Angewandte Chemie International Edition

Fruity like Anapear, green like Dynascone, marine like Calone 1951, floral like Phenoxanol, spicy like Methyl Diantilis, woody like Georgywood, ambery like Spirambrene, and musky like Helvetolide (see picture)—these could be the fragrance trends of the next few years. This report on the trends offers a fascinating journey into the molecular world of scents.

Once just a laboratory curiosity, olefin metathesis has evolved over the last few years into a mature tool for advanced organic, polymer, and bioorganic chemistry (see general reaction scheme). Powerful and highly tolerant new catalysts have been the key to this success. Despite the high standards that have been reached and the tremendous impact of this transformation on the fundamental logic of synthesis, this breathtaking development is not yet over. This review outlines the most recent developments in this timely field of research and discusses some perspectives for future innovations.

Ad-hoc hypotheses are forbidden to rescue threatened theories in Popper's falsification method. Kekulé attempted to save his benzene theory by such a device, but because organic chemists did not apply Popper's rule, Kekulé's theory survived as a cornerstone of modern science.

The development of an efficient, asymmetric variant of the Baylis–Hillman reaction for the synthesis of enantiomerically pure α-methylene-β-hydroxycarbonyl compounds (see scheme) is still a very rewarding goal. Interesting recent approaches for the solution of this problem include the use of chiral auxiliaries and bases and the thorough optimization of reaction conditions.

Targeted three-dimensional networks with unprecedented porosity (nanoporosity) can be designed from first principles. The diversity of nodes and ligands available allows fine tuning of the molecular recognition features of these porous solids. The materials so far prepared, for example [{Cu(4,4′-bipy)₂}_n](SiF₆)_n (see picture; bipy=bipyridine), have a greater surface area accessible to guest molecules than zeolites.

Thermal denaturation and subsequent rapid cooling allow novel supramolecular DNA nanocircles to be obtained in high yields from oligomeric conjugates consisting of bisbiotinylated DNA and streptavidin (see scanning force microscopy image on the left). The endogenous protein molecule within the nanocircles (picture on the right) can be conveniently used for further functionalization.

Three factors are important: the hydrolysis of phospholipids, in this case l-dipalmitolphosphatidylcholine (DPPC), catalyzed by phospholipase A₂ is controlled through 1) enzyme penetration, 2) the cleavage reaction, and 3) the dissolution of reaction products in the subphase. The use of lipid monolayers allows these processes to be studied separately at the air/water interface. Brewster angle microscopy (BAM) is used to follow the enzyme reaction in detail; the picture shows an L-DPPC monolayer at the air/water interface after the hydrolysis reaction.

Minimal structural changes accompany the oxidation of the paramagnetic uranium(V) anion [U(dbabh)₆]⁻ to the neutral, diamagnetic counterpart [U(dbabh)₆] (see structure). These two T_h-symmetric complexes, which were synthesized starting from 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene (Hdbabh), are the first isolable homoleptic hexakisamido complexes of uranium(V) and (VI).

Not only in a chemical sense is elemental gold the paragon of precious. This view needs to be somewhat reassessed; elemental gold disproportionates in the presence of the alkali metals rubidium or cesium and their corresponding oxides. The auride aurates M₇Au₅O₂ (M=Rb, Cs) obtained by this reaction can be interpreted as intergrowths of MAu and M₃AuO₂. Thus, another amazing parallel becomes obvious between the chemistry of gold and the halogens.

A fundamental new type of non-opportunistic hydrogen bond was detected between a proximal C−H group and the selenium atom of a selenocarbonyl group in aldols of chiral N-acyl selones (see structure) by gradient-selected ¹H–⁷⁷Se HMQC NMR spectroscopy and X-ray crystallography. The C−H⋅⋅⋅Se=C interaction provides a possible explanation for the ability of chiral selone groups to interrogate remotely disposed chiral centers by ⁷⁷Se NMR spectroscopy.

Adding benzoic acid and aniline as cocatalysts to the Rh^I/2-amino-3-picoline catalyzed hydroacylation of alkenes with aldehydes to give ketones results in a remarkable improvement in yield and activity [for example, Eq. (1)]. This is one of the few examples of a successful intermolecular hydroacylation reaction.

A chiral Grignard reagent with the metal-bearing carbon atom as the sole stereogenic center has been generated by asymmetric synthesis in >90 % ee (see scheme). Such reagents allow the elucidation of the stereochemical course of subsequent reactions (such as the shown oxidation), which provides hints to the mechanism (electron transfer reactions or polar additions).

