Silyl enol ethers electron-donor properties

These authors also noted that the electron-donor ability of various derivatives of 2,2-dimesityl-1-phenylethenol decreases in the order enolate > enol > enol silyl ether > enol phosphate > enol acetate. As such, a simple derivatization allows the ready modulation of the electron-donor properties of ends. 

Perhaps the most useful type of alkene substrates for these reactions are enol ethers, enol esters and vinyl sulfides. Silyl enol ethers have excellent electron-donor properties, with an ionization potential of about 8 eV and an oxidation potential in various solvents of approximately 1.0-1.5 V vs SCE161. These compounds are easily synthesized by reaction of an enolate with a chlorosilane. (A very recent report synthesized a variety of silyl enol ethers with extremely high stereochemical yield, using the electrogenerated amidate of 2-pyrolidinone as the base.)162 An interesting point is that the use of oxidative or reductive cyclization reactions allows carbonyl functionalities to be ambivalent, either oxidizable or reducible (Scheme 65)163. 

An assessment of the electron donor properties of silyl enol ethers can be made by comparison of their ionization potentials and anodic oxidation potentials with those of related compounds. Scheme 1 summarizes selected examples. 

Rathore, R. and Kochi, J.K, Quantitative assessment of electron-donor properties of enol silyl ethers charge-transfer complex formation, photoelectron spectra and transient electrochemical oxidation. Tetrahedron Lett., 35, 8577, 1994. 

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