Oxidation to selenoxides

Selenides (R2Se) can be oxidized to selenoxides and selenones. It is possible to oxidize a thioether to a sulfoxide in the presence of an alcohol moiety using MnOa/HCl. ... 

Selenides are oxidized to selenoxides that normally suffer an in situ elimination.111 Amines are destroyed,112 although its protection as amides or carbamates prevents the reaction with Collins reagent. Lactols are very quickly oxidized to lactones,113 unless a very great steric hindrance is present.114 Tertiary lactols suffer oxidation via its opened hydroxyketone form.115 The oxidation of tertiary lactols may be slow, so that an alcohol can be selectively oxidized. 

This elimination takes place more easily still when sulfur is replaced by a selenium—PhSe groups can be introduced by the same method, and oxidized to selenoxides with m-CPBA at low temperature. The selenoxides are rarely isolated, because the elimination takes place rapidly at room... 

Aminal 309 was oxidized to selenoxide, and then heated in refluxing toluene with DBU to give the protected 9-substituted azoninone 310 in 75% yield as a result of Claisen rearrangement of the vinyl-substituted intermediate (Equation 44) <1996J(P1)123>. 

Mono- and 1,2-di-substituted alkenes react with PhSeQ/Hg(SCN)2 in benzene (0.5-96 h, at 20 C), giving -tra/if-phenylselenoalkyl isothiocyanates in 70-94% yields.2 Terminal alkenes generally give the product with the selenium terminal (an exception is the prc uct from Bu CH H2) internal alkenes show the expected stereochemistry (cis to threo, trans to erythro). Oxidation to selenoxides could be achieved cleanly only with ozone, and the products cis eliminate in the usual manner to give predominantly the vinylic isothiocyanates (Scheme 73). 

Selenides are also nucleophilic and produce isolable selenonium salts (9) when treated with alkyl halides. They are easily oxidized to selenoxides (10) and further to selenones (11) under more forcing conditions (see Section 4). Reduction of selenides to the corresponding hydrocarbons is most conveniently achieved with nickel boride,or with tri-n-butyl- or triphenyltin hydride under radical conditions. " Other reagents for reductive deselenization include Raney nickel, lithium triethylborohydride, and lithium in ethylamine (Scheme 4). Benzylic selenides undergo radical extrusion reactions under thermal or photolytic conditions to produce... 

When the concentration of hydroperoxide is high, ebselen 3a is oxidized to selenoxide 11a which reacts with one molecule of the thiol to give selenoxysulfide 36. Intermediate 36 and a second molecule of thiol produce disulfide, while the selenenic acid 37 formed is converted back to ebselen. In biological systems, where the concentration of hydroperoxide is low, ebselen and the thiol give selenosulfide 38 which disproportionates into the disulfide and diselenide 39, subsequently oxidized to selenenic anhydride 40 and finally converted into ebselen. 

Like sulfides, selenides can be oxidized to the corresponding oxides, selenoxides, or to selenones. Oxidation to selenoxides is much faster and, with some oxidants, final. Reaction conditions are specified in equation 592 for the oxidation of methyl phenyl selenide [325, 711, 773, 1034]. 

C.vi. Selenoxide Pyrolysis.218 just as sulfides are oxidized to sulfoxides, selenides (R—Se—R) can be oxidized to selenoxides. It is reasonable to assume, therefore, that replacing the sulfoxide with a selenoxide will also lead to thermal syn-elimination to the less substituted alkene. The increased polarity of the Se-O bond of the selenoxide, relative to the S-0 bond of the sulfoxide, and the loss of the unstable R—Se—OH allows even lower temperatures for thermal syn-elimination (typically 0-25 °C). Elimination of PhSeOH from 245 is clearly analogous to the sulfoxide pyrolysis of 243, and the exocyclic-methylene derivative (246) rather than... 

Alkenes may be converted directly into allylic alcohols and ethers by electrolysis in the presence of diphenyldiselenide, water or an alcohol, and a trace amount of tetraethylammonium bromide (Scheme 11). It is assumed that Br, generated electrochemically, reacts with the diselenide to produce PhSeBr which can add electrophilically to the alkene solvolysis, electrochemical oxidation to selenoxide, and in situ elimination would complete the sequence. In accord with this rationale, the products formed are those arising from Markovnikov addition to the alkene. 

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