ansa-Metallocene derivatives XXXIII. 2-Dimethylamino-substituted bis-indenyl zirconium dichloride complexes with and without a dimethylsilyl bridge: syntheses, crystal structures and properties in propene polymerization catalysis

Bis(2-N,N-dimethylamino-indenyl) zirconium dichloride, (2-(CH₃)₂N-C₉H₆)₂ZrCl₂, and dimethylsilyl-bridged bis(2-N,N-dimethylamino-indenyl) zirconium dichloride, (CH₃)₂Si(2-(CH₃)₂N-C₉H₅)₂ZrCl₂, were prepared by reaction of the corresponding ligand lithium salts with ZrCl₄ in toluene. Diffractometric structure determinations reveal C₂-symmetric complex geometries for both complexes. An increased electron density at the Zr center of the dimethylamino-substituted complexes is indicated by reduction potentials which are 0.3–0.4 V more negative than those of their unsubstituted analogs. When activated with methyl aluminoxane in toluene solution, (CH₃)₂Si(2-(CH₃)₂N-C₉H₅)₂ZrCl₂ catalyzes the polymerization of propene to polymers with a microstructure comparable with that of polymers produced with other Me₂Si-bridged bis(indenyl)ZrCl₂ complexes, but with a substantially increased fraction of i-propyl end groups derived from alkyl exchange between Zr-polymer and Al---Me species.     

Method validation for detection and quantification of cocoa butter equivalents in cocoa butter and plain chocolate

A European interlaboratory study was conducted to validate an analytical procedure for the detection and quantification of cocoa butter equivalents in cocoa butter and plain chocolate. In principle, the fat obtained from plain chocolate according to the Soxhlet principle is separated by high-resolution capillary gas chromatography into triacylglycerol fractions according to their acyl-C-numbers, and within a given number, also according to unsaturation. The presence of cocoa butter equivalents is detected by linear regression analysis applied to the relative proportions of the 3 main triacylglycerol fractions of the fat analyzed. The amount of the cocoa butter equivalent admixture is estimated by partial least-squares regression analysis applied to the relative proportions of the 5 main triacylglycerols. Cocoa butter equivalent admixtures were detected down to a level of 2% related to the fat phase, corresponding to 0.6% in chocolate (assumed fat content of chocolate, 30%), without false-positive or -negative results. By using a quantification model based on partial least-squares regression analysis, the predicted cocoa butter equivalent amounts were in close agreement with the actual values. The applied model performed well at the level of the statutory limit of 5% cocoa butter equivalent addition to chocolate with a prediction error of 0.6%, assuming a chocolate fat content of 30%.     

Novel beta-lactam antibiotics synthesized by amination of catechols using fungal laccase

Novel cephalosporins, penicillins, and carbacephems were synthesized by amination of catechols with amino-beta-lactams like cefadroxil, amoxicillin, ampicillin and the structurally related carbacephem loracarbef using laccase from Trametes sp. All isolated monoaminated products inhibited the growth of several Gram positive bacterial strains in the agar diffusion assay, among them methicillin-resistant Staphylococcus aureus strains and vancomycin-resistant Enterococci. Observed differences in the cytotoxicity and in vivo activity in a "Staphylococcus-infected, immune suppressed mouse" model are discussed.     

Insights into the Pamamycin Biosynthesis

Pamamycins are macrodiolides of polyketide origin with antibacterial activities. Their biosynthesis has been proposed to utilize succinate as a building block. However, the mechanism of succinate incorporation into a polyketide was unclear. Here, we report identification of a pamamycin biosynthesis gene cluster by aligning genomes of two pamamycin‐producing strains. This unique cluster contains polyketide synthase (PKS) genes encoding seven discrete ketosynthase (KS) enzymes and one acyl‐carrier protein (ACP)‐encoding gene. A cosmid containing the entire set of genes required for pamamycin biosynthesis was successfully expressed in a heterologous host. Genetic and biochemical studies allowed complete delineation of pamamycin biosynthesis. The pathway proceeds through 3‐oxoadipyl‐CoA, a key intermediate in the primary metabolism of the degradation of aromatic compounds. 3‐Oxoadipyl‐CoA could be used as an extender unit in polyketide assembly to facilitate the incorporation of succinate.     

Enantioselective Alternating Olefin-Carbon Monoxide Copolymerization: A new concept for activity and stereoselectivity

Cationic Pd-complexes modified by dicyclohexyl{(R)-1-[(S)-2-(diphenylphosphino)ferrocenyl]ethyl}phosphine ( 1a ) give very active catalytic systems for the regioregular isotactic specific copolymerization of propene with CO. Other alk-1-enes also give stereoregular and regioregular copolymers, even if with lower productivity. The copolymers are isolated as poly(4-alkyl-tetrahydrofuran-2,2,5,5-tetrayl-2-oxy-2-methylenes) B in the solid state and give the isomeric poly(2-alkyl-1-oxopropane-1,3-diyls) A by dissolution in (CF₃)₂CHOH. Solid polymer A (R = Et) is formed back at least partially when the dissolved material is reprecipitated from MeOH. The use of the related (ferrocenyl)diphosphine ligands 1b ? e and 2 as the catalyst modifier shows that the presence of both elements of chirality and of large substituents on the P-atoms of the ligand is necessary to achieve good stereocontrol, and that the large difference in basicity between the two P-atoms is probably the reason for the good catalytic activity.     

