A fascinating episode in the history of organic chemistry comes alive through examination of Adolf von Baeyer's work. Not only the ring-strain theory, which is now one hundred years old, but also numerous discoveries concerning indigo, benzene, and many other classes of substances are linked with Baeyer's name. The picture on the right shows the scientist at 72 years of age.

The ring-strain theory, which Adolf von Baeyer formulated one hundred years ago, has been expanded in many directions; today, strain is discussed in terms of bond-length and bond-angle distortions as well as nonbonding interactions. Only in such terms can the stability of such highly strained compounds as tetra-tert-butyltetrahedrane and [1.1.1]propellane be understood.

The title compound 1 is a record olefin in more ways than one. Not only the four α-but also the two β-spirocyclopropane groups exert a donor effect on the double bond. The π-ionization potential of 7.3 eV and the oxidation potential of 0.74 V are the lowest values observed so far for monoolefins without heteroatom donor substituents.

Large, substrate-selective cavities are present in the new, water-soluble macrobicyclic compounds 1, X, Y  N, and 2, X  N, Y  C-CH₃. Because the triphenylamine nitrogen can be oxidized electrochemically to the radical cation, these host compounds may be electrochemically modified (“switched on and off”). Compounds such as 1 and 2 are also of interest as models for redox enzymes.

The Mo₂P₂ bicyclobutane complex 2 is formed by diphosphene transfer from the nickel complex 1 to [{C₅H₅)Mo(CO)₃}₂]. In the educt, the (PhP)₂ ligand is trans, in the product cis. The PP bond length in 2 is 213.6 pm, the MoMo bond length 418.7 pm.

Nitromethane has great advantages as a C₁ building block in anthracyclinone synthesis. The 9-OH group is introduced simultaneously with C-10 (R⁴  OH).

The number (n) of double bonds in a cumulene system has a very strong effect on the structure. For cumulenes with n = 3 and 5, both the alternation in the bond lengths and the substituent effects are larger than for allenes (n = 2) and cumulenes with n = 4. The structure analyses of 1a and 1b have now made possible the direct comparison of variously substituted pentatetraenes.

The structure elucidation of the disaccharide-serine derivative 1 demonstrates how the two-dimensional NMR spectroscopic COLOC technique may be used to assign heteronuclear long-range couplings and thus answer the following questions: (1) How are the sugar residues linked? (2) Is the sugar indeed linked to the amino acid through a urethane unit? (3) At which positions are the various protecting groups?

The “aromaticity” of phenol is manifested by an increase in the enolization constant and the CH acidity constant of 2,4-cyclohexadienone by more than 20 orders of magnitude compared with the corresponding constants of acetone. The first experimental estimate of these values was achieved by the combination of kinetic measurements with flash photolysis and ¹H-NMR spectroscopy.

Both enantiomers of the title compound (R,S)-1 are obtainable by crystallization with (S)-()-mandelic acid. (R)- and (S)-1 can be benzoylated on N-1; the derivatives so obtained may be used to prepare branched and nonbranched, proteogenous and nonproteogenous amino acids.

A remarkable pressure dependence is exhibited by the reaction of 11-methylene-1,6-methano[10]annulene with dicyanoacetylene. At normal pressure, one obtains the 1:1 adduct 1a/1b, a fluxional system that is the first example of its kind of a methylenenorcaradiene/heptafulvene valence tautomerism. At 7000 bar, the main product formed is the 2:1 adduct 2. Despite the less favorable geometry, 2 must have been formed by [8+2] cycloaddition of dicyanoacetylene to 1b.

A one-pot reaction afforded the title compound 1, R  Ph, in 60% yield via lithiation of 1-diphenylphosphinopropyne with n BuLi followed by reaction with ClPPh₂. The colorless, crystalline allene 1 is very reactive: for example, it reacts with oxygen, sulfur, and selenium to give the corresponding chalcogenides 2a-c.

That hyperconjugation can result in extreme stabilization is confirmed by the first X-ray structure analysis of an aliphatic carbocation. Until now, only carbocations stabilized by π-electron systems or heteroatoms had been studied. The differences in bond lengths in the 3,5,7-trimethyladamantyl cation 1 (0.18(2) Å) are greater than those in compounds exhibiting the largest anomeric effects. 1′ is therefore the best description of the structure of the cation.

Upon rapid thermal decomposition of GeI₂—an important reaction for the chemical transport of germanium—the metastable subiodide Ge_4.06I, a semiconductor with a clathrate structure, is formed. The clathrate framework is made up of Ge and I³⁺, and I⁻ ions are present in the cavities: [Ge_46-8/3I_8/3]I₈·Ge_4.06I forms dendrite-like intergrown cubes, as shown in the picture on the right.

A serendipitous result was obtained when too much rubidium was used in the synthesis of Rb₃As₇ in a niobium tube: the surprising product is a complex containing the anion [NbAs₈]³⁻ 1. This is the first example of an octacycle acting as an η⁸-ligand to coordinate a metal atom at its center. With Rb⁺, one-dimensional chains [Rb{NbAs₈}]²⁻ are formed, even though the strong complex formers 2,2,2-crypt and ethylenediamine were present during the crystallization.

