Boranylium ions

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BH2NH3 molecule, an example of a borenium cation, a type of boranylium ion with three ligands. Bh2nh3.png
BH2NH3 molecule, an example of a borenium cation, a type of boranylium ion with three ligands.

In chemistry, a boranylium ion is an inorganic cation with the chemical formula BR+
2, where R represents a non-specific substituent. Being electron-deficient, boranylium ions form adducts with Lewis bases. Boranylium ions have historical names that depend on the number of coordinated ligands: [1]

Contents

Borenium ions

Various representations of bonding in borenium ions. Borenium ion fig1.svg
Various representations of bonding in borenium ions.

A borenium ion is an inorganic cation with the chemical formula [BR
2L]+
. In this class of molecules, the electron-deficient boron center has two valence electrons involved in sigma bonding with two ligands, while the third ligand is a two-electron donor such that the overall charge of the complex is +1. [1] Depending on the nature of the ligands around the central boron, this positive charge can be localized on the boron center or delocalized across the entire molecule. [3] Borenium ions can be made in a number of different ways and are of interest for applications in organic synthesis and catalysis. [2]

Synthesis

Synthetic methods for preparing borenium ions include halide abstraction, nucleophilic dissociation, and protic addition to aminoboranes.

Halide or hydride abstraction

Synthetic method involving halide abstraction used by Ryschkewitsch and Wiggins to synthesize a borenium ion. Halide abstraction.png
Synthetic method involving halide abstraction used by Ryschkewitsch and Wiggins to synthesize a borenium ion.

Borenium ions can be made from tetracoordinate Lewis acid-base adducts of boron halides. In this method, halide abstraction by a Lewis acid such as AlCl3 results in a borenium cation and AlCl4− anion. [1] [5] The first borenium ion to be isolated and characterized was made by Ryschkewitsch and Wiggins in 1970 using this method. [4] They found that aluminium chloride dissolved in dichloromethane in the presence of the adduct of 4-methylpyridine and BCl3. A positive charge on boron was then inferred from proton NMR spectroscopy.

Mechanism of halide abstraction used by Ryschkewitsch and Wiggins to synthesize the first borenium ion. Borenium ion.svg
Mechanism of halide abstraction used by Ryschkewitsch and Wiggins to synthesize the first borenium ion.

Similar to the halide abstraction method, borenium ions can be made through abstraction of a hydride from a tetracoordinate boron complex. [6]

Nucleophilic dissociation

Example of the use of nucleophilic displacement to make a borenium ion. Nucleophilic displacement.png
Example of the use of nucleophilic displacement to make a borenium ion.

Displacement of a ligand from a neutral tricoordinate boron halide by a neutral donor such as pyridine results in the generation of a borenium cation. [1] For this reaction to yield the desired borenium cation, the ligand must be a good leaving group and the neutral donor must have enough steric bulk that nucleophilic dissociation is favored over Lewis acid-base adduct formation with the neutral BR3 starting material, as demonstrated by competition experiments. [7]

Protic addition to aminoboranes

Formation of a borenium ion by protonation of an aminoborane Protic attack mech.png
Formation of a borenium ion by protonation of an aminoborane

Aminoboranes can be protonated by various acids to make borenium ions. This synthetic method was developed in 1983 by Narula and Noth who used triflic acid to protonate 1,3-dimethyl-2-(dimethylamino)-1,3,2-diazaborolidine; however, they were unable to crystallize and structurally characterize this particular cation. [7]

Protonation of non-Lewis acidic oxazaborolidines results in the generation of borenium ions that can be used as enantioselective Diels–Alder catalysts. These N-protonated borenium species have been characterized by NMR. [8]

Other methods

Borenium ions can also be made through other methods such as the addition of base to a dicoordinate borinium ion or by metathesis with salts with weakly coordinating anions such as Ag[Al[OC(CF3)3]4] or Li[Al[OC(CF3)3]4]. [1] [9]

