B(C₆F₅)₃-Catalyzed Silation of Alcohols:  A Mild, General Method for Synthesis of Silyl Ethers

Abstract

The commercially available borane tris(pentafluorophenyl)borane, B(C(6)F(5))(3), is an effective catalyst for the dehydrogenative silation of alcohols using a variety of silanes, R(3)SiH, R(2)SiH(2), and R(2)R'SiH. Generally, the reactions occur in a convenient time frame at room temperature using 2 mol % of the borane and are clean and high yielding, with dihydrogen as the only byproduct. Primary aliphatic alcohols are silated cleanly but slowly, with reaction times ranging from 20 to 144 h. Faster reaction times can be achieved by increasing the catalyst loading to 8 mol % or by heating the reaction to approximately 60 degrees C. Secondary and tertiary alcohols react more rapidly, with most reactions being complete in 0.5-2 h. The reaction is tolerant of many functional groups including C=C, C&tbd1;C, -Br, aliphatic ketones, C(O)OR, lactones, furans, OBn, OMe, and NO(2); examples of each are given. Using the phenolic substrate 2,4,6-trimethylphenol, a number of different silanes were tested. Only the most bulky silanes (Bn(3)SiH and Pr(i)()(3)SiH) were not reactive under these conditions. The selectivity of the silation reactions are roughly governed by the relative basicity of the alcohols (and other functions in the molecule) with more basic groups being selectively silated. These observations are rationalized on the basis of a mechanism that invokes borane activation of the silane by hydride abstraction. The resulting intermediate silylium/hydridoborate ion pair then reacts with alcohols to give the observed silyl ether and dihydrogen products.

Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

“…22 29 Si NMR spectroscopy at 233 K. Noting, however, that their procedures did not appear to involve careful drying of the borane before use, it seemed likely that the peak assigned to this adduct is due in fact to Et3SiOSiEt3, formed rapidly when wet borane 35,36 is treated with silane via rapid dehydrogenative silation of water. 37,38 We confirmed this postulate by recording the 29 Si NMR spectrum of an authentic sample of Et3SiOSiEt3. Thus, this report of detection of the Et3Si-HB(C6F5)3 adduct 34 must be viewed as erroneous, and these species remain experimentally elusive.…”

Metal-free transfer hydrogenation of olefins via dehydrocoupling catalysis

“…Overall, this catalytic cycle is computed to be slightly exothermic (ΔH = −6.0 kcal·mol −1 ) with only a small overall ΔG of −0.6 kcal·mol −1 . This dehydrocoupling mechanism is reminiscent of that described for analogous B(C 6 F 5 ) 3 catalyzed reaction in the classic work from Piers and coworkers (29), which subsequently further illuminated and expanded by the groups of Oestreich (30), Brook (31), Rubinsztajn (32), and Paradies (33). In the context of dehydrocoupling, it is also important to note the pioneering work from the Tilley and Manners research groups, who exploited transition metal catalysts for this purpose (34,35), whereas Hill et al (36) described Zn, Mg, Ca, and Sr catalyzed preparations of amino silanes and Baba and coworker (37) have used InBr 3 to dehydrocouple silanes and carboxylic acids (Table 1 and Fig.…”

Investigation of hydrolysis and condensation of methyltriethoxysilane in aqueous systems

“…Recently Stryker's reagent ([Ph3PCuH]6) [8] and tris(pentafluorphenyl) borane (B[C6F5]3) [9][10][11] were reported as effective catalysts of the alcoholysis of hydrosilanes. Solutions of alkoxysilanes in organic solvents are commonly used for impregnation of wood [12,13] and modification of inorganic surfaces [14].…”