Ionic-Liquid-Promoted Decaborane Dehydrogenative Alkyne-Insertion Reactions: A New Route to o-Carboranes

Charge Transfer Complex-Enabled Synthesis of (Hetero)arylated m-Carboranes from m-Carborane Phosphonium Salts

A photoinduced charge transfer complex (CTC)-enabled photoreduction of carborane phosphonium salts for the cage carbon (hetero)arylation of carboranes was developed. It offers a convenient approach for introducing a wide range of aryl and heteroaryl groups, such as pyrroles, thiophenes, indoles, thianaphthenes, benzofurans, pyridines, and benzenes, into carboranes. This strategy offers operational simplicity, mild reaction conditions, and a broad substrate scope, making it highly advantageous.

Carborane clusters increase the potency of bis-substituted cyclam derivatives against Mycobacterium tuberculosis

Bis-substituted cyclam derivatives have recently emerged as a promising new class of antibacterial agents, displaying excellent activity against drug-resistant Mycobacterium tuberculosis (Mtb) and in vivo efficacy in a zebrafish assay. Herein we report the synthesis and biological activity of new carborane derivatives within this class of antitubercular compounds. The resulting carborane-cyclam conjugates incorporating either hydrophobic closo-1,2-carborane or anionic, hydrophilic nido-7,8-carborane clusters display promising activity in an antibacterial assay employing the virulent Mtb strain H37Rv. The most active of these carborane derivatives exhibit MIC50 values of <1 μM, making them the most active compounds in this unique class of antibacterial cyclams reported to date.

Biotinylation of a MRI/Gd BNCT theranostic agent to access a novel tumour-targeted delivery system

A new biotin based BNCT (Boron Neutron Capture Therapy)-MRI theranostic is here reported (Gd-AL01) in order to exploit the high tumour specificity of biotin and the selectivity of BNCT in a synergistic manner. The key is the preparation of an intermediate where an o-carborane is linked to two amino groups orthogonally protected via the exploitation of two consecutive Mitsunobu reactions. The aim is its functionalisation in two different steps with biotin as the biological vector and Gd-DOTA as the MRI probe and GdNCT agent. Cell uptake was evaluated on HeLa tumour cells overexpressing biotin receptors. The internalised boron is proportional to the concentration of the theranostic agent incubated in the presence of cells. A maximum value of 77 ppm is reached and a well detectable signal intensity increase in the T₁ weighted image of HeLa cells was observed, differently from clinically used GdHPDO3A, where no contrast is detected. These excellent results indicate that Gd-AL01 can be applied as a theranostic probe in BNCT studies.

Direct and Regioselective Palladium(II)-Catalyzed B(4)-H Monoacyloxylation and B(4,5)-H Diacetoxylation of o-Carborane Acids with Phenyliodonium Dicarboxylates

A direct B(4)-H monoacyloxylation via a Pd-catalyzed regioselective B(4)-H activation of o-carborane acids with phenyliodonium dicarboxylates was developed, and a series of B(4)-H monoacyloxylated o-carboranes decorated with active groups were synthesized with moderate to good yields as well as excellent selectivity. In addition, a direct B(4,5)-H diacetoxylation from o-carborane acids with phenyliodonium diacetate was demonstrated.

Light-promoted copper-catalyzed cage C-arylation of o-carboranes: facile synthesis of 1-aryl-o-carboranes and o-carborane-fused cyclics

Light-promoted, copper catalyzed cage C–H arylation of o-carboranes with aryl halides has been achieved, leading to the facile synthesis of a variety of 1-aryl-o-carboranes and o-carborane-fused cyclics.

In vitro and in vivo BNCT investigations using a carborane containing sulfonamide targeting CAIX epitopes on malignant pleural mesothelioma and breast cancer cells

This study aims at merging the therapeutic effects associated to the inhibition of Carbonic Anhydrase IX (CAIX), an essential enzyme overexpressed by cancer cells including mesothelioma and breast cancer, with those ones brought by the application of Boron Neutron Capture Therapy (BNCT). This task was pursued by designing a sulfonamido-functionalised-carborane (CA-SF) that acts simultaneously as CAIX inhibitor and boron delivery agent. The CAIX expression, measured by Western blot analysis, resulted high in both mesothelioma and breast tumours. This finding was exploited for the delivery of a therapeutic dose of boron (> 20 μg/g) to the cancer cells. The synergic cytotoxic effects operated by the enzymatic inhibition and neutron irradiation was evaluated in vitro on ZL34, AB22 and MCF7 cancer cells. Next, an in vivo model was prepared by subcutaneous injection of AB22 cells in Balb/c mice and CA-SF was administered as inclusion complex with a β-cyclodextrin oligomer. After irradiation with thermal neutrons tumour growth was evaluated for 25 days by MRI. The obtained results appear very promising as the tumour growth was definitively markedly lower in comparison to controls and the CAIX inhibitor alone. This approach appears promising and it call consideration for the design of new therapeutic routes to cure patients affected by this disease.

