Chemistry - A European Journal

Do the hula-twist!cis–trans Photoisomerization can be achieved either by the more volume-conserving mechanism of hula-twist (HT), the turning over of one C−H unit on the other end of the double bond, or by turning over both substituents on one end of a double bond (one-bond-flip, OBF) as illustrated. The former is believed to be favored for polyene chromophores confined in rigid media.

Sonochemistry and catalysts: The use of sonochemical methods for the preparation of a variety mesoporous materials and for the insertion of amorphous nanosized catalysts into the mesopores is described. The main advantage of the sonication method is that the reaction time is considerably shorter than the conventional methods. When the ultrasonically prepared catalyst–mesoporous composite is used in catalysis a high conversion into product is obtained.

“Skunky” beer! Exposure of beer to light causes the development of an offensive taste and a “skunky” odor termed the “lightstruck flavor”. There is mounting evidence that photolysis of the iso-α-acids (an example is shown here) is particularly important in this process. Time-resolved electron paramagnetic resonance (TREPR) data that give significant details about the primary mechanism of this photochemical reaction are reported.

A variety of enantiopure 1,3-diol building blocks is available on an attractive scale by a stepwise reduction of a chlorinated β,δ-diketo ester (see scheme). The fruitful combination of two highly enantioselective and two highly diastereoselective reduction methods permits free choice of the absolute configuration of the corresponding crystalline dihydroxy ester.

A combination of [Cp*Mo(η⁴-diene)X₂] complexes (diene = isoprene or 2,3-dimethylbutadiene; see scheme) with methylalumoxane (MAO) catalyzes the polymerization of ethylene to yield high molecular weight and highly linear polyethylene, but the activity is very low. Theoretical calculations on this system and on the corresponding Nb system illustrate the role played by the additional unpaired electron.

A remarkably different product distribution from the hydroaminomethylation of ethyl methallylic amine 1 is observed when the reaction is carried out in supercritical carbon dioxide (scCO₂) rather than in organic liquid solvents (see scheme): whereas the cyclic amide 3 is obtained as the major product in conventional solvents, saturated nitrogen heterocycles are formed preferentially in scCO₂. The reaction conditions can be optimized further to yield either the pyrrolidine 2 or the new bicyclic framework 5 as the major saturated heterocyclic product.

Calixarene-based fluoroionophores composed of tert-butylcalix[4]arenes with one or four appended naphthalene moieties, such as the one shown here, exhibit not only red shifts in the absorption and emission spectra upon complexation, but also drastic enhancement of the fluorescence quantum yields. Complexation studies show very high selectivities towards sodium in ethanol and ethanol–water mixtures.

The systematically investigated photoreactivity of 3-alkoxycarbonyl-4-aryl-1,4-dihydropyridines (see figure, R=CH₃, CH₂C₆H₅) substantially differs from that of 3,5-disubstituted derivatives. Novel tetrakishomocubane derivatives were found as well as syn-dimers. The unusual cage formation is discussed with reference to two-color irradiation data.

Orally available pivaloyloxymethyl prodrugs may be activated by pivalase catalytic antibodies. Screening of monoclonal-antibody libraries generated against haptens 2 and 3 with pivaloyloxymethyl-umbelliferone 1 as a fluorogenic substrate led to the isolation of eleven such possible antibodies.

The design of better radiopharmaceuticals can be aided by the investigation of the interactions of metal ions such as In^III and Ga^III, which have useful and increasing applications as tumour-localising agents in nuclear medicine, with certain ligands. The kinetics of the indium(III) binding to 8-quinolinol-5-sulfonic acid (HQSA) and to sulfate have been investigated by both stopped-flow and temperature-jump methods. In the case of HQSA, the observed behaviour is ascribed to the fast equilibrium (see scheme) and only the form on the right is reactive. Concerning the In³⁺−SO₄²⁻ system, the kinetic results are interpreted according to a mechanism in which the step of chelate formation is rate-determining.

A new and efficient high-throughput screening methodology, which combines in situ synthesis of catalysts with an assay by electrospray ionization tandem mass spectrometry (ESI-MS), is introduced in order to investigate the highly active olefin metathesis ruthenium–carbene catalysts [{R₂P(CH₂)_nPR₂-κ²P}XRu=CHR′]⁺. A five-dimensional parameter space was scanned in a total of 1800 experiments (see diagram). The study reveals how structural modifications and the nature of the substrate govern catalyst reactivity towards olefins and uncovers key mechanistic details.

