Polar covalent apex-base bonding in borapyramidanes probed by solid-state NMR and DFT calculations

Pyramidane molecules have attracted chemists for many decades due to their regular shape, high symmetry and their correspondence in the macroscopic world. Recently, experimental access to a number of examples has been reported, in particular the rarely reported square pyramidal bora[4]pyramidanes. To describe the bonding situation of the nonclassical structure of pyramidanes, we present solid-state Nuclear Magnetic Resonance (NMR) as a versatile tool for deciphering such bonding properties for three now accessible bora[4]pyramidane and dibora[5]pyramidane molecules. 11 B solid-state NMR spectra indicate that the apical boron nuclei in these compounds are strongly shielded (around -50 ppm vs. BF3 -Et2 O complex) and possess quadrupolar coupling constants of less than 0.9 MHz pointing to a rather high local symmetry. 13 C-11 B spin-spin coupling constants have been explored as a measure of the bond covalency in the borapyramidanes. While the carbon-boron bond to the -B(C6 F5 )2 substituents of the base serves as an example for a classical covalent 2-center-2-electron (2c-2e) sp2 -carbon-sp2 -boron σ-bond with 1 J(13 C-11 B) coupling constants in the order of 75 Hz, those of the boron(apical)-carbon(basal) bonds in the pyramid are too small to measure. These results suggest that these bonds have a strongly ionic character, which is also supported by quantum-chemical calculations.

Aromatic Metamorphosis of Thiophenes by Means of Desulfurative Dilithiation

A new mode of aromatic metamorphosis has been developed, which allows thiophenes and their benzo-fused derivatives to be converted to a variety of exotic heteroles. This transformation involves 1) the efficient generation of key 1,4-dianions by means of desulfurative dilithiation with lithium powder and 2) the subsequent trapping of the dianions with heteroatom electrophiles in a one-pot manner. Via the desulfurative dilithiation, the sulfur atoms of thiophenes are replaced also with a carbon-carbon double bond or a 1,2-phenylene for the construction of benzene rings.

H2 evolution from H2O via O-H oxidative addition across a 9,10-diboraanthracene

The water reactivity of the boroauride complex ([Au(B₂P₂)][K(18-c-6)]; (B₂P₂, 9,10-bis(2-(diisopropylphosphino)-phenyl)-9,10-dihydroboranthrene) and its corresponding two-electron oxidized complex, Au(B₂P₂)Cl, are presented. Au(B₂P₂)Cl is tolerant to H₂O and forms the hydroxide complex Au(B₂P₂)OH in the presence of H₂O and triethylamine. [Au(B₂P₂)]Cl and [Au(B₂P₂)]OH are poor Lewis acids as judged by the Gutmann-Becket method, with [Au(B₂P₂)]OH displaying facile hydroxide exchange between B atoms of the DBA ring as evidenced by variable temperature NMR spectroscopy. The reduced boroauride complex [Au(B₂P₂)]^- reacts with 1 equivalent of H₂O to produce a hydride/hydroxide product, [Au(B₂P₂)(H)(OH)]^-, that rapidly evolves H₂ upon further H₂O reaction to yield the dihydroxide compound, [Au(B₂P₂)(OH)₂]^-. [Au(B₂P₂)]Cl can be regenerated from [Au(B₂P₂)(OH)₂]^-via HCl·Et₂O, providing a synthetic cycle for H₂ evolution from H₂O enabled by O-H oxidative addition at a diboraanthracene unit.

Understanding the Diverse Reactivity of Pentaphenylborole toward Epoxides

Density functional theory calculations have been performed to study the diverse reactivity of pentaphenylborole toward different epoxides. We systematically investigated the effect of substituents on epoxides for the preference/competition of three experimentally observed pathways, that is, intramolecular proton transfer, direct ring expansion via insertion of one epoxide molecule, and ring expansion via insertion of two epoxide molecules. Our calculations also predicted a high competitivity between the proton transfer and direct ring expansion pathways for the epoxide containing both alkyl and aryl substituents.

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.

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.

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.

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.

Photo electron transfer induced desilylation of N,N-bis(trimethylsilyl)aminodibenzoborole to aminodibenzoborole

Photoinduced desilylation proceeds by single electron transfer and yields the first example of an unsubstituted aminoborole derivative.

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.

Abnormal Tin-Boron Exchange in the Attempted Synthesis of a Borylated Borole

Boroles are important motifs within functional materials. With the aim to prepare a pinacolboryl-substituted derivative, the metallacycle transfer from corresponding zirconium and tin precursors has been explored. We show that the reaction of 1,1-dimethyl-2,3,4,5-tetrapinacolborylstannole with dichloro(phenyl)borane does not provide the desired borole, but instead a stannyl-substituted 1-chloroboracyclopent-3-ene. Spectroscopic and structural details of this highly functionalized boracycle indicate that intramolecular interactions between the tin and oxygen atoms of the boryl substituents may account for the unexpected outcome of the tin-boron exchange reaction.

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.

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.

Reversible water activation driven by contraction and expansion of a 12-vertex-closo-12-vertex-nido biscarborane cluster

The metal-free reversible activation of H–OH bonds of water driven by the rearrangement of a boron cluster is presented.

Ring expansion reactions of anti-aromatic boroles: a promising synthetic avenue to unsaturated boracycles

The ability of anti-aromatic boroles to undergo ring expansion reactions with a broad range of substrates as an effective synthetic protocol towards a diverse array of 6-, 7-, and 8-membered boracycles is detailed in this review.