Intramolecular addition of transient silylium ions 1 to C≡C bonds provides access to vinyl cations 2 (see scheme; R=Me, Ph) which are remarkably stable at room temperature. The pronounced stabilization of the two cations can be explained by the well-documented hyperconjugative effect of two β-silyl substituents.

Broken-symmetry density functional theory shows that there is no intrinsic link between the change in structure and the change in spin state shown by [(Cp*RuCl)₂(μ-Cl)₂]. The bistability observed in this complex should therefore be regarded as an example of bond-stretch isomerism, rather than spin-state isomerism [Eq. (1)]. Cp*=C₅Me₅.

An environmentally benign alternative to the toxic tin hydrides in radical chemistry is offered by the introduction of silylated cyclohexadienes 1. Most of the common radical reactions, such as dehalogenations, deselanations, deoxygenations, cyclizations, and intermolecular radical addition reactions can be conducted by using these readily available new reagents. X=Cl, Br, I, SePh, OCSOPh.

Metathetical exchange between LiN(SiMe₃)₂ and AuCl generated the first tetranuclear (AuN)₄ molecular ring, the gold(I) amide [{Au(μ-N(SiMe₃)₂]}₄]. The cluster is centrosymmetric and comprises an arrangement of four Au atoms (see picture).

The π-electron delocalization in the diazaphospholenes 1 leads not only to a weakening of the P−H bond, but also to an umpolung of its reactivity. Thus, compounds 1 react with acids with elimination of H₂, and display an inverse regioselectivity during the addition to benzaldehyde. R¹=H, Cl; R²=tBu, 2,4,6-Me₃C₆H₂.

Up to three stereocenters are selectively formed in the new domino C,O-cyclodialkylation of dilithiated 1,3-dicarbonyl compounds with 1,4-dibromo-2-butene. This reaction opens an efficient regio- and stereoselective route to a large range of 2-alkylidene-5-vinyltetrahydrofurans (one example shown in scheme). These products were transformed in a new Pd-catalyzed rearrangement reaction into functionalized bicyclo[3.2.1]octan-8-ones, whose skeleton is present in a large number of pharmacologically important natural products. LDA=lithium diisopropylamide, dppe=1,2-bis(diphenylphosphanyl)ethane, DMSO=dimethyl sulfoxide.

Magnetic and structural transitions at low temperatures are exhibited by the novel open-framework cobalt(II) phosphate of the composition [enH₂][Co_3.5(PO₄)₃] (en=ethylenediamine), the structure of which contains 12-membered channels (see picture). In the presence of a magnetic field, the compound undergoes a transition between two ferrimagnetic states.

An icelike chair conformation is adopted by the cyclic water hexamers found in an organic inclusion complex. Self-assembly of the hexamers into one-dimensional tapes (see picture), which interact very weakly with the organic host, confers on them characteristics similar to those of liquid water, as attested by FT-IR spectroscopy.

Assembly of an immobilized tripeptide, platination, and subsequent deprotection and release from the solid support are the key steps in the solid-phase peptide synthesis of the peptide–platinum(II) complex 1. This new methodology should be applicable for the preparation of a library of platinum antitumor compounds by a combinatorial synthesis approach.

A twisted six-membered Al₂N₄ring, which is rationalized by ab initio calculations, is present in compound 1. This compound together with 2 is obtained from the reaction of acetylenes with an aluminum dihydride bearing pyrazolato ligands. Compounds 1 and 2 are the first structurally characterized aluminum compounds with terminal acetylide groups.

The operationally simple and highly efficient metathesis of enynes bearing an acetylenic dioxaborolane moiety provides straightforward access to carbocyclic and heterocyclic 1,3-dienyl-2-boronates [Eq. (1)]. Dienes of this class readily undergo Diels–Alder cycloaddition reactions with high regio- and stereoselectivity.

Gripping compounds! A homodimer is formed by linking two monomers of b-ZIP protein basic regions through a photoresponsive azobenzene unit. By simple irradiation at appropriate wavelenghths, this dimer can be switched between isomers which have moderate (trans) and high (cis) sequence-specific affinity for gripping the major groove of DNA (see scheme). BR=basic region, ds=double-stranded.

