Hydrogenolysis of Haloboranes: from Synthesis of Hydroboranes to Formal Hydroboration Reactions

Hydroboranes are versatile reagents in synthetic chemistry, but their synthesis relies on energy-intensive processes. Herein, we report a new method for the preparation of hydroboranes from hydrogen and the corresponding haloboranes. Triethylamine (NEt3) form with dialkylchloroboranes a Frustrated Lewis Pair (FLP) able to split H2 and afford the desired hydroborane with ammonium salts. Unreactive haloboranes were unlocked using a catalytic amount of Cy2BCl, enabling the synthesis of commonly used hydroboranes such as pinacolborane or catecholborane. The mechanisms of these reactions have been examined by DFT studies, highlighting the importance of the base selection. Finally, the system's robustness has been evaluated in one-pot B-Cl hydrogenolysis/hydroboration reactions of C=C unsaturated bonds.

On the association of frustrated Lewis pairs in ionic liquids: a molecular dynamics simulation study

Sterically hindered frustrated Lewis pairs (FLPs) have the ability to activate hydrogen molecules, and their reactivity is strongly determined by the geometric parameters of the Lewis acids and bases. A recent experimental study showed that ionic liquids (ILs) could largely improve the effective configuration of FLPs. However, the detailed mechanistic profile is still unclear. Herein, we performed molecular dynamics (MD) simulations to reveal the effects of ILs on the structures of FLPs, in particular, the association of Lewis acids and bases. For this purpose, mixed systems were adopted consisting of the ILs [C_nmim][NTf₂] (n = 6, 10, 14), [C₆mim][PF₆] and [C₆mim][CTf₃] and the typical FLP (tBu)₃P/B(C₆F₅)₃ for MD simulations. Radial distribution functions (RDFs) results show that toluene competes with (tBu)₃P to interact with B(C₆F₅)₃, resulting in a relatively low effective (tBu)₃P/B(C₆F₅)₃ complex, while [C₁₀mim][NTf₂] shows less competition with (tBu)₃P, which increases the amount of effective FLPs. Spatial distribution functions (SDFs) results show that toluene forms a continuum solvation-shell, which hinders the interactions between (tBu)₃P and B(C₆F₅)₃, while [C₁₀mim][NTf₂] leaves relatively large empty spaces, which are accessible for (tBu)₃P or B(C₆F₅)₃ molecules, resulting in higher probabilities of effective FLP structures. Lastly, we find that the longer alkyl chain length of [C_nmim]⁺ cations, the higher the amount of effective (tBu)₃P/B(C₆F₅)₃ pairs, and the anion [CTf₃]^- shows negative effects, for which even less effective (tBu)₃P/B(C₆F₅)₃ pairs have been found compared to those of toluene.

Metal-Free Borylation of Heteroarenes Using Ambiphilic Aminoboranes: On the Importance of Sterics in Frustrated Lewis Pair C-H Bond Activation

Two novel frustrated Lewis pair (FLP) aminoboranes, (1-Pip-2-BH₂-C₆H₄)₂ (2; Pip = piperidyl) and (1-NEt₂-2-BH₂-C₆H₄)₂ (3; NEt₂ = diethylamino), were synthesized, and their structural features were elucidated both in solution and in the solid state. The reactivity of these species for the borylation of heteroarenes was investigated and compared to previously reported (1-TMP-2-BH₂-C₆H₄)₂ (1; TMP = tetramethylpiperidyl) and (1-NMe₂-2-BH₂-C₆H₄)₂ (4; NMe₂ = dimethylamino). It was shown that 2 and 3 are more active catalysts for the borylation of heteroarenes than the bulkier analogue 1. Kinetic studies and density functional theory calculations were performed with 1 and 2 to ascertain the influence of the amino group of this FLP-catalyzed transformation. The C-H activation step was found to be more facile with smaller amines at the expense of a more difficult dissociation of the dimeric species. The bench-stable fluoroborate salts of all catalysts (1F-4F) have been synthesized and tested for the borylation reaction. The new precatalysts 2F and 3F are showing higher reaction rates and yields for multigram-scale syntheses.

Replacing C6F5 groups with Cl and H atoms in frustrated Lewis pairs: H2 additions and catalytic hydrogenations

Frustrated Lewis isosteres: 2-aminophenylboranes containing C₆F₅ groups replaced with Cl atoms show similar reactivity with H₂ including catalytic hydrogenations.