Kinetics and Thermodynamics of Phenolate Silylation and Alkylation

Article abstract of DOI:10.1021/ja00231a031

A kinetic and thermodynamic investigation of the silylation reactions of alkali phenolates with several trisubstituted silyl chlorides was performed in tetrahydrofuran, acetonitrile, and dimethyl sulfoxide.Heats and rates of reaction were determined by titration calorimetry and stopped-flow techniques and found to be strongly dependent upon solvent polarity, ion pairing of the alkali phenolate, and steric and electronic influences.Reaction rates were considerably faster in acetonitrile and dimethyl sulfoxide than in tetrahydrofuran where ion-pairing effects were significant.Rates were accelerated by the addition of crown ethers.In most cases, potassium phenolates were more reactive than the corresponding sodium phenolates.The order of reactivity observed for the silyl chlorides increased as follows: i-Pr3 < t-BuPh2 < Me3 < Ph2Me.Substitution with tert-butyl functionalities in the ortho position of the phenolate ring retarded reactivity through steric crowding of the reaction site.Resonance and/or inductive effects of substituents increased the reactivities of phenolate anions in the following order: 2,4,6-(NO2)3 < 2,4-(NO2)2 < 2,6-(t-Bu)2-4-NO2 < 4-NO2 < 4-CN < 4-Cl < 4-F < 4-H < 4-t-Bu < 4-O-Me.Linear relationchips were found between the heats of reaction, the pK_a's of the parent phenols, and the appropriate Hammett ? constants.Hammett plots for the silylation reactions of substituted potassium phenolates with triphenylsilyl fluoride gave values of ρ = 1.24 and 2.40 for the reaction in acetonitrile and dimethyl sulfoxide, respectively.