Christian Hörl

Diborabutatriene: An Electron‐Deficient Cumulene

The complexation of two equivalents of a cyclic (alkyl)(amino)carbene (CAAC) to tetrabromodiborane, followed by reduction with four equivalents of sodium naphthalide, led to the formation of the CAAC-stabilized linear diboracumulene (CAAC)2B2. The capacity of the CAAC ligand to facilitate B2 →CAAC donation of π-electron density resulted in important differences between this species and a previously reported complex featuring a B≡B triple bond stabilized by cyclic di(amino)carbenes, including a longer B-B bond and shorter B-C bonds. Frontier orbital analysis indicated sharing of valence electrons across the entire linear C-B-B-C unit in (CAAC)2B2, which is supported by natural population analysis and cyclic voltammetry.

Boron as a Powerful Reductant: Synthesis of a Stable Boron‐Centered Radical‐Anion Radical‐Cation Pair

Despite the fundamental importance of radical-anion radical-cation pairs in single-electron transfer (SET) reactions, such species are still very rare and transient in nature. Since diborenes have highly electron-rich BB double bonds, which makes them strong neutral reductants, we envisaged a possible realization of a boron-centered radical-anion radical-cation pair by SET from a diborene to a borole species, which are known to form stable radical anions upon one-electron reduction. However, since the reduction potentials of all know diborenes (E1/2 =-1.05/-1.55 V) were not sufficiently negative to reduce MesBC4 Ph4 (E1/2 =-1.69 V), a suitable diborene, IiPr⋅(iPr)BB(iPr)⋅IiPr, was tailor-made to comply with these requirements. With a halfwave potential of E1/2 =-1.95 V, this diborene ranks amongst the most powerful neutral organic reductants known and readily reacted with MesBC4 Ph4 by SET to afford a stable boron-centered radical-anion radical-cation pair.

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.

Direct Hydroboration of BB Bonds: A Mild Strategy for the Proliferation of BB Bonds

Synthetic access to electron-precise boron chains is hampered by the preferential formation of nonclassical structures. The few existing strategies for this involve either strongly reducing reagents or transition-metal catalysts, both with distinct disadvantages. The synthesis of new furyl- and thienyl-substituted diborenes is presented, along with their direct hydroboration with catecholborane (CatBH) to form a new electron-precise B-B bond and a B3 chain. The reaction is diastereoselective and proceeds under mild conditions without the use of strong reducing agents or transition-metal catalysts commonly used in B-B coupling reactions.

Multiple Reduction of 2,5-Bis(borolyl)thiophene: Isolation of a Negative Bipolaron by Comproportionation†

The 2,5-bis(borolyl)thiophene 2, a conjugated acceptor-π-acceptor system, can be reduced to the monoradical anion [2](.-) , the dianion [2](2-) , and the tetraanion [2](4-) . The dianion [2](2-) was also prepared by a comproportionation reaction and features an absorption maximum in the near-IR region (λmax =800 nm), which is characteristic of a bipolaron with a quinoidal structure.

Synthesis of Functionalized 1,4-Azaborinines by the Cyclization of Di-tert-butyliminoborane and Alkynes

Di-tert-butyliminoborane is found to be a very useful synthon for the synthesis of a variety of functionalized 1,4-azaborinines by the Rh-mediated cyclization of iminoboranes with alkynes. The reactions proceed via [2 + 2] cycloaddition of iminoboranes and alkynes in the presence of [RhCl(PiPr3)2]2, which gives a rhodium η(4)-1,2-azaborete complex that yields 1,4-azaborinines upon reaction with acetylene. This reaction is compatible with substrates containing more than one alkynyl unit, cleanly affording compounds containing multiple 1,4-azaborinines. The substitution of terminal alkynes for acetylene also led to 1,4-azaborinines, enabling ring substitution at a predetermined location. We report the first general synthesis of this new methodology, which provides highly regioselective access to valuable 1,4-azaborinines in moderate yields. A mechanistic rationale for this reaction is supported by DFT calculations, which show the observed regioselectivity to arise from steric effects in the B-C bond coupling en route to the rhodium η(4)-1,2-azaborete complex and the selective oxidative cleavage of the B-N bond of the 1,2-azaborete ligand in its subsequent reaction with acetylene.

Synthesis of Zwitterionic Cobaltocenium Borate and Borata-alkene Derivatives from a Borole-Radical Anion

Chemical single-electron reduction of 1-mesityl-2,3,4,5-tetraphenylborole (3) gave a stable radical anion [CoCp*2 ][3] as shown in earlier investigations. Herein, we present the reaction of [CoCp*2 ][3] with the 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO), a common radical trap. Instead of radical recombination, the reaction proceeds through a redox pathway involving oxidation of the borole radical anion combined with reduction of TEMPO. This electron-transfer process is accompanied by a deprotonation reaction of the cobaltocenium counterion by the base TEMPO(-) to give TEMPO-H and a neutral cobalt(I) fulvene complex (7). The latter was not observed directly during the reaction, because it instantaneously reacts as a nucleophile attacking at the boron center of the in situ generated borole 3 to give the borate 6. However, 7 was synthesized independently by deprotonation of [CoCp*2 ][PF6 ]. In addition, the obtained zwitterionic cobaltocenium borate 6 undergoes a photolytic rearrangement to form the borata-alkene derivative 9 that thermally transforms to the chiral cobaltocenium borate 12. Our investigations are based on spectroscopic evidence, X-ray crystallography, elemental analysis, as well as DFT calculations.

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.

Reductive Borylene–CO Coupling with a Bulky Arylborylene Complex†

Partial metal-boron bond cleavage and coupling of a borylene with two CO ligands was observed upon reduction of a new bulky arylborylene complex. Both the borylene precursor and dianionic product were structurally and spectroscopically characterized. In contrast, reduction of an aminoborylene complex led to complete loss of the borylene ligand and classical Hieber reduction. A rationale for these differences based on DFT methods is presented.

Hydridoborylene Complexes and Di‐, Tri‐, and Tetranuclear Borido Complexes with Hydride Ligands

Mono- and dinuclear hydridoborylene complexes were prepared by intermetallic borylene transfer from Group VI borylene or metalloborylene reagents. The hydride and borylene ligands were found to interact with each other significantly, although the boron ligand retains much of its former borylene character. Zero-valent platinum fragments were successively added to the dinuclear hydridoborylene complexes, resulting in tri- and tetranuclear borido complexes, in which the B-H interaction has been lost, and the hydride ligands now bridge two metal centers. The complexes were studied spectroscopically, crystallographically, and by DFT methods, and the unusual bonding situation in the M-B-H triangles of hydridoborylene complexes were evaluated.