Recent Trends in Triarylborane Chemistry: Diversification of Structures and Reactivity via meta-Substitution of the Aryl Groups

Abstract

This Short Review summarizes the synthesis and applications of triarylboranes (BAr₃), including both homoleptic and heteroleptic species, with a focus on the modification of their electronic and structural properties via the introduction of meta-substituents with respect to the B atoms to their Ar groups. This approach constitutes a complementary alternative to conventional strategies for the design of BAr₃, which are usually based on a modification of their ortho- and/or para-substituents. An initial analysis revealed that CH₃ and F are the most common meta-substituents in hitherto reported BAr₃ (apart from the H atom). Thus, an extensive exploration of other substituents, e.g., heavier halogens, longer or functionalized alkyl groups, and aryl groups, will increase our knowledge of the structure and reactivity of BAr₃ and eventually lead to a range of new applications.

1 Introduction

2 Scope of this Review

2.1 The Electronic and Steric Influence of meta-Substituents

2.2 Molecular Transformations Mediated by meta-Substituted Boranes

2.3 Other Examples of meta-Functionalization of BAr₃

3 Conclusions and Perspectives

Key words

boranes - Lewis acids - catalysis - main group elements

1

Introduction

Yoichi Hoshimoto received his M.Sc. and Ph.D. from Osaka University in 2013. He then joined the Frontier Research Base for Global Young Researchers, Osaka University as a tenure-track assistant professor (2013–2018). Subsequently, he was promoted to associate professor in the Department of Applied Chemistry, Faculty of Engineering, Osaka University (2018). Since 2023, he has been recognized as an Outstanding Young Researcher in the Center for Future Innovation (CFi), Faculty of Engineering, Osaka University. He is one of the Thieme Chemistry Journals Awardees 2024.

Triarylboranes (BAr₃), including both homoleptic and heteroleptic species, are typical Lewis acids that are widely used as catalysts, activators, sensors, and bio-imaging agents.[1] In the field of main group catalysis in particular, recent progress in the field of frustrated Lewis pairs (FLPs)[2] has led to a significant structural diversification of halogenated triarylboranes beyond the archetypical B(C₆F₅)₃. This structural diversification can be achieved by the introduction of substituents to the Ar groups at the ortho-, meta-, and/or para-positions, with respect to the boron center, as part of a strategy to control the Lewis acidity of BAr₃.[3] [4] These strategies to control the Lewis acidity focus on regulating the accessibility (a kinetic aspect) and energy (a thermodynamic aspect) of the empty p orbital at the boron center.

Strategies that substitute meta-F and/or para-F atoms in B(C₆F₅)₃ with more or less electron-withdrawing substituents have been applied to prepare more or less electrophilic BAr₃ derivatives through regulation of the electron affinity at the boron center.[1a] [b] [3] [5] Strategies that regulate the steric repulsion between a Lewis base (LB) counterpart (front strain; Figure [1], left) through modulation of the size of the ortho-substituents have also been widely explored.[6] Alternatively, the Lewis acidity of BAr₃ can be modulated by regulating the intramolecular repulsion between the Ar groups of tetrahedral LB–borane adducts (back strain; Figure [1], right). In this context, the classical concept of back strain refers to the repulsion between ortho-substituents.[7] Moreover, Hoshimoto and co-workers recently discussed the concept of ‘remote’ back strain based on the repulsion and supportive non-covalent interactions (NCIs) between meta-substituents themselves and/or between meta-substituents and substituents in the LBs.[8] [9] [10] During our efforts to develop an effective method to finely tune the (catalytic) reactivity of BAr₃, we discovered that only a limited number of substituents had been introduced at the meta-positions for the derivatization of BAr₃.

Figure 1 A schematic illustration of front strain and (remote) back strain generated between BAr₃ and Lewis bases (LBs).

