Nucleophilic Reactivities of Silyl Ketene Acetals and Silyl Enol Ethers Containing (C6F5)3SiO and (C6H5)3SiO Groups

Abstract The kinetics of the reactions of tris(pentafluorophenyl)silyl and triphenylsilyl ketene acetals and enol ethers with benzhydrylium cations have been measured photometrically. The second‐order rate constants (log k 2 ) have been used to determine the nucleophilicity parameters of these π systems, which provide a quantitative comparison of their reactivities with those of other types of nucleophiles. A change from trimethylsilyloxy to triphenylsilyloxy reduces reactivity by only a factor of ten, while replacement of triphenylsilyloxy with tris(pentafluorophenyl)silyloxy decreases the nucleophilicity of the enolate C=C double bond by three to four orders of magnitude. The cation‐stabilizing ability of the tris(pentafluorophenyl)silyloxy group is thus similar to that of a methyl group. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)