Lewis Acidic Boron Research Articles

How Important Are Radical Mechanisms in Frustrated Lewis Pair Chemistry?

Reaction mechanism is the foundation of understanding chemical reactivity. In this issue of Chem , Stephan and co-workers overturn long-held assumptions regarding the reactivity of frustrated Lewis pairs by providing compelling evidence that radical mechanisms might operate in some cases.

Towards a frustrated Lewis pair-ionic liquid system

An ionic liquid encompassing a bulky tertiary amine was synthesized and used as a dual solvent-reactant for the formation of a frustrated Lewis pair with tris(pentafluorophenyl)borane, forming a pseudo-frustrated Lewis pair-ionic liquid system. The ionic liquid-LA system is prone to self-ionization, i.e. with the formation of iminium and boron hydride species, which are able to form adducts with CO2. Compared to frustrated Lewis pairs operating in organic solvents of low polarity, more forcing conditions are required for the ionic liquid system. Calculations indicate that the anion of the ionic liquid interacts with the Lewis acidic boron center, hindering reactivity, and rationalizing the low reactivity observed.

Frustrated Lewis pair chemistry.

Frustrated Lewis pair (FLP) chemistry, a concept that has emerged in the last decade, presents unique systems capable of metal-free hydrogenation by using simple combinations of cooperative main group element components. This finding is particularly interesting given that hydrogenation is a chemical reaction that is performed on a huge scale worldwide with current technologies relying on metal-based catalysts. Over the last 10 years, the range of FLP catalysts has been probed and expanded. At the same time, the concept has also been exploited to employ simple combinations of main group reagents to activate a range of other small molecules. In some cases, these …

Design and reactions of a carbon Lewis base/boron Lewis acid frustrated Lewis pair

The conjugated dienamine 4 selectively adds Piers' borane [HB(C6F5)2] to give the enamine/borane system 5, which features a boratirane structure by internal enamine carbon Lewis base to boron Lewis acid interaction. Compound 5 behaves as a C/B frustrated Lewis pair and undergoes typical addition reactions to benzaldehyde, several nitriles and to sulfur dioxide.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

BArF3-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 (BArF3) is found to be an excellent catalyst for metal-free hydroboration of imines. In the presence of 1.0 mol % of BArF3, several ketimines and aldimines undergo hydroboration with pinacolborane (HBpin) at room temperature without the aid of an external Lewis base. BArF3 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.

A Stable N-Annulated Perylene-Bridged Bisphenoxyl Diradicaloid and the Corresponding Boron Trifluoride Complex.

Organic π-extended radicals display unique electronic structures, and could be used as promising functional materials. However, design and synthesis of stable radicals are challenging for chemists due to their high reactivity. In this work, we synthesized a stable N-annulated perylene-bridged bisphenoxyl diradicaloid, and its complex with Lewis acid boron trifluoride. Their ground-state geometric and electronic structures were systematically studied by various experimental methods, including X-ray crystallographic analysis, variable-temperature NMR spectroscopy, electron-spin resonance spectroscopy (ESR), and superconducting quantum interference device (SQUID) measurements, supported by density functional theory. Both were observed as open-shell singlet diradicaloids in the ground state. The bisphenoxyl diradicaloid demonstrated string features of strong near-infrared (NIR) absorption, closely packed π-dimer structure in crystals, amphoteric redox behavior with a small HOMO-LUMO energy band gap, and a rather small singlet-triplet gap, whereas the complex showed very different photophysical, electrochemical, and magnetic properties. Our studies provide an efficient method of making stable diradicaloids by Lewis acid/base complexation.

Exploring Multistep Continuous‐Flow Hydrosilylation Reactions Catalyzed by Tris(pentafluorophenyl)borane

AbstractExploring the combination of continuous‐flow processes with the boron Lewis acid catalyzed hydrosilylation of aldehydes and ketones has delivered a robust and generally applicable reaction protocol. Notably this approach permits ready access to high temperatures and pressures and thus allows improved reactivity of substrates that were previously recalcitrant under the traditional approach. Efforts to quench the output from the flow reactor with water showed surprising tolerance leading to the application of continuous‐flow systems in multistep imine formation/hydrosilylation processes to generate the corresponding secondary amines from their aldehyde and aniline precursors.magnified image

Intermolecular Redox-Neutral Amine C-H Functionalization Induced by the Strong Boron Lewis Acid B(C6 F5 )3 in the Frustrated Lewis Pair Regime.

N,N-Dimethylmesitylamine undergoes an intermolecular redox-neutral C-H activation/C-C coupling process upon treatment with dimethyl acetylenedicarboxylate and the strong boron Lewis acid B(C6 F5 )3 . Similarly, N,N-dimethylmesitylamine reacts with two molar equivalents of ethyl acrylate to give the respective unsaturated coupling product with H2 transfer to the acrylic ester to form the ethyl propionate/B(C6 F5 )3 adduct. N,N-Dimethylmesitylamine also undergoes a C-H activation at the benzylic ortho sp3 -carbon atom with dihydrogen formation upon treatment with Piers' borane [HB(C6 F5 )2 ]. The last two reactions of N,N-dimethylmesitylamine were analyzed by DFT calculations.

