Diastereoselective alkylations of oxazolidinone vinylogous glycolates

A highly Z-selective isomerization (double bond migration) was observed when oxazolidinone vinylogous glycolate was exposed to a strong base to give N-acyl oxazolidinone, bearing an electron rich olefin. The corresponding enolate was exposed to alkyl halides to provide alkylated compounds on the γ-position with respect to OBn group, with high regioselectivity and moderate diastereoselectivity. However, the nature of the chiral oxazolidinone leads to a significant increase in the reaction diastereoselectivity. A stereospecific formation of cis-olefin was also observed in these alkylated compounds.     

Homo- and hetero-dinuclear anion complexes

The tripodal system 4, in which urea fragments are appended to the three terminal amine nitrogen atoms of a tris(2-aminoethyl)amine (tren) subunit, includes a Cu(II) ion and two anions X-, according to a cascade mechanism through three well defined stepwise equilibria in a DMSO solution. The first anion X- (halide, N3-, NCS-, NO2-, H2PO4-) seeks the Cu(II) centre coordinated by the tren moiety; the second anion X- interacts with the trisurea cavity, but this occurs only if the stronger H-bond acceptors, such as N3- and H2PO4-, are used. Binding of the second X- ion is favoured by the preorganising effect exerted by the metal and disfavoured by the steric and electrostatic repulsions between the anions. Under the appropriate conditions, heterodinuclear complexes of formula [Cu(II)(4)(Cl)(H2PO4)] can be obtained in solution, in which Cl- is bound to the metal centre and H2PO4- interacts with the trisurea compartment.     

Homo- and heterobimetallic niobium(v) and tantalum(v) peroxo-tartrate complexes and their use as molecular precursors for Nb-Ta mixed oxides

New water-soluble bimetallic peroxo-tartrato complexes of niobium(V) and/or tantalum(V) have been prepared, characterized from the structural and spectroscopic point of view, and used as molecular precursors for Nb-Ta mixed oxides. Two new homometallic complexes, (gu)5[Nb2(O2)4(tart)(Htart)] x 4H2O (1a) and (gu)6[Ta2(O2)4(tart)2] x 4H2O (2a), and the corresponding heterometallic complex, (gu)5[NbTa(O2)4(tart)(Htart)] x 4H2O (3), have been obtained. The crystal structures of the homometallic compounds, (gu)5[Nb2(O2)4(tart)(Htart)] x 6H2O x 1H2O2 (1b) and (gu)6[Ta2(O2)4(tart)2] x 6H2O (2b), have been determined, showing, for both cases, two 8-fold-coordinated metal atoms, each surrounded by oxygen atoms belonging to two bidentate peroxides, two monodentate carboxylato, and two alkoxo groups from both bridging tartrato ligands. The coordination polyhedron around each metal atom is a dodecahedron. The thermal treatment of complexes 1a, 2a, and 3 in air at 700 or 800 degrees C, depending of the Ta content, provided Nb2O5, Ta2O5, and the solid solution TaNbO5, respectively. The thermal treatment of a 1:1 Nb/Ta molar ratio mixture of 1a and 2a has also been studied. BET and SEM measurements have been carried out and reveal these oxides possess relatively high specific surface areas and display a porous character. Comparison between the use of homo- and heterometallic precursors is discussed.     

Quantitative self-assembly of a hetero-bimetallic trinuclear square-shaped macrocycle

A new hetero-bimetallic trinuclear macrocycle has been self-assembled quantitatively, in one step, by means of axial Zn^II-N and 90° angular Pt^II-N binding, with the metallated angular bisporphyrin, the cis-square-planar platinum bistriflate complex and the linear 4,4^′-bipyridine, as building block units.     

Homo- and hetero-oxidative coupling of benzyl anions

The regioselective benzylic metalation of substituted toluenes using BuLi/KO-t-Bu/TMP(H) (LiNK metalation conditions) with subsequent in situ oxidative C-C coupling has been developed for the facile generation of 1,2-diarylethanes. A range of oxidants can be used for the oxidative coupling step, with 1,2-dibromoethane proving optimal. Heterocouplings can be achieved starting from a mixture of two different toluenes with a bias toward cross coupling achievable by using a 2-fold excess of one toluene starting material. The utility of this approach is illustrated by the synthesis of several biologically active natural products. A distinct advantage is that the synthetic steps typically required to preactivate the coupling substrates are eliminated and no transition metal is required to facilitate the C-C bond formation.     

