Novel Approach to Aminocarboranes by Mild Amidation of Selected Iodo-carboranes

Advances in the selective functionalization of B(3,6)-H of o-carboranes

This review summarizes the methodologies for the selective functionalization of the B(3,6) vertices of o-carboranes, including the deboration-capitation reaction, the coupling reaction of B-X (X = I, Br) bonds, reactions of 1,3-dehydro-o-carborane and [3-N2-o-C2B10H11][BF4] as well as transition-metal-catalyzed B-H activation. These works offer a versatile toolbox for synthesizing B(3,6)-substituted o-carborane derivatives and will promote their applications in material science, pharmaceutical chemistry, and related disciplines.

Pd-Catalysed synthesis of carborane sulfides from carborane thiols

Carboranes are an interesting class of aromatic molecules with icosahedral geometry, high stability, and unique electronic effects. We herein report a Pd-catalysed coupling reaction of carborane thiols with aryl halides. This protocol was applicable to the controlled synthesis of di(carboranyl) sulfides, and their catalytic performance for aromatic halogenation was examined.

A Pd-catalyzed route to carborane-fused boron heterocycles

Due to the expanding applications of icosahedral carboranes in medicinal and materials chemistry research, their functionalizations have become one of the central themes in boron-rich cluster chemistry. Although several strategies for incorporating nitrogen-containing nucleophiles on a single boron vertex of the icosahedral carboranes (C2B10H12) have been developed, methods for preparing clusters with vicinal B-N moieties are still lacking. The steric bulk of icosahedral carboranes and disparate electronic and steric nature of the N-containing groups have rendered the vicinal diamination challenging. In this article, we show how a developed Pd-catalyzed process is used to incorporate an array of NH-heterocycles, anilines, and heteroanilines with various electronic and steric profiles onto the vicinal boron vertices of a meta-carborane cluster via sequential or one-pot fashion. Importantly, oxidative cyclizations of the cross-coupling products with indoles and pyrroles appended to boron vertices generate a previously unknown class of all-boron-vertex bound carborane-fused six- and seven-membered ring heterocycles. Photophysical studies of the meta-carborane-fused heterocycles show that these structures can exhibit luminescence with high quantum yields and are amenable to further manipulations.

Iridium(III)-Catalyzed Regioselective B(4)-H Amination of o-Carboranes with Sufilimines

Iridium(III)-catalyzed regioselective B(4)-H amination is developed from the reaction of o-carborane acids with sulfilimines without any oxidants under mild conditions, which leads to a wide range of B(4)-H aminated o-carboranes in good yields with a broad substrate scope. Moreover, the selective B(3,6)-diamination reaction of the o-carborane acid was achieved. The present reaction is attractive from a practical point of view because dibenzothiophene is quantitatively recovered and reused.

Metal-Catalyzed and Metal-Free Nucleophilic Substitution of 7-I-B18H21

In this work, two pathways of reactivity are investigated to generate site-specific substitutions at the B7 vertex of the luminescent boron cluster, anti-B₁₈H₂₂. First, a palladium-catalyzed cross-coupling reaction utilizing the precursor 7-I-B₁₈H₂₁ and a series of model nucleophiles was developed, ultimately producing several B-N- and B-O-substituted species. Interestingly, the B-I bond in this cluster can also be substituted in an uncatalyzed fashion, leading to the formation of various B-N, B-O, and B-S products. This work highlights intricate differences corresponding to these two reaction pathways and analyzes the role of solvents and additives on product distributions. As a result of our synthetic studies, seven new B₁₈-based clusters were synthesized, isolated, and characterized by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. The photoluminescence properties of two structurally similar ether and thioether products were further investigated, with both exhibiting blue fluorescence in solution at 298 K and long-lived green or yellow phosphorescence at 77 K. Overall, this work shows, for the first time, the ability to perform substitution of a boron-halogen bond with nucleophiles in a B₁₈-based cluster, resulting in the formation of photoluminescent molecules.

