Oxidative Addition at a Carbene Center: Synthesis of an Iminoboryl-CAAC Adduct

Cyclic (Alkyl)(Amino)Carbene-Iminoboryl Compounds with Three Formal Oxidation States

BN/CC isosterism is an effective strategy to build hybrid functional molecules with unique properties. In contrast to the alkynyl iminium salts derived from cyclic (alkyl)(amino)carbenes (CAACs) that feature only one reversible reduction wave, the isoelectronic cationic CAAC-iminoboryl adducts could be singly and doubly reduced smoothly. Both the resultant neutral radical and anionic azaborataallenes bear NBC-mixed allenic structures. The former radical has a high spin-density of 0.55e at CCAAC carbon, yet exhibits formal boron-centered radical reactivity. The latter azaborataallenes feature the nucleophilic CCAAC center and polar N(δ-)═B(δ+)═C(δ-) unit, and readily undergo nucleophilic substitution, isocyanide insertion, dipolar addition and cycloaddition reactions etc. The N-substituents have been shown to have a significant influence on the solid-state structure, thermal stability, and reactivity of azaborataallenes. This work showcases the allenic BN-unsaturated species as versatile building blocks in organic synthesis.

Synthesis, structure and reactivity of iminoborane radicals

The synthesis, characterization and reactivities of iminoborane radicals were reported. Both X-ray analysis and density functional theory calculations revealed that the unpaired electron delocalizes over the N(1)-C(1)-B(1)-N(2) moiety. Radical trap reactions showed that this radical species acts as a boron radical. These reactions also serve as methods for the synthesis of Lewis base-stabilized oxyl-terminated iminoboranes.

Isolation of (Aryl)-(Imino) Phosphide and (Aryl)-(Phosphaalkene) Amide Complexes of Alkali Metals from Carbene-Phosphinidenes under Reductive-Thermal Rearrangements

Two-electron reduction of cyclic alkyl(amino) carbene (cAAC)-supported chloro-phosphinidene cAAC=P-Cl (1) followed by unprecedented thermal rearrangements afforded the alkali metal complexes of (aryl)-(cyclic alkyl(imino)) phosphides 3 a-3 c, 4 a-4 b through migration of the 2,6-diisopropylphenyl (dipp) group from N to the P centre, and the (aryl)-(cyclic alkyl(phosphaalkene)) amide 5 through cleavage of the CMe2 -N bond followed by energetically favoured 5-exo-tet ring-closure in the presence of the alkali metals Cs (3 a-3 c), K (4 a, 4 b), and Li (5). Compound 3 a was found to be photoluminescent (PL), emitting bright orange light under a laboratory UV lamp of wavelength 365 nm with PL quantum yield (ϕPL ) of 2.6 % (λem =600 nm), and an average lifetime (τ) of 4.8 μs. Reaction of 3 a with CuCl and AgOTf afforded (aryl)-(cyclic alkyl(imino)) phosphide-stabilized tetra-nuclear CuI (6), and octa-nuclear AgI (7) clusters, respectively. Moreover, complexes 3 a-3 c provided a direct route for the stabilization of cyclic alkyl(aminoboryl) phosphaalkenes 8 a-8 c when treated with 1-bromo-N,N,N',N'-tetraisopropylboranediamine.

Iminoboranes With Parent B=NH Entity: Imino Group Metathesis, Nucleophilic Reactivity and N-N Coupling

While R2 C=N-R double bonds in organic imines are well established, the related iminoboranes R-B=N-R are either prone to oligomerization or can only be stabilized at sufficient steric congestion. In particular, the examples of unsubstituted parent B=N-H entity are rare. We demonstrate that the amino imidazoline-2-imine ligand system (HAmIm) not only gives rise to the isolation of a parent (AmIm)B=N-H iminoborane, but also to species of type (AmIm)B=N-SiMe3 with concomitant stabilization by lithium bromide. The double bond character in these systems is unambiguously corroborated by DFT calculations. The steric accessibility of the (AmIm)B=NH unit allows facile reactivity including metathesis reactions with C=O and C=S bonds, nucleophilic addition toward organic and organometallic carbonyl compounds, but also oxidative N-N coupling within a dimeric unit. Thus, the chemical behavior of the (AmIm)B=N-H and (AmIm)B=N-SiMe3 is distinctly different from that of organic imines.