Homo- and heteroflocculation of papermaking fines and fillers

The effects of cationic polyethylenimine (PEI) on the colloidal stability of anionic fines (microcrystalline cellulose or thermomechanical fines), fillers (clay) and their mixtures in deionized and tap water were investigated, using a photometric dispersion analyzer. Measurements confirmed that PEI flocculates all used materials by charge neutralization. As expected, higher additions of PEI lead to electrostatic stabilization of microcellulose and clay suspensions, but it was not possible to stabilize the suspension of fines using high additions of PEI. This is ascribed to the mechanical entanglements of fibrillar fines. In tap water, much more PEI is needed to reach optimum flocculation conditions than in deionized water. Heteroflocculation between PEI-coated clay and fines takes place with a rate which, for high fines concentration and a constant clay concentration, is independent of fines concentration. A theoretical model for the heteroflocculation of fines with PEI-coated clay has been developed, which explains the observed trends. In essence, clay particles can act as bridging agents for fines flocculation.     

Some Homo- and Copolymerization Studies of Dimethyldiallylammonium Chloride

This paper summarizes some of the experimental procedures used for making homopolymers and copolymers of dimethyldiallylammonium chloride, along with a discussion of some of the experimental variables such as effects of catalyst ratio, monomer concentration, and impurities. Also discussed are some of the procedures used for evaluation of the polymers and copolymers in flocculation and sludge dewatering.     

Spectroscopic and electrochemical studies of hetero-bimetallic copper complexes with Schiff's base ligand

Using Schiff's base ligand, several Cu(II) based bimetallic complexes such as Cu-Cu, Cu-Co, Cu-Ni, Cu-Zn, Cu-Mn have been prepared in a stepwise procedure. The structures of these complexes and the ligand have been proposed on the basis of FAB mass, elemental analysis, UV-vis, IR, electron paramagnetic resonance (EPR) and CV studies. EPR parameters, obtained through complete simulation, suggest that the formation of bimetallic complexes forces the Cu(II) centre to increase the flexibility in comparison with the monometallic Cu(II) complex. However, the nature of the second metal ion in the bimetallic complex effects the distortion around the first metal ion. The reduction of the complexes from Cu(II) to Cu(I) involves a large geometrical change and is found to be irreversible. A large positive shift is seen in the cathodic process, which can be ascribed to increased distortion due to bimetallic coordination. These complexes have potential usage in DNA studies.     

The structure, photo-induced magnetization and correlation of the cyano-bridged two-dimensional hetero-bimetallic compounds

Two new 3d-4f cyano-bridged hetero-bimetallic compounds with distinguished two-dimensional honeycomb network structure, {Ln(HP)₂(H₂O)₃(μ-NC)₃Fe(CN)₃}_n, (Ln=Ce, Nd; HP=4-Hydroxypyridine) were synthesized and crystallographically characterized. A novel photo-induced magnetization phenomenon for {Nd(HP)₂(H₂O)₃(μ-NC)₃Fe(CN)₃}_n, was revealed and studied. Furthermore, the correlation between photo-induced magnetization and structure was proposed.     

Temperature Development of Homo- and Hetero-Clustering in Saturated Vapors

Computational findings of temperature increase of clustering degree in saturated vapors are analyzed. A thermodynamic proof is presented and a simple criterion derived. Illustrations are based on saturated steam, magnesium and carbon vapor. The results are applicable to synthesis of fullerenes, metallofullerenes, and heterofullerenes, chemical vapor deposition technique, or atmospheric chemistry.     

Homo- and heterocomplexes of sodium and lithium amides--structures in solution

Addition of the chiral amine (S)-methyl(1-phenyl-2-pyrrolidinoethyl)[15N]amine (1) to a large excess of nBuNa resulted in the formation of a mixed sodium amide/nBuNa complex. This is the first observation of such a complex. Addition of nBuLi to the chiral sodium amide dimer 3 gave a new mixed lithium/sodium amide 5. The use of 15N,6Li coupling constants showed that the lithium in 5 occupied the tetracoordinated site. The use of chiral sodium amide 3 in the desymmetrization of cyclohexene oxide gave a modest enantiomeric excess (ee) of 37%. The corresponding lithium amide gave an ee of 70% of the same enantiomer. This is the first example of the comparison of asymmetric induction by sodium as cation with that of lithium.     

Diastereoselective alkylation of chiral 2-imidazolidinone glycolates: asymmetric synthesis of α-hydroxy carboxylic acids

Sodium enolates of chiral 2-imidazolidinone glycolates reacted with alkyl halides to produce α-alkylated products with high diastereoselectivities, which were readily removed by simple alkaline hydrolysis and were converted to the protected α-hydroxy carboxylic acids. The new stereogenic center was assigned the (R)-configuration by comparison with known compounds.     

