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

Unlocking Heteroaromatic Ring Systems through Chalcogen Insertion into Boroles

In this work, we report the reactivity of various annulated borole derivatives toward chalcogen (O, S, and Se) insertion. Among a series of 9-borafluorenes with different boron substituents (Ph, Br, or o-carboranyl) and a mixed thiophene-benzene-fused derivative, only the 9-o-carboranyl-substituted 9-borafluorene yielded the complete set of chalcogen-containing heteroarenes, including the first 1,2-selenaborinine derivative. To evaluate the aromaticity of this heterocyclic analogue of phenanthrene, nucleus-independent chemical shift (NICS) values were computed and compared to those of its lighter group 16 congeners.

Accessing Boron-Doped Pentaphene Analogues from 12-Boradibenzofluorene

Borole-doped polycyclic aromatic hydrocarbons (PAHs) have garnered attention in recent years due to their attractive photophysical properties and potential utility in electronic devices. In this work, a borole-doped PAH, 12-boradibenzofluorene, is synthesized and formal intermolecular nitrene and oxygen atom insertion reactions were employed to access 1,2-azaborine- and 1,2-oxaborine-containing analogues of the carbonaceous PAH pentaphene. Iodosobenzene is established as a versatile reagent for oxygen atom insertion reactions into a variety of borole species to access 1,2-oxaborine systems.

Highly Colored Boron-Doped Thiazolothiazoles from the Reductive Dimerization of Boron Isothiocyanates

Reduction of (CAAC)BBr2 (NCS) (CAAC=cyclic alkyl(amino)carbene) in the presence of a Lewis base L yields tricoordinate (CAAC)LB(NCS) borylenes which undergo reversible E/Z-isomerization. The same reduction in the absence of L yields deep blue, bis(CAAC)-stabilized, boron-doped, aromatic thiazolothiazoles resulting from the dimerization of dicoordinate (CAAC)B(NCS) borylene intermediates.

Boron and Silicon-Substituted 1,3-Dienes and Dienophiles and Their Use in Diels-Alder Reactions

Boron and silicon-substituted 1,3-dienes and boron and silicon-substituted alkenes and alkynes have been known for years and the last 10 years have seen a number of new reports of their preparation and use in Diels-Alder reactions. This review first covers boron-substituted dienes and dienophiles and then moves on to discuss silicon-substituted dienes and dienophiles.

Ring expansion of alumoles with organic azides: selective formation of six-membered aluminum-nitrogen heterocycles

A cyclopentadienyl-substituted alumole is shown to undergo ring expansions with a series of organic azides, affording 1,2-azaaluminabenzenes either with or without an azo group.

Aside from simple Lewis acid–base chemistry, the reaction chemistry of aluminacyclopentadienes, which are commonly referred to as aluminoles or simply alumoles, remains relatively underdeveloped. To date, few attempts to extend their inherent insertion and cycloaddition reactivity to the construction of stable aluminum-containing heterocycles have been reported. Herein, we demonstrate the selective ring expansion of a cyclopentadienyl-substituted alumole with a series of organic azides to form unsaturated six-membered AlN heterocycles. Depending on the substituent on the azide, the reaction proceeds either with or without loss of dinitrogen, leading to incorporation of only the “NR” unit of the azide or the entire azo substituent into the periphery of the heterocycle. A deeper understanding of these ring expansion reactions is reached through computational studies, illustrating deviations in the mechanism as a function of the organic azide employed.

2,5-Bis-trimethylsilyl substituted boroles

This manuscript includes a comprehensive study of the synthesis and spectroscopic features of 2,5-disilyl boroles.

Dimeric boroles: effective sources of monomeric boroles for heterocycle synthesis

Dimeric boroles as thermal sources of monomers.

Monomeric boroles have been gaining attention as reagents for the synthesis of heterocycles due to their ability to insert atoms into the BC₄ ring in a single step. Although unique boron frameworks can be accessed via this methodology, the products feature aryl substitution on the carbon centers as steric bulk is required to preclude borole dimerization. This work demonstrates that insertion chemistry is possible with Diels–Alder dimeric boroles and that such reactivity is not exclusive to monomeric boroles with bulky groups. With 1-phenyl-2,3,4,5-tetramethylborole dimer, the formal 1,1-insertion of a nitrene and sulfur generate the six-membered aromatic 1,2-azaborine and 1,2-thiaborine, respectively. The isolation of the 1,2-thiaborine enabled the synthesis of an η⁶-chromium complex. Benzophenone and diphenylketene readily insert a CO unit to generate BOC₅ seven-membered rings confirming dimeric boroles can serve as monomeric synthons in 1,2-insertion reactions. An epoxide did not furnish the anticipated eight-membered BOC₆ ring, instead provided a bicyclic system with a BOC₃ ring. The insertion chemistry was demonstrated with two other borole dimers featuring different substitution with diphenylketene as a substrate. This work elevates borole insertion chemistry to a new level to access products that do not require bulky substitution.

