Using “Anhydrous” Hydrolysis To Favor Formation of Hexamethylcyclotrisiloxane from Dimethyldichlorosilane

The reaction between dimethyl sulfoxide and Me2SiCl2 leads to the formation of cyclic siloxanes, principally the more highly strained six-membered ring hexamethylcyclotrisiloxane (D3), via linear α,ω-dichlorosiloxanes. At short reaction times (∼15 min), the reaction was shown not to be undergoing equilibration reactions (ligand metathesis) to a significant degree. A mechanism for the formation of D3 and D4 (octamethylcyclotetrasiloxane) is proposed that invokes conversion of a chlorosilane group into a sulfonium ion intermediate 12. The preferential formation of D3 over D4 is attributed to the greater steric encumbrance of the activated chain termini (SiOS+Me2) than in the corresponding hydrolysis reaction (SiOH). Both the chain extension reactions 12 → 3 and cyclization reactions 12 → D3 are retarded, resulting in a higher selectivity (with DMSO as the oxygen source) for the intramolecular reaction producing D3 than in the case of hydrolysis. The experimental results are inconsistent with silanone formation.