Organoselenium chemistry#Selenoxide oxidations

File:Organoselenium compounds.png

• Selenols ({{chem2|R\sSeH}}) are the selenium equivalents of alcohols and thiols. These compounds are relatively unstable and generally have an unpleasant smell. Benzeneselenol (also called selenophenol or PhSeH) is more acidic (pK_a 5.9) than thiophenol (pK_a 6.5) and also oxidizes more readily to the diselenide. Selenophenol is prepared by reduction of diphenyldiselenide.Organic Syntheses, Coll. Vol. 3, p. 771 (1955); Vol. 24, p. 89 (1944) [http://www.orgsynth.org/orgsyn/prep.asp?prep=cv3p0771 Online Article].
• Diselenides ({{chem2|R\sSe\sSe\sR}}) are the selenium equivalents of peroxides and disulfides. They are useful shelf-stable precursors to more reactive organoselenium reagents such as selenols and selanyl 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) [http://www.orgsynth.org/orgsyn/prep.asp?prep=CV6P0533 Article]
• Selanyl halides ({{chem2|R\sSe\sCl, R\sSe\sBr}}) are prepared by halogenation of diselenides. Bromination of diphenyldiselenide gives phenylselanyl bromide (PhSeBr). These compounds are sources of "{{chem2|PhSe+}}".
• Selenides ({{chem2|R\sSe\sR}}), also called selenoethers, are the selenium equivalents of ethers and sulfides. One example is dimethylselenide ({{chem2|(CH3)2Se}}). 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 typically react as nucleophiles, e.g. with alkyl halides ({{chem2|R'\sX}}) to give selenonium salts {{chem2|[RR'R"Se]+X−}}. Divalent selenium can also interact with soft heteroatoms to form hypervalent selenium centers.{{cite journal | doi = 10.1021/cr900352j | title = Organoselenium Chemistry: Role of Intramolecular Interactions | year = 2010 | last1 = Mukherjee | first1 = Anna J. | last2 = Zade | first2 = Sanjio S. | last3 = Singh | first3 = Harkesh B. | last4 = Sunoj | first4 = Raghavan B. | journal = Chemical Reviews | volume = 110 | issue = 7 | pages = 4357–4416 | pmid = 20384363}} They also react in some circumstances as electrophiles, e.g. with organolithium reagents (R'Li) to the ate complex {{chem2|R'RRSe−Li+}}.
• Selenoxides ({{chem2|R\sSe(\dO)\sR}}) are the selenium equivalents of sulfoxides. Most are unstable, undergoing the selenoxide elimination, but can be notionally oxidized to selenones {{chem2|R\sSe(\dO)2\sR}}, the selenium analogues of sulfones.
• Selenenic acid ({{chem2|R\sSe\sOH}}) are intermediates in the oxidation of selenols. They occur in some selenoenzymes, such as glutathione peroxidase.
• Seleninic acids ({{chem2|R\sSe(\dO)\sOH}}) are analogues of sulfinic acids.
• Selenonic acids ({{chem2|R\sSe(\dO)2\sOH}}) are analogues of sulfonic acids.
• Peroxyseleninic acids ({{chem2|R\sSe(\dO)\sOOH}}) catalyse epoxidation reactions and Baeyer–Villiger oxidations.
• Selenuranes are hypervalent organoselenium compounds, formally derived from the tetrahalides such as {{chem2|SeCl4}}. Examples are of the type {{chem2|Ar\sSeCl3}}.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 (the parent compound is selenirane or selenacyclobutane {{chem2|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.[http://www.arkat-usa.org/home.aspx?VIEW=MANUSCRIPT&MSID=1901 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 ({{chem2|R2C\dSe}}) are the selenium analogues of ketones. They are rare due to their tendency to oligomerize.{{Cite journal| pages = 625–630| issue = 9| year = 2000| pmid = 10995200 | doi = 10.1021/ar980073b| volume = 33| journal = Accounts of Chemical Research | first1 = R.| last2 = Tokitoh | first2 = N.| title = Heavy ketones, the heavier element congeners of a ketone| last1 = Okazaki}} Diselenobenzoquinone is stable as a metal complex.{{cite journal | last1 = Amouri | first1 = H. | last2 = Moussa | first2 = J. | last3 = Renfrew | first3 = A. K. | last4 = Dyson | first4 = P. J. | last5 = Rager | first5 = M. N. | last6 = Chamoreau | first6 = L.-M. | year = 2010 | title = Discovery, Structure, and Anticancer Activity of an Iridium Complex of Diselenobenzoquinone | journal = Angewandte Chemie International Edition | volume = 49 | issue = 41| pages = 7530–7533 | doi = 10.1002/anie.201002532 | pmid=20602399}} Selenourea is an example of a stable compound containing a (formal) C=Se bond.
• Thioselenides ({{chem2|R\sSe\sS\sR}}), compounds with selenium(II)–sulfur(II) bonds, analogous to disulfides. Likewise a selenamide has form R–Se–NR₂.{{cite journal|doi=10.1021/ar50133a004|title=Functional group manipulation using organoselenium reagents|first=Hans J.|last=Reich|author-link=Hans Reich|orig-date=13 Mar 1978|year=1979|publisher=American Chemical Society|p=23|volume=12|journal=Accounts of Chemical Research}}

