Dehydroxylation Step Research Articles

A dipolar cycloaddition approach toward the kopsifoline alkaloid framework

Using a metal-catalyzed domino reaction as the key step, the heterocyclic skeleton of the kopsifoline alkaloid family was constructed by a 1,3-dipolar cycloaddition of a carbonyl ylide dipole derived from a Rh(II)-catalyzed reaction of a diazo ketoester across the indole π-bond. Ring opening of the resulting 1,3-dipolar cycloadduct followed by a reductive dehydroxylation step resulted in the formation of a critical silyl enol ether necessary for the final F-ring closure of the kopsifoline skeleton.

Controlled rate thermal analysis and differential scanning calorimetry of sepiolites and palygorskites

A series of sepiolites, palygorskites and “Rocky Mountain Leather” (RML) clay minerals have been analysed by controlled rate thermal analysis and differential scanning calorimetry (DSC). Eight weight loss steps are observed and are structure and composition dependent. Three dehydration steps and five dehydroxylation steps are observed. The mass spectrometric curve mimicked the differential thermogravimetric (DTGA) curve enabling the detailed determination of the dehydration and dehydroxylation steps.

Controlled rate thermal analysis of nontronite

The thermal decomposition of a series of nontronites and a ferruginous smectite has been studied using a combination of high-resolution thermogravimetric analysis (TGA) with a controlled heating rate and the mass spectrometry (MS) of evolved water vapour. The temperature–mass spectrum curve follows the differential TGA (DTGA) curve with precision, thus providing a second means of calculating the percentage weight loss. Three dehydration steps were observed in the controlled rate thermal analysis (CRTA) experiment at around 86, 117 and 245 °C depending on the nontronite sample. Two overlapping dehydroxylation steps were observed at approximately 350 and 400 °C with a weight loss ratio of 1:2. A comparison is made between the between the results obtained from the dynamic DTGA experiment and the CRTA experiment.

Thermal evolution in air and argon of nanocrystalline MoS 2 synthesized under hydrothermal conditions

Nanocrystalline molybdenum sulfide was synthesized between 150°C and 225°C under hydrothermal conditions starting from ammonium heptamolybdate and thiourea. Samples were characterized with X-ray powder diffraction, electron microscopy, nitrogen adsorption, thermoanalysis and infrared spectroscopy. The hydroxyls involved in the synthesis and adsorbed on crystals surface hindered crystallization and samples still recrystallized after the final dehydroxylation step above 300°C, just when hydroxyls were isolated from each other. This also promoted sulfur bond breaking that gave rise to partial transformation of the MoS 2 into MoO 2 when the annealing atmosphere was argon, and to the total transformation of the sulfide into MoO 3 when it was air. The initial MoS 2 crystals were bend; many of them were isolated, and others associated in bundles that formed worm-like grains interacting with each other to produce spherical grains aggregated in clusters. This morphology gave rise to samples with a low specific surface area.

Structural aspects and thermal properties of takovite-like layered double hydroxides pillared with chromium oxo-anions

Intercalated layered double hydroxides (LDHs) of chromate (CrO42–) and dichromate (Cr2O72–) have been synthesised and characterised. The parent chloride material ([Ni–Al–Cl]) was obtained by coprecipitation at constant pH and further anion-exchanged to incorporate the desired oxo-anions. The physico-chemical properties of all compounds were investigated using X-ray powder diffraction, infrared spectroscopy, thermogravimetric analyses and extended X-ray absorption spectroscopy (EXAFS). The structural evolution of the pristine and pillared materials, following calcination between 100 to 450 °C, was also thoroughly studied. It is shown that the intercalated guests, chromate and dichromate anions, interact with the host hydroxylated sheets at moderate temperatures. This so-called “grafting process” is characterised by a reorganisation of the anionic species within the interlamellar domain associated with a shrinking of the basal spacing. This phenomenon was clearly evidenced with powder X-ray diffraction (contraction of the interlamellar distance) and, more interestingly, by chromium K-edge EXAFS (variation in the local environment of the probe element). On the other hand, thermogravimetric studies indicated that the thermal stability was greatly enhanced following pillaring with oxo-anions. This is reflected through the dehydroxylation step of the LDHs sheets which shifted to much higher temperatures, from 280 °C in the [Ni–Al–Cl] precursor to over 450 °C in the dichromate intercalated compound. Rehydration and reverse exchanges with carbonate anions of the thermally treated materials were both unsuccessful, suggesting that the grafting phenomenon is irreversible. The specific surface areas were also negatively affected following anion exchange and moderate thermal treatment, indicating that the structural changes observed through the different analytical techniques also influence the microtextural properties.

