Boraiminolithium: An Iminoborane-Transfer Reagent

A Base-Free Two-Coordinate Oxoborane

Oxoboranes (R-BO) are transient species that rapidly trimerise to form boroxines. To date, the only method used to stabilise oxoboranes is to add a Lewis base, but this forms a three-coordinate at boron oxoborane that has a different bonding/reactivity profile. Herein we report a base-free, two-coordinate oxoborane that is isolated as a Lewis adduct with AlCl3. This species, Mes*BO-AlCl3 (Mes*=2,4,6-tBu-C6H2), has a ν11ΒΟ stretching frequency of 1843 cm-1, indicating a strong BO bond. Computational analysis indicates this is due to a highly polarised BO bonding interaction combined with modest BO multiple bond character. While the polarisation of the BO bond on AlCl3 coordination enhances the Lewis acidity at boron it also reduces the basicity at oxygen and the latter is key to accessing a base-free oxoborane. Finally, this oxoborane reacts with PhN3 in a unique way to form an unprecedented boron heterocycle.

All-Inorganic sp-Chain Ligands: Isoelectronic B/N Analogues of E. O. Fischer's Alkynylcarbynes

Borylation of a tungsten-bound N2 ligand and halide abstraction provides access to a cationic complex with an unprecedented linear NNBR ligand. This complex undergoes [3+2] cycloaddition with azides, and an unexpected chain-extension reaction with an iminoborane, leading to a complex with a five-atom B/N chain. These two [NNBR]-containing complexes, inorganic analogues of E. O. Fischer's alkynylcarbynes, are very rare examples of molecules containing all-inorganic chains of sp-hybridized atoms.

Synthesis and reactivity of an N-heterocyclic carbene-stabilized diazoborane

Diazo compounds and organic azides are widely used as reagents for accessing valuable molecules in multiple areas of fundamental and applied chemistry. Their capacity to undergo versatile chemical transformations arises from the reactive nature of an incipient dinitrogen molecule at the terminal position. In this work, we report the synthesis and characterization of an N-heterocyclic carbene (NHC)-stabilized diazoborane-a boron-centered analog of organic azides and diazoalkanes. The diazoborane displays a strong tendency to release dinitrogen, thus serving as a borylene source, in analogy to organic azides and diazoalkanes serving as nitrene and carbene sources, respectively. Also reminiscent of diazoalkane and organic azide reactivity, the diazoborane serves as a 1,3-dipole that undergoes uncatalyzed [3+2] cycloaddition with an unactivated terminal alkyne, affording a five-membered heterocycle after a two-step rearrangement.

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.

Strain induced reactivity of cyclic iminoboranes: the (2 + 2) cycloaddition of a 1H-1,3,2-diazaborepine with ethene

Iminoboranes have gathered immense attention due to their reactivity and potential applications as isoelectronic and isosteric alkynes. While cyclic alkynes are well investigated and useful reagents, cyclic iminoboranes are underexplored and their existence was inferred only via trapping experiments. We report the first direct spectroscopic evidence of a cyclic seven-membered iminoborane, 1-(tert-butyldimethylsilyl)-1H-1,3,2-diazaborepine 2, under cryogenic matrix isolation conditions. The amino-iminoborane 2 was photochemically generated in solid argon at 4 K from 2-azido-1-(tert-butyldimethylsilyl)-1,2-dihydro-1,2-azaborinine (3) and was characterized using FT-IR, UV-vis spectroscopy, and computational chemistry. The characteristic BN stretching vibration (1751 cm-1) is shifted by about 240 cm-1 compared to linear amino-iminoboranes indicating a significant weakening of the bond. The Lewis acidity value determined computationally (LAB = 9.1 ± 2.6) is similar to that of boron trichloride, and twelve orders of magnitude lower than that of 1,2-azaborinine (BN-aryne, LAB = 21.5 ± 2.6), a six-membered cyclic iminoborane. In contrast to the latter, the reduced ring strain of 2 precludes nitrogen fixation, but it unexpectedly allows facile (2 + 2) cycloaddition reaction with C2H4 under matrix isolation conditions at 30 K.

Alkylation and Arylation at Boron in NHC-Stabilized Phosphaborenes

Phosphaborenes, featuring a phosphorus-boron multiple bond, remain a relatively untapped area in chemical research due to the limited synthetic methods. Introducing leaving groups as substituents to the phosphorus or boron can pave the way for enhanced functionalization and modification. In this study, we present the synthesis of phosphaborenes featuring an N-heterocyclic boryl group on phosphorus and halogen substituent on boron, with stabilization provided by an N-heterocyclic carbene. Straightforward alkylation/arylation of these phosphaborenes is achieved by substituting the halogen with benzyl and aryl groups at the boron terminus. Our approach offers an efficient route to produce a diverse array of phosphaborene structures.

