Old Key Opens the Lock in Carborane: The in Situ NHC-Palladium Catalytic System for Selective Arylation of B(3,6)–H Bonds of o-Carboranes via B–H Activation

Post-coordination of Ru(ii) controlled regioselective B(4)-H acylmethylation of o-carboranes with sulfoxonium ylides

Despite significant progress in the B-H functionalization of carboranes, the development of cost-effective catalytic systems devoid of noble metals, coupled with mechanistic validation of regioselectivity control, remains a formidable challenge. Herein, we disclose an Ag salt-free, redox-neutral, and inexpensive ruthenium(ii)-catalyzed protocol that enables exclusive B(4)-H acylmethylation of o-carboranes through a novel post-coordination strategy. By exploiting weakly coordinating carboxylic acid as a traceless directing group, this method achieves excellent mono-site selectivity for B-C(sp3) bond formation using diverse sulfoxonium ylides, demonstrating both functional group tolerance and synthetic scalability. This work not only establishes a practical synthetic platform but also addresses critical mechanistic questions unresolved in prior analogous studies. Through deuterium labeling, in situ high-resolution mass spectrometry (HRMS) tracking, and single-crystal X-ray analysis of critical Ru intermediates, we unequivocally demonstrate that the mono-site selectivity originates from a unique post-coordination mode of Ru(ii). The Ru catalyst simultaneously engages both the carboxylic acid and the enolizable acylmethyl moiety in the mono-acylated intermediate, thereby dictating the B(4)-H activation trajectory. Our findings establish a generalizable platform for regiocontrolled carborane functionalization while defining mechanistic paradigms in transition metal-mediated B-H activation chemistry.

Palladium-Catalyzed Regioselective B(3,4,5,6)-H Tetra-Arylation of o-Carboranes

A transition-metal-catalyzed iterative multiple cage B-H activation reaction for constructing multifunctionalization of o-carboranes in one pot is challenging. Herein, palladium-catalyzed regioselective tetra-arylation of a wide range of C(1)-N-aryl-o-carboranyl amides with aryl iodides has been developed. A variety of B(3,4,5,6)-tetra-arylated o-carboranes were synthesized in good-to-excellent yields. Moreover, the mono-, di-, and triarylated intermediate products were isolated, and the exact structures were determined by NMR, high-resolution mass spectrometry, and X-ray analysis, which provide a rationale that the order of introduction of aryl groups into o-carborane is B(4) > B(5) > B(3)/B(6). This protocol represents a powerful synthetic method for constructing polyfunctionalization of o-carborane derivatives under very mild and simple reaction conditions, which offers a valuable reference for the design and synthesis of molecular propellers based on carboranes.

Catalytic regioselective 3,4-difunctionalization of 3-iodo-o-carborane via Pd migration

A Pd-catalyzed one-pot regioselective difunctionalization of 3-iodo-o-carborane has been achieved for the synthesis of a wide variety of 3-alkyl-4-Nu-o-carboranes (Nu = aryl, alkyl, amino, or thio groups) in moderate to excellent yields. This protocol combines the sequential activation of cage B(3)-I and B(4)-H bonds via Pd 1,4-migration.

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.

Dative Bonding Activation Enables Precise Functionalization of the Remote B-H Bond of nido-Carborane Clusters

The innovation of synthetic strategies for selective B-H functionalization is a pivotal objective in the realm of boron cluster chemistry. However, the precise, efficient, and rapid functionalization of a B-H bond of carboranes that is distant from the existing functional groups remains intractable owing to the limited approaches for site-selective control from the established methods. Herein, we report a dative bonding activation strategy for the selective functionalization of a nonclassical remote B-H site of nido-carboranes. By leveraging the electronic effects brought by the exopolyhedral B(9)-dative bond, a cross-nucleophile B-H/S-H coupling protocol of the distal B(5)-H bond has been established. The dative bond not only amplifies the subtle reactivity difference among B-H bonds but also significantly changes the reactive sites, further infusing nido-carboranes with additional structural diversity. This reaction paradigm features mild conditions, rapid conversion, efficient production, broad scope, and excellent group tolerance, thus enabling the applicability to an array of complex bioactive molecules. The efficient and scalable reaction platform is amenable to the modular construction of photofunctional molecules and boron delivery agents for boron neutron capture therapy. This work not only provides an unprecedented solution for the selective diversification of distal B-H sites in nido-carboranes but also holds the potential for expediting the discovery of novel carborane-based functional molecules.

