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An intriguing advance in the rational protein engineering of a newly identified glycosyltransferase from Siraitia grosvenorii (UGT74AC2) has been accomplished by Zhoutong Sun, Yuanxia Sun and coworkers for the targeted pharmaceutically significant glucosylation of the polyhydroxy substrate silybin. Using an advanced version of Focused Rational Iterative Site-specific Mutagenesis (FRISM), they were able to evolve mutants that allow the formation of the 3-OH, 7-OH and the 3,7-O-diglycoside products with 94%, > 99% and > 99% regioselectivity, respectively. This can be viewed as a breakthrough because the wildtype UGT74AC2 delivers a difficult to separate mixture of products in a ratio of 22%:39%:39%. Even N- and S-glucosylations were achieved, a likewise unusual accomplishment.

Directed evolution of an enzyme as catalyst for a given stereoselective transformation provides a mutant for that particular reaction, but organic chemists need catalysts that are characterized by broad substrate acceptance. In a previous study we succeeded in evolving a set of variants of the thermally robust alcohol dehydrogenase TbSADH from Thermoanaerobacter brockii as a catalysts in the ( R )- and ( S )-selective reduction of tetrahydrofuran-3-one, this difficult-to-reduce compound being a sterically small substrate. These mutants were now tested in the asymmetric reduction of seven structurally unrelated and sterically more demanding substrates, including acetophenone, benzyl methyl ketone, 4-phenyl-2-butanone, and 2-oxo-4-phenyl-butanoic acid ethyl ester. The variants clearly out-perform WT TbSADH, but overly bulky substituted benzophenone derivatives are not accepted by WT or mutants.

The achiral cyclohexene derivative dimethyl cis-1,2,3,6-tetrahydrophthalate has been subjected to oxidation catalyzed by cytochrome P450 monooxygenase P450-BM3, leading to diastereoselective epoxidation rather than oxidative hydroxylation. This reaction occurs with 94% diastereoselectivity in favor of the anti-epoxide, in contrast to m-CPBA which delivers unselectively a 70:30 mixture of anti/syn diastereomers. The experimental results are nicely explained on a molecular level by docking experiments and molecular dynamics computations.

The selective partial oxidation of small non-functionalized molecules using biocatalysis based on P450 monooxygenases is known to be difficult due to the expected poor regio- and stereoselectivity, but in this study it was nevertheless attempted. 1-Methylcyclohexene was subjected to oxygen-mediated biocatalytic oxidation using P450-BM3 as the catalyst. Both oxidative hydroxylation and epoxidation were observed, in some cases leading to hydroxy epoxides with high diastereo- and enantioselectivity especially when employing BM3 mutants.

Cold-adapted Pseudomonas aeruginosa LipC is a secreted lipase showing differential properties compared to its well-known counterpart LipA. LipC is fundamentally a cold-acting lipase, capable of tolerating high concentrations of ions and heavy metals, and showing a shift in substrate specificity when incubated at higher temperatures. These properties make LipC an interesting enzyme, well suited for biotechnological or environmental applications, where activity at low temperatures would be required. However, a relatively low thermal resistance constitutes the main drawback for using this enzyme in long-term operational processes. To overcome the lability of LipC, we developed a rational design system to modify specific sites on the enzyme structure to obtain an improved variant of the lipase bearing higher thermal stability, but without loss of its cold-adapted properties. Eight mutant libraries plus two point mutations were constructed affecting those amino acids showing the highest flexibility on the 3D model structure. After screening more than 3000 mutant clones, a LipC variant bearing two amino acid changes and the required thermostability and cold-adapted properties was obtained. The new variant D2_H8, with a 7-fold increased thermal stability in comparison to wild type LipC, will guarantee the use and maintenance of such a lipase in a number of processes being performed at low (4–20 °C) temperatures.

This is a personal account of the author's research in the area of directed evolution of the past 15 years. It focuses on a novel concept first reported in 1997, which has proven to be useful in the development of catalysts for asymmetric transformations in synthetic organic chemistry. When performing such reactions, organic chemists can choose between synthetic transition metal catalysts, organocatalysts or enzymes. However, the latter suffer from traditional disadvantages, such as the often observed insufficient stereoselectivity, limited substrate scope and/or rate. The concept of laboratory evolution of stereoselective mutant enzymes eliminates these long-standing limitations and therefore extends the toolbox of synthetic organic chemistry.

