Controlling Regiochemistry in the Syntheses of Boraindanes from Diborane(4) Starting Materials

C-C bond-forming reactions of 2-isocyanobiphenyl·BX3 adducts: spontaneous construction of polycyclic heteroaromatics

The syntheses and reactivities of 2-isocyanobiphenyl·BX3 adducts (X = I, Br, Cl) are reported. These adducts undergo unexpected C-H bond-functionalizing cyclization upon heating, yielding phenanthridinium-6-trihaloborate zwitterions. Where X = Cl, an unexpected helical polycyclic boronium salt is formed via a competing pathway. Mechanistic details are probed experimentally and computationally.

Crystalline 2π Aromatic Azadiboriridinylium: A BN Analogue of Cyclopropenylium Cation

N-Substitution of a thermally unstable diboratriazole 1 with a trimethylsilyl group affords a remarkably stable diboratriazole derivative 2. Ring contraction of 2 with an N-heterocyclic carbene accompanied by the release of N2 as well as 1,4-hydrogen shift affords a carbene-stabilized azadiboriridine 3. Abstraction of the H-B3mem hydride in 3 with methyl trifluoromethanesulfonate leads to the isolation of a hitherto unknown azadiboriridinylium 4, the first BN analogue of cyclopropenylium cation. X-ray diffraction analysis and computational studies confirmed the delocalization of π electrons over the B2 N three-membered ring, indicating the 2π aromatic feature. Compound 4 undergoes ring expansion reactions with azobenzene and pyridazine to furnish triazadiborolidinylium species 5 and 6, the latter of which possesses a cationic B2 N3 ring with a pronounced 6π aromatic property. Moreover, the reaction of 4 with a diazo compound produces a cationic B2 N3 C pentafulvene derivative 7.

Formation of cyclic (boryl)iminomethane derivatives by the insertion of isocyanides into a boron-carbon bond

Isocyanides are highly valuable reagents in organic synthesis and have been widely used in multicomponent reactions. Although η²-imidoyl metal complexes, which are important intermediates in isocyanide chemistry, have been extensively explored, their boron species analogues have remained elusive. Hererin, we reported the synthesis of cyclic (boryl)iminomethanes via direct isocyanide insertion into the B-C bond of amino alkenyl boranes in a facile synthetic procedure. A family of well-defined cyclic (boryl)iminomethanes are characterized. Furthermore, the intrinsic ring strain of cyclic (boryl)iminomethanes primes them to further react with isocyanides and selectively afford double insertion products. Our results provide new insights into novel isocyanide chemistry involving boron species.

Nucleophilic reactivity of the gold atom in a diarylborylgold(i) complex toward polar multiple bonds

A di(o-tolyl)borylgold complex was synthesized via the metathesis reaction of a gold alkoxide with tetra(o-tolyl)diborane(4). The resulting diarylborylgold complex exhibited a Lewis acidic boron center and a characteristic visible absorption that arises from its HOMO-LUMO excitation, which is narrower than that of a previously reported dioxyborylgold complex. The diarylborylgold complex reacted with isocyanide in a stepwise fashion to afford single- and double-insertion products and a C-C coupled product. Reactions of this diarylborylgold complex with C[double bond, length as m-dash]O/N double bond species furnished addition products under concomitant formation of Au-C and B-O/N bonds, which suggests nucleophilic reactivity of the gold metal center. DFT calculations provided details of the underlying reaction mechanism, which involves an initial coordination of the C[double bond, length as m-dash]O/N bond to the boron vacant p-orbital of the diarylboryl ligand followed by a migration of the gold atom from the tetracoordinate sp3-hybridized boron center, which is analogous to the reactivity of the conventional sp3-hybridized borate species. The DFT calculations also suggested a stepwise mechanism for the reaction of this diarylborylgold complex with isocyanide, which afforded three different reaction products depending on the applied reaction conditions.

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.