Carboranyl Analogues of Mefenamic Acid and Their Biological Evaluation

Mefenamic acid represents a widely used nonsteroidal anti-inflammatory drug (NSAID) to treat the pain of postoperative surgery and heavy menstrual bleeding. Like other NSAIDs, mefenamic acid inhibits the synthesis of prostaglandins by nonselectively blocking cyclooxygenase (COX) isoforms COX-1 and COX-2. For the improved selectivity of the drug and, therefore, reduced related side effects, the carborane analogues of mefenamic acid were evaluated. The ortho-, meta-, and para-carborane derivatives were synthesized in three steps: halogenation of the respective cluster, followed by a Pd-catalyzed B-N coupling and hydrolysis of the nitrile derivatives under acidic conditions. The COX inhibitory activity and cytotoxicity for different cancer cell lines revealed that the carborane analogues have stronger antitumor potential compared to their parent organic compound.

Palladium-Catalyzed Regioselective B(9)-Amination of o-Carboranes and m-Carboranes in HFIP with Broad Nitrogen Sources

Amination of carboranes has a good application prospect in organic and pharmaceutical synthesis. However, the current methods used for this transformation suffer from limitations. Herein, we report a practical method for a highly regioselective formation of a B-N bond by Pd(II)-catalyzed B(9)-H amination of o- and m-carboranes in hexafluoroisopropanol (HFIP) with different nitrogen sources under air atmosphere. The silver salt and HFIP solvent play critical roles in the present protocol. The mechanistic study reveals that the silver salt acts as a Lewis acid to promote the electrophilic palladation step by forming a heterobimetallic active catalyst PdAg(OAc)₃; the strong hydrogen-bond-donating ability and low nucleophilicity of HFIP enhance the electrophilic ability of Pd(II). It is believed that these N-containing carboranes are potentially of great importance in the synthesis of new pharmaceuticals.

New approaches to the functionalization of the 1-carba-closo-decaborate anion

Two new approaches to the functionalization of the 1-carba-closo-decaborate anion [1-CB₉H₁₀]^- at boron atoms via the ring-opening of its 1,4-dioxane derivative with various nucleophiles and Pd-catalysed cross-coupling of its iodo derivative with aromatic amines and heteroaromatics were developed.

Highly selective electrophilic B(9)-amination of o-carborane driven by HOTf and HFIP

An efficient B(9) electrophilic amination of o-carboranes with azodicarboxylates, promoted by a Brønsted acid and HFIP, was developed.

Mechanochemical Iridium(III)-Catalyzed B-Amidation of o-Carboranes with Dioxazolones

Mechanochemistry was successfully applied to the functionalization of carboranes. The mechanochemical iridium(III)-catalyzed regioselective B(3)- and B(4)-amidation of unsubstituted o-carboranes with dioxazolones was developed. In addition, the mechanochemical iridium(III)-catalyzed regioselective B(4)-amidation of substituted o-carboranes was demonstrated. Because mechanochemical B-amidation proceeds smoothly without organic solvents or external heating, the present method is regarded as a sustainable and environmentally friendly surrogate for typical solvent-based reactions.

Electrooxidative B-H Functionalization of nido-Carboranes

An atom-economical method for the direct B-H functionalization of nido-carboranes (7,8-nido-C₂ B₉ H₁₂ ^- ) has been developed under electrochemical reaction conditions. In this reaction system, anodic oxidation serves as a green alternative for traditional chemical oxidants in the oxidation of nido-carboranes. No transition-metal catalyst is required and different heteroatoms bearing a lone pair are reactive in this transformation. Coupling nido-carboranes with thioethers, selenides, tellurides, N-heterocycles, phosphates, phosphines, arsenides and antimonides demonstrates high site-selectivity and efficiency. Importantly, nido-carboranes can be easily incorporated into drug motifs through this reaction protocol.

Expanding the Scope of Palladium-Catalyzed B - N Cross-Coupling Chemistry in Carboranes

Over the past several years, a number of strategies for the functionalization of dicarba-closo-dodecaboranes (carboranes) have emerged. Despite these developments, B - N bond formation on the carborane scaffold remains a challenge due to the propensity of strong nucleophiles to partially deboronate the parent closo-carborane cluster into the corresponding nido form. Here we show that azide, sulfonamide, cyanate, and phosphoramidate nucleophiles can be straightforwardly cross-coupled onto the B(9) vertices of the o- and m-carborane core from readily accessible precursors without significant deboronation by-products, laying the groundwork for further study into the utility and properties of these new B-aminated carborane species. We further showcase select reactivity of the installed functional groups highlighting some unique features stemming from the combination of the electron-donating B(9) position and the large steric profile of the B-connected carborane substituent.