Directed evolution constitutes an ideal method for engineering essentially any catalytic parameter of enzymes for application in synthetic organic chemistry and biotechnology, including thermostability, substrate scope and enantioselectivity. Stereoselectivity is especially important when applying biocatalysts to synthetic organic chemistry. This article focuses on recent methodology developments in laboratory evolution of stereoselective enzymes, hydrolases and monooxygenases serving as the enzymes. Iterative saturation mutagenesis (ISM) has been developed as an unusually effective method to evolve enhanced or reversed enantioselectivity, broader substrate scope and/or higher thermostability of enzymes.

The concept of directed evolution of enantioselective hybrid catalysts was proposed in 2001/2002 and implemented experimentally for the first time in a proof-of-concept study in 2006. The idea is based on directed evolution, which comprises repeating cycles of random gene mutagenesis/expression/screening in a Darwinistic sense for the purpose of improving the catalytic profile of enzymes. In the case of hybrid catalysts, mutagenesis/expression of a protein is first performed with formation of a library of mutants, which are then modified chemically en masse with the introduction of an appropriate achiral ligand system harboring a transition metal. Screening these mutant hybrid catalysts in a given transition metal-catalyzed reaction then leads to an improved catalyst, so that the corresponding gene can be used to start another evolutionary cycle. This process can be repeated as often as needed until the desired catalytic profile has been reached, e.g., enhanced enantioselectivity.

Two new copper(II) and cobalt(II) coordination polymers, {[Cu(Pht)(Im) 2 ] · 1.5H 2 O} n ( 1 ) and [Co(Pht)(Im) 2 ] n ( 2 ) (where Pht 2− = dianion of o -phthalic acid; Im = imidazole) have been synthesized and their crystal structures determined by X-ray crystallography. The structures of 1 and 2 comprise, respectively, [Cu(Im) 2 ] and [Co(Im) 2 ] units linked by bridging 1,6-phthalate ions to form infinite chains. Although there are two independent Cu atoms in the structure of 1 , each copper(II) center adopts an approximately square planar coordination mode with two O atoms from different phthalate ions and two N atoms of Im. In complex 2 , cobalt(II) atoms are tetrahedrally coordinated. Extended hydrogen bonding networks stabilize the three-dimensional structures in both complexes. Preliminary results show that 1 and 2 exhibit catalytic activity on the oxidation of cyclohexene using hydrogen peroxide as oxidant under aerobic conditions at ambient temperature.

The previously proposed concept of using mixtures of two different chiral monodentate P-ligands has been extended to the asymmetric Rh-catalyzed hydrogenation of β-acylamino acrylates with formation of chiral β-amino acid derivatives. Mixtures of BINOL-derived phosphites and phosphonites are particularly effective (ee=94–99% for five different substrates).

Isotherms of Pd–H clusters with different sizes differ from bulk isotherms: they show a largely enhanced solubility in the low-concentration regime. The isotherms resemble those of bulk above the critical point, they show no flat plateau region, but a slope. However, the existence of a hysteresis gives evidence for a phase transition even in small Pd–H clusters. Structural studies of 6.0 nm clusters show a transition between two cubic phases. 3.8 nm Pd–H clusters always show an icosahedral structure in the low and high concentration regime. For an intermediate size of 5.0 nm Pd–H clusters, the lattice structure changes during H-absorption, from cubic to, most probably, icosahedral. The slope in the cluster isotherm’s two-phase region is here attributed to stress emerging between the surfactant shell and the cluster during hydrogen-loading.

The recently described method of combinatorial asymmetric transition metal catalysis based on the use of mixtures of chiral monodentate P-ligands has been extended to include mixtures of chiral and achiral monodentate P-ligands, reversal of enantioselectivity in Rh-catalyzed olefin hydrogenation being possible in appropriate cases.

