Disclosing the structure/activity correlation in trivalent boron-containing compounds: a tendency map

σ-Hole triel bonds in aluminium derivatives

This communication reports the synthesis and X-ray characterization of a new mononuclear aluminium compound exhibiting an intramolecular σ-hole triel bonding interaction. It is compared with a dinuclear aluminium compound, previously reported by us, where the aluminium atoms participate in two concurrent σ-hole triel bonding interactions. To our knowledge, such behaviour has not been previously described in the literature.

Nickel(II)-catalyzed highly selective 1,2-diborylation of non-activated monosubstituted alkenes

A practical method for 1,2-diborylation of non-activated monosubstituted alkenes via nickel catalysis has been developed. The protocol features high functional group tolerance and can be applied for the formal synthesis of drugs and modification of natural product derivatives. Preliminary mechanistic studies imply the involvement of a Ni(II) catalytic cycle.

Robust dicopper(i) μ-boryl complexes supported by a dinucleating naphthyridine-based ligand

Copper boryl species have been widely invoked as reactive intermediates in Cu-catalysed C–H borylation reactions, but their isolation and study have been challenging. Use of the robust dinucleating ligand DPFN (2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine) allowed for the isolation of two very thermally stable dicopper(i) boryl complexes, [(DPFN)Cu₂(μ-Bpin)][NTf₂] (2) and [(DPFN)Cu₂(μ-Bcat)][NTf₂] (4) (pin = 2,3-dimethylbutane-2,3-diol; cat = benzene-1,2-diol). These complexes were prepared by cleavage of the corresponding diborane via reaction with the alkoxide [(DPFN)Cu₂(μ-O^tBu)][NTf₂] (3). Reactivity studies illustrated the exceptional stability of these boryl complexes (thermal stability in solution up to 100 °C) and their role in the activation of C(sp)–H bonds. X-ray diffraction and computational studies provide a detailed description of the bonding and electronic structures in these complexes, and suggest that the dinucleating character of the naphthyridine-based ligand is largely responsible for their remarkable stability.

Cu(i) boryl species have been widely invoked as reactive intermediates in Cu-catalysed C–H borylations, but their isolation has been challenging. In this work, thermally robust dicopper(I) boryl complexes have been synthesized and studied in detail.

Transition-metal-free, one-pot synthesis of benzoxaboroles from o-bromobenzaldehydes via visible-light-promoted borylation

A novel and simple one-pot stepwise method to synthesize benzoxaboroles was demonstrated. This step-by-step synthetic method includes photocatalytic boronization with phenothiazine as a photocatalyst and sequential water-induced reduction in the presence of bis(pinacolato)diboron. A series of o-bromobenzaldehydes were well-tolerated under the standard conditions. In addition, this method has been successfully applied in the synthesis of the anti-tuberculosis candidate drug GSK 3036656 and anti-fungal drug tavaborole.

Alkoxide activation of tetra-alkoxy diboron reagents in C-B bond formation: a decade of unpredictable reactivity

Any attempt to facilitate a new generation of C-B bonds represents a useful tool in organic synthesis. In addition, if that approach highlights the nucleophilic character of boryl moieties in the absence of transition metal complexes, the challenge to create new reactive platforms becomes an opportunity. We have been deeply involved in the experimental and theoretical validation of C-B bond formation by means of alkoxide activation of tetra-alkoxy diboron reagents and here is presented a convenient guide to understand the concept and the applications.

