A Frustrated and Confused Lewis Pair

Hydro-coupling of isocyanates promoted by 1,2-bis(arylimino)acenaphthene aluminum hydrides

The interaction of aluminum hydrides [(dpp-bian)AlH2] (1) and [(ArBIG-bian)AlH2(THF)] (2) (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene; ArBIG-bian = 1,2-bis[(2,6-dibenzhydryl-4-methylphenyl)imino]acenaphthene) with isocyanates RNCO (R = Ph, Cy, 3,5-Cl2Ph) proceeds via insertion of two molecules of isocyanates into each Al-H bond with the formation of unique Al carboxamides [(Ar-bian)Al{OC(H)N(R)C(NR)O}2] (Ar = dpp, R = Ph, 3; Ar = ArBIG, R = Ph, 4; Ar = ArBIG, R = Cy, 5; Ar = ArBIG, R = 3,5-Cl2C6H3, 6). In contrast, the reactions of 1 and 2 with an excess of tert-butylisocyanate afford formimidate derivatives [(Ar-bian)Al{OC(H)N(tBu)}2] (Ar = dpp, 7; Ar = ArBIG, 8). The reactions of N,N'-dicyclohexylcarbodiimide with 1 and 2 give [(dpp-bian)Al{(NCy)2CH}2] (9) and [(ArBIG-bian)Al(H){(NCy)2CH}] (10), correspondingly. New compounds 3-10 have been characterized by ESR spectroscopy; their molecular structures have been established by single-crystal X-ray analysis.

Preferred Electric Field Mechanism for Frustrated Lewis Pair Reactivity

This study employs computational methods to investigate the mechanism of H2 activation by frustrated Lewis pair (FLP) species, including both intermolecular and intramolecular nitrothane/borane FLP systems. Previous studies have proposed two qualitative reactivity mechanism models to explain the facile cleavage of H2 by FLPs. The findings of this study support the electric field mechanism as the favorable pathway for H2 cleavage. Utilizing frontier molecular orbital theory and energy decomposition analysis, the study explores the electronic structure and nature of the reactions under an external electric field (EEF). Analysis using the activation strain model highlights the significant influence of geometrical deformation energies of FLPs on the activation barriers of H2 activation reactions. Computational results suggest that H2 activation by FLP molecules follows the electric field mechanism, indicating the potential of the FLP/EEF combination as an effective activator for inert molecules.

Dinitrogen Activation within Frustrated Lewis Pairs Is Promoted by Adding External Electric Fields

This study uses computational means to explore the feasibility of N2 cleavage by frustrated Lewis pair (FLPs) species. The employed FLP systems are phosphane/borane (1) and carbene/borane (2). Previous studies show that 1 and 2 react with H2 and CO2 but do not activate N2. The present study demonstrates that N2 is indeed inert, and its activation requires augmentation of the FLPs by an external tool. As we demonstrate here, FLP-mediated N2 activation can be achieved by an external electric field oriented along the reaction axis of the FLP. Additionally, the study demonstrates that FLP -N2 activation generates useful nitrogen compound, e.g., hydrazine (H2N-NH2). In summary, we conclude that FLP effectively activates N2 in tandem with oriented external electric fields (OEEFs), which play a crucial role. This FLP/OEEF combination may serve as a general activator of inert molecules.

Controlled reduction of isocyanates to formamides using monomeric magnesium

This work describes a transition metal-free methodology involving an efficient and controlled reduction of isocyanates to only formamide derivatives using pinacolborane (HBpin) as the hydrogenating agent and a bis(phosphino)carbazole ligand stabilized magnesium methyl complex (1) as the catalyst. A large number of substrates undergo selective hydroboration and give exclusively N-boryl formamides.

Hydroboration of isocyanates: cobalt-catalyzed vs. catalyst-free approaches

Hydroboration of isocyanates with HBPin was demonstrated using both catalytic and catalyst-free approaches. In arene solvents, the reactions employed the commercially available and bench-stable Co(acac)₂/dpephos (dpephos = bis[(2-diphenylphosphino)phenyl] ether) pre-catalyst and proved chemodivergent, showing the formation of either formamides or N-methylamines, depending on the concentration of HBPin and the reaction conditions used. Catalytic monohydroboration of isocyanates to formamides was found to be highly chemoselective, tolerating alkenes, alkynes, aryl halides, esters, carboxamides, nitriles, nitroarenes and heteroaromatic functionalities. The catalyst-free hydroboration reactions have been demonstrated in neat HBPin. Whereas monohydroboration proved less selective compared with Co(acac)₂/dpephos-catalyzed transformations, selective deoxygenative hydroboration of isocyanates to N-methylamines was observed under catalyst-free conditions.

