Zintl Clusters as a Platform for Lewis Acid Catalysis

Clusters of the main group elements phosphorus and arsenic, commonly categorized as Zintl clusters, have been known for over a century. And, only now is the application of these systems as catalysts for organic synthesis being investigated. In this work, boranes are tethered via an aliphatic linker to Zintl-based clusters and their Lewis acidity is examined experimentally, by the Gutmann-Beckett test and competency in the hydroborative reduction of six organic substrates, as well as computationally, by fluoride ion affinity and hydride ion affinity methods. The effects of tuning the aliphatic linker length, substituents at the boron, and changing the cluster from a seven-atom phosphorus system to a seven-atom arsenic system on reactivity are studied.

Carbodiimide and Isocyanate Hydroboration by a Cyclic Carbodiphosphorane Catalyst

We report hydroboration of carbodiimide and isocyanate substrates catalyzed by a cyclic carbodiphosphorane catalyst. The cyclic carbodiphosphorane outperformed the other Lewis basic carbon species tested, including other zerovalent carbon compounds, phosphorus ylides, an N-heterocyclic carbene, and an N-heterocyclic olefin. Hydroborations of seven carbodiimides and nine isocyanates were performed at room temperature to form N-boryl formamidine and N-boryl formamide products. Intermolecular competition experiments demonstrated the selective hydroboration of alkyl isocyanates over carbodiimide and ketone substrates. DFT calculations support a proposed mechanism involving activation of pinacolborane by the carbodiphosphorane catalyst, followed by hydride transfer and B-N bond formation.

Photophysical properties of four-membered BN3 heterocyclic compounds: theoretical insights

CONTEXT

Understanding the photochemistry of boron nitrogen (BN)-containing compounds is an important aspect to enhance the various optical and electronic applications. In this work, we have explored the structure, bonding, reactivity, electronic absorption (UV-Vis), and light harvesting efficiency (LHE) of a series of BN3 ring and open-chain systems. The frontier molecular orbitals (FMO) analysis found that ring systems have a low HOMO-LUMO energy gap as compared to the open-chain systems which insinuates the feasibility of ring systems in the optoelectronic materials. Also, the molecular electrostatic potential (MEP) maps have been computed to pursue the electrophilic and nucleophilic sites available at the surface of the compound. Interestingly, we have found that the open-chain compounds show more molecular charge distribution range rather than the ring compounds. The investigation of photophysical properties showed that the UV-Vis absorption significantly red-shifted in BN3 ring systems as compared to open-chain counterparts. Furthermore, light harvesting efficiency (LHE) was also found higher in the ring systems as compared to the BN3 open-chain systems. Moreover, the computed structural parameters are found well corroborated with the available X-ray data.

METHODS

Structures of all compounds were optimized by using density functional theory (DFT) method, with M06-2X/6-31G(d,p) level. All the calculations in this work are carried out in Gaussian 16 program package. GaussView6.1 software was used for the modeling of initial geometries and for the plotting of MEP plots. To account the solvent effect on geometries the polarized continuum model (PCM) was used and tetrahydrofuran (THF) taken as solvent. The NBO6.0 program (incorporated in G16 software) was used for the exploration of bonding nature and stabilization energies of B-N bond. The absorption spectra were simulated by using ORCA 4.2 program.

Multifaceted behavior of a doubly reduced arylborane in B-H-bond activation and hydroboration catalysis

Alkali-metal salts of 9,10-dimethyl-9,10-dihydro-9,10-diboraanthrancene (M₂[DBA-Me₂]; M⁺ = Li⁺, Na⁺, K⁺) activate the H-B bond of pinacolborane (HBpin) in THF already at room temperature. For M⁺ = Na⁺, K⁺, the addition products M₂[4] are formed, which contain one new H-B and one new B-Bpin bond; for M⁺ = Li⁺, the H^- ion is instantaneously transferred from the DBA-Me₂ unit to another equivalent of HBpin to afford Li[5]. Although Li[5] might commonly be considered a [Bpin]^- adduct of neutral DBA-Me₂, it donates a [Bpin]⁺ cation to Li[SiPh₃], generating the silyl borane Ph₃Si-Bpin; Li₂[DBA-Me₂] with an aromatic central B₂C₄ ring acts as the leaving group. Furthermore, Li₂[DBA-Me₂] catalyzes the hydroboration of various unsaturated substrates with HBpin in THF. Quantum-chemical calculations complemented by in situ NMR spectroscopy revealed two different mechanistic scenarios that are governed by the steric demand of the substrate used: in the case of the bulky Ph(H)C[double bond, length as m-dash]NtBu, the reaction requires elevated temperatures of 100 °C, starts with H-Bpin activation which subsequently generates Li[BH₄], so that the mechanism eventually turns into "hidden borohydride catalysis". Ph(H)C[double bond, length as m-dash]NPh, Ph₂C[double bond, length as m-dash]O, Ph₂C[double bond, length as m-dash]CH₂, and iPrN[double bond, length as m-dash]C[double bond, length as m-dash]NiPr undergo hydroboration already at room temperature. Here, the active hydroboration catalyst is the [4 + 2] cycloadduct between the respective substrate and Li₂[DBA-Me₂]: in the key step, attack of HBpin on the bridging unit opens the bicyclo[2.2.2]octadiene scaffold and gives the activated HBpin adduct of the Lewis-basic moiety that was previously coordinated to the DBA-B atom.

