Expedient Synthesis of 1,2‐Thiaborines by Means of Sulfur Insertion into Boroles

The propensity of boroles to undergo ring expansion reactions has been exploited as a route to generate 1,2‐thiaborines, molecules that can be viewed as hybrid inorganic/organic analogues of benzene. Computational studies as well as structural data indicate that the species reported here have a high degree of aromatic character.     

Ring Expansions of Boroles with Diazo Compounds: Steric Control of C or N Insertion and Aromatic/Nonaromatic Products

Access to novel imine‐substituted 1,2‐azaborinines, as well as highly arylated boracyclohexa‐3,5‐dienes has been developed by ring expansion of boroles with diazoalkanes with varying degrees of steric bulk. The formation of a diazoalkane intermediate is also discussed for the reaction of ortho‐brominated p‐tolyl‐azide with 1,2,3,4,5‐pentaphenylborole. Structural details as well as UV/Vis spectroscopic and cyclic voltammetric data are provided. These boron‐containing heterocycles have the potential to serve as building blocks for boron‐containing materials.     

Formation of BN Isosteres of Azo Dyes by Ring Expansion of Boroles with Azides

Herein, we present the results of our investigations on the effect of ortho substitution of aryl azides on the ring‐expansion reaction of boroles, five‐membered unsaturated boron heterocycles. These studies led to the isolation of the first 1,2‐azaborinine‐substituted azo dyes, which are bright yellow solids. One of the derivatives, (E)‐2‐mesityl‐1‐(mesityldiazenyl)‐3,4,5,6‐tetraphenyl‐1,2‐azaborinine, was found to be unstable in solution and to transform through a Jacobsen‐like reaction into an indazole and 1‐hydro‐1,2‐azaborinine. DFT calculations were performed to shed light on possible mechanisms to rationalize the unexpected azo‐azaborinine formation and to draw conclusions about the role played by the ortho substituents in the reaction.     

A Journey from Benzoborirene to Benzoborole-Supported 1,2-Diimine and Antiaromatic Borolediide

The reaction of benzoborirene with one equivalent of isocyanides leads to benzene-fused boretes bearing one imine function, while the reaction with two equivalents of isocyanide affords 2,3-dihydro-2,3-diiminoboroles with perfect regioselectivity. The isocyanide double insertion products represent a novel type of 1,2-diimine with a benzoborole backbone. The reduction chemistry of the benzoborole-supported 1,2-diimine was investigated. Single- and two-electron reduction allow for the isolation and full characterization of a radical anion and a dianion, respectively. In stark contrast to the ordinary boroles, which should turn aromatic by accepting two electrons, the antiaromatic character of the benzoborole backbone is highlighted upon reduction, thus presenting a rare example of antiaromatic borole dianion. Detailed quantum chemical calculations provide a rationale for the observed regioselectivity and the electronic structure of the reduced borole diimine species.     

1,2,3‐Diazaborinine: A BN Analogue of Pyridine Obtained by Ring Expansion of a Borole with an Organic Azide

A new pathway for the ring expansion reaction of antiaromatic boroles with organic azides is reported. While the reaction usually leads to 1,2‐azaborinines, it was diverted to the formation of a 1,2,3‐diazaborinine by changing the electronic characteristics of the reagents. The isolable azo‐azaborinine intermediate initially formed from the reaction of 1‐(2,3,4,5‐tetraphenylborolyl)ferrocene with 4‐azido‐N,N‐dimethylaniline gradually decomposed to a 1,2,3‐diazaborinine and benzonitrile. Both the spectroscopic properties and the reactivity of the heteroaromatic compound show analogies to pyridine, to which it is isoelectronic. Density functional theory (DFT) calculations provided insight into the mechanism of this unusual transformation.     