Structure and electronics

A number of borenium ions have been structurally characterized through x-ray crystallography. The structures of borenium ions generally have two short bonds and one longer bond which is characteristic of a dative bond. The electron-deficient nature of the boron center of many borenium ions has been confirmed by computational and experimental studies. A Natural Population Analysis treatment of many borenium ions show that the boron center does indeed carry a significant positive charge. For example, the BH2NH3+ cation has a natural charge of +0.687 on boron. [10]

Natural Bond Orbital analysis of a series of borenium ions calculated using the M06-2X level of theory and 6-311++G(d,p) basis set as described by Stojanovic and Stojanovic. [10]
Borenium IonNatural Charge on BOccupancy of B 2p Orbital
BH2NH3++0.6870.023
BCl2NH3++0.5660.460
B(CH3)2NH3++1.0870.167
BF2NH3++1.4120.289

Depending on the nature of the ligands around the central boron, this positive charge can be localized on the boron center or delocalized across the entire molecule. In some cases, pi-donating ligands arranged in the plane of the boron's empty p orbital can act to stabilize the electron deficiency of the boron. Density functional theory (DFT) calculations of isolable borenium ions show that the strongly Lewis acidic boron can be stabilized by pi-donation from aromatic substituents such as pyridine. [6]

Contours of deformation density contributions from the pi orbitals of a NHC and BH2 fragment calculated as described by Rezabal and Frison in 2015. The left structure shows loss of electron density; the right structure shows gain of electron density. Bh2nhc nocv.png
Contours of deformation density contributions from the pi orbitals of a NHC and BH2 fragment calculated as described by Rezabal and Frison in 2015. The left structure shows loss of electron density; the right structure shows gain of electron density.

N-heterocyclic carbenes (NHCs) can also be used to stabilize borenium ions through pi-conjugation, albeit acting as weaker pi-donors than neutral N-donors. [12] The interaction energy between a BH2+ fragment and various NHCs has been calculated using the extended transition state method for energy decomposition analysis combined with the natural orbitals for chemical valence (NOCV) theory. This analysis showed a net pi-donating effect of the NHC ligand – in this case, the positive charge is delocalized over the entire pi system rather than localized on the boron. [11]

In other cases the dative ligand has been observed to be twisted out of the BR3 plane due to steric crowding. This nonplanar geometry leads to a reduction in pi-donation to the boron center, making it even more electron-deficient. [1] It has been found that increased localization of charge on the boron increases the Lewis acidity of the borocation. The Gutmann–Beckett method has been used by many researchers in this field to benchmark the Lewis acidities of these cations. [12]

Early crystal structures of borenium cations indicate that the corresponding anion is non-coordinating. [7] Further studies have shown that the reactivity of borocations is highly tied to the identity of its counter ion. In catalytic applications, weakly coordinating anions have allowed for the most active borenium catalysts. A commonly used counter ion for borenium cations is tetrakis(pentafluorophenyl)borate, B(C6F5)4−; however, other counterions such as AlCl4−, halides, and triflate are also possible. [1] [12] The synthetic viability of a borenium ion is often determined by its reactivity relative to its counterion. Halides are often unable to stabilize borenium ions, preferring instead to coordinate to the boron center to make a tetracoordinate species. A systematic evaluation of counterion effects on the synthetic viability of NHC-dicholoroborenium ions was conducted by Muthaiah and coworkers in 2013. [13]

Reactivity and applications

Borenium ions are highly Lewis acidic. Their Lewis acidity is of the boron atom is determined by the electronic and steric effects of its ligands.

Hydrogen activation and FLP chemistry

Catalytic cycle for the hydrogenation of imines facilitated by a NHC-stabilized borenium catalyst. Borenium flp h2 activation.png
Catalytic cycle for the hydrogenation of imines facilitated by a NHC-stabilized borenium catalyst.

