Angewandte Chemie International Edition in English

The cover picture shows the structural formula of a uracil dimer (white) substituted with flavin units (yellow) and superimposed on a photograph of a solar eclipse (printed with permission from Eugene Aeppli, Zurich). This dimer was used to model the light-induced DNA repair reaction—the reversion of pyrimidine dimers, which are photoproducts formed on irradiation of DNA with UV light. For cleavage of the uracil dimer with light of longer wavelength (366 nm), the flavin substituents must be in reduced form; this is also a prerequisite for repair with DNA photolyase. T. Carell et al. report more on these model studies on p. 620 ff.

A rare anniversary gave the impetus for this review: the legendary physicist Friedrich Hund turned 100 years old on February 4, 1996. His significance for chemistry, which does not end with Hund's rules, is acknowledged and honored in this article. Hund and Mulliken cooperated on the development of MO theory, which initially stood in the shadow of VB theory, but eventually took over, becoming generally accepted. The scientific part of this article covers mainly recent insight into Hund's rules, his localization criteria, and coupling cases.

What is the connection between the total synthesis of brevetoxin and the 1994 Soccer World Cup? Apparently irrelevant factors have extensive ramifications for natural product syntheses. The marine biotoxin brevetoxin B, which is responsible for mass poisoning of humans and marine life during “red tides”, was the target of an intense research program in the Nicolaou group. Their perseverance paid off with a wealth of new reactions as dividends. In this “story behind the story” the author describes both the successes and failures along the way.

Lipid dioxygenases, tyrosine hydroxylase, bleomycin, and isopenicillin N synthase are only a few of the O₂-activating mononuclear non-heme iron-containing biomolecules. In recent years a great deal of effort has been dedicated to the elucidation of the structures and the reaction mechanisms of these systems, as well as to the synthesis of model complexes. In many cases, an iron–peroxo species is proposed as a predominant intermediate. The picture on the right shows the structure of an extradiol catechol dioxygenase recently determined by X-ray structure analysis.

Nearly a ton of marine tube worms was processed to obtain milligram quantities of cephalostatin 1. This compound belongs to a family of thirty highly cytotoxic dimeric steroids. Methods for their laboratory synthesis have now been developed.

The enhanced stability provided by two triphenylphosphane oxide ligands has enabled the first crystal structure analysis of a non-heme diironO₂ adduct (1) (structure of the core is shown on the right). Complexes of this type can be activated by introducing a more electron-donating carboxylate ligand. These observations rationalize the carboxylate-rich active sites of non-heme diiron oxygen-activating enzymes such as methane monooxygenase and ribonucleotide reductase.

[Fe₂(μ-1,2,O₂)(N-Et-hptb)(Ph₃PO)₂](BF₄)₃1

Irradiation with white light or monochromatic light at 366 nm causes the cleavage of the uracil dimer 1 at the cyclobutane moiety, providing that the flexin units are reduced prior to the reaction. This cleavage reaction of model compounds such as 1 forms the basis of DNA photolyase mediated DNA repair.

Well-defined poly(ethylene oxide)s with low polydispersities are formed in the anionic polymerization of ethylene oxide with an alkyllithium/phosphazene base initiator system. The base can be regarded as a cryptand for Li⁺ ions. The equilibrium between complexed alkoxide ion pairs and reactive free anions is thus shifted.

Single crystals are not always necessary. The structure of BaHg₂O₂Cl₂ (section shown on the right), which was prepared by a solid-state reaction, was determined from laboratory powder X-ray diffraction data. Isolated Hg₄O₄ squares are the main building blocks.

The first edge-sharing octahedral oxovanadium(IV) dimers with an anti-coplanar and a twist configuration (shown on the right) exhibit ferromagnetic interactions between the metal centers and thus show that it is possible to enforce specific magnetic properties on such systems through choice of ligand. The magneto-structural correlation of this type of compound should aid the interpretation of magnetic interactions in larger vanadium(IV) clusters with similar structural features.

Four negative charges in a π-hydro-carbon are generated by the reduction of rubrene, a tetraphenyl tetracene, at a sodium metal mirror. The black crystals of the contaction quintuple [{Na⁺ (thf)₂}₄(rubrene⁴⁻)] (see structure; the thf molecules have been omitted) contain the first hydrocarbon tetraanion ever isolated. Its structure determination confirms the severe skeletal distortions: the tetracene plane, for instance, is bent up and down by 43°.

A straightforward synthesis provides surprisingly stable, neutral and anionic μ₃-imidorhodium clusters. The unusual complex [Rh₄(μ-MeC₆H₄N)₂(cod)₄], whose structure is shown on the right, contains two bridging imido ligands that cap a triangle of metal atoms. One of these ligands, in addition, coordinates through the p-tolyl ring to a [Rh(cod)] moiety in a n⁵-cyclohexadienyl-like fashion. cod = 1,5-cyclooctadiene.

Not only AuAu but also RhAu interactions are responsible for the unusual trigonal-pyramidal coordination of the oxygen atoms in the complex cation 1. The picture on the right shows the Au₄Rh₂O₂ core of the cluster with norbornadiene (nbd) ligands at the Rh centers; the phosphane ligands have been omitted. The synthesis of this novel heteronuclear cluster is relatively simple.