Hence, a review that summarizes the structure and use of meta-substituted BAr₃ will be a worthwhile addition to previously reported reviews that have predominantly focused on the derivatization of BAr₃ via substitution at the ortho- and para-positions.[1a] [b] Moreover, we previously confirmed that over 80% of ortho- and meta-substituents in BAr₃, along with 50% of para-substituents, consist of H, F, and CH₃ groups, based on our analysis of the 98 homoleptic BAr₃ compounds synthesized up to and including 2020 (as found using SciFinder in February 2024) (Figure [2]). In particular, F or CH₃ ortho-disubstituted compounds were found to have been frequently explored for the kinetic protection of the vacant p orbital on boron by regulation of the front strain. Conversely, the impact of meta-disubstitution on the Lewis acidity of BAr₃ compounds has been less well studied, probably due to the limited number of synthetic routes to meta-F₂- or meta-(CH₃)₂-substituted species.[9] [11] This Short Review thus aims to summarize the structures and applications of meta-substituted homoleptic and heteroleptic BAr₃ species and to shed light on the importance of such structural modifications in the context of regulating the reactivity of BAr₃ compounds.

Figure 2 Analysis of the substituents introduced at the ortho-, meta-, and para-positions in 98 homoleptic BAr₃ species (as found using SciFinder in February 2024); the relative ratio of each substituent type is given, e.g., for a BAr₃ including a 2,4,6-trimethyl group, two ortho- and one para-CH₃ groups are counted.

2

Scope of this Review

This Short Review analyzes both homoleptic and heteroleptic BAr₃ compounds whose synthesis had been reported up until February 2024. BAr₃ species that have merely been explored theoretically are not included. In addition, boranes that have meta-H, -F, and -CH₃ substituents are beyond the scope of this Short Review , even though the corresponding homoleptic species were considered when Figure [2] was prepared, to avoid significant overlap with previous reports.[1a] [b] Furthermore, several BAr₃ compounds with 2,6-dimethylaryl groups (e.g., mesityl groups) and ortho-bridged planar structures have also been omitted, given that these compounds have already been summarized in other critical reviews.[1c] ^, [12] [13] [14]

2.1

The Electronic and Steric Influence of meta-Substituents

A fundamental and highly effective approach to modulate the Lewis acidity of BAr₃ is to regulate the energy levels of the unoccupied p orbital on the boron atom, i.e., modulating the intrinsic electrophilicity. Unsurprisingly, chemists have explored the introduction of (strongly) electron-donating or -withdrawing substituents at the meta- and/or para-positions of the Ar groups. The introduction of the strongly electron-withdrawing CF₃ group in 3,5-(CF₃)₂C₆H₃ (Ar^F) has stimulated the curiosity of many chemists.[15] [16] [17] [18] [19] [20] In 2012, Ashley and co-workers demonstrated the synthesis of BAr^F ₃ (B¹ ) on a practical scale via a reaction between a Grignard­ reagent including the Ar^F ligand and BF₃·OEt₂.[15a] As expected, the Lewis acidity of B¹ was confirmed experimentally, using the Gutmann–Beckett method and Et₃P=O as a probe for a ³¹P NMR analysis, to be higher than that of B(C₆F₅)₃. Conversely, when trans-crotonaldehyde was employed as a probe for the ¹H NMR analysis (Child’s method), B¹ was found to exhibit lower Lewis acidity than B(C₆F₅)₃. Ashley and co-workers also explored the reactivity of an FLP comprised of B¹ and 2,2,6,6-tetramethylpiperidine (TMP) with H₂, which afforded the salt [HTMP][μ-H(B¹ )₂] (Scheme [1a]).[15a] In contrast, Blagg, Lawrence, and Wildgoose reported that the heterolytic cleavage of H₂ did not occur when FLPs of TMP and B(2,5-(CF₃)₂C₆H₃)₃ (B² ) or B(2,4-(CF₃)₂C₆H₃)₃ (B³ ) were employed.[17a] The authors also rationalized the increased electrophilicity of B¹ relative to B² or B³ based on the analysis of the formal reduction potentials of all three compounds. Using B¹ in another catalytic system, Gagné and co-workers showed that different products were obtained when a specific combination of B¹ /EtMe₂SiH or B(C₆F₅)₃/Et₂MeSiH was used in the catalytic reduction of natamycin (Scheme [1b]).[18`] [b] [c] [d]

Scheme 1 (a) Heterolytic cleavage of H₂ via the combination of TMP and B¹ –B³ , with their formal reduction potentials (vs. [FeCp₂]^0/+ [V]) and (b) the reduction of a bioactive compound using B¹ or B(C₆F₅)₃.

