Palladium/Nickel-Cocatalyzed Cycloaddition of 1,3-Dehydro-o-Carborane with Alkynes. Facile Synthesis of C,B-Substituted Carboranes

Rare-Earth Metal Complexes Bearing Electrophilic Carbon and Strongly Polarized Metallacyclopropane Moiety: Synthesis and Diverse Reactivity toward Small Molecules

Metallacyclopropanes are highly strained and very reactive organometallics; the rare-earth metal complexes bearing both highly reactive electrophilic carbon and strongly polarized metallacyclopropanes are extremely rare. This type of rare-earth metal complexes (κ2-L)RE(η2-C2B10H10)·(THF)3 [L = 1-(2-N-C5H10NCH2CH2)-3-(2,6-iPr2C6H3N═CH)-C8H4N, RE = Lu(1a), Yb(1b), Er(1c), Y(1d), Dy(1e)] bearing the indol-2-yl electrophilic carbon and carboryne-based strongly polarized metallacyclopropanes have been synthesized. Structures of complexes 1 are further confirmed by single-crystal X-ray diffraction and DFT theoretical calculations. It is found that complexes 1 have remarkable reactivity toward different polar unsaturated small molecules, elemental sulfur, and selenium to provide different products (2-15) through the selective reactions of the RE-Ccage, and RE-C2-ind bonds with the given small molecules, respectively. The reactivities of these complexes are different from those of the reported rare-earth metallacyclopropenes and d-block metal-carborynes.

Visible-Light-Induced Palladium-Catalyzed Cross-Coupling of Iodocarboranes with (Hetero)Arenes

This work describes a general method for the efficient production of a class of cage B-centered carboranyl radicals at the B3, B4, and B9 sites via a visible-light-promoted palladium(0)/palladium(I) pathway using readily available iodo-o-carboranes as the starting materials. The electrophilicities of these hypervalent boron-centered radicals decrease in the following order: B3 > B4 > B9. They are useful intermediates for the preparation of a family of cage B-(hetero)arylated o-carboranes at ambient temperature.

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.

Functionalization of o-carboranes via carboryne intermediates

Carborynes (1,2-dehydro-o-carborane and 1,3-dehydro-o-carborane), three-dimensional analogues of benzyne, can be generated in situ from the precursors 1-X-2-Li-1,2-C₂B₁₀H₁₀ (X = Br, I, OTs, OTf), or 1-Me₃Si-2-[IPh(OAc)]-1,2-C₂B₁₀H₁₀ or [1-Li-3-N₂-1,2-C₂B₁₀H₁₀][BF₄]. They are a class of very useful synthons for the synthesis of a large variety of functionalized carborane derivatives for potential application in medicine, materials science and organometallic/coordination chemistry. The experimental data demonstrate that there is a correspondence between the reactions of carborynes and those of benzyne with alkenes, dienes, alkynes, aromatics or heteroaromatics in a pericyclic reaction fashion. On the other hand, carborynes have unique properties of their own owing to their steric/electronic features. They undergo regioselective sp²/sp³ C-H bond and N-Li bond insertion reactions, which has not been observed for benzyne. This review provides a comprehensive overview of recent advances in this interesting research field with considerable attention devoted to the reaction modes and the mechanisms involved.

Synthesis of 3-Aryl-ortho-carboranes with Sensitive Functional Groups

A simple and efficient method was developed for the one-pot synthesis of 3-aryl derivatives of ortho-carborane with sensitive functional groups using 3-iodo-ortho-carborane and aryl zinc bromides that were generated in situ. A series of 3-aryl-ortho-carboranes, including those containing nitrile and ester groups, 3-RC₆H₄-1,2-C₂B₁₀H₁₁ (R = p-Me, p-NMe₂, p-OCH₂OMe, p-OMe, o-CN, p-CN, o-COOEt, m-COOEt, p-COOEt) was synthesized using this approach. The solid-state structures of 3-RC₆H₄-1,2-C₂B₁₀H₁₁ (R = p-OMe, o-CN, and p-CN) were determined by single crystal X-ray diffraction. The intramolecular hydrogen bonding involving the ortho-substituents of the aryl ring and the CH and BH groups of carborane was discussed.