N-heterocyclic carbene (NHC)-stabilized borenium ions have been demonstrated to be potent metal-free H2 activation and hydrogenation catalysts. Unlike the neutral boranes typically used in frustrated Lewis pair (FLP) chemistry of this type, borenium ions are inherently electrophilic and do not require electron-withdrawing ligands to perform these small-molecule activations. Because electron-withdrawing substituents can hamper hydride delivery during hydrogenation catalysis, borenium ions can be more potent catalysts than neutral boron species because they are effective hydride donors. Indeed, in 2012, Stephan and coworkers were able to develop a borenium-based FLP system capable of activating H2 stoichiometrically in the presence of phosphine. [14]

In 2015, Devillard et al. synthesized a naphthyl-bridged intramolecular borenium-containing FLP capable of activating H2 with concomitant hydrogenolysis of a mesityl ligand. A second-order perturbation theory analysis of the natural bond orbitals (NBOs) of the intermediate in this reaction involved with H2 activation showed a 281.8 kcal/mol interaction between the sigma bond of H2 and the 2p orbital of the cationic boron. [15]

Borenium ions have also been used catalytically for various hydrogenations. Stephan and coworkers were able to use a borenium ion catalyst to activate H2 catalytically to be used for imine hydrogenation. [14] A similar NHC-stabilized borenium ion was used to catalyze the enantioselective reduction of ketimines. In this example, enantioselectivity was afforded through the use of a chiral NHC ligand. [16]

It has been shown that the steric and electronic properties of the NHC ligand used in these borenium catalysts is of great importance to catalytic activity: NHCs that were too bulky prevented intermolecular hydride delivery and ligands that were highly electron donating weakened the borenium cation's ability to act as a Lewis acid. [12]

Enantioselective catalysis

Borenium ions have been used as metal-free enantioselective catalysts for a number of organic transformations. An early example of such is the Corey–Itsuno reduction. One proposed mechanism for this enantioselective reduction involves the in situ generation of a borenium-like species using BH3 as a Lewis acid. [17]

Further work on borenium ions generated from neutral oxazaborolidines has expanded the scope of their applications. In 2002, it was reported by E. J. Corey and coworkers that N-protonation of non-Lewis acidic oxazaborolidines results in the generation of borenium ions which can catalyze the enantioselective Diels–Alder reaction of 1,3-dienes with 2-methacrolein or 2-bromoacrolein. This particular borenium ion could be made in situ by protonating a neutral oxazaborolidine with triflic acid. Corey and coworkers suggest that the stereoselectivity of this reaction is a result of aldehyde-catalyst association in the pre-transition state which governs stereoselectivity. [8] The use of borenium ions as Diels–Alder catalysts has been further extended to the use of borenium ionic liquids as catalysts for the Diels–Alder reaction by Matuszek et al. in 2017. [18]

Enantioselective Diels-Alder reaction catalyzed by a borenium ion generated in situ by protonation with triflic acid. Borenium catalyzed diels alder.png
Enantioselective Diels–Alder reaction catalyzed by a borenium ion generated in situ by protonation with triflic acid.

Electrophilic aromatic borylation

Borenium ions have also been implicated as intermediates in electrophilic aromatic borylation reactions. [2] In many examples of this reaction, a catalyst is used to activate a borane, producing a highly reactive borenium ion. The formation of this highly electrophilic species drives the formation of the Wheland intermediate, a key step in the electrophilic aromatic addition mechanism. wIn 2013, Stahl et al. used a ruthenium(II) thiolate catalyst to generate borenium ions capable of effecting direct borylation of nitrogen-containing heterocycles. [19]

In 2017, Oestreich and coworkers developed a metal-free method for effecting this transformation. In their work, B(C6H5)3 was used to activate catecholborane, generating a borenium ion capable of borylating various electron-rich heterocycles. [20]

Hydroboration

The electrophilicity of borenium ions can drive the trans-hydroboration of alkynes. In 2016, McGough et al. were able to successfully accomplish metal-free trans-hydroboration with a variety of arylacetylene substrates using a borenium ion electrophile and B(C6F5)3 as a catalyst. [21]

Mechanism of hydroboration with a borenium ion electrophile. Enantioselective hydroboration.png
Mechanism of hydroboration with a borenium ion electrophile.

Polymerization catalysis

Borenium ions have been shown to form ionic liquids capable of catalyzing the polymerization of polyalphaolefins (PAOs). While not yet widely adopted by industry, this technology could provide an alternative to the use of BF3, a toxic and corrosive gas, in the industrial synthesis of PAOs. [22]

Borinium cations

Borinium ions have the formula [BX2]+, [23] where X− is usual a bulky amide (R2N−). They have linear geometry at boron and are coordinatively unsaturated.

