Recent advances in borenium catalysis

Borenium ions are strong Lewis acids because of the positive charge on boron. While their high reactivity had long restricted their role in organic synthesis to stoichiometric reagents, in the past ten years the introduction of suitable supporting ligands, such as N-heterocyclic carbenes, has enabled them to function as competent catalysts for various organic transformations involving the activation of strong covalent bonds, such as H-H, Si-H, B-H, C-H and C-C bonds. This review provides an overview of the recent advances in borenium-catalysed reactions with emphasis on catalyst synthesis, methodology development and mechanistic insight.

Electrochemical oxidative cyclization of alkenes, boronic acids, and dichalcogenides to access chalcogenated boronic esters and 1,3-diols

A sustainable, environmentally benign electrochemical oxidative three-component cyclization of allylic alcohols, boronic acids, and dichalcogenides under metal-free and oxidant-free conditions has been developed.

A free boratriptycene-type Lewis superacid

An exceptionally strong ferrocene-containing, cationic boratriptycene-type Lewis acid is stabilized by a weak Fe⋯B through-space interaction.

[B-Cl-B]+ Cations: Chloroborane Masked Chiral Borenium Ions

A tricoordinate borenium ion has received considerable attention in recent years for its applications in Lewis acid catalysis. Over the years, asymmetric catalysis mediated by a chiral borenium ion has also been developed. To stabilize the electron-deficient boron atom, a series of chloroborane masked borenium ions featuring the symmetrical [B-Cl-B]⁺ linkage are prepared and utilized as the catalyst for the enantioselective Diels-Alder cycloaddition of cyclopentadiene and 2,2,2-trifluoroethyl acrylate. The presence of a Cp* ligand is critical in realizing the cyclic diboron compounds, and the stability of the resulting [B-Cl-B]⁺ cation is dependent on the steric bulkiness of the oxazolidinone moiety. The stereoselectivity of the Diels-Alder cycloaddition is controlled by the substituents of the chiral oxazolidinone ligand and could be further improved via the coordination of SnCl₄ at the bridging chloride of the [B-Cl-B]⁺ cation.

A strongly Lewis-acidic and fluorescent borenium cation supported by a tridentate formazanate ligand

Lewis acids are highly sought after for their applications in sensing, small-molecule activation, and catalysis. When combined with π-conjugated molecular frameworks, Lewis acids with unique optoelectronic properties can be realized. Here, we use a tridentate formazanate ligand to create a planar, redox-active, fluorescent, and strongly Lewis-acidic borenium cation. We also demonstrate that this compound can act as a colourimetric probe for reactivity.

Hydrosilylation and Mukaiyama aldol-type reaction of quinolines and hydrosilylation of imines catalyzed by a mesoionic carbene-stabilized borenium ion

Aldimines and ketimines containing electron-donating and electron-withdrawing groups can be hydrosilylated with borenium catalysts at as low as 1 mol% catalyst loading at room temperature, providing the corresponding secondary amines in excellent yields. Reactions with 2-phenylquinoline gave the 1,4-hydrosilylquinoline product selectively which can be further functionalized in a one-pot synthesis to give unique γ-amino alcohol derivatives. Control experiments suggest that the borenium ion catalyzes both the hydrosilylation and subsequent addition to the aldehyde.

Palladium(0) complexes of diferrocenylmercury diphosphines: synthesis, X-ray structure analyses, catalytic isomerization, and C-Cl bond activation

