A stereoselective synthesis of (+/-)-C-secolimonoid BCDE model compounds related to the insect antifeedant ohchinolide.

A short and stereoselective synthesis of a (+/-)-BCDE C-secolimonid model insect antifeedant related to ohchinolide and nimbolidin was accomplished in 13 (30% overall yield) and 15 (30% overall yield) steps, respectively, from ethyl drimanate. The key steps are the torquoselective electrocyclization of the divinyl ketone 6, induced by perchloric acid, and the stereoselective rearrangement of the hydroxy lactone 12, inspired in a biosynthetic proposal. An alternative route, which provides access to (+/-)-BCDE ohchinolide and nimbolidin isomers, is also described.     

Effect of the Support on the Structural Characteristics of Tin

Tin oxide was supported on alumina, titania, magnesia and silica, treated with hydrogen at different temperatures and characterized by Mössbauer spectroscopy and X-ray diffraction. For the samples calcined at 773 K, tin is present as SnO₂ on alumina, magnesia and silica, but it occupies Ti sites on titania. After hydrogen treatment at high temperatures, tin is reduced from Sn(IV) to Sn(II) on alumina and titania, from Sn(IV) to Sn(0) on silica, but practically not reduced on magnesia. These results show the different degree of interaction between tin and the supporting material.     

The role of carbon and metal in self-assembly of the iron-carbon system at various component ratios

The problem of self-assembly in a metal-carbon system under dynamic conditions (equilibrium and nonequilibrium) is considered using the iron-carbon system as an example. It is proved theoretically and experimentally that the ratio of the components of the system affects the possibility of carbon self-assembly with the formation of fractal iron structures and of metal self-assembly with the participation of polyhapto derivatives of iron and the formation of fullerene-like carbon structures.     

Tin-Platinum catalysts interactions on titania and silica

Pt-Sn was supported on titania and silica, and the resulting interactions between the components in prepared samples and the resulting interactions between the components before and after treatment with hydrogen were characterized by Mössbauer spectroscopy, X-ray diffraction, Rietveld refinement, high-resolution transmission electron microscopy (HRTEM) and catalytic tests data. Results show the presence of Pt and SnO₂ after calcinations, and Pt₃Sn, PtSn and PtSn₃ after reduction. Rietveld analysis shows that some Ti⁴⁺ are replaced by Sn⁴⁺ atoms in the titania structure. Finally, HRTEM and the practically absence of activity observed confirms that metallic platinum is encapsulated.     

Synthesis of aromatic polyethers by Scholl reaction. I. Poly(1,1′-dinaphthyl ether phenyl sulfone)s and poly(1,1′-dinaphthyl ether phenyl ketone)s

A novel synthetic method for the preparation of high molecular weight aromatic polyethers is presented. It consists in the Scholl reaction of di(1-naphthyl) ethers of aromatic derivatives exhibiting lower nucleophilicity and higher oxidation potential than the 1-naphthoxy groups. The examples described in this paper refer to the synthesis of aromatic polyether sulfones and aromatic polyether ketones by the polymerization of 4,4′-di(1-naphthoxy)diphenyl sulfone and respectively 4,4′-di(1-naphthoxy)benzophenone. Both polymerization reactions are performed at room temperature in nitrobenzene, using anhydrous FeCl₃ as catalyst, and apparently follow a “reactive intermediate polycondensation” polymerization mechanism.     

Characterization of the corrosion products formed on mild steel in acidic medium with N-octadecylpyridinium bromide as corrosion inhibitor

The characterization of the corrosion products formed on mild steel SAE 1018 after 2 months exposure in aqueous sulfuric acid with and without corrosion inhibitor N-octadecylpyridinium bromide has been carried out by means of transmission ⁵⁷Fe Mössbauer spectroscopy and X-ray powder diffraction (XRD). The major constituent of the rust formed in this environment without corrosion inhibitor is goethite (??-FeOOH). The samples with N-octadecylpyridinium bromide contain rozenite and large amounts of melanterite in the corrosion layers.     

Modes of interconversion in the cycloheptene ring

The possible modes of interconversion for cycloheptene have been studied by carrying out calculations in the CNDO/2 and MINDO/2 approximations.The transition state of lower energy is found along an asymmetrical pathway from the chair form, which is the most stable conformation, to the twist-boat form; the calculated activation energy agrees with the experimental data obtained from DNMR measures. The possibility of a change in the inversion mechanism for benzocycloheptene is also discussed.     

Intervalley diffusion of hot electrons in germanium

In many-valley semiconductors for certain specific orientations of the applied electric field E, the different drift velocities of electrons in different valleys should give rise to an additional diffusion called intervalley diffusion. Experimental evidence of this effect is presented. This was obtained by measuring the longitudinal diffusion coefficient of electrons in Ge, at 77 and 190 K, for E parallel to the 〈100〉 and 〈111〉 directions. The interpretation of the effect has been confirmed by Monte Carlo calculations.