Organization and nature of fortimicin A (astromicin) biosynthetic genes studied using a cosmid library of Micromonospora olivasterospora DNA.

The cloning of five DNA segments carrying at least seven genes (fms1, fms3, fms4, fms5, fms7, fms11, and fms12) that participate in fortimicin A (astromicin) biosynthesis was described previously. These DNA fragments were used to screen a cosmid library of genomic DNA in order to examine if these biosynthetic genes are clustered in Micromonospora olivasterospora. One cosmid clone (pGLM990) was obtained, which hybridized to all the probes. Complementation analysis, using mutants blocked at various steps and chimeric plasmids subcloned from pGLM990, showed that three additional genes (fms8, fms10, and fms13) are present in pGLM990. A gene conferring self-resistance to the antibiotic, which was independently cloned in Streptomyces lividans, using the plasmid vector pIJ702 was also found to be linked to the cluster of biosynthetic genes. Thus, at least ten biosynthetic genes and a self-defense gene are clustered in a chromosomal region of about 27 kb in M. olivasterospora. Interestingly, the fms8 gene which participates in the dehydroxylation step of fortimicin A biosynthesis was found to have homology with a neomycin resistance gene nmrA from the neomycin-producing Micromonospora sp. MK50. Studies using a cell-free extract of the fms8 mutant and its parent strain showed that the enzyme encoded by fms8 phosphorylates a biosynthetic precursor, fortimicin KK1, in the presence of ATP. Thus the dehydroxylation reaction is suggested to occur via the phosphorylation of the target hydroxyl group. DNA regions homologous to fms genes were found in Micromonospora sp. SF-2098 and Dactylosporangium matsuzakiense, both producers of fortimicin group antibiotics.

Dehydration kinetics of an Indian clay

Decomposition of clay from Garo Hills of Meghalaya (India) was studied by dynamic thermogravimetry. A fourteen kinetic equation procedure was applied for analysis of non-isothermal weight change upto a temperature of 1223 K. Two stages of decomposition were clearly distinguised i.e. the initial dehydration period at temperature 373 K to 673 K and the dehydroxylation period in the temperature range 723 K to 1073 K. First order decomposition mechanism was found to be applicable to both dehydration and dehydroxylation steps. Activation energies of 67 kJ/mol and 278 kJ/mol are estimated for the first and second steps with frequency factors of 16.3 s−1 and 5.16×106s−1 respectively. The weight loss relating to the steps in TG curves allows determination of the contents of the basic mineral in the clay. Percentages of Al2O3.SiO2 and total volatiles as calculated from weight loss data are in accordance with the results of classical chemical analysis.

Comparison of attapulgites from different sources using TG/DTG and FTIR

Abstract Several samples of attapulgites from different sources were analyzed by TG/DTG and FTIR. The thermal behaviour of these clay minerals varies within certain limits according to their origin. For some attapulgite samples, dehydration (bound water) and dehydroxylation occur within the same range of temperature. These differences in thermal behaviour could be related to the various crystal structures, and/or to the chemical compositions of these minerals. We have also observed that associated carbonates in attapulgites decompose in the same temperature zone as the dehydroxylation step. These results could explain the disagreements found in the literature with respect to the quantity of constituent water when comparing the chemical composition of different attapulgite clay minerals.

