Oligomerization of hydrosiloxanes in the presence of tris(pentafluorophenyl) borane

Oligomerization reactions of 1,3-dihydro-1,1,3,3-tetramethyldisiloxane ((MMH)-M-H) and of 1-hydro-1,1,3,3,3- pentamethyldisiloxane ((MM)-M-H) catalyzed by tris( pentafluorophenyl) borane were studied. In the presence of this catalyst, (MMH)-M-H is converted to a series of linear alpha,omega-dihydrooligodimethylsiloxanes of general formula HSiMe2(OSiMe2)(n)OSiHMe2 ((MDnMH)-M-H) and dihydrodimethylsilane (Me2SiH2). In addition to these linear products, cyclic oligodimethylsiloxanes-hexamethylcyclotrisiloxane (D-3) and octamethylcyclotetrasiloxane (D-4)-are also formed. The conversion of (MMH)-M-H follows second-order kinetics to almost full substrate consumption. Trimer 1,5-dihydro-1,1,3,3,5,5-hexamethyltrisiloxane ((MDMH)-M-H) and dihydrodimethylsilane are formed as primary products. Consecutive reactions of (MMH)-M-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 ((MD2MH)-M-H) in a fast consecutive reaction. The dismutation of (MM)-M-H occurs much more slowly than that of (MMH)-M-H and takes a more complex course. Permethyloligosiloxanes Me3Si(OSiMe2)(n)OSiMe3 (MDnM) are the dominant products. Oligomers of the homologous series, alpha-hydro-omega-methylsilyloligodimethylsiloxanes ((MDnM)-M-H), 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.