Couple-close construction of non-classical boron cluster-phosphonium conjugates

Heteropolycyclic molecular systems, which are essential components in the fields of materials and pharmacology, frequently consist of 2D extended organic aromatic rings. Here, we introduce a type of inorganic-organic hybrid 3D conjugates by merging an aromatic boron cluster with a phosphine and a π-conjugated unit. To achieve this, a couple-close synthetic strategy via B-H activation of nido-carboranes with alkynes has been developed, which leads to diverse boron cluster-extended phosphoniums in a twisted structure with high yields under mild conditions. Experimental and theoretical results reveal that the fusion between the boron cluster and the formed borophosphonium heterocycle facilitates electron delocalization throughout the structure. The unusual framework demonstrates distinct properties from bare boron clusters and pure aromatic ring-extended counterparts, such as improved thermal/chemical stability and photophysical properties. Thus, the boron cluster-based 3D conjugates expand the library of aromatic-based heterocyclics, showcasing great potential in functional materials.

Palladium-Catalyzed Regioselective B(3,5)-Dialkenylation and B(4)-Alkenylation of o-Carboranes

Picolyl group directed B(3,5)-dialkenylation and B(4)-monoalkenylation of o-carboranes has been developed with a very low palladium catalyst loading. The degree of substitution is determined by the cage C(2)-substituents due to steric reasons. On the basis of experimental results, a plausible mechanism is proposed including electrophilic palladation and alkyne insertion followed by protonation.

Iridium-catalyzed selective arylation of B(6)-H of 3-aryl-o-carboranes with arylboronic acid via direct B-H activation

This work discloses an iridium-catalyzed selective arylation of B(6)-H of 3-Ar-o-carboranes with arylboronic acid via direct B-H activation for the first time. A series of unsymmetric and symmetric 3,6-diaryl-o-carboranes decorated with diverse active groups have been synthesized with moderate to excellent yields under mild conditions. This work offers an efficient approach for selective arylation of B(6)-H with arylboronic acid and has important value for selective functionalization of o-carboranes.

Ir-Catalyzed B(3)-Amination of o-Carboranes with Amines via Acceptorless Dehydrogenative BH/NH Cross-Coupling

An efficient and convenient strategy for Ir-catalyzed selective B(3)-amination of o-carboranes with amines via acceptorless BH/NH dehydrocoupling was developed, affording a series of B(3)-aminated-o-carboranes in moderate to high isolated yields with H2 gas as a sole by-product. Such an oxidant-free system endues the protocol sustainability, atom-economy and environmental friendliness. A reaction mechanism via an Ir(I)-Ir(III)-Ir(I) catalytic cycle involving oxidative addition, dehydrogenation and reductive elimination was proposed.

Pd-NHC catalysed regioselective activation of B(3,6)-H of o-carborane - a synergy between experiment and theory

Regioselective B-H activation of o-carboranes is an effective way for constructing o-carborane derivatives, which have broad applications in medicine, catalysis and the wider chemical industry. However, the mechanistic basis for the observed selectivities remains unresolved. Herein, a series of density functional theory (DFT) calculations were employed to characterise the palladium N-heterocyclic carbene (Pd-NHC) catalysed regioselective B(3,6)-diarylation of o-carboranes. Computational results at the IDSCRF(ether)-LC-ωPBE/BS1 and IDSCRF(ether)-LC-ωPBE/BS2 levels showed that the reaction undergoes a Pd(0) → Pd(II) → Pd(0) oxidation/reduction cycle, with the regioselective B(3)-H activation being the rate-determining step (RDS) for the full reaction profile. The computed RDS free energy barrier of 24.3 kcal mol-1 agrees well with the 82% yield of B(3,6)-diphenyl-o-carborane in ether solution at 298 K after 24 hours of reaction. The Ag2CO3 additive was shown to play a crucial role in lowering the RDS free energy barrier and facilitating the reaction. Natural charge population (NPA) and molecular surface electrostatic potential (ESP) analyses successfully predicted the experimentally observed regioselectivities, with electronic effects being revealed to be the dominant contributors to product selectivity. Steric hindrance was also shown to impact the reaction rate, as revealed by experimental and computational characterisation studies of substituents and ligand effects. Furthermore, computational predictions aligned with the experimental findings that NHC ligands outperform the phosphine ones for this particular reaction. Overall, the observed trends reported in this work are expected to assist in the rational optimisation of the efficiency and regioselectivity of this and related reactions.

