Synthesis and Reactivity of Carboranylsilylene Stabilized Boranes: Construction of Carborane-Fused Silaboracycles

This work describes that a sterically bulky o-carboranylsilylene can significantly enhance the stability of the resultant carboranylsilylene-borane adducts. Removal of one halide from the boron center via salt metathesis reaction initiates immediately the halide migration from the boron to silicon and subsequent aminidate ligand rearrangement from the silicon to boron, resulting in the formation of a new class of o-carborane-fused silaboracycles in very good yields. The associated reaction mechanism is also discussed

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

o-Carboryne (1,2-dehydro-o-carborane) has been reported as a very reactive intermediate and regarded as a three-dimensional relative of benzyne, whereas the 1,3-dehydro-o-carborane has remained elusive. In this article, we present the preparation of 1,3-dehydro-o-carborane from 3-iodo-1-lithio-o-carborane mediated by palladium(0). This reactive intermediate can be trapped by alkynes via Pd/Ni-cocatalyzed [2 + 2 + 2] cycloaddition reaction, leading to the formation of C,B-substituted-o-carborane derivatives. The possible reaction mechanism involving the formation of metal-1,3-dehydro-o-carborane followed by stepwise insertions of 2 equiv of alkyne and reductive elimination is proposed, and the relative reactivity of M−C versus M−B bond in metal-1,3-dehydro-o-carborane complexes is also discussed. This work offers a new methodology for B-functionalization of carboranes and demonstrates that metal-1,3-dehydro-o-carborane can be viewed as a new kind of boron nucleophile

Reaction of [η¹:η⁵‑(R₂NCH₂CH₂)C₂B₉H₁₀]TaMe₃ with Isonitriles: Effects of Nitrogen Substituents on Product Formation

Tantallacarborane trimethyl complexes show diverse reactivity patterns toward isonitriles. Reaction of [η1:η5-(Me2NCH2CH2)­C2B9H10]­TaMe3 (1) with 1-adamantyl isonitrile (AdNC) led to the clean formation of an imido complex, [σ:η5-(MeNCH2CH2)­C2B9H10]­Ta­(NAd)­(THF) (2) with elimination of methane and 2-methylpropene, whereas treatment of [η1:η5-{(CH2)5NCH2CH2}­C2B9H10]­TaMe3 (3) with AdNC under the same reaction conditions gave the cage B–H activated product {σ:η1:η5-[(CH2)5NCHCH2]­(CHMe2)­C2B9H9}­Ta­(NAd)­(THF) (4). An equimolar reaction of 1 with R1NC (R1 = Cy, Ad), followed by 1 equiv of R2NC (R2 = Xyl, Cy), afforded the imido amido complexes [η1:η5-(Me2NCH2CH2)­C2B9H10]­Ta­(NR1)­[N­(CMeCMe2)­R2] (R1 = Ad, R2 = Cy (5); R1 = Cy, R2 = Xyl (6)). If 2 equiv of 2,6-dimethylphenyl isonitrile (XylNC) was used in the above reaction, the cage B–H alkylation products [η1:η5-(Me2NCH2CH2)­C2B9H9]­Ta­(NR1)­[N­(Xyl)­{CHC­(Me2)­C­(Me)NXyl}] (R1 = Cy (7), Ad (8)) were isolated. On the other hand, η2-iminoacyl imido complexes [η1:η5-(Me2NCH2CH2)­(CHMe2)­C2B9H9]­Ta­(NXyl)­(η2-C,N-MeCNR) (R = iPr (9), Cy (10)) were obtained from an equimolar reaction of 1 with XylNC, followed by 1 equiv of alkyl isonitriles. A double methyl migratory insertion tantallaaziridine species is proposed as a crucial intermediate for all aforementioned reactions, and follow-up steps are dependent upon N-substituents and the type and stoichiometry of isonitriles. All new complexes were characterized by spectroscopic methods and single-crystal X-ray analyses

Atom-Economical Synthesis of <i>N</i>-Heterocycles via Cascade Inter-/Intramolecular C−N Bond-Forming Reactions Catalyzed by Ti Amides

Direct and efficient catalytic reactions with excellent regioselectivity for the preparation of a series of substituted isoindoles, isoquinolines, and imidazoles are reported. Reaction of C6H4(2-CN)CC-R with an array of amines, catalyzed by 10 mol % of [σ:η1:η5-(OCH2)(Me2NCH2)C2B9H9]Ti(NMe2) (1), gives a series of substituted isoindoles in very high yields. In a similar manner, interaction of C6H4(2-CH2CN)CC-Ph with various kinds of amines affords a wide range of substituted isoquinolines. On the other hand, treatment of propargylamines (R′CCCH2NHR′′) with nitriles in the presence of 10 mol % of 1 produces a class of substituted imidazoles in high yields. A possible reaction mechanism is proposed, involving sequential inter- and intramolecular C−N bond formation via hydroamination/cyclization reaction of cyanoalkynes with amines or nitriles with propargylamines catalyzed by titanium amides

