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

Iridium(I)-catalyzed regioselective B(4)-alkenylation has been developed from o-carboranyl sulfoxonium ylides and alkynes through B(4)-H activation. The sequential B(4)- and B(6)-alkenylation afforded B(3,5)-dialkenylated o-carboranyl sulfoxonium ylides in one pot. Eventually, two alkenyl groups, the same or different, were introduced at positions 3 and 5 of the carborane. Sulfoxonium ylide used as a directing group remains available for further functionalization and is converted to B-alkenylated o-carboranyl trichloromethyl ketones.

Facile Construction of New Hybrid Conjugation via Boron Cage Extension

Aromatic polycyclic systems have been extensively utilized as structural subunits for the preparation of various functional molecules. Currently, aromatics-based polycyclic systems are predominantly generated from the extension of two-dimensional (2D) aromatic rings. In contrast, polycyclic compounds based on the extension of three-dimensional (3D) aromatics such as boron clusters are less studied. Here, we report three types of boron cluster-cored tricyclic molecular systems, which are constructed from a 2D aromatic ring, a 3D aromatic nido-carborane, and an alkyne. These new tricyclic compounds can be facilely accessed by Pd-catalyzed B-H activation and the subsequent cascade heteroannulation of carborane and pyridine with an alkyne in an isolated yield of up to 85% under mild conditions without any additives. Computational results indicate that the newly generated ring from the fusion of the 3D carborane, the 2D pyridyl ring, and an alkyne is non-aromatic. However, such fusion not only leads to a ¹H chemical shift considerably downfield shifted owing to the strong diatropic ring current of the embedded carborane but also devotes to new/improved physicochemical properties including increased thermal stability, the emergence of a new absorption band, and a largely red-shifted emission band and enhanced emission efficiency. Besides, a number of bright, color-tunable solid emitters spanning over all visible light are obtained with absolute luminescence efficiency of up to 61%, in contrast to aggregation-caused emission quenching of, e.g., Rhodamine B containing a 2D-aromatics-fused structure. This work demonstrates that the new hybrid conjugated tricyclic systems might be promising structural scaffolds for the construction of functional molecules.

Selective Labeling of Peptides with o-Carboranes via Manganese(I)-Catalyzed C-H Activation

A robust method for the selective labeling of peptides via manganese(I) catalysis was devised to achieve the C-2 alkenylation of tryptophan containing peptides with 1-ethynyl-o-carboranes. The manganese-catalyzed C-H activation was accomplished with high catalytic efficiency, and featured low toxicity, high functional group tolerance and excellent E-stereoselectivity. This approach unravels a promising tool for the assembly of o-carborane with structurally complex peptides of relevance to applications in boron neutron capture therapy.

Iridium(III)-Catalyzed B(4)-Acylmethylation and B(3,5)-Diacylmethylation from o-Carboranes and Sulfoxonium Ylides

An iridium(III)-catalyzed regioselective acylmethylation of the cage B(4)-H bond in o-carborane acids with sulfoxonium ylides is demonstrated through B(4)-H activation in ethanol under very mild conditions, affording a number of B(4)-acylmethylated o-carboranes. Additionally, the selective sequential B(4)- and B(6)-acylmethylation reactions finally gave B(3,5)-diacylmethylated o-carboranes in one pot.

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.

Regioselectivity of Pd-catalyzed o-carborane arylation: a theoretical view

B(3)-Arylation is unfavorable because the steric repulsion between the substituent group on C(2) and the metal moiety would lead to significant distortion of o-carborane and would result in a higher activation energy for reductive elimination.

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.

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.

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.

Theoretical calculation of regioselectivity and solvation effects on B-H activation of O-carborane guided by directing group

This study focuses on uncovering the regioselectivity, directing group, ligand, and solvation effect in B-H activation, which was investigated by DFT calculations. The reaction mechanism was investigated in vacuum, and the advantageous reaction pathway and rate-determining step were determined. Furthermore, the solvation effects and the ligand that coordinated with Pd were studied. The results showed that in neutral and cationic pathways, the anion (OTf)/ligand (1,10-phenanthroline) exerted significant influence on the transition metal catalytic center Pd, thus affecting B-H activation at different sites. The solvation effects also exerted significant influence on the reaction. The greater the polarity of the solvent, the greater the influence on the energies of all stationary points.

Stepwise B–H bond activation of a meta-carborane

Stepwise multiple B–H bond activation is a major challenge in synthetic chemistry.

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.

B–H: site-selective B(3,4)–H arylations were accomplished at room temperature by versatile palladium catalysis enabled by weakly coordinating amides.

Nickel-Catalyzed Transformation of Alkene-Tethered Oxime Ethers to Nitriles by a Traceless Directing Group Strategy

Nickel-catalyzed transformation of alkene-tethered oxime ethers to nitriles using a traceless directing group strategy has been developed. A series of alkene-tethered oxime ethers derived from benzaldehyde and cinnamyl aldehyde derivatives were converted into the corresponding benzonitriles and cinnamonitriles in 46-98% yields using the nickel catalyst system. Control experiments showed that the alkene group tethered to an oxygen atom on the oximes via one methylene unit plays a key role as a traceless directing group during the catalysis.

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.

Palladium-catalyzed intramolecular dehydrogenative coupling of BH and OH: synthesis of carborane-fused benzoxaboroles

A Pd-catalyzed intramolecular dehydrogenative coupling of BH and OH for the construction of cage B–O bonds was developed to afford C,B-carborane-fused heterocycles.

