Photoarylation of Iodocarboranes with Unactivated (Hetero)Arenes: Facile Synthesis of 1,2-[(Hetero)Aryl]n -o-Carboranes (n=1,2) and o-Carborane-Fused Cyclics

Breaking the Base Barrier: Cu(II)-Mediated C-H Heteroarylation of o-Carboranes with Base-Sensitive Heteroaryl Halides

While cage C-arylation reactions using strong bases are among the most frequently used transformations in carborane chemistry, there has been no general solution to allow for the use of weak bases in the reaction. Moreover, base-metal-promoted C-H heteroarylation with base-sensitive heteroaryl halides remained elusive. Herein, copper-mediated cage C-H (hetero)arylation has been achieved without the need for strong bases, leading to the facile synthesis of a wide range of C-(hetero)arylated carboranes in good to excellent yields with a broad substrate scope and good functional group compatibility.

Synthesis of Carboranylated Dihydropyrrolo[1,2-a]quinoxalines and Dihydroindolo[1,2-a]quinoxalines by BF3·OEt2-Catalyzed Heterocyclization of C-Formyl-o-carboranes and Investigation of Their Oxidation Stability

A BF3·OEt2-catalyzed synthesis of carboranylated dihydropyrrolo[1,2-a]quinoxalines and dihydroindolo[1,2-a]quinoxalines in 30-99% yields is presented through the heterocyclization of various C-modified C-formyl-o-carboranes with 1-(2-aminophenyl)-pyrroles/indoles. A systematic comparative investigation of their oxidation stability in air confirmed that 4-carboranyl-4,5-dihydropyrrolo[1,2-a]quinoxaline had better stability than the 4-phenyl analogue. A cage-deboronation reaction for N-acetyl-substituted carboranylated dihydropyrrolo[1,2-a]quinoxaline produced the corresponding 7,8-nido-carborane cesium salt. A kinetic resolution was also realized to obtain an optically pure carboranylated N-heterocycle scaffold bearing a carborane cage carbon-bonded chiral stereocenter.

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.

Synthesis of o-Carborane-Fused Pyrazoles through Sequential C-N Bond Formation

Transition-metal-free synthetic method for o-carborane-fused pyrazoles as a new scaffold has been developed from the reaction of B(4)-acylmethyl or B(3,5)-diacylmethyl o-carborane with 2-azido-1,3-dimethylimidazolinium hexafluorophosphate (ADMP) in the presence of DBU in acetonitrile through sequential diazotization and cyclization reaction in one pot, consequently allowing twofold C-N bond formation under extremely mild conditions and high functional group tolerance.

Visible-Light-Promoted Nickel-Catalyzed Cross-Coupling of Iodocarboranes with (Hetero)Arenes via Boron-Centered Carboranyl Radicals

A general strategy for the generation of hypervalent boron-centered carboranyl radicals at the B(3), B(4), and B(9) positions has been developed for the first time via visible-light-promoted iodine atom abstraction from iodo-o-carboranes by low-valent nickel complex. These radicals react with various (hetero)arenes to afford a wide range of cage B-arylated carborane derivatives at room temperature in very good to excellent yields with a broad substrate scope. Their electrophilicities are dependent on the vertex charges of the cage and follow the order B(3) > B(4) > B(9). Both visible light and nickel catalyst are proved critical to the generation of boron-centered carboranyl radicals. The involvement of boron radicals is supported by control experiments. A reaction mechanism associated with these reactions is also proposed. This strategy offers a new protocol for the generation of boron-centered carboranyl radicals at the selected boron vertex, leading to a facile synthesis of a large class of cage boron substituted carborane molecules.

Light-enabled alkenylation of iodocarboranes with unactivated alkenes

The synthesis of alkenylated-o-carboranes via photoalkenylation of iodocarboranes with unactivated alkenes has been achieved. This strategy features a transition metal-free protocol, a light-promoted reaction under mild reaction conditions, broad substrate scope and good functional group tolerance. Control experiments suggest that the reaction may involve the cage C-centered radical species.

