Tris(pentafluorophenyl)borane and Beyond: Modern Advances in Borylation Chemistry

Triarylborane catalysed N-alkylation of amines with aryl esters

B(C₆F₅)₃ is demonstrated to be an active catalyst for N-alkylation reactions of amine substrates with aryl esters.

Lewis Acid Catalyzed Transfer Hydromethallylation for the Construction of Quaternary Carbon Centers

The design and gram-scale synthesis of a cyclohexa-1,4-diene-based surrogate of isobutene gas is reported. Using the highly electron-deficient Lewis acid B(C6 F5 )3 , application of this surrogate in the hydromethallylation of electron-rich styrene derivatives provided sterically congested quaternary carbon centers. The reaction proceeds by C(sp3 )-C(sp3 ) bond formation at a tertiary carbenium ion that is generated by alkene protonation. The possibility of two concurrent mechanisms is proposed on the basis of mechanistic experiments using a deuterated surrogate.

B(C6F5)3-catalyzed divergent cyanosilylations of chromones dependent on temperature

A B(C₆F₅)₃-catalyzed divergent cyanosilylation of chromones has been successfully realized. A variety of 4-oxochromane-2-carbonitriles were furnished as kinetic products in high yields via 1,4-cyanosilylations. An unexpected C-O bond cyanosilylation was achieved when the temperature was raised to 80 °C, affording 4-oxo-4-(2-hydroxylphenyl)but-2-enenitriles as thermodynamic products in 72-94% yields, which was confirmed by DFT results.

B(C6F5)3-catalyzed dehydrogenative cyclization of N-tosylhydrazones and anilines via a Lewis adduct: a combined experimental and computational investigation

Tris(pentafluorophenyl)borane-catalyzed dehydrogenative-cyclization of N-tosylhydrazones with aromatic amines has been disclosed. This metal-free catalytic protocol is compatible with a range of functional groups to provide both symmetrical and unsymmetrical 3,4,5-triaryl-1,2,4-triazoles. Mechanistic experiments and density functional theory (DFT) studies suggest an initial Lewis adduct formation of N-tosylhydrazone with B(C6F5)3 followed by sequential intermolecular amination of the borane adduct with aniline, intramolecular cyclization and frustrated Lewis pair (FLP)-catalyzed dehydrogenation for the generation of substituted 1,2,4-triazoles.

Functionalized polycarbonates via triphenylborane catalyzed polymerization-hydrosilylation

Triphenylborane catalyzes the copolymerization and terpolymerization of epoxides and CO₂ to yield polycarbonates with excellent dispersity. Via assisted tandem catalysis, these materials could be hydrosilylated in a one-pot fashion yielding modified polymeric materials. Using only a few reagents, materials with glass transition temperatures ranging from 37-110 °C were obtained.

Sterically Unprotected Nucleophilic Boron Cluster Reagents

A cornerstone of modern synthetic chemistry rests on the ability to manipulate the reactivity of a carbon center by rendering it either electrophilic or nucleophilic. However, accessing a similar reactivity spectrum with boron-based reagents has been significantly more challenging. While classical nucleophilic carbon-based reagents normally do not require steric protection, readily accessible, unprotected boron-based nucleophiles have not yet been realized. Herein, we demonstrate that the bench stable closo-hexaborate cluster anion can engage in a nucleophilic substitution reaction with a wide array of organic and main group electrophiles. The resulting molecules containing B‒C bonds can be further converted to tricoordinate boron species widely used in organic synthesis.

Synthesis of a Perfluorinated Phenoxyphosphorane and Conversion to Its Hexacoordinate Anions

We report the synthesis and structural characterization of a neutral P^V Lewis acid, P(OC₆ F₅ )₅ , and salts containing the six-coordinate anions [P(OC₆ F₅ )₅ F]^- and [P(OC₆ F₅ )₆ ]^- . The latter anion exhibits a rare example of F-π_arene interactions in both the solid and the solution phase, which has been quantitatively studied by variable-temperature (VT) NMR spectroscopy. The Lewis acid strength of P(OC₆ F₅ )₅ has been assessed through experimental fluoride ion competition experiments and quantum-chemical calculations of its fluoride ion affinity (FIA) and global electrophilicity index (GEI). Our findings highlight the importance of considering solvent effects in electrophilicity calculations, even when neutral Lewis acids are involved, and show a rare divergence between FIA and GEI trends. The coordinating abilities of the [P(OC₆ F₅ )₆ ]^- and [P(OC₆ F₅ )₅ F]^- anions towards the trityl cation, as a prototypical electrophile, have been assessed.

