Visible-Light-Promoted Photocatalytic B−C Coupling via a Boron-Centered Carboranyl Radical: Facile Synthesis of B(3)-Arylatedo-Carboranes

Charge Transfer Complex-Enabled Synthesis of (Hetero)arylated m-Carboranes from m-Carborane Phosphonium Salts

A photoinduced charge transfer complex (CTC)-enabled photoreduction of carborane phosphonium salts for the cage carbon (hetero)arylation of carboranes was developed. It offers a convenient approach for introducing a wide range of aryl and heteroaryl groups, such as pyrroles, thiophenes, indoles, thianaphthenes, benzofurans, pyridines, and benzenes, into carboranes. This strategy offers operational simplicity, mild reaction conditions, and a broad substrate scope, making it highly advantageous.

Site-Selective Functionalization of Carboranes at the Electron-Rich Boron Vertex: Photocatalytic B-C Coupling via a Carboranyl Cage Radical

Functionalization of carboranes in a vertex-specific manner is a perennial challenge. Here, we report a photocatalytic B-C coupling for the selective functionalization of carboranes at the boron site which is most distal to carbon. This reaction was achieved by the photo-induced decarboxylation of carborane carboxylic acids to generate boron vertex-centered carboranyl radicals. Theoretical calculations also demonstrate that the reaction more easily occurs at the boron site bearing higher electron density owing to the lower energy barrier for a single-electron transfer to generate a carboranyl radical. By using this strategy, a number of functionalized carboranes could be accessed through alkylation, alkenylation, and heteroarylation under mild conditions. Moreover, both a highly efficient blue emitter with a solid-state luminous efficiency of 42 % and a drug candidate for boron neutron capture therapy (BNCT) containing targeting and fluorine units were obtained.

N-Ligand-Enabled Aromatic Nucleophilic Amination of 1,2-Diaryl-o-Carboranes with (R2 N)2 Mg for Selective Synthesis of 4-R2 N-o-Carboranes and 2-R2 N-m-Carboranes

The nucleophilic aromatic BH substitution reaction of carboranes is uncommon, compared to the electrophilic one. This work reported a pyridine-enabled transition-metal-free regioselective nucleophilic aromatic cage B(4)-H amination of 1,2-diaryl-o-carboranes with magnesium bisamides, giving a series of B(4)-aminated o-carboranes. DFT calculations showcased a stepwise B-N formation/B-H cleavage process, in which Mg-H formation/cage closure is the rate-determining step. Unprecedentedly, in the presence of 4,4'-di-tert-butyl-2,2'-dipyridyl (dtbpy), a tandem B(4)-amination/cage isomerization reaction of o-carboranes was discovered for the facile preparation of B(2)-aminated m-carboranes. Control experiments indicated that magnesium complex, bidentate ligand (dtbpy) and reaction temperature were crucial in the cage isomerization process. This direct nucleophilic aromatic cage B-H amination reaction offers an alternative strategy for selective amination of o- and m-carboranes.

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.