BAr^F₃‑Catalyzed Imine Hydroboration with Pinacolborane Not Requiring the Assistance of an Additional Lewis Base

The rarely used boron Lewis acid tris­[3,5-bis­(trifluoromethyl)­phenyl]­borane (BAr^F₃) is found to be an excellent catalyst for metal-free hydroboration of imines. In the presence of 1.0 mol % of BAr^F₃, several ketimines and aldimines undergo hydroboration with pinacolborane (HBpin) at room temperature without the aid of an external Lewis base. BAr^F₃ is more reactive than other Lewis acidic boranes, including the often-used tris­(pentafluorophenyl)­borane (B­(C₆F₅)₃). The steric hindrance imparted by the six fluorine atoms ortho to the boron center in B­(C₆F₅)₃ accounts for this. Mechanistic control experiments indicate conventional Lewis acid catalysis involving imine activation and hydride transfer from HBpin

Accessing Highly Substituted Indoles via B(C₆F₅)₃‑Catalyzed Secondary Alkyl Group Transfer

Herein, we report a synthetic method to access a range of highly substituted indoles via the B(C6F5)3-catalyzed transfer of 2° alkyl groups from amines. The transition-metal-free catalytic approach has been demonstrated across a broad range of indoles and amine 2° alkyl donors, including various substituents on both reacting components, to access useful C(3)-alkylated indole products. The alkyl transfer process can be performed using Schlenk line techniques in combination with commercially available B(C6F5)3·nH2O and solvents, which obviates the requirement for specialized equipment (e.g., glovebox)

Pathways to Functionalized Heterocycles: Propargyl Rearrangement using B(C₆F₅)₃

The reactions of propargyl amides, ureas, carbamates, and carbonates with B­(C₆F₅)₃ proceed via an intramolecular 5-<i>exo</i>-<i>dig</i> cyclization across the alkyne unit to yield the corresponding vinyl borate species. The generated sp² carbocation is stabilized by the flanking heteroatoms, allowing for isolation of oxazoline intermediates. The fate of these intermediates is strongly dependent upon the propargyl-functionalized starting material, with the carbamates and carbonates undergoing a ring-opening mechanism (propargyl rearrangement) to give cyclic allylboron compounds, while prolonged heating of the urea derivatives shows evidence of oxazole formation. In a deviation away from the reactivity of carbamates stated previously, the benzyl carbamate substrate undergoes dealkylation at the benzylic position, liberating 5-methyloxazol-2-(3<i>H</i>)-one

Diverging Pathways in the Activation of Allenes with Lewis Acids and Bases: Addition, 1,2-Carboboration, and Cyclization

The reactions of allenes with frustrated (or cooperative) Lewis acid/base pairs result in the 1,4-addition of the base pair to the allene. The reactions of allenyl ketones and esters just in the presence of the strong Lewis acid B­(C₆F₅)₃ afford the selective formation of the 1,2-carboboration products. In both cases the Lewis acid activates the allene to either a C₆F₅ migration or nucleophilic attack by the Lewis base. In addition to the 1,2-carboboration pathway, which can be viewed as being triggered by activation of the ketone (σ-activation), in the case of allenyl esters the corresponding cyclization products are observed in the presence of water

Pathways to Functionalized Heterocycles: Propargyl Rearrangement using B(C₆F₅)₃

The reactions of propargyl amides, ureas, carbamates, and carbonates with B­(C₆F₅)₃ proceed via an intramolecular 5-<i>exo</i>-<i>dig</i> cyclization across the alkyne unit to yield the corresponding vinyl borate species. The generated sp² carbocation is stabilized by the flanking heteroatoms, allowing for isolation of oxazoline intermediates. The fate of these intermediates is strongly dependent upon the propargyl-functionalized starting material, with the carbamates and carbonates undergoing a ring-opening mechanism (propargyl rearrangement) to give cyclic allylboron compounds, while prolonged heating of the urea derivatives shows evidence of oxazole formation. In a deviation away from the reactivity of carbamates stated previously, the benzyl carbamate substrate undergoes dealkylation at the benzylic position, liberating 5-methyloxazol-2-(3<i>H</i>)-one

Structure and Bonding of the Manganese(II) Phosphide Complex (<i>t</i>-BuPH₂)(η⁵-Cp)Mn{μ-(<i>t</i>-BuPH)}₂Mn(Cp)(<i>t</i>-BuPH₂)

Rather than achieving bis-deprotonation of the phosphine, reaction of Cp₂Mn (Cp = cyclopentadienyl) with <i>t</i>-BuPH₂ at room temperature yields monodeprotonation of half of the available phosphine in the product (<i>t</i>-BuPH₂)­(η⁵-Cp)­Mn­{μ-(<i>t</i>-BuPH)}₂Mn­(Cp)­(<i>t</i>-BuPH₂) (<b>1</b>). This complex comprises a Mn­(II) phosphide and is a dimer in the solid state, containing a Mn₂P₂ diamond core. Consistent with the observation of a relatively short intermetal distance of 2.8717(4) Å in <b>1</b>, DFT analysis of the full structure points to a singlet ground state stabilized by a direct Mn–Mn single bond. This is in line with the diamagnetic character of <b>1</b> and an 18-electron count at Mn

Structure and Bonding of the Manganese(II) Phosphide Complex (<i>t</i>-BuPH₂)(η⁵-Cp)Mn{μ-(<i>t</i>-BuPH)}₂Mn(Cp)(<i>t</i>-BuPH₂)

Rather than achieving bis-deprotonation of the phosphine, reaction of Cp₂Mn (Cp = cyclopentadienyl) with <i>t</i>-BuPH₂ at room temperature yields monodeprotonation of half of the available phosphine in the product (<i>t</i>-BuPH₂)­(η⁵-Cp)­Mn­{μ-(<i>t</i>-BuPH)}₂Mn­(Cp)­(<i>t</i>-BuPH₂) (<b>1</b>). This complex comprises a Mn­(II) phosphide and is a dimer in the solid state, containing a Mn₂P₂ diamond core. Consistent with the observation of a relatively short intermetal distance of 2.8717(4) Å in <b>1</b>, DFT analysis of the full structure points to a singlet ground state stabilized by a direct Mn–Mn single bond. This is in line with the diamagnetic character of <b>1</b> and an 18-electron count at Mn