Synthesis, properties, and hydroboration activity of the highly electrophilic borane bis(pentafluorophenyl)borane, HB(C6F5)(2)

Two reliable and efficient routes to bis(pentafluorophenyl)borane, 1, are d escribed. A three-step procedure uses the -C6F5 transfer agent Me2Sn(C6F5)( 2) to produce the chloroborane ClB(C6F5)(2), which is subsequently converte d 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 degrees C for 3 days in benzene. Borane 1 is dimeric in the solid state, as determined by X-ray crystallographic analys is. However, in aromatic solvents, detectable amounts of monomeric borane a re present (ratio of dimer:monomer approximate to 4.5: 1). The ease of dime r dissociation to monomer coupled with the high electrophilicity of the bor ane makes 1 a very reactive hydroboration reagent in aromatic solvents. Hyd roborations do not proceed in donor solvents such as tetrahydrofuran. A sur vey of a variety of olefin and alkyne substrates shows that 1 hydroborates with comparable regio- and chemoselectivities to commonly used reagents suc h as 9-BBN, but at a much faster rate. A second unique feature of the reage nt is the facility with which boryl migration takes place in the products o f olefin hydroboration. This property can be used to access thermodynamic p roducts of hydroboration where other reagents give diastereomeric kinetic p roducts. Alkynes can be selectively monohydroborated; terminal alkyne subst rates will react with a second equivalent of 1, while internal alkynes are immune to further hydroboration. Two procedures for the oxidation of the pr oducts of hydroboration were developed. Since the organobis(pentafluorpheny l)boranes are susceptible to protonolyis, oxidation must be carried out in a two-phase system using highly alkaline hydrogen peroxide or with a nonaqu eous procedure using Me-3-NO 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.