Predicting dinitrogen activation by borenium and borinium cations

The activation of thermodynamically stable and kinetically inert dinitrogen (N₂) has been a great challenge due to the significant strength of the triple bond. Recently, in an experimental study on N₂ activation by boron species, a highly reactive two-coordinated borylene broke through the limitations of traditional strategies of N₂ activation by metal species. Still, studies on metal-free N₂ activation remain underdeveloped. Here, we systematically investigate a frustrated Lewis pair (FLP) combining carbene and borenium (or borinium) cations to screen potential candidates for N₂ activation via density functional theory calculations. As a result, we found that two FLPs (closed form FLP, borenium and open form FLP, borinium) are able to activate N₂ in a thermodynamically and kinetically favorable manner, with a low energy barrier of 9.6 and 7.3 kcal mol^-1, respectively. Furthermore, aromaticity was found to play an important role in the stabilization of the products, supported by nucleus-independent chemical shift (NICS), anisotropy of the current-induced density (ACID) and electron density of delocalized bonds (EDDB) analysis. Our findings provide an alternative approach for metal-free N₂ activation, highlighting the importance of FLP chemistry and aromaticity in N₂ activation.

Reported Catalytic Hydrofunctionalizations that Proceed in the Absence of Catalysts: The Importance of Control Experiments

The enlarged landscape of catalysis lies in the heart of chemistry. As the journey has set a milestone in organic synthesis, its darker side has not entered into the limelight. Studies disclose that the reported reactions by using catalysts were also attainable in the absence of catalysts in many cases. This article presents a literature collection that includes the significance of control experiments in hydrofunctionalization reactions. Systematic analysis reveals that the catalysts are ambiguous and might be unessential in chemical reactions enlisted here.

On 1,3-phosphaazaallenes and their diverse reactivity

1,3-Phosphaazaallenes are heteroallenes of the type RP Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CNR′ and little is known about their reactivity. In here we describe the straightforward synthesis of ArPCNR (Ar = Mes*, 2,4,6-tBu-C₆H₂; ^MesTer, 2.6-(2,4,6-Me₃C₆H₂)–C₆H₃; ^DipTer, 2.6-(2,6-iPr₂C₆H₂)–C₆H₃; R = tBu; Xyl, 2,6-Me₂C₆H₃) starting from phospha-Wittig reagents ArPPMe₃ and isonitriles CNR. It is further shown that ArPCNtBu are thermally labile with respect to the loss of iso-butene and it is shown that the cyanophosphines ArP(H)CN are synthetically feasible and form the corresponding phosphanitrilium borates with B(C₆F₅)₃, whereas deprotonation of ^DipTerP(H)CN was shown to give an isolable cyanidophosphide. Lastly, the reactivity of ArPCNR towards Pier's borane was investigated, showing hydroboration of the CN bond in Mes*PCNtBu to give a hetero-butadiene, while with ^DipTerPCNXyl the formation of the Lewis acid–base adduct with a B–P linkage was observed.

The combination of phospha-Wittig reagents with isonitriles affords 1,3-phosphaazaallenes and their diverse reactivity has been studied in detail.

B(C6F5)3- and HB(C6F5)2-mediated transformations of isothiocyanates

This contribution reports on the reactivity of isothiocyanates towards the boranes B(C6F5)3 and HB(C6F5)2. The reactions of alkyl-substituted isothiocyanates with B(C6F5)3 were found to result in rearrangement reactions to yield stable thiocyanate-B(C6F5)3 adducts. Treatment of isothiocyanates with HB(C6F5)2 leads to 1,2-hydroboration and thus, B,N,C,S heterocycles are formed, which react further under non-inert conditions. Hydrolysis of the hydroboration products leads to a new access to thioformamides.

Investigation of interactions in Lewis pairs between phosphines and boranes by analyzing crystal structures from the Cambridge Structural Database

The interactions between phosphines and boranes in crystal structures have been investigated by analyzing data from the Cambridge Structural Database (CSD). The interactions between phosphines and boranes were classified into three types; two types depend on groups on the boron atom, whereas the third one involves frustrated Lewis pairs (FLPs). The data enabled geometric parameters in structures to be compared with phosphine-borane FLPs with classical Lewis pairs. Most of the crystal structures (78.1%) contain BH₃ as the borane group. In these systems, the boron-phosphorus distance is shorter than systems where the boron atom is surrounded by groups other than hydrogen atoms. The analysis of the CSD data has shown that FLPs have a tendency for the longest boron-phosphorus distance among all phosphine-borane pairs, as well as different other geometrical parameters. The results show that most of the frustrated phosphine-borane pairs found in crystal structures are bridged ones. The minority of non-bridged FLP structures contain, beside phosphorus and boron atoms, other heteroatoms (O, N, S for instance).