Transition-metal-free direct nucleophilic substitution of carboranyllithium and 2-halopyridines

A practical and efficient C(cage)-heteroarylation of carborane is presented, via direct nucleophilic substitution of carboranyllithium with 2-halopyridines. This reaction does not need the aid of any transition metal and utilizes readily available carboranyllithium nucleophiles, thereby avoiding transmetalation of carboranyllithium. The process exhibits a broad scope, and a vast array of 2-halopyridines have proven to be suitable substrates. The method serves as a complement to C(cage)-arylation reactions and may find wide applications in materials science and medicinal and coordination chemistry.

Carborane-Induced Excimer Emission of Severely Twisted Bis-o-Carboranyl Chrysene

The synthesis of a highly twisted chrysene derivative incorporating two electron deficient o-carboranyl groups is reported. The molecule exhibits a complex, excitation-dependent photoluminescence, including aggregation-induced emission (AIE) with good quantum efficiency and an exceptionally long singlet excited state lifetime. Through a combination of detailed optical studies and theoretical calculations, the excited state species are identified, including an unusual excimer induced by the presence of o-carborane. This is the first time that o-carborane has been shown to induce excimer formation ab initio, as well as the first observation of excimer emission by a chrysene-based small molecule in solution. Bis-o-carboranyl chrysene is thus an initial member of a new family of o-carboranyl phenacenes exhibiting a novel architecture for highly-efficient multi-luminescent fluorophores.

Decaborane anion tautomerism: ion pairing and proton transfer control

The reaction of 1,8-bis(dimethylamino)naphthalene-which is often referred by the trade name Proton-Sponge (PS)-with decaborane in hexane afforded [HPS][B10H13] (1) salt as a pale-yellow precipitate. Variable-temperature NMR studies allowed the full assignment of 1H and 11B spectra for this familiar ten-vertex polyhedral anion. In addition, this work reveals that an increase in the temperature leads to the intramolecular exchange of three B-H-B hydrogen atoms around the hexagonal face of the boat-shaped cluster. This previously unrecognised H-tautomerism complements the long-known low-energy proton exchange of only one of the bridging hydrogen atoms. The temperature dependent proton fluxional behaviour controls the molecular environment of the polyhedral cage, averaging the negative charge of the anion. The result is a debilitation of the cation-anion interactions in solution, favouring the transfer of the proton from the organic aromatic cation, [HPS]+, to the polyhedral anion, [B10H13]-. This proton transfer affords Proton-Sponge and decaborane, increasing the entropy of the system and sustaining an equilibrium which at high temperatures shifts toward the neutral reactants and at low temperatures moves toward the ionic products. A single X-ray diffraction analysis of 1 is discussed.

Direct synthesis of dicarbollides

A substantially new synthesis of dicarbollides based on the dicarbon insertion of alkynes into the nine-vertex 4-Et₃N-arachno-B₉H₁₃ framework is reported.

Design of a catalyst through Fe doping of the boron cage B10H14 for CO2 hydrogenation and investigation of the catalytic character of iron hydride (Fe-H)

