Formation and Cycloaddition Reactions of a Reactive Boraalkene Stabilized Internally by N-Heterocyclic Carbene

Nitrous oxide as diazo transfer reagent

Nitrous oxide, commonly known as "laughing gas", is formed as a by-product in several industrial processes. It is also readily available by thermal decomposition of ammonium nitrate. Traditionally, the chemical valorization of N2O is achieved via oxidation chemistry, where N2O acts as a selective oxygen atom transfer reagent. Recent results have shown that N2O can also function as an efficient diazo transfer reagent. Synthetically useful methods for synthesizing triazenes, N-heterocycles, and azo- or diazo compounds were developed. This review article summarizes significant advancements in this emerging field.

Germaborene reactivity study - addition of carbon nucleophiles, cycloaddition reactions, coordination chemistry

MeNHC substituted germaborenium cation 2 was synthesized directly in reaction of bromo-substituted germaborene 1b with MeNHC. The adamantyl isonitrile substituted germaborenium cation 4 was obtained stepwise: substitution of the chloride atom against adamantyl isonitrile at the B-Cl unit in 1a, simultaneous migration of the chloride to the germanium atom followed by chloride abstraction using Na[BArF 4] gives the germaborenium cation 4. Substitution of the bromide atom in 1b against carbon monoxide followed by bromide abstraction using Ag[Al(OtBuF)4] leads to compound 6 exhibiting a B[double bond, length as m-dash]C double bond substituted at the boron atom by a germylium cation. Treating the germaborene [Ge[double bond, length as m-dash]B-Ph] (1c) with selenium, a cycloaddition product 7 was characterised featuring a GeBSe heterocycle. Carbon dioxide reacts with 1b to give a four membered ring molecule 8 as the product of a B-C and Ge-O bond formation. In reaction of 1b with dimethylbutadiene, a product 9 of a [2 + 4] cycloaddition was isolated. Transition metal fragments [Fe(CO)4 (10), CuBr (11), AuCl (12)] show coordination at the germaborene double bond. Molecular structures of the germaborene coordination compounds 10-12 are presented and the ligand properties are discussed. After treating the germaborene [Ge[double bond, length as m-dash]B-Br] (1b) with [Cp*Al]4, insertion of a Cp*Al moiety into the B-Br bond was found (13).

The Lewis superacidic aluminium cation: [(NHC)Al(C6F5)2]. Chem Commun (Camb) 2024;60:698-701. [PMID: 38111304 DOI: 10.1039/d3cc05440c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The aluminium salt [(NHC)Al(tol)(C6F5)2][B(C6F5)4], (NHC = C3H2(N(iPr2C6H3))2) is shown to behave as a Lewis superacid as it abstracts fluoride from [SbF6]-. It also acts as a Lewis acid catalyst for hydrosilyation, hydrodefluorination and Friedel-Crafts reactions.

Derivatization of an Alkylideneborane with B═C Bond Cleavage

We present the synthesis, structural characterization, and reactivity of alkylideneborane 2, supported by π-donating N-heterocyclic imino and σ-donating N-heterocyclic carbene (NHC) ligands. The incorporation of these ligands effectively weakens the B═C bond strength, leading to enhanced reactivity. Consequently, selective cleavage of the B═C bond can be achieved using pyridine-N-oxide, sulfur, and selenium, resulting in the formation of 1,3-dioxa-2,4-diboretane 3, thioxoborane 4, and selenoborane 5, respectively. Furthermore, intriguing B═C bond insertions with CO2 and CS2 are observed, affording zwitterionic borenium/fluorenide 6 and dithiaboretane 7. The former species 6 is readily converted to transient oxoborane and imidazolium enolate, showcasing the bora-Wittig reaction of alkylideneborane. This investigation highlights the potential of alkylideneborane as a versatile building block for synthesizing novel organoboron compounds through unconventional transformations.

Insertion of carbon monoxide into an unsymmetrical diborene to form an oxaborirane

The insertion of carbon monoxide (CO) into an unsymmetrical diborene, concomitant with B-O bond formation under ambient conditions, gives an oxaborirane species. A similar insertion reaction with isocyanide, the isoelectronic species of CO, generates azaboriridine derivatives.