Palladium(0) phosphine complexes are of great importance as catalysts in numerous bond formation reactions that involve oxidative addition of substrates. Highly active catalysts with labile ligands are of particular interest but can be challenging to isolate and structurally characterize. We investigate here the synthesis and chemical reactivity of Pd⁰ complexes that contain geometrically adaptable diferrocenylmercury-bridged diphosphine chelate ligands (L) in combination with a labile dibenzylideneacetone (dba) ligand. The diastereomeric diphosphines 1a (pSpR, meso-isomer) and 1b (pSpS-isomer) differ in the orientation of the ferrocene moieties relative to the central Ph₂PC₅H₃-Hg-C₅H₃PPh₂ bridging entity. The structurally distinct trigonal LPd⁰(dba) complexes 2a (meso) and 2b (pSpS) are obtained upon treatment with Pd(dba)₂. A competition reaction reveals that 1b reacts faster than 1a with Pd(dba)₂. Unexpectedly, catalytic interconversion of 1a (meso) into 1b (rac) is observed at room temperature in the presence of only catalytic amounts of Pd(dba)₂. Both Pd⁰ complexes, 2a and 2b, readily undergo oxidative addition into the C-Cl bond of CH₂Cl₂ at moderate temperatures with formation of the square-planar trans-chelate complexes LPd^IICl(CH₂Cl) (3a, 3b). Kinetic studies reveal a significantly higher reaction rate for the meso-isomer 2a in comparison to (pSpS)-2b.

A Cationic NHC-Supported Borole

This work describes the synthesis and characterization of a highly reactive cationic borole. Halide abstraction with Li{Al[OC(CF3 )3 ]4 } from the NHC-chloroborole adduct yields the first stable NHC-supported 1-(Me NHC)-2,5-(SiMe3 )2 -3,4-(Ph*)2 -borole cation. Electronically, it features both a five-membered cyclic conjugated 4 π-electron system and a cationic charge and thus resembles the yet elusive cyclopentadienyl cation. The borole cation was characterized crystallographically, spectroscopically (NMR, UV/Vis), by cyclovoltammetry, microanalysis and mass-spectrometry and its electronic structure was probed computationally. The cation reacts with tolane and reversibly binds carbon monoxide. Direct comparison with the structurally related, yet neutral, 1-mesityl borole reveals strong Lewis acidity, reduced HOMO-LUMO gaps, and increased anti-aromatic character.

Hydrosilylation of Carbonyls Catalyzed by Hydridoborenium Borate Salts: Lewis Acid Activation and Anion Mediated Pathways

The electronically unsaturated three-coordinated hydridoborenium cations [LBH]⁺[HB(C₆F₅)₃]^- (1) and [LBH]⁺[B(C₆F₅)₄]^- (2), supported by a bis(phosphinimino)amide ligand, were found to be excellent catalysts for hydrosilylation of a range of aliphatic and aromatic aldehydes and ketones under mild reaction conditions (L = [{(2,4,6-Me₃C₆H₂N)P(Ph₂)}₂N]). The key steps of the catalytic cycle for hydrosilylation of PhCHO were monitored via in situ multinuclear NMR measurements for catalysts 1 and 2. The combined effect of carbonyl activation via the Lewis acidic hydridoborenium cation and the hydridic nature of the borate counteranion in 1 makes it a more efficient catalyst in comparison to that of carbonyl activation via the predominant Lewis acid activation pathway operating with catalyst 2. The catalytic cycle of 1 showed hydride transfer from the borate moiety [HB(C₆F₅)₃]^- to PhCHO in the first step, forming [PhCH₂-O-B(C₆F₅)₃]^-, which subsequently underwent σ-bond metathesis with Et₃SiH to form the product, PhCH₂-O-SiEt₃. Quantum chemical calculations also support the borate anion mediated mechanism with 1. In contrast, the reaction catalyzed by 2 proceeds predominantly via the Lewis acid activation of the carbonyl group involving [LB(H)←OC(H)Ph]⁺[B(C₆F₅)₄]^- as the transition state and [LBOCH₂Ph]⁺[B(C₆F₅)₄]^- as the intermediate.

