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[reaction: see text] The first enantioselective synthesis of (+)-trans-195A is described. The structure has been constructed by ring-rearrangement metathesis (RRM) and zirconium-mediated Negishi-coupling, used for the first time to prepare 6,6-membered heterocycles, as key steps. By comparison of the synthesized material with the isolated natural product, the absolute configuration of natural trans-195A was determined to be (2R,4aS,5R,8aS)-(-).
[reaction: see text] The repetitive use of Cp(2)ZrBu(2) (Negishi reagent) was applied in the synthesis of three 3-methoxyestra-1,3,5(10)-trienone isomers within four steps from the advanced intermediate 11. The overall synthesis is based on three zirconium-mediated reactions: (a) oxidative addition of a benzyl ether, (b) cyclization of an allyl-ene compound, and (c) cyclocarbonylation of a diene. The presented synthesis demonstrates that complex organic compounds can be prepared by the repetitive use of one reagent.
Electron-deficient imines are shown to insert into the zirconium-carbon bond of azazirconacyclobutenes to generate new six-membered ring metallacycles. On heating, these expanded zirconacycles undergo a retro-[4 + 2] cycloaddition to generate alpha,beta-unsaturated imines in excellent yields and novel, fully characterized electron-deficient imidozirconocene complexes.
The aim of the present experimental study was to evaluate the tissue response to hafnium (Hf) a reactive metal closely related to titanium (Ti) and zirconium (Zr). Hf has not been previously evaluated as implant material in a biologic environment. In a first experiment, 21 machined Hf non-threaded implants (test) and 21 similar Ti implants (control) were inserted in the abdominal wall of 21 rats. Animals were sacrificed after 8 days (6 rats), 6 (7 rats) and 12 weeks (8 rats). In a second experiment, 18 rabbits received 18 Hf and 18 Ti threaded implants in their tibiae, one implant in each tibia. The rabbits were sacrificed after 6, 12 and 24 weeks (6 animals/time interval). The bulk metal of the abdominal wall implants, embedded together with the surrounding tissue, was electrolytically dissolved and semithin (1 microm) sections of the intact tissue-implant interface were evaluated by light microscopy (morphometry). Bone-implant contact and bone area within threads were evaluated in ground sections. In soft tissues, a fluid space containing predominantly monocytes/macrophages surrounded the abdominal implants at 8 days. At 6 and 12 weeks, a fibrous capsule, consisting of layers of macrophages and fibroblasts, surrounded the implants. Macrophages, including multinuclear giant cells, always formed the innermost layer in contact with the implant surface. No quantitative or qualitative difference in the tissue organization was detected between Ti and Hf implants. In rabbits, 6 weeks after insertion, the proximal two threads located within the cortical bone were filled with bone in contact with Hf and Ti. The distal threads contained bone marrow. After 12 and 24 weeks, mature bone was present in the proximal 3-4 implant threads. No statistically significant difference was found between Hf and Ti implants at any time periods. It is concluded that Hf is an interesting metal for biomedical applications in bone and soft tissue.
Different metals are increasingly being used to manufacture implants, especially in the fields of dentistry and orthopedics. No metal or alloy is completely inert in vivo. The metal and the organic fluids interact releasing, for example, metallic products. Several hypotheses regarding the probable dissemination routes of titanium have been postulated, but its valence, the organic nature of its ligands and its potential toxicity have yet to be established. In a previous experimental study we demonstrated that i.p. injected titanium and zirconium oxides disseminate and deposit in organs such as liver and lung. The aim of this work was to study the eventual participation of blood cells in the transport mechanism of titanium employing the intraperitoneal injection of titanium oxide in rats as the experimental model. Twenty male Wistar rats, x: 100 g body weight, were intraperitoneally injected with 16x10(3) mg/kg b.w. of TiO(2) in saline solution. Blood samples were taken by heart puncture at 3 and 6 months; blood smears were performed and stained with safranin evidencing monocytes containing titanium particles. The results obtained in this study would indicate that one of the ways in which titanium is disseminated is through the blood stream, via blood cells.
Lead has recently been expelled from many commercial applications and materials (for example, from solder, glass and pottery glaze) owing to concerns regarding its toxicity. Lead zirconium titanate (PZT) ceramics are high-performance piezoelectric materials, which are widely used in sensors, actuators and other electronic devices; they contain more than 60 weight per cent lead. Although there has been a concerted effort to develop lead-free piezoelectric ceramics, no effective alternative to PZT has yet been found. Here we report a lead-free piezoelectric ceramic with an electric-field-induced strain comparable to typical actuator-grade PZT. We achieved this through the combination of the discovery of a morphotropic phase boundary in an alkaline niobate-based perovskite solid solution, and the development of a processing route leading to highly <001> textured polycrystals. The ceramic exhibits a piezoelectric constant d33 (the induced charge per unit force applied in the same direction) of above 300 picocoulombs per newton (pC N(-1)), and texturing the material leads to a peak d33 of 416 pC N(-1). The textured material also exhibits temperature-independent field-induced strain characteristics.