Boronium cations

Boronium ions have the formula [L2BR2]+ (L = Lewis base). Boronium ions are tetrahedral and coordinatively saturated.

A well-known example is [(H3N)2BH2]+. Reaction of diborane with ammonia mainly gives [H2B(NH3)2]+ [BH4]− (diammoniodihydroboronium tetrahydroborate). [24] [25]

Other non-classical boron cations are mononuclear boron di- and tri-cations with formula [L3BX]2+ and [L4B]3+, respectively. [26]

Other reported boron cations are dibora-dications (bis(borenium) dications), some examples are depicted below. [27] [28]

Dibora dication.png

Related Research Articles

In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons.

<span class="mw-page-title-main">Corey–Itsuno reduction</span>

The Corey–Itsuno reduction, also known as the Corey–Bakshi–Shibata (CBS) reduction, is a chemical reaction in which a prochiral ketone is enantioselectively reduced to produce the corresponding chiral, non-racemic alcohol. The oxazaborolidine reagent which mediates the enantioselective reduction of ketones was previously developed by the laboratory of Itsuno and thus this transformation may more properly be called the Itsuno-Corey oxazaborolidine reduction.

<span class="mw-page-title-main">Organocatalysis</span> Method in organic chemistry

In organic chemistry, organocatalysis is a form of catalysis in which the rate of a chemical reaction is increased by an organic catalyst. This "organocatalyst" consists of carbon, hydrogen, sulfur and other nonmetal elements found in organic compounds. Because of their similarity in composition and description, they are often mistaken as a misnomer for enzymes due to their comparable effects on reaction rates and forms of catalysis involved.

A carbometallation is any reaction where a carbon-metal bond reacts with a carbon-carbon π-bond to produce a new carbon-carbon σ-bond and a carbon-metal σ-bond. The resulting carbon-metal bond can undergo further carbometallation reactions or it can be reacted with a variety of electrophiles including halogenating reagents, carbonyls, oxygen, and inorganic salts to produce different organometallic reagents. Carbometallations can be performed on alkynes and alkenes to form products with high geometric purity or enantioselectivity, respectively. Some metals prefer to give the anti-addition product with high selectivity and some yield the syn-addition product. The outcome of syn and anti- addition products is determined by the mechanism of the carbometallation.

A frustrated Lewis pair (FLP) is a compound or mixture containing a Lewis acid and a Lewis base that, because of steric hindrance, cannot combine to form a classical adduct. Many kinds of FLPs have been devised, and many simple substrates exhibit activation.

Asymmetric hydrogenation is a chemical reaction that adds two atoms of hydrogen to a target (substrate) molecule with three-dimensional spatial selectivity. Critically, this selectivity does not come from the target molecule itself, but from other reagents or catalysts present in the reaction. This allows spatial information to transfer from one molecule to the target, forming the product as a single enantiomer. The chiral information is most commonly contained in a catalyst and, in this case, the information in a single molecule of catalyst may be transferred to many substrate molecules, amplifying the amount of chiral information present. Similar processes occur in nature, where a chiral molecule like an enzyme can catalyse the introduction of a chiral centre to give a product as a single enantiomer, such as amino acids, that a cell needs to function. By imitating this process, chemists can generate many novel synthetic molecules that interact with biological systems in specific ways, leading to new pharmaceutical agents and agrochemicals. The importance of asymmetric hydrogenation in both academia and industry contributed to two of its pioneers — William Standish Knowles and Ryōji Noyori — being collectively awarded one half of the 2001 Nobel Prize in Chemistry.

