Angewandte Chemie International Edition in English

The cover picture shows a space-filling model of C₁₃₉₈H₁₂₇₈, a dendrimer constructed from 127 phenylacetylene units, and one of the largest pure hydrocarbon synthesized to date. Moore et al. have thus broken their own record of C₁₁₃₄H₁₁₄₆ (Angew. Chem. Int. Ed. Engl. 1993, 32, 246). The rapid construction of this huge, shape-persistent(!) molecule—the monomers of each generation are identically colored—is the result of a novel synthetic strategy which combines a linear increase in monomer size per generation with the conventional convergent dendrimer growth method. The size of this fourth generation dendrimer spans over 125 Å, as indicated by molecular models and confirmed by size-exclusion chromatography. Dendrimers are not only interesting because of their unusual molecular architecture—the representation here, for example, is also reminiscent of objects whose size is orders of magnitude different such as snowflakes—but, above all, also as starting materials for novel functional materials. More on the production and characterization of this and related dendrimers are reported by Moore et al. on p.1354ff.; the boom in dendrimer chemistry is also illustrated by the work of Mülhaupt et al. (p.1306) and de Brabander-van den Berg et al. (p. 1308). Also from inorganic chemistry further adventurous sallies into the realm of “nanostructures” are reported: Fenske et al. (p.1303) have synthesized the discrete cluster complex [Cu₁₄₆Se₇₃(PPh₃)₃₀], which can be described as a phosphane-wrapped Cu₂Se particle.

Slowly the veil of secrecy is lifting, which has for so long shrouded the precise mechanism of the biosynthesis of vitamin B₁₂. The identification of the genes for the participating enzymes, their overexpression, ¹³C labeling, and ¹³C NMR spectroscopy were and are crucial in the cracking of this chemical mystery. Thus, it is not inappropriate that the main chapters in this review begin with quotes from Sherlock Holmes.

Increased metabolic stability, better bioavailability, and longer duration of action characterize peptidomimetics, compounds that can replace native peptides in their interaction with receptors. Peptidomimetics can be used as aids in the investigation of peptidergic systems and also as therapeutic agents. Until recently their design has been predominantly empirical (modification of native peptides, optimization of lead structures), but a rational design based on studies of structure and conformation energies is gradually gaining support.

Ideal for the study of electrode reactions under unconventional conditions, such as in solutions almost free of electrolytes, in nonpolar solvents, in solids, and even in the gas phase, “ultramicroelectrodes” have diameters in the μm range. In addition, in cyclic voltammetry the study of very rapid homogeneous and heterogeneous electrode processes are made possible by the different time scale that arises for low scan rates because of the size of the electrode, and complementary to this effect, by the tiny iR effects at high scan rates.

The supramolecular chemistry of boron–carbon clusters has received significant new impulses from two old hands at boron chemistry: Hawthorne et al. reported in a series of papers on “mercuracarborands”, macrocycles from three or four C₂B₁₀ carboranes linked by an equal number of Hg atoms, whose complexation can be considered as that of an anti-crown (that is, a crown compound with reversed charge distribution); Wade et al. (see this issue p.1328) have achieved the preparation of a macrocycle, in which three meta-phenylene and three meta-carboranediyl units alternate. This macrocycle promises interesting chemistry not only as an unusual host, but also as a starting material for novel metal complexes.

In the deliberate stereoselective control of reactions and for application in therapy, the chemistry of the antibody catalysts have provided new impulses. Furthermore, recent developments in the field of immunology give rise to the hope that in the future semisynthetic, catalytically active antibodies can be directly prepared by molecular biology so that the classical immunological methods, which require testing on animals, can be replaced. For example, the antibodycatalyzed hydrolysis of cocaine is very promising.

The bent structure of [C₅Me₅Ru(acac)] in the solid state (Hacac = acetylacetone) arises from the interaction of the ruthenium center with the γ-carbon atom of the acac ligand of a second, symmetry-equivalent molecule of [C₅Me₅Ru(acac)] and is not an intrinsic characteristic of the complex as claimed by Kölle et al. in 1990 in a communication in this journal. Reinterpretation of the X-ray data provided these results.