An Aluminium Imide as a Transfer Agent for the [NR]2- Function via Metathesis Chemistry

The reactions of a terminal aluminium imide with a range of oxygen-containing substrates have been probed with a view to developing its use as a novel main group transfer agent for the [NR]^2- fragment. We demonstrate transfer of the imide moiety to [N₂ ], [CO] and [Ph(H)C] units driven thermodynamically by Al-O bond formation. N₂ O reacts rapidly to generate the organoazide DippN₃ (Dipp=2,6-ⁱ Pr₂ C₆ H₃ ), while CO₂ (under dilute reaction conditions) yields the corresponding isocyanate, DippNCO. Mechanistic studies, using both experimental and quantum chemical techniques, identify a carbamate complex K₂ [(NON)Al-{κ² -(N,O)-N(Dipp)CO₂ }]₂ (formed via [2+2] cycloaddition) as an intermediate in the formation of DippNCO, and also in an alternative reaction leading to the generation of the amino-dicarboxylate complex K₂ [(NON)Al{κ² -(O,O')-(O₂ C)₂ N-(Dipp)}] (via the take-up of a second equivalent of CO₂ ). In the case of benzaldehyde, a similar [2+2] cycloaddition process generates the metallacyclic hemi-aminal complex, K_n [(NON)Al{κ² -(N,O)-(N(Dipp)C(Ph)(H)O}]_n . Extrusion of the imine, PhC(H)NDipp, via cyclo-reversion is disfavoured thermally, due to the high energy of the putative aluminium oxide co-product, K₂ [(NON)Al(O)]₂ . However, addition of CO₂ allows the imine to be released, driven by the formation of the thermodynamically more stable aluminium carbonate co-product, K₂ [(NON)Al(κ² -(O,O')-CO₃ )]₂ .

Stereoselective Activation of Small Molecules by a Stable Chiral Silene

The reaction of the dilithium salt of the enantiopure (S)-BINOL (1,1'-bi-2-naphthol) with two equivalents of the amidinate-stabilized chlorosilylene [L^Ph SiCl] (L^Ph =PhC(NtBu)₂ ) led to the formation of the first example of a chiral cyclic silene species comprising an (S)-BINOL ligand. The reactivity of the Si=C bond was investigated by reaction with elemental sulfur, CO₂ and HCl. The reaction with S₈ led to a Si=C bond cleavage and concomitantly to a ring-opened product with imine and silanethione functional groups. The reaction with CO₂ resulted in the cleavage of the CO₂ molecule into a carbonyl group and an isolated O atom, while a new stereocenter is formed in a highly selective manner. According to DFT calculations, the [2+2] cycloaddition product is the key intermediate. Further reactivity studies of the chiral cyclic silene with HCl resulted in a stereoselective addition to the Si=C bond, while the fully selective formation of two stereocenters was achieved. The quantitative stereoselective addition of CO₂ and HCl to a Si=C bond is unprecedented.

Introducing N-Heterocyclic Iminophosphoranes (NHIPs): Synthesis by [3 + 2] Cycloaddition of Azophosphines with Alkynes and Reactivity Studies

Azophosphines (Ar-N═N-PR₂) were prepared from N-aryl-N'-(trimethylsilyl)diazenes (Ar-N═N-SiMe₃) and R₂PCl by Me₃SiCl elimination or oxidation of phosphinohydrazines (Ar-NH-NH-PR₂) by 2,5-dialkyl-1,4-benzoquinones. Azophosphines underwent 1,3-dipolar cycloaddition with cyclooctyne and dimethylacetylene dicarboxylate to give N-heterocyclic iminophosphoranes (NHIPs), which are structurally similar to cyclic (alkyl)(amino)carbenes. The cycloaddition reaction is compatible with various phosphorus atom substituents including phenyl (NHIP-1,4,6), isopropyl (NHIP-2), cyclohexyl (NHIP-3), and dimethylamino (NHIP-5) groups. The pK_BH⁺ values of the NHIPs in acetonitrile range from 13.13 to 23.14. On the basis of the Huynh electronic parameter, NHIP-1 and NHIP-2 have σ-donor strengths comparable with that of 1,8-diazabicyclo[5.4.0]undec-7-ene. NHIP-1 underwent facile 1,2-addition with pentafluoropyridine to form a rare fluorophosphorane. The treatment of NHIP-1 with triphenylsilane resulted in P-N bond cleavage, accompanied by the reduction of phosphorus(V) to phosphorus(III). A homoleptic, cationic Cu^I-NHIP-1 complex was also prepared. The potential utility of π-donating NHIPs was demonstrated by the stabilization of a reactive iminoborane (Cl-B≡N-SiMe₃). The facile scalable synthesis, tunability of steric demands, and basicity of NHIPs suggest that this new heterocycle class may find a wide range of applications in synthetic chemistry.

