Chemistry - A European Journal

Spider-like oligothiophenes… are designed, synthesized and characterized by F. Sannicolò and co-workers (see page 459 ff.). A linear oligothiophene “body” decorated with 5-(2,2′-bithiophenyl) “legs” provides an excellent compromise between increasing molecular complexity and improvement of electronic properties. The electrochemical and optical properties of the “spiders” and those of the films prepared by their electrooxidative coupling are detailed.

Yb wires: Bimetallic ytterbocene–polypyridyl complexes with various bridging polypyridyl ligands have been prepared and are being studied as potential candidates as single molecular magnets. A variety of ligand geometries (see scheme) have been studied with the aim of tuning electronic and magnetic properties.

Seeing red! Looking into the invisible is not a paradox any more thanks to technological advances that permit us to substitute our retina with matrices of IR detectors exhibiting a greater and greater number of pixels, while functioning at room temperature.

Do ferrocenes communicate? Distinct intervalence charge-transfer bands for the mixed-valent cation [(FeCp)₂(μ-C₁₀H₆(BPh)₂)]⁺ (see graphic) in solution indicate that the rigid diboron bridge strongly promotes electronic communication between the ferrocene moieties. A pronounced counterion dependence of the Mössbauer results in the solid state can be traced back to subtle structural differences and crystal-lattice effects.

Spider decoration: Design, synthesis and characterization of α-penta-, hexa- and heptathiophenes (the latter is shown) decorated with 2-(5,2′-bi(thienyl) pendants are reported. Their electrochemical and optical properties and those of the electroactive films prepared by electrooxidative coupling are described.

Building bromide bridges: A new bromo-bridged Ni^III compound has been synthesized. We succeeded in the electrostatic carrier doping of a single crystalline sample by using a field-effect transistor (FET) device (see figure). This compound displayed n-type semiconductor behavior, which can be reasonably rationalized by the existence of a small amount of Ni^II impurities.

Fucosyltransferase inhibition assay: A fluorescence resonance energy transfer (FRET)-based assay method for the characterization of human fucosyltransferases (see scheme) was established by using a novel GDP-fucose analogue that was derived by chemical modification of naturally abundant and inexpensive D-galactose.

Where to fasten a hairpin? A simple peptide system was used to study the contribution of disulfide bonds to β- hairpin stability. The effect depends on its location within the β-hairpin structure (see graphic), non-hydrogen-bonded vs hydrogen-bonded site, proximity to the turn region, and packing with its surrounding neighbors' side chains. This context-dependent behavior might be extendable to other chemical cross-links.

Metal-catalysed oxidation processes: A study of the coordination behaviour of an original ligand containing both thiosemicarbazone and tosylamide moieties has evidenced the versatility of systems of this type towards a wide range of metals. Reactions with manganese and copper yield novel products derived from metal-catalysed oxidation processes. A low-coordinated lead complex is formed, whilst the assembly with Zn²⁺ ions induces a particular zeolite-type structure (see picture).

A transfer of chirality from dinucleotides to quaterthiophene is observed for conjugates ^5′TA^3′-t4-^3′AT^5′, ^5′AA^3′-t4-^3′AA^5′, and ^5′TT^3′-t4-^3′TT^5′ at high ionic strength and in cast films (TA: thymidine–adenosine, AA: adenosine–adenosine, TT: thymidine–thymidine). Circular dichroism, photoluminescence, and current density/voltage plots display significant variation on changing the dinucleotide scaffold.

A new type of FRET-based fluorescent probe for detecting intra- and extracellular HO^.: Au NPs of 15 nm diameter were decorated with DNA oligomers terminating in thiol functions at their 3′-positions and possessing 5′ fluorophore modifications. The Au NPs in the fluorophore–Au NP composites function as excellent fluorescent quenchers, but fluorescence is restored as a result of HO^.-induced strand breaks in the single-stranded DNA linkers (see figure). The probe features excellent selectivity for HO^. and is cell-permeable and intracellular HO^.-responsive.

A domino pathway to highly fluorescent spirocycles: An insertion–coupling–isomerization-Diels–Alder hetero domino reaction furnishes (tetrahydroisobenzofuran)-spiro-benzofuranones and -spiro-dihydroindolones in good yields. Many compounds of this new class of spirocycles possess large Stokes shifts and fluoresce intensively with blue over green to orange colors (see figure).

“Cryptic” heteroditopic receptors: Tripodal amido caps have been efficiently grafted onto the narrow rim of a calix[6]arene through peptide-coupling reactions. The resulting calix[6]cryptamides display a well-defined hydrophobic cavity closed by convergent amide groups, which gives two distinct binding sites. These receptors can bind neutral guests or organic ion-pair salts, and even show chiral guest recognition in the calixarene cavity (see figure).

The unique fatty acid pattern and a 3-deoxy-D-manno-oct-2-ulosonic acid (KDO) moiety account for the high toxicity of N. meningitidis lipooligosaccharides. A highly convergent approach for the synthesis of a lipid A derivative (see graphic) containing KDO has been described.

Restrain yourself! Two conformationally restrained epothilone analogues (e.g. see scheme) designed to restrict the conformational mobility of the aromatic side chain were made by introducing a methylene bridge between C-14 and C-17 of epothilone in the form of a cyclopentene ring. Preliminary cytotoxicity studies revealed strong and selective growth inhibitory activity of one of the analogues on two leukemia cell lines over solid human tumor cell lines.

Record breaker! The present paper describes the moment when the record for the largest covalently linked porphyrin-based macroring was broken. It could well be the record size for any ring of pure synthetic origin.

To mature nicely: The effects of mutations on the backbone protein of a Diels–Alderase catalytic antibody (CA), 39 A-11, during maturation are investigated. Structural analysis and energy calculations determined that the matured catalyst had a fourfold enhanced catalytic rate constant.