Pápai, Soós, and co-workers also investigated the electronic effects of meta-substituents through a comparison of the Lewis acidity of a series of heteroleptic BAr₃ compounds (B⁴ –B¹⁵ ) based on their hydride ion affinity (HIA) and the Gutmann–Beckett method (Figure [3]).[11] These authors showed that the replacement of the meta-H atom with an F atom significantly enhances the Lewis acidity, whereas a replacement of a meta-Cl atom in either the Mes or 2,6-Cl₂-aryl moieties results in negligible changes.

Figure 3 Comparison of the theoretical HIA (kcal mol^–1) and relative Lewis acidity (%LA) of B⁴ –B¹⁵ ; %LA values were determined using the Gutmann–Beckett method with Et₃P=O and are calculated with respect to B(C₆F₅)₃ (%LA = 100).

The steric effects imparted by meta-substituents have been examined from three main perspectives: (1) the buttressing effect; (2) London dispersion forces; and (3) remote back strain as a sum of the electronic/steric repulsion and NCIs. For example, Wada and co-workers synthesized B(3-Br-2,6-(MeO)₂C₆H₂)₃ (B¹⁶ ) through the direct bromination of air-stable B(2,6-(MeO)₂C₆H₂)₃ with N-bromosuccinimide (NBS) (Scheme [2]).[21] While B(2,6-(MeO)₂C₆H₂)₃ forms isolable adducts with primary amines and ammonia, B¹⁶ does not form such adducts with the same amines. The authors attributed this reactivity difference to the buttressing effect caused by the meta-Br atoms, i.e., the Br atoms push the adjacent MeO groups closer to the boron atom thus hindering access of the amines to the boron center. The electron-withdrawing nature of the Br atoms was not considered in this case.

Scheme 2 Synthesis of B¹⁶ .

Slootweg and co-workers recently proposed that London dispersion forces play a critical role in stabilizing encounter complexes formed between BAr₃ and LBs (Scheme [3a]).[22a] The authors observed the formation of 1:1 co-crystals comprised of B(3,5- ^t Bu₂C₆H₃)₃ (B¹⁷ ) and N(3,5- ^t Bu₂C₆H₃)₃. Based on theoretical calculations, the authors concluded that interaction energies between the meta- ^t Bu groups in B¹⁷ and those in N(3,5- ^t Bu₂C₆H₃)₃ are significantly larger than the corresponding energies formed between B¹ and N(3,5- ^t Bu₂C₆H₃)₃. Hansen, Paradies, and co-workers further studied the importance of dispersion forces based on a combined experimental and theoretical approach.[22b] These authors expanded the discussion to include the B(3,5-R₂C₆H₃)₃ (R = ^t Bu, B¹⁷ ; Me, B¹⁸ ) and P(3,5-R′₂C₆H₃)₃ (R′ = ^t Bu, ⁱ Pr, Me) pairs and concluded that the Lewis acid/base adducts generally become more stable as the size of the dispersion-energy donor increases, albeit that their stability is sensitive to the solvation conditions (Scheme [3b]). In this study, the reactivity of B(3,5- ⁱ Pr₂C₆H₃)₃ was not explored due to difficulties associated with synthesis and stability.

Scheme 3 London dispersion energies stabilize (a) the amine-borane encounter complex involving B¹⁷ , and (b) the phosphine-borane adducts involving B¹⁷ and B¹⁸ ; the reported P–B bond lengths (Å) and association energies of the Lewis pairs (ΔG _exp in kcal mol^–1) are shown.

Scheme 4 Comparison of theoretical parameters for the formation of Et₃P=O–B ⁿ (n = 19–21) adducts, i.e. relative Gibbs energies (ΔG° in kcal mol^–1 with respect to [B ⁿ + Et₃P=O]), and deformation energies (E _DEF in kcal mol^–1). The chemical shifts in ³¹P NMR, δ_P, obtained in the reaction between B¹⁹ –B²¹ and Et₃P=O (0.3 eq.) in CH₂Cl₂ are also given.