Diverse Reactivity of Dienes with Pentaphenylborole and 1-Phenyl-2,3,4,5-Tetramethylborole Dimer

The reactions of a monomeric borole and a dimeric borole with 2,3-dimethyl-1,3-butadiene and 1,3-cyclohexadiene were investigated. The monomeric borole reacted at ambient temperature whereas heat was required to crack the dimer to form the monomer and induce reactivity. 2,3-Dimethyl-1,3-butadiene reacts to give diverse products resulting from a cycloaddition process with the B−C moiety of the boroles acting as a dienophile, followed by rearrangements to furnish bicyclic species. For 1,3-cyclohexadiene, a [4+2] process is observed in which 1,3-cyclohexadiene serves as the dienophile and the boroles as the diene partner. The experimental results are corroborated with mechanistic theoretical calculations that indicate boroles can serve as either a diene or dienophile in cycloaddition reactions with dienes.     

(Hetero)arene-fused boroles: a broad spectrum of applications

(Hetero)arene-fused boroles are a class of compounds containing a 5-membered boron diene-ring. Based on their molecular framework, the (hetero)arene-fused boroles can be considered as boron-doped polycyclic antiaromatic hydrocarbons and are thus of great interest. Due to the vacant p_z orbital on the 3-coordinate boron atom, the antiaromaticity and strain of the 5-membered borole ring, (hetero)arene-fused boroles possess strong electron accepting abilities and Lewis acidity. By functionalization, they can be tuned to optimize different properties for specific applications. Herein, we summarize synthetic methodologies, different strategies for their functionalization, and applications of (hetero)arene-fused boroles.

(Hetero)arene-fused boroles, ‘antiaromatic’ 2n-electron π-systems, more stable and more functionalizable than boroles, offer greater potential for a variety of applications.     

Thiophene-fused ladder boroles with high antiaromaticity

A series of polycyclic thiophene-fused boroles were synthesized on the basis of stepwise substitution reactions from thienylboronic ester precursors. In these ladder-type π-conjugated systems, the thiophene-fused structure enhances the antiaromaticity of the borole ring. This trend is opposite to the conventional understanding that the arene-fused structure decreases the antiaromaticity of the 4π-electron ring skeletons. The ladder boroles exhibited characteristic properties such as long-wavelength absorptions and low reduction potentials.     

Recent developments in the chemistry of antiaromatic boroles

First isolated in 1969, most progress in the chemistry of the antiaromatic 4π electron borole system has been made during the 1980s. However, besides the fundamental aspects of the electronic structure and reactivity, boroles have not encountered serious research efforts for a rather long timeframe. This is somewhat surprising given the fact that boroles feature a unique combination of antiaromaticity, strong electrophilicity and unusual electronic properties. It was not until 2008 that interest was resparked. Since then, tremendous progress has been achieved in this area, particularly with respect to synthetic access, structural characterization and reactivity. Various differently substituted borole derivatives have been successfully isolated and characterized both in solution and in the solid state, which provided a more thorough understanding of the structure/reactivity relationship. This feature article is intended to provide a general overview on the electronic structure and the consequences of antiaromaticity on the inherent properties of these highly reactive species. The different synthetic methodologies to generate boroles and their divergent reactivity patterns will be described in great detail, which will emphasize their high potential and relevance in modern chemistry.     

Taming the beast: fluoromesityl groups induce a dramatic stability enhancement in boroles

The electron-deficient pentaarylborole 1-(2′,4′,6′-tris(trifluoromethyl)phenyl)-2,3,4,5-tetraphenylborole (1) has been synthesised with the long-term aim of developing borole-based optoelectronic materials. The bulky 2,4,6-tris(trifluoromethyl)phenyl (FMes) group on the boron atom of 1 significantly improves (>600 times) its air stability relative to its mesityl analogue. Moreover, 1 shows good thermal stability without undergoing the dimerisation or isomerisation reactions reported for some other boroles. A triarylborole analogue (2), belonging to a new class of borole with the 3- and 4-positions of the BC₄ ring linked by a –(CH₂)₃– group, has also been synthesised to elucidate the influence of carbon-bonded substituents on the stability of boroles. Both boroles were prepared through the reaction of Li[FMesBF₃] and divinyldilithium reagents, a new and general method for borole syntheses. Compound 2 was found to isomerise through a [1,3]-H shift and double-bond rearrangement to an s-trans-butadienylborane species under highly basic (NaOH) conditions. The increased steric crowding at the boron centre through incorporation of the FMes group does not preclude binding of Lewis bases to either 1 or 2, as demonstrated by their fully reversible binding of pyridine. Interestingly, 1 exhibits a blue-shifted absorption spectrum, as compared with its mesityl analogue, a result contrary to previous understanding of the influence of substituent electronics on the absorption spectra of boroles. Most importantly, these boroles exhibit much greater air-stability than previously reported analogues without sacrificing the strong electron-accepting ability that makes boroles so attractive; indeed, 1 and 2 have very low reduction potentials of –1.52 and –1.69 eV vs. Fc/Fc⁺, respectively.     

