Mechanism studies of the conversion of 13C-labeled n-butane on zeolite H-ZSM-5 by using 13C magic angle spinning NMR spectroscopy and GC-MS analysis

By using 13C MAS NMR spectroscopy (MAS = magic angle spinning), the conversion of selectively 13C-labeled n-butane on zeolite H-ZSM-5 at 430-470 K has been demonstrated to proceed through two pathways: 1) scrambling of the selective 13C-label in the n-butane molecule, and 2) oligomerization-cracking and conjunct polymerization. The latter processes (2) produce isobutane and propane simultaneously with alkyl-substituted cyclopentenyl cations and condensed aromatic compounds. In situ 13C MAS NMR and complementary ex situ GC-MS data provided evidence for a monomolecular mechanism of the 13C-label scrambling, whereas both isobutane and propane are formed through intermolecular pathways. According to 13C MAS NMR kinetic measurements, both pathways proceed with nearly the same activation energies (E(a) = 75 kJ mol(-1) for the scrambling and 71 kJ mol(-1) for isobutane and propane formation). This can be rationalized by considering the intermolecular hydride transfer between a primarily initiated carbenium ion and n-butane as being the rate-determining stage of the n-butane conversion on zeolite H-ZSM-5.     

Photodegradation of Polychlorinated Dibenzo-p-dioxins in Liquid Samples by Near UV Light Irradiation

Photodegradation of 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TCDD) in hexane solution was studied under controlled near-UV light exposure in the spectral region from 325 to 269 nm. GC-MS was used to detect the amount of unreacted dioxin and to characterize the relevant degradation products. Irradiation experiments carried out at a constant light energy (700 mJ) showed that the percentage of 1,2,3,4-TCDD left in the solution after irradiation changed from about 55 to 75%, with a minimum of 55% at 310 nm. Further irradiation experiments carried out at two wavelengths, namely 310 and 269 nm, and light energy ranging from 0 to 4000 mJ, showed that the photodegradation reaction of the TCDD always followed a pseudo-first-order kinetic, with a rate constant of 8 × 10⁻⁴and 5 × 10⁻⁴mJ⁻¹, respectively. These experiments also showed that trichloro- and dichloro-dibenzo-p-dioxins were produced with less than 15% of the initial quantity of TCDD, leading to the conclusion that the dechlorination process is a minor photolysis pathway.     

Zinc-catalyzed oxidation of 5-S-cysteinyldopa to 2,2'-bi(2H-1,4-benzothiazine): tracking the biosynthetic pathway of trichochromes, the characteristic pigments of red hair

Trichochromes, the peculiar pigments of red human hair, featuring the Delta(2,2)(')-bi(2H-1,4-benzothiazine) skeleton, are known to arise from cysteinyldopas, mainly the 5-S-isomer (5). However, the mode of formation and the direct precursors have remained largely undefined. To fill this gap, we investigated the oxidation of 5 in air or with chemical and enzymatic agents under biomimetic conditions. In the presence of zinc ions, which occur in epidermal tissues at significant concentrations, the reaction course is diverted toward the formation of a labile 3-carboxy-2H-1,4-benzothiazine intermediate (11), which was identified by direct NMR analysis. Structural formulation was supported by characterization of the analogous compound 13 isolated from oxidation of the model 5-methyl-3-S-cysteinylcatechol (12) after methylation. In the further stages of the oxidation, diastereomeric 2,2'-bi(2H-1,4-benzothiazine) 15 and 14 were obtained from 5 and 12, respectively, the reaction proceeding at a higher rate and to a greater extent in the presence of acids. The dimers were shown to readily convert to each other in the presence of acids. In the case of the methylated dimers 14, a 2,2'-bi(4H-1,4-benzothiazine) intermediate (16) was isolated and characterized. In acidic media, trichochrome C (1a), the most abundant in red human hair, was smoothly formed from aerial oxidation of 15, and under similar conditions, trichochrome-related products (17 and 18) were obtained from 14 prior to or after methylation. The presence of 1a and precursors 5 and 15 was investigated by HPLC analysis of red hair samples following mild proteolytic digestion. On the basis of these data, a likely biosynthetic route to trichochrome pigments of red human hair is depicted.     

Preparation and self-assembly of chiral porphyrin diads on the gold electrodes of quartz crystal microbalances: a novel potential approach to the development of enantioselective chemical sensors

Porphyrin diad 1 was synthesized by reaction of the acyl chloride of porphyrin 2 and trans-1,2-dithiane-4,5-diol. The Co complex of this diad was studied as a potential enantioselective receptor for chiral recognition in solution and in the solid state. In solution both enantiomers of limonene induce significant changes in the visible and circular dichroism (CD) spectra of [Co2(1)], while a different behavior is observed in the case of the enantiomeric pair of trans-1,2-diaminocyclohexane. A different efficiency of [Co2(1)] chiral recognition is obtained for these compounds, with a remarkable degree of enantiodiscrimination observed in the case of limonene. Self-assembled monolayers of [Co2(1)] were deposited onto the gold electrodes of quartz crystal microbalances to be used as sensing materials of nanogravimetric sensors operating in the gas phase. The enantiodiscrimination properties of these sensors towards the enantiomeric pairs of chiral analytes have been studied. While in the case of analytes bearing donor ligand atoms we did not observe a remarkable enantioselectivity, a significant degree of chiral discrimination was observed in the case of limonene; this result is particularly encouraging for the potential development of enantioselective chemical sensors for use in an array configuration.     

Syntheses, structures, and surface aromaticity of the new carbaalane [(AlH)(6)(AlNMe(3))(2)(CCH(2)R)(6)] (R = Ph, CH(2)SiMe(3)) and a stepwise functionalization of the inner and outer sphere of the cluster

The reaction of the acetylene RC triple bond CH (R = Ph, CH(2)SiMe(3)) with an excess of AlH(3).NMe(3) in boiling toluene leads to the carbaalane [(AlH)(6)(AlNMe(3))(2)(CCH(2)R)(6)] (R = Ph 1, CH(2)SiMe(3) 2) in good yield. Treatment of 2 with BCl(3) under varying conditions gives the chlorinated products [(AlCl)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(3))(6)] 3 and [(AlCl)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(2)Cl)(6)] 4, respectively. The latter clearly demonstrates that the cluster can be stepwise functionalized within the inner and outer sphere. The X-ray single-crystal structures of 1, 2, and 4 have been determined. All compounds have in common that the central core consists of a cluster having eight aluminum and six carbon atoms. The bonding properties in this cluster are described as a new manifestation of three-dimensional surface aromaticity. Each Al(4)C fragment of the cube is formed by four bonds with three electron pairs, thus leading to a strong delocalization of the electrons. A phenomenological modeling using a three-dimensional Hückel scheme with fitted parameters to reproduce the energies from ab initio calculations revealed that the orbital scheme localized at one Al(4)C fragment possesses an orbital sextet with a large HOMO-LUMO gap. This is in line with the criteria of aromaticity. The idea of aromaticity was sustained also by qualitative valence bond reasons enumerating the different resonance structures by means of graph theoretical methods.     

A NEW ONE-STEP REDUCTIVE N-ALKYLATION OF AMINO ACIDS AND THEIR ESTERS

The reductive alkylation of amine with carbonyl compounds usingsodium hydrogen telluride has been investigated in recent years.Application of this method to the synthesis of N-alkyl derivativesof biologically important amino acids and their esters is described.