Phage display can be used as a protein engineering tool to select proteins with desirable binding properties from a library of randomly constructed mutants. Here, we describe the development of this method for the directed evolution of Bacillus subtilis lipase A, an enzyme that has marked properties for the preparation of pharmaceutically relevant chiral compounds. The lipase gene was cloned upstream of the phage g3p encoding sequence and downstream of a modified g3p signal sequence. Consequently, the enzyme was displayed at the surface of bacteriophage fd as a fusion to its minor coat protein g3p. The phage-bound lipase was correctly folded and fully enzymatically active as determined from the hydrolysis of p-nitrophenylcaprylate with K(m)-values of 0.38 and 0.33 mM for the phage displayed and soluble lipase, respectively. Both soluble lipase and lipase expressed on bacteriophages reacted covalently with a phosphonate suicide inhibitor. The phage does not hamper lipase binding, since both soluble and phage-bound lipase have a similar half-life of inactivation of approximately 5 min. Therefore, we conclude that the Bacillus lipase can be functionally expressed on bacteriophages as a fusion to the phage coat protein g3p. The specific interaction with the suicide inhibitor offers a fast and reproducible method for the future selection of mutant enzymes with an enantioselectivity towards new substrates.

Deprotonation of phenol by tetra-n-butylammonium hydroxide and subsequent removal of water by P2O5 results in quantitative formation of tetra-n-butylammonium phenol-phenolate. The compound crystallizes as an ion pair, with the solute phenol molecule intimately attached to the phenolate anion by an O–H⋯O− hydrogen bond (O⋯O 2.471(5) A). A short contact between one α-C atom of the tetra-n-butylammonium cation and the phenolate O atom indicates that the ion pair interaction is strengthened by an additional C–H⋯O− hydrogen-bond, despite the attached phenol.

The methods of directed evolution, developed in the 1990s, can be applied successfully to the creation of enantioselective enzymes for use in synthetic organic chemistry. The combination of appropriate molecular biological methods for random mutagenesis and expression coupled with high-throughput screening systems for the determination of ee-values forms the basis of this novel approach to asymmetric catalysis. The principle is illustrated by the dramatic enhancement of enantioselectivity of a lipase as the catalyst in the hydrolytic kinetic resolution of a chiral ester, the selectivity factor improving from E=1.1 to E=51. Reversal of enantioselectivity is also possible. Finally, the concept of directed evolution of selective hybrid catalysts has been delineated.

Lipases are the most used enzymes in synthetic organic chemistry, catalyzing the hydrolysis of carboxylic acid esters in aqueous medium or the reverse reaction in organic solvents. Recent methodological advancements regarding practical factors affecting lipase activity and enantioselectivity are reviewed. Select practical examples concerning the use of lipases in the production of chiral intermediates are also highlighted.

Starting from commercially available ( S )-1-(2-bromophenyl)ethanol, novel phosphite/phosphonite ligands have been prepared in which a BINOL moiety is attached to each phosphorus centre. The four diasteromeric ligands were tested in Cu-catalysed conjugate addition of Et 2 Zn, Pd-catalysed allylic substitution and Rh-catalysed hydrogenation. Very different results regarding activity and enantioselectivity as a function of the particular diastereomer were observed.

Current efficiencies of CO 2 during methanol oxidation were determined by differential electrochemical mass spectrometry (DEMS). The influences of methanol concentration, electrode structure and ruthenium on methanol oxidation have been investigated. CO 2 is formed via CO ad oxidation in a serial reaction, while dissolved intermediates, i.e. formaldehyde and formic acid are also formed in a parallel reaction. The chance that these intermediates can further be oxidised depends on the diffusion and convection conditions in connection with the surface structure of the Pt electrode. High methanol concentrations favour the reaction path via dissolved intermediates. Ru promotes methanol oxidation to CO 2 via CO ad , leading to a higher current efficiency of CO 2 .

By combining two gas chromatography instruments (each containing a column with a chiral stationary phase) with the proper robotics and software, it is possible to construct an instrumental configuration which makes possible high-throughput screening of the enantioselectivity of a given catalytic reaction. As an example, the acylation-based catalytic kinetic resolution of racemic 2-phenyl-1-propanol catalyzed by mutant lipases can be about assayed, 700 exact E- and ee-determinations being possible per day. The method is, therefore, of interest in the directed evolution of enantioselective enzymes and/or in the combinatorial search for asymmetric transition metal catalysts.

The helical diol 2,15-dihydroxy-hexahelicene (HELIXOL) is an excellent fluorescence sensor for chiral amines and amino alcohols. The fluorescence of HELIXOL can be efficiently as well as enantioselectively quenched by the enantiomers of several chiral amines and amino alcohols. Its high fluorescence intensity and its ability to enantiodiscriminate effectively makes it potentially useful for high-throughput screening of ee.