Iridium-catalyzed selective amination of B(4)-H for the synthesis of o-carborane-fused indolines

An iridium-catalyzed selective amination of B(4)-H via dehydrogenative cross-coupling of B-H/N-H bonds for the synthesis of o-carborane-fused indolines has been developed for the first time. Various types of unprecedented o-carborane-fused indolines have been synthesized, which would be potential candidates for applications in drug discovery, pharmaceutical chemistry and functional materials. This work offers a valuable reference for the designing and synthesis of o-carborane-fused heterocycles.

Iridium-Catalyzed Selective B(4)-H Amination of o-Carboranes with Anthranils

We report here a catalytic selective cage B4-H amination of o-carboranes employing an Ir(III) complex as a catalyst and anthranils as aminating agents, leading to a large class of B4-aminated o-carboranes with very high yields and a broad substrate scope under mild conditions without any oxidants. In these reactions, the carboxyl group serves as a traceless directing unit to determine the site selectivity and degree of substitution.

Cu(OTf)2/NBS-Mediated Tandem Reaction of 1-Cinnamyl Alcohol-o-Carboranes via Ring Opening of Oxetane with Arenes: An Alternative Approach for the Synthesis of C-Alkenyl-o-Carboranes

The Cu(OTf)₂/NBS-mediated tandem reaction of 1-cinnamyl alcohol-o-carboranes for the synthesis of C-alkenyl-o-carboranes has been developed. Mechanism studies demonstrated that the Cu(OTf)₂-promoted ring opening of oxetane with electron-rich arenes as soft nucleophiles was involved and was the key step for the transformation. This work provided an alternative strategy for the synthesis of C-alkenyl-o-carboranes, which has an important reference for the synthesis of o-carborane derivatives with diversity.

Transition metal catalyzed selective B(3)-H or B(4)-H amination of o-carboranes via dehydrogenative BH/NH cross-coupling

A unique approach to vertex-selective catalytic B-H amination at either the B(3)- or B(4)-position in o-carboranes has been developed. Using different transition metal catalysts, dehydrogenative BH/NH cross-coupling of o-carboranes and free amines has been achieved, leading to a wide variety of cage B(3)- or B(4)-aminated o-carboranes in moderate to high yields with excellent regioselectivity, where carboranyl carboxylic acids and amines can serve as competent coupling partners without any pre-functionalization. The isolation and structural identification of a key intermediate provide an insight into the reaction mechanism in the catalytic B(4)-H amination.

Practical Synthesis of B(9)-Halogenated Carboranes with N-Haloamides in Hexafluoroisopropanol

The B(9)-H halogenation of o-carborane and m-carborane was achieved with excellent selectivities in hexafluoroisopropanol (HFIP) under simple reaction conditions: single reagent [trichloroisocyanuric acid (TCCA), tribromoisocyanuric acid (TBCA) or N-iodosuccinimide (NIS)], catalyst-free, air-/moisture-tolerant, and convenient work-up. With this method, a variety of 9-halogenated o-carboranes and m-carboranes were obtained in good to excellent yields with broad tolerance of functional groups.

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.

Palladium-Catalyzed Regioselective B(3,4)-H Acyloxylation of o-Carboranes

We disclose herein an efficient regioselective B(3,4)-H activation via a ligand strategy, affording B(3)-monoacyloxylated and B(3,4)-diacyloxylated o-carboranes. The identification of amino acid and phosphoric acid ligands is crucial for the success of B(3)-mono- and B(3,4)-diacyloxylation, respectively. This ligand approach is compatible with a broad range of carboxylic acids. The functionalization of complex drug molecules is demonstrated. Other acyloxyl sources, including sodium benzoate, benzoic anhydride, and iodobenzene diacetate, are also tolerated.