[{(nbd)Rh[μ-O(AuPPh₃)₂]}₂]²⁺1

All six NH functionalities in hexa(cyclohexylamino)cyclotriphosphazene [NP(NHCy)₂]₃ (Cy = cyclohexyl) can be deprotonated with n-butyllithium. The hexalithiated compound 1 has a dimeric structure in which two hexaanions are complexed to twelve lithium counterions. The high solubility in aprotic solvents makes 1 a potential new ligand system for a variety of metals.

In two consecutive one-electron oxidations, oligopyrroles substituted with phenyl capping groups (PhPy_nPh, n = 2–4) can be oxidized reversibly to give stable cation radicals and dications. Spectroelectrochemical studies give direct evidence that diamagnetic π-dimers of cation radicals are formed in solution. Such dimers may be involved as charge carriers in conducting polypyrrole.

The number and availability of starting compounds determine the structure diversity possible in a multicomponent reaction. For example, in a set of parallel reactions an amino-functionalized resin (P) was treated with 8 aldehydes, 12 carboxylic acids, and 1 isocyanide to provide 88 of the desired α-acyl amino amides [Eq. (a)]. Additional isocyanides not available commercially can be prepared from α-lithiated benzylisonitrile and used without purification. R¹, R² = alkyl, aryl.

Adapted from biocatalytic reactions the principle of the membrane reactor (represented schematically on the right) is applicable to the continuous asymmetric addition of diethylzinc to benzaldehyde. α,α-Diphenyl-L-prolinol is coupled to a homogeneously soluble polymer (1), which enables it to be retained in the reaction vessel by an ultrafiltration membrane. By decoupling the residence time of reactants and the catalyst the total turnover number could be increased by a factor of 10.

Ammonium and borate substituents polarize π-electron systems, which then exhibit internal CT bands in the UV/Vis spectrum. These systems (for example 1) are distinctly more transparent than analogous push–pull-substituted compounds. Since their first-order hyperpolarizabilities are nevertheless similar to push–pull compounds, they are better suited to applications for second harmonic generation.

The 3′-phosphonate building block 1 of a platinated nucleoside has been synthesized for the first time. A platinated nucleotide was then incorporated with specificity into an oligonucleotide by using standard procedures of automated DNA synthesis. This method should make it possible to prepare defined platinated oligonucleotides, and should facilitate studies of the mechanisms by which platinum complexes are biologically active.

A “charm bracelet” is a graphic description of the polymer prepared from cyclopentadithiophene with fullerene pendants. The interaction of the readily reducible fullerene moieties with the readily oxidizable polythiophene backbone makes 1 an interesting new material. Cyclic voltammetric studies have shown that the C₆₀ substituents and the polythiophene chain retain their unique electrochemical properties in 1.

Diversomer libraries consisting of substituted cyclopentanes and cyclohexanes 3 were produced by Knoevenagel reaction of polymer-bound malonate 1 and aldehydes 2, subsequent ene reaction, and cleavage from the support. In this domino solid-phase procedure the simple and induced diastereoselectivities of the ene reaction are generally very satisfactory (>99:1). R = H, Me, nBu, Ph.

An intact organonitrogen bridge is formed when ethyl diazoacetate or phenyl azide add across a ZrIr bond (for example, 1; Cp = C₅H₅, Cp^* = C₅Me₅). The ethyl diazoacetate complex is thermally stable, but heating the phenyl azide complex 1 leads to first-order loss of N₂ and formation of the bridging phenylimido complex.

The body temperature inside a living organism can be determined by measuring the ¹HNMR chemical shifts of the paramagnetic praseodymium chelate complex 1. The complex is very stable and well-tolerated. In a first in vivo application the temperature inside the liver of a living rat was determined with an accuracy better than 0.5 K.

Strong competitive complex formation by H₂O is one of the significant results in this NMR spectroscopic study on the reactive intermediates in the Heck reaction of methyl acrylate. Compounds 1–3 were identified unambiguously as key intermediates with the aid of ¹³C-labeled methyl acrylate (• indicates ¹³C atom).

Deselenation of the sterically congested cyclic polyselenide TbtCHSe_n (n = 5.1) provided new access to selenobenzaldehyde 1a, which is stable in solution but on concentration dimerizes to give the head-to-tail dimer 2. Thermolysis of 2 (45°) resulted in formation of 1a and its rotational isomer 1b. Pentacarbonyltungsten complexes of both isomers were synthesized and characterized.

Electronic and steric factors influence the enantioselectivity of the formation of pyrrolidines 1 in an asymmetric variant of the well-known Pd-catalyzed cycloiso-merization of 1,6-enynes to give 1,4-dienes [Eq. (a)]. First hints to the importance of these factors were obtained by varying the R group in the enyne and the substituents on the P-aryl groups in the chiral ligand bis[(diarylphosphanyl)ethyl]biferrocene (TRAP^*). R = Si(alkyl)₃, alkyl.