Probing the Source of Enhanced Activity in Multiborylated Silsesquioxane Catalysts for C–O Bond Reduction

A family of variably borylated silsesquioxanes can be conveniently synthesized by the hydroboration of vinyl- and allyl-modified silsesquioxanes using Piers’ borane (HB­(C6F5)2). The catalytic activity of these Lewis acidic catalysts has been examined for the reduction of isochroman with 1,1,3,3-tetramethyldisiloxane, and loadings as low as 0.05 mol % boron are feasible. Despite scaling all catalytic reactions to the boron Lewis acid, the multiborylated silsesquioxanes showed exceptional catalytic activity compared to the monoborylated silsesquioxanes. Even at a catalyst loading of 0.05 mol %, the multiborylated catalyst could achieve a TOF of 7 min–1. The ideal position for boron on the silsesquioxanes was at the C2 position, as this position did not inhibit Lewis acidity via the β-silicon effect (at C1) or limit the inductive electron-withdrawing ability of the silsesquioxane core (at C3). The high catalyst activity is attributed to the increased Lewis acidity of the multiborylated silsesquioxanes.