{{Main|Selenium in biology}}

Selenium, in the form of organoselenium compounds, is an essential micronutrient whose absence from the diet causes cardiac muscle and skeletal dysfunction. Organoselenium compounds are required for cellular defense against oxidative damage and for the correct functioning of the immune system. They may also play a role in prevention of premature aging and cancer. The source of Se used in biosynthesis is selenophosphate.

Glutathione oxidase is an enzyme with a selenol at its active site. Organoselenium compounds have been found in higher plants. For example, upon analysis of garlic using the technique of high-performance liquid chromatography combined with inductively coupled plasma mass spectrometry (HPLC-ICP-MS), it was found that γ-glutamyl-Se-methylselenocysteine was the major Se-containing component, along with lesser amounts of Se-methylselenocysteine. Trace quantities of dimethyl selenide and allyl methyl selenide are found in human breath after consuming raw garlic.{{cite book|author=Block, E.|title=Garlic and Other Alliums: The Lore and the Science|url=https://books.google.com/books?id=6AB89RHV9ucC|publisher=Royal Society of Chemistry|year=2010|isbn=978-0-85404-190-9}}

Selenocysteine, called the twenty-first amino acid, is essential for ribosome-directed protein synthesis in some organisms.{{cite journal|title=Catalytic properties of an Escherichia coli formate dehydrogenase mutant in which sulfur replaces selenium|year=1991|last1=Axley|first1=M.J.|last2=Böck|first2=A.|last3=Stadtman|first3=T.C.|journal=Proc. Natl. Acad. Sci. U.S.A.|volume=88|pages=8450–8454|doi=10.1073/pnas.88.19.8450|pmid=1924303|issue=19|pmc=52526|bibcode = 1991PNAS...88.8450A |doi-access=free}} More than 25 selenium-containing proteins (selenoproteins) are now known.{{cite journal|title=From selenium to selenoproteins: synthesis, identity, and their role in human health|year=2007|last1=Papp|first1 =L.V.|last2=Lu|first2=J.|last3=Holmgren|first3=A.|last4=Khanna|first4=K.K.|journal=Antioxidants & Redox Signaling|volume=9|pages=775–806|doi=10.1089/ars.2007.1528|issue=7|pmid=17508906}} Most selenium-dependent enzymes contain selenocysteine, which is related to cysteine analog but with selenium replacing sulfur. This amino acid is encoded in a special manner by DNA. Selenosulfides are also proposed as biochemical intermediates.

Selenomethionine is a selenide-containing amino acid that also occurs naturally, but is generated by post-transcriptional modification.

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

• the weakness of the {{chem2|C\sSe}} bond and
• the easy oxidation of divalent selenium compounds.

Contrary to theoretical productions, selenium stablizes geminal carbanions slightly less than the corresponding sulfur compounds. Moreover, selenium is so nucleophilic that alkyl halides preferentially alkylate the selenium in many selenoether anions, before the halide collapses the resulting ylide in a nucleophilic substitution. Nevertheless, propargylic selenoether anions alkylate without deselenation, and then oxidize to α-selenoenones.{{sfn|Reich|1979|pp=26-27,29}}