Carborane Guests for Cucurbit[7]uril Facilitate Strong Binding and On-Demand Removal

High-affinity guests have been reported for the macrocyclic host cucurbit[7]uril (CB[7]), enabling widespread applications, but hindering CB[7] materials from being returned to their guest-free state for reuse. Here, we present polyhedral boron clusters (carboranes) as strongly binding, yet easily removable, guests for CB[7]. Aided by a Pd-catalyzed coupling of an azide anion, we prepared boron-functionalized 9-amino-ortho-carborane that binds to CB[7] with a K_a ≈ 10¹⁰ M^-1. Upon basic treatment, ortho-carborane readily undergoes deboronation to yield anionic nido-carborane, a poor guest for CB[7], facilitating recovery of guest-free CB[7]. We showcase the utility of the modified ortho-carborane guest by recycling a CB[7]-functionalized resin. With this report, we introduce stimuli-responsive decomplexation as an additional consideration in the design of high-affinity host-guest complexes.

Off-Cycle Processes in Pd-Catalyzed Cross-Coupling of Carboranes

Off-cycle processes in catalytic reactions can dramatically influence the outcome of the chemical transformation and affect its yield, selectivity, rate, and product distribution. While the generation of off-cycle intermediates can complicate reaction coordinate analyses or hamper catalytic efficiency, the generation of such species may also open new routes to unique chemical products. Recently, we reported the Pd-mediated functionalization of carboranes with a range of O-, N-, and C-based nucleophiles. By utilizing a Pd-based catalytic system supported by a biaryl phosphine ligand developed by Buchwald and co-workers, we discovered an off-cycle isomerization process ("cage-walking") that generates four regioisomeric products from a single halogenated boron cluster isomer. Here we describe how several off-cycle processes affect the regioisomer yield and distribution during Pd-catalyzed tandem cage-walking/cross-coupling. In particular, tuning the transmetallation step in the catalytic cycle allowed us to incorporate the cage-walking process into Pd-catalyzed cross-coupling of sterically unencumbered substrates, including cyanide. This work demonstrates the feasibility of using tandem cage-walking/cross-coupling as a unique low-temperature method for producing regioisomers of mono-substituted carboranes.

Improved synthesis of icosahedral carboranes containing exopolyhedral B-C and C-C bonds

Carboranes are boron-rich molecular clusters possessing electronic characteristics that allow for orthogonal approaches to vertex-selective modifications. We report improved functionalization methods utilizing orthogonal chemistry to achieve efficient substitution at electron-rich B-vertices and electron-poor C-vertices of carborane. Functionalization of B-vertices with alkyl and (hetero)aryl groups using the corresponding Grignard reagents has been improved through the use of a Pd-based precatalyst featuring an electron-rich biaryl phosphine ligand, resulting in reduced reaction times. Importantly, this method is tolerant towards alkyl-based Grignard reagents containing β-hydrogens. Furthermore, a transition metal-free approach to the substitution of carborane C-vertices with (hetero)aryl substrates has been developed under nucleophilic aromatic substitution (SNAr) conditions. The selective substitution of carboranes afforded by these methods holds potential for the rational synthesis of heterofunctionalized boron clusters with substituents on both boron and carbon-based vertices.

Metal-catalyzed cross-coupling chemistry with polyhedral boranes

Over the past several decades, metal-catalyzed cross-coupling has emerged as a very powerful strategy to functionalize carbon-based molecules. More recently, some of the cross-coupling methodologies have been adapted to inorganic compounds including boron-rich clusters. The development of this chemistry relies on the ability to synthesize halogenated boron-rich clusters which can serve as electrophilic cross-coupling partners with nucleophilic substrates in the presence of a metal catalyst. While the cross-coupling chemistry with boron-clusters is conceptually reminiscent of that of its hydrocarbon counterparts, several key aspects including the spheroidal bulk of clusters and the distinct nature of boron-halogen/boron-heteroatom bonds make this chemistry unique. The utility of metal-catalyzed cross-coupling can be extended to several classes of polyhedral boranes including neutral and anionic carboranes, metallaboranes, and carbon-free boranes. Importantly, cross-coupling enables a suite of boron-heteroatom (C, N, O, P, S) couplings to prepare boron cluster-based systems that can be used for ligand design, medicinal chemistry, and materials applications.

