Angewandte Chemie International Edition in English

The cover picture shows the three-dimensional scanning force microscopic image of the supercoiling of a pBR322-DNA molecule. The height information is coded by shades of purple (the lighter, the higher). The two turns of the DNA chains about the superhelical axis and the right-handedness of the supercoiling are well distinguished. Supercoiled and relaxed circular DNA molecules are now routinely imaged by scanning force microscopy; however, the three-dimensional resolution of the images now achieved was previously not possible. Scanning force microscopy should also allow the imaging of the three-dimensional structure of polymers other than DNA. More about the possibilities and problems with the technique is reported by B. Samori et al, on p. 1461 ff. That double-helical structures play a role not only in biochemistry but also in complex chemistry is demonstrated in the communications by A. F. Williams et al. (p. 1463 ff.) and E. C Constable et al. (p. 1465 ff.).

Not exactly from the elements but nevertheless from simple precursors, the total synthesis of calicheamicin γ (1) was achieved in a remarkably short time. The definitive, highly complicated structure was only determined in 1987/1988. For chemists engaged in natural product synthesis this compound presented a challenge matched previously, perhaps, only by that of vitamin B₁₂ and palytoxin; thus, it is not surprising that several research groups entered into a race that was packed with excitement and drama. Apropos simple components, the Greek word chaliche (χαλιξ) means “pebble made of limestone” more about this in this very personal recollection.

Trialkylaluminum compounds and elemental aluminum were for a long time the only products obtained from attempts to synthesize organoelement compounds with Al in the oxidation state II and with one AlAl bond. With the synthesis [see Eq. (a)] of 1, the first compound of this type that could be fully characterized, and other tetraalkyldielement compounds R₂EER₂ (E Al, Ga, In), the chemical properties of this class of compound and, in particular, the reactivity of the metal–metal bond can now be studied in detail.

Bacteria's natural potential for degrading synthetic compounds is greater than typically assumed—it extends even to heteroarenes, chlorinated arenes, and, as shown below, highly toxic nitrophenols. Only when the aromatic ring is multisubstituted or has a specific substitution pattern are these compounds transformed into xenobiotics that are difficult or impossible to degrade further. In this case, the process may be directed or the cooperation of several bacterial strains may be employed for complete degradation. As a last resort, hybrid metabolic pathways may be constructed by gene transfer. TCA tricarboxylic acid cycle.

The investigation of organic multispin systems—with the triplet state as the initial member—has contributed considerably to the understanding of chemical bonding and to the examination of quantum mechanical model concepts. Structural information is accessible from the form and strength of the interaction between unpaired electrons in triplet ground states; in photochemistry triplet excited states play an important role both from a mechanistic and a preparative point of view. Besides triplet states, spin states of higher multiplicity (that is, molecules with more than two unpaired electrons) are attracting increasing interest. One way to superparamagnetic organic molecules has been shown by the synthesis of a branched nonacarbene with 18 unpaired electrons (S = 9 state).

Solid-state synthesis by microwave irradiation allows the “fascinatingly simple” preparation of crystalline CuInS₂ from the elements. The product is not of “semiconductor quality” and can therefore not be used directly for solar cells. The method, however, which has been established for a long time in solvent and molecular chemistry, should rapidly find wider applications in solid-state chemistry.

“Next to nothing is known about how fullerenes actually grow and form.” This statement was published in spring. Soon thereafter Bowers et al. published a series of articles that seem to be the light at the end of the tunnel. According to these studies, in graphite vaporization primarily C, clusters form from single C atoms. These species are linear (for C to C), monocyclic (for C to C), and planar-bicyclic cations (for C to about C); fullerenes are also present above C. The most important new insight is that the mono- and bicyclic C species isomerize directly to fullerenes on addition of energy; „cooling” of the „excited” fullerenes takes place by loss of C₁, C₂, or C₃ units.

With the help of reverse genetics the gene whose changes cause Huntington chorea, also known as St. Vitus's dance, was identified from a 500 000 base-pair sequence of the short arm of chromosome 4 several months ago. This rare hereditary disease usually breaks out between the ages of 35 and 50, and is characterized by disturbances of the motor and neural systems, and inevitably leads to death. The genetic reason appears to be repetitions of a certain base triplet; the greater their number, the earlier the disease breaks out. How this “molecular disease” was tracked down is described in an intelligible and absorbing way.

The hitherto unknown exo/exo conformation for bis(imino)phosphoranes is exhibited by the halogen derivatives XP(NtBu₃C₆H₂)₂ (X Cl, Br, I(1)). The P atom is in a trigonal-planar environment, and the phenyl rings are orthogonal to the N₂PI plane. The crystal structure of 1 is shown below.

