenium compounds.

Vinylic selenides

Vinylic selenides are organoselenium compounds that play a role in organic synthesis, especially in the development of convenient stereoselective routes to functionalized alkenes.[14] 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.[15][16][17][18] For example, the nucleophilic addition of selenophenol 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.[19] On the other hand, the adducts depend 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; the 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

Selenium dioxide is useful in organic oxidation. Specifically, SeO2 will convert an allylic methylene group into the corresponding alcohol. A number of other reagents bring about this reaction. .

Scheme 1. Selenium dioxide oxidation

In terms of reaction mechanism, SeO2 and the allylic substrate react via pericyclic process beginning with an ene reaction that activates the C-H bond. 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.

SeO2-based oxidations sometimes afford carbonyl compounds such as ketones.,[20] β-Pinene[21] and cyclohexanone oxidation to 1,2-cyclohexanedione [22] Oxidation of ketones having α-methylene groups affords diketones. This type of oxidation with selenium oxide is called Riley oxidation.[23]

Selenoxide eliminations

Main article: Selenoxide elimination

In presence of a β-proton, a selenide will give an elimination reaction after oxidation, to leave behind an alkene and a selenol. In the elimination reaction, all five participating reaction centers are coplanar and, therefore, the reaction stereochemistry is syn. Oxidizing agents used are hydrogen peroxide, ozone or MCPBA. This reaction type is often used with ketones leading to enones. An example is acetylcyclohexanone elimination with benzeneselenylchloride and sodium hydride [24]

Scheme 2. Selenoxide elimination of carbonyl compounds

The Grieco elimination is a similar selenoxide elimination using o-nitrophenylselenocyanate and tributylphosphine to cause the elimination of the elements of H2O.

See also

  • The chemistry of carbon bonded to other elements in the periodic table:
CH He
CLi CBe CB CC CN CO CF Ne
CNa CMg CAl CSi CP CS CCl CAr
CK CCa CSc CTi CV CCr CMn CFe CCo CNi CCu CZn CGa CGe CAs CSe CBr CKr
CRb CSr CY CZr CNb CMo CTc CRu CRh CPd CAg CCd CIn CSn CSb CTe CI CXe
CCs CBa CHf CTa CW CRe COs CIr CPt CAu CHg CTl CPb CBi CPo CAt Rn
Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Uuq Uup Uuh Uus Uuo
↓
CLa CCe CPr CNd CPm CSm CEu CGd CTb CDy CHo CEr CTm CYb CLu
Ac Th Pa CU Np Pu Am Cm Bk Cf Es Fm Md No Lr
Chemical bonds to carbon
Core organic chemistry Many uses in chemistry
Academic research, but no widespread use Bond unknown / not assessed

