Phenylselenyl chloride oxidant

Phenylselenyl chloride adds to alkenes in a stereoselective trans manner950. The adduct is oxidized by chlorine and the seleno moiety is displaced by chloride with inversion of the configuration to yield the cis dichloro adduct (equation 142)951. 

Phenylselenyl chloride, C HjSeCI, and phenylselenyl bromide, C Hs eBr, in connection with oxidizing agents such as hydrogen peroxide or sodium periodate, are used for the conversion of aldehydes, ketones, and esters into their a,p-unsaturated analogues. The key intermediate is alkyl phenyl selenoxide, which decomposes via a five-membered transition state [167] (equation 27). 

The phenylselenyl chloride rram-addnct from addition to alkenes can be oxidized and the seleno moiety displaced by chloride to give the cw-dichlorides in high yield137. 

There are similar procedures involving organic selenides. Reaction of ketones and aldehydes with phenylselenyl chloride occurs readily to give the a-seleno derivative, presumably by a mechanism similar to a-halogenation. Alternatively, ketones and esters can be converted to the a-seleno derivatives by reaction of the enolates with phenylselenyl chloride. The a-seleno compounds serve as precursors to a,jS-unsaturated carbonyl compounds because oxidation with hydrogen peroxide leads to spontaneous elimination of PhSeOH. 

Among the applications of allyl silanes reported this year their reaction with phenylselenyl chloride and subsequent oxidation of the resulting allyl selenides to allylic alcoholshas been used in the sequence outlined in Scheme IS. 3-Bromoallyltrimethylsilane therefore behaves as a hydroxypropenyl synthon. Nitroalkenes react with allylsilanes to give, after hydrolysis, y,S-unsaturated ketones, and a synthesis of substituted cyclopentenones based on this procedure has been described.Allyl trimethylsilyl ethers undergo a palladium-promoted coupling with aryl iodides to give /3-aryl-a,/3-unsaturated ketones, and palladium also catalyses the reaction of various aryl bromides with trimethylsilyl acetylene to produce ethynylated aromatics. 

Sequential addition of methyl-lithium and phenylselenyl chloride to the double bond in (192) results in the selenyl derivative (193), which when treated with bromine and hydrogen peroxide yields (+)-Prelog-Djerassi lactonic acid (194). Similarly, addition of a methyl group to (195), by LiMezCu, and oxidative elimination of the phenylselenyl group provides a useful route to dihydrojasmone and c(5-jasmone as shown in Scheme 35. ... 

The transformation of saturated ketones into a,/8-unsaturated ketones is often carried out by the introduction of an a-seleno-substituent, followed by oxidation and elimination. An alternative method for selenation of ketones (and esters) has been described wherein the lithium enolate reacts with selenium metal (rather than a selenyl halide) followed by methyl iodide (Scheme 29). Yields are comparable to the classical method. Phenylselenyl chloride reacts with enamines derived from aldehydes to give a-phenylselenoaldehydes, and hence o(,/3-unsaturated aldehydes by oxidative elimination [equation (21)]. ... 

From enolates via selenoxidesA Lithium enolates derived from ketones, lactones, and esters react with PhSe-SePh or with phenylselenyl bromide or chloride (PhSeX) to form a-(phenylseleno)carbonyl compounds. These can be oxidized to the corresponding selenoxides with subsequent 5yn-elimination of benzeneselenic acid to form enones. 

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