Nickel-Catalyzed Regioselective [2+2+2] Cycloaddition of Carboryne with Alkynes

Borirenes and Boriranes: Development and Perspectives

Strained compounds constitute a highly topical area of research in chemistry. Borirene and borirane both feature a BC2 three-membered ring. They can be viewed as the structural analogues of cyclopropane and cyclopropene, where a CH2 unit of the carbonaceous counterparts is replaced with BH, respectively. Indeed, this structural variation introduces numerous intriguing aspects. For instance, borirane and borirene are both Lewis acidic due to the presence of a tricoordinate borane center. In addition, borirene is 2π aromatic according to Hückel's rule. In addition to their ability to form adducts with Lewis bases and the capacity of borirenes to act as ligands in coordination with metals, both borirenes and boriranes exhibit ring-opening reactivity due to the considerable ring strain. Under specific conditions, coordinated boriranes can even cleave two BC bonds to serve as formal borylene sources (although the reaction mechanisms are quite complex). On the other hand, recent successful syntheses of benzoborienes and their carborane-based three-dimensional analogues (also referred to as carborane-fused boriranes) have introduced novel perspectives to this field. For instance, they display excellent ring-expanding reactivity, possibly attributed to the boosted ring strain arising from the fusion of borirenes with benzene and boriranes with o-carborane. Importantly, their applications as valuable "BC2 " synthons have become increasingly evident along with the newly disclosed reactivity. Additionally, the boosted Lewis acidity of carborane-fused boriranes, thanks to the potent electron-withdrawing effect of o-carborane, combined with their readiness for ring enlargement, makes them promising candidates as electron-accepting building blocks in the construction of chemically responsive luminescent materials. This review provides a summary of the synthesis and reactivity of borirene and borirane derivatives, with the aim of encouraging the design of new borierene- and borirane-based molecules and inspiring further exploration of their potential applications.

A Three-Dimensional Inorganic Analogue of 9,10-Diazido-9,10-Diboraanthracene: A Lewis Superacidic Azido Borane with Reactivity and Stability

Herein, we report the facile synthesis of a three-dimensional (3D) inorganic analogue of 9,10-diazido-9,10-dihydrodiboraantracene, which turned out to be a monomer in both the solid and solution state, and thermally stable up to 230 °C, representing a rare example of azido borane with boosted Lewis acidity and stability in one. Apart from the classical acid-base and Staudinger reactions, E-H bond activation (E=B, Si, Ge) was investigated. While the reaction with B-H (9-borabicyclo[3.3.1]nonane) led directly to the 1,1-addition on Nα upon N2 elimination, the Si-H (Et3 SiH, PhMe2 SiH) activation proceeded stepwise via 1,2-addition, with the key intermediates 5int and 6int being isolated and characterized. In contrast, the cooperative Ge-H was reversible and stayed at the 1,2-addition step.

3D and 2D aromatic units behave like oil and water in the case of benzocarborane derivatives

A large number of 2D/2D and 3D/3D aromatic fusions that keep their aromaticity in the fused compounds have been synthesized. In addition, we have previously proven the electronic relationship between the 3D aromaticity of boron hydrides and the 2D aromaticity of PAHs. Here we report the possible existence of 3D/2D aromatic fusions that retain the whole aromaticity of the two units. Our conclusion is that such a 3D/2D aromatic combination is not possible due to the ineffective overlap between the π-MOs of the planar species and the n + 1 molecular orbitals in the aromatic cage that deter an effective electronic delocalization between the two fused units. We have also proven the necessary conditions for 3D/3D fusions to take place, and how aromaticity of each unit is decreased in 2D/2D and 3D/3D fusions.

2D/2D fusion of aromatic halves leading to a global aromatic is found in many polycyclic aromatic hydrocarbons, whereas 2D/3D aromaticity is difficult to achieve. Here the authors report a computational chemistry investigation showing that 3D/2D aromatic combination is not possible.

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.