Acid/Base-Free Acyclic Anionic Oxoborane and Iminoborane Bearing Diboryl Groups

Anionic oxoboranes and neutral iminoboranes, which are isoelectronic to ketones and alkynes, respectively, have attracted much attention because of their unique structures and various reactivity. However, acid/base-free oxoboranes and iminoboranes are still limited, and readily accessible examples with diverse electronic and steric characteristics are highly desirable. Herein, we report the first syntheses of the acyclic anionic oxoborane 2 and iminoborane 4 bearing two boryl ligands, both of which are acid/base-free. Spectroscopic analysis, X-ray crystallography, and theoretical calculations reveal that 2 and 4 possess a polarized terminal B═O double bond and central B≡N triple bond, respectively.

Unraveling the reactivity of a cationic iminoborane: avenues to unusual boron cations

A cationic terminal iminoborane [Mes*N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> B ← IPr₂Me₂][AlBr₄] (3⁺[AlBr₄]⁻) (Mes* = 2,4,6-tri-tert-butylphenyl and IPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) has been synthesized and characterized. The employment of an aryl group and N-heterocyclic carbene (NHC) ligand enables 3⁺[AlBr₄]⁻ to exhibit both B-centered Lewis acidity and BN multiple bond reactivities, thus allowing for the construction of tri-coordinate boron cations 5⁺–12⁺. More importantly, initial reactions involving coordination, addition, and [2 + 3] cycloadditions have been observed for the cationic iminoborane, demonstrating the potential to build numerous organoboron species via several synthetic routes.

An NHC-stabilized aryliminoboryl cation exhibits both boron-centered Lewis acidity and multiple bond reactivity and could be utilized as an effective synthon for unusual cationic boron species.

Contrasting E-H Bond Activation Pathways of a Phosphanyl-Phosphagallene

The reactivity of the phosphanyl‐phosphagallene, [H₂C{N(Dipp)}]₂PP=Ga(Nacnac) (Nacnac=HC[C(Me)N(Dipp)]₂; Dipp=2,6‐ⁱPr₂C₆H₃) towards a series of reagents possessing E−H bonds (primary amines, ammonia, water, phenylacetylene, phenylphosphine, and phenylsilane) is reported. Two contrasting reaction pathways are observed, determined by the polarity of the E−H bonds of the substrates. In the case of protic reagents (^δ−E−H^δ+), a frustrated Lewis pair type of mechanism is operational at room temperature, in which the gallium metal centre acts as a Lewis acid and the pendant phosphanyl moiety deprotonates the substrates. Interestingly, at elevated temperatures both NH₂ⁱPr and ammonia can react via a second, higher energy, pathway resulting in the hydroamination of the Ga=P bond. By contrast, with hydridic reagents (^δ+E−H^δ−), such as phenylsilane, hydroelementation of the Ga=P bond is exclusively observed, in line with the polarisation of the Si−H and Ga=P bonds.

A Cyclic Iminoborane-NHC Adduct: Synthesis, Reactivity, and Bonding Analysis

Lewis-base coordinated iminoborane adducts, in contrast to their isoelectronic analogue imines, remain largely unexplored given the lack of efficient synthetic strategies for generating robust compounds. Herein, we report the preparation of a cyclic amino iminoborane carbene complex 2 obtained in quantitative yield by adding NHC to the 1,8-(trimethylsilyl)aminonaphthalene complex of boron 1 to induce the elimination of trimethylsilyl chloride (TMSCl). The iminoborane-NHC adduct 2 shows unprecedented thermal stability both in the solid and solution phases, due to the rigid, pre-established geometry of the 1,8-diaminonaphthalene scaffold. Theoretical calculations reveal an exceptionally strong iminoborane-NHC bond as a consequence of the enhanced boron-center acidity in combination with the lower steric and electronic shielding. We show that the chemical bond can be understood as donor-acceptor interaction, leading to a different kind of electronic situation of the B═N π-bond. The high conjugation between the p_z-lone pair of the tricoordinated sp² hybridized N atom and the B═N π-system results in a particularly long B═N double bond distance. Taking advantage of the pendant lone pair of the dicoordinated sp² hybridized N atom, the iminoborane-NHC adduct gives access to NHC-stabilized borenium cation 3 through the reaction with trimethylsilyl triflate (Me₃SiOTf) or to the gallium adduct 4 by reacting with GaCl₃. Incorporating an iminoborane functional group into a π-conjugated system brings a new bonding situation for broadening the scope of BN-containing polyaromatic systems.

