Chemistry - A European Journal

Novel dendrimers are described composed of natural metabolites or biocompatible building blocks. These dendrimers (see scheme) are synthesized in high yield using a divergent approach and possess favorable chemical and physical properties for tissue engineering applications.

The magnitudes of intermolecular edge-to-face aromatic interactions vary by 6 kJ mol⁻¹ depending on the nature of the substituents. The trends can be explained based on the electrostatic interactions between the two π systems as depicted.

A molecular mechanics force field that explicitly represents the anisotropic charge distribution at atomic centres was tested against conventional atom-centred charge force fields and found to be superior in its ability to reproduce experimental trends in intermolecular aromatic interaction energies. The model complexes used for this investigation are shown.

Oxidised fishbone carbon nanofibres form covalent bonds to a rhodium(III) complex with anthranilic acid, the structure of which was investigated by means of EXAFS spectroscopy and molecular modelling (see figure). Upon reduction with sodium borohydride, small rhodium metal particles (1.5–2 nm) are formed that are catalytically active in the liquid-phase hydrogenation of cyclohexene.

Base-catalyzed anti–syn isomerization can be used to synthesize syn-trialkylated truxenes by alkylation of the truxene trianion from the anti isomers as depicted. Some of these syn-derivatives self-associate in solution through arene–arene interactions.

Site-specifically labelled redox-active DNA oligomers have been prepared containing a ferrocenylthymidine derivative by using automated solid-phase synthesis (see picture). X-ray crystallography and DFT calculations of the series of related metallocene-nucleoside monomers show that there is a high degree of interaction between the metal-containing group and the nucleobase.

Phenols are essentially carbon bases according to studies on the protonation of phenol (1), catechol (2), resorcinol (3), and hydroquinone (4) in the gas phase, both experimentally by high-pressure mass spectrometry and theoretically by molecular orbital calculations up to the G2(MP2,SVP) level. Enthalpies and entropies of protonation are also reported.

A simple, mild, and straightforward route to phosphonothioates from alkyl bromides and H-phosphonates (see scheme), in a two-step procedure based on thiocyanate chemistry, is described.

A new difluoromethylcarboxyl building block has been generated. Its reactivity towards various electrophiles allows the synthesis of various difluoromethylated compounds.

Ring-closing metathesis of 2-hydroxy-2,6-dialkenylated cyclohexanones followed by oxidative cleavage of the resultant keto bridge affords eight- or nine-membered carbocycles. Using an enyne instead of a diene, metathesis gives conjugated dienes capable of Diels–Alder addition to activated dienophiles. Cleavage of the keto-bridging tether produces 8–6 and 9–6 fused bicarbocycles with up to four stereocenters (see scheme).

Membrane transport by enantioselective receptors has potential for the preparative-scale resolution of racemates. The steroidal guanidinium cation 1 has been used to transport N-acetylphenylalanine, with multiple turnovers and in up to 70 % enantiomeric excess.

Morphogenesis of barium chromate particles has been successfully demonstrated by using double-hydrophilic block copolymers (DHBC) as crystal modifiers under various experimental conditions. Well-defined morphologies of BaCrO₄ particles and complex superstructures can be conveniently produced, such as lancet-like, elongated X-shaped, rodlike particles, ellipsoids, spheres, conelike nanofiber bundles and multi-cone-like superstructures (see scheme).

Construction of hexameric metallomacrocycles using direct and single-pot methods is described. Ru^II and Fe^II were used separately to connect homo- and heteroleptic bis(terpyridine) ligands thereby forming the benzenoid shapes such as that depicted.

A remarkable concentration dependence of propan-2-ol was observed with a maximum at 24 equivalents in the transfer hydrogenation of a variety of different imines to the corresponding amines by propan-2-ol in benzene catalyzed by [Ru₂(CO)₄(μ-H)(C₄Ph₄COHOCC₄Ph₄)]. The reaction is highly efficient with high turnover frequencies, and the product amines were obtained in excellent yields. Also unexpectedly ketimines react faster than aldimines, which is reversed to all previous hydrogenations of imines.