From a different perspective, Hoshimoto and co-workers also explored how the physical and electronic properties of meta-substituents increase/decrease the stability of borane-LB adducts, and hence, prevent/promote the dissociation of LBs from the adducts to generate free BAr₃ (or FLP species) (Scheme [4]).[8] [9] [10] The authors focused on the concept of ‘remote’ back strain (Figure [1b]), which is defined as the sum of repulsive (steric/electronic) and attractive (NCIs) interactions that occur between the meta-substituents of the tetrahedral BAr₃ units in the borane-LB adducts (or a transition state of the borane deformation). In general, these meta-substituents must be effectively separated in the free trigonal planar BAr₃ structure.[9] To quantify the remote back strain, homoleptic boranes bearing 2,6-F₂-3,5-allyl₂C₆H₃ (B²⁰ ) and 2,6-F₂-3,5-TMS₂C₆H₃ (B²¹ ) were synthesized for a comparison with B(2,6-F₂C₆H₃)₃ (B¹⁹ ), as these boranes exhibit nearly identical intrinsic Lewis acidity (the LUMO energy levels) and front strain toward LBs. The relative Gibbs energy values (ΔG°) for the formation of an adduct with a LB (the LBs used in the corresponding work were Et₃P=O, H₂O, CO, THF, and NMe₃) and the deformation energy (E _DEF),[3a] which is an energetic penalty paid for the geometrical change at the boron center upon adduct formation, were evaluated for each of these three boranes. For example, when Et₃P=O was used as the LB, the E _DEF values increased in the order B¹⁹ < B²¹ < B²⁰ , which is consistent with the trend determined via the Gutmann–Beckett method. However, the ΔG° values showed a different trend and increased in the order B²¹ < B¹⁹ < B²⁰ . This discrepancy was rationalized by considering the multiple NCIs formed between the meta-TMS groups themselves and the meta-TMS and P-Et groups in the Et₃P=O–B²¹ adducts. Finally, the authors concluded that repulsion and the NCIs generated between meta-substituents are essential for estimating and regulating the remote back strain for fine-tuning the catalytic activity of BAr₃. It should also be noted that B²⁰ is a rare example of a liquid BAr_3.

In another report, meta-substituents were used as a tool for monitoring the stereoisomerization of BAr₃. Mislow and co-workers demonstrated that the stereoisomerization in B(3- ⁱ Pr-2,4,6-Me₃C₆H)(2,6-Me₂C₆H₃)₂ proceeds via a two-ring flip mechanism, as confirmed by temperature-dependent ¹H NMR spectroscopy based on the diastereotopic nature of the meta- ⁱ Pr group.[23]

2.2

Molecular Transformations Mediated by meta-Substituted Boranes

Some meta-substituted BAr₃ compounds have been applied as aryl-transfer reagents. Frohn and co-workers studied the migration of an aryl group from an electrophilic BAr₃ species, such as B(C₆F₅)₃ and B(3-CF₃C₆H₄)₃ (B²² ), to XeF₂ in CH₂Cl₂, eventually affording [ArXe][ArBF₃] (Ar = C₆F₅ or 3-CF₃C₆H₄) (Scheme [5a]).[24] In 2019, Melen, Wirth, and co-workers developed an aryl-transfer reaction from BAr₃ to various α-aryl-α-diazoacetates (Scheme [5b]).[25] They found that the number of aryl groups that are transferred depends on the Lewis acidity of BAr₃, e.g., B(C₆F₅)₃ and B(3,4,5-F₃C₆H₂)₃ can transfer all three Ar groups, whereas B(3,4-Cl₂C₆H₃)₃ (B²³ ) can only transfer one of its three 3,4-Cl₂C₆H₃ groups. Ishida, Iwamoto, and co-workers also reported on the migration of aryl groups from BAr₃, including B(C₆F₅)₃, B¹ , and BAr*₃ (Ar* = 3,5- ^t Bu₂-4-MeOC₆H₂; B²⁴ ), to a dialkylsilanone, which was proposed to proceed via the formation of a Si=O–B adduct (Scheme [5c]).[26] This report nicely demonstrates that BAr₃ can enhance the reactivity of an unsaturated silicon center upon adduct formation, which is often seen in the activation of carbonyl compounds by Lewis acids.