Electron-Precise Dicationic Tetraboranes: Syntheses, Structures and Rearrangement to an Alkylidene Borate-Borenium Zwitterion and a 1,3-Azaborinine

Carbene-stabilized symmetrical and unsymmetrical dicationic tetraboranes, featuring an electron-precise tetraborane chain, were synthesized and fully characterized. Reactions of these tetraboranes with reductants/bases give rise to different outcomes according to the conditions employed, including: 1) reduction and rearrangement of the tetraborane chain to give a zwitterionic alkylidene borate-borenium species; 2) cleavage of the tetraborane chain to afford a 1,3-azaborinine; and 3) reduction of the supporting ligands to provide a diamino dipotassium salt. The zwitterionic alkylidene borate-borenium species can be viewed as an analogue of the base-stabilized diborenes. NMR spectroscopy and DFT calculations reveal a highly polarized B-B bond in the zwitterionic alkylidene borate-borenium, in which the formal oxidation states of the boron atoms can be considered as -1 and +2. These results suggest the considerable potential of tetraboranes as synthons for low-valent boron species.

Boraalkenes Made by a Hydroboration Route: Cycloaddition and B=C Bond Cleavage Reactions

Hydroboration of styrene or vinylcyclohexane with the IMes(C₆ F₅ )BH⁺ cation followed by deprotonation provided a convenient synthetic entry to the [B]=CHCH₂ R boraalkenes 9 a and 9 b. The in situ generated IMes(SCN)BH⁺ system reacted similarly with 1,1-diphenylethene followed by deprotonation to give the isothiocyanato substituted boraalkene 9 c. The boraalkenes underwent [2+2] cycloaddition reactions with a small series of heterocumulenes to give the respective four-membered heterocycles. The [B]=CHCH₂ R+CO₂ cycloadducts 13 a and 13 b added the borane HB(C₆ F₅ )₂ with cleavage of the central B-C σ-bond. CS₂ underwent an unusual reaction with the boraalkenes, namely insertion into the B=C bond with formation of the borylated dithioketene acetal under complete rupture of the strong B=C double bond. The intermediate dithiobora-β-lactone type intermediate was isolated in the case of the isothiocyanato-boraalkene reaction and characterized by X-ray diffraction.

Ligand-stabilized heteronuclear diatomics of group 13 and 15

A theoretical investigation of ligand-stabilized MX diatomics (M = group 13, X = group 15 element) with N-heterocyclic carbene (NHC) ligands has been carried out to assess bonding and electronic structure. Binding of two ligands in the form L-MX-L is generally preferred over binding of a single ligand as L-MX or MX-L. Binding of carbene donor ligands is predicted to be thermodynamically favorable for all the systems, and is very favorable for the lighter group 15 systems (nitrogen and phosphorus). Detailed analysis of the bonding in these complexes has been carried out with energy decomposition analysis (EDA). In all cases, the carbene to boron and carbene to nitrogen bonding is described as an electron-sharing double bond with both σ and π bonding interactions. For the heavier elements, bonding to C (except for PC interactions) is best described as a donor-acceptor σ single bond.

Deoxygenating Reduction of CO2 by [Cp*Al]4 to Form a (Al3O2C)2 Cluster Featuring Two Ketene Moieties

Herein we report that the reaction of the low-valent aluminum(I) species [Cp*Al]₄ (Cp* = pentamethylcyclopentadienyl) with CO₂ exhibits complete cleavages of the C═O bonds. The deoxygenating reduction reaction of [Cp*Al]₄ with CO₂ at 120 °C afforded [(Cp*)₃Al₃O₂C(CO)]₂ (1), which featured two stacked (Al₃O₂C)₂ units and two C═C═O ketene moieties. Moreover, the isoelectronic analogues of diimine and isothiocyanate with CO₂ were also investigated, and the reactions of [Cp*Al]₄ with Dipp*-N═C═N-Dipp* and Dipp-C═N═S [Dipp* = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl; Dipp = 2,6-diisopropylphenyl] afforded dialuminylimine (2) and tetrameric [Cp*AlS]₄ (3), respectively.

Activation of CS2 with the 2H-Phosphindole Complex to Construct P,S-Polycycles

The activation of CS₂ by the 2H-phosphindole complex with a low-coordinate phosphadiene moiety is reported. The successive hetero-Diels-Alder reaction between 2H-phosphindoles and CS₂ constructs two bridged rings and one spirocycle simultaneously, affording structurally complex P,S-polycyclic products. The two 2H-phosphindoles approach the C═S bond in a head-to-head disposition to minimize steric hindrance. This work reveals the unique reactivity of low-coordinate organophosphorus species and their potential applications in small molecule activation.