An effective chirality transfer from an enantiopure sulfinyl quinone allows the one-pot preparation of tetrahydro[4]helicene quinones, which contain both central and helical chiralities, with excellent optical purities or dihydro[4]helicene derivatives, which contain only helical chirality (see scheme). In the latter, the configurational stability is mainly controlled by the size of the substituent at C-12.

A three-step sequence starting from N-Boc p-anisidines allowed a short synthesis of 4-aminotropones (see scheme; Boc=tert-butoxycarbonyl). Regioselective introduction of substituents at the tropone ring was controlled by choosing adequate substitution at the aniline derivative and/or the sulfoxide or sulfone used in the second step. When this is dimethyl sulfone an atom economy process occurred in which SO₂ or the methtyl sulfinate anion are the byproducts.

Synthesis of multifunctional pyrroles: Although one might think that metal complexes may be the reagents of choice for highly selective transformations, a novel metal-free reaction sequence in allene-β-lactams that leads to the biologically relevant pyrrole frame has been developed by using a sodium methoxide/methanol system (see figure).

The transformation of N₂ to 2 NH₃ through the Chatt cycle is investigated with DFT calculations, which provided an energy profile and mechanism of this biomimetic process of ammonia synthesis.

A triplet aryl cation is the primary intermediate from the photogenotoxic drug lomefloxacin (see scheme). Two modes of intermolecular reaction were demonstrated by experiment and computation and explain the biological effect of this drug.

Swapping protons! Regioselective hydrogen–deuterium exchange reaction at the benzylic site was accomplished by using heterogeneous Pd/C as a catalyst and D₂O as a deuterium source in the presence of a small amount of H₂ gas. Furthermore, the Pd/C-ethylenediamine complex [Pd/C(en)]-catalyzed deputation on the benzylic position of the O-benzyl protective group was also developed.

Pacman dyads transfer charge! Novel Pacman-type face-to-face zinc–porphyrin–fullerene dyads have been synthesized and studied (see figure). Strong π–π intramolecular interactions between the porphyrin and fullerene entities resulted in new charge-transfer absorption and emission bands.

Fourteen ligands including a linkage between the C2 of 8-hydroxyquinoline were synthesized. They favor metal complexes where two 8-hydroxyquinolines chelate the same metal ion. The complexes inhibit the precipitation of Aβ-peptide induced by Cu^II and Zn^II but also the toxic formation of H₂O₂ due to a Aβ–Cu complex. The copper(II) and zinc(II) complexes of these ligands were also characterized.

Novel iron spin-crossover polymer: Structural and physical studies suggest that non-covalent intermolecular interactions, rather than interactions through bridging ligands, are mainly responsible for the cooperativity in some spin-crossover polymers (see figure) described herein.

IsomEriZation in the crystal! [Zn(TA)₂(H₂O)₂]⋅4 H₂O (H-TA=tiglic acid) has been embedded in a framework composed of CECR (CECR= C-ethylcalix[4]resorcinarene) molecules to give the first quantitative diffraction study of the kinetics of single-crystal-to-single-crystal E→Z isomerization within a supramolecular framework (see scheme).

The latest in TCNE chemistry: [Fe^IICl₂(NCMe)₂] dissolves in MeCN forming [Fe^IICl₃(NCMe)]⁻ and [Fe^II(NCMe)₆]²⁺. This anion reduces TCNE to [TCNE]^.−, which reacts with [Fe^II(NCMe)₆]²⁺ to precipitate [Fe^II{C₄(CN)₈}(NCMe)₂] (1, shown here) that possesses the σ-dimer of [TCNE]^.−, [C₄(CN)₈]²⁻. [FeCl₄]²⁻ also reduces TCNE, but since [Fe^II(NCMe)₆]²⁺ is not present, 1 does not form.

Through-space coupling by design: A heterobimetallic Gd–Co complex of a hexadentate chelating ligand that bears a proximal antipodal phosphine donor exhibits antiferromagnetic through-space exchange coupling of similar magnitude to that observed in complexes with more conventional superexchange pathways. The picture shows a calculated spin-density isosurface associated with the unpaired electron on cobalt, along with a simplified depiction of the antibonding interaction between the phosphine donor and the singly occupied d orbital on cobalt that places unpaired density proximal to the chelated lanthanide.

Ambiphilic ligands: The coordination of the diphosphine–borane ligand DPB to RhCl(CO), PtCl₂ and PdCl₂ (see graphic) allows a spectroscopic estimation of the σ-acceptor character of the borane moiety and highlights a pronounced influence of the metal basicity on the magnitude of the dative M→B interaction in this isoelectronic series.

Enantiopure benzylic alkynes can be synthesized from racemic benzyl bromides by means of an enantioselective nickel-catalyzed cross-coupling reaction using trialkynylindium reagents (see scheme; DMA=dimethylacetamide). The reaction is stereospecific and takes place with high atom economy, as only 40 mol % of R₃In is used.

Seven up! Isostructurality encompassing seven halogen-bonded cocrystals has been achieved by different pairings of six molecules that contain structurally equivalent halogen bond donor (I and Br) and acceptor groups (O, NH, and S); each cocrystal exhibited distinct physical properties that could be systematically varied without significant changes to the structure.

Three independent movements can be performed by a rotaxane-based molecular shuttle (see picture) consisting of a thread with NH₂⁺ (red) and amide (pink) recognition sites and a macrocycle composed of pyridine/aniline and oligo(ethylene glycol) units (dark red, thick and thin, respectively), driven by acid/base and metal-ion complexation/decomplexation, each of which is accompanied by a different magnitude of fluorescent response from the anthracene fluorophore (blue). 12-C-4=[12]crown-4.