B(C6F5)(3)-catalyzed silation of alcohols: A mild, general method for synthesis of silyl ethers

The commercially available borane tris(pentafluorophenyl)borane, B(C6F5)(3) , is an effective catalyst for the dehydrogenative silation of alcohols usi ng a variety of silanes, R3SiH, R2SiH2, and R2R'SiH. Generally, the reactio ns occur in a convenient time frame at room temperature using 2 mol % of th e borane and are clean and high yielding, with dihydrogen as the only bypro duct. Primary aliphatic alcohols are silated cleanly but slowly, with react ion times ranging from 20 to 144 h. Faster reaction times can be achieved b y increasing the catalyst loading to 8 mol % or by heating the reaction to similar to 60 degrees C. Secondary and tertiary alcohols react more rapidly , with most reactions being complete in 0.5-2 h. The reaction is tolerant o f many functional groups including C=C, C=C, -Br, aliphatic ketones, C(O)OR , lactones, furans, OBn, OMe, and NO2; examples of each are given. Using th e phenolic substrate 2,4,6-trimethylphenol, a number of different silanes w ere tested. Only the most bulky silanes (Bn(3)sSiH and (Pr3SiH)-Si-i) were not reactive under these conditions. The selectivity of the silation reacti ons are roughly governed by the relative basicity of the alcohols land othe r functions in the molecule) with more basic groups being selectively silat ed. These observations are rationalized on the basis of a mechanism that in vokes 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.