Base‐Free Palladium‐Catalyzed Hydrodechlorination of Aryl Chlorides with Pinacol Borane

AbstractBase‐free palladium‐catalyzed reactions of aryl chlorides with pinacol borane have been found to give the hydrodechlorinated products while similar reactions of aryl halides with pinacol borane in the presence of a base were reported to result in borylation. The base‐free conditions offer reasonable functional group compatibility. Experimental studies suggest that smooth and selective hydride transfer from pinacol borane to arylpalladium chloride under base‐free conditions is promoted by a strong interaction between the Lewis acidic boron and the chlorine on the palladium.

Ruthenium-Catalyzed Ammonia Borane Dehydrogenation: Mechanism and Utility.

One of the greatest challenges in using H2 as a fuel source is finding a safe, efficient, and inexpensive method for its storage. Ammonia borane (AB) is a solid hydrogen storage material that has garnered attention for its high hydrogen weight density (19.6 wt %) and ease of handling and transport. Hydrogen release from ammonia borane is mediated by either hydrolysis, thus giving borate products that are difficult to rereduce, or direct dehydrogenation. Catalytic AB dehydrogenation has thus been a popular topic in recent years, motivated both by applications in hydrogen storage and main group synthetic chemistry. This Account is a complete description of work from our laboratory in ruthenium-catalyzed ammonia borane dehydrogenation over the last 6 years, beginning with the Shvo catalyst and resulting ultimately in the development of optimized, leading catalysts for efficient hydrogen release. We have studied AB dehydrogenation with Shvo's catalyst extensively and generated a detailed understanding of the role that borazine, a dehydrogenation product, plays in the reaction: it is a poison for both Shvo's catalyst and PEM fuel cells. Through independent syntheses of Shvo derivatives, we found a protective mechanism wherein catalyst deactivation by borazine is prevented by coordination of a ligand that might otherwise be a catalytic poison. These studies showed how a bidentate N-N ligand can transform the Shvo into a more reactive species for AB dehydrogenation that minimizes accumulation of borazine. Simultaneously, we designed novel ruthenium catalysts that contain a Lewis acidic boron to replace the Shvo -OH proton, thus offering more flexibility to optimize hydrogen release and take on more general problems in hydride abstraction. Our scorpionate-ligated ruthenium species (12) is a best-of-class catalyst for homogeneous dehydrogenation of ammonia borane in terms of its extent of hydrogen release (4.6 wt %), air tolerance, and reusability. Moreover, a synthetically simplified ruthenium complex supported by the inexpensive bis(pyrazolyl)borate ligand is a comparably good catalyst for AB dehydrogenation, among other reactions. In this Account, we present a detailed, concise description of how our work with the Shvo system progressed to the development of our very reactive and flexible dual-site boron-ruthenium catalysts.

ChemInform Abstract: Tiffeneau—Demjanov Rearrangement by Using α‐Chloro‐α‐diazoacetate: A Direct Access to α‐Chlorinated Medium‐Size‐Ring Ketones via Ring Enlargement.

α-Chloro(diazo)acetate can be readily used in an intermolecular ring expansion catalyzed by a boron Lewis acid to construct α-chlorinated medium-size cyclic ketones. This intermolecular Tiffeneau–Demjanov reaction proceeds in good yield from easily prepared or commercially available starting materials and allows the concomitant formation of a tetrasubstituted stereocenter.

3-tert-Butyl-Substituted Cyclohexa-1,4-dienes as Isobutane Equivalents in the B(C6 F5 )3 -Catalyzed Transfer Hydro-tert-Butylation of Alkenes.

Cyclohexa-1,4-dienes with a tert-butyl group at C3 are shown to function as isobutane equivalents when activated by the strong boron Lewis acid tris(pentafluorophenyl)borane. The hitherto unprecedented transfer hydro-tert-butylation from one unsaturated hydrocarbon to another is achieved with 1,1-diarylalkenes as substrates, thereby presenting itself as a new way of incorporating tertiary alkyl groups into carbon frameworks. Transient carbocation intermediates give rise to competing reaction pathways that could not be fully suppressed.

A theoretical analysis of substituent electronic effects on phosphine-borane bonds.

Phosphine-borane adducts are a well-known moiety in synthetic and coordination chemistry. These complexes form a dative bond in which the Lewis basic phosphorus atom donates electron density into an empty p-orbital of the Lewis acidic boron atom. However, donation of the phosphorus lone pair is not the only stabilizing interaction, as hyperconjugation and electrostatic interaction also play important roles in bonding. This paper describes a detailed density functional theory level (B3LYP) study completed to determine the impact electron-donating and withdrawing substituents have on phosphine-borane bonds through the investigation of a series of para-substituted PAr3-BH3 and PH3-BAr3 phosphine-borane adducts. Natural bond orbital (NBO) partitioning was used to calculate the distribution of electron density between the phosphine and borane fragments. Extended transition state and natural orbitals for chemical valence (ETS-NOCV) analysis was used to isolate contributions to the overall electronic interaction of the phosphine-borane adducts. Molecular orbital composition and charge donation was calculated using AOMix. The resulting data was correlated with Hammett σ constants.

