Palladium-catalyzed intermolecular coupling of o-carborane with aromatics by direct cage B−H bond activation has been achieved, leading to the synthesis of a series of cage B(4,5)-diarylated-o-carboranes in high yields with excellent regioselectivity. Traceless directing group -COOH plays a crucial role for site- and di-selectivity of such intermolecular coupling reaction. A Pd^II–Pd^IV–Pd^II catalytic cycle is proposed to be responsible for the stepwise arylation.

A visible-light-mediated in situ generation of a boron-centered carboranyl radical (o-C₂B₁₀H₁₁^.) has been described. With eosin Y as a photoredox catalyst, 3-diazonium-o-carborane tetrafluoroborate [3-N₂-o-C₂B₁₀H₁₁][BF₄] was converted into the corresponding boron-centered carboranyl radical intermediate, which can undergo efficient electrophilic substitution reaction with a wide range of (hetero)arenes. This general and simple procedure provides a metal-free alternative for the synthesis of 3-(hetero)arylated-o-carboranes.

Palladium-catalyzed intermolecular coupling of o-carborane with aromatics by direct cage B−H bond activation has been achieved, leading to the synthesis of a series of cage B(4,5)-diarylated-o-carboranes in high yields with excellent regioselectivity. Traceless directing group -COOH plays a crucial role for site- and di-selectivity of such intermolecular coupling reaction. A Pd^II–Pd^IV–Pd^II catalytic cycle is proposed to be responsible for the stepwise arylation.

A visible-light-mediated in situ generation of a boron-centered carboranyl radical (o-C₂B₁₀H₁₁^.) has been described. With eosin Y as a photoredox catalyst, 3-diazonium-o-carborane tetrafluoroborate [3-N₂-o-C₂B₁₀H₁₁][BF₄] was converted into the corresponding boron-centered carboranyl radical intermediate, which can undergo efficient electrophilic substitution reaction with a wide range of (hetero)arenes. This general and simple procedure provides a metal-free alternative for the synthesis of 3-(hetero)arylated-o-carboranes.

1,3-Dehydro-o-carborane is a useful synthon for selective cage boron functionalization of o-carboranes. It reacts readily with alkenes or alkynes to give a variety of cage B(3)-alkenyl/allenyl o-carboranes by ene reactions in very high yields and excellent regioselectivity. This can be ascribed to the highly polarized cage CB multiple bond, which lowers the activation barriers of the ene reaction.

A nickel-catalyzed arylation at the carbon center of o-carborane cages has been developed, thus leading to the preparation of a series of 1-aryl-o-carboranes and 1,2-diaryl-o-carboranes in high yields upon isolation. This method represents the first example of transition metal catalyzed C,C′-diarylation by cross-coupling reactions of o-carboranyl with aryl iodides.

Palladium-catalyzed direct dialkenylation of cage B(4,5)H bonds in o-carboranes has been achieved with the help of a carboxylic acid directing group, leading to the preparation of a series of 4,5-[trans-(ArCHCH)]₂-ocarboranes in high yields with excellent regioselectivity. The traceless directing group, eliminated during the course of the reaction, is responsible for controlling regioselectivity and dialkenylation. A possible catalytic cycle is proposed, involving a tandem sequence of Pd^II-initiated cage BH activation, alkene insertion, β-H elimination, reductive elimination, and decarboxylation.

A nickel-catalyzed arylation at the carbon center of o-carborane cages has been developed, thus leading to the preparation of a series of 1-aryl-o-carboranes and 1,2-diaryl-o-carboranes in high yields upon isolation. This method represents the first example of transition metal catalyzed C,C′-diarylation by cross-coupling reactions of o-carboranyl with aryl iodides.

Palladium-catalyzed direct dialkenylation of cage B(4,5)H bonds in o-carboranes has been achieved with the help of a carboxylic acid directing group, leading to the preparation of a series of 4,5-[trans-(ArCHCH)]₂-ocarboranes in high yields with excellent regioselectivity. The traceless directing group, eliminated during the course of the reaction, is responsible for controlling regioselectivity and dialkenylation. A possible catalytic cycle is proposed, involving a tandem sequence of Pd^II-initiated cage BH activation, alkene insertion, β-H elimination, reductive elimination, and decarboxylation.

The reasons for facile double desilylation of 13-vertex carborane 1,2-Me₂Si(CH₂)₂-1,2-C₂B₁₁H₁₁ (2) are discussed in this article. New 13- and 14-vertex ruthenacarboranes bearing the same -CH₂SiMe₂CH₂- linkage have been prepared and structurally characterized for comparison. Structural analyses of 13- and 14-vertex heteroboranes as well as control experiments suggest that the facile double desilylation of 2 on silica gel can be attributed to the joint actions of several factors involving the high ring-strain of exo five-membered C₄Si ring, Lewis acidity of Si atom and Br⊘nsted acidity of silica surface.

o-Carboryne can undergo α-CH 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-CH proton of tertiary amines. This approach serves as an efficient methodology for the preparation of a series of 1-aminoalkyl-o-carboranes.

o-Carboryne can undergo α-CH 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-CH proton of tertiary amines. This approach serves as an efficient methodology for the preparation of a series of 1-aminoalkyl-o-carboranes.