Preferential growth along the 〈110〉 axes occurs in the preparation of Pt nanowires by reduction of [Pt(NH₃)₄]⁺ ions in the mesopores of silica MCM-41. The nanowires, uniformly 3.0 nm in diameter and with lengths of tens to hundreds of nanometers, consist of single crystals and are stable up to 500°C. Unsupported nanowires, released by dissolving the matrix in aqueous HF, are stable up to 300°C.

Upon doping with iodine vapor, the electrical conductivities of network polysilanes [RSi]_n and [MeSi(OMe)_l(R)_m]_n increase to the order of 10⁻³ S cm⁻¹ when the R group is N,N-dialkylaminophenyl. The polysilanes also show photoconducting properties due to generation of the photocarrier upon irradiation with a pulsed laser light (λ=425 nm; see picture, I=intensity): the polysilane is the electron donor and 9,10-dicyanoanthracene (DCA) is the acceptor.

A ligand with a linear arrangement of three coordination pockets produces a Cu₉O₁₂, alkoxide-bridged, square grid of closely spaced copper centers (see picture) in high yield by a strict self-assembly process. Orthogonal bridging connections within the grid lead to dominant intramolecular ferromagnetic coupling with a high-spin ground state.

A 10⁹-fold increase in efficiency is observed when the hydrolysis of sulfur-containing pesticides of the parathion and coumaphos type is carried out in the presence of cycloplatinated aryl oximes (see reaction scheme), even when the catalyst concentration is as low as 10⁻⁴ M. These robust catalysts are thus comparable in activity to organophosphate hydrolases, which detoxify the organophosphoric species by the same reaction.

H/D exchange between the hydrido ligands in the highly unsaturated dinuclear iron tetrahydrido complex 1 occurs on addition of dihydrogen or C₆D₆. Complex 1, which is formed by treatment of [(η⁵-C₅Me₅)FeCl(NMe₂CH₂CH₂NMe₂)] with LiAlH₄, reacts with Ph₂SiH₂ or PPh₂H with retention of the diiron framework to afford a μ-silylene complex or a bis-μ-phosphido complex, respectively.

Cyclic meso-2-ene-1,4-diol diesters undergo desymmetrization and cycloalkenyl esters undergo dynamic kinetic asymmetric transformations with excellent enantioselectivity when nitromethane and 2-nitropropane are used in the reaction (see scheme). A short synthesis of carbanucleosides also emerges.

An achiral Cs⁺ion acts as a template in the formation of homochiral, hydrogen-bonded pentamers from a racemic isoguanosine derivative 1. The meso decamer [(D)-1]₅⋅Cs⁺⋅[(L)-1]₅ (see picture) has one homochiral pentamer composed of only (D)-1 and the other pentamer is made up of (L)-1. Such enantiomeric self-recognition does not occur for the analogous G-quartets.

Iron centers in highly siliceous environments have been isolated by the reaction of incompletely condensed polyhedral silasesquioxanes with iron phosphane complexes in benzene. Among the new compounds is the oxo-bridged dianionic dimer 1. Single-crystal X-ray structures and preliminary reactivity studies are described.

Interpenetration is avoided in the graphite-like structure constructed by the complex ligand [Ni^II(bpca)₂] (bpca=bis(2-pyridylcarbonyl)amine) and iron(II) ions (see picture). The structure contains large cavities at the centers of the rings. Controlling the reaction stoichiometry and the choice of solvents enables this complex to be prepared selectively in preference to related discrete trinuclear and one-dimensional complexes.

Three pairs of groups can be added sequentially to C₆₀ with excellent regioselectivity giving compounds with similar spatial distribution (1 a and 1 b, see picture), but with permutated edge-to-face relationships. The structures of these isomers can be entirely deduced from the chemical shift changes induced by the magnetically anisotropic 4,5-diazafluorene moieties.

Stable gigaohm seals were obtained between a microstructured silicon chip (1) and an electrophoretically autopositioned giant vesicle (2). The low electrical background noise (<80 fA) of the resulting planar lipid bilayer (3) allows the observation of single ion channel events.

Attaching branched alkyl chains directly to the phenyl ring of substituent R in hexaalkynylbenzene (see picture) proved to be the key in the synthesis of the first room-temperature discotic nematic liquid crystals. A liquid crystal device having discotic nematic liquid crystals shows advantages over conventional devices with calamitic nematic liquid crystals in that it has a wide and symmetrical viewing angle and no reversal of the contrast ratio in any direction.