Catalytic Synthesis of Oligosiloxanes Mediated by an Air Stable Catalyst, (C6F5)3B(OH2)

Abstract

The utility of (C 6 F 5) 3 B(OH 2) 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 H 2 O-tolerance of (C 6 F 5) 3 B(OH 2) promoted a catalytic route to disiloxanes which showed >99% conversion of three tertiary silanes, Et 3 SiH, PhMe 2 SiH, and Ph 3 SiH. 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 (C 6 F 5) 3 B(OH 2) 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 (R 3 SiH) as the sole starting material, the fate of the reaction largely depends on the creation of silanol (R 3 SiOH) from R 3 SiH 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.

Silylation of Alcohols, Phenols, and Silanols with Alkynylsilanes – an Efficient Route to Silyl Ethers and Unsymmetrical Siloxanes

“…[6][7][8][9][10] to their most known function as a temporary protective group for hydroxyl moieties (e.g., natural products synthesis, derivatization, etc.). [11][12][13][14] There are several methods of their synthesis, starting with classical condensation between alcohols and halosilanes, [15] and finishing on catalytic coupling reactions with hydro-, [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] vinyl- [33] or allylsilanes, [34,35] and more recently also with silyl formates. [36][37][38] However, in the case of O-silylation of alcohols with alkynylsilanes -this process has never been comprehensively studied, and previous investigations have mainly referred to the problem of C sp -Si bond stability rather than the application of this transformation as an efficient method for obtaining silyl ethers.…”

Direct cross-linking of silyl-functionalized cage siloxanes via nonhydrolytic siloxane bond formation for preparing nanoporous materials

“…The Q 3 /(Q 3 + Q 4 ) integral ratio suggested that 2.6% of the Si(OSi) 4 units underwent cleavage, probably due to hydrolysis with a trace of water in the reaction system. 17 The integral ratio of the 29 Si signals of D 2 /(M 1 + D 1 + D 2 ) was 0.91, indicating that the percentage of the condensed silyl groups ( P cond ) was 91% (Table 1). Although the P–R reaction is strongly exothermic, 9 the FT-IR and 29 Si MAS NMR analyses confirmed that the reaction proceeded while retaining the D4R cage structure.…”

When Attempting Chain Extension, Even Without Solvent, It Is Not Possible to Avoid Chojnowski Metathesis Giving D3

“…Previous experience with polymer chain extension of HSi-telechelic polymers with water did not lead to the formation of cyclics. Under these conditions with low catalyst concentrations (0.02%) (BCF/H 2 O and B(C 6 F 5 ) 3 ∙OH 2 [ 38 ]), redistribution reactions are similarly not efficient. While the reactions here were complete in 30 min, there was no change in the cyclics profile between 30–180 min (SM); redistribution would favor the formation of D 4 which, of the cyclics characterized, was formed in the lowest concentration.…”