Mono- and Bis-Phosphine Promoted Incorporation of Boron, Nitrogen, and Phosphorus into Heterocycles via Staudinger Reactions of Borafluorene Azides

We report the synthesis and characterization of a series of BNP-incorporated borafluorenate heterocycles formed via thermolysis reactions of pyridylphosphine and bis(phosphine)-coordinated borafluorene azides. The use of diphenyl-2-pyridylphosphine (PyPh2P), trans-1,2-bis(diphenylphosphino)ethylene (Ph2P(H)C═C(H)PPh2), and bis(diphenylphosphino)methane (Ph2PC(H2)PPh2) as stabilizing ligands resulted in Staudinger reactions to form complex heterocycles with four- (BN2P, BNPC, P2N2) and five-membered (BNP2C and BN2PC) rings, which were successfully isolated and fully characterized by multinuclear NMR and X-ray crystallography. However, when bis(diphenylphosphino)benzene (Ph2P-Ph-PPh2) was used as the ligand in a reaction with 9-bromo-9-borafluorene (BF-Br), due to the close proximity of the donor P atoms, the diphosphine-stabilized borafluoronium ion with an unusual borafluorene dibromide anion was formed. Reaction of the borafluoronium ion with trimethylsilyl azide left the cation intact, and the dibromide anion was substituted by a diazide. Density functional theory calculations were used to provide mechanistic insight into the formation of these new boracyclic compounds. This work highlights a new method in which donor phosphine ligands may be used to promote dimerization, cyclization, and ring contraction reactions to produce boracycles via Staudinger reactions.

One-Pot Access to Boron-Doped Fused Heterocycles via Domino Cyclization of Bis-Diazidoboranes with Isonitrile

Boron-doped fused heterocycles have shown great potential in the field of functional materials. This study reports on the synthesis of a new class of bis-diazidoboranes and the discovery of their cycloaddition reaction with isonitriles. Triply fused boron-doped heterocyclic compounds were constructed in a one-pot process through a domino cycloaddition, providing an effective route for constructing complex boron-doped heterocyclic systems.

Photochemically and Thermally Generated BN-Doped Borafluorenate Heterocycles via Intramolecular Staudinger-Type Reactions

A series of BN-incorporated borafluorenate heterocycles, bis(borafluorene-phosphinimine)s (11-15), have been formed via intramolecular Staudinger-type reactions. The reactions were promoted by light or heat using monodentate phosphine-stabilized 9-azido-9-borafluorenes (R3P-BF-N3; 6-10) and involve the release of dinitrogen (N2), migration of phosphine from boron to nitrogen, and oxidation of the phosphorus center (PIII to PV). Density functional theory (DFT) calculations provide mechanistic insight into the formation of these compounds. Compounds 11-15 are blue emissive in the solution and solid states with absolute quantum yields (ΦF) ranging from 12 to 68%.

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.

Distance-Triggered Distinct Aryl Migrations on Azidodiboranes

Derived from structurally similar precursors, two different azidodiboranes went through distinct aryl migration reactions triggered by different boron-boron separation distances. Biphenylene based diborane with a shorter boron-boron distance underwent heterolateral aryl migration to form a seven-membered azadiborepin, while xanthrene based diborane with a longer boron-boron distance afforded a stable bis-azidoborane scaffold. The pyrolysis of such a bis-azidoborane led to eight-membered oxazadiborocine through homolateral aryl migration and subsequent [3+2] cycloaddition. Density functional theory (DFT) calculations unveiled that the boron-boron separation distances were the intrinsic factors for the distinct migrations.

A Three‐Dimensional Inorganic Analogue of 9,10‐Diazido‐9,10‐Diboraanthracene: A Lewis Superacidic Azido Borane with Reactivity and Stability

Herein, we report the facile synthesis of a three‐dimensional (3D) inorganic analogue of 9,10‐diazido‐9,10‐dihydrodiboraantracene, which turned out to be a monomer in both the solid and solution state, and thermally stable up to 230 °C, representing a rare example of azido borane with boosted Lewis acidity and stability in one. Apart from the classical acid‐base and Staudinger reactions, E−H bond activation (E=B, Si, Ge) was investigated. While the reaction with B−H (9‐borabicyclo[3.3.1]nonane) led directly to the 1,1‐addition on N_α upon N₂ elimination, the Si−H (Et₃SiH, PhMe₂SiH) activation proceeded stepwise via 1,2‐addition, with the key intermediates 5_int and 6_int being isolated and characterized. In contrast, the cooperative Ge−H was reversible and stayed at the 1,2‐addition step.

