UV-induced immobilization of tethered zirconocenes on H-terminated silicon surfaces

A tethered ethylenebis(indenyl) zirconocene was covalently immobilized on H-terminated Si(111) surfaces using UV-mediated alkene hydrosilylation, thus making possible the development of structured catalytic surfaces with highly controlled properties.     

Cysteine-S-trityl a key derivative to prepare N-methyl cysteines

S-Trt Cys are used as precursors for the synthesis of protected NMe-Cys. N-Methylation of Alloc-Cys(Trt)-OH and Boc-Cys(Trt)-OH gives the corresponding N-methylated derivatives in good yields and purities, which can be further derivatized in solution to obtain a myriad of S-protected derivatives. To further broaden the scope of this methodology, the N (alpha)-amino protecting group of the NMe- S-protected Cys can be replaced easily either on the solid phase (from the Alloc precursor) or in solution (from the Boc precursor). Thus, this convenient route allows us to obtain many different protected NMe-Cys, which were of limited accessibility until now.     

Semisynthesis of new D-seco-C-nor-taxane derivatives containing a polyfunctionalized furanosyl or cyclopentenyl or cyclopentyl C-ring

The synthesis of new D-seco-C-nor-taxane derivatives in which the D-ring has been deleted and the C-ring has been transformed into a new pentatomic ring, i.e., the polyfunctionalized tetrahydrofuranosyl and cyclopentenyl or cyclopentyl ring, was performed starting from baccatin III derivatives. The synthetic strategy adopted took advantage of the oxetane ring opening and disconnection of the C4-C5 bond, followed by an intramolecular condensation. The formation of furanosyl or cyclopentyl rings is strictly dependent on the presence of unprotected or protected oxygen at C-7 in the starting material. The reactions proceeded with good diastereoselectivity with control of the stereochemistry of one or two stereocenters.     

Application of chemical and chemometric analytical techniques to the study of ancient ceramics from Dougga (Tunisia)

In the present paper chemical characterization has been carried out on 67 shards of archaeological pottery from Dougga (North Tunisia). The analysed shards, dated to the Byzantine period (VI–VII century A.D.), belong to the three ceramic classes African Red Slip Ware, Dougga Ware and African cooking Ware. Fourteen elements have been determined by both atomic emission spectroscopy with flame as source (AES) and by using an inductively coupled plasma source (ICP-OES). The data acquired have been treated by statistical techniques in order to define grouping for the examined shards. Both unsupervised and supervised methods have been employed in order to define groups of different pottery shards. As a comparison, some samples (control group) coming from Southern Tunisia have been examined. All the statistical methods employed have evidenced how the control group, as concerns the chemical composition, is clearly distinguishable from Ain Wassel samples which are highly homogeneous. In fact because of the compositional homogeneity of the Northern Tunisia productions, it is quite difficult to establish a good classification and distribution of the samples in well defined cluster. Nevertheless supervised analysis has evidenced how, among the three classes, the African cooking Ware is the more distinguishable one confirming the archaeologists' hypothesis that Dougga Ware is an imitation of African Red Slip Ware.     

Hydrolytic and photoinduced degradation of tribenuron methyl studied by HPLC-DAD-MS/MS

The paper studies, with the help of HPLC-DAD-MS/MS technique, the hydrolytic and photoinduced degradation processes that take place in aqueous solutions of tribenuron methyl, both when preserved in the dark and when undergoing solar box irradiation under conditions that simulate sun light. The results indicate that the degradation products formed by hydrolysis alone and by photoirradiation are the same, but kinetics of the hydrolysis reaction is much slower. The degradation products are identified as 2-methoxy-4-methylamino-6-methyl-1,3,5-triazine (P1), methyl 2-aminosulfonylbenzoate (P2), and saccharin (P3) and quantified. Ecotoxicological biotests performed on 0.1 microg L(-1) photoirradiated solutions of the herbicide give a border line toxicity situation comparable to that of the precursor and indicate that the herbicide is characterized by low persistence in the environment, as required. Its degradation, however, does not lead to mineralization but to the formation of products of comparable toxicity. To evaluate the matrix effects, the photodegradation of the herbicide is also studied in the presence of rice paddy waters: the process is slower than in ultrapure water but leads to the same products. Experiments performed for comparison by irradiating ultrapure water solutions with UV lamp (254 nm) show that the degradation process is not only faster with respect to sunlight, but gives a different pathway, without in anyway leading to mineralization.     