Electrooxidative B-H Functionalization of nido-Carboranes

An atom-economical method for the direct B-H functionalization of nido-carboranes (7,8-nido-C₂ B₉ H₁₂ ^- ) has been developed under electrochemical reaction conditions. In this reaction system, anodic oxidation serves as a green alternative for traditional chemical oxidants in the oxidation of nido-carboranes. No transition-metal catalyst is required and different heteroatoms bearing a lone pair are reactive in this transformation. Coupling nido-carboranes with thioethers, selenides, tellurides, N-heterocycles, phosphates, phosphines, arsenides and antimonides demonstrates high site-selectivity and efficiency. Importantly, nido-carboranes can be easily incorporated into drug motifs through this reaction protocol.

Strong σ-Hole Activation on Icosahedral Carborane Derivatives for a Directional Halide Recognition

Crystal engineering based on σ-hole interactions is an emerging approach for realization of new materials with higher complexity. Neutral inorganic clusters derived from 1,2-dicarba-closo-dodecaborane, substituted with -SeMe, -TeMe, and -I moieties on both skeletal carbon vertices are experimentally demonstrated herein as outstanding chalcogen- and halogen-bond donors. In particular, these new molecules strongly interact with halide anions in the solid-state. The halide ions are coordinated by one or two donor groups (μ₁ - and μ₂ -coordinations), to stabilize a discrete monomer or dimer motifs to 1D supramolecular zig-zag chains. Crucially, the observed chalcogen bond and halogen bond interactions feature remarkably short distances and high directionality. Electrostatic potential calculations further demonstrate the efficiency of the carborane derivatives, with V_s,max being similar or even superior to that of reference organic halogen-bond donors, such as iodopentafluorobenzene.

Light-promoted copper-catalyzed cage C-arylation of o-carboranes: facile synthesis of 1-aryl-o-carboranes and o-carborane-fused cyclics

Light-promoted, copper catalyzed cage C–H arylation of o-carboranes with aryl halides has been achieved, leading to the facile synthesis of a variety of 1-aryl-o-carboranes and o-carborane-fused cyclics.

Transition-Metal-Free Cross-Coupling Reaction of Iodocarboranes with Terminal Alkynes Enabled by UV Light: Synthesis of 1-Alkynyl-o-Carboranes and Carborane-Fused Cyclics

A transition-metal-free coupling protocol between iodocarboranes and terminal alkynes enabled by light at room temperature has been developed, leading to the synthesis of a variety of 1-alkynyl-o-carboranes. Moreover, following this strategy, the introduction of 1-I-3-aryl-o-carboranes or 1-I-2-aryl-o-carboranes results in the formation of o-carborane-fused cyclics. Interestingly, when 1-I-3-(p-R-C₆H₄)-o-carboranes are chosen as coupling partners, unexpected R-group migration products are also isolated. On the basis of the results of control experiments and isolation of the key intermediates, a possible reaction mechanism is then proposed, involving the formation of spiro radical species.

Perylene Bisimide Derivative-Based Fluorescent Film Sensors: From Sensory Materials to Device Fabrication

Film-based fluorescent sensors have become an important field of sensor research due to abundant acquirable signals, real-time monitoring, and ease of miniaturization and integration, where chemically sensitive films are the most vital component of the sensor devices. In this feature article, we introduce hardware structures of film-based fluorescent sensors following the examination/investigation of the recent progress of such sensors with perylene bisimide (PBI) derivatives as sensing fluorophores in the films. PBI derivatives were specially chosen because of their outstanding chemical, photochemical, and thermal stabilities as well as their unusual high-fluorescence quantum yields. And finally, we provide a prediction for the future developments and challenges of this emerging field.

