Synthesis, Properties, and Hydroboration Activity of the Highly Electrophilic Borane Bis(pentafluorophenyl)borane, HB(C₆F₅)₂¹

Two reliable and efficient routes to bis(pentafluorophenyl)borane, 1, are described. A three-step procedure uses the −C₆F₅ transfer agent Me₂Sn(C₆F₅)₂ to produce the chloroborane ClB(C₆F₅)₂, which is subsequently converted to 1 by treatment with a silane, and proceeds with an overall yield of 62%. Alternatively, 1 can be made in 69% yield from B(C₆F₅)₃ and Et₃SiH by heating the two reagents at 60 °C for 3 days in benzene. Borane 1 is dimeric in the solid state, as determined by X-ray crystallographic analysis. However, in aromatic solvents, detectable amounts of monomeric borane are present (ratio of dimer:monomer ≈4.5:1). The ease of dimer dissociation to monomer coupled with the high electrophilicity of the borane makes 1 a very reactive hydroboration reagent in aromatic solvents. Hydroborations do not proceed in donor solvents such as tetrahydrofuran. A survey of a variety of olefin and alkyne substrates shows that 1 hydroborates with comparable regio- and chemoselectivities to commonly used reagents such as 9-BBN, but at a much faster rate. A second unique feature of the reagent is the facility with which boryl migration takes place in the products of olefin hydroboration. This property can be used to access thermodynamic products of hydroboration where other reagents give diastereomeric kinetic products. Alkynes can be selectively monohydroborated; terminal alkyne substrates will react with a second equivalent of 1, while internal alkynes are immune to further hydroboration. Two procedures for the oxidation of the products of hydroboration were developed. Since the organobis(pentafluorphenyl)boranes are susceptible to protonolyis, oxidation must be carried out in a two-phase system using highly alkaline hydrogen peroxide or with a nonaqueous procedure using Me₃NO as the oxidant. Hydroboration/oxidation can be carried out rapidly in a one-pot procedure which gives alcohol or carbonyl products in good to excellent yields.