Borole-Derived Spirocyclic Tetraorganoborate

Preparation of a novel conjugated tetraorganoborate is presented in a facile two-step procedure. Successful utilization as a halide abstraction reagent is demonstrated in a metathesis reaction with the platinum­(II) boryl complex [(Cy₃P)₂Pt­(Br)­(BC₄Ph₄)], which was obtained by oxidative addition of 1-bromo-2,3,4,5-tetraphenylborole to a platinum(0) species. The novel compounds were investigated by means of spectroscopic and X-ray diffraction techniques

Borole-Derived Spirocyclic Tetraorganoborate

Preparation of a novel conjugated tetraorganoborate is presented in a facile two-step procedure. Successful utilization as a halide abstraction reagent is demonstrated in a metathesis reaction with the platinum­(II) boryl complex [(Cy₃P)₂Pt­(Br)­(BC₄Ph₄)], which was obtained by oxidative addition of 1-bromo-2,3,4,5-tetraphenylborole to a platinum(0) species. The novel compounds were investigated by means of spectroscopic and X-ray diffraction techniques

Si–H Bond Activation at the Boron Center of Pentaphenylborole

Si–H bond activation is usually considered a domain of transition-metal complexes, and only few metal-free systems have proven suitable for this task. We have now found that Et₃SiH readily reacts with pentaphenylborole to afford 1-bora-3-cyclopentenes as the <i>syn</i> and <i>anti</i> addition products. Here, Si–H bond cleavage is accomplished at a single boron center, a reactivity that is facilitated by a combination of high electrophilicity and loss of antiaromaticity. The mechanism of this transformation most likely involves a sequence of adduct formation, σ-bond metathesis, and conrotatory ring closure, similar to that observed for H/D exchange between H₂ and silanes mediated by HB­(C₆F₅)₂ and heterolytic H₂ splitting by boroles, respectively

Si–H Bond Activation at the Boron Center of Pentaphenylborole

Si–H bond activation is usually considered a domain of transition-metal complexes, and only few metal-free systems have proven suitable for this task. We have now found that Et₃SiH readily reacts with pentaphenylborole to afford 1-bora-3-cyclopentenes as the <i>syn</i> and <i>anti</i> addition products. Here, Si–H bond cleavage is accomplished at a single boron center, a reactivity that is facilitated by a combination of high electrophilicity and loss of antiaromaticity. The mechanism of this transformation most likely involves a sequence of adduct formation, σ-bond metathesis, and conrotatory ring closure, similar to that observed for H/D exchange between H₂ and silanes mediated by HB­(C₆F₅)₂ and heterolytic H₂ splitting by boroles, respectively

Tin-Bridged <i>ansa</i>-Metallocenes of the Late Transition Metals Cobalt and Nickel: Preparation, Molecular and Electronic Structures, and Redox Chemistry

Using the flytrap approach, paramagnetic <i>ansa</i>-metallocenes of the late transition metals cobalt and nickel containing a tetra-<i>tert</i>-butyldistannane bridge have been prepared. The complexes were identified using a combination of analytical methods (NMR, EPR, cyclic voltammetry, and X-ray crystallography) and further converted to their corresponding cations by one-electron oxidation with ferrocenium hexafluorophosphate. Spectral and structural analyses of the ionic products are consistent with metal-based oxidations

Tin-Bridged <i>ansa</i>-Metallocenes of the Late Transition Metals Cobalt and Nickel: Preparation, Molecular and Electronic Structures, and Redox Chemistry

Using the flytrap approach, paramagnetic <i>ansa</i>-metallocenes of the late transition metals cobalt and nickel containing a tetra-<i>tert</i>-butyldistannane bridge have been prepared. The complexes were identified using a combination of analytical methods (NMR, EPR, cyclic voltammetry, and X-ray crystallography) and further converted to their corresponding cations by one-electron oxidation with ferrocenium hexafluorophosphate. Spectral and structural analyses of the ionic products are consistent with metal-based oxidations

1‑Heteroaromatic-Substituted Tetraphenylboroles: π–π Interactions Between Aromatic and Antiaromatic Rings Through a B–C Bond

A series of 2,3,4,5-tetraphenylboroles substituted with different aromatic heterocycles (thiophene, furan, pyrrole, and dithiophene) in the 1-position were synthesized and characterized by means of NMR, elemental analysis, and X-ray crystallography. In contrast to known 2,3,4,5-tetraphenylboroles, X-ray diffraction revealed a nearly coplanar arrangement of the aromatic heterocycles and the antiaromatic borole scaffold as a result of π-conjugation, which could be substantiated by DFT calculations. Furthermore, the 2,2′-dithiophene-bridged bisborole (<b>14</b>) exhibits a large bathochromic shift in the absorption spectrum, demonstrating the exceptional Lewis acidity of the nonannulated borolyl moiety

1‑Heteroaromatic-Substituted Tetraphenylboroles: π–π Interactions Between Aromatic and Antiaromatic Rings Through a B–C Bond

A series of 2,3,4,5-tetraphenylboroles substituted with different aromatic heterocycles (thiophene, furan, pyrrole, and dithiophene) in the 1-position were synthesized and characterized by means of NMR, elemental analysis, and X-ray crystallography. In contrast to known 2,3,4,5-tetraphenylboroles, X-ray diffraction revealed a nearly coplanar arrangement of the aromatic heterocycles and the antiaromatic borole scaffold as a result of π-conjugation, which could be substantiated by DFT calculations. Furthermore, the 2,2′-dithiophene-bridged bisborole (<b>14</b>) exhibits a large bathochromic shift in the absorption spectrum, demonstrating the exceptional Lewis acidity of the nonannulated borolyl moiety

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

Di-<i>tert</i>-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(P<i>i</i>Pr₃)₂]₂, which gives a rhodium η⁴-1,2-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 η⁴-1,2-azaborete complex and the selective oxidative cleavage of the B–N bond of the 1,2-azaborete ligand in its subsequent reaction with acetylene