Chemical Reduction and Dimerization of 1‐Chloro‐2,3,4,5‐tetraphenylborole

As neutral isoelectronic analogues of the elusive cyclopentadienyl cation, boroles have been of interest for their prospective applications as strong Lewis acids, chromophores, and electron acceptors. Recently our group discovered a π‐nucleophilic boryl anion based on the borole system. In an effort to extend borole chemistry, we now report the molecular structure of 1‐chloro‐2,3,4,5‐tetraphenylborole ( 1 ) and its corresponding borole dianion resulting from the two‐electron reduction of 1 with KC₈. The thermally induced dimerization of 1 yields an unprecedented boracyclohexadiene/borolene spiro‐bicyclic compound and the resulting dimer was fully characterized including a single‐crystal X‐ray analysis.     

Oligo(borolyl)benzenes—Synthesis and Properties

Herein, we report the synthesis of boroles that are linked by a conjugated phenylene spacer. The characterization of these compounds was completed by NMR and UV/Vis spectroscopy, as well as X‐ray crystal diffraction. Furthermore, the coordination behavior of these oligoboroles towards five electronically and sterically disparate pyridine derivatives was studied and revealed fundamental differences in the properties of the resulting adducts. The experimental results were supported by density functional theory (DFT) calculations that showed a charge‐transfer effect upon formation of the pyridine‐4‐carbonitrile adduct. By chemical reduction of a tris(borolyl)‐substituted benzene derivative, a hexaanion was isolated as a result of a two‐electron reduction of each borolyl moiety. The interaction of the borolyl units through the aryl spacer, and the possible increase of the Lewis acidity due to the conjugation of the borolyl moieties, were investigated by base transfer reactions.     

BN-Butafulvenes and Their Application in the Synthesis of Highly Substituted BN-9,1-Naphthalenes

BN-butafulvenes, mono-BN isosteres of butafulvene and highly strained isomers of azaborines and B-amino boroles, have been synthesized via hydrolysis of the urana-borabicyclic complexes obtained from the reactions of bis(alkynyl)boranes with an uranacyclopropene complex. Their 4-dimethylaminopyridine (DMAP) adducts can further isomerize to 1,2,4,6-multisubstituted BN-9,1-naphthalenes. Both NMR reaction monitoring and theoretical calculations point to a reaction mechanism involving dearomative insertion of DMAP followed by two consecutive 1,2-hydrogen shifts. The photophysical studies of the highly substituted BN-9,1-naphthalenes reveal a notable redshift in both the UV/Vis absorption and emission spectra. The (TD)-DFT calculations corroborate the experimental data, suggesting that the strong π-donating amino substitution at the 1- and/or 6-positions destabilizes the HOMO, and thus leading to a notable decrease of the HOMO–LUMO gap.     

Reductive Carbocyclization of Homoallylic Alcohols to syn‐Cyclobutanes by a Boron‐Catalyzed Dual Ring‐Closing Pathway

The organoborane‐catalyzed reductive carbocyclization of homoallylic alcohols has been developed by using hydrosilanes as reducing reagents to provide a range of 1,2‐disubstituted arylcyclobutanes. The reaction proceeds in a cis‐selective manner with high efficiency under mild conditions. Mechanistic studies, including deuterium scrambling and Hammett studies, and DFT calculations, suggest a dual ring‐closing pathway.     