Reactions of main group compounds with azides forming organic nitrogen-containing species

Since the seminal discovery of phenyl azide by Grieß in 1864, a variety of organic azides (R-N3) have been developed and extensively studied. The amenability of azides to a number of reactions has expanded their utility as building blocks not only in organic synthesis but also in bioorthogonal chemistry and materials science. Over the decades, it has been demonstrated that the reactions of main group compounds with azides lead to diverse N-containing main group molecules. In view of the pronounced progress in this area, this review summarizes the reactions of main group compounds with azides, emphatically introducing their reaction patterns and mechanisms. The reactions of forming inorganic nitrogen species are not included in this review.

Frustrated Lewis Pair Chelation and Reactivity of Complexed Parent Iminoborane and Aminoborane

An intramolecular phosphine-borane frustrated Lewis pair (FLP) chelate, ⁱPr₂P(C₆H₄)BCy₂ or PB (Cy = cyclohexyl), was used to coordinate aminoborane (H₂BNH₂) and iminoborane (HBNH) units via donor-acceptor stabilization. Attempts to induce dehydrogenation from these B-N adducts with known metal catalysts (or pre-catalysts) have been unsuccessful thus far, and related observations were noted with an H₂BNH₂ complex supported by a modified FLP chelate bearing a geometrically constrained bicyclic 9-borabicyclo(3.3.1)nonane (BBN) unit. Treatment of the iminoborane adduct [PB{HBNH}] with a chlorinating agent led to ligand activation via B-C bond cleavage instead of the expected H/Cl exchange at boron to give [PB{ClBNH}]. Nucleophilic attack at the boron center in [PB{HBNH}] was observed upon addition of BnK (Bn = benzyl), yielding the amidoborate complex [PB{H(Bn)BNH}{K(THF)₂}].

Metathesis Reactions of a NHC-Stabilized Phosphaborene

The BP unsaturated unit is a very attractive functional group as it provides novel reactivity and unique physical properties. Nonetheless, applications remain limited so far due to the bulky nature of B/P-protecting groups, required to prevent oligomerization. Herein, we report the synthesis and isolation of a N-heterocyclic carbene (NHC)-stabilized phosphaborene, bearing a trimethylsilyl (TMS) functionality at the P-terminal, as a room-temperature-stable crystalline solid accessible via facile NHC-induced trimethylsilyl chloride (TMSCl) elimination from its phosphinoborane precursor. This phosphaborene compound, bearing a genuine B=P bond, exhibits a remarkable ability for undergoing P-centre metathesis reactions, which allows the isolation of a series of unprecedented phosphaborenes. X-ray crystallographic analysis, UV/Vis spectroscopy, and DFT calculations provide insights into the B=P bonding situation.

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.

Synthesis of Iminoboryl o-Carboranes by Lewis Base Promoted Aminoborirane-to-Iminoborane Isomerization

The iminoboryl o-carboranes (Me₃Si)-Cb-B≡N-R (Cb = B₁₀C₂H₁₀, 3a, R = SiMe₃; 3b, R = ^tBu) have been successfully synthesized by tetrahydrofuran (THF)-promoted isomerization from the corresponding o-carborane-fused aminoboriranes Cb{BN(SiMe₃)R} (2). The synthetic protocol of the previously reported borirane 2a was optimized. The borirane Cb{BN(SiMe₃)^tBu} (2b) and the iminoboranes 3a and 3b were fully characterized by NMR, IR, and single-crystal X-ray diffraction analyses. The borirane 2a isomerizes more readily than 2b. The kinetics study revealed a bimolecular mechanism between borirane and THF, which is in good agreement with the computationally proposed reaction pathway. The title compounds are thermally robust, but compound 3a dimerized in the presence of a catalytic amount of ^tBuNC to give the cyclodimer 4. Quick equilibrium between 4 and the isonitrile adduct 4·^tBuNC was observed in solution.

Access to adducts of parent iminoborane isomers, HBNH and NBH2, using frustrated Lewis pair chelation

Adducts of the parent iminoborane isomers, HBNH and NBH₂, have been prepared, each stabilized by the frustrated Lewis pair (FLP) chelate ⁱPr₂P(C₆H₄)BCy₂ (PB). PB{HBNH} was accessed via dehydrohalogenation, while the corresponding isomer PB{NBH₂} was obtained from the borylation of the formal nitrene-FLP complex PB{NH}.