Fe⁺ is the only cation that mediates the activation of molecular oxygen as well as that of the CH and CC bonds of the olefin when the complexes [M(C₂H₄)]⁺ of the transition metals (M ScZn) are allowed to react with O₂ under single-collision conditions in the gas phase. This unique behavior parallels the role of iron complexes in important condensed-phase processes in many chemical and biological systems.

As a result of their different symmetry, the four structural isomers of the title compound (the C₄h isomer 1 is depicted on the right) could be characterized by ¹H NMR spectroscopy. With these complexes the complete chromatographic separation of the four structural isomers of tetrasubstituted metal phthalocyanines was achieved for the first time. R 2-EtC₆H₁₂O.

Cleavage of the PS bond of an R₂PS function (R cyclohexyl), bound in a η² fashion to Mn(CO)₄, through insertion of R¹CP, followed by subsequent [2+2] cycloaddition of the product and a second molecule of R¹CP, leads to the thiadiphosphole 1. By decreasing the size of R stepwise (→ Ph → Me), the bicycle 2 and the pentacycle 3 can be formed. [Mn] Mn(CO)₃, R1 tBu.

Hitherto unknown for main group elements was the structural element M₆O₁₃, which has now been found in the cation 1, taci = 1,3,5-triamino-1,3,5-trideoxyinositol. In the In₆O₁₃ core with a μ₆-oxo center and approximate O_h symmetry, the OIn₆ unit is completely shielded by the six-coordinate (taci–3 H) ligands, which explains the surprising stability of 1 toward further hydrolysis.

An aluminum polyhedron with six equivalent Al atoms occurs in the radical anion [AltBu] according to the EPR spectrum. Ab initio calculations reveal that the Al framework has a distorted octahedral structure with D_3d symmetry (picture shown on the right).

A pseudo seven-coordinate central phosphorus atom is observed in the phosphanes 1 and 2. In solution this coordination is retained in the rigid phosphane 1, whereas in 2 an equilibrium exists between the opened and closed forms; as a result, MeI reacts with 1 at the P atom, but with 2 at the N atom.

A lower change in entropy upon duplex formation and a higher propensity for the triplex formation with complementary natural DNA, together with a higher resistance to degradation by phosphodiesterases are the main characteristics of the novel DNA analogues 1 and 2. The concept of reducing pairing entropy by reducing the conformational flexibility of an oligonucleotide single strand (preorganization) may be useful in “antisense” research.

Up to three Cu¹ ions and three catenane rings can be interlocked on the arms of the bicyclic central unit (shown schematically on the right); thus, the first multiring catenanes not based on macromonocycles, but on macrobicycles are formed. The synthesis of these supramolecular complexes uses the three-dimensional template effect of the Cu¹ ions.

Despite significantly different molecular structures and redox potentials, the electronic properties of the oxoferryl cation radical of 1 (M Fe^IV O instead of M Fe^IIICl, obtained by oxidation of 1 with m-chloroperoxyben zoic acid), which shows saddle-shaped distortions as a result of its peripheral chloro substituents, are similar to those of the corresponding cation radical of the unsubstituted “planar” porphyrin. These results help in the understanding of the different chemoand regioselectivities of halogenated and halogen-free iron porphyries in the catalytic hydroxylation of alkanes. R 2,4,6 Me₃C₆H₂.

Without problems and from “within”, that is, by Li–As multicenter bonds and LiHC interactions, is the description of how the electronic stabilization of the Li atoms appears to function in the Ge₂As₆Li₆ framework of 1, which is shielded by bulky tBu and SiiPr₃ substituents. The lithium-rich main group element aggregate 1 is formed in the reaction of tBuGeF₃ with LiAs(SiiPr₃)H in the presence of tBuLi. • As, Ge, ◯Li, ⊗ R¹ SiiPr₃, ⊙ R² tBu.

Unusual pK_a values for amino acids in the active sites of proteins play a large role in their function. A new explanation for these pK_a shifts, which considers not electrostatic but mainly hydrophobic interactions as the cause, is presented here. This is substantiated by the correlation between the titrimetrically determined pK_a values of a family of polypentapeptides containing aspartic acid and the aspartic acid content in the polypeptide.

4 GPa pressure, 600 °C, and a reaction time of 5 minutes—under these conditions LiSi can be prepared from the elements. Like the homologous silicides MSi (M NaCs), which have been known for a long time, LiSi also belongs to the Zintl phases. The Si⁻ ions form a three-dimensional network in a structure comprising puckered Si₈ rings. Li⁺ ions are located in the cavities (shown on the right). ⊗ Si, ◯ Li.

