Oligomerization of Hydrosiloxanes in the Presence of Tris(pentafluorophenyl)borane

Abstract

Oligomerization reactions of 1,3-dihydro-1,1,3,3-tetramethyldisiloxane ( H MM H ) and of 1-hydro-1,1,3,3,3-pentamethyldisiloxane ( H MM) catalyzed by tris(pentafluorophenyl)borane were studied. In the presence of this catalyst, H MM H is converted to a series of linear α,ω-dihydrooligodimethylsiloxanes of general formula HSiMe 2 (OSiMe 2 ) n OSiHMe 2 ( H MD n M H ) and dihydrodimethylsilane (Me 2 SiH 2 ). In addition to these linear products, cyclic oligodimethylsiloxanes hexamethylcyclotrisiloxane (D 3 ) and octamethylcyclotetrasiloxane (D 4 ) are also formed. The conversion of H MM H follows second-order kinetics to almost full substrate consumption. Trimer 1,5-dihydro-1,1,3,3,5,5-hexamethyltrisiloxane ( H MDM H ) and dihydrodimethylsilane are formed as primary products. Consecutive reactions of H MM H and the reactions between oligomeric products, i.e., higher oligomers, occur much more slowly. An exception is formation of D 3 that is generated from linear tetramer 1,7-dihydrooctamethyltetrasiloxane ( H MD 2 M H ) in a fast consecutive reaction. The dismutation of H MM occurs much more slowly than that of H MM H and takes a more complex course. Permethyloligosiloxanes Me 3 Si(OSiMe 2 ) n OSiMe 3 (MD n M) are the dominant products. Oligomers of the homologous series, α-hydro-ω-methylsilyloligodimethylsiloxanes ( H MD n M), are also formed, but they appear at lower concentrations than corresponding oligomers MD n M. The metathetic mechanism of this oligomerization, which includes transient formation of trisilyloxonium ion is discussed.