Hauke Kelch

Synthesis, Coordination Behavior, and Reduction Chemistry of Cymantrenyl-1,3-bis(2,3,4,5-tetraphenyl)borole

We describe the synthesis of base-free bisborole [Cym(†)(BC(4)Ph(4))(2)]-Cym(†)=(OC)(3)Mn(η(5)-C(5)H(3))-and its transformation into two fully characterized Lewis acid-base adducts with pyridine bases of the type 4-R-NC(5)H(4) (R=tBu, NMe(2)). The results of electrochemical, as well as NMR and UV/Vis spectroscopic studies on [Cym(†)(BC(4 Ph(4))(2)] and the related monoborole derivative [Cym(BC(4)Ph(4))]-Cym=(OC)(3)Mn(η(5)-C(5) H(4))-provided conclusive evidence for 1) the enhanced Lewis acidity of the two boron centers that result from conjugation of two borole fragments, and 2) the fact that Mn-B bonding interactions between the Lewis acidic borole moieties and the Mn center are considerably less pronounced for bisborole [Cym(†)(BC(4)Ph(4))(2)]. In addition, the reduction chemistry of [Cym(†)(BC(4)Ph(4))(2)] has been studied in detail, both electrochemically and chemically. Accordingly, chemical reduction of [Cym(†)(BC(4)Ph(4))(2)] with magnesium anthracene afforded the corresponding tetraanion, which features a rare Mg-OC bonding mode in the solid state.

Engineering a Small HOMO-LUMO Gap and Intramolecular C-H Borylation by Diborene/Anthracene Orbital Intercalation

The diborene 1 was synthesized by reduction of a mixture of 1,2-di-9-anthryl-1,2-dibromodiborane(4) (6) and trimethylphosphine with potassium graphite. The X-ray structure of 1 shows the two anthryl rings to be parallel and their π(C14 ) systems perpendicular to the diborene π(B=B) system. This twisted conformation allows for intercalation of the relatively high-lying π(B=B) orbital and the low-lying π* orbital of the anthryl moiety with no significant conjugation, resulting in a small HOMO-LUMO gap (HLG) and ultimately a C-H borylation of the anthryl unit. The HLG of 1 was estimated to be 1.57 eV from the onset of the long wavelength band in its UV/Vis absorption spectrum (THF, λonset =788 nm). The oxidation of 1 with elemental selenium afforded diboraselenirane 8 in quantitative yield. By oxidative abstraction of one phosphine ligand by another equivalent of elemental selenium, the B-B and C1 -H bonds of 8 were cleaved to give the cyclic 1,9-diborylanthracene 9.

Elucidating the Reactivity of Vicinal Dicarbenoids: From Lewis Adduct Formation to B-C Bond Activation.

The reactivity of the diaminoacetylene Pip-C≡C-Pip (Pip=piperidyl=NC5 H10 ) towards phenyldichloro- and triphenylborane is presented. In the case of the less Lewis acidic PhBCl2 , the first example of a double Lewis adduct of a vicinal dicarbenoid is reported. For the more Lewis acidic triphenylborane, coordination to the bifunctional carbene leads to a mild B-C bond activation, resulting in a syn-1,2-carboboration. Ensuing cis/trans isomerization yields a novel ethylene-bridged frustrated Lewis pair (FLP). The compounds were characterized using multinuclear NMR spectroscopy, structural analysis, and mass spectrometry. Reactivity studies of both isomers with the N-heterocyclic carbene 1,3-dimethylimidazol-2-ylidene (IMe) aided in elucidating the proposed isomerization pathway. DFT calculations were carried out to elucidate the reaction mechanism. The rather low free energy of activation is consistent with the observation that the reaction proceeds smoothly at room temperature.

Platinum trans-Bis(borirene) Complexes Displaying Coplanarity and Communication Across a Platinum Metal Center

Ambient-temperature photolysis of the aminoborylene complex [(OC)5 Cr=B=N(SiMe3 )2 ] in the presence of a series of trans-bis(alkynyl)platinum(II) precursors of the type trans-[Pt(CCAr)2 (PEt3 )2 ] (Ar=Ph, p-C6 H4 OMe, and p-C6 H4 CF3 ) successfully leads to twofold transfer of the borylene moiety [:B=N(SiMe3 )2 ] onto the alkyne functionalities. The alkynyl precursors and resultant bis(borirene)platinum(II) complexes formed are of the type trans-[Pt(B{=N(SiMe3 )2 }C=CAr)2 (PEt3 )2 ] (Ar=Ph, p-C6 H4 OMe, and p-C6 H4 CF3 ). These species have all been successfully characterized by NMR, IR, and UV/Vis spectroscopy as well as by elemental analysis. Single-crystal X-ray diffraction has verified that these trans-bis(borirene)platinum(II) complexes display coplanarity between the twin three-membered rings across the platinum core in the solid state and stand as the first examples of coplanar conformations of twin borirene systems. These complexes were modeled using density functional theory (DFT), providing information helpful in determining the ability of the transition metal core to interact with each individual borirene ring system and allowing for the observed coplanarity of these rings in the solid state. This proposed transition metal interaction with the twin borirene systems is manifested in the electronic characterization of these borirene species, which display divergent photophysical UV/Vis spectroscopic profiles compared to a previously published mono(borirene)platinum(II) complex.

Scope of the Thermal Ring-Expansion Reaction of Boroles with Organoazides

Electronic and steric factors have been investigated in the thermal ring expansion of boroles with organic azides, a reaction that provides access to highly arylated 1,2-azaborinines, BN analogues of benzene. Reactions of a variety of boroles and organic azides demonstrate that the synthetic method is quite general in furnishing 1,2-azaborinines, but the respective reaction rates reveal a strong dependence on the substituents on the two reactants. The products have been characterized by UV/Vis, electrochemical, NMR, and X-ray diffraction methods, clarifying their constitutions and highlighting substituent effects on the electronic structure of the 1,2-azaborinines. Furthermore, analysis of several possible mechanistic pathways for 1,2-azaborinine formation, as studied by DFT, revealed that a two-step mechanism involving azide-borole adduct formation and nitrene insertion is favored.

Cyclisation of biscarbenoids - a novel mode of cyclobutadiene stabilisation

The reaction of the vicinal biscarbenoid Pip-C[triple bond, length as m-dash]C-Pip with dimethyltin dichloride yields a unique tetraamino-substituted cyclobutadienyl system featuring a dative C-Sn interaction. DFT investigation of the reaction mechanism revealed that coordination of the stannyl fragment to the nucleophilic carbon leads to a metal-stabilised zwitterion, allowing for [2+2] cycloaddition under thermal conditions. The compound features a homoaromatic π-system comprising the three sp2-hybridised carbon atoms of the four-membered ring as a consequence of charge separation.

Borabicyclo[3.2.0]heptadiene: A Fused Bicyclic Isomer of Borepin

A new isomer of borepin, identified as 2-borabicyclo[3.2.0]hepta-3,6-diene by single-crystal X-ray diffraction and heteronuclear NMR spectroscopy, was obtained by reaction of 1-mesityl-2,3,4,5-tetraphenylborole with the diaminoalkyne bis(piperidyl)acetylene. Analogous reactions of the alkyne with other borole derivatives generated seven-membered borepins, in which a second equivalent of the alkyne was found to insert into the exocyclic B-C bond. Results of mechanistic DFT studies as well as differences in the reactivity of the boroles toward the diaminoalkyne are discussed.

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.