Chemistry - A European Journal

Strong low molecular weight gelators can be prepared by using 4,6-O-benzylidene monosaccharides (see scheme) in various organic solvents. They are accessible in a variety of homologues, each with a unique molecular architecture, which allows them to be used for systematic studies of gelation phenomena. Analysis of the molecular arrangement in a single crystal was found to be a valuable tool to explain the differences in gelation ability.

Oligoarylamides with backbones rigidified by intramolecular hydrogen bonds are found to adopt stable, folded conformations. Short oligomers fold into crescent-shaped molecules, long oligomers fold into hollow helices such as the 9-mer shown here. A large (≈10 Å), hydrophilic cavity exists at the center of the folded molecules.

Two diastereomers of thymine glycol are formed by γ-irradiation of DNA. Phosphoramidite building blocks of (5R,6S)- and (5S,6R)-thymine glycol (see picture) were synthesized, and both isomers were incorporated into oligonucleotides separately. These oligonucleotides were used to analyze the effects of the oxidized base and its stereochemistry on the thermodynamic stability of the duplex.

A positive cooperativity between the two two-center components of the three-center H-bond for compound 1 has been revealed from NMR and IR spectroscopy, X-ray crystallography, and ab initio calculations of 1. The stability of the three-center hydrogen bond in amide 1 has been examined by comparing 1 with its structural isomers, related isomers (different substituents), and conformational isomers.

How do transition metal complexes affect the opto-electronic properties of conjugated polymers? A series of conjugated polymers (as shown) functionalized with ruthenium and rhenium complexes has been synthesized, and their photosensitivity, light-emitting properties, and charge-transport properties have been studied.

A novel heterodimer is formed from a chiral lithium amide and the carbenoid 2-lithium-1-methylimidazole. The heterodimer is found to deprotonate cyclohexene oxide with a stereoselectivity of 96 % ee and a 96% yield as shown in the scheme.

The cyclodienynes 1 have been prepared by a five-step sequence from readily available starting materials. On heating they cyclize to the benzocycloalkenes 2.

The thermal cycloisomerization of carbocyclic 1,3-dien-5-ynes (ring size 7–14; see scheme) and of both parent and benzannelated hexa-1,3-dien-5-yne was investigated by ab initio and experimental methods. In analogy to the Bergman cyclization of cyclic enediynes, some of the smaller strained dienyne carbocycles showed lower cyclization barriers than the parent system.

A multichromophoric supermolecule has been designed by assembling seven push–pull chromophores on a β-cyclodextrin ring, as shown. This favours a polar arrangement of the dipolar chromophores and promotes interactions that affect their absorption, emission and nonlinear optical properties.

A rapid change in the directionality of the hydrogen-bonding belt in self-assembled tetraurea calixarene capsules occurs when a tetraethylammonium cation is included as guest, while the capsule itself is kinetically very stable. A rationale for this surprising internal motion is provided by MD simulations which predict a connection between the two hemispheres by only one hydrogen bond per urea function but an additional stabilisation of the capsule by nondirectional cation–π interactions. Variable-temperature NMR measurements revealed that the rotation of the guest cation around an equatorial pseudo-C₂ axis is restricted, whereas its rotation around the molecular C₄-axis is fast.

Ideal probes made of well-defined conjugates have been synthesised (see for example figure) using di-, tri- and tetrasaccharide fragments, related to the capsular polysaccharide of Streptococcus pneumoniae type 3, and different protein carriers. These conjugates were then used for the evaluation of the influence of the different structural parameters in immunological tests.

Unprecedented polysulfides with pendant carbonyl complexes of formula [(CO)₄Mn{(PPh₂)₂C-S_n−C(PPh₂)₂}Mn(CO)₄] have been prepared by nucleophilic degradation of S₈ with the diphosphanylmethanide derivative [Mn(CO)₄{(PPh₂)₂CH}]. Control can be established over the sulfur chain length as well as over the acid–base behavior of the C−S_n−C bridging unit, which can be reversibly protonated at the carbon atoms (structure of the doubly protonated form of S₂ derivative shown).

