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

Abstract

Oligomerization reactions of 1,3-dihydro-1,1,3,3-tetramethyldisiloxane (HMMH) and of 1-hydro-1,1,3,3,3-pentamethyldisiloxane (HMM) catalyzed by tris(pentafluorophenyl)borane were studied. In the presence of this catalyst, HMMH is converted to a series of linear α,ω-dihydrooligodimethylsiloxanes of general formula HSiMe2(OSiMe2)nOSiHMe2 (HMDnMH) and dihydrodimethylsilane (Me2SiH2). In addition to these linear products, cyclic oligodimethylsiloxaneshexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4)are also formed. The conversion of HMMH follows second-order kinetics to almost full substrate consumption. Trimer 1,5-dihydro-1,1,3,3,5,5-hexamethyltrisiloxane (HMDMH) and dihydrodimethylsilane are formed as primary products. Consecutive reactions of HMMH and the reactions between oligomeric products, i.e., higher oligomers, occur much more slowly. An exception is formation of D3 that is generated from linear tetramer 1,7-dihydrooctamethyltetrasiloxane (HMD2MH) in a fast consecutive reaction. The dismutation of HMM occurs much more slowly than that of HMMH and takes a more complex course. Permethyloligosiloxanes Me3Si(OSiMe2)nOSiMe3 (MDnM) are the dominant products. Oligomers of the homologous series, α-hydro-ω-methylsilyloligodimethylsiloxanes (HMDnM), are also formed, but they appear at lower concentrations than corresponding oligomers MDnM. The metathetic mechanism of this oligomerization, which includes transient formation of trisilyloxonium ion is discussed.