The bent structure of [C₅Me₅Ru(acac)] in the solid state (Hacac = acetylacetone) arises from the interaction of the ruthenium center with the y-carbon atom of the acac ligand of a second, symmetry-equivalent molecule of [C₅Me₅Ru(acac)] and is not an intrinsic characteristic of the complex as claimed by Kölle et al. in 1990 in a communication in this journal. Reinterpretation of the X-ray data provided these results.

A reversible mechanical control of the mobility of the interlocked rings is possible in photoswitchable catenanes like 1. Furthermore, the crown ether guest influences the thermal isomerization of the azobenzene host: the catenane 1 isomerizes ten times faster than the unoccupied host macrocycle from the (Z) to the (E) configuration.

Two para-, two meta-, or one para- and one meta-xylyl group as spacers A and B are found in the isomeric catenanes 1; the compounds with nonidentical spacers have now also been obtained by self assembly. The effects of minimal constitutional changes in the catenane components on the structure and dynamic behavior have been studied for the first time and may provide important information about the control of catenane properties.

The unthreading and rethreading of pseudorotaxane 1, which contains two p-phenylene spacers linking two bipyridinium units and a central 1, 5-naphthalenediyl unit, is reversible when an initially oxygen-free solution of 1 in water also containing 9-anthracenecarboxylic acid as a photosensitizes and triethanolamine as a “sacrificial” reluctant is irradiated and then aerated. The absorption and emission spectra of the solution provide conclusive evidence.

The largest cluster characterized to date by a crystal structure analysis, [Cu₁₄₆Se₇₃(PPh₃)₃₀], forms in the reaction of CuCL with Se(SiMe₃)₂ in the presence of PPh₃ in THF. The Cu-Se core of this huge cluster is shielded perfectly by the PPh₃ ligands. In this way, a molecule with a diameter of about 40 Å is formed. As a result of a certain structural similarity with Cu₂Se, the cluster could be considered as a Cu₂Se particle protected by ligands.

By quantitative cyanoethylation of polyamines combined with catalytic hydrogenation (Raney nickel) of the resulting polynitriles, poly(trimethylene imine) cascade molecules are accessible in high yields and high purities. Starting from ammonia, five generations of highly symmetrical dendrimers without structural defects were prepared with this reaction sequence.

In kilogram quantities; pure poly(propylene imine) dendrimers can be prepared in an extremely simple reaction sequence comprising Michael addition (primary amines to acrylonitrile) and heterogeneous hydrogenation with a Raney cobalt catalyst. Both steps proceed quantitatively and selectively and can be employed with many core and end groups.

The silicon atom is pentacoordinate in cations of salts like 1, which can be prepared, for instance, in a redox reaction of the corresponding silane (with I₂, X = I) or transformation with electrophiles like PhCoCl (X = Cl) or Ph₃C⁺BF (X= BF₄). These compounds were completely characterized; conductivity titrations confirm that they are ionic.

An effective access to compounds with three-coordinate silicon is provided by the title reaction [Eq. (a)], which occurs in two successive steps. Compounds 1 and 3 were completely characterized, but compound 2 could only be detected by IR spectroscopy.

An alkynyl(vinylidene) complex like 1 as intermediate in the Rh-mediated coupling of two alkyne molecules makes it possible for two products, enynes and the isomeric butatrienes, to be formed. The product selectivity can be controlled very easily by switching the order of addition of HX and CO.

Nine Cl atoms bridge the edges of the compressed trigonal bipyramid (stabilized by the incorporated carbon pair) of the central [{Pr₅(C₂){(Cl₂₁)] unit of the title compound (picture on the right). In addition three terminal Cl atoms bind to each apex of the bipyramid and two each to the Pr atoms of the pyramidal base. Till now only tetrahedral and octahedral clusters of metal-rich lanthanide halides were known.

