In spite of relatively few examples, two-center catalysis is proving to be a clever and effective means of performing diverse reactions with both high regio- and enantioselectivity. A simple scheme allows classification of the efficient M¹–M²–binol complexes 1 as well as the enzymes urease and carbon monoxide dehydrogenase. M¹ = La, Al,…; M² = Li, Na, K,….

Unactivated gold powder readily reacts with Me₃EI₂ (E  P, As) in diethyl ether to form the planar complex [AuI₃(AsMe₃)] and, surprisingly, the trigonal-bipyramidal complex [AuI₃-(PMe₃)₂] (structure shown on the right); the latter readily hydrolyzes to form [(Me₃PO)₂H][AuI₂]. The oxidation of this noble metal to Au³⁺ and the production of complexes of different stoichiometries and structures illustrates the oxidizing power and the subtlety of these Me₃EI₂ reagents.

Activities twice as good as those of sialyl Lewis^x are displayed by sialyl Lewis^x mimetics such as 1 which are made up of an amino acid, a galactose amine, and a carboxylate-bearing side chain.

Two enzymes are used for the efficient synthesis of 6′-oxo-N-acetyl-D-lactosamine. GalβOpNP, with the good p-nitrophenyl (pNP) leaving group, is first oxidized to the 6-oxo derivative with galactose oxidase. Subsequent transglycosylation with GlcNAcβOMe, catalyzed by β-galactosidase from Bacillus circulans, leads to the desired disaccharide in good yield [Eq. (a)].

A partial structure of many glycoproteins, the complex undecasaccharide–asparagine conjugate 1 was obtained for the first time by total synthesis. Crucial to the synthesis was a combination of modern chemical and enzymatic methods, which reduced the overall number of steps and allowed efficient deprotonation.

An excellent functionalized enzyme model for the active site of methane monooxygenase and ribonucleotide reductase is represented by the μ-ace-tato- and μ-oxo-bridged diiron complex 1. Reaction with H₂O₂ or O₂ in the presence of ascorbate leads to hydroxylation of one phenyl ring to a phenol. This then coordindates to the metal center with formation of a mononuclear complex.

Trigonal-planar coordination is seen for the indium center in In(InTrip₂)₃ (1) (Trip  2,4,6-iPr₃C₆H₂), the first indane with three unusually short bonds to indium substituents. The compound is obtained by the reduction of the diindane Trip₂InInTrip₂ with lithium metal. In contrast, reduction of the aluminum and gallium analogs gives the [Trip₂MMTrip₂]⁻ anions, which have a MM π-bond order of 0.5.

Tetrathiocin 1 and the tetramer 2 could be synthesized oxidatively from ethenedithiolate and were successfully characterized. Compound 2 with its cage structure can be considered as a new type of host molecule. It also forms from 1, which has a twisted structure, in acetonitrile/chloroform.

Pd^II/Pd^IV or Pd^II/Pd⁰? The unusual Pd^II catalyst 1 was reduced to a Pd⁰ complex by reaction with an amine and a base. β-Hydrogen elimination and subsequent CH bond-forming reductive elmination generate a coordinated tris(o-tolyl)phosphane. The same palladium(II) metallacycle was reduced to Pd⁰ in a cross-coupling reaction by CC bond-forming reductive elimination to form coordinated P(oTol)₂-(C₆H₄CH₂Ar). Thus, these compounds can react by a Pd^II/Pd⁰-containing catalytic cycle.

The platinium complex 1 is the first homogeneous catalyst that can be used in reactions that proceed analogously to the Claus process. The novel intermediate 2, which contains a 2-oxotrisulfido chelating ligand, catalyzes the transformation of toxic H₂S into sulfur.

Unique reactivity is shown by the fluorotitanium complex obtained from TiF₄ and 1,1′-bi-2-naphthol. It is more reactive than the chloro-, bromo-, and alkoxytitanium complexes previously employed and catalyzes the enantioselective additions of allyltrimethylsilane to aldehydes [Eq. (a)], providing an alternative to known methods involving the corresponding organotin reagents.

Not one, but two structures are adopted by the metalation product of 1-triorganosilyl-2-butyne in solution. An equilibrium between the allenyllithium (1) and propargyllithium (2) reagents is rapidly established and has a free energy of activation ΔG^* of less than 4 kcal mol⁻¹ at −150°C.

The first C₁₂H₁₂ hydrocarbon with a central tricyclic skeleton and four double bonds (1) has been synthesized. In the most successful approach 2, obtained from the corresponding ketone by treatment with MeLi, was transformed into the tetraene 1 by elimination of water under mild conditions with (COCl)₂/DMSO/NEt₃ (the Swern conditions).