Simple silver(I)-salt catalyzed selective hydroboration of isocyanates, pyridines, and quinolines

AgSbF₆ has been established as an effective catalyst for the hydroboration of structurally and electronically diverse isocyanates under ligand- and solvent-free conditions which selectively yielded either N-boryl formamides or N-boryl methylamines under different conditions. Further, various N-heterocycles can be selectively hydroborated using this simple catalytic system; pyridine derivatives undergo preferential 1,4 hydroboration whereas the formation of tetrahydroquinoline (after hydrolysis) via complete heterocycle hydrogenation was observed for quinolines.

Ferrocene tethered boramidinate frustrated Lewis pairs: stepwise capture of CO2 and CO

The synthesis and reactivity of novel ferrocene tethered boramidinate frustrated Lewis pairs (FLPs), capable of the sequential capture of small molecules, is reported. Reactions of 1,1'-dicarbodiimidoferrocenes with different boranes provides access to metallocene tethered FLPs. The reactivity of the boramidinate moieties can be tuned by the nature of the carbodiimido substituents (alkyl vs. aryl) and the borane used in the reduction (9-borabicyclo[3.3.1]nonane [(C₈H₁₄)₂BH]₂vs. bis-pentafluorophenyl borane [(C₆F₅)₂BH]₂). The boramidinate FLP arms do not engage in intramolecular reactions, allowing for independent small molecule capture by each FLP. By careful synthetic control, sequential capture of different gaseous small molecules (CO₂ and CO or CO₂ and CNtBu) by the same bis(boramidinate)ferrocene molecule has been demonstrated.

Synthesis of Four-Membered BN3 Heterocycles by the Borylation of Triazenes

Borylated triazenes were synthesized by the dehydrocoupling of triazenes with 9-borabicyclo(3.3.1)nonane, by the condensation of triazenes with BEt₃, or by the reaction of sodium triazenides with dialkyl- or diarylboron halides. The structures of the products were found to depend on the size of the substituents. Sterically demanding mesityl groups at boron or nitrogen gave rise to open-chain structures, whereas smaller substituents led to the formation of novel BN₃ heterocycles.

Zinc Hydride Catalyzed Chemoselective Hydroboration of Isocyanates: Amide Bond Formation and C=O Bond Cleavage

Herein, a remarkable conjugated bis-guanidinate (CBG) supported zinc hydride, [{LZnH}₂ ; L={(ArHN)(ArN)-C=N-C=(NAr)(NHAr); Ar=2,6-Et₂ -C₆ H₃ }] (I) catalyzed partial reduction of heteroallenes via hydroboration is reported. A large number of aryl and alkyl isocyanates, including electron-donating and withdrawing groups, undergo reduction to obtain selectively N-boryl formamide, bis(boryl) hemiaminal and N-boryl methyl amine products. The compound I effectively catalyzes the chemoselective reduction of various isocyanates, in which the construction of the amide bond occurs. Isocyanates undergo a deoxygenation hydroboration reaction, in which the C=O bond cleaves, leading to N-boryl methyl amines. Several functionalities such as nitro, cyano, halide, and alkene groups are well-tolerated. Furthermore, a series of kinetic, control experiments and structurally characterized intermediates suggest that the zinc hydride species are responsible for all reduction steps and breaking the C=O bond.

Iron-catalysed hydroboration of non-activated imines and nitriles: kinetic and mechanistic studies

Iron-catalysed hydroboration of imines and nitriles has been developed under low catalyst loading (1 mol%) in the presence of HBpin. A wide scope of substrate was found to smoothly undergo hydroboration, including electron releasing/withdrawing and halogen substitution patterns and cyclic substrates which all afforded the corresponding amines in good to excellent yields. Dihydroboration of nitriles was achieved conveniently under solvent free and additive free conditions. Promisingly, this catalytic system is also capable of the hydroboration of challenging ketimine substrates. Preliminary kinetic analysis of imine hydroboration reveals a first-order dependence on catalyst concentration. Both HBpin and 4-fluorophenyl-N-phenylmethanimine (1b) appear to exhibit saturation kinetics with first order dependence up to 0.5 mmol HBpin and 0.75 mmol imine, respectively. Temperature-dependent rate experiments for imine hydroboration have also been explored. Activation parameters for the hydroboration of FPhC[double bond, length as m-dash]NPh (1b) were determined from the Eyring and Arrhenius plots with ΔS ≠, ΔH ≠, and E a values of -28.69 (±0.3) e.u., 12.95 (±0.04) kcal mol-1, and 15.22 (±0.09) kcal mol-1, respectively.

Organoaluminum Compounds as Catalysts for Monohydroboration of Carbodiimides

The effective catalytic activity of organoaluminum compounds for the monohydroboration of carbodiimides has been demonstrated. Two aluminum complexes, 2 and 3, were synthesized and characterized. The efficient catalytic performances of four aluminum hydride complexes L1 AlH2 (L1 =HC(CMeNAr)2 , Ar=2,6-Et2 C6 H3 ; 1), L2 AlH2 (NMe3 ) (L2 =o-C6 H4 F(CH=N-Ar), Ar=2,6-Et2 C6 H3 ; 2), L3 AlH (L3 =2,6-bis(1-methylethyl)-N-(2-pyridinylmethylene)phenylamine; 3), and L4 AlH(NMe3 ) (L4 =o-C6 H4 (N-Dipp)(CH=N-Dipp), Dipp=2,6-iPr2 C6 H3 ; 4), and an aluminum alkyl complex L1 AlMe2 (5) were used for the monohydroboration of carbodiimides investigated under solvent-free and mild conditions. Compounds 1-3 and 5 can produce monohydroborated N-borylformamidine, whereas 4 can afford the C-borylformamidine product. A suggested mechanism of this reaction was explored, and the aluminum formamidinate compound 6 was characterized by single-crystal X-ray, also a stoichiometric reaction was investigated.