Ammonium eneselenolates: Stereochemistry and electronic properties

Ammonium eneselenolates were generated with high efficiency by reacting sel enothioic acid S-esters with a THF solution of TBAF. The methylation of amm onium eneselenolates gave ketene selenothioacetals as stereoisomeric mixtur es. The ratio of the two stereoisomers depended on the duration of the reac tion 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 a s 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 sele nothioacetals. NMR spectra of ammonium eneselenolates implied that the elec trons at the selenium atom are somewhat delocalized to the carbon-carbon do uble bond and the carbon-selenium bond shows partial double-bond character.