Isolation of ligand-centered borocations in molybdenum complexes containing a triaminoborane-bridged diphosphorus ligand

Metal complexes that form isolable, ligand-centered borenium ions (i.e. reactive three-coordinate boron cations) are rare, especially with highly-versatile diphosphorus ligands. Here we report the first structurally-characterized examples of ligand-centered borocations in a class of diphosphorus ligands derived from the bicyclic triaminoborane 1,8,10,9-triazaboradecalin (TBD). Treating (PhTBDPhos)Mo(CO)4 (1) with HOTf or HNTf2 resulted in protonation of the bridgehead α-nitrogen on the TBD backbone and formation of ligand-centered borenium ions (1-HOTf and 1-HNTf2, respectively), whereas reaction of 1 with HBF4·Et2O resulted in protonation of two α-nitrogen atoms and fluoride abstraction from BF4- to form a four-coordinate borocation in [1-H2F][BF4]. Single-crystal XRD data confirmed the formation of the borocations, and multinuclear NMR and IR spectroscopy studies were used to interrogate the electronic environment at molybdenum and boron. The 11B NMR resonances for the borenium ions in 1-HOTf and 1-HNTf2 (δ 29.5 and 29.6 ppm) were more deshielded than the resonance for 1 (δ 25.9 ppm), consistent with the decreased electron density at boron. The 31P NMR data revealed similar trends in response to increasing protonation on the TBD backbone, and aligned well with small, stepwise increases in Mo-CO stretching frequencies that followed the order borane (1) < borenium (1-HOTf and 1-HNTf2) < boronium ([1-H2F][BF4]). Density functional theory calculations conducted on 1, 1-HOTf, and [1-H2F][BF4] revealed subtle changes in boron and nitrogen atomic charges consistent with those calculated for more well-established borenium ions in metal-free systems. Overall, the results confirm previous observations of latent borenium ion reactivity in TBDPhos complexes.

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.

Functional conjugated pyridines via main-group element tuning

The functional properties arising from a combination of main-group elements with pyridine-based organic conjugated scaffolds are highlighted.

Using Ylide Functionalization to Stabilize Boron Cations

The metalated ylide YNa [Y=(Ph3 PCSO2 Tol)- ] was employed as X,L-donor ligand for the preparation of a series of boron cations. Treatment of the bis-ylide functionalized borane Y2 BH with different trityl salts or B(C6 F5 )3 for hydride abstraction readily results in the formation of the bis-ylide functionalized boron cation [Y-B-Y]+ (2). The high donor capacity of the ylide ligands allowed the isolation of the cationic species and its characterization in solution as well as in solid state. DFT calculations demonstrate that the cation is efficiently stabilized through electrostatic effects as well as π-donation from the ylide ligands, which results in its high stability. Despite the high stability of 2 [Y-B-Y]+ serves as viable source for the preparation of further borenium cations of type Y2 B+ ←LB by addition of Lewis bases such as amines and amides. Primary and secondary amines react to tris(amino)boranes via N-H activation across the B-C bond.

Planar-Chiral 1,1'-Diboryl Metallocenes: Diastereoselective Synthesis from Boryl Cyclopentadienides and Spin Density Analysis of a Diborylcobaltocene

The reaction of nonsubstituted alkali metal cyclopentadienides with haloboranes leads to ∼90:10 mixtures of isomeric diene products that can be deprotonated to give simple boryl cyclopentadienides. We extended this transformation to the sterically hindered lithium tert-butylcyclopentadienide 1 using FBMes₂ (Mes = 2,4,6-trimethylphenyl) and ClBCy₂ as electrophiles. The boryl group is selectively introduced in the remote position to minimize steric congestion. The new boryl dienes are obtained as mixtures of isomers, and subsequent deprotonation with MeLi or LiHMDS affords the lithium 1,3-disubstituted cyclopentadienides 5a,b in yields over 95%. Direct assembling of tert-butylated boryl cyclopentadienides with MCl₂ (M = Fe, Co) selectively leads to 1,1'-planar chiral ferrocenes 6a,b and cobaltocene 7. To shed light into the diastereoselective formation of 6a, DFT calculations were performed. The potential energy surface was scrutinized so as to identify and compare its diastereoisomers and conformers. This stereoselectivity is attributed to minimized steric repulsions between the tert-butyl and the BMes₂ groups in the eclipsed conformation of the racemic diastereoisomers. The X-ray structures of boryl diene 2a and diboryl ferrocene 6a are reported. The electronic structure of cobaltocene 7 was analyzed by EPR and DFT calculations. The spin density of this unique open-shell complex is mainly localized on the Co center, but significant spin density is also found on the boron atoms, indicating substantial delocalization of the unpaired electron over the Lewis acid moieties. Consistently, the singly occupied molecular orbital is a combination of a Co-centered 3d orbital with π(BC) orbitals on each CpBMes₂ rings. There is only weak, if any, direct M···B interaction in 6 and 7.

