Reduktion von Ferrocenylborol

Highly Stable, Readily Reducible, Fluorescent, Trifluoromethylated 9-Borafluorenes

Three different perfluoroalkylated borafluorenes (F Bf) were prepared and their electronic and photophysical properties were investigated. The systems have four trifluoromethyl moieties on the borafluorene moiety as well as two trifluoromethyl groups at the ortho positions of their exo-aryl moieties. They differ with regard to the para substituents on their exo-aryl moieties, being a proton (F XylF Bf, F Xyl: 2,6-bis(trifluoromethyl)phenyl), a trifluoromethyl group (F MesF Bf, F Mes: 2,4,6-tris(trifluoromethyl)phenyl) or a dimethylamino group (p-NMe2 -F XylF Bf, p-NMe2 -F Xyl: 4-(dimethylamino)-2,6-bis(trifluoromethyl)phenyl), respectively. All derivatives exhibit extraordinarily low reduction potentials, comparable to those of perylenediimides. The most electron-deficient derivative F MesF Bf was also chemically reduced and its radical anion isolated and characterized. Furthermore, all compounds exhibit very long fluorescent lifetimes of about 250 ns up to 1.6 μs; however, the underlying mechanisms responsible for this differ. The donor-substituted derivative p-NMe2 -F XylF Bf exhibits thermally activated delayed fluorescence (TADF) from a charge-transfer (CT) state, whereas the F MesF Bf and F XylF Bf borafluorenes exhibit only weakly allowed locally excited (LE) transitions due to their symmetry and low transition-dipole moments.

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 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.

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.

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.

Homolytic, Heterolytic, Mesolytic - As You Like It: Steering the Cleavage of a HC(sp3 )-C(sp3 )H Bond in Bis(1H-2,1-benzazaborole) Derivatives

A set of (3,3')-bis(1-Ph-2-R-1H-2,1-benzazaborole) compounds, in which R=tBu (Bab-tBu)₂ , R=Dipp (Bab-Dipp)₂ or R=tBu and Dipp (Bab-Dipp)(Bab-tBu), was synthesized and fully characterized using ¹ H, ¹¹ B, ¹³ C, and ¹⁵ N NMR spectroscopy as well as single-crystal X-ray diffraction analysis. The central HC(sp³ )-C(sp³ )H bond with restricted rotation at the junction of both 1H-2,1-benzazaborole rings displayed an intriguing reactivity. It was demonstrated that this bond is easily mesolytically cleaved using alkali metals to form the respective aromatic 1Ph-2R-1H-2,1-benzazaborolyl anions M⁺ (THF)_n (Bab-tBu)^- (M=Li, Na, K) and K⁺ (THF)_n (Bab-Dipp)^- . Furthermore, the central HC(sp³ )-C(sp³ )H bond of bis(1H-2,1-benzazaborole)s is also homolytically cleaved either by heating or photochemical means, giving corresponding 1Ph-2R-1H-2,1-benzazaborolyl radicals (Bab-tBu)^. and (Bab-Dipp)^. , which rapidly self-terminate. Nevertheless, their formation was unambiguously established by NMR analysis of the reaction mixtures containing products of the self-termination of the radicals after heating or irradiation. (Bab-Dipp)^. radical was also characterized using EPR spectroscopy. Importantly, it turned out that the essentially non-polarized HC(sp³ )-C(sp³ )H bond in (Bab-tBu)₂ is also cleaved heterolytically with 2 equiv of MeLi, giving the mixture of Li⁺ (SOL)_n (Bab-tBu)^- (SOL=THF or Et₂ O) and lithium methyl-substituted borate complex Li⁺ (SOL)_n (Bab-tBu-Me)^- in a diastereoselective fashion.

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.

A combined experimental and theoretical study on the isomers of 2,3,4,5-tetracarba-nido-hexaborane(6) derivatives and their photophysical properties

Building upon our ongoing studies on the high-yield synthetic route to 2,3,4,5-tetracarba-1,6-nido hexaborane derivative 5, we continue to explore the chemistry of this system to isolate a range of perphenylated C4 B2 -type carborane isomers. As a result, the photolysis of 5 for an elongated period yielded 6, which exhibits an intense luminescence upon excitation with a UV lamp (λex =366 nm). In contrast, the conversion of 5 under thermal conditions (microwave heating) generated the isomer 7, which does not show any luminescence. The structural disparity among the three isomers 5-7 is the position of the butadiene moiety with respect to the C4 B2 -carborane cage as deduced from single-crystal X-ray diffraction analyses. In an attempt to generate the classical structural motif of 5, an equimolar amount of KOtBu was added to a THF solution of 5, which immediately provided the boratacyclopentadiene derivative 8 in good yield. The observed rearrangement reactions were further investigated by quantum chemical calculations to suggest a plausible reaction pathway. According to the DFT calculations, the energetically favored reaction mechanism most likely involves tricyclic transition states and classical intermediate structures. In addition, the fluorescence emission properties of 6 were investigated in solution as well as in the solid state.

Antiaromaticity to aromaticity: from boroles to 1,2-azaborinines by ring expansion with azides

We have exploited the reactivity of antiaromatic boroles, gaining access to aryl-substituted monocyclic 1,2-azaborinines. The observed ring-expansion reaction of inherently electron-deficient boroles with organometallic and organic azides is demonstrated for representative examples. This substance class is expected to provide a new avenue into 1,2-azaborinine chemistry, especially in the area of functional organoboron materials. Our results are based on NMR and UV/Vis spectroscopy as well as single-crystal X-ray crystallography and provide a virtually quantitative approach that also offers numerous points of variation.

Redox chemistry of a hydroxyphenyl-substituted borane

Chemical reduction of a hydroxyphenyl-substituted borane triggers a sequential electron- and intramolecular hydrogen-atom-transfer process to afford a hydridoborate phenoxide dianion. On the other hand, hydrogen-atom abstraction of the borane leads to the isolation of a neutral borylated phenoxyl radical, which can be transformed to the corresponding benzoquinone borataalkene derivative by reduction with cobaltocene.

Evidence for extensive single-electron-transfer chemistry in boryl anions: isolation and reactivity of a neutral borole radical

Despite the synthesis of a boryl anion by Yamashita et al. in 2006, compounds that show boron-centered nucleophilicity are still rare and sought-after synthetic goals. A number of such boryl anions have since been prepared, two of which were reported to react with methyl iodide in apparent nucleophilic substitution reactions. One of these, a borolyl anion based on the borole framework, has now been found to display single-electron-transfer (SET) reactivity in its reaction with triorganotetrel halides, which was confirmed by the isolation of the first neutral borole-based radical. The radical was characterized by elemental analysis, single-crystal X-ray crystallography, and EPR spectroscopy, and has implications for the understanding of boron-based nucleophilic behavior and the emergent role of boron radicals in synthesis. This radical reactivity was also exploited in the synthesis of compounds with rare B-Sn and B-Pb bonds, the latter of which was the first isolated and structurally characterized compound with a "noncluster" B-Pb bond.