Eike G. Hübner

Selective Mono- to Perarylations of Tetrabromothiophene by a Cyclobutene-1,2-diylbisimidazolium Preligand

(Cyclobut-1-ene-1,2-diyl)bis(1-methylimidazolium)tetrafluoroborate is applied as preligand in palladium-catalyzed cross-coupling reactions starting from tetrabromothiophene for the synthesis of mono-, bi-, tri-, and tetraaryl-substituted thiophenes bearing up to four different aryl rings. A synthetic kit for preparations of nine different substitution patterns of arylated thiophenes is presented by application of only one single catalyst system. In agreement with DFT calculations, which predict energetically low rotational barriers in triaryl-3-bromothiophenes and tetraarylthiophenes, no NOE effects between adjacent aryl rings are detectable. The regioselectivity of their syntheses has therefore been elucidated by reduction of triaryl-3-bromothiophene to 2,3,5-triarylthiophene followed by HMBC, HSQC, and NOESY NMR measurements. Additionally, results of an X-ray single structure analysis are presented.

Formation of N-Heterocyclic Carbenes by Tautomerization of Mesomeric Betaines: Cyclic Boron Adducts and Palladium Complexes From 2-(Imidazolium-1-yl)phenolates

2-(Imidazolium-1-yl)phenolates are conjugated heterocyclic mesomeric betaines in tautomeric equilibrium with the corresponding N-heterocyclic carbenes (NHCs), 3-(2-hydroxyphenyl)-imidazol-2-ylidenes. The carbene tautomers can be trapped as thiones (X-ray analysis). Moreover, bis(triphenylphosphine)palladium(II) dichloride in THF trapped the carbene tautomer as a palladium complex without participation of the phenolate group (X-ray analysis). The corresponding anionic NHCs, 2-phenolate-substituted imidazol-2-ylidenes, can be trapped by triethylborane or triphenylborane to form 4,4-diethyl- or 4,4-diphenyl-4H-benzo[e]imidazo[2,1-c][1,4,2]oxaza-borininium-4-ides, respectively (two X-ray analyses). These tricyclic systems are the first representatives of a new heterocyclic ring system. The results of DFT calculations concerning the HOMO/LUMO profiles and partial charges are also presented.

Hybrid single-chain nanoparticles via the metal induced crosslinking of N-donor functionalized polymer chains

Single-chain nanoparticles (SCNPs) formed via the crosslinking of isolated polymer chains in highly diluted solutions represent a conveniently achievable class of well-defined small particles. Due to their partly swollen geometry, they provide huge surface areas along the polymer chain. Herein, we present a set of copolymers based on poly(n-butyl acrylate-co-4-vinylpyridine) with molecular weights ranging from 6000 to 70 000 g mol−1 and an amount of metal binding sites ranging from 3 to 22 mol%. These organosoluble precursors with hydrophobic side chains were treated with Cu2+, Co2+, Ni2+, Zn2+, Fe2+, Mg2+, and Pd2+. The coordination of metal ions to pyridine moieties as versatile N-donors leads to intrachain crosslinking and subsequent chain collapse. Overall, a set of 34 hybrid SCNPs was prepared. SCNP formation was monitored by dynamic light scattering and diffusion ordered NMR spectroscopy. The influence of the molecular weight, the molecular weight distribution, and the amount of binding sites on size reduction during SCNP formation is discussed in the context of models given in the literature. Additionally, an indication for the dependency of size reduction on the coordinative bond strength is given. Selected crosslinking metal centres have been analyzed by theoretical investigations. A significant freedom for the coordination geometry which can induce chain collapse is revealed.

Sonogashira coupling in 3D-printed NMR cuvettes: synthesis and properties of arylnaphthylalkynes

3D-Printing has been used to build hollow NMR tube/spinner combinations out of NMR-transparent polyamide. The whole 3D-print has been processed inside a glove box and during pauses of the 3D-printing process, all reactants for the palladium-catalyzed decarboxylative Sonogashira coupling of aryl halides with arylpropiolic acids have been inserted. Within the totally gas tight and pressure resistant tubes, a set of arylnaphthylalkynes has been synthesized and the progress of the reaction has been monitored via NMR spectroscopy. The (nonlinear) optical properties of the push–pull substituted bis(aryl)alkynes have been investigated by fluorescence spectroscopy and DFT calculations. A significant correlation between the intensity of the triple-bond stretching vibration and the calculated first hyperpolarizability is reported.

Dipolar Bent and Linear Acetylenes Substituted by Cationic Quinolinium and Anionic Benzoates. Formation of Mesomeric Betaines

3-Ethynylquinoline was subjected to a Sonogashira-Hagihara reaction with methyl 2-, 3-, and 4-bromobenzoates, respectively, and then N-methylated to give 3-[((methoxycarbonyl)phenyl)ethynyl]-1-methylquinolinium salts (two X-ray analyses). On saponification of the 3- and 4-substituted benzoates, the mesomeric betaines 3- and 4-[(1-methylquinolinium-3-yl)ethynyl]benzoates were formed. By contrast, the 2-benzoate derivative gave either the corresponding (1-oxo-1H-isochromen-3-yl)quinolinium derivative or the mesomeric betaine 2-(1-methylquinolinium-3-yl)-1,3-dioxo-2,3-dihydro-1H-inden-2-ide depending on the reaction conditions. A DFT calculation predicts a transoid conformation of the acetylene bond in the intermediate 2-[(1-methylquinolinium-3-yl)ethynyl]benzoate which is due to a strong hydrogen bond between the carboxylate group and 2H of the quinolinium ring, in addition to a 1,5-interaction between the carboxylate group and the CC triple bond. The bond angles of the transoid CC triple bond were calculated to be 211.6° and -175.1° in vacuo. The corresponding linear triple bond is 50.4 kJ/mol less stable in vacuo according to the calculation, and the N-heterocyclic carbene quinoline-2-ylidene is not formed as a tautomer.

Sydnone anions and abnormal N-heterocyclic carbenes of O-ethylsydnones

Deprotonated sydnones, which can be represented as anionic N-heterocyclic carbenes, were prepared as Li adducts and compared with deprotonated O-ethylsydnones (5-ethoxy-1,2,3-oxadiazol-4-ylidenes) which belong to the class of abnormal NHCs. The Pd complexes of the sydnone anions (X-ray analysis) as well as of the O-ethylsydnone carbenes proved to be efficient catalysts in aryl couplings of thiophenes.