Borocation catalysis

The synthesis, stability and catalytic reactivity of borocations are described in the context of their reaction in frustrated Lewis pair-type processes.

Imidazole-stabilized, electron-deficient boron cations

Highly Lewis acidic borenium cations were prepared and are shown to activate H₂ and effect 1,1-carborations of alkynes.

Reactive p-block cations stabilized by weakly coordinating anions

The chemistry of the p-block elements is a huge playground for fundamental and applied work. With their bonding from electron deficient to hypercoordinate and formally hypervalent, the p-block elements represent an area to find terra incognita. Often, the formation of cations that contain p-block elements as central ingredient is desired, for example to make a compound more Lewis acidic for an application or simply to prove an idea. This review has collected the reactive p-block cations (rPBC) with a comprehensive focus on those that have been published since the year 2000, but including the milestones and key citations of earlier work. We include an overview on the weakly coordinating anions (WCAs) used to stabilize the rPBC and give an overview to WCA selection, ionization strategies for rPBC-formation and finally list the rPBC ordered in their respective group from 13 to 18. However, typical, often more organic ion classes that constitute for example ionic liquids (imidazolium, ammonium, etc.) were omitted, as were those that do not fulfill the - naturally subjective -"reactive"-criterion of the rPBC. As a rule, we only included rPBC with crystal structure and only rarely refer to important cations published without crystal structure. This collection is intended for those who are simply interested what has been done or what is possible, as well as those who seek advice on preparative issues, up to people having a certain application in mind, where the knowledge on the existence of a rPBC that might play a role as an intermediate or active center may be useful.

Diverse reactivity of borenium cations with >N–H compounds

The P-stabilized borenium 1 displays rich reactivity towards >N–H compounds: substitution reaction at a borenium center with Ph₂NH, Lewis adduct formation and subsequent deprotonation with NH₃, B–Mes cleavage and formation of a dicationic species with HNTf₂.

Tautomerization of 2,6-lutidines in the presence of B(C6F5)3 using catecholborane as a precatalyst

The Lewis pair of 2,6-lutidine and B(C₆F₅)₃ can undergo tautomerization to produce enamine B(C₆F₅)₃ adducts when catecholborane is applied as a precatalyst.

Unsolvated Al(C6F5)3: structural features and electronic interaction with ferrocene

Alkyl/aryl ligand exchange between AlEt3 and B(C6F5)3 in hexanes enables the formation and isolation of the unsolvated Al(C6F5)3 as a crystalline solid, the structure of which has been determined by single-crystal X-ray diffraction analysis. Instead of forming the anticipated AlF contacts with the seemingly more accessible meta- and para-F's of -C6F5 groups, two Al(C6F5)3 molecules form a dimeric structure with double AlF interactions between the Al center of one molecule and the ortho-F atom of the -C6F5 group on the other molecule. This mode of interactions is apparently linked to the thermal and shock sensitivity of the unsolvated Al(C6F5)3 in the solid state. To compare with the B(C6F5)3/ferrocene frustrated Lewis pair system, the complexation between Al(C6F5)3 and ferrocene has also been studied, which affords a stable adduct formed through the η(1)-coordination of Al to one of the CCp atoms, similar to the alane-toluene or benzene complex.