The innovative catalyst Fe@B₁₀H₁₄ is designed through Fe doping of the boron cage B₁₀H₁₄ and is employed to catalyze CO₂ hydrogenation using a quantum mechanical method. First, the structure of the Fe@B₁₀H₁₄ complex is characterized through calculated ¹¹B NMR chemical shifts and Raman spectra, and the interactions between Fe and the four H atoms of the opening in the cage are analyzed, which show that various iron hydride (Fe-H) characteristics exist. Subsequently, the potential of Fe@B₁₀H₁₄ as a catalyst for the hydrogenative reduction of CO₂ in the gas phase is computationally evaluated. We find that an equivalent of Fe@B₁₀H₁₄ can consecutively reduce double CO₂ to obtain the double product HCOOH through a two-step reduction, and Fe@B₁₀H₁₂ and Fe@B₁₀H₁₀ are successively obtained. The Fe presents single-atom character in the reduction of CO₂, which is different from the common iron(ii) catalyzed CO₂ reduction. The calculated total free energy barrier of the first CO₂ reduction is only 8.79 kcal mol^-1, and that of the second CO₂ reduction is 25.71 kcal mol^-1. Every reduction reaction undergoes two key transition states TSC-H and TSO-H. Moreover, the transition state of the C-H bond formation TSC-H is the rate-determining step, where the interaction between π_{C[double bond, length as m-dash]O}* and the weak σ_Fe-H bond plays an important role. Furthermore, the hydrogenations of Fe@B₁₀H₁₂ and Fe@B₁₀H₁₀ are investigated, which aim at determining the ability of Fe-H circulation in the Fe doped decaborane complex. We find that the hydrogenation of Fe@B₁₀H₁₀ undergoes a one-step H₂-adsorbed transition state TSH-adsorb with an energy barrier of 6.42 kcal mol^-1 from Fe@B₁₀H₁₂. Comparing with the hydrogenation of Fe@B₁₀H₁₀, it is slightly more difficult for the hydrogenation of Fe@B₁₀H₁₂, where the rate-determining step is the H₂-cleaved transition state TS2H-H with an energy barrier of 17.38 kcal mol^-1.

Concise and Versatile Synthesis of Sulfoquinovosyl Acyl Glycerol Derivatives for Biological Applications

Sulfoquinovosyl acylpropanediol (SQAP), a chemically modified analogue of sulfoquinovosyl acylglycerol (SQAG) that occurs in sea algae, has been reported to show a variety of biological activities, including accumulation in tumor cells and the inhibition of tumor cell growth. We report herein on a new concise and versatile synthesis of SQAP itself and derivatives bearing iodoaryl groups and boronclusters. This method should be useful for the design and synthesis of SQAG/SQAP derivatives for diagnosis and the treatment of cancer and related diseases.

Below the 12-vertex: 10-vertex carborane anions as non-corrosive, halide free, electrolytes for rechargeable Mg batteries

The synthesis and application of the first high voltage, non-corrosive, Mg battery electrolyte based on small carborane anions are reported. This electrolyte displays equal oxidative stability compared to its larger 12-vertex cousin, but is more cost effective to prepare.

Photoarylation of Iodocarboranes with Unactivated (Hetero)Arenes: Facile Synthesis of 1,2-[(Hetero)Aryl]n -o-Carboranes (n=1,2) and o-Carborane-Fused Cyclics

Photoarylation of iodocarboranes with unactivated arenes/heteroarenes at room temperature has been achieved, for the first time, thus leading to the facile synthesis of a large variety of cage carbon mono(hetero)arylated and di(hetero)arylated o-carboranes. This work represents a clean, efficient, transition-metal-free, and cheap synthesis of functionalized carboranes, which has significant advantages over the known methods.

Dehydrogenation of icosahedral carborane anions via gas-phase collisional activation

RATIONALE

The icosahedral carborane anion HCB₁₁ H₁₁^-1 is well known for its exceptional stability in solution and the solid state, but the gas-phase properties of this molecule have not previously been explored.

METHODS

Electrospray ionization ion trap mass spectrometry, collisional activation, and isotopic labelling were used to examine the formation and reaction pathways of covalent and noncovalent carborane clusters.

RESULTS

Covalent attachment of an amino group to the C-vertex of HCB₁₁ H₁₁^-1 dramatically alters its stability in the gas phase. Interestingly, covalently bound carborane dimers are produced during electrospray ionization. Subsequent collisional activation of these dimers leads to facile hydrogen losses (7 H₂ molecules). The loss of H₂ does not follow a stochastic pattern, but occurs preferentially by loss of 3 H₂ , followed by loss of 2 H₂ and then another 2 H₂ molecules.

CONCLUSIONS

This study provides insight into new possible modes of B-H activation and functionalization in carborane cages. Copyright © 2016 John Wiley & Sons, Ltd.

Inductive effects of 10 and 12-vertex closo-carborane anions: cluster size and charge make a difference

The inductive effects of 10 and 12-vertex closo-carborane anion ligand substituents are elucidated for the first time. It is found that both of these cluster substituents are potent electron donating groups, which is in contrast to C-functionalized o-carborane. The fact that the 10-vertex cluster displays the strongest electron donating ability can be rationalized by its charge and size.