Desulfurization with tributylphosphane converts the ferrocenepolysulfide 1 into three-dimensional network polymers. These consist of ferrocene subunits interconnected by oligosulfide crosslinks. Gel permeation chromatography on an analogous polymerized alkyl derivative of 1 indicates molecular weights in excess of 10⁶ Da. •=Fe.

Small change-large effect. When two Nmethyl groups on one N atom are replaced by hydrogen atoms in each of the diamino hgands in 1, M = Co, the bridging aqua ligand is ousted and an asymmetric dicobalt complex with one five-coordinate, one six-coordinate metal center, and three bridging carboxylato ligands results. This type of structural tuning is important for modeling, for example, the metal coordination environment in diiron(ii) non-heme proteins in which a coordination site on one metal is open.

2,10-Bis(phenylethynyl)-1,6-methano[10]annulene, a compound with “parallel” triple bonds, reacts with [CpCo(PPh₃)₂] and 2,6-dimethylphenyl isocyanide via a metallacycle to provide the 1,6-methano[10]annulene complex 1, which has a bisnorcaradiene structure. In addition, the methylene bridge of a 1,6-methano[10]annulene derivative was cleaved under unusually mild conditions to afford the novel naphthalene derivative 2.

A simple access to histidines that contain a carbon branch at the C2 atom of the imidazole ring is provided by reaction (a). The protected monoiodo histidine 1 is therefore a versatile coupling partner.

Three icosahedral and three planar building blocks (C₂B₁₀ carboranes and benzene rings, respectively) alternate in the novel macrocycle 1. This contains a cavity that is surrounded by hydrogen atoms, of which six are bound to boron and three to carbon; this cavity thus offers potential guests an unusual environment. Such “cluster-cycles” offer some scope for use as frameworks on which to hold bi- and trinuclear arrays of metal atoms.

For more than 50 years, the accepted opinion was that the dihydroxylation of olefins with OsO₄ proceeded by a concerted mechanism [Eq. (a)]. However, if the influence of the reaction temperature on the enantiomeric excesses in asymmetric dihydroxylations is studied, Eyring plots are obtained with inversion points, which is consistent with a stepwise mechanism.

A simultaneous discrimination between 3′- and 5′-phosphates, 2′-oxy- and 2′-deoxyriboses and nucleobases of nucleotides is made possible by the use of β-cyclodextrins with ammonio substituents in the 6 position; the corresponding host–guest complex (shown schematically) forms in water in the process. NMR spectroscopic investigations show that the sugar unit is found inside the cyclodextrin cavity.

From the as yet unknown [22]porphyrins(1.2.1.2) are derived the [22]annulenoquinones 1, which are accessible by a double Wittig reaction. All data indicate that 1 are real quinoid compounds and that the dianions are aromatic 22-π-electron systems (hydroquinones).

2-Aminomethylthiazol-4-carboxylic acid 1 is the result of one of a total of eight posttranslational backbone modifications in a very active gyrase inhibitor, microcin B 17. The structure of this highly modified 43-peptide was elucidated through a combination of analytical and spectroscopic techniques, especially two-dimensional NMR spectroscopy of a ¹⁵N-labeled peptide.

Information about the reactivity of the corresponding monoradicals is provided by the D parameters of the biradicals 1. The zero-field splitting parameter D, which can be obtained from the EPR spectrum of a localized triplet biradical, increases with the inverse cube of the average distance between the unpaired spins. For para-substituted, bisbenzylic triplet 1,3-cyclopentanediyls 1, the D parameter varies measurably depending on the substituents X.

Molecular rods such as 1 with a row of up to 18 atoms were obtained in preparative amounts by evaporation of graphite in the presence of helium and dicyanogen. The same experiments with chlorine as the reaction partner for the C plasma led to perchlorinated aromatic compounds. The carbon frameworks of these compounds are sections of the C₆₀ structure.