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.

Computational Investigation of Nickel-Mediated B–H Activation and Regioselective Cage B–C(sp2) Coupling of o-Carborane

Density functional theory (DFT) methods including LC-ωPBE, CAM-B3LYP, B3LYP, and B3LYP-D3, combined with double Zeta all-electron DZVP basis set, have been employed to conduct computational investigations on nickel-mediated reaction of o-carboranylzirconacycle, n-hexene, and 2-bromophenyltrimethylsilylacetylene in toluene solution. A multistep mechanism leading to the C,C,B-substituted carborane-fused tricyclics, including (1) sequential insertion of alkene and alkyne into Ni–C bonds; (2) double 1,2-migration of the TMS group; (3) B–H activation assisted by Cs2CO3 additive; and (4) reduction cage B–C (sp2) coupling, was proposed. Among these steps, the B–H activation of o-carborane was located as rate-determining step (RDS). With assistance of Cs2CO3 additive (replaced by K2CO3 in simulation), the RDS free-energy barrier at PCM-LC-ωPBE/DZVP level was calculated to be 23.1–23.9 kcal·mol−1, transferring to a half-life of 3.9–15.1 h at 298 K. The predicted half-life coincides well with 80% experimental yields of C,C,B-substituted carborane-fused tricyclics after 12 h. Kinetic data obtained by employing LC-ωPBE method also reproduced the experimental diastereoselective ratio well. Various B–H activation pathways with and without Cs2CO3 additive were taken into consideration, which illustrates Cs2CO3 as an essential guarantee for smooth occurrence of this reaction at room temperature.

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.

Reversible Photothermal Isomerization of Carborane-Fused Azaborole to Borirane: Synthesis and Reactivity of Carbene-Stabilized Carborane-Fused Borirane

A fully reversible photothermal isomerization between carborane-fused trigonal-planar azaborole (dark-purple) and tetrahedral borirane (pale-yellow) has been observed, leading to the isolation and structural characterization of the first example of carborane-fused borirane. DFT calculations indicate that the azaborole is thermodynamically more stable than the borirane by 11.2 kcal mol^-1 , and the energy barrier for the thermal conversion from azaborole to borirane is 35.5 kcal mol^-1 . The reactivity studies show that the B-C(cage) bond in borirane can be broken in the reaction with CuCl, HCl, or elemental sulfur.

(BB)-Carboryne Complex of Ruthenium: Synthesis by Double B-H Activation at a Single Metal Center

The first example of a transition metal (BB)-carboryne complex containing two boron atoms of the icosahedral cage connected to a single exohedral metal center (POBBOP)Ru(CO)2 (POBBOP = 1,7-OP(i-Pr)2-2,6-dehydro-m-carborane) was synthesized by double B-H activation within the strained m-carboranyl pincer framework. Theoretical calculations revealed that the unique three-membered (BB)>Ru metalacycle is formed by two bent B-Ru σ-bonds with the concomitant increase of the bond order between the two metalated boron atoms. The reactivity of the highly strained electron-rich (BB)-carboryne fragment with small molecules was probed by reactions with electrophiles. The carboryne-carboranyl transformations reported herein represent a new mode of cooperative metal-ligand reactivity of boron-based complexes.

The Application of Copper/Iron Cocatalysis in Cross-Coupling Reactions

For conventional cross-couplings in organic chemistry, precious metal (such as Pd or Rh) complexes have been the preferable choices as catalysts. However, their high cost, toxicity, and potential contamination of products limit their massive applications on some occasions, particularly in the pharmaceutical industry, where close monitoring of the metal contamination of products is required. Therefore, the use of metals that are less expensive and less toxic than Pd or Rh can be greatly advantageous and earth abundant metal (such Fe or Cu) catalysts have shown promise for replacing the precious metals. Interestingly, a certain copper catalyst combined with an iron catalyst displays higher catalytic efficiency than itself in various coupling reactions. Notably, ligand-free conditions make such protocols more useful and practical in many cases. In this account, we summarize the recent progress made in this increasingly attractive topic by describing successful examples, including our own work in the literature, regarding effective copper/iron cocatalysis. In addition, a few examples involving a magnetic and readily recyclable CuFe2 O4 nanoparticle cocatalyst are also included.