A slight but definitely superior chemoselectivity to that provided by a commonly employed commercial catalyst under identical hydrogenation conditions is shown by palladium supported on highly lipophilic functional resins, as shown. This finding demonstrates the potential of variations of the lipophilic/hydrophilic character of the support as a tool for the improvement of the chemoselectivity of resin-based metal catalysts.

High catalytic activity and enantioselectivity are obtained in the asymmetric Michael addition of α-cyanopropionates and acrolein with a new chiral Phebox-derived rhodium(III) catalyst (Phebox = 2,6-bis(oxazolinyl)phenyl), [(Phebox)Rh^III(SnMe₃)Cl], generated in situ from [(Phebox)SnMe₃] and [{RhCl(c-octene)₂}₂] by oxidative addition.

A very useful and versatile potential tridentate ligand based on anthracene with two dimethylamino groups at the 1,8-positions and with a Br atom at the 9-position has been prepared. This ligand can easily be lithiated with nBuLi to introduce a boron atom to the 9-position and converted to the corresponding palladium compound (see scheme).

The coordination preferences of the metal components seem to be important factors that determine the structure (and properties) of thiophosphates. The nonlinear PS₄³⁻ ligand in coordinated to uranium leads to the formation of metal thiophosphates with helical structure elements (see figure). The synthesis, structure, and magnetic properties of two such compounds are investigated.

Carbohydrate-based dendritic compounds (see diagram) composed of 21 and 27 monosaccharide residues have been synthesized in a convergent manner from trisaccharide AB₂ monomers based on a maltosyl-β-(1→6)-galactose structure, whereby the two primary positions of the glucosyl residues which carry the methylamino groups (B) in one monomer can be made to undergo reductive alkylation with the masked formyl function (A) in another monomer.

A oligonucleotide analogue with a highly unusual stereochemistry, α-L-LNA (see picture), is investigated. Two α-L-LNA:DNA duplexes, a nonamer and a decamer duplex that incorporate three and four α-L-LNA nucleotides, respectively, have been characterised by CD and NMR spectroscopy. An NMR structure of the decamer duplex reveals that the nucleic acid backbone absorbs the constraints imposed by the stereochemistry of α-L-LNA nucleotides.

The first metal sulfonate network with a cationic skeleton is reported. The structure is that of pillared layers with Cl⁻ anions residing in channels in the interlayer region (see figure). Significantly, the Cl⁻ anions are exchangeable for F⁻ in 80 % yield with structural integrity; however, the F⁻ analogue cannot be synthesized directly.

Nitrogen inversion and hydrogen-bond dissociation occur as a concerted process as shown by molecular mechanics and ab initio calculations for the new piperidines possessing an intramolecular OH⋅⋅⋅N bond.

The electronic interactions in bis-porphyrin systems connected by different oligothiophene spacers (as shown in the scheme) have been studied and proved to be highly dependent on the bridge connectivity. In asymmetrical dyads, the photoinduced energy transfer from the zinc porphyrin excited state to the free-base porphyrin is very efficient, and the largest rate was found in the dyad bridged with the longest spacer: the bis-ethynyl quaterthiophene.

Highly active catalyst precursors, based on ruthenium–benzylidene complexes bearing one or two imidazol-2-ylidene ligands (shown here), for atom transfer radical polymerization (ATRP) of methyl methacrylate and styrene were investigated. They also allow the polymerization of n-butyl acrylate and vinyl acetate, most probably by a redox-initiated free-radical process.

The kinetics of olefin epoxidation by the catalytically relevant mpRe and dpRe complexes of methyltrioxorhenium (MTO) were investigated in ionic liquid solvents . UV/Vis and ²H NMR spectroscopy were used to study the reactions in these new media. Alkene oxidation by dpRe is five times faster than alkene oxidation by mpRe (i.e., k₄≈5×k₃; see scheme).