Scheme 5 Aryl-transfer reactions from BAr₃ to (a) XeF₂, (b) a diazo ester, and (c) a dialkylsilanone.

Chiu and co-workers reported heteroleptic BAr₃ species B²⁵ and B²⁶ , which bear para-OH groups (Scheme [6]).[27] Trivalent boron compounds such as these contain Lewis basic and Brønsted acidic functional groups and it is not always facile for these moieties to co-exist within the same compound. Interestingly, ^t Bu groups were introduced at the meta-positions of bulky 1,3,5-R₃C₆H₂ (B²⁵ : R = Me (Mes); B²⁶ : R = ^t Bu (Mes*)) group at the boron center. Moreover, these authors explored the oxidation of these boranes and confirmed that boryl analogues of the galvinoxy radical, such as [B²⁵ ]^•– and [B²⁶ ]^•– , were generated. The introduction of boron decreases the quinoidal character of the phenoxyl radical and activates the open-shell species.

Scheme 6 Oxidation of B²⁵ and B²⁶ (Mes* = 1,3,5- ^t Bu₃C₆H₂).

Bourissou and co-workers reported the preparation of the ortho-phenylene bridged phosphine-borane compound B²⁷ , which contains two Ar^F groups at the boron center (Scheme [7]).[28] Based on NMR, single-crystal X-ray diffraction, and DFT analyses, an intramolecular coordination interaction between the phosphorus and the boron centers was proposed. The catalytic reactivity of B²⁷ was evaluated in the dehydrogenation of acyclic and cyclic (di)amine-boranes.

Scheme 7 Dehydrogenation of cyclic amine-boranes catalyzed by B²⁷ .

One of the most extensively explored fields in main group catalysis is the reduction of unsaturated compounds with BAr₃ catalysts.[2] [29] Needless to say, meta-substituted BAr₃ have also contributed to the significant development of this important field. For example, Pápai, Soós, and co-workers reported the catalytic hydrogenation of aldehydes, ketones, and enones using H₂ and B¹² –B¹⁵ .[30]

The use of hydrosilanes as a reductant is another practical example of the main-group-catalyzed reduction of unsaturated compounds. For example, Ingleson and co-workers pioneered the development of a main-group-catalyzed reductive alkylation of amines with carbonyl compounds and hydrosilanes in the presence of B(C₆F₅)₃, BPh₃, or B(3,5-Cl₂C₆H₃)₃ (B²⁸ ).[31] In particular, the in situ generation of B²⁸ from Na[B(3,5-Cl₂C₆H₃)₄] enabled the transformation of primary amines whose conjugate acids span pK _a values of 10.6 to 18.5 in MeCN (Scheme [8]).[31a]

Scheme 8 Reductive alkylation of amines catalyzed by B²⁸ , generated from Na[B(3,5-Cl₂C₆H₃)₄] in situ.

Between 2017 and 2018, Hoshimoto and co-workers and Soós and co-workers independently reported the BAr₃-catalyzed reductive alkylation of amines with aldehydes and H₂, where H₂O is generated as the sole byproduct.[7] [32] In the former case, a catalyst-controlled reaction system that generates an active FLP species comprising B¹³ and THF was extensively applied to the reductive alkylation of multisubstituted aniline derivatives. However, the direct use of amino acids was still found to be challenging even under harsh reaction conditions.[32] Meanwhile, Soós and co-workers constructed a substrate-controlled system, that furnishes an FLP from B(2-Cl-6-FC₆H₃)(2,6-Cl₂C₆H₃)₂ and in situ generated imine intermediates, which was predominantly applied to the functionalization of N-alkyl amines.[7] To further expand the utility of such BAr₃-catalyzed reductive functionalization methods using H₂, Hoshimoto and co-workers recently demonstrated an in silico assisted strategy to significantly shorten the lengthy trial-and-error processes usually used for the optimization of BAr₃ (Scheme [9]).[33] In this study, B²⁹ –B³⁸ were prepared for the construction of an in silico library of BAr₃ for the collection of the experimental parameters required for machine learning. Eventually, the optimal reaction system was discovered to be B³⁴ and 4-methyltetrahydropyran (MTHP) and this was successfully applied to the reductive alkylation of aniline-based amino acids and C-terminal-protected peptides.