Chiral Lewis acids (CLAs) are a type of Lewis acid catalyst. These acids affect the chirality of the substrate as they react with it. In such reactions, synthesis favors the formation of a specific enantiomer or diastereomer. The method is an enantioselective asymmetric synthesis reaction. Since they affect chirality, they produce optically active products from optically inactive or mixed starting materials. This type of preferential formation of one enantiomer or diastereomer over the other is formally known as asymmetric induction. In this kind of Lewis acid, the electron-accepting atom is typically a metal, such as indium, zinc, lithium, aluminium, titanium, or boron. The chiral-altering ligands employed for synthesizing these acids often have multiple Lewis basic sites that allow the formation of a ring structure involving the metal atom.

<span class="mw-page-title-main">Liebeskind–Srogl coupling</span>

The Liebeskind–Srogl coupling reaction is an organic reaction forming a new carbon–carbon bond from a thioester and a boronic acid using a metal catalyst. It is a cross-coupling reaction. This reaction was invented by and named after Jiri Srogl from the Academy of Sciences, Czech Republic, and Lanny S. Liebeskind from Emory University, Atlanta, Georgia, USA. There are three generations of this reaction, with the first generation shown below. The original transformation used catalytic Pd(0), TFP = tris(2-furyl)phosphine as an additional ligand and stoichiometric CuTC = copper(I) thiophene-2-carboxylate as a co-metal catalyst. The overall reaction scheme is shown below.

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In Lewis acid catalysis of organic reactions, a metal-based Lewis acid acts as an electron pair acceptor to increase the reactivity of a substrate. Common Lewis acid catalysts are based on main group metals such as aluminum, boron, silicon, and tin, as well as many early and late d-block metals. The metal atom forms an adduct with a lone-pair bearing electronegative atom in the substrate, such as oxygen, nitrogen, sulfur, and halogens. The complexation has partial charge-transfer character and makes the lone-pair donor effectively more electronegative, activating the substrate toward nucleophilic attack, heterolytic bond cleavage, or cycloaddition with 1,3-dienes and 1,3-dipoles.

<span class="mw-page-title-main">Hydrogen-bond catalysis</span>

Hydrogen-bond catalysis is a type of organocatalysis that relies on use of hydrogen bonding interactions to accelerate and control organic reactions. In biological systems, hydrogen bonding plays a key role in many enzymatic reactions, both in orienting the substrate molecules and lowering barriers to reaction. The field is relatively undeveloped compared to research in Lewis acid catalysis.

Metal-catalyzed C–H borylation reactions are transition metal catalyzed organic reactions that produce an organoboron compound through functionalization of aliphatic and aromatic C–H bonds and are therefore useful reactions for carbon–hydrogen bond activation. Metal-catalyzed C–H borylation reactions utilize transition metals to directly convert a C–H bond into a C–B bond. This route can be advantageous compared to traditional borylation reactions by making use of cheap and abundant hydrocarbon starting material, limiting prefunctionalized organic compounds, reducing toxic byproducts, and streamlining the synthesis of biologically important molecules. Boronic acids, and boronic esters are common boryl groups incorporated into organic molecules through borylation reactions. Boronic acids are trivalent boron-containing organic compounds that possess one alkyl substituent and two hydroxyl groups. Similarly, boronic esters possess one alkyl substituent and two ester groups. Boronic acids and esters are classified depending on the type of carbon group (R) directly bonded to boron, for example alkyl-, alkenyl-, alkynyl-, and aryl-boronic esters. The most common type of starting materials that incorporate boronic esters into organic compounds for transition metal catalyzed borylation reactions have the general formula (RO)2B-B(OR)2. For example, bis(pinacolato)diboron (B2Pin2), and bis(catecholato)diborane (B2Cat2) are common boron sources of this general formula.

<span class="mw-page-title-main">Photoredox catalysis</span>

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β-Carbon elimination is a type of reaction in organometallic chemistry wherein an allyl ligand bonded to a metal center is broken into the corresponding metal-bonded alkyl (aryl) ligand and an alkene. It is a subgroup of elimination reactions. Though less common and less understood than β-hydride elimination, it is an important step involved in some olefin polymerization processes and transition-metal-catalyzed organic reactions.

