Substitution reactions at silicon under strongly acidic conditions: Ligand metathesis between methyltrichlorosilane and octamethylcyclotetrasiloxane

Abstract A study of the reactions between methyltrichlorosilane (MeSiCl 3 ) and octamethylcyclotetrasiloxane (D 4 ) in the presence of trifluoromethanesulfonic acid (HOTf) showed that nucleophilic substitution in CHCl 3 solution at the trifunctional silicon was surprisingly slow in comparison with the rate of equilibration/polymerization of D 4 . The reaction rate at MeSiCl 3 was significantly faster with water present and faster still in THF. In these reactions, there was no evidence for SiOH groups ( 29 Si NMR), even in the presence of excess water. The overriding criterion for relative reactivity toward nucleophilic substitution under strongly acidic conditions is the relative leaving group ability of the groups on silicon. Once MeSiCl 3 begins to participate in the redistribution reactions, mono‐ 3 and tri‐ 5 , but not di‐substituted 4 systems are formed (MeSiCl 3−x (D n SiMe 2 Z (Z = OTf, Cl)) x , x = 1, 3), likely through an intramolecular reaction. The mechanisms of nucleophilic substitution at silicon under these conditions are discussed.