Electronic versus steric control in palladium complexes of carboranyl phosphine-iminophosphorane ligands

A new family of carboranyl phosphine-iminophosphorane ligands was prepared and characterized. The new ligands present a carboranyl group directly attached to the iminophosphorane nitrogen atom through a cage carbon atom (C-carboranyl derivatives L1-L3) or through the B3 boron atom (B-carboranyl derivatives L4 and L5), and the phosphine group on a side chain derived from the diphosphine dppm, i.e. with a two-atom spacer between the P and N donor atoms. The non-carboranyl analogue L6, with a biphenyl group on the nitrogen atom, was also synthesized for comparison. These potential (P, N) ligands were used to obtain palladium complexes (Pd1-Pd6) and, thus, study how the different inductive effect of the carboranyl substituents can modify the coordinating ability of the nitrogen atom. The structural analysis of the complexes revealed two different coordination modes for the ligands: the (P, N) chelate coordination and the unexpected P-terminal coordination, which is not observed for non-carboranyl phosphine-iminophosphoranes. These unexpected structural differences led us to perform DFT calculations on the ligands and metal complexes. The calculations show that the final coordination modes depend on the balance between the electronic and steric properties of the particular carboranyl group.

A facile approach for the synthesis of nido-carborane fused oxazoles via one pot deboronation/cyclization of 9-amide-o-carboranes

One pot deboronation/cyclization of 9-amide-o-carboranes for construction of nido-7,8-carborane fused oxazoles.

Quantitative Assessment of Substitution NMR Effects in the Model Series of o-Carborane Derivatives: α-Shift Correlation Method

The principles of a new α-shift correlation (ASC) NMR method are demonstrated on a model series of substituted derivatives of o-carborane for which reliable NMR data are available. This graphical method revealed an acceptable linear correlation between α(¹¹B) or α(¹³C) shifts and those induced by substituents in unsubstituted (u) positions of the carborane cluster. The linearity holds for all nuclei involved in skeletal bonding: Δδ(N)_u = g × α (where N = ¹¹B, ¹³C, and ¹H). The factor g (slope of the correlation line × 10²) becomes an important measure of sensitivity of a given cage position to substituent changes. The β, γ, and δ = A (= antipodal) shifts can be therefore derived from the α-shift, are linearly proportional, and reflect additive character in double substitution. The ASC method appears to be an important tool for quantitative assessment of substituent NMR effects in all exo-substituted boron-cluster systems.

Dehydrogenative cross-coupling of o-carborane with thiophenes via Ir-catalyzed regioselective cage B–H and C(sp2)–H activation

Iridium-catalyzed carboxylic acid directed site-selective B–H/C–H dehydrogenative cross-coupling of o-carborane with thiophenes has been achieved for the first time, leading to the preparation of 4-thienyl-o-carboranes in a simple one-pot process for potential applications in materials.

B–H functionalization of the monocarba-closo-dodecaborate anion by rhodium and iridium catalysis

The regioselective derivatization of the monocarba-closo-dodecaborate anion via catalytic B–H bond activation is reported.

Blue Phosphorescent Zwitterionic Iridium(III) Complexes Featuring Weakly Coordinating nido-Carborane-Based Ligands

We report the development of a new class of phosphorescent zwitterionic bis(heteroleptic) Ir(III) compounds containing pyridyl ligands with weakly coordinating nido-carboranyl substituents. Treatment of phenylpyridine-based Ir(III) precursors with C-substituted ortho-carboranylpyridines in 2-ethoxyethanol results in a facile carborane deboronation and the formation of robust and highly luminescent metal complexes. The resulting nido-carboranyl fragments associate with the cationic Ir(III) center through primarily electrostatic interactions. These compounds phosphoresce at blue wavelengths (450-470 nm) both in a poly(methyl methacrylate) (PMMA) matrix and in solution at 77 K. These complexes display structural stability at temperatures beyond 300 °C and quantum yields greater than 40%. Importantly, the observed quantum yields correspond to a dramatic 10-fold enhancement over the previously reported Ir(III) congeners featuring carboranyl-containing ligands in which the boron cluster is covalently attached to the metal. Ultimately, this work suggests that the use of a ligand framework containing a weakly coordinating anionic component can provide a new avenue for designing efficient Ir(III)-based phosphorescent emitters.