Mapping the Electronic Structure and the Reactivity Trends for Stabilized α-Boryl Carbanions

The chemistry of stabilized α-boryl carbanions shows remarkable diversity, and can enable many different synthetic routes towards efficient C-C bond formation. The electron-deficient, trivalent boron center stabilizes the carbanion facilitating its generation and tuning its reactivity. Here, the electronic structure and the reactivity trends of a large dataset of α-boryl carbanions are described. DFT-derived parameters were used to capture their electronic and steric properties, computational reactivity towards model substrates, and crystallographic analysis within the Cambridge Structural Dataset. This study maps the reactivity space by systematically varying the nature of the boryl moiety, the substituents of the carbanionic center, the number of α-boryl motifs, and the metal counterion. In general, the free carbanionic intermediates are described as borata-alkene species with C-B π interactions polarized towards the carbon. Furthermore, it was possible to classify the α-boryl alkylidene metal precursors into three classes directly related to their reactivity: 1) nucleophilic borata-alkene salts with alkali and alkaline earth metals, 2) nucleophilic η² -(C-B) borata-alkene complexes with early transition metals, Cu and Ag, and 3) α-boryl alkyl complexes with late transition metals. This trend map aids selection of the appropriate reactive synthon depending on the reactivity sought.

Stability and decomposition of copper(i) boryl complexes: [(IDipp)Cu–Bneop], [(IDipp*)Cu–Bneop] and copper clusters

Synthesis and characterisation of NHC copper boryl complexes [(NHC)Cu–Bneop] and their decomposition to low-valent copper clusters.

Nucleophilic reactivity of the gold atom in a diarylborylgold(i) complex toward polar multiple bonds

A di(o-tolyl)borylgold complex was synthesized via the metathesis reaction of a gold alkoxide with tetra(o-tolyl)diborane(4). The resulting diarylborylgold complex exhibited a Lewis acidic boron center and a characteristic visible absorption that arises from its HOMO-LUMO excitation, which is narrower than that of a previously reported dioxyborylgold complex. The diarylborylgold complex reacted with isocyanide in a stepwise fashion to afford single- and double-insertion products and a C-C coupled product. Reactions of this diarylborylgold complex with C[double bond, length as m-dash]O/N double bond species furnished addition products under concomitant formation of Au-C and B-O/N bonds, which suggests nucleophilic reactivity of the gold metal center. DFT calculations provided details of the underlying reaction mechanism, which involves an initial coordination of the C[double bond, length as m-dash]O/N bond to the boron vacant p-orbital of the diarylboryl ligand followed by a migration of the gold atom from the tetracoordinate sp3-hybridized boron center, which is analogous to the reactivity of the conventional sp3-hybridized borate species. The DFT calculations also suggested a stepwise mechanism for the reaction of this diarylborylgold complex with isocyanide, which afforded three different reaction products depending on the applied reaction conditions.

Vibrationally resolved valence and core photoionization and photoexcitation spectra of an electron-deficient trivalent boron compound: the case of catecholborane

Compounds containing trivalent boron (TB) as the electron-deficient site(s) find numerous practical uses ranging from Lewis bases in organic synthesis to high-tech industry, with a number of novel applications anticipated. We present an experimental and theoretical study of the gas-phase valence photoionization (VUV-PES), core photoionization (XPS) and photoexcitation (NEXAFS) spectra of a representative TB compound catecholborane (CB). For modelling and assigning the spectra we used the ΔDFT and restricted single excitation space TD-DFT methods for the XPS and NEXAFS, and OVGF and EOM-CCSD for the VUV-PES. The vibrationally resolved structure was computed in the Franck-Condon (FC) and Herzberg-Teller (FCHT) approximations generally resulting in a good agreement with the observed spectral features. For the prediction of core-electron binding energies (CEBEs) several density functionals were tested. The best performance overall was furnished by ωB97X-D suggesting that including the dispersion correction is beneficial. The FCHT vibronic intensities are in clear discrepancy with the B 1s NEXAFS spectrum if the harmonic approximation is used for the B-H wagging mode both in the ground and in the first core-excited state. Instead, a much better agreement is obtained if the excited state potential is approximated to a symmetric double-well. The observed vibronic pattern could be a general fingerprint of the presence of TB centre(s), specifically, the transfer of the (core) density to the vacant boron p-orbital in the excited state.