Deammoniation and dehydroxylation of calcium ammonium chabazites

The deammoniation and dehydroxylation of calcium ammonium chabazites has been studied by thermal analysis and infrared spectroscopy. At low ammonium-exchange levels dehydroxylation starts only at extremely high temperatures (ca. 1200 K) owing to the occupancy of the crystallographic SI cation sites by Ca and hence to the existence of isolated hydroxyls at 3630 cm–1 vibrating in the plane of the eight-membered rings. At higher exchange levels a second band at 3550 cm–1 and a dehydroxylation step at much lower temperatures (890–990 K) develop, pointing to the existence of hydroxyls in close proximity which vibrate in the hexagonal prisms or in the six-membered rings. Steaming and calcination results in the appearance of new i.r. bands at 3670 and 3750 cm–1 which are assigned to hydroxyls vibrating in the large cavities and to lattice-terminating OH groups, respectively. Steaming of highly ammonium-exchanged chabazite shifts the frequencies of the acidic hydroxyls to higher values but does not result (as in the case of other zeolites) in a substantial release of aluminium from the framework. The structure of the highly exchanged but steamed H-chabazites is in contrast to the non-steamed samples, which are thermostable after rehydration, indicating that during such steam treatment less acidic hydroxyl groups are formed.

In vitro occurrence of m-octopamine in the cultured cephalic ganglion of Locusta migratoria L. after l-dopa administration

1. Cephalic ganglia of the locust Locusta migratoria L. were cultured in the presence of l-DOPA and inhibitors of dopa decarboxylase or dopamine β hydroxylase. 2. The addition of l-DOPA to the culture medium resulted in a marked increase of m-octopamine, following higher levels of dopamine. 3. l-DOPA produced an increase of m-tyramine. 4. RO 44602 considerably reduced the levels of p and m-octopamines, m-tyramine, dopamine and noradrenaline while fusaric acid reduced only those of p and m-octopamines, m-tyramine and noradrenaline. 5. Though not normally found in the locust nervous system, m-octopamine appears to be closely related to catecholamines and to be produced via their biosynthesizing enzymes through a hypothetical dehydroxylation step.

Studies on Metal Hydroxy Compounds. XIV. Thermal Analyses, Calorimetry, and Decomposition Kinetics of Heterocationic Hydroxy Chloride Compounds

Studies were carried out on two series of mixed metal hydroxy chloride compounds of the type xMe(OH)2•yMCl2 where Me = Cd, Cu and M = Mn, Co, Ni, Cu, Zn, Cd, Mg, or Ca.Thermal analyses for the Cd–M series reveal that the initial mode of decomposition involves a dehydroxylation step with a concomitant metathetical reaction between MCl2 and CdO. The compound CdCl2•2CdO is formed and decomposes at higher temperature leading to the eventual volatilization of CdCl2. The kinetics of thermal decomposition follow a simple first-order rate equation. The sole exception to this general pattern of behavior is the Cd–Zn compound.The initial mode of thermal decomposition of the Cu–M series compounds follows a parallel pattern described for the Cd–M series. The kinetic data for the Cu–M series are fitted into three groups: (i) phase boundary control embracing the contracting sphere and contracting plate model equations, (ii) A-E type equation with n = 2, and (iii) first-order rate equation.

Studies on metal hydroxy compounds. X. Thermal analyses, decomposition kinetics, and infrared spectra of lead halide (Cl, Br, I) derivatives

The thermal analyses, thermogravimetry (TG), and differential thermal analysis (DTA) of PbOHCl, PbOHBr, and PbOHI reveal that the initial mode of decomposition is via dehydroxylation. Calorimetric measurements along with related enthalpy values for the decomposition reaction are given.The infrared spectra of these compounds are interpreted in terms of folded bands of (PbOH+)n tied together by halide ions consistent with their crystal structures.The kinetics of thermal decomposition of lead hydroxyhalides follow a diffusion-type rate equation which is in contrast to the simple first-order rate equation observed for most Cd, Zn, and Cu compounds. These two distinct rates are interpreted in terms of mobile OH or H species in the dehydroxylation step.