Atom-Economical Synthesis of <i>N</i>-Heterocycles via Cascade Inter-/Intramolecular C−N Bond-Forming Reactions Catalyzed by Ti Amides

Direct and efficient catalytic reactions with excellent regioselectivity for the preparation of a series of substituted isoindoles, isoquinolines, and imidazoles are reported. Reaction of C6H4(2-CN)CC-R with an array of amines, catalyzed by 10 mol % of [σ:η1:η5-(OCH2)(Me2NCH2)C2B9H9]Ti(NMe2) (1), gives a series of substituted isoindoles in very high yields. In a similar manner, interaction of C6H4(2-CH2CN)CC-Ph with various kinds of amines affords a wide range of substituted isoquinolines. On the other hand, treatment of propargylamines (R′CCCH2NHR′′) with nitriles in the presence of 10 mol % of 1 produces a class of substituted imidazoles in high yields. A possible reaction mechanism is proposed, involving sequential inter- and intramolecular C−N bond formation via hydroamination/cyclization reaction of cyanoalkynes with amines or nitriles with propargylamines catalyzed by titanium amides

Titanacarborane Amide Catalyzed Transamination of Guanidines

This work describes a catalytic transamination of guanidines with a broad substrate scope of primary, secondary, heterocyclic, aliphatic, and aromatic amines, using 5−10 mol % of the half-sandwich titanacarborane amide [σ:η1:η5-(OCH2)(Me2NCH2)C2B9H9]Ti(NMe2) as catalyst. This reaction tolerates common functional groups. The reaction mechanism is also proposed

[2 + 2] Cycloaddition of <i>o</i>‑Carboryne with Vinyl Ethers: Synthesis of Carborane-Fused Cyclobutanes

o-Carboryne (1,2-dehydro-o-carborane) is a very useful synthon for the synthesis of a variety of carborane-functionalized molecules. Using 1-Li-2-OTf-o-C2B10H10 as a precursor, o-carboryne undergoes an efficient [2 + 2] cycloaddition with a large variety of vinyl ethers at room temperature to give a series of carborane-fused cyclobutanes in very good to high isolated yields. This reaction is compatible with many functional groups and has a very broad substrate scope ranging from alkyl- to aryl- and to silyl-substituted vinyl ethers. A stepwise reaction mechanism is proposed based on the control experiments, which is supported by DFT calculations. All new compounds have been fully characterized by 1H, 13C, and 11B NMR spectroscopy as well as HRMS spectrometry. Some are further confirmed by single-crystal X-ray analyses

Reaction of a Zirconocene–Carboryne Complex with Pyridines: Ligand C–H Activation

Reactions of Cp2Zr(μ-Cl)(μ-C2B10H10)Li(OEt2)2 (1) with various N-heterocycles derived from pyridine were studied. Treatment of 1 with pyridine, 2-bromopyridine, 2,4-lutidine, quinoline, and 2-(1-hexynyl)pyridine generated α-C–H activation (σ-bond metathesis) products Cp2Zr(η2-C,N-C5H4N)(σ-C2B10H11) (2), Cp2Zr[η2-C,N-(6-Br-C5H3N)](σ-C2B10H11) (3), Cp2Zr[η2-C,N-(4,6-Me2-C5H2N)](σ-C2B10H11) (4), Cp2Zr(η2-C,N-C9H6N)(σ-C2B10H11) (5), and Cp2Zr{η2-C,N-[6-(nBuCC)-C5H3N]}(σ-C2B10H11) (7), respectively. On the other hand, reaction of 1 with acridine gave the addition product 1,2-[Cp2Zr(10,9-C13H9N)]-1,2-C2B10H10 (6) in 85% isolated yield. Complex 1 reacted with 3-(1-hexynyl)pyridine to afford α-C–H activation species Cp2Zr{η2-C,N-[5-(nBuCC)C5H3N]}(σ-C2B10H11) (8a) and Cp2Zr{η2-C,N-[3-(nBuCC)C5H3N]}(σ-C2B10H11) (8b) in a molar ratio of 42:58, as determined by the 1H NMR spectrum. In the presence of CuI, however, the CC insertion products zirconacyclopentenes 1,2-[Cp2ZrC(2-C5H4N)CR]-1,2-C2B10H10 [R = Bun (9), Ph (10)] were obtained in 74–77% yields. It is suggested that the coordination of pyridine to the Zr atom is crucial for α-C–H activation (σ-bond metathesis). The presence of CuI can alter the reaction path by preventing the coordination of pyridine to the Zr atom, which blocks the α-C–H activation path, leading to the alkyne insertion reaction. All complexes were characterized by 1H, 13C, and 11B NMR spectra as well as elemental analyses. Their structures were further confirmed by single-crystal X-ray analyses

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

Nickel-Mediated Three-Component Cycloaddition Reaction of Carboryne, Alkenes, and Alkynes

Nickel-Mediated Three-Component Cycloaddition Reaction of Carboryne, Alkenes, and Alkyne