Anionic and zwitterionic carboranyl N-heterocyclic carbene Au(i) complexes

The syntheses of the first carboranyl N-heterocyclic carbene complexes with transition metals are reported.

Straightforward synthesis of radioiodinated Cc-substituted o-carboranes: towards a versatile platform to enable the in vivo assessment of BNCT drug candidates

Due to their high boron content and rich chemistry, dicarba-closo-dodecaboranes (carboranes) are promising building blocks for the development of drug candidates with application in Boron Neutron Capture Therapy. However, the non-invasive determination of their pharmacokinetic properties to predict therapeutic efficacy is still a challenge. Herein, we have reported the unprecedented preparation of mono-[(125)I] iodinated decaborane via a catalyst-assisted isotopic exchange. Subsequent reactions of the radiolabelled species with acetylenes in acetonitrile under microwave heating yield the corresponding (125)I-labelled, Cc-substituted o-carboranes with good overall radiochemical yields in short reaction times. The same synthetic strategy was successfully applied to the preparation of (131)I-labelled analogues, and further extension to other radioisotopes of iodine such as (124)I (positron emitter) or (123)I (gamma emitter) can be envisaged. Hence, the general strategy reported here is suitable for the preparation of a wide range of radiolabelled Cc-substituted o-carborane derivatives. The labelled compounds might be subsequently investigated in vivo by using nuclear imaging techniques such as Single Photon Emission Computerized Tomography or Positron Emission Tomography.

Carborane–β-cyclodextrin complexes as a supramolecular connector for bioactive surfaces

The supramolecular carborane–β-cyclodextrin system allows for effective monovalent immobilization of biologically active peptides resulting in efficient cell adhesion and spreading.

Cation reduction and comproportionation as novel strategies to produce high voltage, halide free, carborane based electrolytes for rechargeable Mg batteries

The chemical reduction of reactive cations and comproportionation are two new strategies reported to produce halide free carborane based electrolytes for rechargeable Mg batteries.

Synthesis of unsymmetrical N-carboranyl NHCs: directing effect of the carborane anion

The selective synthesis of unsymmetrically substituted anionic C-2 and dianionic C-2/C-5 N-carboranyl NHCs is reported, revealing a directing effect conveyed by the anionic carborane substituent.

Regioselective insertion of o-carborynes into the α-C-H bond of tertiary amines: synthesis of α-carboranylated amines

o-Carboryne can undergo α-CH bond insertion with tertiary amines, thus affording α-carboranylated amines in very good regioselectivity and isolated yields. In this process, the nucleophilic addition of tertiary amines to the multiple bond of o-carboryne generates a zwitterionic intermediate. An intramolecular proton transfer, followed by a nucleophilic attack leads to the formation of the final product. Thus, regioselectivity is highly dependent upon the acidity of α-CH proton of tertiary amines. This approach serves as an efficient methodology for the preparation of a series of 1-aminoalkyl-o-carboranes.

High yielding synthesis of carboranes under mild reaction conditions using a homogeneous silver(I) catalyst: direct evidence of a bimetallic intermediate

Methods used to prepare functionalized carboranes generally require heating to high temperatures, and thus limits the range of derivatives which can be prepared directly from alkynes. We show here that by using a homogeneous silver(I) catalyst it is now possible to prepare carboranes in good to excellent yield at temperatures below 40 °C, including at room temperature. The process is general and provides an important new synthetic strategy for the preparation of functionalized boron clusters.

High yielding preparation of dicarba-closo-dodecaboranes using a silver(I) mediated dehydrogenative alkyne-insertion reaction

The synthesis of 1,2-dicarba-closo-dodecaboranes (ortho-carboranes) is often low yielding which is a critical issue given the increasing use of boron clusters in material science and medicinal chemistry. To address this barrier, a series of Cu, Ag, and Au salts were screened to identify compounds that would enhance the yields of ortho-caboranes produced when treating alkynes with B10H12(CH3CN)2. Using a variety of functionalized ligands including mono- and polyfunctional internal and terminal alkynes, significant increases in yield were observed when AgNO3 was used in catalytic amounts. AgNO3 appears to prevent unwanted reduction/hydroboration of the alkyne prior to carborane formation, and the process is compatible with aryl, halo, hydroxy, nitrile, carbamate, and carbonyl functionalized alkynes.