Selenoxides, alkyl

Sulfonesand sulfonic esters can also be alkylated in the a position if strong enough bases are used. Alkylation at the a position of selenoxides allows the formation of alkenes, since selenoxides easily undergo elimination (17-11). 

In another elegant approach (Scheme 18), a synthesis of 5-alkenyl-substituted 1,2,4-oxadiazoles relies upon a selenoxide. -elimination at the 5-a-carbon of the selenium resin-supported 1,2,4-oxadiazole 152. Access to compound 152 was achieved in two steps from the supported oxadiazole 150, which underwent deprotonation and alkylation at the 5-a-carbon to give the a-alkylated selenium resin 151. 1,3-Dipolar cycloaddition then gave the selenium resin-supported 1,2,4-oxadiazole 152 <2005JC0726>. 

Selenoxides telluroxides. Diaryl or alkyl aryl sclcnidcs and diaryl or dialkyl tclluridcs arc oxidized to oxides in 40 95 yield by treatment with r-hutyl... 

The selenoxide elimination of primary alkyl 2-pyridyl selenides to give terminal alkcnes also proceeds in higher yield than the corresponding reaction with alkyl phenyl selenides.2 Example ... 

Oxidation of alkyl iodides, bearing electron-withdrawing groups such as car-bomethoxy and sulfonyl at the a-carbon, with m-chloroperbenzoic acid results in clean elimination to give olefins [Eq. (27)]. This reaction involves reductive / -elimination of the intermediate iodosylalkanes, as observed in thermal peri-cyclic -elimination of sulfoxides and selenoxides. Exclusive syn stereochemistry in the reductive /1-elimination was established by deuterium labeling... 

Chiral sulfoxides or selenoxides.1 This oxaziridine (1) is generally more effective than the modified Sharpless reagent of Kagan (13, 52) for enantioselective oxidation of alkyl aryl sulfides or selenides to the corresponding sulfoxides or selenoxides. The polar Cl groups of 1 improve both rate and the enantioselectivity. 

Propargyl phenyl selenide is a versatile multifunctional acrylate synthon, as shown in Scheme 12. The (Uanion is prepared and reacted successively with an alkylating agent (R— X) and an electrophile (E ). The oxidative rearrangement of the propargylic selenoxide (35) to an allenic selenenate (36), and thence to the a-phenylselenoenone (37), forms the keystone of this synthetic method, and ovendl yields firom propargyl phenyl selenide are in the range of 38-68%. Further elaboration of (37) is possible... 

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... 

Until quite recently the isolation of optically active selenoxides has been limited to those contained in steroids (isolated as diastereoisomers). The difficulty in obtaining these compounds was attributed to the racemization through the achiral hydrated intermediates. Simple optically active selenoxides (5-11% ee) were first prepared by kinetic resolution. Direct oxidation of selenides to selenoxides was first reported using optically active oxaziridine derivatives under anhydrous conditions, but the extent of the asymmetric induction was somewhat unsatisfactory with methyl phenyl selenide as substrate (8-9% ee). Recently much improved enantiomeric excesses (45-73%) were achieved with new oxaziridine reagents such as (70). An attempt at the asymmetric oxidation of more bulky selenides was independently carried out using Bu OCl in the presence of (-)-2-octanol (equation 55),2 but resulted in unsatisfactory enantioselectivities (ee 1%). Much better results were obtained by the oxidation of P-oxyalkyl aryl selenides (ee 18-40% equation 56) and alkyl aryl selenides (ee 1-28%) 2S using TBHP in the presence of (+)- or (-)-diisopropyl tartarate (DIPT) and titanium(IV) alkoxide. 

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