Organoborazines. III. Homo- and copolymerization of p-vinylphenylcyclotriborazines

The polymerization and copolymerization with styrene of a series of unsymmetrically B-p-vinylphenyl-N-methyl and N-phenyl borazines [R³(R²)₂B₃N₃(R¹)₃; R¹ = methyl, phenyl, R² = methyl phenyl, R³ = p-vinylphenyl] has been studied. The polymerization of these monomers yielded both tractable and crosslinked materials. The polymers obtained were characterized by ¹H- and ¹³C-NMR spectroscopy, elemental analysis, gel permeation chromatography, and thermogravimeteric analysis. The reactivity ratios for the copolymerization reaction were calculated by the Mortimer-Tidwell method.     

Homo- and copolymerization of butadiene and styrene with neodymium tricarboxylate catalysts

Homo- and copolymerizations of butadiene (BD) and styrene (St) with rare-earth metal catalysts, including the most active neodymium (Nd)-based catalysts, have been examined, and the cis-1,4 polymerization mechanism was investigated by the diad analysis of copolymers. Polymerization activity of BD was markedly affected not only by the ligands of the catalysts but also by the central rare-earth metals, whereas that of St was mainly affected by the ligands. In the series of Nd-based catalysts [Nd(OCOR)₃:R = CF₃, CCl₃, CHCl₂, CH₂Cl, CH₃], Nd(OCOCCl₃)₃ gave a maximum polymerization activity of BD, which decreased with increasing or decreasing the pKa value of the ligands. This tendency was different from that for Gd(OCOR)₃ catalysts, where the CF₃ derivative led to the highest polymerization activity of BD. For the polymerization of St and its copolymerization with BD, the maximum activities were attained at R = CCl₃ for both Nd- and Gd-based catalysts. The copolymerization of BD and St with Nd(OCOCCl₃)₃ catalyst was also carried out at various monomer feed ratios, to evaluate the monomer reactivity ratios as r_BD = 5.66 and r_St = 0.86. The cis-1,4 content in BD unit decreased with increasing St content in copolymers. From the diad analysis of copolymers, it was indicated that Nd(OCOCCl₃)₃ catalyst controls the cis-1,4 structure of the BD unit by a back-biting coordination of the penultimate BD unit. Furthermore, the long range coordination of polymer chain by the neodymium catalyst was suggested to assist the cis-1,4 polymerization. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 241–247, 1998     

Compton scattering study on the electronic properties of niobium carbide and niobium nitride

The electronic structure, experimental Compton profile and the chemical bonding mechanism of niobium carbide and niobium nitride are studied on the basis of the band structure calculations, using a self-consistent, all-electron, linear combination of Gaussian orbitals (LCGO) calculation based on density functional theory. Isotropic Compton profiles of niobium carbide have been measured, using a conventional technique based on 59.54 keV gamma-radiation with a solid state detector. The agreement between the experimental and the theoretical momentum density is very good. It is shown that the anisotropies at low momentum are heavily influenced by the particular shape of the Fermi surface. The charge distributions resulting from valence bands in different regions of the unit cell are discussed. The limitations of the rigid band structure model are illustrated and general trends in the chemical bonding are discussed.     

Novel Cofacial Porphyrin-Based Homo- and Heterotrimetallic Complexes of Transition Metals

We present a straightforward and generally applicable synthesis route for cofacially linked homo- and heterotrimetallic trisporphyin complexes. The protocol encompasses synthesising the first aryl-based, trans-o-phenylene trisporphyrin starting from pyrrole and benzaldehyde with an overall yield of 3.6 %. It also allows investigating the respective cis-isomer as the first conformationally restricted planar-chiral trisporphyrin. The free-base ligand was used in subsequent metalation reactions to afford the corresponding homotrimetallic Mn(III)-, Fe(III)-, Ni(II)-, Cu(II)-, Zn(II)- and Pd(II) complexes – additionally, a small adaptation of the protocol resulted in the defined Ni(II)Fe(III)Ni(II) complex in a total yield of 2.3 %. By monitoring Ni(II) insertion into the empty trimeric ligands, we affirmed that the outer porphyrin rings are filled before the internal ring. The molecular species were characterised by ¹H NMR, UV-Vis, photoluminescence, IR, MS, CID, and high-resolution IMS measurements.     

Thermodynamically controlled self-sorting of hetero-bimetallic metallo-supramolecular macrocycles: what a difference a methylene group makes!

(31)P NMR and tandem MS experiments provide clear evidence for the thermodynamically controlled self-assembly of hetero-bimetallic metallo-supramolecular macrocycles through self-sorting caused by different ancillary ligands.