A New Mode of Chemical Reactivity for Metal-Free Hydrogen Activation by Lewis Acidic Boranes

We herein explore whether tris(aryl)borane Lewis acids are capable of cleaving H2 outside of the usual Lewis acid/base chemistry described by the concept of frustrated Lewis pairs (FLPs). Instead of a Lewis base we use a chemical reductant to generate stable radical anions of two highly hindered boranes: tris(3,5-dinitromesityl)borane and tris(mesityl)borane. NMR spectroscopic characterization reveals that the corresponding borane radical anions activate (cleave) dihydrogen, whilst EPR spectroscopic characterization, supported by computational analysis, reveals the intermediates along the hydrogen activation pathway. This radical-based, redox pathway involves the homolytic cleavage of H2 , in contrast to conventional models of FLP chemistry, which invoke a heterolytic cleavage pathway. This represents a new mode of chemical reactivity for hydrogen activation by borane Lewis acids.

Bulky cationic β-diketiminate magnesium complexes

Cationic β-diketiminate Mg complexes with the bulky tBuBDI ligand and the weakly coordinating anion B(C6F5)4- have been prepared by the reaction of (tBuBDI)MgnBu with [Ph3C]+[B(C6F5)4]-; tBuBDI = CH[C(tBu)N-Dipp]2 and Dipp = 2,6-diisopropylphenyl. Their structures are compared to the previously reported cationic (MeBDI)Mg+ complexes; MeBDI = CH[C(Me)N-Dipp]2. Crystallization of [(tBuBDI)Mg]+[B(C6F5)4]- from chlorobenzene gave a unique (tBuBDI)Mg+·ClC6H5 cation with a rather short MgCl and consequently long C-Cl bond. Crystallization from chlorobenzene/arene solvent mixtures gave (tBuBDI)Mg+·arene complexes (arene = benzene, toluene, m-xylene) but in the presence of mesitylene the chlorobenzene complex was formed. Due to the greater shielding of the metal, none of these complexes display Mg(F5C6)4B- interactions. Crystal structures of the arene complexes show in all cases η2-coordination of the arene ligands. Ring slippage from a more favorable η2-coordination can be explained by the steric bulk of the tBuBDI ligand. The smaller arenes, benzene and toluene, also bind to (tBuBDI)Mg+ in bromobenzene solution. The Lewis acidity of these cationic Mg complexes was determined by the Gutmann-Beckett test. The acceptor number for (tBuBDI)Mg+ (AN = 76.0) is substantially higher than that estimated for (MeBDI)Mg+ (AN = 70.3). Calculation of the atomic NPA charges by DFT shows that the Mg2+ ion in (tBuBDI)Mg+ is slightly more positively charged than the metal in (MeBDI)Mg+, confirming its higher Lewis acidity. The lower benzene complexation energy calculated for (tBuBDI)Mg+versus (MeBDI)Mg+ is due to steric congestion of the metal in the (tBuBDI)Mg+ cation which allows only for Mg(η1)C6H6 instead of Mg(η6)C6H6 bonding. This ring slippage, however, results in a significant polarization of the electron density in the benzene ring, making it susceptible for nucleophilic attack.

Revisiting the Perfluorinated Trityl Cation

Although ultimately not isolable for X-ray structural characterization, the free perfluorinated trityl cation was shown to be observable in neat triflic acid, which represents milder conditions than previous reports of this cation in "magic acid" or oleum. A triflate-bound species could be generated in organic solvents using stoichiometric amounts of triflic acid and was shown to be synthetically viable for hydride abstraction from Et₃ SiH. It was demonstrated that the para-position on the -C₆ F₅ rings is the primary point of attack for decomposition of the cation.

Halogenoborane mediated allene cyclooligomerization

Allene reacts with the strongly electrophilic halogenoboranes XB(C₆F₅)₂ (X: Cl or Br) by forming a mixture of 1,3,5-trimethylenecyclohexane and the stoichiometric halogeno-borated tetramerization products.

The halogenoboranes XB(C₆F₅)₂ (X: Cl, Br) react with allene to give a mixture of the cyclotrimer 1,3,5-trimethylenecyclohexane (1) and the related halogenoborylated cyclotetramerization products 3a and 3b. Alkyl-substituted allenes were catalytically cyclotrimerized metal-free by XB(C₆F₅)₂ (X: Cl, Br) to give cis,trans-2,4,6-trialkyl-1,3,5-trimethylenecyclohexanes under mild conditions.