Simply and economically in large quantities the title complex 1 can be prepared from lactose as galactosyl donor and N-acetylglucosamine as acceptor. β-Galactosidase from Bacillus circulans functions as biocatalyst in this transgalactosylation.

The first intra- and intermolecular carbene insertions within any constrained molecular reaction vessel was detected in the photolysis of the solid inclusion complex 1·β-cyclodextrin(CD). NMR and/or FAB mass spectra reveal the formation of products such as 2·β-CD and 3·β-CD.

A nematic phase of the conjugated polymers 1 was proven by investigations with polarization microscopy, X-ray diffraction, and differential scanning calorimetry. Previously the conductivity and nonlinear optical properties of these compounds seemed to be their only points of interest.

The structure hardly changes when the cluster [Ir₁₄(CO)₂₇]⁻ (1) is reduced to the dianion or oxidized to the neutral compound. The 14 Jr atoms form a trigonal bipyramid, which contains an Jr atom at the center of every edge (see picture). The terminal CO ligands cover the metal framework completely.

Within only ten minutes the necessary structural information regarding the sequence of the monosaccharide units in 1 and the position of the glycoside linkages is obtained by selective TOCSY-ROESY experiments.

Within only ten minutes the necessary structural information regarding the sequence of the monosaccharide units in 1 and the position of the glycoside linkages is obtained by selective TOCSY-ROESY experiments.

Two rotational isomers of an ethylene derivative could for the first time be prepared and structurally characterized: the almost planar 1 and the twisted 2, which probably has the largest twist angle (approximately 50°) of all symmetrically substituted ethylenes yet synthesized. The longest wavelength absorption maximum of the twisted compound 2 undergoes a bathochromic shift of 105 nm relative to that of 1.

Only five generations of growth are necessary to generate the phenylacetylene dendrimer 1, the largest pure hydrocarbon (C₁₃₉₈H₁₂₇₈) and one of the largest dendrimers (diameter: 12.5 nm). Architectures prepared by the innovative scheme described in this communication quickly acquire a large size due to the unique pattern of nonlinear growth. In the picture, the circles symbolize benzene rings, the lines CC bridges, and the branched groups on the periphery tBu substituents.

Pores of uniform size are found in the title compound (a structural model is shown on the right), in which parallel layers of a zirconium phosphate are joined by biphenyl “pillars”. The structure of this polymer, which may have potential as a shape-selective catalyst or a molecular sieve, is reminiscent of the architecture of a Greek temple.

Electrocyclic ring closure of the methylenephosphonium ion 1, which is in equilibrium with 2, leads to the base-induced formation of 3 with the novel four-membered ring. Compounds of type 2 are considerably more stable than the corresponding pure hydrocarbons and can thus be isolated and characterized by X-ray crystallography.

Unequivocal answers to the questions in the title are not possible yet, but the strongest stable uncharged base, the P_S compound 1, is 28-29 pK units more basic than triethylamine. Compound 1 is easily prepared, readily soluble, and can be stored almost indefinitely at room temperature in the absence of moisture.

A textbook example of the dynamic Jahn–Teller effect is provided by the Jahn-Teller components of the v₆ mode of the benzene cation. They were resolved to the rotational level for the first time by zero kinetic energy (ZEKE) photoelectron spectroscopy. The picture on the right shows the ZEKE spec-trum of the 6¹(j= ± 1/2) band of the electronic ground state of the benzene cation via the intermediate state S₁ 6¹(J′ =1, K′ =1, +l).

A tetrameric contact ion pair structure with a distorted cubane (NNa)₄ core is revealed for the title complex 1. The structures of such α-nitrile “carbanions”–important intermediates in organic chemistry–are dependent on the identity of the metal employed, as a comparison with the related complex 2 with an (NLi)₂ ring shows.

Faster than the competing attack at the carbonyl group is the Cu-catalyzed 1,4-addition of trialkylaluminum compounds to α, β-unsaturated ketones [Eq. (a)]. Previously this transformation succeeded only with stoichiometric amounts of cuprate reagents. R = Me, Et; X = Br, Br₂, CN.