Neutral cyclic sp2-sp3 and sp3-sp3 diboranes from N,N'-dicyclohexylcarbodiimide insertion into 1,2-dichlorodiboranes(4)

We report the synthesis and structural characterization of new neutral cyclic sp2-sp3 and sp3-sp3 diboranes from the reaction of N,N'-dicyclohexylcarbodiimide with 1,2-dichlorodiboranes(4) at room temperature. The present study is the first example of an insertion reaction of carbodiimide into diborane(4).

Consecutive borylcupration/C–C coupling of γ-alkenyl aldehydes towards diastereoselective 2-(borylmethyl)cycloalkanols

Copper(i) catalyzes the borylative cyclization of γ-alkenyl aldehydes through chemo- and regioselective addition of Cu–B to CC and concomitant intramolecular 1,2-addition of Cu–C on CO.

Hexahalodiborate Dianions: A New Family of Binary Boron Halides

The electron‐precise binary boron subhalide species [B₂X₆]²⁻ X=F, Br, I) were synthesized and their structures confirmed by X‐ray crystallography. The existence of the previously claimed [B₂Cl₆]²⁻, which had been questioned, was also confirmed by X‐ray crystallography. The dianions are isoelectronic to hexahaloethanes, are subhalide analogues of the well‐known tetrahaloborate anions (BX₄⁻), and are rare examples of molecular electron‐precise binary boron species beyond B₂X₄, BX₃, and [BX₄]⁻.

Terminal versus Bridging Boryl Coordination in N-Heterocyclic Carbene Copper(I) Boryl Complexes: Syntheses, Structures, and Dynamic Behavior

The B-B bond activation of the diborane(4) derivatives B₂cat₂ with the copper(I) alkoxido complex [(SIDipp)Cu-O tBu] delivers, depending on the solvent, either the linear boryl complex [(SIDipp)Cu-Bcat] from PhMe or the μ-boryl complex [((SIDipp)Cu)₂Bcat][cat₂B] from THF. The relevant conversion of the linear boryl complex to the μ-boryl complex occurs in the polar solvent via formal boryl anion abstraction by the Lewis acid catB-O tBu, concomitantly formed during the B-B activation. With Lewis acids such as BPh₃ or [CPh₃][BArF] (reversible), boryl abstraction from the linear complexes [(SIDipp)Cu-Bcat] or [(SIDipp)Cu-Bdmab] occurs and results in the μ-boryl complexes [((SIDipp)Cu)₂Bcat/dmab][Ph₃B-Bcat/dmab] and [((SIDipp)Cu)₂Bcat][BArF]. The formation of [((SIDipp)Cu)₂Bcat][cat₂B] is generally accompanied by the concomitant formation of the μ-hydrido complex [((SIDipp)Cu)₂H][cat₂B]. The spiroborate [cat₂B]^- is formed from the initially formed Lewis acid/base adduct [catB-B(O tBu)cat]^- presumably in a process that involves the glass surface of the reaction vessel. All complexes are thoroughly characterized structurally as well as spectroscopically, in particular with respect to the dynamic behavior of the μ-boryl complexes in solution.

Square-Planar Pt(II) and Ir(I) Complexes as the Lewis Bases: Donor-Acceptor Adducts with Group 13 Trihalides and Trihydrides

The stability and properties of donor-acceptor adducts of square-planar Pt(II) and Ir(I) complexes (designated as PtX, IrX, or generally MX complexes) with trihydrides and trihalides of group 13 elements of general formula YZ₃ (Y = B, Al, Ga; Z = H, F, Cl, Br) were studied theoretically using DFT methodology in the gas phase. MX complexes were represented by wide range of the ligand environment which included model complexes [Ir(NH₃)₃X]⁰ and cis-[Pt(NH₃)₂X₂]⁰ (X = H, CH₃, F, Cl, Br) and isoelectronic complexes [Ir(NNN)(CH₃)]⁰ and [Pt(NCN)(CH₃)]⁰ with tridentate NNN and NCN pincer ligands. MX complexes acted as the Lewis bases donating electron density from the doubly occupied 5d _z² atomic orbital of the metal M atom to the empty valence p _z orbital of Y whose evidence was clearly provided by the natural atomic orbital (NAO) analysis. This charge transfer led to the formation of pentacoordinated square pyramidal MX·(YZ₃) adducts with M·Y dative bond. Binding energies were -44.7 and -75.2 kcal/mol for interaction of GaF₃ as the strongest acid with PtNCN and IrNNN pincer ligands complexes. Only M·B bonds had covalent character although MX·BZ₃ adducts were the least stable due to large values of Pauli repulsion and deformation energies. The highest degree of covalent character was found for adducts of BH₃ in all series of structures studied. Al and Ga adducts showed remarkably similar behavior with respect to geometry and binding energies.

