Angewandte Chemie International Edition in English

The cover picture shows a computer-generated space-filling representation of the product of photochemical chlorination of decamethyl-1,12-dicarba-closo-dodecaborane(12), decakis(dichloromethyl)-1,12-dicarba-closo-dodecaborane(12). The semitransparent Cl atoms (yellow) allow the visualization of the underlying and camouflaged icosahedral 1,12-dicarba-closo-dodecaborane framework, in which B is red, H is white, and C is gray. The CH vertices are buried in an array of five axially oriented Cl atoms arising from nearby CHCl₂ substituents. Apparent hydrocarbons of the type described here by M. F. Hawthorne et al. on p. 2536 ff. constitute a new class of carborane derivatives replete with unique chemistry and applications. The picture was produced by Darren M. Ricciardi, Imperative Images, Aurora, CO, USA, using the program Lightwave 3D by NEWTEK.

The addition of diazonaphthoquinones to novolak resins controls the dissolution of solid films of these materials by perturbing their percolation field (shown on the right). This rather unexpected effect is the basis of a photolithographic resist (light-sensitive varnish) used in the manufacture of almost all of today's integrated circuits (“computer chips”).

Continuous improvement of the McMurry reaction since its discovery in the 1970s has led to impressive applications in syntheses of natural products and pharmaceuticals. In addition, the exact mode of action of the titanium reagents has been scrutinized. In the latest variant the active titanium species is not generated before but during the reductive coupling of the carbonyl-containing substrate.

The subject of an exciting debate—does electron transfer play a role in bimolecular nucleophilic substitution (a)? How would this reaction differ from the classical S_N2 reaction (b)? A number of research groups have employed electrochemical and computational methods to determine if alternatives exist for one of the best known mechanisms in organic chemistry.

Long-chain-branched copolymers 2 can be prepared from ethene and acrylates (R′ Me, tBu, CH₂(CF₂)₆CF₃) with the Pd^II catalysts 1. Copolymers of this type were previously produced exclusively by radical routes in high-pressure processes. Propene/carbon monoxide copolymers (polyketones) have been obtained with other Pd^II complexes.

Even on a large scale an aqueous solution of 1,7-dioxaspiro[5.5]undecane and a tailored cyclodextrin selectively yields the inclusion complex with the (R) guest (shown on the right), the structure of which was studied both in the solid state and in solution. The yields are higher than would be expected for the racemate because the (S) enantiomer of the guest is transformed into the (R) enantiomer, which forms the more stable host–guest complex.

A structure reminiscent of the YBa₂Cu₃O₇ superconductor is shown by the novel reduced manganese perovskite YBaMn₂O₅ (on the right), in which double layers of MnO₅ square pyramids enclose the Ba²⁺ ions (large spheres), while Y³⁺ ions (medium-sized black spheres) lie between the layers. Unlike many oxidized Mn perovskites, YBaMn₂O₅ does not exhibit magnetoresistive effects. Instead a ferrimagnetic transition occurs at 167 K which may be due to Mn²⁺/Mn³⁺ valence and spin ordering.

In new bridging ligands the chelating groups listed in the title are fused to the termini of rigid molecular spacers. The resulting bisbidentate ligands such as those shown below have been prepared with matching and different terminal groups, and the coordination centers are arranged with varying separations and orientations. Selected bridging ligands and polypyridylruthenium(II) moieties were used to prepare dinuclear complexes, which were examined in electrochemical experiments. E CO₂CH₃.

Four stereogenic centers are generated with high diastereoselectivity in only a few steps starting from 2-naphthoic acid (1). Meyers oxazoline synthesis, nucleophilic addition of phenyllithium, and epoxidation afford rac-2 in 63% overall yield. The stereochemical course of the reaction is directed by the phenyl group.

Remarkable news concerning the long-known and well-investigated reaction system naphthalene/sodium in THF is reported: At room temperature a reversible intermolecular transfer of hydrogen takes place both between naphthalene rings and also with THF molecules. In contrast, the corresponding lithium and potassium systems show no such behavior. In addition, alkanes and cycloalkanes as solvent additives effect a significant increase in the H/D exchange rate in the sodium system. Benzene, toluene, ethene, and even methane (!) also take part in the H/D exchange reactions in the naphthalene/sodium system.

Complexes with terminal pnictide ligands are interesting in themselves and as ligands for the complexation of Lewis acids. Herein the syntheses of complexes such as 1 (E P, As) and 2 (M Cr, W) are described, and detailed theoretical studies on the relative thermodynamic stability of these and analogously constructed complexes are presented.

Oxidation at position 10 converts the title compounds into dimers. In an extension of this reaction a 15,15′-p-phenylene-linked bis(oxophlorin) produces the robust supramolecular array tetrakis(isooxophlorin) 1. In the crystal 1 has an overall helical structure in which the groove is filled by molecules of n-hexane.

The highest negative charge of a 16-electron system known to date is shown by the CO₂-analogous anion (CBC)⁵⁻ in Al₃BC₃. This has been confirmed by the vibrational spectroscopic and structural characterization of this compound, which revealed the linearity and bond lengths given below. Al₃BC₃ can be regarded as the first carbidecarbidoborate of aluminum the first.

Desilylations can trigger the intramolecular aldol reaction of alicyclic tricarbonyl compounds. Ester enolates are formed selectively and cyclize to give functionalized hydrindanones [Eq. (a); R¹ = (−)-menthyl, (−)- and (+)-phenylmenthyl; SiR₃ SiMe₃, SiPh₂Me]. The cyclization products are extremely important and useful synthetic building blocks for the synthesis of steroids.

