USING ANHYDROUS HYDROLYSIS TO FAVOR FORMATION OF HEXAMETHYLCYCLOTRISILOXANE FROM DIMETHYLDICHLOROSILANE

The reaction between dimethyl sulfoxide and Me2SiCl2 leads to the form ation of cyclic siloxanes, principally the more highly strained six-me mbered ring hexamethylcyclotrisiloxane (D-3), via linear alpha,omega-d ichlorosiloxanes. At short reaction times (similar to 15 min), the rea ction was shown not to be undergoing equilibration reactions (ligand m etathesis) to a significant degree. A mechanism for the formation of D -3 and D-4 (octamethylcyclotetrasiloxane) is proposed that invokes con version of a chlorosilane group into a sulfonium ion intermediate 12. The preferential formation of D-3 over D-4 is attributed to the greate r steric encumbrance of the activated chain termini (SiOS+Me2) than in the corresponding hydrolysis reaction (SiOH). Both the chain extensio n reactions 12 --> 3 and cyclization reactions 12 --> D-3 are retarded , resulting in a higher selectivity (with DMSO as the oxygen source) f or the intramolecular reaction producing D-3 than in the case of hydro lysis. The experimental results are inconsistent with silanone formati on.