Synthesis and reactivity of [Au(NHC)(Bpin)] complexes

A new class of [Au(NHC)(Bpin)] complexes has been synthesized and their unusual reactivity was investigated using computational and experimental methods.

Benzoxaborole compounds for therapeutic uses: a patent review (2010- 2018)

INTRODUCTION

Benzoxaborole is a versatile boron-heterocyclic scaffold which has found in the last 10 years a broad spectrum of applications in medicinal chemistry, due to its physicochemical and drug-like properties. Use of benzoxaborole moiety in the design of compounds led to the discovery of new classes of anti-bacterial, anti-fungal, anti-protozoal, anti-viral as well as anti-inflammatory agents with interesting drug development perspectives.

AREAS COVERED

This article reviews the patent literature as well as chemistry literature during the period 2010-2018 where in several benzoxaborole derivatives with therapeutic options were reported.

EXPERT OPINION

Two benzoxaborole derivatives are already clinically used for the treatment of onychomycosis (tavaborole) and atopic dermatitis (crisaborole), with several others in various phases of clinical trials. By inhibiting enzymes essential in the life cycle of fungal, protozoan, bacterial and viral pathogens, it is probable that other compounds may soon enter the armamentarium of anti-infective agents. On the other hand, phosphodiesterase 4 seems to be the human target responsible of the anti-inflammatory action of some benzoxaboroles. The chemical versatility, peculiar mechanism of action related to the electron deficient nature of the boron atom, and ease of preparation make benzoxaboroles a highly interesting field for the pharmaceutical industry.

Computational mechanistic study of Ru-catalyzed CO2 reduction by pinacolborane revealing the σ-π coupling mechanism for CO2 decarbonylation

It has been reported that RuH2(η2-H2)2(PCy3)2 (1) could mediate CO2 reduction by pinacolborane (HBpin), affording pinBOBpin (7), pinBOCH3 (8), pinBOCHO (9), pinBOCH2OBpin (10), and an unprecedented C2 species pinBOCH2OCHO (11), which meanwhile is converted to the Ru complexes, including the transient 3 (RuH(κ2-O2CH)(CO)(PCy3)2) and 5 (RuH{(μ-H)2Bpin}(CO)(PCy3)2), and the persistent 4 (RuH(κ2-O2CH)(CO)2(PCy3)2) and 6 (RuH2(CO)2(PCy3)2). To gain an insight into the catalysis, a DFT study was carried out. The study identified the key active catalyst to be the hydride 13 (RuH2(CO)(PCy3)2) and characterized the mechanisms leading to the experimentally observed species (3-11). By investigating the experimental system, we learned a new mechanism called σ-π coupling for CO2 decarbonylation. Under this mechanism, CO2 and HBpin first co-coordinate to the Ru center of 13, then σ-π coupling takes place, forming a B-O bond between CO2 and HBpin, Ru-H and Ru-C bonds, and simultaneously breaking the H-Bpin bond, followed by -OBpin group migration to the Ru center, completing the CO2 decarbonylation. An interesting feature regarding the Ru catalysis was the involvement of η1-Hη1-H → η2-H2 and η1-Hη1-Bpin → η2-HBpin reductions, which facilitated the oxidative H-Bpin addition or the coordination mode change of CO2 from η1-O to η2-CO for CO2 activation or σ-π coupling. The facilitation effects could be attributed to the reductions enhancing the electron donations from the Ru center to the antibonding orbitals of the activating bonds.

