No abstract

The coupling reaction of silyl hydrides with alkoxysilanes to produce siloxanes and hydrocarbons catalyzed by tris(pentafluorophenyl)borane was studied by gas chromatography and UV spectroscopy using model reagent systems:  Ph2MeSiH + Ph2MeSiOn-Oct (I) and Ph2MeSiH + Me3SiOn-Oct (II). Detailed kinetic studies performed for system I showed that the reaction is first order in both substrates and the rate is proportional to the catalyst concentration. A highly negative apparent entropy of activation points to a crowded transition state structure, leading to a significant dependence of the rate on steric effects. Studies of system II demonstrated that the exchange of the Si−H and Si−OR functionality accompanies the coupling process and in many cases is the dominating reaction in this system. Ultraviolet spectra recorded during the reaction show a distinct strong absorption band with λmax = 303−306 nm, which is due to an allowed electronic transition in the uncomplexed B(C6F5)3 molecule. This absorption also gives rise to intense fluorescence with a maximum of the emission band at 460 nm. When the borane is complexed by oxygen nucleophiles, such as water, alcohol, or silanol and is not active as a catalyst, it does not show the absorption in the 303−306 nm region. This absorption may serve as a measure of the concentration of the active uncomplexed catalyst in the reaction system. Since complexes of B(C6F5)3 with the alkoxysilane substrates and the disiloxane products are relatively weak, the catalyst appears in the reaction system mostly as an uncomplexed species and its concentration is not significantly changed during the reaction. The mechanism proposed includes the transient formation of a complex between hydrosilane, borane, and alkoxysilane in which H- is transferred from silicon to boron and an oxonium ion moiety is generated by interaction of alkoxysilane with positive silicon. The decomposition of the complex occurs by the H- transfer to one of the three electrophilic centers of the oxonium structure, which explains the competition between the siloxane formation and the Si−H/Si−OR exchange. In the case of alkoxysilanes derived from primary alcohols, H- is preferably transferred to silicon. However, for alkoxysilanes derived from a secondary alcohol, such as isopropyl alcohol, the secondary carbon is more readily attacked than silicon by H-, which leads to a high yield of mixed disiloxane.

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) n OSiHMe2 (HMD n MH) and dihydrodimethylsilane (Me2SiH2). In addition to these linear products, cyclic oligodimethylsiloxaneshexamethylcyclotrisiloxane (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) n OSiMe3 (MD n M) are the dominant products. Oligomers of the homologous series, α-hydro-ω-methylsilyloligodimethylsiloxanes (HMD 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.

Polyaryloxysilanes and siloxanes (PAS) are an interesting and useful class of polymers with excellent flammability characteristics. The B(C 6 F 5 ) 3 catalyzed dehydrocondensation of bis-phenols or their simple alkylethers with dihydrosilanes and siloxanes provides ready access to these polymers. The reaction proceeds readily even when hindered phenol or silane substrates are employed affording sterically protected PAS derivatives. The combination of steric hindrance around the silyl ether linkage, the absence of ionic impurities and the use of very low levels of the boron catalyst in these reactions produces thermally and hydrolytically robust materials, which should further enhance their utility. Phenol functional macromonomers, such as polycarbonate and PPO derivatives, are readily coupled, providing easy access to siloxane copolymers. The only severe limitation of this process for preparing PAS derivatives is that the reaction is intolerant of base functionality in either the monomers or solvents used in the reactions.