Reactivity of Tetrahalo- and Difluorodiboranes(4) toward Lewis Basic Platinum(0): Bis(boryl), Borylborato, and Doubly Boryl-Bridged Platinum Complexes

The reaction of the tetrahalodiboranes(4) B₂F₄, B₂Cl₄, and B₂Br₄ with a Lewis basic platinum(0) complex led to the isolation of the cis-bis(difluoroboryl) complex cis-[(Cy₃P)₂Pt(BF₂)₂] (1) and the novel borylborato complexes trans-[(Cy₃P)₂Pt{B(X)-BX₃}] (2, X = Cl; 3, X = Br), respectively. The trans influence of the borylborato group was found to be one of the strongest ever observed experimentally. Furthermore, the reactivity of little-explored diaryldifluorodiboranes(4) F₂B-BMes₂ and the new derivative F₂B-BAn₂ (An = 9-anthryl) toward a range of platinum(0) complexes was investigated. Reactions with relatively nonbulky platinum(0) complexes led to the formation of unsymmetrical cis-bis(boryl) complexes cis-[(R₃P)₂Pt(BF₂)(BMes₂)] (6, R = Me; 7, R = Et) as well as the first example of a fourfold-unsymmetrical bis(boryl) complex, [(Me₃P)(Cy₃P)Pt(BF₂)(BMes₂)] (12). The use of a more bulky Pt complex provided access to the unprecedented dinuclear bis(boryl) complexes [{( iPr₃P)Pt}₂(μ-BF₂)(μ-BAr₂)] (8, Ar = Mes; 9, Ar = An), which feature two different μ₂-bridging boryl ligands.

Effects of the Substituents of Boron Atoms on Conjugated Polymers Containing B←N Units

Organoboron chemistry is a new tool to tune the electronic structures and properties of conjugated polymers, which are important for applications in organic optoelectronic devices. To investigate the effects of substituents of boron atoms on conjugated polymers, we synthesized three conjugated polymers based on double B←N-bridged bipyridine (BNBP) with various substituents on the boron atoms. By changing the substituents from four phenyl groups and two phenyl groups/two fluorine atoms to four fluorine atoms, the BNBP-based polymers show blue-shifted absorption spectra, decreased LUMO/HOMO energy levels, and enhanced electron affinities, as well as increased electron mobilities. Moreover, these BNBP-based polymers can be used as electron acceptors for all-polymer solar cells. These results demonstrate that substituents of boron atoms can effectively modulate the electronic properties and applications of conjugated polymers.

B-B bond activation and NHC ring-expansion reactions of diboron(4) compounds, and accurate molecular structures of B2(NMe2)4, B2eg2, B2neop2 and B2pin2

In this detailed study we report on the structures of the widely employed diboron(4) compounds bis(pinacolato)diboron (B₂pin₂) and bis(neopentyl glycolato)diboron (B₂neop₂), as well as bis(ethylene glycolato)diboron (B₂eg₂) and tetrakis(dimethylamino)diboron (B₂(NMe₂)₄), and their reactivity, along with that of bis(catecholato)diboron (B₂cat₂) with backbone saturated and backbone unsaturared N-heterocyclic carbenes (NHCs) of different steric demand. Depending on the nature of the diboron(4) compound and the NHC used, Lewis-acid/Lewis-base adducts or NHC ring-expansion products stemming from B-B and C-N bond activation have been observed. The corresponding NHC adducts and NHC ring-expanded products were isolated and characterised via solid-state and solution NMR spectroscopy and X-ray diffraction. In general, we observed B-B bond and C-N bond activation at low temperature for B₂eg₂, at room temperature for B₂neop₂ and at higher temperature for B₂cat₂. The reactivity strongly depends on steric effects of the NHCs and the diboron(4) compounds, as well as on the corresponding Lewis-basicity and Lewis-acidity. Our results provide profound insight into the chemistry of these diboron(4) reagents with the nowadays ubiquitous NHCs, the stability of the corresponding NHC adducts and on B-B bond activation using Lewis-bases in general. We demonstrate that B-B bond activation may be triggered even at temperatures as low as -40 °C to -30 °C without any metal species involved. For example, the reactions of B₂eg₂ with sterically less demanding NHCs such as Me₂Im^Me and iPr₂Im in solution led to the corresponding ring-expanded products at low temperatures. Furthermore, boronium [L₂B(OR)₂]⁺ and borenium [LB(OR)₂]⁺ cations have been observed from the reaction of the bis-borate B₂eg₃ with the NHCs iPr₂Im and Me₂Im^Me, which led to the conclusion that the activation of bis-borates with NHCs (or Lewis-bases in general) might be a facile and simple route to access such species.