Boron Lewis Acid Promoted Ruthenium‐Catalyzed Hydrogenation of Amides: An Efficient Approach to Secondary Amines

AbstractThe hydrogenation of amides to amines has been developed by using the catalyst [Ru(H)2(CO)(Triphos)] (Triphos=1,1,1‐tri(diphenylphosphinomethyl)ethane) and catalytic boron Lewis acids such as B(C6F5)3 or BF3⋅Et2O as additives. The reaction provides an efficient method for the preparation of secondary amines from amides in good yields with high selectivity.

Boron Lewis Acid-Catalyzed Hydroboration of Alkenes with Pinacolborane: BArF3 Does What B(C6 F5 )3 Cannot Do!

The transition-metal-free hydroboration of various alkenes with pinacolborane (HBpin) initiated by tris[3,5-bis(trifluoromethyl)phenyl]borane (BArF3 ) is reported. The choice of the boron Lewis acid is crucial as the more prominent boron Lewis acid tris(pentafluorophenyl)borane (B(C6 F5 )3 ) is reluctant to react. Unlike B(C6 F5 )3 , BArF3 is found to engage in substituent redistribution with HBpin, resulting in the formation of ArF Bpin and the electron-deficient diboranes [H2 BArF ]2 and [(ArF )(H)B(μ-H)2 BArF2 ]. These in situ-generated hydroboranes undergo regioselective hydroboration of styrene derivatives as well as aliphatic alkenes with cis diastereoselectivity. Another ligand metathesis of these adducts with HBpin subsequently affords the corresponding HBpin-derived anti-Markovnikov adducts. The reactive hydroboranes are regenerated in this step, thereby closing the catalytic cycle.

ChemInform Abstract: Enantioselective Diels—Alder Reaction Induced by Chiral Supramolecular Lewis Acid Catalysts Based on CN···B and PO···B Coordination Bonds.

Chiral supramolecular boron Lewis acid catalysts were prepared from chiral 3-phosphoryl-1,1′-bi-2-naphthols, (2-cyanophenyl)boronic acids, and tris(pentafluorophenyl)borane, bound through CN···B and PO···B coordination bonds. In particular, the coordinated tris(pentafluorophenyl)boranes increase the Lewis acidity of the active center in the manner of a Lewis acid assisted Lewis acid catalyst system. A possible cavity in these catalysts was highly suitable for several Diels–Alder probe reactions of acroleins with cyclic or acyclic dienes, which gave the corresponding adducts in good to high yields and high enantio­selectivities.

Benzodipyrrole-based Donor-Acceptor-type Boron Complexes as Tunable Near-infrared-Absorbing Materials.

Benzodipyrrole-based donor-acceptor boron complexes were designed and synthesized as near-infrared-absorbing materials. The electron-rich organic framework combined with the Lewis acidic boron co-ordination enabled us to tune the LUMO energy level and the HOMO-LUMO gap (i.e.,the absorption wavelength) by changing the organic acceptor units, the number of boron atoms, and the substituents on the boron atoms.

Base‐Free Palladium‐Catalyzed Borylation of Aryl Chlorides with Diborons

AbstractThe base‐free palladium‐catalyzed borylation of aryl chlorides with diborons was achieved. The base‐free conditions offered acceptable functional group compatibility. Based on experimental and computational studies, it was shown that smooth boryl transfer from the diborons to the arylpalladium chloride was promoted by strong interaction between the Lewis acidic boron and the chlorine atom on palladium.

Rapid Dihydrogen Cleavage by Persistent Nitroxide Radicals under Frustrated Lewis Pair Conditions

Persistent radicals undergo hydrogen atom abstraction reactions with a great variety of substrates, but not with dihydrogen. It has now been found that the TEMPO radical splits dihydrogen under mild conditions in the presence of the strong bulky B(C6 F5 )3 boron Lewis acid. The reaction is thought to proceed by a typical frustrated Lewis pair mechanism with the TEMPO radical acting as the active Lewis base. The reaction was analyzed by DFT, which indicates that no significant spin density on the hydrogen atoms is accumulated along the H2 splitting reaction path.

Recent Advances in Boron-Containing Conjugated Porous Polymers.

Porous polymers, integrating the advantages of porous materials and conventional polymers, have been well developed and exhibited tremendous attention in the fields of material, chemistry and biology. Of these, boron-containing conjugated porous polymers, featuring tunable geometric structures, unique Lewis acid boron centers and very rich physical properties, such as high specific surface, chargeable scaffold, strong photoluminescence and intramolecular charge transfer, have emerged as one of the most promising functional materials for optoelectronics, catalysis and sensing, etc. Furthermore, upon thermal treatment, some of them can be effectively converted to boron-doped porous carbon materials with good electrochemical performance in energy storage and conversion, extensively enlarging the applicable scope of such kinds of polymers. In this review, the synthetic approaches, structure analyses and various applications of the boron-containing conjugated porous polymers reported very recently are summarized.