Accessing 1,2‐Substituted Cyclobutanes through 1,2‐Azaborine Photoisomerization

We provide a seminal example of the utility of the 1,2‐azaborine motif as a 4C+1N+1B synthon in organic synthesis. Specifically, conditions for the practically scalable photoisomerization of 1,2‐azaborine in a flow reactor are reported that furnish aminoborylated cyclobutane derivatives. The C?B bonds could also be functionalized to furnish a diverse set of highly substituted cyclobutanes.     

Highly Linear Selective Cobalt‐Catalyzed Addition of Aryl Imines to Styrenes: Reversing Intrinsic Regioselectivity by Ligand Elaboration

Highly linear selective, imine‐directed hydroarylation of styrene has been achieved with cobalt‐based catalytic systems featuring bis(2,4‐dimethoxyphenyl)(phenyl)phosphine and either 2‐methoxypyridine or DBU as a ligand and a Lewis base additive, respectively, thus affording a variety of 1,2‐diarylethanes (bibenzyls) in good yields under mild reaction conditions. The triarylphosphine controls the regioselectivity, while the Lewis base significantly accelerates the reaction. Ligand screening and deuterium‐labeling studies provide implications about the roles of the ligand and the Lewis base in the crucial C? C reductive elimination step.     

[8+2] and [8+3] Cyclization Reactions of Alkenyl Carbenes and 8‐Azaheptafulvenes: Direct Access to Tetrahydro‐1‐azaazulene and Cyclohepta[b]pyridinone Derivatives

The reactivity of Fischer alkenyl carbenes toward 8‐azaheptafulvenes is examined. Alkenyl carbenes react with 8‐azaheptafulvenes with complete regio‐ and stereoselectivity through formal [8+3] and [8+2] heterocyclization reactions, which show an unprecedented dependence on the C_β substituent at the alkenyl carbene complex. Thus, the formal [8+3] heterocyclization reaction is completely favored in carbene complexes that bear a coordinating moiety to give tetrahydrocyclohepta[b]pyridin‐2‐ones. Otherwise, alkenyl carbenes that lack appropriate coordinating groups undergo a formal [8+2] cyclization with 8‐azaheptafulvenes to give compounds that bear a tetrahydroazaazulene structure. A likely mechanism for these reactions would follow well‐established models and would involve a 1,4‐addition/cyclization in the case of the [8+2] cyclization or a 1,2‐addition/[1,2] shift–metal‐promoted cyclization for the [8+3] reaction. The presence of a coordinating moiety in the carbene would favor the [1,2] metal shift through transition‐state stabilization to lead to the [8+3] product. All these processes provide an entry into the tetrahydroazaazulene and cycloheptapyridone frameworks present in the structure of biologically active molecules.     

Blocking Deprotonation with Retention of Aromaticity in a Plant ent‐Copalyl Diphosphate Synthase Leads to Product Rearrangement

Substitution of a histidine, comprising part of the catalytic base group in the ent‐copalyl diphosphate synthases found in all seed plants for gibberellin phytohormone metabolism, by a larger aromatic residue leads to rearrangements. Through a series of 1,2‐hydride and methyl shifts of the initially formed bicycle predominant formation of (?)‐kolavenyl diphosphate is observed. Further mutational analysis and quantum chemical calculations provide mechanistic insight into the basis for this profound effect on product outcome.     

Perfluoropentaphenylborole: A New Approach to Lewis Acidic,Electron‐Deficient Compounds

Zirconocene is the key : A new synthetic method, which utilizes zirconocene‐mediated coupling of alkynes, has been developed for the preparation of a new class of highly Lewis acidic boroles (see scheme). Such compounds hold potential for applications in catalysis and the field of electron‐deficient organic materials.

     

Highly Stereoselective Synthesis of 1,2‐Diorganothio‐1‐alkenes via Hydrothiolation of Alkynyl Sulfides Catalyzed by Cesium Hydroxide

In the presence of catalytic amount of cesium hydroxide, the hydrothiolation of alkynyl sulfides occurred at room temperature in DMF under nitrogen atmosphere to afford exclusive (Z)‐1,2‐diorganothio‐1‐alkene in excellent yields. It could provide a new and expedient way for the preparation of symmetrical and unsymmetrical (Z)‐1,2‐diorganolthio1‐1‐alkenes.