<span class="mw-page-title-main">Organoberyllium chemistry</span> Organoberyllium Complex in Main Group Chemistry

Organoberyllium chemistry involves the synthesis and properties of organometallic compounds featuring the group 2 alkaline earth metal beryllium (Be). The area remains understudied, relative to the chemistry of other main-group elements, because although metallic beryllium is relatively unreactive, its dust causes berylliosis and compounds are toxic. Organoberyllium compounds are typically prepared by transmetallation or alkylation of beryllium chloride.

<span class="mw-page-title-main">Boraacenes</span> Boron containing acene compounds

Boraacenes are polycyclic aromatic hydrocarbons containing at least one boron atom. Structurally, they are related to acenes, linearly fused benzene rings. However, the boron atom is electron deficient and may act as a Lewis Acid when compared to carbon. This results in slightly less negative charge within the ring, smaller HOMO-LUMO gaps, as well as differences in redox chemistry when compared to their acene analogues. When incorporated into acenes, Boron maintains the planarity and aromaticity of carbon acenes, while adding an empty p-orbital, which can be utilized for the fine tuning of organic semiconductor band gaps. Due to this empty p orbital, however, it is also highly reactive when exposed to nucleophiles like water or normal atmosphere, as it will readily be attacked by oxygen, which must be addressed to maintain its stability.

Organogermanium compounds in cross-coupling reactions refers to a type of cross-coupling reaction where one of the coupling partners is an organogermanium compound. Usually these reactions are catalyzed by transition metal complexes.