Solution-Phase Synthesis of a Base-Free Benzoborirene and a Three-Dimensional Inorganic Analogue

The base-free benzoborirene 1,2-BR-1,2-C₆H₄ (7) and its three-dimensional inorganic analogue 1,2-BR-1,2-C₂B₁₀H₁₀ (13) have been successfully synthesized by Cp₂ZrBr₂ and LiCl elimination, respectively. The Cl analogue of the key intermediate for the formation of benzoborirene 7 has been isolated and structurally characterized, thus suggesting the reaction pathway via benzyne Zr complex formation, B-Br/C_benzyne-Zr σ-bond metathesis, and a Cp₂ZrBr₂ elimination/ring-closing process. The rationality of the reaction pathway has been confirmed by DFT calculations. In addition, the title compounds shared the same reactivity pattern (i.e., 1,3-silyl migration) toward ^MeIiPr (8), thus allowing for the synthetic approach to the first carborane-substituted iminoborane 14.

The pivotal role of electronics in preferred alkene over alkyne Ni-carboryne insertions and absolute regioselectivities

The in situ formation mechanisms of active Ni-carboryne species (COM1) and subsequent alkene/alkyne Ni-C bond insertion priorities, as well as relevant cycloaddition regioselectivities and kinetics, were investigated using the IDSCRF-B3LYP density functional theory (DFT) method, and all atoms were equitably treated at the DGDZVP level. The results reveal the o-carborane species to be energetically hedged into a four-step path (barrier heights 5.3, 19.7, 18.4 and 0.3 kcal mol-1, respectively) prior to being transferred into the active Ni-carboryne species (COM1) with the assistance of nBuLi and NiCl2(PPh3)2 at room temperature. In direct agreement with empirical trends, alkene insertion into Ni-C bonds on COM1 is exclusively favoured over the competing alkyne insertion. Electronic structure analyses of the corresponding transition structures showed that the preference of alkenes to alkynes is due to different bonding characteristics during this insertion process, namely, back donation for alkenes but donation for alkyne insertion, as evidenced by molecular graphics and NBO charge distributions. Subsequent alkyne additions (i.e. post alkene insertion) arise as the rate-determining step (RDS) for each of the five different reactions (a-e) explored. The solution free-energy barriers of these RDSs (30.5-38.5 kcal mol-1) were in quantitative agreement with their corresponding experimental yields, evidencing the reliability of the DFT results to reproduce chemical phenomena and energetic trends in real Ni-catalysed carboryne-alkene/alkyne cycloadditions.

Rhodium-catalysed synthesis of multi-substituted silylindenes from aryl alkynes and hydrosilanes via C–H bond activation

We successfully developed rhodium-catalysed synthesis of multi-substituted silylindenes from 2 equivalents of aryl alkynes and 1 equivalent of hydrosilanes in moderate to good yields via C–H bond activation for the first time.

Regioselective insertion of o-carborynes into the α-C-H bond of tertiary amines: synthesis of α-carboranylated amines

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

Palladium-catalyzed annulation of arynes by o-halobenzamides: synthesis of phenanthridinones

The palladium-catalyzed annulation of arynes by substituted o-halobenzamides produces N-substituted phenanthridinones in good yields. This methodology provides this important heterocyclic ring system in a single step by simultaneous C-C and C-N bond formation, under relatively mild reaction conditions, and tolerates a variety of functional groups.

Novel Zn-Sn-O nanocactus with excellent transport properties as photoanode material for high performance dye-sensitized solar cells

A novel chemically stable Zn-Sn-O nanocactus structure has been synthesized for the first time using a hydrothermal method. The Zn-Sn-O nanocactus structure comprises a Zn poor-Zn(2)SnO(4) plate and Zn-doped SnO(2) nanothorns growing on the plate, both of which have high electron mobilities. The nanocactus is used as the photoanode of dye-sensitized solar cells (DSSCs). The overall power conversion efficiency (PCE) for the Zn-Sn-O nanocactus film reaches 2.21%, which is twice the previous reported efficiency of pure SnO(2). Electrochemical impedance spectroscopy (EIS) measurements show that the Zn-Sn-O nanocactus film has a good effective diffusion length and high intrinsic electron mobility. After TiCl(4) treatment of the Zn-Sn-O nanocactus film, the current density increases nearly three times and the PCE increases to 6.62%, which compares favourably with the P25 DSSCs (6.97%) and is much higher than that of the SnO(2) (1.04%) or Zn(2)SnO(4) (3.7%)-based DSSCs.