The integration of materials having a high dielectric constant (high-kappa) into carbon-nanotube transistors promises to push the performance limit for molecular electronics. Here, high-kappa (approximately 25) zirconium oxide thin-films (approximately 8 nm) are formed on top of individual single-walled carbon nanotubes by atomic-layer deposition and used as gate dielectrics for nanotube field-effect transistors. The p-type transistors exhibit subthreshold swings of S approximately 70 mV per decade, approaching the room-temperature theoretical limit for field-effect transistors. Key transistor performance parameters, transconductance and carrier mobility reach 6,000 S x m(-1) (12 microS per tube) and 3,000 cm2 x V(-1) x s(-1) respectively. N-type field-effect transistors obtained by annealing the devices in hydrogen exhibit S approximately 90 mV per decade. High voltage gains of up to 60 are obtained for complementary nanotube-based inverters. The atomic-layer deposition process affords gate insulators with high capacitance while being chemically benign to nanotubes, a key to the integration of advanced dielectrics into molecular electronics.
Residues of the tetracycline group of antibiotics were quantified in pork and chicken muscle tissue that had previously been screened with a microbiological inhibition test and an immunological method. Pieces of frozen pork and chicken meat were screened on a pH 6 culture medium seeded with Bacillus subtilis. An aqueous extract of the inhibitor-positive samples was then screened with a group-specific commercial ELISA kit, able to detect levels of oxytetracycline, chlortetracycline, tetracycline and doxycycline corresponding with the European MRL or lower. The cut-off value of the ELISA was set at a B/B0 value of 75%. Finally, confirmation and quantification were performed using a validated HPLC method with fluorescence detection. The fluorescence was induced by complexation of the tetracyclines with the zirconium cation which is added post-column to the HPLC eluate. This fluorescence makes it possible to quantitate residues below one-half of the MRL. To gain additional qualitative information some samples were also analysed with LC-MS-MS. ELISA analysis demonstrated the presence of residues of tetracyclines in 12 out of 19 inhibitor-positive pork samples and in 19 out of 21 inhibitor-positive chicken samples. Doxycycline was detected with HPLC in 10 of these 12 pork samples and in 18 out of 19 chicken samples. The two other ELISA positive pork samples contained oxytetracycline, while no tetracyclines were found in one ELISA positive chicken meat sample. The correlation between the ELISA B/B0 values and the actual levels determined with the HPLC method was poor, whereas a better correlation was observed between the inhibition zones and the doxycycline levels. Our results indicate that an inhibition test with a medium at pH 6 and B. subtilis as test organism is well suited to screen pork and chicken muscle tissue for residues of tetracycline antibiotics. Since many positive samples contained doxycycline levels below the MRL, a confirmatory method is necessary to quantify the residues.
A novel, sensitive, high-performance liquid chromatographic method for the residue determination of tetracyclines in bovine milk has been developed. Clean-up of the sample extracts is performed with metal chelate affinity chromatography. Detection is based upon the measurement of fluorescence induced by complexation of tetracyclines with the zirconium cation, which is added post-column to the high-performance liquid chromatographic eluate. The method has been validated for oxytetracycline, tetracycline and chlortetracycline. Calibration curves using 'blank' milk fortified at the 10-200 ng ml-1 level showed good linear correlation. Accuracy was measured at the 100 ng ml-1 level and was found to be within acceptable limits using a t-test (alpha = 0.05). At this level both within- and between-day precision, as measured by relative standard deviation (S(r)), were less than 16% for oxytetracycline and chlortetracycline, and less than 20% for tetracycline. The most striking advantages of the proposed method, over previously reported methods, are the higher sensitivity and specificity achieved. Using a signal-to-noise ratio of 4:1, the detection limits of the assay were 1 ng ml-1 for oxytetracycline, 2 ng ml-1 for tetracycline and 4 ng ml-1 for chlortetracycline.
A neutral polystyrene resin column, dynamically loaded with dipicolinic acid at a concentration of 0.1 mM in 1 M potassium nitrate eluent, was investigated for the separation characteristics of a number of high valence metal cations over the pH range 0-3. The metal species studied were Th(IV), U(VI), Zr(IV), Hf(IV), Ti(IV), Sn(IV), V(IV) and V(V), Fe(III) and Bi(III), of which Ti(IV), Sn(IV), V(IV) and Fe(III) did not show any retention. For the remaining metal ions, significant retention was obtained with good peak shapes, except for Th(IV), which moved only slightly from the solvent front with some tailing. The retention order at pH 0.3 was Th(IV) < V(V) < Bi(III) < U(VI) < Hf(IV) < Zr(IV). A notable feature of this separation system was the high selectivity shown for uranium, zirconium and hafnium, the last two being nearly resolved in 15 min on the relatively short 10 cm column.