References

  1. 1 2 3 4 5 6 7 8 Koelle, P.; Noeth, H. (1985-10-01). "The chemistry of borinium and borenium ions". Chemical Reviews. 85 (5): 399–418. doi:10.1021/cr00069a004. ISSN   0009-2665.
  2. 1 2 3 De Vries, Timothy S.; Prokofjevs, Aleksandrs; Vedejs, Edwin (2012-07-11). "Cationic Tricoordinate Boron Intermediates: Borenium Chemistry from the Organic Perspective". Chemical Reviews. 112 (7): 4246–4282. doi:10.1021/cr200133c. ISSN   0009-2665. PMC   3394883 . PMID   22519545.
  3. ↑ Lee, Kyounghoon; Kirkvold, Clara; Vlaisavljevich, Bess; Daly, Scott R. (2018-11-05). "Ligand-Centered Borenium Reactivity in Triaminoborane-Bridged Diphosphine Complexes". Inorganic Chemistry. 57 (21): 13188–13200. doi:10.1021/acs.inorgchem.8b01601. ISSN   0020-1669. PMID   30351072. S2CID   53036749.
  4. 1 2 3 Ryschkewitsch, George E.; Wiggins, J. W. (1970-03-01). "Trigonal boron cation". Journal of the American Chemical Society. 92 (6): 1790–1791. doi:10.1021/ja00709a079. ISSN   0002-7863.
  5. ↑ Clark, Ewan R.; Ingleson, Michael J. (2013-11-25). "[(acridine)BCl2]+: A Borenium Cation That Is a Strong Boron- and Carbon-Based Lewis Acid". Organometallics. 32 (22): 6712–6717. doi:10.1021/om400463r. ISSN   0276-7333.
  6. 1 2 Bentivegna, BriAnne; Mariani, Christine I.; Smith, Jason R.; Ma, Shuhua; Rheingold, Arnold L.; Brunker, Tim J. (2014-06-09). "Formation, Stability, and Structures of Borenium and Boronium Cations Derived from Pentamethylazaferrocene–Boranes by Hydride or Chloride Abstraction Reactions". Organometallics. 33 (11): 2820–2830. doi:10.1021/om500348u. ISSN   0276-7333.
  7. 1 2 3 Narula, Chaitanya (1984). "Preparation and Characterization of Salts Containing Cations of Tricoordinate Boron". Inorganic Chemistry. 23 (25): 4147–4152. doi:10.1021/ic00193a009.
  8. 1 2 3 Corey, E. J.; Shibata, Takanori; Lee, Thomas W. (2002-04-01). "Asymmetric Diels−Alder Reactions Catalyzed by a Triflic Acid Activated Chiral Oxazaborolidine". Journal of the American Chemical Society. 124 (15): 3808–3809. doi:10.1021/ja025848x. ISSN   0002-7863. PMID   11942799.
  9. ↑ Chen, Jiawei; Lalancette, Roger A.; Jäkle, Frieder (2013-05-02). "Synthesis and Lewis acid properties of a ferrocene-based planar-chiral borenium cation". Chemical Communications. 49 (43): 4893–4895. doi:10.1039/C3CC41556B. ISSN   1364-548X. PMID   23571677.
  10. 1 2 Stojanović, Milovan; Baranac-Stojanović, Marija (2015-09-08). "A theoretical study on borenium ion affinities toward ammonia, formaldehyde and chloride anions". RSC Advances. 5 (93): 75895–75910. Bibcode:2015RSCAd...575895S. doi: 10.1039/C5RA13825F . ISSN   2046-2069.
  11. 1 2 Rezabal, Elixabete; Frison, Gilles (2015). "Estimating π binding energy of N-Heterocyclic carbenes: The role of polarization" (PDF). Journal of Computational Chemistry. 36 (8): 564–572. doi:10.1002/jcc.23852. ISSN   1096-987X. PMID   25708019. S2CID   22398660.
  12. 1 2 3 4 Eisenberger, P.; Crudden, C. M. (2017-04-10). "Borocation catalysis". Dalton Transactions. 46 (15): 4874–4887. doi:10.1039/C6DT04232E. ISSN   1477-9234. PMID   28294211.
  13. ↑ Muthaiah, Senthilkumar; Do, Dinh Cao Huan; Ganguly, Rakesh; Vidović, Dragoslav (2013-11-25). "Counterion Dependence on the Synthetic Viability of NHC-stabilized Dichloroborenium Cations". Organometallics. 32 (22): 6718–6724. doi:10.1021/om400541q. hdl: 10356/98250 . ISSN   0276-7333.
  14. 1 2 3 Farrell, Jeffrey M.; Hatnean, Jillian A.; Stephan, Douglas W. (2012-09-13). "Activation of Hydrogen and Hydrogenation Catalysis by a Borenium Cation". Journal of the American Chemical Society. 134 (38): 15728–15731. doi:10.1021/ja307995f. ISSN   0002-7863. PMID   22931196.
  15. ↑ Devillard, Marc; Brousses, Rémy; Miqueu, Karinne; Bouhadir, Ghenwa; Bourissou, Didier (2015-05-04). "A Stable but Highly Reactive Phosphine‐Coordinated Borenium: Metal‐free Dihydrogen Activation and Alkyne 1,2‐Carboboration". Angewandte Chemie International Edition. 54 (19): 5722–5726. doi:10.1002/anie.201500959. ISSN   1521-3773. PMID   25800957.