Substrate-Controlled Direct α-Stereoselective Synthesis of Deoxyglycosides from Glycals Using B(C6F5)3 as Catalyst

B(C₆F₅)₃ enables the metal-free unprecedented substrate-controlled direct α-stereoselective synthesis of deoxyglycosides from glycals. 2,3-Unsaturated α-O-glycoside products are obtained with deactivated glycals at 75 °C in the presence of the catalyst, while 2-deoxyglycosides are formed using activated glycals that bear no leaving group at C-3 at lower temperatures. The reaction proceeds in good to excellent yields via concomitant borane activation of glycal donor and nucleophile acceptor. The method is exemplified with the synthesis of a series of rare and biologically relevant glycoside analogues.

Metallomimetic Chemistry of Boron

The study of main-group molecules that behave and react similarly to transition-metal (TM) complexes has attracted significant interest in recent decades. Most notably, the attractive idea of replacing the all-too-often rare and costly metals from catalysis has motivated efforts to develop main-group-element-mediated reactions. Main-group elements, however, lack the electronic flexibility of TM complexes that arises from combinations of empty and filled d orbitals and that seem ideally suited to bind and activate many substrates. In this review, we look at boron, an element that despite its nonmetal nature, low atomic weight, and relative redox staticity has achieved great milestones in terms of TM-like reactivity. We show how in interelement cooperative systems, diboron molecules, and hypovalent complexes the fifth element can acquire a truly metallomimetic character. As we discuss, this character is powerfully demonstrated by the reactivity of boron-based molecules with H₂, CO, alkynes, alkenes and even with N₂.

Radicals derived from Lewis acid/base pairs

While conventional approaches to stabilizing main group radicals have involved the use of Lewis acids or bases, this tutorial review focuses on new avenues to main group radicals derived from combinations of donor and acceptor molecules.

Carboboration of isocyanates with tris(pentafluorophenyl)borane and evidence for dissociative FLP chemistry of an acid-base pair

We report on the reactivity of isocyanates towards tris(pentafluorophenyl)borane which was found to result in the 1,2-carboboration of the isocyanate C[double bond, length as m-dash]O bond. The resulting six-membered heterocyclic compounds can be rationalized as adducts of imino-boranes with isocyanates and react accordingly, allowing for the dissociative displacement of the trapped isocyanate moiety in a series of exchange reactions. Related carboelementation reactions with the heavier group 13 analogues, Al(C6F5)3 and Ga(C6F5)3, result in a dimerization of the carboboration products to afford eight-membered heterocycles.

Consecutive intermolecular 1,1-carboboration reactions of Me3Si-substituted alkynes with the halogeno-B(C6F5)2 reagents

1-(Trimethylsilyl)propyne 2a reacts with halogenoboranes XB(C₆F₅)₂ (X: Cl, Br) in a 2 : 1 molar ratio under mild conditions by consecutive 1,1-carboboration reactions to give the highly substituted boryldienes 4.

B(C6F5)3-Catalyzed formal (4+1)-annulation of ortho-quinone methides with diazoacetates: access to 2,3-dihydrobenzofurans

A novel B(C₆F₅)₃-catalyzed formal (4+1)-cycloaddition reaction of alkyne-tethered ortho-quinone methides with diazoacetates has been well established.

Catalyst-free and solvent-free hydroboration of ketones

The hydroboration of a wide range of ketones with HBpin under catalyst-free and solvent-free conditions was reported in high yields.

Borylative cyclisation of diynes using BCl3 and borocations

Products from the borylative cyclisation of diynes using BCl₃ is dependent on substituent effects, however, some control of product outcome is achieved using borocations or BCl₃ in the presence of [BCl₄]⁻.

Cationic Complexes of Boron and Aluminum: An Early 21st Century Viewpoint

Boron and aluminum are lighter Group 13 elements, found in daily life commodities, and considered environmentally benign. Nevertheless, they markedly differ in their elemental properties (e.g., metal character, atomic radius). The use of Lewis acidic complexes of boron and aluminum for methods of bond activation and catalysis (e.g., hydrogenation of unsaturated substrates, polymerization of olefins and epoxides) is quickly expanding. The introduction of cationic charge may boost the metalloid-centered Lewis acidity and allow for its fine-tuning particularly with regard to preference for "hard" or "soft" Lewis bases (i.e., substrates). Especially the isolation of low-coordinate cations (number of ligand atoms smaller than four) demands elaborate techniques of thermodynamic and kinetic stabilization (i.e., electronic saturation and steric shielding) by a ligand system. Furthermore, the properties of the solvent and the counteranion must be considered with care. Here, selected examples of boron and aluminum cations are described.