B-B bond activation and NHC ring-expansion reactions of diboron(4) compounds, and accurate molecular structures of B2(NMe2)4, B2eg2, B2neop2 and B2pin2

In this detailed study we report on the structures of the widely employed diboron(4) compounds bis(pinacolato)diboron (B₂pin₂) and bis(neopentyl glycolato)diboron (B₂neop₂), as well as bis(ethylene glycolato)diboron (B₂eg₂) and tetrakis(dimethylamino)diboron (B₂(NMe₂)₄), and their reactivity, along with that of bis(catecholato)diboron (B₂cat₂) with backbone saturated and backbone unsaturared N-heterocyclic carbenes (NHCs) of different steric demand. Depending on the nature of the diboron(4) compound and the NHC used, Lewis-acid/Lewis-base adducts or NHC ring-expansion products stemming from B-B and C-N bond activation have been observed. The corresponding NHC adducts and NHC ring-expanded products were isolated and characterised via solid-state and solution NMR spectroscopy and X-ray diffraction. In general, we observed B-B bond and C-N bond activation at low temperature for B₂eg₂, at room temperature for B₂neop₂ and at higher temperature for B₂cat₂. The reactivity strongly depends on steric effects of the NHCs and the diboron(4) compounds, as well as on the corresponding Lewis-basicity and Lewis-acidity. Our results provide profound insight into the chemistry of these diboron(4) reagents with the nowadays ubiquitous NHCs, the stability of the corresponding NHC adducts and on B-B bond activation using Lewis-bases in general. We demonstrate that B-B bond activation may be triggered even at temperatures as low as -40 °C to -30 °C without any metal species involved. For example, the reactions of B₂eg₂ with sterically less demanding NHCs such as Me₂Im^Me and iPr₂Im in solution led to the corresponding ring-expanded products at low temperatures. Furthermore, boronium [L₂B(OR)₂]⁺ and borenium [LB(OR)₂]⁺ cations have been observed from the reaction of the bis-borate B₂eg₃ with the NHCs iPr₂Im and Me₂Im^Me, which led to the conclusion that the activation of bis-borates with NHCs (or Lewis-bases in general) might be a facile and simple route to access such species.

Addition of dihydrogen to a borylborenium center

The activation of a H-H bond, the simplest covalent bond, is a fundamentally important process. Addition of H2 to an elemental center typically occurs on low valent transition metal or main group complexes through oxidative addition to afford metal dihydride complexes. In contrast, activation of H2 on a high valent center generally results in heterolytic cleavage of the H-H bond to a proton and hydride. Here, we report experimental and computational evidence for the addition of H2 to a borenium center in an N-heterocyclic carbene (NHC) coordinated borylborenium cation, which leads to the formation of a dihydroborenium complex accompanied by the elimination of two σ-bonded substituents, namely mesityl (Mes) and pinacolboryl (Bpin) groups, as mesitylboronic ester.

Understanding the mechanism of transition metal-free anti addition to alkynes: the selenoboration case

A novel mechanism was proposed for the stereoselective anti-addition of selenoboranes to alkynes using catalytic amounts of PCy₃.

Acylboranes: synthetic strategies and applications

Acylboranes are an attractive class of compounds, of which the synthesis has very recently been documented as summarised in this review. Access to these compounds provides a path to study their properties and reactivity.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B-C bonds and other processes.

Homolytic Cleavage of a B-B Bond by the Cooperative Catalysis of Two Lewis Bases: Computational Design and Experimental Verification

Density functional theory (DFT) investigations revealed that 4-cyanopyridine was capable of homolytically cleaving the B-B σ bond of diborane via the cooperative coordination to the two boron atoms of the diborane to generate pyridine boryl radicals. Our experimental verification provides supportive evidence for this new B-B activation mode. With this novel activation strategy, we have experimentally realized the catalytic reduction of azo-compounds to hydrazine derivatives, deoxygenation of sulfoxides to sulfides, and reduction of quinones with B2 (pin)2 at mild conditions.