  16. ↑ Mercea, Dan M.; Howlett, Michael G.; Piascik, Adam D.; Scott, Daniel J.; Steven, Alan; Ashley, Andrew E.; Fuchter, Matthew J. (2019-06-13). "Enantioselective reduction of N-alkyl ketimines with frustrated Lewis pair catalysis using chiral borenium ions". Chemical Communications. 55 (49): 7077–7080. doi:10.1039/C9CC02900A. hdl: 10044/1/70396 . ISSN   1364-548X. PMID   31149679. S2CID   206141629.
  17. ↑ Corey, E. J.; Bakshi, Raman K.; Shibata, Saizo (1987-09-01). "Highly enantioselective borane reduction of ketones catalyzed by chiral oxazaborolidines. Mechanism and synthetic implications". Journal of the American Chemical Society. 109 (18): 5551–5553. doi:10.1021/ja00252a056. ISSN   0002-7863.
  18. ↑ Matuszek, K.; Coffie, S.; Chrobok, A.; Swadźba-Kwaśny, M. (2017-03-06). "Borenium ionic liquids as catalysts for Diels–Alder reaction: tuneable Lewis superacids for catalytic applications" (PDF). Catalysis Science & Technology. 7 (5): 1045–1049. doi:10.1039/C7CY00106A. ISSN   2044-4761.
  19. ↑ Stahl, Timo; Müther, Kristine; Ohki, Yasuhiro; Tatsumi, Kazuyuki; Oestreich, Martin (2013-07-31). "Catalytic Generation of Borenium Ions by Cooperative B–H Bond Activation: The Elusive Direct Electrophilic Borylation of Nitrogen Heterocycles with Pinacolborane". Journal of the American Chemical Society. 135 (30): 10978–10981. doi:10.1021/ja405925w. ISSN   0002-7863. PMID   23855894.
  20. ↑ Yin, Qin; Klare, Hendrik F. T.; Oestreich, Martin (2017). "Catalytic Friedel–Crafts C−H Borylation of Electron-Rich Arenes: Dramatic Rate Acceleration by Added Alkenes". Angewandte Chemie International Edition. 56 (13): 3712–3717. doi:10.1002/anie.201611536. ISSN   1521-3773. PMID   28244623.
  21. 1 2 McGough, John S.; Butler, Samuel M.; Cade, Ian A.; Ingleson, Michael J. (2016-04-26). "Highly selective catalytic trans-hydroboration of alkynes mediated by borenium cations and B(C6F5)3". Chemical Science. 7 (5): 3384–3389. doi: 10.1039/C5SC04798F . ISSN   2041-6539. PMC   6006955 . PMID   29997833.
  22. ↑ Hogg, James M.; Ferrer-Ugalde, Albert; Coleman, Fergal; Swadźba-Kwaśny, Małgorzata (2019-08-08). "Borenium Ionic Liquids as Alternative to BF3 in Polyalphaolefins (PAOs) Synthesis" (PDF). ACS Sustainable Chemistry & Engineering. 7 (17): 15044–15052. doi:10.1021/acssuschemeng.9b03621. ISSN   2168-0485. S2CID   202069606.
  23. ↑ Shoji, Yoshiaki; Tanaka, Naoki; Mikami, Koichiro; Uchiyama, Masanobu; Fukushima, Takanori (2014-05-11). "A two-coordinate boron cation featuring C–B+–C bonding". Nature Chemistry. 6 (6): 498–503. Bibcode:2014NatCh...6..498S. doi:10.1038/nchem.1948. ISSN   1755-4330. PMID   24848235.
  24. ↑ De Vries, Timothy S.; Prokofjevs, Aleksandrs; Vedejs, Edwin (2012-04-20). "Cationic Tricoordinate Boron Intermediates: Borenium Chemistry from the Organic Perspective". Chemical Reviews. 112 (7): 4246–4282. doi:10.1021/cr200133c. ISSN   0009-2665. PMC   3394883 . PMID   22519545.
  25. ↑ Piers, Warren E.; Bourke, Sara C.; Conroy, Korey D. (2005). "Borinium, Borenium, and Boronium Ions: Synthesis, Reactivity, and Applications". Angewandte Chemie International Edition. 44 (32): 5016–5036. doi:10.1002/anie.200500402. PMID   16086441.
  26. ↑ Vargas-Baca, Ignacio; Findlater, Michael; Powell, Adam; Vasudevan, Kalyan V.; Cowley, Alan H. (2008). "Boron di- and tri-cations". Dalton Transactions (45): 6421–6. doi:10.1039/b810575h. ISSN   1477-9226. PMID   19002329.
  27. ↑ Prokofjevs, Aleksandrs; Kampf, Jeff W.; Solovyev, Andrey; Curran, Dennis P.; Vedejs, Edwin (2013-10-10). "Weakly Stabilized Primary Borenium Cations and Their Dicationic Dimers". Journal of the American Chemical Society. 135 (42): 15686–15689. doi:10.1021/ja407458k. ISSN   0002-7863. PMC   3857331 . PMID   24087933.
  28. ↑ Arnold, Nicole; Braunschweig, Holger; Dewhurst, Rian D.; Hupp, Florian; Radacki, Krzysztof; Trumpp, Alexandra (2016-08-12). "Desymmetrizing Electron-Deficient Diboranes(4): Diverse Products and Their Reactivity". Chemistry - A European Journal. 22 (39): 13927–13934. doi:10.1002/chem.201602805. ISSN   0947-6539. PMID   27514500.
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