Vinylic selenides are organoselenium compounds that play a role in organic synthesis, especially in the development of convenient stereoselective routes to functionalized alkenes.{{cite journal|doi=10.1055/s-1997-1210|title=Vinylic Selenides and Tellurides - Preparation, Reactivity and Synthetic Applications|year=1997|last1=Comasseto|first1=João Valdir|last2=Ling|first2=Lo Wai|last3=Petragnani|first3=Nicola|last4=Stefani|first4=Helio Alexandre|journal=Synthesis|volume=1997|pages=373|issue=4|s2cid=260336441}} 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.{{cite journal|doi=10.1016/0022-328X(83)80118-1|title=Vinylic selenides|year=1983|last1=Comasseto|first1=J|journal=Journal of Organometallic Chemistry|volume=253|pages=131–181|issue=2}}{{cite journal|doi=10.1021/ol0498904|title=Hydroselenation of Alkynes by Lithium Butylselenolate: an Approach in the Synthesis of Vinylic Selenides|pmid=15040741|year=2004|last1=Zeni|first1=Gilson|last2=Stracke|first2=Marcelo P.|last3=Nogueira|first3=Cristina W.|last4=Braga|first4=Antonio L.|last5=Menezes|first5=Paulo H.|last6=Stefani|first6=Helio A.|author-link4=Antonio Luiz Braga|journal=Organic Letters|volume=6|issue=7|pages=1135–8}}{{cite journal|doi=10.1016/S0040-4020(01)00337-4|title=Synthesis of (Z)-1-phenylseleno-1,4-diorganyl-1-buten-3-ynes: hydroselenation of symmetrical and unsymmetrical 1,4-diorganyl-1,3-butadiynes|year=2001|last1=Dabdoub|first1=M|journal=Tetrahedron|volume=57|pages=4271–4276|issue=20}}{{cite journal|doi=10.1016/S0040-4039(02)01904-4|title=Indium(I) iodide-mediated chemio-, regio-, and stereoselective hydroselenation of 2-alkyn-1-ol derivatives|year=2002|last1=Doregobarros|first1=O|last2=Lang|first2=E|last3=Deoliveira|first3=C|last4=Peppe|first4=C|last5=Zeni|first5=G|journal=Tetrahedron Letters|volume=43|pages=7921|issue=44}} 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.{{cite journal|doi=10.1016/S0022-328X(00)85812-X|title=Stereoselective synthesis of vinylic selenides|year=1981|last1=Comasseto|first1=J|journal=Journal of Organometallic Chemistry|volume=216|pages=287–294|issue=3}} 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 Markovnikov 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.

{{main|Riley oxidation}}

Selenium dioxide is useful in organic oxidation. Specifically, {{chem2|SeO2}} will convert an allylic methylene group into the corresponding alcohol. A number of other reagents bring about this reaction.

File:Seleniumdioxide oxidation.svg

In terms of reaction mechanism, {{chem2|SeO2}} and the allylic substrate react via pericyclic process beginning with an ene reaction that activates the {{chem2|C\sH}} 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.

{{chem2|SeO2}}-based oxidations sometimes afford carbonyl compounds such as ketones,Organic Syntheses Coll. Vol. 9, p. 396 (1998); Vol. 71, p. 181 (1993) [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv9p0396 Online article] {{Webarchive|url=https://web.archive.org/web/20051024020101/http://www.orgsyn.org/orgsyn/prep.asp?prep=cv9p0396 |date=2005-10-24}} β-PineneOrganic Syntheses Coll. Vol. 6, p. 946 (1988); Vol. 56, p. 25 (1977). [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0946 Online article] {{Webarchive|url=https://web.archive.org/web/20051101031113/http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0946 |date=2005-11-01}} and cyclohexanone oxidation to 1,2-cyclohexanedione.Organic Syntheses, Coll. Vol. 4, p. 229 (1963); Vol. 32, p. 35 (1952). [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv4p0229 Online article] {{Webarchive|url=https://web.archive.org/web/20051127035818/http://www.orgsyn.org/orgsyn/prep.asp?prep=cv4p0229 |date=2005-11-27}} Oxidation of ketones having α-methylene groups affords diketones. This type of oxidation with selenium oxide is called Riley oxidation.{{cite journal|doi=10.1039/JR9320001875|title=255. Selenium dioxide, a new oxidising agent. Part I. Its reaction with aldehydes and ketones|year=1932|last1=Riley|first1=Harry Lister|last2=Morley|first2=John Frederick|last3=Friend|first3=Norman Alfred Child|journal=Journal of the Chemical Society (Resumed)|pages=1875}}

{{main|Selenoxide elimination}}

In presence of a β-hydrogen, a selenide will give an elimination reaction after oxidation, to leave behind an alkene and a SeO-selenoperoxol. The SeO-selenoperoxol is highly reactive and is not isolated as such. 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.Organic Syntheses Coll. Vol. 6, p. 23 (1988); Vol. 59, p. 58 (1979) [http://www.orgsynth.org/orgsyn/prep.asp?prep=cv6p0023 Online Article]

File:SelenoxideElimination.png

The Grieco elimination is a similar selenoxide elimination using o-nitrophenylselenocyanate and tributylphosphine to cause the elimination of the elements of {{chem2|H2O}}.

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