Synthesis of new 3,5-diarylisoxazolidines by cycloaddition of oxaziridines and alkenes

This article reports a novel process of cycloaddition of C-aryloxaziridines with a variety of alkenes to afford stable, five-membered heterocycles 13–24. The steric hindrance of the tert-butyl group on the nitrogen atom of the oxaziridine is responsible for the high stereoselectivity of the cycloaddition reaction.     

Methane Direct Conversion on Mo/ZSM-5 Catalysts Modified by Pd and Ru

The effect of addition of Ru and Pd to Mo/HZSM-5 catalysts used in the dehydroaromatiza-tion of methane was investigated. Catalytic tests and temperature-programmed oxidation results showed that Pd-based catalysts were more selective to naphthalene and suffered strong deactivation. The presence of Ru improved the activity and stability, with a decrease in the carbonaceous deposit probably because of a mechanism of protection of the Mo2C surface.     

Silver coordination chemistry of a new versatile "Janus"-type N(2),O(2)-bichelating donor, formation of an unprecedented supramolecular network of binuclear silver building blocks containing a five-coordinate beta-diketonate, and isolation of unexpected silver-tin-silver heterotrimetallic complexes from silver metathesis reactions

Synthetic, spectroscopic, and single-crystal X-ray studies are reported for several complexes of silver(I) with the N(2),O(2)-bichelating Q(py) ligand (HQ(py) = 1-(2-pyridyl)-3-methyl-4-trifluoroacetylpyrazol-5-one). Direct interaction between HQ(py) and AgNO(3) in methanol, in the presence of NaOCH(3), affords derivative Ag(Q(py)), showing a polynuclear structure composed of dinuclear building blocks with two different Ag environments and two Q(py) donors differently connected. By adding neutral ligands such as PR(3) (R = Ph, Cy, C(6)H(4)-o-CH(3), C(6)H(4)-p-F, Bu(i)) to Ag(Q(py)), dinuclear Ag(Q(py))(PR(3)) derivatives have been isolated, containing bridging N(2),O-exotridentate Q(py) donors spanning a pair of AgPR(3) moieties. Reaction of Ag(Q(py))(PPh(3)) with excess PPh(3) produces the mononuclear Ag(Q(py))(PPh(3))(2) containing N(2)-chelate Q(py). Ag(Q(py)) interacts with 1,2-bis(diphenylphosphino)ethane (dppe) yielding the derivative Ag(Q(py))(dppe), having a polynuclear structure in the solid state which is seemingly disrupted in solution, with the formation of two new species, a mononuclear neutral compound and a dinuclear ionic one. By the interaction of Ag(Q(py)) with nitrogen donors L (L = imidazole (imH), 1-methylimidazole (Meim), 1-methyl-2-mercaptoimidazole (Hmimt), 1,10-phenanthroline (phen)), mononuclear species Ag(Q(py))(L) have been obtained, where Q(py) is coordinated to silver in N(2)-chelating mode. Ag(Q(py))(PPh(3))(2) reacts with SnRCl(3) (R = Ph, Bu(n)) affording heterotrimetallic [[(Ph(3)P)(2)AgCl](2)SnRCl(3)] derivatives.     

Bis-qtpy (qtpy = 2,2':6',2'':6'',2'"-quaterpyridine) metal complexes, [M(qtpy)(2)]2+

Reactions of perchlorates of iron(II), nickel(II), and zinc(II) with 2,2':6',2':6',2'"-quaterpyridine (qtpy) gave the first crystallographically established bis-qtpy metal complexes of formula [M(qtpy)(2)][ClO(4)](2) (M = Fe, Ni, Zn). Coordination of two terdentate quaterpyridines to the same center produces a distorted octahedron of six nitrogen atoms around the metal, leaving two pendant pyridyl groups, one for each quaterpyridine. For the diamagnetic zinc system, an NMR investigation has been carried out in order to establish the conditions to obtain the intermediate mono-qtpy complex, of formula [Zn(qtpy)(H(2)O)(2)][ClO(4)](2), which has also been crystallographically established. The corresponding hexafluorophosphate derivatives [M(qtpy)(2)][PF(6)](2) (M = Ni and Zn) were prepared in DMF at room temperature.     

