Alkyl diselenides

For the determination of the value for each compound, defined as the ratio of response given by the electron capture detector to that given by the hydrogen flame ionization detector ( ( ) + EC/FI), peak areas were measured with the disc chart integrator. Injections of pure alkyl monoselenides, and solutions of alkyl diselenides and ethylselenocyanate in cyclohexane were used for determinations with the electron capture detector. 

In conclusion, while 6-alkyl-2-selenouracil compounds (RSeU) are stable in various solvents, including water and other polar or non-polar solvents, spoke c.t. complexes of formnlae [(RSeU)IJ are formed in dichloromethane solutions, but are unstable in methanol/acetonitrile and/or acetone solntions. [(RSeU)IJ is transformed to 6-alkyl-2-nracil in methanolic/acetonitrile solntions. Upon re-crystallization in acetone the diselenides are formed possibly throngh the formation of a substituted selenouracil as indicated by H, NMR spectra and ESI-MS spectra. The whole process may be hydrolytic (Scheme 13.5). 

AUcylSe and alkylTe groups can be introduced by reaction with diselenides and ditellurides, but the desired products can be prepared in a more practical and economical way, by successive addition of the elements and the alkyl halide. The insertion of the elements... 

The diselenides, RSe.SeR, are prepared by the interaction of potassium diselenide and dialkyl sulphates or alkyl halides ... 

If the diselenide acid is treated with phosphorus pentachloride the free acid chloride is not isolated, but the compound (HCl.Se.C6H4.COCl)2. The normal acid chloride, however, is formed by using thionvl chloride it crystallises from benzene or toluene in needles, M.pt. 178° to 174° C. If the hydrochloride, M.pt. 65c to 66° C., be boiled with methyl alcohol, a compound is obtained, M.pt. 74° to 75° C., which is the hydrochloride of the methyl ester of the diselenide acid, (Se.C6H4.COMe)2, which may be liberated by dilute sodium hydroxide. The free ester melts at 143° to 144° C. The corresponding ethyl ester melts at 129° to 130° C. and its hydrochloride at 91° to 92c C. Both esters may be formed also from the silver salt of the acid by heating with the alkyl iodide for two to three hours at 1X0° to 1203 C. 

Cyclization of olefinic Se,0-heteroacetals 133 in the presence of titanium tetrachloride affords 4-chloroselenanes 134 in good yields (Scheme 13) <1994JOC1011>. The reaction proceeds through the formation of an a-selenocarbocation which reacts with the olefinic bond followed by chloride addition. The required precursors for the reaction can be readily prepared by reduction of the corresponding diselenides followed by alkylation. 

AlkylSe and alkylTe groups can be introduced by reaction with diselenides and ditellurides, but the desired products can be prepared in a more practical and economical way, by successive addition of the elements and the alkyl halide. The insertion of the elements proceeds most easily in liquid ammonia, provided that grey Te powder and red Se powder is used (black Te powder, obtained by precipitation is unreactive, probably because of the presence of an oxide coating, while the black modification of Se,"selenium nigrum" is also less reactive). In Et20 or THF, temperatures in the range 0-20 C are necessary for the dissolution of the elements. 

The same photolytic treatment of O-acyl esters (2) in the presence of disulfides, diselenides, and ditellurides effectively produces the corresponding alkyl sulfides, alkyl selenides, and alkyl tellurides respectively, through SHi reaction on the chalcogen atoms by alkyl radicals, as shown in eq. 8.9. The reactivities somewhat depend on the kind of chalcogenides. Thus, the effective formation of alkyl sulfides requires 30 eq. of disulfides, that of alkyl selenides requires 10 eq. of diselenides, and that of alkyl tellurides requires 2 eq. of ditellurides [27, 28]. 

Diselenides. These compounds can be prepared in satisfactory yield by reaction of 1 with alkyl halides, epoxides, or lactones in THF at 20°. 

Although phenylsenenyl radicals can be generated easily by irradiation of diphenyl diselenide, their low reactivity toward unactivated alkenes and alkynes has prevented their extensive use in synthesis.267,268 But diphenyl diselenide can be used as an efficient trap for alkyl radicals. The products are alkyl phenyl selenides. This trapping reaction has found application to different radical reactions and the selenides obtained by this reaction can be used in subsequent reactions. 

The reactions of selenocarbonyl compounds with electrophiles are also well-established procedures. Alkylations or acylations of the selenium atom of selenoamides409 or selenoureas410 are known. Selenonium salts are formed initially they can then be converted into diselenides, selenazoles, or cyclic selenides depending on their structure. Reactions of selenocarbonyls with bromine and iodine have also been widely exploited. Selenocarbonates, sele-nothiocarbonates,411-415 and selenoureas416-418 can be employed, the reaction of 209 with 1 equiv. bromine led to the hypervalent 10-Se-3 complex 210, whereas an excess of bromine gave rise to a cleavage of the carbon-selenium double bond and formation of product 211 (Scheme 64). 

The lithiation of 1,3-oxathiane (296) takes place with s-BuLi at —78 °C to give 2-lithio-1,3-oxathiane (315), an analogue of 2-lithio-l,3-dithiane (161), but with lower stability487. This intermediate reacts with different electrophiles, such as alkyl halides, carbonyl compounds, benzonitrile, dimethyl disulfide, dimethyl diselenide, trimethylplumbyl acetate and trimethylsilyl, germyl and stannyl chlorides488,489. However, further deprotection of 2-substituted 1,3-oxathianes has not been reported yet. 

Diselenides are generally prepared by the aerial oxidation of selenols or selenolates (see Section 2) or by the reaction of Li2Sc2, Na2Se2, or other 802 species with alkyl or aryl halides. In the latter case, elevated temperatures and DMF as the solvent are recommended. Aryl diselenides may also be obtained from the reaction of 802 with diazonium salts (Scheme 6). Relatively few unsymmetrical diselenides have been reported. Triselenides are also known but have not been as widely studied. 

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