Abstract
The utility of (C6F5)3B(OH2) as catalyst for the simple and environmentally benign synthesis of oligosiloxanes directly from hydrosilanes, is reported. This protocol offers several advantages compared to other methods of synthesizing siloxanes, such as mild reaction conditions, low catalyst loading, and a short reaction time with high yields and purity. The considerable H2O-tolerance of (C6F5)3B(OH2) promoted a catalytic route to disiloxanes which showed >99% conversion of three tertiary silanes, Et3SiH, PhMe2SiH, and Ph3SiH. Preliminary data on the synthesis of unsymmetrical disiloxanes (Si-O-Si') suggests that by modifying the reaction conditions and/or using a 1:1 combination of silane to silanol the cross-product can be favored. Intramolecular reactions of disilyl compounds with catalytic (C6F5)3B(OH2) led to the formation of novel bridged siloxanes, containing a Si-O-Si linkage within a cyclic structure, as the major product. Moreover, the reaction conditions enabled recovery and recycling of the catalyst. The catalyst was re-used 5 times and demonstrated excellent conversion for each substrate at 1.0 mol% catalyst loading. This seemingly simple reaction has a rather complicated mechanism. With the hydrosilane (R3SiH) as the sole starting material, the fate of the reaction largely depends on the creation of silanol (R3SiOH) from R3SiH as these two undergo dehydrocoupling to yield a disiloxane product. Generation of the silanol is based on a modified Piers-Rubinsztajn reaction. Once the silanol has been produced, the mechanism involves a series of competitive reactions with multiple catalytically relevant species involving water, silane, and silanol interacting with the Lewis acid and the favored reaction cycle depends on the concentration of various species in solution.
Highlights
Organo(poly)siloxanes, bearing the repeating Si-O bond motif, are considered one of the most important classes of functional materials that have influenced many technological industries (Sawama et al, 2016; Wang et al, 2017)
Transition metal catalyzed routes to unsymmetrical disiloxanes have been achieved through Pd-catalyzed arylation of hydroxysiloxanes (Kurihara et al, 2013), nonhydrolytic Pd/Ccatalyzed cross-coupling reactions (Igarashi et al, 2014), Bamediated dehydrocoupling of hydrosilanes and silanols (Le Coz et al, 2019) and silylation of silanols catalyzed by a ruthenium complex (Marciniec et al, 2008)
For 1a and 1b, the siloxane product was isolated by dissolving the reaction mixture in n-pentane to selectively precipitate out the (C6F5)3B(OH2) catalyst. 1c is a solid so the catalytic reactions were enabled by the addition of minimum amount of dry toluene (∼1 mL)
Summary
Organo(poly)siloxanes (silicones), bearing the repeating Si-O bond motif, are considered one of the most important classes of functional materials that have influenced many technological industries (Sawama et al, 2016; Wang et al, 2017). Polysiloxanes are generated by acid- or base-catalyzed ring opening polymerization of cyclic siloxanes, or by hydrolysis of chlorosilanes (Grubb, 1954; Brinker and Scherer, 1990). These methods have limited control over the oligomeric or polymeric siloxane sequence being formed. The synthesis of symmetrical disiloxanes directly from hydrosilanes have been reported using InBr3, a reaction which involves Lewis acid-catalyzed air oxidation of hydrosilanes (Scheme 1, A, conditions I) (Sridhar et al, 2009). Other routes to form Si-O-Si’ bonds include the use of fluoride and azidosilanes (Abele et al, 2003) as well as a catalyst-free, chlorine-free option using disilazanes (Kucinski and Hreczycho, 2019b) (Scheme 1, B, conditions IX–X)
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