Palladium-catalyzed regioselective synthesis of B(4,5)- or B(4)-substituted o-carboranes containing α,β-unsaturated carbonyls

B(4,5)- or B(4)-Substituted o-carboranes containing α,β-unsaturated carbonyls are regioselectively synthesized through a Pd-catalyzed decarboxylation cross coupling reaction.

Catalytic Cascade Dehydrogenative Cross-Coupling of BH/CH and BH/NH: One-Pot Process to Carborano-Isoquinolinone

A proof-of-principle study of cascade dehydrogenative cross-coupling of carboranyl carboxylic acid with readily available benzamide has been achieved, resulting in the facile synthesis of previously inaccessible carborano-isoquinolinone derivatives in a simple one-pot process, in which two cage B-H, one aryl C-H, and one N-H bond were sequentially activated to construct efficiently new B-C and B-N bonds, respectively. Under suitable reaction conditions, such cascade cyclization can be stopped at the first B-H/C-H cross-coupling step to give a series of α-carboranyl benzamides, suggesting the preferential occurrence of B-C cross-coupling over that of B-N. The carboxylic acid directing group plays a key role in the B-C cross-coupling step, which is then removed through in situ decarboxylation. The CV results combined with control experiments indicate that high-valent Ir(V)-species may be involved in the reaction pathways, which is crucial for such cascade dehydrogenative cross-coupling reactions. The isolation and structural identification of a key intermediate, its controlled transformations, and deuterium labeling experiments support a new Ir-nitrene-mediated amination for B-H/N-H dehydrocoupling.

Computational Investigation of Nickel-Mediated B–H Activation and Regioselective Cage B–C(sp2) Coupling of o-Carborane

Density functional theory (DFT) methods including LC-ωPBE, CAM-B3LYP, B3LYP, and B3LYP-D3, combined with double Zeta all-electron DZVP basis set, have been employed to conduct computational investigations on nickel-mediated reaction of o-carboranylzirconacycle, n-hexene, and 2-bromophenyltrimethylsilylacetylene in toluene solution. A multistep mechanism leading to the C,C,B-substituted carborane-fused tricyclics, including (1) sequential insertion of alkene and alkyne into Ni–C bonds; (2) double 1,2-migration of the TMS group; (3) B–H activation assisted by Cs2CO3 additive; and (4) reduction cage B–C (sp2) coupling, was proposed. Among these steps, the B–H activation of o-carborane was located as rate-determining step (RDS). With assistance of Cs2CO3 additive (replaced by K2CO3 in simulation), the RDS free-energy barrier at PCM-LC-ωPBE/DZVP level was calculated to be 23.1–23.9 kcal·mol−1, transferring to a half-life of 3.9–15.1 h at 298 K. The predicted half-life coincides well with 80% experimental yields of C,C,B-substituted carborane-fused tricyclics after 12 h. Kinetic data obtained by employing LC-ωPBE method also reproduced the experimental diastereoselective ratio well. Various B–H activation pathways with and without Cs2CO3 additive were taken into consideration, which illustrates Cs2CO3 as an essential guarantee for smooth occurrence of this reaction at room temperature.

Nucleophilic substitution: a facile strategy for selective B-H functionalization of carboranes

Vertex-specific functionalization of carboranes has received considerable research interest, due to the valuable application of carborane derivatives in medicine, coordination/organometallic chemistry and materials. In comparison with a protic cage C-H bond, cage B-H is hydridic and generally less polar, with a bond dissociation energy of ca. 108 kcal mol^-1. These features make B-H activation quite different from that of the C-H bond. In addition, selectivity among ten very similar BH vertices in o-carborane is challenging yet crucial to the effective construction of carborane-based functional molecules. To address these issues, a brand new strategy of cage B-H nucleophilic substitution has recently been presented and developed for straightforward and regioselective cage B-H functionalization. The regioselectivity can be controlled by the electronic/steric properties of the cage carbon substituents. This Frontier article highlights the recent advancement in the nucleophilic cage B-H substitution of carboranes.

Highly selective palladium-catalyzed one-pot, five-fold B-H/C-H cross coupling of monocarboranes with alkenes

Palladium-catalyzed dehydrogenative B-H/C-H cross coupling of monocarborane anions with alkenes is reported, allowing for the first time the isolation of selectively penta-alkenylated boron clusters. The reaction cascade is regioselective for the cage positions, leading directly to B2-6 functionalization. Under mild and convenient conditions, styrenes, benzylic alkenes and aliphatic alkenes are demonstrated to be viable coupling partners with exclusive vinyl-type B-C bond formation. Multiple subsequent transformations provide access to directing group-free products, chiral derivatives and penta-alkylated cages. The five-fold coupling, combined with the latter reactions, represents a powerful methodology for the straightforward synthesis of new classes of boron clusters.

Iridium-induced regioselective B–H and C–H activations at azo-substitutedm-carboranes: facile access to polynuclear complexes

Iridium-induced selective C–H and B(2,3)–H bond activations have been achieved atm-carboranes featuring azobenzene directing groups. A series of mononuclear, dinuclear and trinuclear complexes have been obtained.

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.

Controlled functionalization of o-carborane via transition metal catalyzed B–H activation

This review summarizes recent advances in transition metal catalyzed vertex-specific BH functionalization of o-carborane for controlled synthesis of its derivatives.