Scheme 9 (a) Heteroleptic BAr₃ species B²⁹ –B³⁸ designed for the machine-learning-assisted optimization of borane catalysts and (b) B³⁴ -catalyzed reductive alkylation of amino acids and peptides using H₂. ^a 60 atm H₂; MTHP = 4-methyltetrahydropyran.

Recently, Hoshimoto and co-workers also disclosed a conceptually novel approach for the direct use of ‘crude’ H₂ (a gaseous mixture of H₂, CO, CO₂, and/or CH₄) for the catalytic hydrogenation of unsaturated molecules (Scheme [10a]).[8] [9] [10] Given that a huge amount of H₂ will be produced through the production of crude H₂ from hydrocarbon resources (i.e., natural gas, biomass, or food waste), the development of a technology bypassing the energy- and cost-intensive multistep purification processes of crude H₂ will be valuable.[34] The authors found that the hydrogenation of 2-methylquinoline (MeQin), used as a model liquid organic hydrogen carrier (LOHC), proceeded in the presence of 0.1 mol% of B(2,6-Cl₂C₆H₃)(2,6-F₂-3,5-X₂C₆H)₂ [X = F (B¹³ ), Cl (B³⁹ ), Br (B⁴⁰ ), and Ar^F (B⁴¹ )] under solvent-free conditions.

Scheme 10 (a) Simplified schemes of typical contemporary routes for H₂ purification for the hydrogenation of unsaturated compounds; (b) direct use of crude H₂ for the catalytic hydrogenation of MeQin using B¹³ and B³⁹ –B⁴¹ ; and (c) direct use of crude H₂ for the catalytic hydrogenation of 1-naphthaldehyde using B⁴² –B⁴⁴ . ^a 1,4-Dioxane used as the solvent.

The catalyst turnover number (TON) increased in the order B¹³ (1000) < B⁴¹ (1340) < B³⁹ (1400) < B⁴⁰ (1520) (Scheme [10b]).[8] It should be noted here that the intrinsic Lewis acidity of B¹³ , B⁴⁰ , and B⁴¹ is nearly identical, and thus, such a significant difference in TON should be attributed to the size of the meta-substituents (e.g., the degree of their remote back strain). B⁴⁰ and B⁴¹ were also applied to the catalytic hydrogenation of 2,6-lutidine in the presence of gaseous mixtures of H₂/CO (40/4 atom each) and H₂/CO₂ (40/4 atom each) to afford 2,6-dimethylpiperidine. Finally, by taking advantage of the catalytic activity of B⁴¹ in the dehydrogenation of 2-methyl-1,2,3,4-tetrahydroquinoline (H₄-MeQin), the authors demonstrated a molecular-based H₂ purification via the hydrogenation of MeQin with crude H₂ and the subsequent dehydrogenation of H₄-MeQin to afford highly pure H₂. Subsequently, Hoshimoto and co-workers also demonstrated that B²⁰ could be successfully applied to the catalytic hydrogenation of unsubstituted quinoline under mixed gas (H₂/CO/CO₂) conditions.[9]

The aforementioned approach for the direct use of crude H₂ has recently been expanded to the catalytic hydrogenation of carbonyl compounds. In this case, a BAr₃ compound of the type, B(2,6-F₂-3,5-X₂C₆H)₃, was used, and the alcohol yield was found to increase when the meta-substituents were changed from X = F (B⁴² ) to Cl (B⁴³ ) to Br (B⁴⁴ ) (Scheme [10c]).[10] Notably, this trend in reaction efficiency is consistent with the increase in the E _DEF values. Therefore, the increased remote back strain seems to provide higher reaction efficiency by preventing the formation of an adduct with the LBs involved in the system. Importantly, the formyl groups in the aromatic and aliphatic aldehydes that also contain halogen and olefinic substituents can be selectively hydrogenated under mixed gas conditions. Furthermore, B⁴⁴ catalyzed the hydrogenation of undec-10-enal in the presence of a gaseous mixture of H₂, CO, CO₂, and CH₄ (76/0.2/20/3.2 molar ratio), which is produced from CH₄ via desulfurization, stream reforming, and CO-shift conversion processes in industry. These examples showcase (i) the power that can be extracted from BAr₃ catalysis via the fine-tuning of their Lewis acidity based on meta-substitution and (ii) the advantages of BAr₃ relative to the simple alternative of transition metal catalysts that require the use of purified H₂.