Aromaticity of unsaturated BEC4 heterocycles (E = N, P, As, Sb, O, S, Se, Te)

A compendium of pnictogen and chalcogen substituted boron heterocycles were assessed for their aromatic character by first principles density functional theory. Group-15 and Group-16 elements were placed at the ortho-, meta-, and para-positions of six-membered rings relative to boron to assess their impact on the aromaticity of the unsaturated heterocycles. Aromaticity was analyzed by a multidimensional approach using nuclear independent chemical shifts, gauge-including magnetically induced current, as well as natural bond orbital and natural resonance theory analyses. Based on these methods, we observe a general decline of aromaticity in heavier pnictaborines while the chalcogen analogues maintain relatively strong aromatic character. These general trends result from complementary π-π* natural bond order interactions that sustain resonance within the ring of each heterocycle establishing a pattern of cyclic delocalization. Consequently, natural resonance theory displays strong resonance, which is corroborated with the signed modulus of ring current, toroidal vortices of current maps, and elevated average induced current throughout the ring. The 1,3-configurations for pnictaborines and chalcogenaborines are generally more aromatic compared to the 1,2- and 1,4-isomers, which contain π-holes that limit diatropism within the heterocycles. However, an energetic trend favors the 1,2-heterocycles in both groups, with a few exceptions driven in large-part by π-donation of the lone pair on the heteroatom to the pz orbital on the adjacent boron resulting in stabilization. The importance of planarity for high aromaticity is demonstrated, especially in the pnictaborine isomers where pyramidalization at the pnictogen is favored, while bond regularity seems a less important criterion.

Synthesis and Reactivity of Cyclic Borane-Amidine Conjugated Molecules Formed by Direct 1,2-Carboboration of Carbodiimides with 9-Borafluorenes

Efficient 1,2-carboboration reactions to the C=N bond of carbodiimides with 9-borafluorenes, which give rise to cyclic borane-amidine conjugates with a seven-membered BNC₅ ring, are reported. The resulting cyclic borane-amidine conjugates can be hydrolyzed into an acyclic bifunctional biaryl compound carrying both boronic acid and amidine groups, rendering the utility of the two-step protocol for the synthesis of multi-functionalized molecular systems with a potential as a supramolecular building block. Furthermore, the conjugated structure of the cyclic boron-amidine compounds can be changed upon alkylation of the boron atom that increases the coordination number of boron. The combination of Lewis acid (borane) and conjugated base (amidine) provides rich structural diversity of heteroatom-containing π-conjugated systems.

Electronic Push-Pull Modulation by Peripheral Substituents in Pentaaryl Boroles

Establishing access to a bulky tetraaryl dilithiobutadiene (Ph*C)₄ Li₂ (Ph*=3,5-tBu₂ (C₆ H₃ )) allowed for the synthesis of five-membered heterocycles with incorporated main-group elements. Along with an amino borole, a set of substituted pentaaryl boroles (Ph*C)₄ BAr has been synthesized. The examination of their absorption spectra and computational studies by means of DFT granted insight into the influence of peripheral substituents on the electronic features of the parent pentaphenyl borole (PhC)₄ BPh. Introduction of the more electron-rich Ph* residue at the carbon atoms increases the HOMO energy, redshifting the visible π/π*-absorption bands compared with the parent pentaphenyl borole. The influence on the frontier orbitals of three different boron-bound aryls with electronically modulating substituents in the remote 3,5-positions Ar=3,5-R₂ -C₆ H₃ (R=Me, H, CF₃ ) was studied. The substituents were found to increase (+I effect, Me) or decrease (-I effect, CF₃ ) the LUMO energy, thus directly affecting the visible absorption spectra. This represents the first study on HOMO-LUMO-gap adjustments by a combined push-pull approach of a substituted pentaphenylborole.