Ir-catalyzed selective dehydrogenative cross-coupling of aryls with o-carboranes via a mixed directing-group strategy

Ir-catalyzed highly selective B–H/C–H cross dehydrogenative coupling between o-carboranes and (hetero)aryls has been achieved using a mixed directing-group strategy.

A self-catalyzed reaction of 1,2-dibenzoyl-o-carborane with hydrosilanes - formation of new hydrofuranes

The activation of Si-H bonds is a very important transformation both in organic and inorganic chemistry. Herein we report that 1,2-dibenzoyl-o-carborane (1) reacts with Si-H bonds, yielding new hydrofurane-type products. The mechanism of this Si-H bond activation was studied both experimentally and by DFT calculations, and supposedly proceeds in an FLP-type manner.

Electronic Effect-Guided, Palladium-Catalyzed Regioselective B-H Activation and Multistep Diarylation of o-Carboranes with Aryl Iodides

Density functional theory calculations at IDSCRF-B3LYP/DZVP computational level were conducted on palladium-catalyzed regioselective B-H activation and diarylation of o-carboranes with aryl iodides in solution. Computational results indicate that this reaction follows a multistep mechanism and needs to get over several transition states before the final B(4,5)-diarylated o-carborane derivatives are formed. B-H activation, oxidation addition, and successive reduction of the Pd(II) catalyst involving a Pd(II)-Pd(IV)-Pd(II) catalytic cycle has been confirmed, in which AgOAc plays a crucial role. Electron-donating group on the cage carbon of o-carboranes is verified to be beneficial for its B-H activation and diarylation, while steric hindrance between the aryl and o-carboranyl groups retards it. Natural population analysis and Gibbs free energetic results predict consistent regioselectivities with experiments and manifest the pivotal role of electronic effect in controlling regioselective B-H activation of o-carboranes. These results are expected to shed some light on further improvement of experimental conditions and better controlling of regioselectivities.

Synthesis, structure and aromaticity of carborane-fused carbo- and heterocycles

The results of molecular structures, NMR data, and NICS (nucleus-independent chemical shift) and ISE (isomerization stabilization energy) values as well as molecular orbital analyses clearly suggest the presence of considerable aromatic character in the exo five-membered ring of carborane-fused carbo- and heterocycles and considerable conjugation between a 3-D carborane and a fused 2-D π-ring system.

Conjugation between a 3-D icosahedral carborane and a fused 2-D π-ring system is ambiguous. To address this issue, we prepared several carborane-fused carbo- and heterocycles. Detailed studies on their molecular structures, NMR data, and NICS (nucleus-independent chemical shift) and ISE (isomerization stabilization energy) values as well as molecular orbital analyses clearly suggest the presence of (1) considerable aromatic character in the exo five-membered ring of carborane-fused carbo- and heterocycles and (2) considerable conjugation between a 3-D carborane and a fused 2-D π-ring system. These results will shed some light on the design of new carborane-based materials.

Synthesis of carborane-fused carbo- and heterocycles via zirconacyclopentane intermediates

Carborane-fused zirconacyclopentane serves as an important synthon for the preparation of a variety of carborane-fused carbo- and heterocycles via insertion and transmetalation reactions.

Dehydrogenative cross-coupling of o-carborane with thiophenes via Ir-catalyzed regioselective cage B–H and C(sp2)–H activation

Iridium-catalyzed carboxylic acid directed site-selective B–H/C–H dehydrogenative cross-coupling of o-carborane with thiophenes has been achieved for the first time, leading to the preparation of 4-thienyl-o-carboranes in a simple one-pot process for potential applications in materials.

The synthesis and reactivity of 16-electron half-sandwich iridium complexes bearing a carboranylthioamide ligand

The 16-electron complex 2 has been synthesized. It can react with different donor ligands and undergoes M–S bond insertion reactions with DMAD. Complex 2 is shown to react with [Cp*Ir] unit to form a M–M-bound binuclear complex. Interestingly, the metal–metal bond could be broken by HCl to produce the selective B(6)–H activation species.