Background : The biocatalytic production of enantiopure compounds is of steadily increasing importance to the chemical and biotechnological industry. In most cases, however, it is impossible to identify an enzyme that possesses the desired enantioselectivity. Therefore, there is a strong need to create by molecular biological methods novel enzymes which display high enantioselectivity. Results : A bacterial lipase from Pseudomonas aeruginosa (PAL) was evolved to catalyze with high enantioselectivity the hydrolysis of the chiral model substrate 2-methyldecanoic acid p -nitrophenyl ester. Successive rounds of random mutagenesis by ep-PCR and saturation mutagenesis resulted in an increase in enantioselectivity from E =1.1 for the wild-type enzyme to E =25.8 for the best variant which carried five amino acid substitutions. The recently solved three-dimensional structure of PAL allowed us to analyze the structural consequences of these substitutions. Conclusions : A highly enantioselective lipase was created by increasing the flexibility of distinct loops of the enzyme. Our results demonstrate that enantioselective enzymes can be created by directed evolution, thereby opening up a large area of novel applications in biotechnology.

Surprisingly high enantioselectivities in the Rh-catalyzed hydrogenation of itaconic acid dimethyl ester and methyl-2-acetamido acrylate are observed upon using chiral monophosphonite ligands derived from binaphthol (ee up to 94%). Although appropriate chelating diphosphonites are more effective, the easy modular synthesis of the chiral monophosphonites may allow for the optimization of a given asymmetric hydrogenation reaction.

The chiral helical diphosphine 2,15-bis(diphenylphosphino)-hexahelicene (PHelix) is an excellent ligand in palladium catalyzed kinetic resolution involving allylic substitution. Depending upon the substrate, enantioselectivities (ee) of up to 99% are observed. Although formally a diphosphine, PHelix appears not to undergo chelation, which means that it behaves as a chiral monophosphine.

The production of enantiopure compounds is of steadily increasing importance to the chemical and biotechnological industries. In principal, the application of directed evolution in combination with newly developed screening methods enables the generation of enzymes with improved enantioselectivity. The first and most advanced example relates to a bacterial lipase from Pseudomonas aeruginosa. This enzyme was evolved towards a model substrate to yield in a lipase mutant showing > 90% enantiomeric excess as compared to 2% for the wild-type lipase. The creation of enantioselective enzymes by directed evolution will become an important technology in the near future.

Thermodynamic properties of the Pd–H system are studied on 3.1- and 3.6-nm clusters by performing gasvolumetric measurements. To avoid interfacial stress that occurs between an adhering particle and the substrate, the clusters are embedded in an elastic soft polymer matrix and in a surfactant shell, respectively. A largely enhanced H-solubility was found in the α-phase region. Additionally the solubility limit of the α-phase is shifted to a higher H-concentration whereas the minimum α′-phase solubility is lowered compared to that of bulk Pd. This can be attributed to an occupation of subsurface sites. Sorption–desorption isotherms show the existence of a small hysteresis. This hysteresis can not be related to the formation of misfit dislocations because of the small cluster size, but can be explained with regard to the thermodynamics of an open two-phase system with coherent interfaces.

Double lithiation of bis(tetra-hydroindenyl)iron followed by phosphorylation using Ph 2 PCl affords two different chiral diphosphines having respectively C 1 - and C 2 -symmetry. Following antipode separation, the compounds can be used as ligands in Rh-catalyzed hydrogenation (up to 97% ee ) and hydroboration (up to 84% ee ) of olefins and in Ir-catalyzed hydrogenation of imines (up to 79% ee ).

Starting from 1,2-dibromobenzene, the synthesis of N,N,N ′, N ′-tetraethyl-[2-(diphenylphosphino)phenyl]phosphonous acid tetraamide is possible in two simple steps. This key compound reacts with a variety of chiral diols such as ( R )- and ( S )-binaphthol, 1,2:5,6-diisopropylidene- d -mannitol or (1 R ,2 R )-1,2-diphenyl-1,2-ethane diol to form the corresponding non- C 2 -symmetric phosphine-phosphonite compounds. These ligands react with Rh(COD) 2 BF 4 to form bidentate Rh-complexes which serve as catalysts in the asymmetric hydrogenation of dimethyl itaconate with ee values of up to 88%.

N,N -Dibenzylamino ketones of the type Bn 2 N(R 1 )CHC(O)R 2 , prepared in enantiomerically pure form from α-amino acids, undergo non-chelation controlled Grignard-type reactions with RLi, RMgX or RCeCl 2 without any undesired racemization.