New outcomes of Lewis base addition to diboranes(4): electronic effects override strong steric disincentives

Two surprising new outcomes of the reaction of Lewis bases with dihalodiboranes(4) are presented, including sp(2)-sp(3) diboranes in which the Lewis base unit is bound to a highly sterically congested boron atom, and a rearranged double base adduct. The results provide a fuller understanding of the reactivity of diboranes, a poorly-understood class of molecule of critical importance to synthetic organic chemistry.

Transition-Metal-Free Borylation of Allylic and Propargylic Alcohols

The base-catalyzed allylic borylation of tertiary allylic alcohols allows the synthesis of 1,1-disubstituted allyl boronates, in moderate to high yield. The unexpected tandem performance of the Lewis acid-base adduct, [Hbase](+) [MeO-B2 pin2 ](-) favored the formation of 1,2,3-triborylated species from the tertiary allylic alcohols and 1-propargylic cyclohexanol at 90 °C.

Isolation, structure and reactivity of a scandium boryl oxycarbene complex

The reaction of a half-sandwich scandium boryl complex 1 with CO (1 atm) afforded a novel boryl oxycarbene complex 2. The structure of 2 was characterized by 1H, 13C and 11B NMR, X-ray diffraction, and DFT analysis. Further reaction of 2 with CO (1 atm) yielded a phenylamido- and boryl-substituted enediolate complex 3 through C-C bond formation between CO and the carbene unit in 2 and cleavage and rearrangement of the Si-N bond in the silylene-linked Cp-amido ligand. Upon heating, insertion of the carbene atom into a methine C-H bond in the boryl ligand of 2 took place to give an alkoxide complex 4. The reactions of 2 with pyridine and 2-methylpyridine led to insertion of the carbene atom into an ortho-C-H bond of pyridine and into a methyl C-H bond of 2-methylpyridine, respectively. The reaction of 2 with ethylene yielded a borylcyclopropyloxy complex 7 through cycloaddition of the carbene atom to ethylene.

A mild carbon-boron bond formation from diaryliodonium salts

The direct metal-free borylation of diaryliodonium salts with diboron reagents is now demonstrated to be a feasible process toward formation of aryl boronic esters without any additive or catalysts, and it can be extended to a two-step C-C coupling of both aryl groups of the initial diaryliodonium reagent.

sp(2)-sp(3) diboranes: astounding structural variability and mild sources of nucleophilic boron for organic synthesis

Despite the widespread use of organoborane reagents in organic synthesis and catalysis, a major challenge still remains: very few boron-centered nucleophiles exist for the direct construction of B-C bonds. Perhaps the most promising emerging solution to this problem is the use of sp(2)-sp(3) diboranes, in which one boron atom of a conventional diborane(4) is quaternised by either a neutral or anionic nucleophile. These compounds, either isolated or generated in situ, serve as relatively mild and convenient sources of the boryl anion [BR2](-) for use in organic synthesis and have already proven their efficacy in metal-free as well as metal-catalysed borylation reactions. This Feature article documents the history of sp(2)-sp(3) diborane synthesis, their properties and surprising structural variability, and their burgeoning utility in organic synthesis.

Application of a nucleophilic boryl complex in the frustrated Lewis pair: activation of H-H, B-H and C=C bonds with B(C6F5)3 and boryl-borate lithium

The frustrated Lewis pair comprised of B(C6F5)3 and a boryl-borate lithium salt Li[pinBB(Ph)pin] can efficiently activate dihydrogen, pinacolborane and ethylene at ambient temperature. Theoretical studies suggest that the nucleophilic sp(2) boryl moiety of Li[pinBB(Ph)pin] plays different roles in these reactions.

Mixed diboration of alkenes in a metal-free context

Experimental and theoretical rationalization on regioselective mixed diboration of alkenes, with the unsymmetrical diboron reagent Bpin-Bdan, providing the protecting Bdan moiety in the internal position.