Investigation of the thermal degradation of fully aromatic regular polyesters: Poly(oxy-1,4-phenyleneoxy-fumaroyl-bis-4-oxybenzoate)

The thermal behaviour and degradation mechanism of fully aromatic polyester, poly(oxy-1,4-phenyleneoxy-fumaroyl-bis-4-oxybenzoate), were studied by pyrolysis-gas chromatography and pyrolysis-gas chromatography/mass spectrometry at 500-700 °C, and by thermogravimetry. The influence of fullerene C₆₀ additives on thermal behaviour and thermal degradation was investigated. On the basis of pyrolysis products determined, the origin of the main degradation products (maleic anhydride, phenol, hydroquinone, phenyl ether, p-hydroxybenzoate-p′-phenol, etc.) was estimated. The fullerene is a well-known efficient acceptor of radicals and its presence influences the thermal degradation process of polymers shifting the decomposition from a radical pathway to a non-radical mechanism. Thermal degradation mechanism of poly(oxy-1,4-phenyleneoxy-fumaroyl-bis-4-oxybenzoate) is discussed in detail.     

Determination of furanic compounds in traditional balsamic vinegars by ion-exclusion liquid chromatography and diode-array detection

A method for the determination of furanic compounds in traditional balsamic vinegars is proposed. It is based on ion-exclusion chromatographic separation and diode-array detection of furans through an isocratic elution with 0.01 N phosphoric acid and 16% acetonitrile. Preliminary trials on standard compounds stability in heat-acidic conditions are also performed. In all the 19 samples analyzed, 2-furoic acid, 5 HMF, and furfural are found. No sample contains 4-hydroxy-2,5-dimethyl-3-(2H)-furanone (DHMF); 2-acetylfuran; or furfuryl alcohol. Three unknown compounds are also detected. The last eluting of these compounds is identified as 5-acethoxymethylfurfural, and, notwithstanding a partial hydrolysis in our chromatographic conditions, its quantitation can be carried out.     

Self-assembled monolayers of cobalt(II)- (4-tert-butylphenyl)-porphyrins: the influence of the electronic dipole on scanning tunneling microscopy images

Self-assembled monolayers (SAMs) of cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin, a promising material for optical, photoelectrochemical, and chemical sensor applications, were prepared on Au(111) via axial ligation to 4-aminothiophenol, and studied by several surface science techniques. Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements showed the apparent topology of the Au(111) herringbone structure reconstruction, but with bias-dependent contrast images and asymmetric I/V characteristics. Photoelectron spectroscopy confirmed the presence of metalloporphyrins on the surface, whereas near-edge X-ray absorption (NEXAFS) measurements revealed that the porphyrin ring was tilted by about 70 degrees with respect to the surface plane. The above effects are ascribed to the presence of oriented molecular dipole layers between the metal and the organic material as confirmed by a comparison with first-principles density-functional theory calculations. The measured bias-dependent STM profiles have been reproduced by a simple monodimensional tunneling model.     

Progress toward a biomimetic synthesis of phomoidride B

[reaction: see text]. An intramolecular cyclization strategy for effecting a biomimetic synthesis of the core structure of the fungal secondary metabolites phomoidrides A and B is described. The cyclization substrate 20 is prepared in eight steps from dibromide 10. Treatment of 20 with triethylamine in acetonitrile results in a rapid cyclization to give 21 and 22 in good yield.     

Nature of the chemical bonding in D3h [MH3M]+ cations (M = Be,Mg)

Motivated by the particularly short metal-metal distance that has been predicted for the D_3h [BeH₃Be]⁺ cation, comparable to those anticipated for triple bonds, we investigate the nature of the bonding interactions in the D_3h [MH₃M]⁺ cations (M = Be, Mg). CCSD(T)/cc-pVQZ calculations are used to determine optimized geometries for all of the various species, including those “capped” by He or Ne atoms (as proxies for an inert gas matrix). The primary tools that are then used to investigate the nature of the chemical bonding are spin-coupled generalized valence bond calculations and the analysis of localized natural orbitals and of domain-averaged Fermi holes. The various results for all of the systems considered indicate the presence of highly polar three-center two-electron M─H─M bonding character instead of any significant direct metal-metal bonding.