[3-N2-o-C2B10H11][BF4]: a useful synthon for multiple cage boron functionalizations of o-carborane

A simple and efficient method for selective cage B(3) multiple functionalization of o-carborane is described. Reaction of [3-N2-o-C2B10H11][BF4] with various kinds of nucleophiles gave a very broad spectrum of cage B(3)-substituted o-carborane derivatives, 3-X-o-C2B10H11 (X = OH, SCN, NH2, NO2, N3, CF3, PO(C6H5)2, etc). This reaction may serve as another efficient [18F]-radiolabeling method of carborane clusters for positron emission tomography applications.

Synthesis and 11C-Radiolabelling of 2-Carboranyl Benzothiazoles

Dicarba-closo-dodecaboranes, commonly known as carboranes, possess unique physico-chemical properties and can be used as hydrophobic moieties during the design of new drugs or radiotracers. In this work, we report the synthesis of two analogues of 2-(4-aminophenyl)benzothiazole (a compound that was found to elicit pronounced inhibitory effects against certain breast cancer cell lines in vitro) in which the phenyl ring has been substituted by a m-carborane cage. Two different synthetic strategies have been used. For the preparation of 1-(9-amino-1,7-dicarba-closo-dodecaboran-1-yl)-benzo-thiazole, the benzothiazole group was first introduced on one of the cluster carbon atoms of m-carborane and the amine group was further attached in three steps. For the synthesis of 1-(9-amino-1,7-dicarba-closo-dodecaboran-1-yl)-6-hydroxybenzothiazole, iodination was performed before introducing the benzothiazole group, and the amino group was subsequently introduced in six steps. Both compounds were radiolabelled with carbon-11 using [¹¹C]CH₃OTf as the labelling agent. Radiolabelling yields and radiochemical purities achieved should enable subsequent in vitro and in vivo investigations.

Carborane-Containing Hydroxyamidine Scaffolds as Novel Inhibitors of Indoleamine 2,3-Dioxygenase 1 (IDO1)

Two new carborane-containing hydroxyamidines were prepared as potential inhibitors of the indoleamine 2,3-dioxygenase 1 (IDO1) enzyme. One compound (3) displayed low micromolar (1.90 μM) inhibition of IDO1, with the related compound (4) displaying >5-fold lower inhibitory activity, i.e. subtle differences in structure between the two carborane compounds led to dramatic changes in inhibitor binding. In silico docking experiments unravel a possible molecular mechanism that is consistent with the observed difference in IDO1 binding for 3 and 4 and also for the phenyl bioisosteres 1 and 2.

Syntheses and structural characterization of o-carboranylamides with direct cage-amide bond

Reactions of lithio-o-carborane with isocyanates under various conditions were studied, and the structural features of the resulting carboranylamides are described. The reactions of o-carborane (o-C2B10H12), n-BuLi (two equiv.) and two equiv. of (substituted) phenylisocyanate, pentylisocyanate and p-ethylphenylthioisocyanate in diethyl ether, respectively, led, after workup, to the corresponding mono-substituted carboranylamide 2a-g and carboranylthioamide 5 in low to moderate yields, and only with RNCO (R = Ph, m-MeOC6H4, pentyl) could disubstituted products 3a-c be isolated. The reaction with phenylisocyanate afforded the mono-amide and di-amide products in a ratio of approximately 1 : 2, whereas in the other two reactions the ratios are approximately 4 : 1 and 3 : 2, respectively. In tetrahydrofuran all the reactions attempted with RNCO (R = Ph, p-IC6H4, m-NCC6H4 and pentyl) gave more monoamide products than those in diethyl ether. With phenylisocyanate no diamide product was isolated and with pentylisocyanate the ratio between monoamide and diamide is approximately 3.5 : 1. The new carboranylamides were characterized by means of elemental analyses, IR and NMR spectroscopy and mass spectrometry, as well as single-crystal X-ray diffraction analyses of 2a-f, 3a and 5.