The activity of highly oxygenated derivatives of vitamin D₃ in cancer and psoriasis therapies makes these compounds attractive synthetic goals. Here the vitamin D₃ building block 3 was synthesized enantiospecifcally from (+)-(R)pulegone (1). The intramolecular cycloaddition of 2 is a key step in the reaction.

Formulated as an η³ complex, compound 2 is synthesized from the centrosymmetric ligand 1 and [Fe₂(CO)₉]. On photolysis 2 evolves CO and is converted into 3, in which the P atoms have an almost planar coordination environment. mes 2,4,6-Me₃C₆H₂.

The versatile building block, methoxyallene reveals another a new facet: The titanium derivative 1 derived from it reacts with aldehydes at the γ position. Thus the methoxyalkynes anti-3 arise with high anti-selectivity on reaction of 1 with α-N,N-dibenzylamino aldehydes 2. These activated carboxylic acid derivatives can be transformed into the homostatine lactones anti-4 by hydrolysis. R CH₃, CH₂Ph, CH₂CHMe₂.

Ever more useful is the triamido ligand [(Me₃Si)NCH₂CH₂]3N³⁻, which now has shown its worth in stabilizing a terminal selenido and telluride unit at a tantalum center. Treatment of the dichloride 1 with (thf)₂LiESi(SiMe₃)₃, E Se or Te, affords the stable complexes 2 and 3 in good yield. The products can be characterized by spectroscopy and X-ray crystallography.

Enantio- and diastereomerically pure samples of the methyl ether 1 of ACRL toxin IIIA can be prepared in 16 or 20 steps from methyl 3-hydroxy-2-methylpropionate. As an alternative to the aldol method the anti-configurated propionate units are stereoselectively produced by carbanion-aldehyde addition with a subsequent oxidation–reduction sequence.

“BaCuO₂” is not a simple ternary oxide but a complex oxocarbonate. It is a common component of superconducting oxocuprates. The complete elucidation of the structure by simultaneous X-ray and neutron powder diffraction on samples of “BaCuO₂” gave a statistically disordered carbonate group at the heart of the structure and resulted in the composition of this supposed oxide given in the title.

A perfluorinated diisocyanide was stabilized for the first time in complex 1 (cis form is depicted). The carbon atoms of the isocyanides are susceptible to nucleophilic attack, and 1 reacts with Me₂NH with the elimination of a [Cr(CO)₅] substituent to give an imidazole derivative with an annelated fluoro-substituted four-membered ring.

Light-induced charge separation, rapid intrarotaxane electron transfer over a short distance, migration of the two rotaxane components relative to each other, and a slow intrarotaxane electron transfer over a longer distance are the steps that enable molecular shuttle motion (see scheme on the right). Ferrocene, bipyridinium, and dialkoxybenzene units play a crucial role in this process.

Until now only planar projections of the molecular shape of supercoiled circular DNA molecules could be imaged with scanning force microscopy (SFM). In the three-dimensional image of a supercoiled pBR322 DNA molecule in this report, the chirality and writhing number can be “read” directly. Local changes in the supercoiling in individual DNA molecules can be monitored, which should be important for our understanding of biological processes such as DNA transcription.

Not intra-, but intermolecular stacking interactions between imidazole rings are responsible for the self assembly of the ainuclear Cu¹ complex [Cu₂L₂]²⁺ to an infinite double-helical polymer (picture on the right) in the solid state. In recent years a range of double-helical complexes of low-molecular weight has been prepared; however, [Cu₂L₂] is the first polymeric complex of this type.

The head-to-head self assembly of two [CoAgL₂]³⁺ units in the solid state results in a tetranuclear complex (shown on the right), which is not held together by AgAg interactions, but by stacking interactions between pyridine rings. This and the preceeding communication show that supramolecular complex chemistry is booming.

New possibilities for increasing the stability of the configuration of α-heteroatomsubstituted alkyllithium compounds follow from the unexpected finding that ortho methyl groups influence the rate of racemization of α-arylselenoalkyllithium compounds. According to this observation, rotation about the CX bond (X Se) [Eq. (a)] can be the rate-determining step.

What affects the configurational stability of chiral organolithium reagents? It is frequently proposed that ion pair separation plays a key role in the configurational inversion of alkyllithium compounds. But NMR studies of 1 and 2 show that the configurational stability of the solvent-separated ion pair is greater than that of the contact ion pair.

Evidently a σ complex and not a free uncomplexed silyl cation is present in the crystals of 1· toluene. This follows from ab initio studies into the structure of the cationic complexes [Me₃Sitoluene]⁺, [Me₃Sibenzene]⁺, and [H₃Sibenzene]⁺ as well as from IGLO calculations of the δ(²⁹Si) values of these complexes. The differences between these and σ complexes with alkyl groups can be ascribed to the more effective CSi hyperconjugation. Compound 1. toluene was recently described as “the first free silyl cation”.