Even in the absence of base, compounds trans-[IrX(=C=C=CPh₂)(PiPr₃)₂] (X=Cl, I) react with CH₃I to give the butatriene complexes trans-[IrX(η²-CH₂=C=C=CPh₂)(PiPr₃)₂] in excellent yields. While the reactions of 1 (R=Ph) and 2 (R=tBu) with HCl lead to the formation of the octahedral hydridoiridium(III) compounds 3 and 4 by oxidative addition at the metal center, treatment of the same starting materials with CF₃CO₂H affords the Fischer-type iridium carbenes 5 and 6, which react with NaBPh₄ to generate the cationic carbynes 7 and 8 by abstraction of the trifluoroacetato unit from the α-carbon atom.

In aqueous solution, enolether radical cations were generated by photoionization (λ≤222 nm) or by electron transfer to radiation-chemically produced oxidizing radicals. The radical cations exhibit electrophilic reactivity with respect to nucleophiles such as water or phosphate as well as electron transfer reactivity towards one-electron reductants such as phenols, amines, vitamins C and E, and guanine nucleosides. The oxidizing power of the radical cation of 2,3-dihydrofuran, which can be considered as a model for the enolether formed on strand breakage of DNA, is estimated as 1.27–1.44 V/NHE.

Glycosynthases with different substrate specificity are used in sequence with glycosyl fluoride donors to assemble complex oligosaccharides (see scheme).

Controlling the cation–anion interaction for complexes [Au₂(dcpm)₂]Y₂ (Y = ClO₄⁻, PF₆⁻, CF₃SO₃⁻, Au(CN)₂⁻, Cl⁻, SCN⁻ and I⁻) can change their emission energy from the near-UV to the visible region. The triplet-state difference absorption spectra of these complexes exhibit bands at λ≈350 and 420 nm. These bands probably originate from the excited states of [Au₂(dcpm)₂]²⁺ and its complexes with Y/solvent, respectively (see scheme).

The very dangerous mushroomAmanita phalloides contains both amatoxins and phallotoxins: two families of toxic bicyclic peptides. A series of synthetic analogues differing from natural phalloidin (see figure) by substitutions at 2-, 3-, and 7-positions have been investigated in solution by circular dichroism and NMR spectroscopy. The corresponding molecular models, obtained by RMD simulations based on experimental NMR data, have been used to investigate the structural requirements for the interaction with F-actin.

The primary intermediate in the reaction of MnO₃Cl with tetramethylethylene was isolated by means of matrix-isolation techniques and identified as 1 by means of IR spectroscopy and density functional theory calculations on all conceivable products of the reaction. The reasons for the preference of the epoxidation route to 1 as opposed to the [2+3] cycloaddition leading to 2 are discussed.

Novel zwitterionic complexes are generated from the reaction of triphenyltin carboxylates, derived from [18]crown-6- and [15]crown-5-(benzo-4-carboxylate), and sodium thiocyanate (an example is illustrated here). In these zwitterions the sodium cation is complexed by the crown ether ring, the tin atom is five-coordinate due to coordination with the thiocyanate anion, and there is an intramolecular cation–anion separation of well over 9 Å.

A sphere of twelve fluorine atoms around the tetrahedral BC₄ skeleton forms a nearly perfect inert anion (see figure). The ¹¹B NMR spectrum of K[B(CF₃)₄] dissolved in CD₃CN is also shown.

Fast parallel synthesis was used to prepare a family of eighteen triisopropylsilyl-capped phenylene ethynylene pentamers for organic electroluminescence. The pentamers were grown from a polymer support by using a series of palladium-catalysed coupling reactions between aryl iodides and alkynes. Pentamer 1 was found to have the highest quantum efficiency in dichloromethane, and so was incorporated into organic electroluminescent devices; blue emission was observed.

Conformational rules for spatial orientation of the ring substituents in 3-piperideines have been formulated on the basis of molecular mechanics and high level of theory ab initio calculations (see scheme, R=Me, Ph, tBu).

The oxaspirocyclopropanation and the one-pot hydroxycyclopropanation of simple alkenes have been accomplished by thermal treatment with 2- and 3-haloalkoxy(alkenyl)carbene complexes of chromium followed by halogen–lithium exchange reactions (see scheme). Depending on the nature of both the halogen atom and the lithiating reagent, the formation of either a radical or ionic intermediate is presumed to account for these results.

Individual dendritic branches can be used to solubilise dyes by using simple acid–base hydrogen-bonding interactions (see figure). Both the extent of the dendritic branching and the functionality at the focal point are proven to be essential in controlling the efficiency of solubilisation and the nature of the microenvironment experienced by the dye.