BNN-1,3-dipoles: isolation and intramolecular cycloaddition with unactivated arenes

The mono-base-stabilized 1,2-diboranylidenehydrazine derivatives featuring a 1,3-dipolar BNN skeleton are obtained by dehydrobromination of [ArB(Br)NH]₂ (Ar = 2,6-diphenylphenyl (Dpp), Ar = 2,6-bis(2,4,6-trimethylphenyl)phenyl (Dmp) or Ar = 2,4,6-tri-tert-butylphenyl (Mes*)) with N-heterocyclic carbenes (NHCs). Depending on the Ar substituents, such species can be isolated as a crystalline solid (Ar = Mes*) or generated as reactive intermediates undergoing spontaneous intramolecular aminoboration of the proximal arene rings via [3 + 2] cycloaddition (Ar = Dpp or Dmp). The latter reactions showcase the 1,3-dipolar reactivity toward unactivated arenes at ambient temperature. In addition, double cycloaddition of the isolable BNN species with two CO₂ molecules affords a bicyclic species consisting of two fused five-membered BN₂CO rings. The electronic structures of these BNN species and the mechanisms of these cascade reactions are interrogated through density functional theory (DFT) calculations.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

CO2 Binding and Splitting by Boron-Boron Multiple Bonds

The room-temperature, ambient-pressure reactions of CO₂ with two species containing boron-boron multiple bonds led to the incorporation of either one or two CO₂ molecules. The structural characterization of a thermally unstable intermediate in one case indicates that an initial [2+2] cycloaddition is the key step in the reaction mechanism.

Cleavage of BN triple bonds by main group reagents

A silylene and a 1,2-diaminoalkyne respectively cleave the strong BN triple bond in iminoboranes.

Synthesis and Structure of N,N-Dinitroamidoborane Complexes

A general approach to the synthesis of borohydride complexes containing one or two dinitroamide fragments has been suggested. Based on a smooth substitution of halide in haloborane or dibromoborane complexes with N,N-dinitroamide salts, this method provides various N,N-dinitroamidoboranes complexes in good yields and in analytically pure form. By means of spectroscopic and computational methods, it was demonstrated that dinitroamidoborane complexes could form as both B,N- and B,O-isomers, which did not interconvert at ambient temperature.

Reactivity of a coordinated inorganic acetylene unit, HBNH, and the azidoborane cation [HB(N3)]. Chem Sci 2017;8:2337-2343. [PMID: 28451338 PMCID: PMC5365008 DOI: 10.1039/c6sc04893e]

A donor-acceptor complex of HBNH was prepared via thermolysis of a carbene-stabilized azidoborane. The reactivity of the fundamentally important HBNH unit (inorganic alkyne analogue) was explored in detail, including attempts to convert this species and related hydrido(azido)borane cations into molecular complexes of BN. This work provides added impetus for the development of molecular precursors that can release bulk boron nitride (a desirable insulator and thermal conductor) under mild conditions, and from solution.

Even the normal is abnormal: N-heterocyclic carbene C2 binding to a phosphaalkene without breaking the PC π-bond

The reaction of MesPCPh₂ with the least sterically demanding N-heterocyclic carbene (NHC = IMe) results in formation of the ‘abnormal’ (C⁴-substituted) 4-phosphino-NHC (1).

Mono- and di-cationic hydrido boron compounds

Stable mono- and di-cationic hydrido boron compounds featuring CH₂BH₂(μ-H)BH₂CH₂ and CH₂BH(μ-H)₂BHCH₂ cores are readily accessible by dehydrogenative hydride abstractions. NBO calculations revealed the occurrence of two B–H–B 3c–2e bonds in the HB(μ-H)₂BH moiety of di-cations (7 and 8), where 43% is located at the H bridges and ∼28% at each boron atom.