Ring closure to the [15]crown-5 derivative is prevented in the reaction of an enediolate with tetraethyleneglycol dimesylate by participation of an acetal side chain (reaction intermediate depicted on the right). The attack of a second enediolate initiates the formation of the corresponding [30]crown-10 derivative in up to 50% yield.

Efficient enantiomeric resolution of the antiallergenic agent (±)-ST, which shows crystalline polymorphism, is accomplished by simple recrystallization. Both stable crystals of the racemic compound and metastable mixed crystals are formed, and the enantiomeric enrichment of the mother liquor virtually reaches 100% ee.

Monodentate phosphorus amidites like 1 and Cu(OTf)₂ provide access to the first chiral complexes that catalyze the enantioselective conjugate addition of organozinc reagents to cyclic as well as acyclic enones. The ligand-accelerated process affords β-substituted ketones in excellent yields and with high ee values. Functional groups in the zinc reagent are tolerated.

The catalyst decides whether the syn or anti product is formed! In the cyclization of 1, the ring-closing metathesis of two double bonds that are located at a chiral and a prochiral center leads to the α,α′-disubstituted pyrrolidine derivative 2 with viable diastereoselectivity. Tfa = trifluoroacetyl, [Ru] and [Mo] = Ru- and Mo-carbene complexes, respectively.

Compounds with the structural element -CH₂NR₂ are easily accessible from (phenylthiomethyl)amines by reductive CS bond cleavage and subsequent addition or substitution (see below). One advantage of this shorter method than tin–lithium exchange is the ready availability of the starting materials, which need not be purified by chromatography.

Endohedral fullerene compounds with isomeric carbon frameworks have been identified for the first time. The three stable isomers of Tm²⁺@C were separated by HPLC, and characterized by cyclic voltammetry and by UV/Vis/NIR and ¹³C NMR spectroscopy as probably having C₂, C_s, and C_3v symmetry (see picture).

Kekulene is indeed a “normal” aromatic molecule. This conclusion is reached in a theoretical study in which a number of calculated properties of kekulene and reference compounds such as benzene, phenan-threne, and 1,2:7,8-dibenzanthracene were compared. Structure 1 is the most apt description of this molecule, and claims of superaromaticity can finally be laid to rest.

Simple chromatography using SiO₂ impregnated with (S)-4-chlorophenyl-carbamoyllactic acid ((S)-2) can be used to separate the enantiomers of receptor 1 (R_f((R)-1/R_f((S)-1) = 0.8/0.1)). The very large association constant of the more stable complex [(R)-1·(S)-2] (5.7 × 10⁵ M⁻¹) was determined in a series of competitive NMR titrations with α-amino acid derivatives, which form less stable complexes with 1.

The asymmetric oxa-Michael addition of (-)-N-formylnorephedrine, a chiral oxygen nucleophile with a removable auxiliary, to (E)-nitroalkenes 1 permits the enantioselective synthesis of protected vicinal amino alcohols (R)-3 with high enantiomeric excesses after reduction of the 1,4-adducts 2. This new procedure opens an efficient entry to synthetic building blocks for natural products and drugs.

Stable and interconvertible are the NiFe compounds 1 and 2 in which the cis-dithiolato sulfur atoms bind to the Fe⁰–CO and Fe^II–CO complex fragments, respectively. Unlike the Ni – S distances, which are essentially independent of the oxidation state of the iron center and the denticity of the metallothiolato ligand, the NiFe distances differ by 0.7 Å.

Organocobalt(III) chelates with tridentate Schiff bases, in particular 1, readily generate free radicals in mildly acidic (pH 5 – 7) solutions. These compounds display certain antitumor action and substantially enhance the efficacy of chemo-, radio-, and thermo-therapy in cancer treatment.

The strong pyramidalization of the radical intermediates (shown on the right) is the reason why anti Cram chelation products are obtained in the reduction of 1,2-dialkyl-1,2-dioxy-substituted radicals, even though a chelate is formed. The main diastereoisomer results from an attack syn to the large substituent R². R¹  CH₂CHCHCH₂SnBu₃, R₂  tBu, R  para-methoxyphenyl.

The same mechanism as taxol but a completely different, simpler structure is displayed by the recently described epothilones A and B. These features immediately attracted the attention of synthetic chemists. An approach to the macrocyclic framework of the epothilones has now been devised through use of ring-closing metathesis with the Grubbs catalyst [RuCl₂(CHPh){P(C₆H₁₁)₃}₂].