The mechanism of stereodifferentiation was elucidated for the triplet ring-closure reaction of 1,2-dimethyltrimethylene (shown schematically on the right). The combined analysis of potential energy surfaces and spin-orbit coupling surfaces indicated that stereodifferentiation reflects the energetic behavior of the triplet state. This could well be a general feature of triplet photoreactions. ISC = inter-system crossing.

Pairwise exchange of geminal protons in unsaturated carbonic acid derivatives of the type H₂CC(R)COOR′ during the rhodium(I)-catalyzed hydrogenation [Eq. (a)] has been proven conclusively for the first time by using in situ NMR spectroscopy and parahydrogen (PHIP method). The most important conclusion is that the formation of the hydridoalkyl intermediates must be a reversible reaction.

A considerable extension of the synthetic potential of lithium arsenides is likely through the alanates [LiAl(AsHR)₄] (RH, triorganosilyl), which can be synthesized from LiAlH₄ and H₂AsR. For example, the Zintl compound [Li(tmeda)]₃ As₇1 was obtained by thermal decomposition in the presence of tmeda, and the first As₄Si₄ cubane was prepared by mild arsanylation of RSiCl₃ (R2,4,6-iPr₃C₆H₂). The novel complex 2 with an adamantane framework was formed from RAsH₂ (RMe₂C(iPr)SiMe₂) and LiAlH₄. tmedaMe₂NCH₂CH₂NMe₂, dmeMeOCH₂CH₂OMe.

Only a single purification step is required to isolate the glycoconjugate-related oligosaccharides prepared by a new strategy, which is shown schematically below. Prior to purification the support-bound oligosaccharide, which is labeled with a hydrophobic tag, is cleaved from the polymeric support and deprotected.

139 rotational lines of the ²⁸Si isotopomer of free silaethene H₂CSiH₂ (1) have been observed in the gas phase by millimeter wave spectroscopy. The search was guided by ab initio calculations at the MP₂, CCSD, and CCSD(T) levels with TZ2Pf basis sets. The rotational constants determined experimentally and by ab initio methods for 1 with a planar C_2v-symmetric structure agree within about 0.5%. Compound 1 was best produced by vacuum pyrolysis of 5,6-bis(trifluoromethyl)-2-silabicyclo[2.2.2]octa-5,7-diene in an Ar stream; 1 decomposes exponentially with a 1/e lifetime at ambient temperature of 30±2 ms.

The anionic benzyl carbon atom is sp² hybridized in 1, the first lithiated phosphane oxide that has been characterized by X-ray crystallography. In the solid state as well as in THF solution, 1 is shown to exist as a dimer with no LiC contacts. Unlike related phosphonamides, 1 prefers an orthogonal conformation of the benzylic group in the solid state.

Macrocyclization by ether formation (retrosynthesis shown below), the key step in a conceptually novel approach to cyclopeptide alkaloids, relies on an intramolecular nucleophilic substitution reaction. This new strategy was demonstrated for the synthesis of model compounds for the 14-membered cyclopeptide alkaloids, an important class of natural products.

A complex sequence of ferro- and antiferromagnetic interactions occurs in the compound [Ni(N,N′-dmen)(μ-N₃)₂] (N,N′-dmen N,N′-dimethylethylenediamine), which exists as a one-dimensional chain in the solid state (shown below; C atoms of the amine ligands are omitted). The magnetic behavior of this unusual spin system has been analyzed in detail.

The simultaneous activation of carbon dioxide and benzene by SiF in the gas phase has been investigated by FT-ICR spectrometry. The key step is the formation of a tightly bound SiF-(CO₂) adduct, which converts the unreactive carbon dioxide into an electrophile able to carbonylate benzene efficiently. At low pressures the benzoyl cation is formed.

Even the metal–ligand coordination in the molecular square 1 is retained in the fragments generated by fast atom bombardment in the mass spectroscopic method described. In this example the M–2OTf peak can be identified unequivocally owing to the peak separation of 0.5 m/z units in the isotopic distribution pattern.

Never failing to surprise—radical reactions with amazing selectivity. Photocyclization of the modified dipeptides 1 proceeds via biradicals that cyclize to give the peptidomimetics 2. Z benzyloxycarbonyl.

The sulfur atom, the nitrogen atom of the deprotonated amide functionality, and an oxygen atom of the carboxylate group are the ligating atoms of the dipeptide N-(2-mercaptopropionyl)glycine (H₃mpg) in its complex with the VO²⁺ center in the anion of 1 (shown on the right). This complex salt is accessible from [VOCl₂(phen)] (phen = 1,10-phenanthroline) and H₃mpg in the presence of Et₃N. This study provides insight into the mode of binding of a dipeptide to an oxovanadium(IV) unit.

Slow sublimation of the dithiadiazolyl radical 1 yields crystals of a new polymorph 1β (orthorhombic, space group Fdd2). Below 36 K this new phase exhibits a spontaneous magnetic moment, which is described in terms of canted antiferromagnetism.

Complete halogenation of decamethyl-1,12-dicarba-closo-dodecaborane(12) under radical halogenation conditions affords the title compound 1 in high yields (structure shown along the CC axis of the carborane framework). This molecule introduces an important new concept for the modular construction of structurally well-defined molecular systems.

A surprisingly simple, systematic route to the title compounds is achieved by ligand transfer from [Cp₂TiS₇NH] to the difunctional sulfanes Sn_nCl₂ (n = 1, 2). This should provide new impetus to sulfur–nitrogen chemistry. Whereas the structure of S₉NH (shown on the right) is significantly different from that of S₁₀, the structure of S₈NH resembles that of S₉.