Nucleophilic addition reactions selenium nucleophiles

Well before the wide use of organoselenium compounds in chemistry, it was discovered that electrophilic selenium compounds of the type RSeX add stereospecifically to alkenes.45 Since that time this reaction has been an important tool in the portfolio of organic chemists and has been used even for the construction of complex molecules. Comprehensive reviews on this chemistry have appeared46-49 and in recent times the synthesis of chiral selenium electrophiles and their application in asymmetric synthesis has emerged. As shown in Scheme 1, the addition reactions of selenium electrophiles to alkenes are stereospecific anti additions. They involve the initial formation of seleniranium ion intermediates 1 which are immediately opened in the presence of nucleophiles. External nucleophiles lead to the formation of addition products 2. The addition to unsymmetrically substituted alkenes follows the thermodynamically favored Markovnikov orientation. The seleniranium ion intermediates of alkenes with internal nucleophiles such as 3 will be attacked intramolecularly to yield cyclic products 4 and 5 via either an endo or an exo pathway. Depending on the reaction conditions, the formation of the seleniranium ions can be reversible. 

There are various ways to generate and synthesize selenium electrophiles and some of these compounds are commercially available. The addition reaction can also be dependent on the counterion X of these reagents and several protocols have been developed to exchange the counterions. The most commonly used electrophile is the phenylselenyl electrophile and compounds like phenylselenenyl chloride 6 and phenylselenenyl bromide 7 are commercially available. They can also be easily generated from diphenyl diselenide 8 by treatment with sulfuryl chloride or elementary chlorine or bromine, respectively. Diselenides in general are very versatile precursors for selenium electrophiles. For addition reactions using external nucleophiles the use of selenenyl halides can lead to complications, because chloride or bromide ions can also act as nucleophiles and lead to undesired side-products. An... 

Additions of selenium electrophiles to double bonds have most frequently been used as part of a synthetic sequence, because the addition products are very versatile building blocks in synthesis. They can undergo a variety of subsequent transformations and can, therefore, serve as precursors for the generation of radicals 27 in radical reactions. Using a selenoxide elimination, new double bonds as shown in 28 can be generated. After oxidation of the selenide to the seleneone the selenium moiety can be replaced by a second nucleophile to generate compounds of type 29 (Scheme 2). 

But due to the importance of nitrogen functional groups, the addition reactions of selenium electrophiles to alkenes using nitrogen nucleophiles represent another synthetically relevant process. Nitriles have been used as versatile... 

Due to the important role of nitrogen functional groups, the addition reactions of an electrophilic selenium reagent and a nitrogen nucleophile to a carbon-carbon double bond represent a synthetically relevant process with potential practical applications. Among the reactions of this type which have been described already, perhaps the most important contribution is represented by the Ritter-type amide synthesis described by Toshimitsu and Uemura [48a, 48b]. The addition of a phenylselenyl and of an acylamino group to a mono or a 1,2-disubstituted olefin was accomplished by treating the olefin with PhSeCl in acetonitrile and water in the presence of trifluoromethanesulfonic acid. 

By further evaluation of the redox system SeBr4/SeBr2/Se/Br in aprotic solvents (Section III,A), the first examples of bromoselenates-(II,IV) containing selenium in both oxidation states +2 and +4 were prepared. Three dinuclear, trinuclear, and tetranuclear types of anions are reported in this novel class of mixed-valence compounds Se2Brg, SeaBrio , and Se4Bri2 . They are shown in Fig. 31. Similar to the bromoselenates(II), they were obtained by nucleophilic addition reactions of bromide ions within the complex reaction mixtures of selenium bromides. Evidently, the formation of Bra" is important for the stabilization of Se(II) in addition to Se(IV), shifting the equilibrium... 

The addition of selenium reagents to double bonds has already been discussed above. However, a special case of such additions has received particular attention in the past five years this is in the cyclization of olefins. In this context phenyl-selenyl iodide is described as a new rea nt for the cyclization of dienes to carbo-cyclic compounds. The authors claim that phenylselenyl iodide is far more effective than the corresponding bromide or chloride for this transformation. The intramolecular attack of the carbon nucleophile in (287) onto the double bond in the cyclopentene ring is another example of a cyclization reaction and forms the basis for a synthesis of ( )-hirsutene, In this case, the tricycle (288)... 

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