Ammonium eneselenolates: stereochemistry and electronic properties

Area of Science:

• Organic Chemistry
• Organoselenium Chemistry

Background:

• Selenothioic acid S-esters are versatile precursors in organoselenium chemistry.
• Understanding the stereochemical outcomes of eneselenolate reactions is crucial for synthetic applications.

Purpose of the Study:

• To investigate the efficient generation of ammonium eneselenolates.
• To explore the stereoselectivity of ketene selenothioacetal formation via methylation of ammonium eneselenolates.
• To elucidate the isomerization pathways of eneselenolates and their impact on product stereochemistry.

Main Methods:

• Reaction of selenothioic acid S-esters with tetrabutylammonium fluoride (TBAF) in tetrahydrofuran (THF).
• Methylation of in-situ generated ammonium eneselenolates using methyl iodide (MeI).
• Characterization of intermediates and products using Nuclear Magnetic Resonance (NMR) spectroscopy (¹H, ¹³C, and ⁷⁷Se).

Main Results:

• Ammonium eneselenolates were generated with high efficiency.
• Ketene selenothioacetals were formed as stereoisomeric mixtures, with ratios dependent on reaction duration prior to methylation.
• NMR analysis indicated that ammonium eneselenolates exist predominantly as Z-isomers, suggesting kinetic formation of stereoisomeric mixtures followed by isomerization.
• Methylation of Z-ammonium eneselenolates led to isomerization to E-isomers, resulting in stereoisomeric ketene selenothioacetals.
• Evidence of electron delocalization from selenium to the carbon-carbon double bond and partial double-bond character in the carbon-selenium bond was observed.

Conclusions:

• Ammonium eneselenolates can be efficiently synthesized and undergo stereoselective methylation.
• The observed stereoisomer ratios in ketene selenothioacetals are governed by kinetic control and subsequent isomerization processes.
• The electronic structure of ammonium eneselenolates features delocalized electrons and a partially double-bonded C-Se linkage.