Selenides, alkyl vinyl

Depending on the nature of the R groups of the alkynyl selenides 99, vinylic species with different regio- and stereochemistry are obtained (Scheme 62). When R is an alkyl group, mixture of products of different regiochemistry is obtained,179 which makes this method unsuitable for the preparation of this class of compounds. A solution for this problem consists in the use of lithium alkynyl selenolates 102 as precursors of the vinylzirconium species 103, as shown in Scheme 63.1... 

Perveev et al.3S found that they could obtain alkyl, vinyl, and hydroxyalkyl selenophenes by the reaction of acetylenic a-epoxides, vinylacetylenic-a-epoxides, or hydroxy-a-epoxides with hydrogen selenide in the presence of barium hydroxide at 20°C. The yield of selenophene (30-80%) depends on the structure of the initial epoxide and decreases with the increase in the chain length or branching. 

Reich et al. observed that the LDA treatment of an allyl selenide and subsequent alkylation with benzyl bromide afforded an isomeric mixture of allylic and vinylic selenides. The vinylic selenide, corresponding to the y-attack of the allylic carbanion, was the minor product [5] (Scheme 3). After addition of l-bromo-3-phenylpropane, instead of benzyl bromide, the allylic selenide was isolated in only 68% yield [6]. 

Transformation of alkylated vinyl sulfides and selenides to carbonyl compounds... 

M-1 s-1 alkyl phenyl selenides, aryl bromides, vinyl bromides, a-chloro esters, a-thiophenyl esters 104-102M 1 s-1 alkyl chlorides, alkyl phenyl sulfides, a-chloro and a-thiophenyl ethers. 

Trialkylboranes were converted into alkyl phenyl selenides and alkyl phenyl tellurides by reactions with PhSeSePh and PhTeTePh in the presence of stoichiometric amounts of air.550 Vinyl selenides and tellurides were synthesized by treating vinylboronic acids or esters with phenylselenyl chloride in ionic liquids (Equation (114))551 or by palladium-catalyzed coupling reaction of diorgano ditellurides (Equation (115)).552... 

Vinyl selenides have been lithiated at the a-position by LDA983,984 at —78 °C in THF to give a-(arylselanyl)vinyllithiums 680, a-(methylselanyl)vinyllithiums 681 being obtained by selenium-lithium transmetallation from l,l-bis(methylselanyl)alkenes with n-BuLi in THF or t-BuLi in ether at —78 °C985 986. These intermediates reacted with alkyl halides, epoxides, carbonyl compounds and DMF985, the final deprotection being performed by mercury(II) salts986. 

The synthetic utility of this base (1) was demonstrated in the preparation of vinyl iodides in high yields from simple ketohydrazones and iodine (Table), a process that normally gives mixtures of vinyl iodides and geminal diiodides if less hindered bases are employed.6 This base has also been used in the elimination of sulfonic acids from the corresponding sulfonates, the alkylation of compounds containing active methylene groups, the conversion of hydrazones to vinyl selenides, and the preparation of esters from sterically hindered acids.4 5... 

The first example of the use of the catalytic one-pot procedure described above, in which the deselenenylation occurs with substitution, is represented by the conversion of vinyl halides into a-alkoxy acetals [116]. This is illustrated in Scheme 38 in the case of -bromostyrene 235. The regioselective methoxy-selenenylation affords the a-bromo selenide 236, which undergoes a rapid solvolysis, through a selenium-stabilized carbocation to produce the selenide 237. Oxidation of this alkyl phenyl selenide with ammonium persulfate produces an oxygen stabilized carbocation, which affords the final product 238, and, at the same time, regenerates the selenenylating agent. 

The a-deprotonation of vinylic aryl selenides is strongly dependent on the substituents [4]. For phenyl vinyl selenide, the reaction was successfully carried out with LDA or KDA at -78°C in THF (Scheme 50, reaction 1). When a alkyl group is present, the use of LDA in THF leads to the abstraction of both vinyl and allyl protons. The a-deprotonation was improved using KDA in THF at -78°C (Scheme 50, reaction 2). With two )9,)9 -alkyl groups, the metalation occurred only at an allylic position (Scheme 50, reaction 3). 

Dabdoud et al. have shown that the hydrozirconation of alkyn-l-ylbutyl selenides was complete with two equivalents of Cp2Zr(H)Cl. The nature of the products, formed after reaction with BuTeBr, is dependent on the substituent R [78] (Scheme 56, reaction 1). With R=Ph, the ketene butylselanyl(butyl-telluranyl)acetal was obtained exclusively. In the other cases (R = alkyl, MeOCH2), small amounts of (Rj-(2-butyltelluranyl)vinyl butyl selenides were also produced. The hydrozirconation of substituted acetylenic butyl selenides could be considered as a convenient approach for the synthesis of... 

The effects of aryl groups were kinetically analyzed by comparing the rate constants of both steps (ki for oxidation step and /C2 for elimination step) which were determined by NMR analysis of the concentration of vinyl selenides, the intermediate selenoxide, and allenic sulfones [16b]. This kinetic study indicates that the rates of both oxidation and elimination steps were accelerated by the introduction of an electron-withdrawing group. Such acceleration has been known in the overall selenoxide elimination as well as in the selenoxide elimination step of alkyl aryl selenides. As a result, it was disclosed that the ratio of these rate constants (/C1//C2) was closely related to the enantiomeric excess of the products the smaller the ratio, the larger the enantiomeric excess becomes. Thus, the introduction of o-nitrophenyl group as an aryl moiety, which suppresses sterically the racemization of the intermediate chiral selenoxide and accelerates the selenoxide elimination step, is necessary to achieve a higher asymmetric induction. 

Increasing the substitution of the alkyl group on the 3-carbon of 1-alkenyl phenyl selenides again favors metaliation at the vinylic site (Scheme 48). > KDA in THF has proved to be, without contest, the most successful reagent for this purpose. The reaction takes place at low temperature (-78 C) and the 1-phenylselenoalkenyl metal, produced in almost quantitative yield, retains its stereochemistry (Scheme 48, b-d). 

The reaction involving the alkylation of p-hydroxyalkyl selenides to give p-hydroxyalkylselenonium salts which are then cyclized with a base is by far the most general. It allows Ae synthesis of a large variety of epoxides such as tenninal, a,a- and a,p-disubstituted, tri-33-> and tetra-substituted, 3,i88 as oxaspiro[2.0.n]-hexanes, -heptanes and -octanes (Scheme 161, g Scheme 162, d Scheme 164, d Scheme 165, b) - and vinyl oxiranes (Schemes 166 and 181)33 -239 from both p-hydroxy-alkyl methyl33- 3 3 22>.222j36,263 phenyl selenides. ... 

Vinyl sulfides and selenides can be alkylated efficiently at the a-position in a two step-one pot reaction which involves their a-metallation and further reaction of the resulting anion with alkyl halides. 

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