Mixed Co/Fe oxide nanoparticles in block copolymer micelles

Small iron oxide and Co-doped iron oxide nanoparticles (NPs) were synthesized in a commercial amphiphilic block copolymer, poly(ethylene oxide)-b-poly(methacrylic acid) (PEO 68-b-PMAA8), in aqueous solutions. The structure and composition of the micelles containing guest molecules (metal salts) or NPs (metal oxides) were studied using transmission electron microscopy, dynamic light scattering, X-ray photoelectron spectroscopy, and X-ray powder diffraction. The enlarged micelle cores after incorporation of metal salts are believed to be formed by both PMAA blocks containing metal species and penetrating PEO chains. The nanoparticle size distributions in PEO 68-b-PMAA8 were determined using small-angle X-ray scattering (SAXS) in bulk. Two independent methods for SAXS data interpretation for comprehensive analysis of volume distributions of metal oxide NPs showed presence of both small particles and larger entities containing metal species which are ascribed to organization of block copolymer micelles in bulk. The magnetometry measurements revealed that the NPs are superparamagnetic and their characteristics depend on the method of the NP synthesis. The important advantage of the PEO 68-b-PMAA8 stabilized magnetic nanoparticles described in this paper is their remarkable solubility and stability in water and buffers.     

A crystallographic and DFT study on a NHC complex of niobium oxide trifluoride

The complex [NbOF₃(Ipr)]₂, 1, was afforded in crystalline form by reaction of NbF₅ with the bulky NHC ligand 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) in toluene by slow contact with moisture air. The molecular structure of 1 was ascertained by X-ray diffraction, providing the first example of a dinuclear NbOF₃ derivative and also a rare case of niobium compound with a monodentate NHC. A DFT investigation has shown that the Nb–C bond consists of a weak NHC to Nb σ donation, reinforced by an electrostatic contribution presumably favored by the presence of the ancillary fluoride ligands. The computed enthalpy for the dissociation of one Ipr from 1 is ca. 36 kcal mol^?1. The presence of bulky 2,6-diisopropylphenyl substituents on the carbene ligand has negligible influence on the Nb–C bond, as highlighted by DFT analyses on simplified models.     

Effect of Titanium on Fluxional Behavior of Unbridged Metallocene Catalysts

Summary: Thanks to the use of a ligand consisting of a saturated ring fused to the cyclopentadienyl moiety, which sensibly increases the stability of titanocenes, it was possible to investigate the oscillating behavior of titanium catalysts in propylene polymerization for the first time. The titanium‐based catalysts yield poly(propylene)s with new and interesting microstructures. Indeed, in spite of the significantly lower isotactic pentad content [mmmm], the average lengths of the isotactic blocks (N_iso) are similar or even higher than that obtained with the zirconium‐based homologues. Accordingly, differential scanning calorimetry (DSC) analysis shows melting peaks which are more evident than those obtained with the zirconium homologues. The study of the microstructure of the poly(propylene)s obtained with these catalysts allowed us to participate in the still open discussion on the oscillating mechanism of unbridged metallocenes.

The titanium‐based catalysts employed in this study.     

Persistence of the Feigenbaum Attractor in One-Parameter Families

Consider a map ψ₀ of class C ^r for large r of a manifold of dimension n greater than or equal to 2 having a Feigenbaum attractor. We prove that any such ψ₀ is a point of a local codimension-one manifold of C ^r transformations also exhibiting Feigenbaum attractors. In particular, the attractor persists when perturbing a one-parameter family transversal to that manifold at ψ₀. We also construct such a transversal family for any given ψ₀, and apply this construction to prove a conjecture by J. Palis stating that a map exhibiting a Feigenbaum attractor can be perturbed to obtain homoclinic tangencies. Received: 4 August 1998 / Accepted: 11 May 1999