Angewandte Chemie International Edition in English

The cover picture shows the structure of hexakis(trimethylsilylethynyl)[3]radialene, a carbon-rich compound with unusual electronic properties. This trimethylsilyl derivative of a C₁₈H₆ hydrocarbon, which could serve as a monomer for a two-dimensional carbon network, can be prepared from readily available compounds in only a few steps. The [3]radialene forms deep red crystals, and unexpectedly its absorption is strongly red-shifted. Solutions of the radialene in dichloromethane are purple-red; electrochemical reduction proceeds through a rainbow of colors from purple-red over violet to light green. F. Diederich et al. report more on this interesting compound on pages 805–809.

This issue of Angewandte Chemie serves as a launching pad for the first issue of Chemistry—A European Journal, which is bound at the back. A separate Table of Contents at the beginning of the “Chemistry” section gives more information about the contributions in the inaugural issue.

Cyclic oligomers in which phenol or resorcinol units are linked by methylene or alkylidene bridges are called calixarenes and resorcarenes, respectively. They can be readily prepared in large quantities and are ideal starting materials for the synthesis of a large number of host molecules, in which these basic structures are modified or are combined to give larger units as in 1. Potential applications of calixarenes include their use as highly specific ligands for analytical chemistry, the decontamination of waste water, the construction of artificial enzymes, and the preparation of new materials.

A hydrogen abstraction as first step is the common feature of the nonenzymatic DNA cleavages presented here. The cleavage reagents are redox-active transition metal complexes endowed with affinity for DNA and cytotoxic natural substances like enediynes and bleomycins. They all attack the deoxyribose sugar unit of DNA, and any one of the five hydrogen atoms could be abstracted. After a survey of the modes of activation and binding to DNA, current ideas on the molecular mechanisms of the DNA degradation by these substances are discussed.

A CuCu bond in the mixed-valent Cu_A center of cytochrome c oxidase of Bacillus subtilis was postulated last year, though with necessary caveats, on the basis of EXAFS measurements. The deduced arrangement (bottom left) contradicts another structural proposal (bottom right) from the same year. A critical appraisal of all evidence available at present still suggests a negative answer to the question posed in the title.

Three total syntheses of zaragozic acids, a new group of natural products with blood cholesterol lowering properties, were successfully developed within a short time and published almost simultaneously. Convergence, skilled manipulation of protecting groups, and highly stereoselective reactions are the appealing features of these first syntheses of zaragozic acid A (Nicolaou) and zaragozic acid C (1) (Carreira and Evans).

The arrangement of charged and uncharged guests (N and two H₂O molecules; see structure on the right) and their interaction in the cavity of the anion of the title compound 1 can be controlled by deliberately exchanging particles in the truncated cone outside the host (not shown), which resembles an ion channel. Since 1 can incorporate a diversity of guests, continuing studies promise exciting insights into these interactions.

Crowns of thiophenes of type 2 with five, six, and eight heterocycles were obtained in moderate to good yields in the reaction of the appropriate [n]rotanes of type 1 and Na₂S. In the crystal 2 adopts a chairlike conformation with three thiophene S atoms above and three below the equatorial plane of the macrocycle.

Geminal dialkylated hydroporphyrins are important as intermediates of the vitamin B₁₂ biosynthesis or as cofactors of reducing enzymes in bacteria and plants. They can be synthesized enantioselectively by asymmetric reduction of acylporphyrins and amide acetal Claisen rearrangement with transfer of chirality, and can form complexes like 1 with metals.

Umpolung of the π-allyl group in π-allylpalladium intermediates is possible by alkylallyl exchange with diethylzinc. The resulting nucleophilic allylzinc species can attack benzaldehyde. Thus reaction of the cyclohexenyl ester trans-1 yields cis-syn-2 selectively. This stereoselectivity can be explained with transition state structure I. 1,3-Disubstituted allyl compounds undergo analogous reactions with regio- and stereoselectivity. LPPh₃.

Not only aesthetic, but also structurally interesting are the novel bridged π systems like 1 with “π boat” units. They can be synthesized by cyclization of 1,6-diazacyclodeca-3,8-diyne with dihalides. Whether the trimer 1 or a monomer or dimer forms depends on the geometrical properties of the bridge monomers employed (in 1para-substituted benzene).

The exothermic formation of a Ca–CO complex [Eq.(a), ΔH^o = −25 kJ mol⁻¹] confirms that CO can bind to a metal center without π back donation from filled d orbitals. In its spectroscopic properties, the calcocene–carbonyl complex resembles “nonclassical” noble metal carbonyl complexes. Cp^* = η⁵-C₅Me₅.