Metal-catalyzed B-H acylmethylation of pyridylcarboranes: access to carborane-fused indoliziniums and quinoliziniums

Metal-catalyzed mono-acylmethylation of pyridylcarboranes has been realized using α-carbonyl sulfoxonium ylides as a coupling partner. The reaction features high efficiency, excellent site-selectivity and good functional group tolerance. In the presence of pyridyl and enolizable acylmethyl groups, a post-coordination mode has been proposed and validated by in situ high resolution mass spectroscopy (HRMS) to rationalize the unique mono-substitution. Post-functionalization at the newly incorporated alkyl site provides additional utility of this method, including the construction of carborane-fused indoliziniums and quinoliziniums. We believe that these mono-alkylated carboranes, together with their post-functionalized derivatives, may find applications in luminescent materials and drug discovery in the near future.

Synthesis of 3-Aryl-ortho-carboranes with Sensitive Functional Groups

A simple and efficient method was developed for the one-pot synthesis of 3-aryl derivatives of ortho-carborane with sensitive functional groups using 3-iodo-ortho-carborane and aryl zinc bromides that were generated in situ. A series of 3-aryl-ortho-carboranes, including those containing nitrile and ester groups, 3-RC₆H₄-1,2-C₂B₁₀H₁₁ (R = p-Me, p-NMe₂, p-OCH₂OMe, p-OMe, o-CN, p-CN, o-COOEt, m-COOEt, p-COOEt) was synthesized using this approach. The solid-state structures of 3-RC₆H₄-1,2-C₂B₁₀H₁₁ (R = p-OMe, o-CN, and p-CN) were determined by single crystal X-ray diffraction. The intramolecular hydrogen bonding involving the ortho-substituents of the aryl ring and the CH and BH groups of carborane was discussed.

A Strategy for Selective Catalytic B-H Functionalization of o-Carboranes

Carboranes are a class of polyhedral carbon-boron molecular clusters featuring three-dimensional aromaticity, which are often considered as 3D analogues of benzene. Their unique structural and electronic properties make them invaluable building blocks for applications ranging from functional materials to versatile ligands to pharmaceuticals. Thus, selective functionalization of carboranes has received tremendous research interest. In earlier days, the vast majority of the works in this area were focused on cage carbon functionalization via facile deprotonation of cage CH, followed by reaction with electrophiles. On the contrary, cage B-H activation is very challenging since the 10 B-H bonds on o-carborane are very similar, and how to achieve the desired transformation at specific boron vertex is a long-standing issue.As carbon is considered more electronegative than boron, this property results in different vertex charges on the o-carborane cage, which follow the order B(3,6)-H ≪ B(4,5,7,11)-H < B(8,10)-H < B(9,12)-H. We thought that this difference may trigger the favorite interaction of a proper transition metal complex with a specific B-H bond of carborane, which could be utilized to solve the selectivity issue. Accordingly, our strategy is described as follows: (1) electron-rich transition metal catalysts are good for the activation of the most electron-deficient B(3,6)-H bonds (connected to both cage C-H vertices); (2) electron-deficient transition metal catalysts are good for the activation of the relatively electron-rich B(8,9,10,12)-H bonds (with no bonding to either cage C-H vertices); and (3) directing-group-assisted transition metal catalysis is appropriate for the activation of the B(4,5,7,11)-H bonds (connected to only one cage C-H vertex), whose vertex charges lie in the middle of the range for the 10 B-H bonds. This strategy has been successfully applied by our laboratory and other groups in the development of a series of synthetic routes for catalytic selective activation of B-H bonds of the carborane cage, resulting in the synthesis of a large number of cage-boron-functionalized carborane derivatives in a regioselective and catalytic fashion. Subsequently, significant progress in this emerging area has been made.In 2013 we reported the selective tetrafluorination of o-carboranes at the B(8,9,10,12)-H bonds using an electron-deficient Pd(II) salt, [Pd(MeCN)₄][BF₄], as the catalyst. In 2014 we disclosed the first example of carboxy-directed alkenylation of o-carboranes at the B(4) vertex promoted by an Ir(III) catalyst. Subsequently, in 2017 we presented an electron-rich Ir(I)-catalyzed diborylation of o-carboranes at the B(3,6)-H bonds. We also uncovered the first example of Pd-catalyzed asymmetric synthesis of chiral-at-cage o-carboranes in 2018. These proof-of-principle studies have greatly stimulated research activities in selective B-H activation of carboranes and boron clusters enabled by transition metal catalysts. We have so far developed a toolbox of synthetic methods for selective catalytic cage B-olefination, -arylation, -alkenylation, -alkynylation, -oxygenation, -sulfenylation, -borylation, -halogenation, and -amination. We have recently expanded our research to base metal catalysis. As the field progresses, we expect that other methods for regioselective cage B-H activation will be invented, and the results detailed in this Account will promote these efforts.