Like the importance of benzyne, witnessed in modern arene chemistry for decades, 1,2-dehydro-o-carborane (o-carboryne), a three-dimensional relative of benzyne, has been used as a synthon for generating a wide range of cage, carbon-functionalized carboranes over the past 20 years. However, the selective B functionalization of the cage still represents a challenging task. Disclosed herein is the first example of 1,3-dehydro-o-carborane featuring a cage CB bond having multiple bonding characters, and is successfully generated by treatment of 3-diazonium-o-carborane tetrafluoroborate with non-nucleophilic bases. This presents a new methodology for simultaneous functionalization of both cage carbon and boron vertices.

Like the importance of benzyne, witnessed in modern arene chemistry for decades, 1,2-dehydro-o-carborane (o-carboryne), a three-dimensional relative of benzyne, has been used as a synthon for generating a wide range of cage, carbon-functionalized carboranes over the past 20 years. However, the selective B functionalization of the cage still represents a challenging task. Disclosed herein is the first example of 1,3-dehydro-o-carborane featuring a cage CB bond having multiple bonding characters, and is successfully generated by treatment of 3-diazonium-o-carborane tetrafluoroborate with non-nucleophilic bases. This presents a new methodology for simultaneous functionalization of both cage carbon and boron vertices.

Several monoanions of 13-vertex carboranes were prepared in high yields from the reactions of C,C′-linked 13-vertex carboranes with tBuOK or NaH in dry THF at room temperature. These monoanions were characterized by various spectroscopic methods, elemental analysis, and single-crystal X-ray diffraction. The results showed substantial double-bond character between the cage-carbon atom and the exo vicinal carbon atom, thus leading to charge delocalization into the cage. As a result, the atomatom distances within the cage were elongated, with one broken CB bond. However, the cage geometry of the monoanions remained very similar to that of their corresponding neutral 13-vertex closo-carboranes. These monoanions represent the first examples of 13-vertex carboranes with exo-π bonding to hypercarbon atoms.

A new organic–inorganic hybrid ligand H₂C(C₅Me₄H)(C₂B₁₀H₁₁) (2) has been prepared. A selective deboration reaction with piperidine in ethanol resulted in the isolation of [Me₃NH][H₂C(C₅Me₄H)(C₂B₉H₁₁)] (3). An amine-elimination reaction of 3 with [Zr(NMe₂)₄] produced a neutral metal-amide complex [η⁵:η⁵-H₂C(C₅Me₄)(C₂B₉H₁₀)]Zr(NMe₂)(NHMe₂) (4). The double-ring-opening reaction of 4 with tetrahydrofuran (THF) afforded [η⁵:η⁵-H₂C(C₅Me₄)(C₂B₉H₁₀)]Zr (OCH₂CH₂CH₂CH₂)₂N(CH₃)₂ (5). Treatment of the trianionic salt of 3 with [MCl₄(thf)₂] gave complexes [{η⁵:η⁵-H₂C(C₅Me₄)(C₂B₉H₁₀)}M(μ-Cl)₂][Li(thf)₂] (M=Zr (6), Hf (7)). Reaction of 6 with LiCH₂TMS generated the ionic species [{η⁵:η⁵-H₂C(C₅Me₄)(C₂B₉H₁₀)}Zr(CH₂TMS)₂][Li(thf)₃] (9). Interaction of complex 6 with KCH₂(NMe₂)-o-C₆H₄ led to the formation of the neutral metal alkyl complex [η⁵:η⁵-H₂C(C₅Me₄)(C₂B₉H₁₀)]Zr[σ:σ-CH₂(NMe₂)-o-C₆H₄] (10). All complexes have been fully characterized by various spectroscopic techniques and elemental analyses. Some were further confirmed by single-crystal X-ray analyses.

The chemistry of boron clusters has been dominated by icosahedral carboranes for over half a century. Only in recent years has significant progress been made in the chemistry of supercarboranes (carboranes with more than 12 vertices). A number of CAd (carbon-atoms-adjacent) 13- and 14-vertex carboranes, and CAp (carbon-atoms-apart) 13-vertex carboranes as well as their corresponding 14- and 15-vertex metallacarboranes have been successfully prepared and structurally characterized. This breakthrough relied on the use of CAd nido-carborane dianions as starting materials. These supercarboranes can undergo single-electron reduction to give stable supercarborane radical monoanions with [2n+3] framework electrons, and electrophilic substitution reaction to afford hexasubstituted supercarboranes. They can react with nucleophiles to offer monocarba-closo-dodecaborate monoanions from cage-carbon extrusion reactions. Their unique chemical properties make the chemistry of supercarboranes distinct from that of their 12-vertex analogues. These studies open up new possibilities for the development of polyhedral clusters of extraordinary size. This focus review offers an overview of recent advances in this growing research field.