Small molecule activation by boron-containing heterocycles

Most of the chemical and biological processes involving the fixation and transformation of small molecules have long been exclusive for metal complexes. Meanwhile, the last decades have seen a significant advance in main group chemistry that mimics transition-metal complexes, among which various boron-containing systems have been successful in mediating the small molecule activation. In this review, we focus on boron-containing heterocycles enabling the activation of σ- and π-bonds in small molecules, in conjunction with the proposed mechanisms.

Successive carbene insertion into 9-phenyl-9-borafluorene

The reactions of 9-phenyl-9-borafluorene with trimethylsilyldiazomethane in a 1 : 1 and 1 : 2 stoichiometry furnished the corresponding BC5 and BC6 heterocycles via the formal insertion of one and two carbene units.

C-H addition reactivity of 2-phenylpyridine and 2,2'-bipyridine with pentaphenylborole

The reactions of pentaphenylborole with pyridines with a pendent aryl group in the 2-position, specifically 2-phenylpyridine and 2,2'-bipyridine, readily afforded adducts. Upon thermolysis, the adducts converted to ortho C-H addition products with the two substituents introduced onto the diene on the boracyclic carbon atoms adjacent to boron in a syn fashion. Photolysis of the syn 2-phenypyridine product led to isomerization to the anti conformer which did not readily occur for the 2,2'-bipyridine complex. Such C-H bond reactivity is uncommon for main group elements and provides insight into methodologies to access boron frameworks.

Intermolecular insertion reactions of azides into 9-borafluorenes to generate 9,10-B,N-phenanthrenes

The reactions of 9-borafluorenes with organic azides result in either the insertion of the α-nitrogen and elimination of N₂ or the insertion of the γ-nitrogen to generate the corresponding phenanthrene analogues with boron and nitrogen in the 9- and 10-positions, respectively.

DFT Studies of Dimerization Reactions of Boroles

Boroles undergo dimerization reactions to give Diels-Alder (DA) dimers, bridged-bicyclic (BB) dimers or spiro dimers (SD) depending on the substituents on the borole. We performed DFT calculations to investigate how different substituents at the carbon atoms of the butadiene backbone as well as at the boron atom influence the dimerization reaction pathways. The DFT results show that, in general, both the DA and BB dimers are easily accessible kinetically, and the DA dimers are thermodynamically more stable than the BB dimers. When the substituent-substituent repulsive steric interactions are alleviated to a certain extent, the BB dimers are more stable than the DA dimer, and become accessible. The SD dimers are generally kinetically difficult to obtain. However, we found that aryl substituents promote the formation of the SD dimers.

Facile Route to Quadruply Annulated Borepins

A two-step synthesis sequence furnishes quadruply annulated borepins in high yields. The first step involves a nucleophilic substitution reaction between aryl-BF₃K salts (aryl = mesityl, phenyl) and lithiated bromonapthalene derivatives LiNaph^Br,R (HNaph^Br,R = 8-bromonaphthalene (a), 5-bromoacenaphthene (b), 5-bromoacenaphthylene (c)). In the second step, the resulting heteroleptic triarylboranes aryl-B(Naph^Br,R)₂ (3a-c) are subjected to an intramolecular Ni-mediated Yamamoto reaction to close the seven-membered rings and create the borepins 4a-c. Only in the case of 3b is the Yamamoto reaction accompanied by a C-H activation reaction furnishing the 7-hydro-7-borabenzo[de]anthracene derivative 5. The product ratio 4b/5 can be influenced by control of the local Ni(0) concentration. The borepins 4a-c are benchtop stable and highly soluble even in hexane. Compounds 4a-c undergo reversible one-electron reduction; 4c is also able to accept a second electron in a reversible manner and already at moderate potential values (E_1/2 = -1.49 V and -1.84 V (vs FcH/FcH⁺)). 4a, 4b, and 5 show photoluminescence in the blue-green region of the spectrum, while 4c is nonfluorescent, which is likely attributable to an intramolecular charge-transfer transition.

Facile access to unsymmetrically substituted tellurium–boron based heterocycles

Protonolysis and alkyne exchange reactions are used to give unsymmetrically substituted Te–B based heterocycles.

DFT studies on reactions of boroles with carbon monoxide

DFT calculations were performed to study systematically the influence of borole substituents on the reaction of boroles with carbon monoxide.