Selenium atoms, reactions

In some reactions intramolecular chalcogen nitrogen interactions may lead to stereochemical control. For example, selenenyl bromides react with C=C double bonds, providing a convenient method of introducing various functional groups. The reaction proceeds readily, but affords a racemic mixture. The modified reagent 15.22 contains a chiral amine in close interaction with the selenium atom. It reacts with olefins affording up to 97% ee of isomer A (Scheme 15.2). ... 

Selenoaldehydes 104, like thioaldehydes, have also been generated in situ from acetals and then directly trapped with dienes, thus offering a useful one-pot procedure for preparing cyclic seleno-compounds [103,104], The construction of a carbon-selenium double bond was achieved by reacting acetal derivatives with dimethylaluminum selenide (Equation 2.30). Cycloadditions of seleno aldehydes occur even at 0 °C. In these reactions, however, the carbon-selenium bond formed by the nucleophilic attack of the electronegative selenium atom in 105 to the aluminum-coordinated acetal carbon, may require a high reaction temperature [103], The cycloaddition with cyclopentadiene preferentially gave the kinetically favorable endo isomer. 

Substitutional ring openings seem to be more unusual (Eqs. 22-24, Scheme 5). The reactions of 2,5-dimethoxyselenophene with butyl- and phenyllithium have been examined.85 In view of the formation of dibutyl-and diphenylselenide, respectively, together with some dienes, a logical reaction sequence could be formulated in which the first step is an attack on the selenium atom by the lithium reagent. Nonfused selenophenes are also cleaved in a substitution-type reaction. 

The reaction of dabco (l,4-diazobicyclo[2.2.2]octane) with Me2Cd yields a 1 1 adduct 197, which adopts a linear polymeric structure (Figure 34).255 The cadmium atom is coordinated by two dabco units and two methyl carbon atoms giving rise to a distorted tetrahedral environment. Finally, the organocadmium adduct 198 (Figure 35) has been isolated from the reaction of Me2Cd with Cd[(SeP-/-Pr2)2N]2.256 The solid-state structure consists of dimeric units where each methylcadmium unit is coordinated to three selenium atoms. The geometry about the cadmium center is tetrahedral with a Cd-C distance of 2.16 A, which is comparable to that observed in other cadmium alkyl complexes. 

Reports regarding selenoamides and selenoureas are less than the corresponding thioamides, thioureas, amides and ureas because of instability of compounds including selenium atom and their difficulty in the preparation. Recently, preparation methods to overcome some difficulties have been developed. Their reactions, preparation, application to heterocycles or biological assay systems have been actively investigated. 

Atoms of S and Se can sufficiently structurally influence fragments of CH3 that are frequently located on the ends of hydrocarbon chains or in the form of free radicals. The data given confirm high reactivity of sulfur and selenium atoms as retardants of chain reactions of free radicals as elements drawing back impaired valence electrons of free radicals, but at the same time preserving the basic structure of hydrocarbon chain. 

Fig. 4. HPLC-Chromatogram of the selenium sulfide mixture obtained from the reaction of SClj and ScjCh with potassium iodide. The figures give the numbers of selenium atoms in the molecule...

A mechanism involving nucleophilic displacement of the hydroxyl group on C-2 by the selenium atom at C-5 has been proposed for the formation of the acetal 73. This mechanism does not, however, account for the presence of two products, epimeric at C-2, in the reaction mixture. 

The absence of an EPR signal in solution or in the solid state is indicative of a singlet ground state for the diradical ( BuBP Pr2)2- An indication of the radical character of this derivative is provided by a variety of facile oxidative addition reactions (Scheme 9.12). " For example, the treatment of ( BuBP Pr2)2 with diphenyl diselenide (or elemental selenium) produces a bicyclic compound in which a selenium atom bridges the two boron atoms. Trimethyl tin hydride reacts rapidly with ( BuBP Pr2)2 to give the trans adduct. Finally, ( BuBP Pr2)2 is slowly oxidised by deuterated chloroform to produce a R,R -dichloro adduct as a mixture of cis and trans isomers. 

The fact that selenides with the selenium atom attached to the more highly substituted carbon of the double bond were identified among the products of the oxidation of bis (l-methyl-2-acetoxypropyl) selenide in the presence of 2-methyl-2-butene suggested that the reaction might proceed partially by the alternate mechanism shown opposite. 

The reaction of tetrachlorodisilane with chalcogenation reagents is one of the synthetic methods to bis-nor-adamantane or double-decker type compounds. When a solution of di-terf-butyltetrachlorodisilane 66 in THF is refluxed together with one equivalent of lithium sulfide or stirred with lithium selenide at room temperature, the tetra(ferf-butylsilicon)pentachalcogenides 67 and 68, respectively, are formed (Scheme 18)6. Each pentachalcogenide exhibits one resonance of a ferf-butyl group in the H and 13C NMR spectra. Most likely, the bis-nor-adamantane derivatives 69 and 70 are initially formed. Insertion of a sulfur or selenium atom into one of the two strained Si—Si bonds would then lead to the observed products. 

The aspects of thermal and photochemical decomposition reactions of 1,2,3-selenadiazoles with nitrogen and selenium atoms have been described in CHEC(1984) and CHEG-II(1996) <1984CHEC(6)333, 1996CHEC-II(4)743>. The ready thermal decomposition reactions of 1,2,3-selenadiazoles have been continuously utilized. 

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