Phosphine-catalyzed cage carbon functionalization of o-carborane: facile synthesis of alkenylcarboranes

Phosphines can efficiently catalyze alkenylation of o-carborane with electron-deficient alkynes, which represents the first example of organocatalysis in o-carborane functionalization.

Synthesis, structure, and alkyne insertion of a mixed-sandwich zirconacarborane alkyl

A neutral mixed-sandwich zirconacarborane alkyl reacts with both internal and terminal alkynes to give the Zr–C σ bond mono-insertion products, in which both electronic and steric factors play a role in the regioselectivity.

A combined experimental and theoretical study on the isomers of 2,3,4,5-tetracarba-nido-hexaborane(6) derivatives and their photophysical properties

Building upon our ongoing studies on the high-yield synthetic route to 2,3,4,5-tetracarba-1,6-nido hexaborane derivative 5, we continue to explore the chemistry of this system to isolate a range of perphenylated C4 B2 -type carborane isomers. As a result, the photolysis of 5 for an elongated period yielded 6, which exhibits an intense luminescence upon excitation with a UV lamp (λex =366 nm). In contrast, the conversion of 5 under thermal conditions (microwave heating) generated the isomer 7, which does not show any luminescence. The structural disparity among the three isomers 5-7 is the position of the butadiene moiety with respect to the C4 B2 -carborane cage as deduced from single-crystal X-ray diffraction analyses. In an attempt to generate the classical structural motif of 5, an equimolar amount of KOtBu was added to a THF solution of 5, which immediately provided the boratacyclopentadiene derivative 8 in good yield. The observed rearrangement reactions were further investigated by quantum chemical calculations to suggest a plausible reaction pathway. According to the DFT calculations, the energetically favored reaction mechanism most likely involves tricyclic transition states and classical intermediate structures. In addition, the fluorescence emission properties of 6 were investigated in solution as well as in the solid state.

Methods to produce B-C, B-P, B-N and B-S bonds in boron clusters

Boranes, heteroboranes and metallacarboranes, all named as boron clusters, offer an alternative to typical organic molecules or organic molecular materials. Carbon and boron share the important property of self-catenation thus these elements can produce individually large and sophisticated molecules. Boron clusters and organic molecules display electronic, physical, chemical and geometrical characteristics manifestly different. These differences highlight the complementarity of organic molecules and boron clusters, and therefore the feasibility or necessity to produce hybrid molecules incorporating both types of fragments. To join these two types of fragments, or alternatively these two types of molecular compounds, tools are needed. In this review the current methods of producing boron clusters with carbon, B-C, nitrogen, B-N, phosphorus, B-P and sulphur bonds, B-S, are indicated. As there are many existing borane clusters of different sizes, heteroboranes and metallacarboranes, the revision of methods to generate the B-C, B-P, B-S, and B-N bonds has been restricted to the most widely used boron clusters; [B12H12](2-), dianionic and an example of a borane, 1,2-C2B10H12, neutral and an example of a heteroborane, and [Co(C2B9H11)2](-), monoanionic and an example of a metallacarborane.

