Ammonium Eneselenolates: Stereochemistry and Electronic Properties
摘要:
Ammonium eneselenolates were generated with high efficiency by reacting selenothioic acid S-esters with a THF solution of TBAF. The methylation of ammonium eneselenolates gave ketene selenothioacetals as stereoisomeric mixtures. The ratio of the two stereoisomers depended on the duration of the reaction before the addition of Mel. Ammonium eneselenolates were characterized by examining their H-1, C-13, and Se-77 NMR spectra, which indicated that ammonium eneselenolates were present almost exclusively as Z-isomers. These results suggested that ammonium eneselenolates are kinetically generated as stereoisomeric mixtures, and isomerization of E-isomers to Z-isomers then takes place to result in the exclusive formation of Z-isomers. During the methylation of Z-isomers of ammonium eneselenolates, the isomerization of Z-isomers to E-isomers occurs to give stereoisomeric mixtures of ketene selenothioacetals. NMR spectra of ammonium eneselenolates implied that the electrons at the selenium atom are somewhat delocalized to the carbon-carbon double bond and the carbon-selenium bond shows partial double-bond character.
Highly Efficient Generation of Ammonium Eneselenolates, Their Reactions and Electronic Properties
作者:Toshiaki Murai、Shuuya Hayakawa、Shinzi Kato
DOI:10.1246/cl.2000.368
日期:2000.4
The ammonium eneselenolates generated from selenothioic acid S-esters and ammonium fluoride were reacted with carbon electrophiles to furnish ketene selenothioacetals with high stereoselectivities. The spectroscopic properties of the ammonium eneselenolates have suggested that the electrons on the selenium atom efficiently delocalize on the carbon-carbon double bond.
The reaction of lithium alkyneselenolates generated from lithium alkynides and selenium powder with a variety of thiols afforded selenothioic acid S-alkyl esters in moderate to high yields. The product yields were highly dependent on the substituents of the starting acetylenes. The formation of unstable S-aryl ester was confirmed by the enolization followed by alkylation.
Selenothioic acid S-alkyl esters were treated with Et3N and Cd(OAc)2·2H2O to give symmetrically substituted selenophenes, whereas the similar reaction in the presence of alkyl halides afforded ketene selenothioacetals in moderate yields.