The strong organoborane Lewis acid B(C(6)F(5))(3) catalyzes the hydrosilation (using R(3)SiH) of aromatic and aliphatic carbonyl functions at convenient rates with loadings of 1-4%. For aldehydes and ketones, the product silyl ethers are isolated in 75-96% yield; for esters, the aldehydes produced upon workup of the silyl acetal products can be obtained in 45-70% yield. Extensive mechanistic studies point to an unusual silane activation mechanism rather than one involving borane activation of the carbonyl function. Quantitative kinetic studies show that the least basic substrates are hydrosilated at the fastest rates; furthermore, increased concentrations of substrate have an inhibitory effect on the observed reaction rate. Paradoxically, the most basic substrates are reduced selectively, albeit at a slower rate, in competition experiments. The borane thus must dissociate from the carbonyl to activate the silane via hydride abstraction; the incipient silylium species then coordinates the most basic function, which is selectively reduced by [HB(C(6)F(5))(3)](-). In addition to the kinetic data, this mechanistic proposal is supported by a kinetic isotope effect of 1.4(5) for the hydrosilation of acetophenone, the observation that B(C(6)F(5))(3) catalyzes H/D and H/H scrambling in silanes in the absence of substrate, computational investigations, the synthesis of models for proposed intermediates, and other isotope labeling and crossover experiments.

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The frustrated Lewis pair system consisting of 2 equiv of 2,2,6,6-tetramethylpiperidine (TMP) and tris(pentafluorophenyl)borane [B(C(6)F(5))(3)] activates carbon dioxide to form a boratocarbamate-TMPH ion pair. In the presence of triethylsilane, this species is converted to a silyl carbamate and the known ion pair [TMPH](+)[HB(C(6)F(5))(3)](-), which recently was shown to react with CO(2) via transfer of the hydride from the hydridoborate to form the formatoborate [TMPH](+)[HC(O)OB(C(6)F(5))(3)](-). In the presence of extra B(C(6)F(5))(3) (0.1-1.0 equiv) and excess triethylsilane, the formatoborate is rapidly hydrosilated to form a formatosilane and regenerate [TMPH](+)[HB(C(6)F(5))(3)](-). The formatosilane in turn is rapidly hydrosilated by the B(C(6)F(5))(3)/Et(3)SiH system to CH(4), with (Et(3)Si)(2)O as the byproduct. At low [Et(3)SiH], intermediate CO(2) reduction products are observed; addition of more CO(2)/Et(3)SiH results in resumed hydrosilylation, indicating that this is a robust, living tandem catalytic system for the deoxygenative reduction of CO(2) to CH(4).

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.

Boron cations are elusive and highly electrophilic species that play a key role in the chemistry of boron. Despite early interest in the chemistry of boron cations, until recently they have largely remained chemical curiosities. However, hints at harnessing their potential as potent electrophiles have begun to appear and developments in weakly coordinating anion technology suggest that this is an area of research that is ripe for exploration. It has been nearly 20 years since the last major review on boron cations; herein we summarize the progress in the area since that time.

The substitution of isoelectronic B–N units for C–C units in aromatic hydrocarbons produces novel heterocycles with structural similarities to the all-carbon frameworks, but with fundamentally altered electronic properties and chemistry. Since the pioneering work of Dewar some 50 years ago, the relationship between B–N and C–C and the wealth of parent all-carbon aromatics has captured the imagination of organic, inorganic, materials, and computational chemists alike, particularly in recent years. New applications in biological chemistry, new materials, and novel ligands for transition-metal complexes have emerged from these studies. This review is aimed at surveying activity in the area in the past couple of decades. Its organization is based on ring size and type of the all-carbon or heterocyclic subunit that the B–N analog is derived from. Structural aspects pertaining to the retention of aromaticity are emphasized, along with delineation of significant differences in physical properties of the B–N compound as compared to the C–C parent.Key words: boron-nitrogen heterocycles, aromaticity, organic materials, main-group chemistry.

A broad range of benzaldimines and ketimines can be hydrosilated efficiently, employing B(C(6)F(5))(3) as a catalyst in conjunction with PhMe(2)SiH. Spectral evidence supports the intermediacy of a silyliminium cation with a hydridoborate counterion formed via abstraction of a hydride from PhMe(2)SiH by B(C(6)F(5))(3) in the presence of imines.