Figure 4 meta-Substituted BAr₃ B⁴⁵ –B⁴⁸ with 2,4,6-Me₃-aryl units.

2.3

Other Examples of meta-Functionalization of BAr₃

The introduction of Mes groups into BAr₃ can significantly increase their stability toward LBs such as H₂O. For example, in 1960, B(2,4,6-Me₃-3,5-(NO₂)₂C₆)₃ (B⁴⁵ ) was prepared by treatment of B(Mes)₃ with a mixed acid under cryogenic conditions (Figure [4]).[35] Later, Ashley, Wildgoose, Slootweg, and co-workers used B⁴⁵ to accomplish the homolytic H₂ cleavage through a one-electron reduction in the absence of an external Lewis base.[36] In 1981, Wilson and co-workers reported the synthesis of B(3-MeOC₆H₄)(2,4,6-Me₃C₆H₂)₂ (B⁴⁶ ) and B(3-ClC₆H₄)(2,4,6-Me₃C₆H₂)₂ (B⁴⁷ ).[37] More recently, Ito and co-workers demonstrated the synthesis of B(3-Br-4-MeC₆H₃)(2,4,6-Me₃C₆H₂)₂ (B⁴⁸ ) via the reaction between 2,4-dibromo-1-methylbenzene and Ph₂MeSi–BMes₂ in the presence of Na(O ^t Bu).[38]

3

Conclusions and Perspectives

This Short Review summarizes previously reported meta-substituted triarylboranes (BAr₃) and classifies them based on the roles that the meta-substituents, with respect to the boron centers, on the aryl groups play. The electronic and steric effects imparted by the meta-substituents have been used to tune the electronic/physical properties and reactivity of BAr₃ with respect to Lewis bases (i.e., the Lewis acidity). The introduction of electron-donating or -withdrawing groups at the meta-positions can change the intrinsic electrophilicity (e.g., the energy level of the empty p orbital of the boron center and the charge) of the boron atoms. A buttressing effect caused by a meta-substituent that pushes an adjacent ortho group closer to the boron atom has also been discussed. Importantly, recent progress in theoretical calculations has enabled the detailed consideration of non-covalent interactions (NCIs) related to the aryl meta-substituents. In this context, regulation of the stability of (pre-organized) Lewis adducts by London dispersion energies and remote back strain can be taken into consideration.

It should be noted here that the substitution of the ortho- and/or para-positions significantly impacts the reactivity of BAr₃ by modulating the intrinsic electrophilicity and front strain (intermolecular repulsion caused by the ortho-substituents). After such a relatively rough modulation, fine-tuning the Lewis acidity via meta-substitution should work better and eventually play a critical role in affording a desired reactivity to BAr₃. In fact, meta-designed BAr₃ compounds have been applied in challenging catalytic molecular transformations, such as reductive alkylation of valuable amines (including amino acids and peptides), as well as molecular-based H₂ purification systems. However, the preparation of BAr₃ species with unprecedented substitution patterns is typically laborious and time-consuming when one attempts it for the first time. Thus, cheminformatics-based prediction and optimization of the target BAr₃ will likely be an area of future research.[33] [39] [40] The authors anticipate further diversification of the structures and applications of BAr₃ through their ortho-, para-, and meta-functionalization. Given there are a maximum of six slots for each ortho- and meta-position in a single triarylborane molecule, along with three slots for the para-position, the exploration of BAr₃ will continue.

Conflict of Interest

The authors declare no conflict of interest.

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Publication History

Received: 11 June 2024

Accepted after revision: 03 July 2024

Article published online:
19 August 2024