A new type of hydrogen-bonded, basket-weave-like network structure has been isolated for the compounds [M(H₂pdc)₂(H₂O)₂]⋅2 H₂O (M = Mn, Fe, Co, Ni, Zn; H₃pdc = 3,5-pyrazoledicarboxylic acid; see figure). Through heating–cooling (hydration–dehydration) cycles, these compounds undergo a highly reversible structural interconversion associated with distinct color changes.

Two independent chemical processes are linked with mediation by electron-transfer reactions in a reversible and highly repeatable cycle that resembles a regulatory function (see schematic diagram). Addition of Zn²⁺ to Fcdpa⁺ produces the complex Fcdpa⁺-Zn²⁺, which oxidizes FcCrypt⁺-Na⁺, the ability of which to coordinate Na⁺ is drastically reduced. Removal of Zn²⁺ from Fcdpa-Zn²⁺ with the stronger ligand cyclam gives Fcdpa, which reduces FcCrypt⁺ to FcCrypt, whose strong binding capacity for Na⁺ is thus restored. A single cycle is now complete. FcCrypt denotes a ferrocene cryptand, and Fcdpa a bis(dipicolylamino)ferrocene.

Hexarhenium(III) clusters of the general formula, [Re₆(μ₃-Se)₈I_6−m(L)_m]^m−4 (m=3–6; a pair of geometrical isomers is depicted), containing monodentate diphosphine ligands L (Ph₂PCH₂PPh₂ or Ph₂P(CH₂)_nPOPh₂; n=1–5) have been prepared. During the synthetic procedures, the η¹-bonded diphosphine Ph₂P(CH₂)_nPPh₂ ligands are oxygenated. The complexes display strong luminescence with microsecond scale emission lifetimes at ambient temperature.

The formation of chiral, enantiopure aggregates of n-butyllithium and anisyl fenchols is controlled by ortho substituents (X) in the anisyl moieties and is elucidated by X-ray and computational analyses for X=H, SiMe₃, tBu, SiMe₂(tBu) and Me (see structure).

Ask more of your catalyst! Herein is described the evolution of chiral rhodium catalysts that not only are able to form carbonyl ylides from diazo substrates, but are also capable of remaining associated with the ylide to effect efficient asymmetric cycloaddition (see scheme).

A new, solvent-free method for the synthesis of thioureas LC=S that eliminates the toxic and highly flammable CS₂ is provided by the oxidation of cyclic aminals LCH₂ (L=RNCH₂CH₂CH₂-NR and RNCH₂CH₂NR, R=Me, Et, iPr, tBu, or Ph) by S₈ at 150 °C (see scheme).

Can one really compare chemical shifts calculated for isolated, static molecules to those obtained from experiments in solution? At least for aqueous [H₂VO₄]⁻ (1) and other typical vanadates, thermal and solvent effects on δ(⁵¹V) should indeed be quite small, as indicated by density functional based dynamic simulations. These simulations also reveal interesting details of the solvation shell (see figure, a typical snapshot of 1 with eight H-bonded water molecules).

The efficient fabrication of patterned materials under ambient, near-aqueous conditions on polychloromethylstyrene films, involving patterned irradiation (60 mJ cm⁻², 193 nm), reductive amination of the aldehyde photoproduct, and selective grafting of dye or metal to the resulting amine, is described. The picture shows micron-scale fluorescent Cy3.5 dye patterns prepared according to the method.

Attaching chromophores to a binaphthyl framework (see figure) results in materials that resist crystallization. Molecular topology therefore can be manipulated to control bulk morphology.

Its capability for hydrogen binding and the versatile use of its photocycloaddition products make the dihydropyridone 2 an interesting starting material for the Paternò–Büchi reaction (see scheme). The use of compound 2 and its 4-substituted derivatives in this reaction was probed. Excellent facial diastereoselectivity was recorded by the hydrogen-bond-mediated photocycloaddition to the aldehyde host 1.