A deprotonation pathway to reactive [B]CH2 boraalkenes

The BH compounds IMes(Ar^F)BH(NTf₂) (Ar^F: C₆F₅ or FpXyl) were converted to the IMes(Ar^F)BCH₂ boraalkenes in a three step reaction sequence of B-methylation with methyllithium, hydride abstraction and deprotonation. The BCH₂ boraalkenes reacted with elemental sulfur to give a thiaborirane product. They underwent [2+2] cycloaddition reactions with carbon dioxide or sulfur dioxide to give four-membered boron containing heterocycles. The boraalkenes added strongly Lewis acidic boranes at their CH₂ carbon atoms. The corresponding HB(C₆F₅)₂/boraalkene adduct reduced carbon monoxide to a -OCH(C₆F₅)- moiety inside a five-membered heterocycle at the B-CH₂-B template. The boraalkenes reacted with the [(Me₂S)AuCl] reagent to form the corresponding (boraalkene)AuCl complexes.

An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small-Molecule Activation and Rearrangement Chemistry

A one-pot synthesis of a CAAC-stabilized, unsymmetrical, cyclic diborene was achieved via consecutive two-electron reduction steps from an adduct of CAAC and B2 Br4 (SMe2 )2 . Theoretical studies revealed that this diborene has a considerably smaller HOMO-LUMO gap than those of reported NHC- and phosphine-supported diborenes. Complexation of the diborene with [AuCl(PCy3 )] afforded two diborene-AuI π complexes, while reaction with DurBH2 , P4 and a terminal acetylene led to the cleavage of B-H, P-P, and C-C π bonds, respectively. Thermal rearrangement of the diborene gave an electron-rich cyclic alkylideneborane, which readily coordinated to AgI via its B=C double bond.

Generation of Boryl-nitroxide Radicals from a Boraalkene via the Nitroso Ene Reaction

Examples of isolated boron substituted nitroxide radicals are rare. The reaction of the reactive cyclic boraalkene 3 with nitrosobenzene yields a mixture of the [2 + 2] cycloaddition product 4a, the B-nitroxide radicals 5a and 6a and the azoxybenzene co-product 7avia a bora nitroso ene reaction pathway, the boron analogue of the nitroso ene reaction. The products were separated by flash chromatography, and the B-nitroxide radicals were characterized by X-ray diffraction and EPR spectroscopy. Radical 5a was shown to be a hydrogen atom abstractor. Both the B-nitroxide radicals are more easily oxidized compared to e.g. TEMPO, as shown by cyclic voltammetry.

The NHC-stabilized boraalkene reacts with nitrosobenzene to give a [2+2] cycloaddition product plus a pair of persistent borylnitroxide radicals. These are thought to be formed by means of a bora nitroso ene reaction followed by H-atom transfer.

N-Heterocyclic Carbene Stabilized 1-Bora-1,3-butadienes

Deprotonation of [(NHC)(Fmes)B-allyl]⁺ borenium cations (NHC, IMes (a) or IMe₂ (b); Fmes, 2,4,6-(CF₃)₃C₆H₂) provides an easy entry to the NHC-stabilized 1-bora-1,3-butadienes. They feature a planar s-trans-conformation just like 1,3-butadiene. The 1-borabutadiene 7a undergoes hydroboration reactions; the HB(C₆F₅)₂ hydroboration product is trapped with CO or an isonitrile to give the respective cyclic zwitterionic borenium-borate enolate or enamide products. 1-Borabutadiene 7b undergoes 1,4-chalcogenation with elemental sulfur or selenium, and it gives the six-membered heterocyclic 1,4-addition product with the S═O bond of sulfur dioxide. Compound 7b served as a precursor for the formation of a borylated η³-allyl ligand at Ru. 7b formed a Rh complex by reaction with [Rh(ethylene)₂Cl]₂. It subsequently underwent an intramolecular C-H activation reaction to a mixture of η³-methyl-boraallyl Rh complex isomers.

Competition for Hydride between Silicon and Boron: Synthesis and Characterization of a Hydroborane-Stabilized Silylium Ion

Potent main‐group Lewis acids are capable of activating element‐hydrogen bonds. To probe the rivalry for hydride between silylium‐ and borenium‐ion centers, a neutral precursor with the hydrosilane and hydroborane units in close proximity on a naphthalene‐1,8‐diyl platform was designed. Abstraction of one hydride leads to a hydroborane‐stabilized silylium ion rather than a hydrosilane‐coordinated borenium ion paired with [B(C₆F₅)₄]⁻ or [HCB₁₁Cl₁₁]⁻ as counteranions. Characterization by multinuclear NMR spectroscopy and X‐ray diffraction supported by DFT calculations reveals a cationic, unsymmetrical open three‐center, two‐electron (3c2e) Si−H−B linkage.