References

  1. ^ A. Krief, L. Hevesi, Organoselenium Chemistry I. Functional Group Transformations., Springer, Berlin, 1988 ISBN 3540186298
  2. ^ 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
  3. ^ Paulmier, C. Selenium Reagents and Intermediates in Organic Synthesis; Baldwin, J. E., Ed.; Pergamon Books Ltd.: New York, 1986 ISBN 0-08-032484-3
  4. ^ Freudendahl, Diana M.; Santoro, Stefano; Shahzad, Sohail A.; Santi, Claudio; Wirth, Thomas (2009). "Green Chemistry with Selenium Reagents: Development of Efficient Catalytic Reactions". Angewandte Chemie International Edition 48 (45): 8409. doi:10.1002/anie.200903893. PMID 19802863. 
  5. ^ Wallschläger, D.; Feldmann, F. (2010). Formation, Occurrence, Significance, and Analysis of Organoselenium and Organotellurium Compounds in the Environment. Metal Ions in Life Sciences. 7, Organometallics in Environment and Toxicology. RSC Publishing. pp. 319–364. ISBN 978-1-84755-177-1. 
  6. ^ Lwig, C. J. Pogg. Ann. 1836, 37, 552
  7. ^ Organic Syntheses, Coll. Vol. 3, p. 771 (1955); Vol. 24, p. 89 (1944) Online Article.
  8. ^ Organic Syntheses, Coll. Vol. 6, p. 533 (1988); Vol. 59, p. 141 (1979) Article
  9. ^ Organoselenium Chemistry: Role of Intramolecular Interactions 2010 Anna J. Mukherjee, Sanjio S. Zade, Harkesh B. Singh and Raghavan B. Sunoj Chem. Rev., Article ASAP doi:10.1021/cr900352j
  10. ^ Chemistry of hypervalent compounds (1999) Kin-ya Akiba ISBN 978-0-471-24019-8
  11. ^ 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
  12. ^ Okazaki, R.; Tokitoh, N. (2000). "Heavy ketones, the heavier element congeners of a ketone". Accounts of chemical research 33 (9): 625–630. doi:10.1021/ar980073b. PMID 10995200.  edit
  13. ^ Amouri, H., Moussa, J., Renfrew, A. K., Dyson, P. J., Rager, M. N. and Chamoreau, L.-M. (2010), Discovery, Structure, and Anticancer Activity of an Iridium Complex of Diselenobenzoquinone. Angewandte Chemie International Edition, 49: 7530–7533. doi:10.1002/anie.201002532
  14. ^ Comasseto, João Valdir; Ling, Lo Wai; Petragnani, Nicola; Stefani, Helio Alexandre (1997). "Vinylic Selenides and Tellurides - Preparation, Reactivity and Synthetic Applications". Synthesis 1997 (04): 373. doi:10.1055/s-1997-1210. 
  15. ^ Comasseto, J (1983). "Vinylic selenides". Journal of Organometallic Chemistry 253 (2): 131. doi:10.1016/0022-328X(83)80118-1. 
  16. ^ Zeni, Gilson; Stracke, Marcelo P.; Nogueira, Cristina W.; Braga, Antonio L.; Menezes, Paulo H.; Stefani, Helio A. (2004). "Hydroselenation of Alkynes by Lithium Butylselenolate:  an Approach in the Synthesis of Vinylic Selenides". Organic Letters 6 (7): 1135. doi:10.1021/ol0498904. PMID 15040741. 
  17. ^ Dabdoub, M (2001). "Synthesis of (Z)-1-phenylseleno-1,4-diorganyl-1-buten-3-ynes: hydroselenation of symmetrical and unsymmetrical 1,4-diorganyl-1,3-butadiynes". Tetrahedron 57 (20): 4271. doi:10.1016/S0040-4020(01)00337-4. 
  18. ^ Doregobarros, O; Lang, E; Deoliveira, C; Peppe, C; Zeni, G (2002). "Indium(I) iodide-mediated chemio-, regio-, and stereoselective hydroselenation of 2-alkyn-1-ol derivatives". Tetrahedron Letters 43 (44): 7921. doi:10.1016/S0040-4039(02)01904-4. 
  19. ^ Comasseto, J (1981). "Stereoselective synthesis of vinylic selenides". Journal of Organometallic Chemistry 216 (3): 287. doi:10.1016/S0022-328X(00)85812-X. 
  20. ^ organic Syntheses Coll. Vol. 9, p. 396 (1998); Vol. 71, p. 181 (1993) Online article
  21. ^ organic Syntheses Coll. Vol. 6, p. 946 (1988); Vol. 56, p. 25 (1977). Online article
  22. ^ Organic Syntheses, Coll. Vol. 4, p. 229 (1963); Vol. 32, p. 35 (1952). Online article
  23. ^ Riley, Harry Lister; Morley, John Frederick; Friend, Norman Alfred Child (1932). "255. Selenium dioxide, a new oxidising agent. Part I. Its reaction with aldehydes and ketones". Journal of the Chemical Society (Resumed): 1875. doi:10.1039/JR9320001875. 
  24. ^ Organic Syntheses Coll. Vol. 6, p. 23 (1988); Vol. 59, p. 58 (1979) Online Article

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