Synthesis of a diboryl-N-heterocycle and its conversion to a bidentate cationic Lewis acid

Sequential reaction of 2-lithio-1-methylimidazole with 9-borabicyclo[3.3.1]nonane (9-BBN) dimer and 9-Cl-9-BBN yields diboryl-N-heterocycle C4H5N2(H)(BC8H14)2 (1). Reaction of 1 with I2 results in the net substitution of chelated hydride for a singly boron-bound iodide to produce C4H5N2(I)(BC8H14)2 (2). Conversely, reaction of 1 with [Ph3C][B(C6F5)4] results in the formation of the bidentate cationic Lewis acid [(C4H5N2)(BC8H14)2][B(C6F5)4] (3). Compound 3 catalyzes the hydrogenation of N-benzylidene-tert-butylamine at room-temperature.

Reactivity of Amine/E(C6F5)3 (E = B, Al) Lewis Pairs toward Linear and Cyclic Acrylic Monomers: Hydrogenation vs. Polymerization

This work reveals the contrasting reactivity of amine/E(C6F5)3 (E = B, Al) Lewis pairs toward linear and cyclic acrylic monomers, methyl methacrylate (MMA) and biorenewable γ-methyl-α-methylene-γ-butyrolactone (γMMBL). While mixing of 2,2,6,6-tetramethylpiperidine (TMP) and B(C6F5)3 leads to a frustrated Lewis pair (FLP), Et3N reacts with B(C6F5)3 to form disproportionation products, ammonium hydridoborate ionic pair and iminium zwitterion. On the other hand, the stoichiometric reaction of either TMP or Et3N with Al(C6F5)3 leads to clean formation of a classic Lewis adduct (CLA). Neither TMP nor Et3N, when paired with E(C6F5)3, polymerizes MMA, but the Et3N/2B(C6F5)3 pair promotes transfer hydrogenation of MMA to form methyl isobutyrate. In contrast, the amine/E(C6F5)3 pairs promote rapid polymerization of γMMBL carrying the more reactive exocyclic methylene moiety, achieving full conversion in less than 3 min even at a low catalyst loading of 0.0625 mol %. TMP is more effective than Et3N for the polymerization when paired with either the borane or the alane, while the alane exhibits higher polymerization activity than the borane when paired with Et3N. Overall, the TMP/Al(C6F5)3 system exhibits the highest polymerization activity, achieving a maximum turn-over frequency of 96,000 h-1 at 0.125 mol % of catalyst loading, producing high molecular weight PγMMBL with Mn = 1.29 × 105 g∙mol-1.

A family of N-heterocyclic carbene-stabilized borenium ions for metal-free imine hydrogenation catalysis

Room-temperature metal-free hydrogenation catalysis.

This manuscript probes the steric and electronic attributes that lead to “frustrated Lewis pair” (FLP)-type catalysis of imine hydrogenation by borenium ions. Hydride abstraction from (ItBu)HB(C₆F₅)₂2 prompts intramolecular C–H bond activation to give (CHN)₂(tBu) (CMe₂CH₂)CB(C₆F₅)₂3, defining an upper limit of Lewis acidity for FLP hydrogenation catalysis. A series of seven N-heterocyclic carbene–borane (NHC–borane) adducts ((R′CNR)₂C)(HBC₈H₁₄) (R′ = H, R = dipp 4a, Mes 5a, Me 8a; R = Me R′ = Me 9a, Cl, 10a) and ((HC)₂(NMe)(NR)C)(HBC₈H₁₄) (R = tBu, 6a, Ph 7a) are prepared and converted to corresponding borenium salts. These species are evaluated as catalysts for metal-free imine hydrogenation at room temperature. Systematic tuning of the carbene donor for the hydrogenation of archetypal substrate N-benzylidene-tert-butylamine achieves the highest reported turn-over frequencies for FLP-catalyzed hydrogenation at amongst the lowest reported catalyst loadings. The most active NHC–borenium catalyst of this series, derived from 10a, is readily isolable, crystallographically characterized and shown to be effective in the hydrogenation catalysis of functional group-containing imines and N-heterocycles.