Dendrimers are a rather new class of macromolecules, which are by now at the edge of technical application. A number of dendritic catalysts have been described, but in most cases catalyst recyclization is an unsolved problem. The present article describes the retention of diaminopropyl-type dendrimers bearing palladium phosphine complexes by ultra- or nanofiltration membranes and their use as catalysts for the allylic substitution in a continuously operating chemical membrane reactor. Retention rates are higher than 99.9% resulting in a sixfold increase of the total turnover number for the Pd catalyst.

Palladium salts such as Pd(OAc) 2 or PdCl 2 (PhCN) 2 in the presence of N,N -dimethylglycine (DMG) constitute the simplest and one of the most reactive and selective catalyst systems for the Heck reaction of aryl bromides with olefins known to date.

The linear polyether dialdehydes (2-hydroxybenzaldehyde-3-yl)-O-(CH 2 CH 2 O) n -(3-yl-2-hydroxybenzaldehyde), where n = 3 ( 1 ), 4 (2), react with S -methylisothiosemicarbazide hydroiodide (or hydrochloride) in the presence of Ba(CF 3 SO 3 ) 2 and zinc(II) acetate and/or copper(II) acetate in methanol/THF to give [ZnBaL 1 l(CF 3 SO 3 )(CH 3 OH)] 2 ·0.5H 2 O ( 5 ). [CuBaL 1 (CF 3 SO 3 ) 2 ] 2 ( 8 ) and [CuBaL 2 (CF 3 SO 3 ) 2 ] 2 ( 10 ). Treatment of 5 with AgCF 3 SO 3 in methanol and subsequent recrystallization from ethanol results in loss of I − to give [ZnBal 1 (CF 3 SO 3 ) 2 (H 2 O)] 2 ( 6 ). Removal of the Ba 2− from 6,8 and 10 by using an aqueous solution of guanidinium sulfate affords ZnL 1 ·H 2 O ( 7 ), CuL 1 ·2H 2 O ( 9 ) and CuL 2 ·2H 2 O ( 11 ). The molecular structure of 5 has been determined by X-ray crystallography. Crystal data for 5 : monoclinic, P2 1 /c,a=9.331(1), b=15.293(2), c=25.940(2) A , β=97.94(1)°, Z=2 . Electronic and NMR spectra, the magnetic behavior, ESR and mass spectra, and the electrochemistry of the new compounds are discussed.

The synthesis of 2-pyrimidyldiphenylphosphine (2), bis-2-pyrimidylphenyl-phosphine (3) and tris-2-pyrimidylphosphine (4) is described. Ligand 2 forms cationic complexes with palladium(II) in which four-membered P/N-chelates are involved, leading to a very active catalyst for the carbonylation of alkynes.

The simultaneous entrapment of a lipase such as Pseudomonas cepacia or Candida antarctica and nanostructured superparamagnetic magnetite (Fe3O4) in hydrophobic sol–gel materials derived from CH3Si(OCH3)3 (MTMOS) or other hydrophobic precursors leads to catalytically active, mechanically stable and magnetically separable heterogeneous biocatalysts. The relative enzyme activity in the test reaction involving the esterification of lauric acid by n-octanol in isooctane is typically 200–300%, with respect to the same reaction using a conventional suspension of the non-immobilized enzyme. Separation of the catalyst by applying a simple magnet poses no problems. In the kinetic resolution of 2-pentylamine, enantioselectivity is essentially complete (ee=97–99%).

The Morita–Baylis–Hillman reaction of cyclohexenone and p -nitro benzaldehyde is catalyzed by carrier proteins such as serum albumins or enzymes such as certain lipases, conversion of up to 35% and enantioselectivities of up to 19% being observed.

The concept of covalently connecting a catalytically active transition metal center with a water-soluble receptor (host molecule) makes a new type of supramolecular catalysis possible in which the features of molecular recognition, phase transfer catalysis and transition metal catalysis are combined in a single system. The first examples of this principle make use of the commercially available β-cyclodextrin (β-CD) as the receptor and rhodium complexes of diphosphanes as the catalytically active center, these being covalently connected to one another via a spacer. In competitive hydrogenation of certain olefins unusual degrees of substrate selectivity based on the molecular recognition are observed, not possible by conventional transition metal catalysts. The two-phase (H2O/organic) hydrogenation of nitro-aromatics also is a smooth process with these supramolecular catalysts. They also constitute an unusually active catalyst system for the selective hydroformylation of higher olefins such as 1-octene in a two-phase system.

