New mechanisms for the base-catalyzed cleavage of SiSi bonds in organopolysilanes: the base-catalyzed solvolysis of pentaphenyldisilanecarboxylic acid and pentaphenyldisilanol in ethanol/water media

Abstract

A kinetic investigation of the base-catalyzed decomposition of pentaphenyldisilanecarboxylic acid ( 1) and pentaphenyldisilanol ( 2) in ethanol/water media is reported. The solvolysis of the SiSi bond in 2, which also is formed on the base-catalyzed decarbonylation of 1, proceeds by concurrent first-order and second-order processes. At low base concentrations where the first-order process predominates, the intermediate, triphenylsilane ( 3), has been isolated. Solvent isotope effects and activation parameters have been determined. Mechanisms are proposed for the two kinetically distinguishable processes for SiSi bond cleavage in which the pentaphenyldisilanolate ion undergoes either an internal nucleophilic displacement reaction or nucleophilic attack at Si by base in the rate-determining step. A general mechanistic approach for the cleavage of Si bonds in polysilanes by aqueous-alcoholic base is proposed in which polysilanolate ions are formed by nucleophilic attack by base at Si which undergo internal nucleophilic attack resulting in cleavage of the SiSi bond adjacent to the anionic termini. Subsequently, polysilanolate ions are regenerated in which the number of Si atoms is reduced by one.