Mechanism of the B(C₆F₅)₃-Catalyzed Reaction of Silyl Hydrides with Alkoxysilanes. Kinetic and Spectroscopic Studies

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.

The Power of Non-Hydrolytic Sol-Gel Chemistry: A Review

“…It is catalyzed by tris(pentafluorophenyl)borane and utilized mainly in the synthesis of siloxane polymers, but its application to the preparation of hybrid mixed oxide (Si-O-B) systems was shown recently [25,26,[52][53][54].…”

“…Alkane is eliminated in the course of the reaction, which is catalyzed by the strong Lewis acid tris(pentafluorophenyl)borane, B(C6F5)3. The catalyst plays an important role for hydride ion transfer [52,130]. Similar to the other NHSG syntheses, this reaction suffers from ligand exchange and Ester elimination is a minor route, which was successfully applied to siloxane preparation [121,122].…”

Synthesis of vinyl substitute poly(silphenylene‐siloxane) via silyl hydride‐dialkoxysilane process

“…31 In our investigation, the 1 H-NMR and UV spectra of the polymer clearly indicate that no side hydrosilation reaction was observed, and the vinyl group remained undamaged during the reaction, which allow us in preparing vinyl substitute poly(silphenylene-siloxane), i.e., crosslinkable poly(silphenylene-siloxane), through this new efficient silyl hydride-dialkoxysilane process. The hydrosilation of olefins need relative high level of B(C 6 F 5 ) 3 , 5 mol %, 22 which is much more higher than the level of B(C 6 F 5 ) 3 in the silyl hydride-dialkoxysilane couple reaction, which is only 0.1 mol % (6.2 Â 10 À4 mol/L).…”

“…This phenomenon is common in the B(C 6 F 5 ) 3 catalyzed reaction of silyl hydride and alkoxysilane. 30,31 …”

“…[20][21][22][23][24][25][26][27][28][29] B(C 6 F 5 ) 3 catalyzed coupling reaction of silyl hydrides with alkoxysilanes was a newly developed methods to prepare linear polysiloxanes by Rubinsztajn and coworkers. 30,31 This reaction takes place at or below room temperature in the presence of very low levels of B(C 6 F 5 ) 3 . The rate of the reaction can be controlled by the rate of addition of reagents.…”