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(C₆F₅)₃, is an effective catalyst for the dehydrogenative silation of alcohols using a variety of silanes, R₃SiH, R₂SiH₂, and R₂R‘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 ∼60 °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 CC, C⋮C, −Br, aliphatic ketones, C(O)OR, lactones, furans, OBn, OMe, and NO₂; 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 (Bn₃SiH and Prⁱ₃SiH) 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.