Pseudomonas lipases play an important role in biotechnology both as hydrolases for detergent additives and as synthases catalyzing the kinetic resolution of racemic compounds. Large-scale production of Pseudomonas lipases requires correct folding and secretion through the bacterial membranes. Controllable expression of the gene lipH encoding a lipase-specific foldase proves to be important for overexpression in the homologous host Escherichia coli. Construction of appropriate His-tagged fusion proteins permitted overexpression, secretion and one-step purification of lipase from culture supernatants of the homologous host Pseudomonas aeruginosa. The immobilization of lipases in hydrophobic sol-gel materials derived from alkylsilane precursors of the type RSi(OCH3)3 or mixtures of RSi(OCH3)3 and Si(OCH3)4 provides highly active chemically and thermally stable heterogeneous biocatalysts. The entrapped lipases are excellent catalysts in a variety of synthetic organic transformations. Using directed evolution based on error prone PCR, the enantioselectivity of the hydrolysis of a chiral ester, catalyzed by the lipase from P. aeruginosa, can be increased from ee 2 to ee 81% in just four mutagenesis cycles.

Preformed Ni-clusters stabilized by tetraoctyl- or tetradodecylammonium bromide are readily available by electrochemical fabrication, the average size of the particles being 2.5 nm. Colloidal solutions of such clusters effectively catalyze the [3+2]cycloaddition of methylenecyclopropane to methyl acrylate. The preformed clusters can also be immobilized on solid supports, resulting in heterogeneous catalysts which also catalyze the cycloaddition reaction.

Rhodium-complexes of bis(diarylphosphino)methylamino ligands RN(CH2PPh2)2 are active catalysts in the hydroformylation of 1-octene and 2-vinylnaphthalene, regio-selectivity (n/iso-ratio) and catalyst activity (TOF) depending upon the nature of the parasubstituent in the N-aryl moiety (R = C6H5, p-CF3C6H4, p-NMe2C6H4).

Starting from R -configurated binaphthol the R -form of the title compound was readily prepared in two steps and characterized by X-ray structural analysis. It was used as the chiral ligand in the Ti-catalyzed asymmetric oxidation of methyl-p-tolylsulfide, the S -enantiomeric form of the sulfoxide being formed preferentially (ee = 86%).

Lipase genes originating from the Gram-negative bacteria Serratia marcescens and Pseudomonas aeruginosa were cloned. S. marcescens lipase was overexpressed in Escherichia coli yielding inclusion bodies which were purified and finally refolded to give enzymatically active lipase. The lipase operon of P. aeruginosa consisting of genes lipA and lipH was cloned behind the T7 φ10 promoter and overexpressed in a lipase-negative P. aeruginosa strain carrying a chromosomal insertion of the gene encoding T7 RNA polymerase. A 3D structural model was built for P. aeruginosa lipase using the coordinates of the Burkholderia cepacia lipase structure which has recently been solved in its open conformation by X-ray crystallography. Both lipases have been purified to homogeneity and were tested for their potential to catalyze biotechnologically important reactions. S. marcescens lipase stereoselectively hydrolyzed racemic isopropylideneglycerol acetate which is a basic building block in a variety of organic synthesis reactions. P. aeruginosa lipase was successfully used for kinetic resolution of chiral alcohols and amines giving enantiomeric excess values of ≥ 95% at reaction rates of 40–50%. Our results demonstrate that both lipases can be produced at levels of 100 mg/l for S. marcescens and 150 mg/l for P. aeruginosa . The recombinant lipase proteins are promising candidates for biotechnological applications.

The synthesis of 2,15-bis(diphenylphosphino)-hexahelicene (PHelix) in enantiomerically pure form and its use as a helical ligand for enantioselective rhodiumcatalyzed hydrogenation are described.

Optimal stereoselectivities in the OsO4-catalyzed dihydroxylation of chiral γ-amino α,β-unsaturated esters are achieved by the proper choice of the protective groups at nitrogen [t-butoxycarbonyl (Boc) versus dibenzyl] and the correct choice of the chiral ligand of the catalyst (AD-mix-α versus AD-mix-β).

Soluble palladium clusters and palladium nickel bimetallic clusters stabilized by tetraalkylammonium salts or poly(vinylpyrrolidone) are effective catalysts in Suzuki and Heck reactions involving iodo-, bromo- or activated chloroaromatics, whereas chlorobenzene is not a suitable reaction partner.