Steric blocking of an intramolecular 1,2-migratory insertion reaction of a zirconium salicylaldiminato complex leads to a long-lived catalyst for ethene polymerisation, but promotes a new radical catalyst decomposition mechanism in certain instances; kinetic and thermodynamic parameters for both pathways have been established.
Spraying cerium and zirconium precursors dissolved in carboxylic acids into a methane-oxygen flame resulted in well-structured nanocrystals of ceria-zirconia mixed oxides with high temperature stability and surface area.
A successful strategy has been developed for the synthesis of a cationic pentamethylcyclopentadienyl zirconium amidinate initiator on a lightly crosslinked polystyrene-divinylbenzene support that can be used to polymerize alpha-olefins in a living and stereospecific fashion.
Alkylation of ferrocene bis-imines Fc[N=C(H)Ar]2 (Ar = Ph, p-Tol) with MeLi was found to proceed in a highly diastereoselective fashion producing the C2-symmetric ferrocene diamines Fc[NC(H)(Me)Ar]2-H2 with 90% diastereomeric excess. This process allowed for the synthesis of C2-symmetric zirconium complexes Fc[NC(Me)Ar]ZrBn2.
Treatment of the complexes [(C(5)H(4)PR(2))(2)Zr(CH(3))(2)](b: R = isopropyl; c: R = cyclohexyl) with the reagent HIr(CO)(PPh(3))(3) (2b) yield the heterobimetallic complexes [mu-C(5)H(4)PR(2))(2)(H(3)C-Zr-Ir(CO)(PPh(3)))] (4b, 4c) with evolution of methane. The reaction of the -PPh(2) substituted analogue with initially yields an intermediate [(H(3)C)(2)Zr(mu-C(5)H(4)PPh(2))(2)Ir(H)(CO)(PPh(3))] 5a, that still contains both methyl groups at zirconium and does not contain a metal-metal bond. At room temperature, the intermediate reacts further with methane formation to eventually yield the (Zr-Ir) complex 4a. The corresponding [mu-C(5)H(4)PR(2))(2)(H(3)C-Zr-Rh(CO)(PPh(3)))] complexes 3a (R = Ph) and 3b (R = isopropyl) react cleanly with isopropyl alcohol to liberate methane and yield the corresponding [mu-C(5)H(4)PR(2))(2)(Me(2)CHO-Zr-Rh(CO)(PPh(3)))] products (7a, 7b). Carefully monitoring the reaction of with Me(2)CHOH by NMR revealed that the Zr-Rh functionality is attacked first to give the intermediate [Me(Me(2)CHO)Zr([micro sign]-C(5)H(4)PR(2))(2)Rh(H)(CO)(PPh(3))] (6b). This intermediate then reacts further to cleave off methane and re-form the (Zr-Rh) metal-metal bond to yield the product 7b. The tetrametallic mu-oxo-(Zr-Rh) metallocene derivate 11a was obtained starting from the (Zr-Rh) complex 3a and it was characterized by X-ray diffraction. It may be that this reaction is also initiated by H-OH addition to the [Zr-Rh] metal-metal bond.
This tutorial review surveys recent advances in the field of zirconium promoted coupling reactions of unsaturated molecules (and/or organolithium compounds) and heterosubstituted alkenes. This reaction has recently emerged as a powerful tool in organic chemistry to access a variety of synthetically useful building blocks not easily available by other methods. In particular, three different reaction pathways are discussed: i) the reaction involving alkyne-zirconocene and aryne-zirconocene complexes which allows access to dienyl and aryl zirconocene complexes; ii) the reaction of alkene-zirconocene complexes and enol ethers which has shown to be highly dependent on the structure of the enol ether used, allowing the synthesis of simple vinyl zirconocenes or multicomponent coupling products, and; iii) the reaction involving imine-zirconocene complexes which supposes a new entry to allylic amines.
Homogeneous catalysts for olefin polymerization such as metallocene or half-sandwich complexes containing the metals titanium, zirconium and hafnium, or other transition-metal coordination complexes can be functionalized with alkenyl groups and have then the potential to copolymerize with olefins to give heterogeneous catalysts. In a similar manner metallacyclic metallocene complexes with a metal-carbon sigma bond allow the catalytic insertion of olefins into this bond and produce heterogeneous catalysts. It is also possible to functionalize the active species of the cocatalyst methylalumoxane (MAO) and use it for self-immobilization processes. The high excess of MAO that is necessary in homogeneous solution can be reduced by more than 90% with this method.
Octahedral titanium and zirconium complexes based on salicyloxazoline ligands with sterically demanding ortho-substituents provide a new family of extremely active ethene polymerization catalysts [up to 10(8) g PE (mol bar h)(-1)] which are in some cases "single site".