Reactivity of a Phosphaalkyne with Pentaarylboroles

A donor-supported silavinylidene and silylium ylides: boroles as a flexible platform for versatile Si(ii) chemistry

Electron-deficient, anti-aromatic 2,5-disilyl boroles are shown to be a flexibly adaptive molecular platform with regards to SiMe3 mobility in their reaction with the nucleophilic donor-stabilised precursor dichloro silylene SiCl2(IDipp). Depending on the substitution pattern, selective formation of two fundamentally different products of rivalling formation pathways is achieved. Formal addition of the dichlorosilylene gives the 5,5-dichloro-5-sila-6-borabicyclo[2.1.1]hex-2-ene derivatives. Under kinetically controlled conditions, SiCl2(IDipp) induces 1,3-trimethylsilyl migration and adds exocyclically to the generated carbene fragment giving an NHC-supported silylium ylide. In some cases interconversion between these compound classes was triggered by temperature or NHC-addition. Reduction of silaborabicyclo[2.1.1]hex-2-ene derivatives under forcing conditions gave clean access to recently described nido-type cluster Si(ii) half-sandwich complexes of boroles. Reduction of a NHC-supported silylium ylide gave an unprecedented NHC-supported silavinylidene which rearranges to the nido-type cluster at elevated temperatures.

Diverse Reactions of o-Carborane-Fused Silylenes with C≡E (E = C, P) Triple Bonds

The reactivities of o-carborane-fused silylenes toward molecules with C≡E (E = C, P) bonds are reported. The reactions of bis(silylene) [(LSi:)C]₂B₁₀H₁₀ (1a, L = PhC(NtBu)₂) with arylalkynes afforded bis(silylium) carborane adducts 2 and 3, showing a Si(μ-C₂)Si structure with an open-cage nido-carborane backbone. In contrast, the reaction of 1a with a phosphaalkyne AdC≡P (Ad = 1-adamantyl) smoothly furnished compound 4, comprising fused CPSi rings with a C=Si double bond and Si-Si single bond, and the related formation mechanism was investigated by DFT calculations. Furthermore, when monosilylene [(LSi:)C]CHB₁₀H₁₀ (1b) was employed to react with AdC≡P, compound 5 was isolated. The structure of 5 features a 1,2,3-triphosphetene core. All products were characterized by NMR spectroscopy and/or X-ray crystallography.

Probing borafluorene B-C bond insertion with gold acetylide and azide

The reaction of Ph₃PAuN₃ with 9-Ph-9-borafluorene resulted in complexation of the azide to boron while a gold acetylide reacted with 9-Ph-9-borafluorene to insert the acetylide carbon to access a six-membered boracycle with an exocyclic double bond.

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.

Li deposited on LiCl: an efficient reducing agent

Lithium was deposited onto lithium chloride powder in a ∼5% weight/weight ratio and demonstrated to be an effective reducing reagent.

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.

Linkage Isomerism Leading to Contrasting Carboboration Chemistry: Access to Three Constitutional Isomers of a Borylated Phosphaalkene

We describe the reactivity of two linkage isomers of a boryl-phosphaethynolate, [B]OCP and [B]PCO (where [B]=N,N'-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl), towards tris- (pentafluorophenyl)borane (BCF). These reactions afforded three constitutional isomers all of which contain a phosphaalkene core. [B]OCP reacts with BCF through a 1,2 carboboration reaction to afford a novel phosphaalkene, E-[B]O{(C6 F5 )2 B}C=P(C6 F5 ), which subsequently undergoes a rearrangement process involving migration of both the boryloxy and pentafluorophenyl substituents to afford Z-{(C6 F5 )2 B}(C6 F5 )C=PO[B]. By contrast, [B]PCO undergoes a 1,3-carboboration process accompanied by migration of the N,N'-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl to the carbon centre.

A Cationic NHC-Supported Borole

This work describes the synthesis and characterization of a highly reactive cationic borole. Halide abstraction with Li{Al[OC(CF3 )3 ]4 } from the NHC-chloroborole adduct yields the first stable NHC-supported 1-(Me NHC)-2,5-(SiMe3 )2 -3,4-(Ph*)2 -borole cation. Electronically, it features both a five-membered cyclic conjugated 4 π-electron system and a cationic charge and thus resembles the yet elusive cyclopentadienyl cation. The borole cation was characterized crystallographically, spectroscopically (NMR, UV/Vis), by cyclovoltammetry, microanalysis and mass-spectrometry and its electronic structure was probed computationally. The cation reacts with tolane and reversibly binds carbon monoxide. Direct comparison with the structurally related, yet neutral, 1-mesityl borole reveals strong Lewis acidity, reduced HOMO-LUMO gaps, and increased anti-aromatic character.

A Phosphinine-Derived 1-Phospha-7-Bora-Norbornadiene: Frustrated Lewis Pair Type Activation of Triple Bonds

Salt metathesis of 1-methyl-2,4,6-triphenylphosphacyclohexadienyl lithium and chlorobis(pentafluorophenyl)borane affords a 1-phospha-7-bora-norbornadiene derivative 2. The C≡N triple bonds of nitriles insert into the P-B bond of 2 with concomitant C-B bond cleavage, whereas the C≡C bonds of phenylacetylenes react with 2 to form λ4 -phosphabarrelenes. Even though 2 must formally be regarded as a classical Lewis adduct, the C≡N and C≡C activation processes observed (and the mild conditions under which they occur) are reminiscent of the reactivity of frustrated Lewis pairs. Indeed, NMR and computational studies give insight into the mechanism of the reactions and reveal the labile nature of the phosphorus-boron bond in 2, which is also suggested by detailed NMR spectroscopic studies on this compound. Nitrile insertion is thus preceded by ring opening of the bicycle of 2 through P-B bond splitting with a low energy barrier. By contrast, the reaction with alkynes involves formation of a reactive zwitterionic methylphosphininium borate intermediate, which readily undergoes alkyne 1,4-addition.

2,5-Bis-trimethylsilyl substituted boroles

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

A Neutral "Aluminocene" Sandwich Complex: η1 - versus η5 -Coordination Modes of a Pentaarylborole with ECp* (E=Al, Ga; Cp*=C5 Me5 )

The pentaaryl borole (Ph*C)₄ BXyl^F [Ph*=3,5-tBu₂ (C₆ H₃ ); Xyl^F =3,5-(CF₃ )₂ (C₆ H₃ )] reacts with low-valent Group 13 precursors AlCp* and GaCp* by two divergent routes. In the case of [AlCp*]₄ , the borole reacts as an oxidising agent and accepts two electrons. Structural, spectroscopic, and computational analysis of the resulting unprecedented neutral η⁵ -Cp*,η⁵ -[(Ph*C)₄ BXyl^F ] complex of Al^III revealed a strong, ionic bonding interaction. The formation of the heteroleptic borole-cyclopentadienyl "aluminocene" leads to significant changes in the ¹³ C NMR chemical shifts within the borole unit. In the case of the less-reductive GaCp*, borole (Ph*C)₄ BXyl^F reacts as a Lewis acid to form a dynamic adduct with a dative 2-center-2-electron Ga-B bond. The Lewis adduct was also studied structurally, spectroscopically, and computationally.

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.

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.

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.

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.

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.

Oxygen insertion into boroles as a route to 1,2-oxaborines

The synthesis of 1,2-oxaborines is accomplishedviathe reaction of pentaarylboroles withN-methylmorpholine-N-oxideviaa 1,1-insertion reaction.