Electrooxidative o-carborane chalcogenations without directing groups: cage activation by copper catalysis at room temperature

Copper-catalyzed electrochemical direct chalcogenations of o-carboranes was established at room temperature. Thereby, a series of cage C-sulfenylated and C-selenylated o-carboranes anchored with valuable functional groups was accessed with high levels of position- and chemo-selectivity control. The cupraelectrocatalysis provided efficient means to activate otherwise inert cage C-H bonds for the late-stage diversification of o-carboranes.

Photoredox B-H functionalization to selective B-N(sp3) coupling of nido-carborane with primary and secondary amines

Access to nido-carborane site-selective B-N(sp³) coupling by photoredox catalysed B-H activation has been achieved for the first time, which leads to the synthesis of a series of nitrogen-containing nido-carboranes with moderate to good yields. This protocol is applicable to primary and secondary amines containing alkyl, or heteroaryl groups as well as sulfonamides. Furthermore, the open to air and metal-free conditions with excellent site-selectivity represent a significant improvement for B-H functionalization of nido-carboranes with organic functionalities.

Pd-catalyzed selective tetrafunctionalization of diiodo-o-carboranes

A palladium-catalyzed highly selective tetrafunctionalization of 3,6-I2-o-carborane and 4,7-I2-o-carborane has been developed, leading to the preparation of 3,6-dialkenyl-4,11-R2-o-carboranes and 4,7-dialkenyl-5,11-R2-o-carboranes (R = alkyl, allyl and aryl) in moderate to excellent yields. This represents a new strategy for selective synthesis of polyfunctionalized o-carborane derivatives via a one-pot process.

Progress in the medicinal chemistry of organoboron compounds

The review aims to draw attention to the latest advances in the organoboron chemistry and therapeutic use of organoboron compounds. The synthetic strategies towards boron-containing compounds with proven in vitro and/or in vivo biological activities, including derivatives of boronic acids, benzoxaboroles, benzoxaborines and benzodiazaborines, are summarized. Approaches to the synthesis of hybrid structures containing an organoboron moiety as one of the pharmacophores are considered, and the effect of this modification on the pharmacological activity of the initial molecules is analyzed. On the basis of analysis of the published data, the most promising areas of research in the field of organoboron compounds are identified, including the latest methods of synthesis, modification and design of effective therapeutic agents.

The bibliography includes 246 references.

Selective B(5,8,9)-Triarylation Reaction of o-Carboranes through Determination of the Order of Introduction of Aryl Groups into B(4)-Acylamino-o-carboranes

Palladium-catalyzed iterative cage B-H arylation reaction of a wide range of B(4)-acylamino-o-carboranes with aryl iodides has been developed, leading to the formation of B(5,8,9)-triarylated B(4)-acylamino-o-carboranes with excellent regioselectivity. Moreover, B(5,8,9)-triarylated carboranes bearing three different aryl groups were synthesized from B(4)-acylamino-o-carborane and three different aryl iodides. The order of introduction [B(9) > B(8) > B(5)] of aryl groups into the B(5,8,9)-triarylation reaction was determined for the first time through NMR monitoring and X-ray analyses.