Mixtures of BINOL-derived monodentate phosphites and phosphonites have been reacted with Rh-salts to form three (pre)catalysts, which are in equilibrium; two homo-combinations ML a L a and ML b L b as well as the hetero-combination ML a L b . In these cases in which the latter is more active and more enantioselective than the former, enhanced asymmetric induction results in appropriate transition metal catalyzed reactions. This principle has been extended to include mixtures of BINOL-derived monodentate phosphites, phosphonites and phosphines as ligands in the asymmetric Rh-catalyzed hydrogenation of N -acyl enamines leading to ee values of >97%.

Co(acac) 3 catalyzes the aerobic oxidation of vinyl aromatic compounds ArCH=CH 2 with formation of ArCO 2 H and ArCHO, and also the aerobic oxidation of cyclic ethers with formation of the corresponding lactones.

New isothiosemicarbazide-based heteronucleating ligands have been synthesised by Ba2+-templated (1:1) macrocyclisation of the polyetherdialdehydes, 3,3′-(3,6-dioxaoctane-1,8-diyldioxy)bis(2-hydroxybenzaldehyde) and 3,3′-(3,6,9-trioxaundecane-1,11-diyldioxy)bis(2-hydroxybenzaldehyde), by using S-methylisothiosemicarbazide as the diamine building block. Thus, the linear polyether dialdehydes (2-hydroxybenzaldehyde-3-yl)-O-(CH2CH2O)n-(3-yl-2-hydroxybenzaldehyde), with n = 3 and 4, react wit S-methylisothiosemicarbazide in the presence of Ba(CF3SO3)2 and Ni(CH3COO)2·4H2O to give two nickel/barium complexes 5 and 6, the crystal structures of which are described. 5: triclinic, P 1 , a=9.886(3), b=12.376(4), c=13.845(6) A , α=73.97(3), β=88.58(3), γ=83.26(3)°, Z=1 ; 6: triclinic, P 1 , a=10.767(2), b=13.221(2), c=15.986(2) A , α=102.57(1), β=101.13(1), γ=109.91(1)°, Z=2 . Treatment of these complexes with an aqueous solution of guanidinium sulfate yields the respective Ba free metallomacrocycles. The crystal structure of one of these (3·H2O) has been determined. 3·H2O: triclinic, P 1 , a=8.106(3), b=11.836(4), c=13.174(4) A , α=88.22(3), β=76.64(3), γ=70.30(3)°, Z = 2 . Its geometry is compared with its parent compound 5. It reveals that coordination of the Ba2+ cation (0.6 A above the mean plane of the O atoms) has little influence on the conformation of the macrocycle. The polyether ring in the two Ba complexes differs in size yet the Ba…Ni distances are remarkably similar (3.664(5) versus 3.723(3) A) suggesting that the two metal atoms although not bonding may be touching.

Nickel(II) and copper(II) complexes MeL, where H2LL = [9-(2′-hydroxyphenyl)-6-methyl-3-acetyl-5,7,8-triazanona-3,6,8-trien-2-one], have been synthesized by template reaction of salicylaldehyde acetamidrazone with corresponding Me(acac)2 and Hacac in the presence of the orthoformic ester at 110°C. The crystal structure of CuL has been determined by X-ray diffraction. The square-planar mode of coordination is realized in CuL. Comparison of geometrical parameters of CuL with those of the corresponding derivative based on S--allylisothiosemicarbazide showed that substitution of the -SR group by methyl in the quadridentate ligand does not affect the mode of binding nor the main interatomic distances and angles in the ligand. The data from magnetic measurements, 1HH NMR and EI mass spectra indicate that NiL has a similarly structured coordination polyhedron. Epoxidation of norbornene can be performed efficiently with molecular oxygen (1 atm) in THF (or THF-EtOAc) in the presence of CuL at 70°C. The corresponding copper(II) derivatives based on S--substituted isothiosemicarbazides are much less active as catalysts.

Chiral β-N,N-dibenzylamino nitriles Bn 2 NCH(R)CH 2 CN, prepared in enantiomerically pure form from α-amino acids, can be deprotonated and stereoselectively alkylated to afford β-amino nitriles Bn 2 NCH(R)CH(R′)CN with two stereogenic centers. LiAlH 4 -reduction leads to the corresponding 1,3-diamines.