The title reaction, a transformation that theoretically should be difficult, has been successfully carried out. The exploitation of two “β effects” was crucial: β-silicon stabilizes the intermediate allyl cation and two geminal methyl groups in the β position control the course of the reaction. An example is the transformation of 1 into 2, which is converted into 3 under acid catalysis.

The coordination number 5, two MnC σ-bonds and two hard donor ligands characterize the organomanganese(II) complex 1. It is accessible by reaction of [MnCl₂(thf)₂] with phenylthiomethyllithium and is stable at room temperature (X-ray structure analysis). Compound 1 reacts like a typical Grignard reagent with benzaldehyde.

Binuclear WFe complexes containing a phosphaallyl bridge can be synthesized from W(CO)₅-stabilized vinylphosphanes such as 1 (R  H) by reaction with the Fe₂ complex 2. In the product 3, the bridging ligand functions simultaneously as a 2e and 3e donor. The structure was elucidated by X-ray diffraction analysis.

A metallacycle that is both an alkyl and a carbene complex is the active catalyst in the metathesis of cyclopentene and 1-octene with [Cl₃(dme)WCCMe₃] (dme = dimethoxyethane). Poly-1-pentenylene (70%) is formed from cyclopentene in 3 h at 20°C; complexes such as 1 are assumed to be key intermediates. These results explain, in particular, the steric course of metathesis reactions.

A milestone for the chemistry of phosphorus-containing complexes is the synthesis of the black-red cyclo-P₅ complex 1, which is metallic in appearance. In light of the overwhelming significance of the cyclopentadienyl ligand in organometallic chemistry, the isovalent isoelectronic cyclo-P₅ would appear to hold much promise. ESR investigations indicate that 1 is a mixed valence complex (d⁴/d⁵ system), which can be readily oxidized and reduced.

The hafnacyclobutane 1 takes up 1.5 equivalents of CO very rapidly even under normal conditions, forming the polycyclic complex 4 as sole product in good yield. 4 is a stable 1:1 adduct of [(η²-ccyclobutanone)HfCp₂] 2 and the hafnocene enediolate complex 3, the formation of which involves the coupling of two CO molecules——possibly via the intermediacy of the η²-ketone complex 2.

Coenzyme A analogues with CH₂ instead of S are model compounds that may be used to gain insight into the mechanism of action of methylmalonyl-CoA mutase. Propionylcarba(dethia)coenzyme A 1 was synthesized and enzymatically carboxylated to the methylmalonate derivative 2. The coenzyme B₁₂-dependent enzymatic rearrangement of the β-keto acid 2 to the γ-keto acid 3 is the first of its kind to be described.

The [24]annulene 1 containing four CH₂ bridges is surprisingly formed in the twostep organometallic title reaction. The key intermediate is a bismethano-bridged nickelacyclotridecahexaene with two PMe₃ ligands. Upon treatment of this complex with PMe₃, the final product 1 is formed by reductive elimination.

Structural elements of sugars and fatty alcohols are present in the title compounds, which form thermotropic liquid crystals (for n≥6). This is substantiated by two series with L-ido and D-gluco configuration. The value of n was varied from 5 to 17.

The smallest isoiable paracyclophane, the title compound 1, was obtained by thermal valence isomerization of the Dewar benzene isomer 2. The ¹H-NMR and, in particular, the UV spectra indicate that the benzene ring in 1 deviates much more strongly from planarity than in [6]paracyclophane.

Four centers of chirality—2 × C, 1 × N, 1 × Mo—are present in the title compound 1, R^* = (R)-1-CHMePh. Complex 1 is characterized, among other things, by a slightly bent CO group, which is in close contact to Rh. A complicated series of ring openings and epimerizations is suggested as a plausible route to 1.

A “ketene iminate” and not a carbanion is present in the title compound 1·C₆H₆. The bond lengths in the CCN group deviate considerably from those in ketene imines such as 2. In comparison, enolates have practically the same bond lengths as enol ethers. This difference is due to the different stabilization of a negative charge by nitriles and carbonyl compounds.

The spontaneous crystallization of the enantiomers of the title compound 1 (space group P2₁) has been observed. The X-ray structure analysis reveals a pyramidal arrangement of the substituents on the nitrogen atoms. The large barrier to racemization (27.1 kcal mol⁻¹) is largely due to the gauche effect of the vicinal lone pairs of electrons.

A quantitative separation of calcium and strontium is made possible by the reaction of M(OtBu)₂ (M  Sr, Ca) with [Sn(OtBu)₂]₂ in benzene (!). The strontium salt dissolves quantitatively; the calcium salt is insoluble. In the process, the polycycle shown on the right is formed; it is even soluble in hexane and its structure is reminiscent of that of SrO.

The retrocleavage of the adducts 1–5 proceeds according to a synchronous mechanism. The resonance energy gain in the products is 11 to 40 kcal/mol; in each case, 40% of this gain is reflected in a reduction of the energy of the transition state.