Die Reaktionen von Eisen(III)-chlorid mit Trithiazylchlorid. Die Kristallstruktur von [S4N4Cl]+[FeCl4]⊖

Reactions of Iron Trichloride with Trithyazyl Chloride. Crystal Structure of [S₄N₄Cl]⁺[FeCl₄]^? Iron trichloride reacts with (NSCl)₃ yielding S₄N₄[FeCl₄]₂, S₃N₃Cl₂[FeCl₄] or S₄N₄Cl[FeCl₄], depending on the reaction conditions. The i.r. spectra prove the presence of [FeCl₄]^? ions for all three compounds. The ⁵⁷Fe-Mössbauer spectra show a slight quadrupole splitting at 80 K for S₃N₃Cl₂[FeCl₄] (ΔE^Q = 0.42 mm · s^?1) and S₄N₄Cl[FeCl₄] (ΔE^Q = 0.23 mm · s^?1), which indicates a slight deformation of the FeCl₄^? tetrahedra. The crystal structure of S₄N₄Cl[FeCl₄] was determined and refined with X-ray diffraction data (2549 independent reflexions, R = 0.026). S₄N₄Cl[FeCl₄] crystallizes in the triclinic space group P1 with two formula units per unit cell. The lattice constants are a = 712, b = 911, c = 1006 pm, α = 76.5°, β = 83.8° and γ = 80.5°. The structure consists of the so far unknown [S₄N₄Cl]^⊕ cations and slightly deformed FeCl₄^? ions. The [S₄N₄Cl]^⊕ ion consists of a S₄N₄ ring built up of two nearly planar S₃N₂ fragments having a dihedral angle of 136°. The average SN bond length is 157 pm, the SCI bond length 214 pm.     

Synthesis and Characterisation of the Sodium and Lithium Cryptates of Macrobicyclic Ligands incorporating pyridine,bipyridine, and biisoquinoline units

Sythetic procedures have been deweloped for the preparation of sodium and lithium cryptates of the macrabicyclic ligands 1–11 containing pyridine, bipyridine, and biisoquinoline groups. They involve stepwise construction of the bicyclic system as will as direct macrobicyclisation procedures (Scheme 1) and give access to both symmetrical and dissymmetrical structures. Marked cation template effects have been found that facilitate the cyclisation processes. The ligands 1–11 were isolated as their cryptates with Na⁺ or Li⁺ cations.     

Oxovanadium(V) tetrathiacalix[4]arene complexes and their activity as oxidation catalysts

With the aim of modeling reactive moieties and relevant intermediates on the surfaces of vanadium oxide based catalysts during oxygenation/dehydrogenation of organic substrates, mono- and dinuclear vanadium oxo complexes of doubly deprotonated p-tert-butylated tetrathiacalix[4]arene (H4TC) have been synthesized and characterized: PPh4[(H2TC)VOCl(2)] (1) and (PPh4)2[{(H2TC)V(O)(mu-O)}2] (2). According to the NMR spectra of the dissolved complexes they both retain the structures adopted in the crystalline state, as revealed by single-crystal X-ray crystallography. Compounds 1 and 2 were tested as catalysts for the oxidation of alcohols with O(2) at 80 degrees C. Both 1 and 2 efficiently catalyze the oxidation of benzyl alcohol, crotyl alcohol, 1-phenyl-1-propanol, and fluorenol, and in most cases dinuclear complex 2 is more active than mononuclear complex 1. Moreover, the two thiacalixarene complexes 1 and 2 are in many instances more active than oxovanadium(V) complexes containing "classical" calixarene ligands tested previously. Complexes 1 and 2 also show significant activity in the oxidation of dihydroanthracene. Further investigations led to the conclusion that 1 acts as precatalyst that is converted to the active species PPh4[(TC)V==O] (3) at 80 degrees C by double intramolecular HCl elimination. For complex 2, the results of mechanistic investigations indicated that the oxidation chemistry takes place at the bridging oxo ligands and that the two vanadium centers cooperate during the process. The intermediate (PPh4)2[{H2TCV(O)}2(mu-OH)(mu-OC13H9)] (4) was isolated and characterized, also with respect to its reactivity, and the results afforded a mechanistic proposal for a reasonable catalytic cycle. The implications which these findings gathered in solution may have for oxidation mechanisms on the surfaces of V-based heterogeneous catalysts are discussed.