PubMed Health⌕ Search

Biomedical subjects

Holger Braunschweig

Publications and source records attributed to Holger Braunschweig.

At least 19 recordsLinked to original sources
PubMed2006

Transition-metal complexes of boron-new insights and novel coordination modes.

Since the publication of the last review in 1998, the transition-metal chemistry of boron has continued to raise unceasing interest. Boryl complexes, representing the most extensive subclass, have remained a focus of intense research, particularly for their implication in the metal-mediated functionalization of organic substrates. Absolute novelties such as borane complexes and terminal borylene complexes have been structurally authenticated. Upon further elaboration of these compounds, the known coordination modes of boron-based ligands have grown considerably. Combined structural and theoretical investigations have contributed to elucidate the fundamental electronic characteristics of the transition-metal-boron bond and are leading to applications of these compounds. The most useful synthetic strategies for the generation of transition-metal-boron bonds are highlighted here, and the most recent and intriguing compounds that have been reported are outlined and discussed.

Journal Article↗
PubMed2006

Synthesis of ansa-[n]silacyclopentadienyl-cycloheptatrienyl-chromium complexes (n = 1, 2): novel precursors for polymers bearing chromium in the backbone.

Reaction of [(eta5-C5H4Li)(eta7-C7H6Li)Cr]tmeda with a variety of dialkyl(dichloro)silanes in aliphatic solvents afforded the corresponding [1]silatrochrocenophanes. Structural characterization by X-ray diffraction analysis of the [1]silatrochrocenophanes bearing Me2Si, (iPr)2Si, and silacyclobutane bridges revealed tilt angles alpha of 15.56(12) degrees , 15.8(1) degrees , and 16.33(17) degrees , respectively. Analogously, a [2]silatrochrocenophane (6) was prepared in excellent yield by reaction of [(eta5-C5H4Li)(eta7-C7H6Li)Cr]tmeda with 1,2-dichloro-1,1,2,2-tetramethyldisilane. This complex also was characterized structurally and exhibited a tilt angle alpha of 2.60(15) degrees. The [1]silatrochrocenophane bearing the Me2Si bridge underwent facile and regioselective carbon-silicon bond cleavage with [Pt(PEt3)4] to give a very high yield of an oxidative addition product. The ring-opening polymerization of these novel [1]silatrochrocenophanes afforded ring-opened chromium-based polymers.

Journal Article↗
PubMed2006

6H,13H-Pyrazino[1,2-a;4,5-a']diindole analogs: probing the pharmacophore for allosteric ligands of muscarinic M2 receptors.

A series of 6H,13H-pyrazino[1,2-a;4,5-a']diindole analogs was synthesized in order to probe the pharmacophore hypothesis for allosteric ligands of muscarinic M(2) receptors. The 3D structure of the novel ring system was determined by means of NMR spectroscopy and X-ray diffraction revealing a totally flat geometry. Low binding affinities for the [(3)H]N-methylscopolamine-occupied M(2) receptors (reflected by EC(50,diss)) indicated that the spatial arrangement of the pharmacophore elements (two aromatic rings flanked by two cationic centers) incorporated in the bisquaternary analogs 5 and 6 is unfavorable for strong ligand-receptor interactions. Due to the structural similarity of the novel compounds to neuromuscular-blocking agents, their affinities (reflected by K(i)) to the muscle type of nicotinic acetylcholine receptors were also determined. The dimethyl and diallyl analogs 5 and 6 exhibited rather high affinities to the muscle type of nicotinic acetylcholine receptors, suggesting a pronounced neuromuscular-blocking activity. Compound 5 showed a 34-fold higher affinity for the muscle type nAChR than for the allosteric site of M(2) receptors.

Allosteric Regulation↗
PubMed2005

Synthesis and structure of novel organocycloborates.

A series of alpha,omega-boryl(bromo)alkanes of the general formula R2B-(CH2)n-Br (n = 3, 4, 5, 6) was obtained in high yield following a standard hydroboration protocol. Upon treatment with Mg turnings and formation of the respective Grignard species, all alkanes with n = 4 to 6 underwent an unprecedented boron-centered cyclisation reaction with formation of boratacyclopentanes, -hexanes, and -heptanes, respectively. All new compounds were isolated in high yields as colourless, crystalline materials and characterised in solution by multinuclear NMR spectroscopy. Four representative examples were chosen for X-ray diffraction studies, thus providing the first structurally characterised ring systems of that size at a tetraalkyl borate centre.

Journal Article↗
PubMed2005

Boron as a bridging ligand.

The Lewis acidity of [(eta5-C5R5)Fe(CO)2(BX2)] (1: R = H, X = Cl) and the robust nature of the constituent Fe--B bond have been demonstrated by reaction with 4-methylpyridine to yield the corresponding Lewis acid-base adduct. These properties have subsequently been utilised in the construction of heterodinuclear bridging borylene, bridging boryl and mu(2)-boride complexes from analogues 3 (R = Me, X = Br) and 5 (R = Me, X = Cl). All the bimetallic species so formed exhibited unprecedented coordination modes for boron.

Journal Article↗
PubMed2005

[2]Borametallocenophanes of Zr and Hf: synthesis, structure, and polymerization activity.

The first [2]borametallocenophanes of Zr and Hf are reported, which were obtained from 1,2-bis(dimethylamino)-1,2-dicyclopentadienyldiborane(4). The constitution of these complexes in solution and, for the Hf complex, in the crystalline state is discussed. The title complexes were studied as Ziegler-Natta-type catalyst precursors for the polymerization of ethylene. Both complexes produced PE of high molecular mass up to 1.6 x 106 g/mol, and very interestingly, the Hf complex proved to be as active as its Zr analogue.

Journal Article↗