Catalytic Regioselective Cage B(8)–H Arylation of o-Carboranes via “Cage-Walking” Strategy

Synthesis and structure of 3-arylazo derivatives of ortho-carborane

A series of B-substituted arylazo derivatives of ortho-carborane 3-XC₆H₄-NN-1,2-C₂B₁₀H₁₁ (X = p-NMe₂, p-OMe, o-Me) were synthesized by reaction of its diazonium derivative [3-N₂-o-C₂B₁₀H₁₁][BF₄] with the corresponding Grignard reagents.

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.

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.

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.

New global minima of 6-vertex dicarboranes: classical but unexpected

Dicarboranes generally adopt global minimum predicted by the well-known Wade-Mingos rules, although one classical non-closo structure in the benzvalene form has long been pursued and later synthesized. Here we predicted two new non-closo global minima for 6-vertex dicarboranes (C2B4R6), i.e., trigonal bipyramid (R = SH) and butterfly (R = Cl, NH2, OH, F). The long expected classical benzvalene-like structure, however, is not the global minimum for any of the nine substituents.

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.