B(C6F5)3-catalyzed Markovnikov addition of indoles to aryl alkynes: an approach toward bis(indolyl)alkanes

The first example of the metal- and solvent-free B(C6F5)3-catalyzed Markovnikov addition of indoles to aryl alkynes was disclosed. Both N-H and N-protected indoles were tolerated, leading to a wide spectrum of versatile bis(indolyl)alkanes in moderate to good yields with high regioselectivities.

Selective Boryl-Anion Migration in a Vinyl sp2 -sp3 Diborane Induced by Soft Borane Lewis Acids

An intramolecular 1,2‐boryl‐anion migration from boron to carbon has been achieved by selective activation of the π system in [(vinyl)B₂Pin₂)]⁻ using “soft” BR₃ electrophiles (BR₃=BPh₃ or 9‐aryl‐BBN). The soft character is key to ensure 1,2‐migration proceeds instead of oxygen coordination/B−O activation. The BR₃‐induced 1,2‐boryl‐anion migration represents a triple borylation of a vinyl Grignard reagent using only B₂Pin₂ and BR₃ and forms differentially protected 1,1,2‐triborylated alkanes. Notably, by increasing the steric bulk at the β position of the vinyl Grignard reagent used to activate B₂Pin₂, 1,2‐boryl‐anion migration can be suppressed in favor of intermolecular {BPin}⁻ transfer to BPh₃, thus enabling simple access to unsymmetrical sp²−sp³ diboranes.

Metal-Free Acetylene Coupling by the (C6 F5 )2 B-X 1,2-Halogenoboration Reaction

(C₆ F₅ )₂ B-halides were conveniently prepared by treatment of (C₆ F₅ )₂ BH with tritylchloride or -bromide, respectively. With cyclopropylacetylene, (C₆ F₅ )₂ BBr underwent sequential cis-1,2-halogenoboration followed by 1,2-carboboration to give the 4-bromo-2,4-dicyclopropylbutadienyl-B(C₆ F₅ )₂ product. It reacted further with additional cyclopropylacetylene to give the linear triene and tetraene products in a metal-free alkyne oligomerization reaction. The pyridine adduct of the initial diene product was characterized by X-ray diffraction. (C₆ F₅ )₂ BCl reacted analogously. Similar (C₆ F₅ )₂ BX induced oligomerization reactions were carried out with two conjugated enynes.

Interfacial Lewis Acid-Base Adduct Formation Probed by Vibrational Spectroscopy

Understanding Lewis pair (LP) interactions at heterogeneous environments is important for controlling surface reactions. We report the formation of interfacial Lewis adducts with tris(pentafluorophenyl)borane as the Lewis acid and 4-mercaptobenzonitrile attached to gold as the Lewis base. We use the nitrile vibrational frequency as a probe of adduct strength, with stronger adducts leading to larger frequencies. The vibrational frequency shifts of the surface adducts were measured via sum frequency generation spectroscopy and compared to the frequency shifts of bulk adducts. Our results show a distinctly smaller frequency shift for the surface adducts compared to the bulk, indicating a weaker Lewis acid-base interaction near the surface. We explore three possible origins of this difference: interfacial frustration, surface electric fields, and electronic energy level alignment. We highlight the relevance of each and note that likely more than one of them affect the observed surface LP interactions.

The global electrophilicity index as a metric for Lewis acidity

The global electrophilicity index, GEI, is used as a general, quantitative and base-independent metric of Lewis acidity. This parameter has been benchmarked against the established fluoride ion affinity, FIA, for a range of 22 neutral and cationic Lewis acids from across the p-block, including boranes, trityl derivatives, phosphonium cations and sulfoxonium cations. As a demonstration of utility, the GEI is used to catalogue a library of fluoroaryl boranes as it provides a rapid tool for the comparison of a series of Lewis acids featuring peripheral substituent variation.

Divergent Elementoboration: 1,3-Haloboration versus 1,1-Carboboration of Propargyl Esters

This work showcases the 1,3-haloboration reaction of alkynes in which boron and chlorine add to propargyl systems in a proposed sequential oxazoliumborate formation with subsequent ring-opening and chloride migration. In addition, the functionalization of these propargyl esters with dimethyl groups in the propargylic position leads to stark differences in reactivity whereby a formal 1,1-carboboration prevails to give the 2,2-dichloro-3,4-dihydrodioxaborinine products as an intramolecular chelate. Density functional theory calculations are used to rationalize the distinct carboboration and haloboration pathways. Significantly, this method represents a metal-free route to highly functionalized compounds in a single step to give structurally complex products.