The asymmetric alkylation of N-propionylisoxazolidines (2a and 2b), which are prepared by treatment of 1a and 1b with propionyl chloride and triethylamine, proceeds with high diastereoselectivity, even with β-branched alkyl triflates. The alkylated products 3 can be converted into the alcohols 4, aldehydes 5, and ketones 6 in high yields in one step with recovery of the auxiliary 1.

Both discotic molecules and certain linear polymers form columnar hexagonal mesophases. Whereas the dislike structure of the macrocycle 1 is dictated by the molecular shape, in polymer 2 it is presumably enforced by a helical folding of the polymer's main chain induced by the side chains (bottom right).

Alone a tailor-made chiral cationic Lewis acid is not sufficient to achieve high enantiomic excesses in Diels–Alder reactions; the counterion can dramatically affect the efficiency of the chiral catalyst. Thus, for example, the enantioselectivities reached in these reactions in the presence of the copper complex 1 with X = SbF₆ as counterion are in some cases substantially greater than those with X = OSO₂CF₃.

Good access to the title compounds is provided by the reaction shown below. Compound 1 reacts readily with secondary amines to give N,N-dialkylaminocyclopentadienes 2, whose Li salts can be treated, for example, with FeCl₂ to give the ferrocene derivatives 3.

An efficient solid-phase synthesis of glycopeptides is achieved with the new anchor HYCRON, which combines an allylic ester with the flexible, solubilizing triethyleneglycol spacer. Acid- and base-stability of the allylic anchor enables both Fmoc and Boc strategies to be used. The partially protected glycopeptides can be cleaved from the support by Pd⁰ catalysis under virtually neutral conditions in high yields (about 95%) and purity (>95%). AMPS = aminomethylpolystyrene.

Hexaethynyl[3]radialene (1 a) is isoelectronic and isosteric with the known hexacyano[3]radialene and a molecular precursor for a two-dimensional all-carbon network. The hexakis(trimethylsilyl) derivative 1 b was now prepared by addition of tetrachlorocyclopropene to deprotonated 1,5-bis(trimethylsilyl)penta-1,4-diyne, followed by oxidation, and was characterized by X-ray crystallography. In solution, 1 b shows an unexpected purple-red color and is reduced in two reversible one-electron steps to the violet radical anion 1 b and dianion 1 b²⁻.

A facile route provides the title compound 1 in only a few steps starting from bromopentafluorobenzene. Borane 1 is a very reactive hydroborating agent for a range of alkenes and alkynes. Reactions with unsymmetrical substrates display excellent regioselectivity, and interesting selectivities with silicon-containing substrates are rationalized by electronic effects.

The direct sulfonation of arylphosphanes with H₂SO₄/SO₃/boric acid offers numerous advantages, the most significant being that P oxidation is largely avoided. In addition, selective sulfonation is possible. The products, for example 1, are water-soluble and interesting with respect to two-phase catalysis. Ar = C₆H₄-m-SO₃Na.

By multistep resonance ionization with lasers, followed by mass spectrometry, the first ionization potential of neutral atoms can be measured with quantities of only about one nanogram. The ionization potentials of americium (5.9736(3) eV) and of curium (5.9914(2) eV) were thus determined for the first time. The figure on the right shows a plot of the number of ions N versus the total photon energy E_tot of three laser beams, which are essential for the experiment. The arrow points to the ionization threshold.

Self-diffusion coefficients of a series of polycyclic charged systems and their parent compounds were measured by using the PGSE NMR method. These data suggest that the dianions of naphthacene 2 and pyracylene 3 may exist as dimers with sandwich structure, just like the lithium salt of the tetraanion of corannulene 1.

The ambiphilic reactivity exhibited by the silicon compound 1 contrasts the behavior of neutral silanes and simple silyl anions. This reactivtiy is an indication of silylenoid character, which can be accounted for by the two limiting electronic structures 1 a and 1 b.

A new three-dimensional coordination network with cubic symmetry is displayed by the structure of 1 (shown on the right). Compound 1 is obtained from tri(4-pyridyl)triazine ligands (tpt) and Hg²⁺ ions, and has a system of interconnected spacious cavities which are occupied by large numbers of solvent molecules and counterions. The coordination sphere of the Hg center is formed by six pyridine substituents, which is very rare for complexes.

Complex 2 is formed by two discrete ions which originate during hydrogenolysis of 1 according to Equation (a). The cation is dinuclear with three bridging hydrido ligands. The mixed-valent anion of 2 is also dinuclear and contains two bridging nitrido moieties, which cannot be the product of the hydrogenolysis of NSi bonds since silanes are not formed in the reaction.

The stable borylene-bridged Mn₂ complexes 1 and 2 are formed in the reaction of K[(η⁵-C₅H₄R)-Mn(SiMePh₂)(CO)₂] with the diboranes(4) B₂X₂Cl₂. The mechanism of formation still remains to be clarified. The molecular structure of 1 (R = H) shows similarities with dimetallacyclopropane derivatives.