Reaction of B2 (o-tol)4 with CO and Isocyanides: Cleavage of the C≡O Triple Bond and Direct C-H Borylations

The reaction of highly Lewis acidic tetra(o-tolyl)diborane(4) with CO afforded a mixture of boraindane and boroxine by the cleavage of the C≡O triple bond. ¹³ C labeling experiments confirmed that the carbon atom in the boraindane stems from CO. Simultaneously, formation of boroxine 3 could be considered as borylene transfer to capture the oxygen atom from CO. The reaction of diborane(4) with ^t Bu-NC afforded an azaallene, while the reaction with Xyl-NC furnished cyclic compounds by direct C-H borylations.

Insertion reactions of small unsaturated molecules in the N-B bonds of boron guanidinates

We report here 1,1- and 1,2-insertion reactions of small unsaturated molecules in the N-B bonds of two boron guanidinates, (Me₂N)C(NⁱPr)₂BCy₂ (1) and {ⁱPr(H)N}C(NⁱPr){N(p-^tBu-C₆H₄)}BCy₂ (2), and two bisboron guanidinates(2-), {ⁱPr(BCy₂)N}C(NⁱPr){N(p-^tBu-C₆H₄)}BCy₂ (3) and {ⁱPr(C₈H₁₄B)N}C(NⁱPr){N(p-Me-C₆H₄)}BC₈H₁₄ (4), the latter being prepared for the first time by double deprotonation of the corresponding guanidine with the 9-borabicyclo[3.3.1]nonane dimer, (H-BC₈H₁₄)₂. Compounds 1-4 easily insert aromatic isonitriles, XylNC (Xyl = 2,6-Me₂-C₆H₃) and (p-MeO-C₆H₄)NC, to give the expected diazaboroles 5-12, some of them being structurally characterised by X-ray diffraction. Interestingly, the BC₈H₁₄ derivatives 11 and 12 are in a fast temperature-dependent equilibrium with the de-insertion products, whose thermodynamic parameters are reported here. A correlation between these equilibria and the puckered heterocyclic structure found in the solid state for 11, and confirmed by DFT calculations, is also established. Reactions of the aforementioned guanidinates with CO are more sluggish or even precluded, and only one product, {ⁱPr(H)N}C{N(p-^tBu-C₆H₄)}(NⁱPr)(CO)BCy₂ (13), could be isolated in moderate yields. The 1,2-insertions of benzaldehyde in compounds 1, 2 and 4 are reversible reactions in all cases, and only one of the insertion products, {ⁱPr(H)N}C{N(p-^tBu-C₆H₄)}(NⁱPr)(PhHCO)BCy₂ (16a), was isolated and diffractrometrically characterised. Likewise, CO₂ reversibly inserts into a N-B bond of 2 to give {ⁱPr(H)N}C{N(p-^tBu-C₆H₄)}(NⁱPr)(CO₂)BCy₂ (19) with a conversion of ca. 9%. In all these equilibria, de-insertion is always favoured upon increasing the temperature.

Hydride oxidation from a titanium-aluminum bimetallic complex: insertion, thermal and electrochemical reactivity

We report the synthesis and reactivity of paramagnetic heterometallic complexes containing a Ti(iii)-μ-H-Al(iii) moiety. Combining different stoichiometries amounts of Cp2TiCl and KH3AlC(TMS)3 (Cp = cyclopentadienyl, TMS = trimethylsilyl) resulted in the formation of either bimetallic Cp2Ti(μ-H)2(H)AlC(TMS)3 (2) or trimetallic (Cp2Ti)2(μ-H)3(H)AlC(TMS)3 (3) via salt metathesis pathways. While these complexes were indefinitely stable at room temperature, the bridging hydrides were readily activated upon exposure to heteroallenes, heating, or electrochemical oxidation. In each case, formal hydride oxidation occurred, but the isolated product maintained the +3 oxidation state at both metal centers. The nature of this reactivity was explored using deuterium labelling experiments and Density Functional Theory (DFT) calculations. It was found that while C-H activation from the Ti(iii) bimetallic may occur through a σ-bond metathesis pathway, chemical oxidation to Ti(iv) promotes bimolecular reductive elimination of dihydrogen to form a Ti(iii) product.

The phosphinoboration of carbodiimides, isocyanates, isothiocyanates and CO2

The transition metal-free addition of phosphinoboronate ester Ph₂PBpin (pin = 1,2-O₂C₂Me₄) to heterocumulenes including carbodiimides, isocyanates, isothiocyanates and carbon dioxide proceeds with remarkable selectivity to give products in high yield.