Intramolecular bissilylation under high pressure yields the ethene derivative 1, whose structure is not twisted about the CC bond. Its UV spectra and crystal structure suggests that the weak absorption at 361 nm is attributable to the fourfold Si substitution and not, as previously suggested for tetrakis(organosilyl)ethenes, from twisting.

Two independent isomeric molecules in one crystal. This was the surprising result of the X-ray structural analysis of 1. Both molecules can be considered as representatives of transition states of the η² ⇇η² rearrangement that is common in Ni^o–arene complexes.

Since it remains unchanged after several days in air and several months under H₂O-free argon, the hydrazone 1 is unexpectedly stable; however, methyleneamines RNCH₂ rapidly di-, tri-, or polymerize. Compound 1 reacts with (iPr₂N)CF₃SO as an imine not as a hydrazone-and affords the cationic heterocycles 2 and 3 (counterion is CF₃).

That compounds bearing homologous ligands—here PiPr₃ and SbiPr₃—can behave differently is underlined by the synthesis of the first carbenerhodium(i) complexes in which the carbene is not stabilized by a heteroatom. Compound 1 (characterized by X-ray structural analysis) undergoes a variety of interesting ligand displacement reactions, giving, for example, 2 (L SbiPr₃, CO, CNtBu) and 3. Diphenylketene is obtained by coupling of the carbene ligand with CO.

CF₃ groups stabilize the Cu^III center in the complex anion [Cu(CF₃)₄]⁻ (1). Salts of 1 with large cations are easy to prepare by oxidation of Cu^I-CF₃ compounds with, for example, I₂. The Cu^III center in 1 has approximately square-planar coordination geometry, and the F atoms form one planar hexagon above the CuC₄ plane and one below. • Cu, ◯ C, F.

The unprecedented ring size in tetramer 1 arises from the alternation of the

coordinating units in the sixteen-membered macrocycle. This alternation spaces out the ring components of the OxO⁻ ligands, and the flat aromatic rings of the ligands align themselves in near-parallel pairs (shown on the right). Tetramer 1 is prepared from Me₃A1 and OxOH in toluene.

The enantioselectivity increases from 4.5 to 20.4% ee in favor of the (–)-enantiomer in the intramolecular hetero-Diels-Alder reaction of 1, which provides 2 and 3 when the pressure is increased from 1 to 5000 bar. A chiral Lewis acid is also employed in this reaction.

A Mo^IV anion, a neutral Mo^V complex, and a mixed-valence Mo^III/Mo^IV cation are found within the same crystal of the title compound 1, which is prepared by the reaction of [(Cp^*MoBr₂)₂] with bromine in benzene. The coexistence of three oxidation states of molybdenum within 1 and the formation of 1 is rationalized based on the particular redox behavior of the individual constituents of the redox system Cp^*MoBr/Br₂. The anion [Cp^*MoBr₄]⁻ in 1 is shown on the right. Cp^* η⁵-C₅Me₅.

Maximum sensitivity and excellent suppression of the water signal is achieved in 3D triple-resonance NMR experiments by the application of a heteronuclear gradient echo in phase-modulating pulse sequences. This principle is demonstrated for an HNCO experiment applied to a complex of a uniformly ¹³C/¹⁵Nlabeled protein (calmodulin) and a peptide (C2OW).

When the primary products are condensed in a cold trap to prevent further reaction, CH₄/NH₃ and C₂H₆/NH₃ mixtures can be converted into imines in the presence of Hg vapor on UV irradiation [Eq. (a)]. Slowing the gas circulation rate during the photolysis leads to CC bond formation; for example, CH₃CHNH is formed from CH₄/NH₃. R H, CH₃.

For 65 years it has been discussed whether hexatungstate ions exist. Now the confirmation of their existence and the structural analysis of Na₅[H₃W₆O₂₂] · 18H₂O have been achieved; in aqueous solution the latter is probably in equilibrium with the known [W₇O₂₄]⁶⁻ ion. The polyhedral representa tion of the structure of [H₃W₆O₂₂]⁵⁻ is shown on the right.

The higher melting component of β-Na₃Hg, namely the Na atoms, are mobile in the solid at relatively low temperature (36–60 °C) throughout the range of existence of this structural modification. In contrast, the Hg atoms show no unusual mobility. β-Na₃Hg crystallizes in a distorted rhombohedral Li₃Bi-type structure.

Silver nitrate and succinic acid react to form the pale yellow prisms of succinatodisilver(I) (1). The structure of 1 (shown on the right) contains Ag₄ clusters parallel to the ac plane, each of which are bound to four other clusters through succinato bridges.