THEORETICAL STUDY ON THE SECOND-ORDER NONLINEAR OPTICAL PROPERTIES OF C,B-SUBSTITUTED CARBORANE CONJUGATED DERIVATIVES

To systemically investigate structure–property relationship and design excellent nonlinear optical (NLO) material, the second-order NLO properties of a series of C,B -substituted carborane conjugated derivatives have been studied by density functional theory (DFT). The static first hyperpolarizabilities (βtot) were calculated at the M05-2X/6-31+G* level of theory. The results show that the βtot values gradually increase with the increasing of the conjugation length, especially the introduction of ferrocene. It is found that 1,3-benzo-o-carborane-ferrocene (2h) has the largest first hyperpolarizability (55.968 × 10-30 esu), which is 150 times larger than that of benzocarborane (1a). This means that the static first hyperpolarizabilities of the studied compounds can be substantially increased by structural modification. A basis for understanding the origin of these large NLO responses is proposed based on consideration of the frontier molecular orbitals (FMOs), orbital energy, transition energy of the studied compounds, and the two-state mode. The lower transition energy and larger oscillator strength play an important role in increasing the first hyperpolarizability value. This study may evoke possible ways to design preferable NLO materials.

Transition metal-carboryne complexes: synthesis, bonding, and reactivity

The construction and transformation of metal-carbon (M-C) bonds constitute the central themes of organometallic chemistry. Most of the work in this field has focused on traditional M-C bonds involving tetravalent carbon: relatively little attention has been paid to the chemistry of nontraditional metal-carbon (M-C(cage)) bonds, such as carborane cages, in which the carbon is hypervalent. We therefore initiated a research program to study the chemistry of these nontraditional M-C(cage) bonds, with a view toward developing synthetic methodologies for functional carborane derivatives. In this Account, we describe our results in constructing and elucidating the chemistry of transition metal-carboryne complexes. Our work has shown that the M-C(cage) bonds in transition metal-carboranyl complexes are generally inert toward electrophiles, and hence significantly different from traditional M-C bonds. This lack of reactivity can be ascribed to steric effects resulting from the carboranyl moiety. To overcome this steric problem and to activate the nontraditional M-C(cage) bonds, we prepared a series of group 4 and group 10 transition metal-carboryne complexes (where carboryne is 1,2-dehydro-o-carborane), because the formation of metallacyclopropane opens up the coordination sphere and creates ring strain, facilitating the reactions of M-C(cage) bonds with electrophiles. Structural and theoretical studies on metal-carboryne complexes suggest that the bonding interaction between the metal atom and the carboryne unit is best described as a resonance hybrid of the M-C σ and M-C π bonds, similar to that observed in metal-benzyne complexes. The nickel-carboryne complex (η(2)-C(2)B(10)H(10))Ni(PPh(3))(2) can (i) undergo regioselective [2 + 2 + 2] cycloaddition reactions with 2 equiv of alkyne to afford benzocarboranes, (ii) react with 1 equiv of alkene to generate alkenylcarborane coupling products, and (iii) also undergo a three-component [2 + 2 + 2] cyclotrimerization with 1 equiv of activated alkene and 1 equiv of alkyne to give dihydrobenzocarboranes. The reaction of carboryne with alkynes is also catalyzed by Ni species. Subsequently, a Pd/Ni co-catalyzed [2 + 2 + 2] cycloaddition reaction of 1,3-dehydro-o-carborane with 2 equiv of alkyne was developed, leading to the efficient formation of C,B-substituted benzocarboranes in a single process. In contrast, the zirconium-carboryne species, generated in situ from Cp(2)Zr(μ-Cl)(μ-C(2)B(10)H(10))Li(OEt(2))(2), reacts with only 1 equiv of alkyne or polar unsaturated organic substrates (such as carbodiimides, nitriles, and azides) to give monoinsertion metallacycles, even in the presence of excess substrates. The resultant five-membered zirconacyclopentenes, incorporating a carboranyl unit, are an important class of intermediates for the synthesis of a variety of functionalized carboranes. Transmetalation of zirconacyclopentenes with other metals, such as Ni and Cu, was also found to be a very useful tool for various chemical transformations. Studies of metal-carboryne complexes remain a relatively young research area, particularly in comparison to the rich literature of metal-benzyne complexes. Other transition metal-carborynes are expected to be prepared and structurally characterized as the field progresses, and the results detailed here will further that effort by providing easy access to a wide range of functionalized carborane derivatives.