Palladium-catalyzed selective B(3)-H arylation of o-carboranes with arylboronic acids at room temperature

A palladium-catalyzed selective B(3)-H arylation of o-carboranes at room temperature has been developed using readily available arylboronic acids as the aryl source, and the corresponding 3-aryl-o-carboranes were obtained in good to excellent yields. This method provides a powerful synthetic route for constructing polysubstituted o-carborane derivatives.

Rh-Catalyzed Decarbonylative Cross-Coupling between o-Carboranes and Twisted Amides: A Regioselective, Additive-Free, and Concise Late-Stage Carboranylation

The convenient cross-coupling of sp² or sp³ carbons with a specific boron vertex on carborane cage represents significant synthetic values and insurmountable challenges. In this work, we report an Rh-catalyzed reaction between o-carborane and N-acyl-glutarimides to construct various B_cage -C bonds. Under the optimized condition, the removable imine directing group (DG) leads to B(3)- or B(3,6)-C couplings, while the pyridyl DG leads to B(3,5)-Ar coupling. In particular, an unexpected rearrangement of amide reagent is observed in pyridyl directed B(4)-C(sp³ ) formation. This scalable protocol has many advantages, including easy access, the use of cheap and widely available coupling agents, no requirement of an external ligand, base or oxidant, high efficiency, and a broad substrate scope. Leveraging the Rh^I dimer and twisted amides, this method enables straightforward access to diversely substituted and therapeutically important carborane derivatives at boron site, and provides a highly valuable vista for carborane-based drug screening.

Pd-Catalyzed sequential B(3)–I/B(4)–H bond activation for the synthesis of 3,4-benzo-o-carboranes

A Pd-catalyzed sequential B(3)–I and B(4)–H bond activation was developed for the synthesis of 3,4-benzo-o-carboranes via a formal [2 + 2 + 2] cycloaddition.

The in Situ NHC-Palladium Catalyzed Selective Activation of B(3)-H or B(6)-H Bonds of o-Carboranes for Hydroboration of Alkynes: An Efficient Approach to Alkenyl-o-carboranes

An in situ Pd-NHC catalyzed selective B(3,6)-H activation for hydroboration of internal alkynes has been accomplished under mild conditions. This work offers a facile approach for the synthesis of alkenyl-o-carboranes and has important reference for selective functionalization of B(3,6)-H bonds.

Palladium Catalyzed Selective B(3)-H Activation/Oxidative Dehydrogenative Coupling for the Synthesis of Bis(o-carborane)s

A palladium catalyzed selective B(3)-H activation/oxidative dehydrogenative coupling for the synthesis of bis(o-carborane)s connected with B(3)-B(3') and B(3)-B(6') bonds has been developed for the first time. A plausible mechanism involving stepwise activation of B(3)-H and B(3'/6')-H bonds by Pd^II and Pd^IV was proposed. This work is the first example and the most efficient protocol for synthesis of bis(o-carborane)s connected with B(3)-B(3') and B(3)-B(6') bonds, which has important reference for design, synthesis, and application of bis(o-carborane)s in related fields.

Regioselective B(3,4)-H arylation of o-carboranes by weak amide coordination at room temperature

Palladium-catalyzed regioselective di- or mono-arylation of o-carboranes was achieved using weakly coordinating amides at room temperature. Therefore, a series of B(3,4)-diarylated and B(3)-monoarylated o-carboranes anchored with valuable functional groups were accessed for the first time. This strategy provided an efficient approach for the selective activation of B(3,4)-H bonds for regioselective functionalizations of o-carboranes.

Synthesis of Polyhedral Borane Cluster Fused Heterocycles via Transition Metal Catalyzed B-H Activation

Aromatic heterocycles are ubiquitous building blocks in bioactive natural products, pharmaceutical and agrochemical industries. Accordingly, the carborane-fused heterocycles would be potential candidates in drug discovery, nanomaterials, metallacarboranes, as well as photoluminescent materials. In recent years, the transition metal catalyzed B-H activation has been proved to be an effective protocol for selective functionalization of B-H bond of o-carboranes, which has been further extended for the synthesis of polyhedral borane cluster-fused heterocycles via cascade B-H functionalization/annulation process. This article summarizes the recent progress in construction of polyhedral borane cluster-fused heterocycles via B-H activation.