Diborane(4) Azides: Surprisingly Stable Sources of Transient Iminoboranes

Herein we describe the first examples of isolable electron-precise diboranes(4) that bear azide moieties: the acyclic 1,2-diazido-1,2-bis(dimethylamino)diborane(4) and the cyclic 1,4-diaryl-2,3-diazido-1,4-diaza-2,3-diborinines (aryl=mesityl, 2,6-xylyl, 4-tolyl). The reported examples are not only stable enough to be observed and isolated (putative transient diborane(4) azides previously reported by our group spontaneously decompose even below room temperature), but some of them are even robust enough to undergo controlled pyrolysis without explosive decomposition at temperatures well above 100 °C. In two cases, the controlled pyrolysis allows the isolation of complex diazaboretidines, which are the apparent dimerization products of endocyclic boryl-iminoboranes.

Synthesis of Complex Boron-Nitrogen Heterocycles Comprising Borylated Triazenes and Tetrazenes Under Mild Conditions

The reactions of organic azides with diaryl(dihalo)diboranes(4) were explored, resulting in the observation of a number of surprising reactivity patterns. The reaction of phenyl azide with 1,2-diaryl-1,2-dihalodiboranes(4) resulted in the formation of five-membered rings comprising diboryl-triazenes with retention of the boron-boron bond, while the reaction of the peculiar 1,1-di(9-anthryl)-2,2-difluorodiborane(4) with phenyl azide yielded a six-membered ring bearing a diboryl-triazene, whereby the B-B bond was ruptured by the insertion of an arylnitrene-like reactive intermediate. Both types of heterocycles feature unprecedented connectivity patterns and are very rare examples of boryl triazenes beyond the more common 1,2,3-triazolatoboranes. They are also the product of a unique type of aryl migration from a boron center to the phenyl azide γ-nitrogen center. Lastly, the substitution of 1,2-diaryl-1,2-dihalodiboranes(4) with azide groups, using trimethylsilyl azide as the transfer reagent, yielded boryl-tetrazaboroles and diboryldiazadiboretidines (as side-products), invoking the intermediacy of the first N-boryl-substituted iminoboranes, which are BN isosteres of monoborylated alkynes. The synthetic results are complemented with mechanistic proposals derived from quantum-chemical calculations.

Recent advances in transition metal-free catalytic hydroelementation (E = B, Si, Ge, and Sn) of alkynes

This review describes the recent advances in the transition metal-free hydroelementation of alkynes with various metalloid hydrides.

Catalyst free boron carbon bond cleavage and facile formation of five-membered PNBCC heterocycles

The [3 + 2] cycloaddition reaction of phosphanyl aminoborane [N(2,6-iPr₂C₆H₃)(PPh₂)(BCy₂)] (1) with activated alkynes led to boron and phosphorus containing five-membered heterocycles [(2,6-iPr₂C₆H₃)NPPh₂(CO₂R)C-C(Cy)(CO₂R)(BCy)] [R = Me (2), Et (3) and H (4)] with facile cleavage of the B-C bond and concomitant formation of a P-C bond with an ylidic character. DFT calculations indicate that 1 can be considered as a non-conjugated 1,3-dipole having two reaction centers viz., a nucleophilic P-center and an electrophilic B-center. The reaction of 1 with the alkynes proceeds through a stepwise dipolar addition mechanism, followed by the migration of the cyclohexyl group from the B-atom to the adjacent C-atom.

PNPCB heterocycles via thermal and Lewis acid catalyzed trans-hydroborations

Reactions of iPr₂P(BH₃)N₃ and R₂PCCR affords compounds of the form R₂P(CCR)NP(BH₃)iPr₂ which undergo subsequent thermally induced or Lewis acid catalyzed intramolecular hydroboration to give PNPCB heterocycles R₂P(CCHPh)NP(BH₂)iPr₂.

Cycloaddition reactions of (C6F5)2BN3 with dialkyl acetylenedicarboxylates

The 1,3-dipolar cycloaddition reactions of the electron deficient boron azide, (C6F5)2BN3 (1) with the electron-poor acetylenes RO2CC≡CCO2R (R = Me, Et) afforded the new mono- and bis-1,2,3-triazole derivatives and .

Applications of pentafluorophenyl boron reagents in the synthesis of heterocyclic and aromatic compounds

Recently, main group reagents have attracted a lot of attention in bond-forming reactions in organic synthesis. This article highlights the use of pentafluorophenyl substituted boron reagents in their reactions with C=C and C≡C π-bonds for the synthesis of heterocyclic and aromatic compounds. These cyclisation reactions fall into four general classes although there is some overlap between classes and often combinations of these different types of reactivity are observed in the formation of the final heterocyclic product: (i) 1,2- (and 1,4-) additions of nucleophile and Lewis-acidic boron centre, (ii) 1,1-carboboration, (iii) carbocation rearrangements and (iv) cycloaddition chemistry. In addition, the prospect of using such boron reagents catalytically in the synthesis of aromatic compounds such as oxazoles and dibenzopentalene derivatives is emphasised.