The determination of tellurium by nondispersive atomic fluorescence spectrometry using the hydride generation technique

Atomic fluorescence spectrometry with a nondispersive measuring system is combined with a hydride generation technique for the determination of tellurium. Atomic fluorescence measurement is based on the reduction of tellurium by either metallic zinc or sodium borohydride, introduction of the generated tellurium hydride into a premixed argon (entrained air)-hydrogen flame, and excitation with a tellurium electrodeless discharge lamp. The comparison of the zinc and the sodium borohydride reduction methods is discussed in terms of detection limit, precision and interference. The best attainable detection limits for tellurium are 2ng (0.1 $\text{ng}\text{ml}$) and 30 ng (1.5 $\text{ng}\text{ml}$) with the zinc and the sodium borohydride methods respectively. Analytical working curves obtained from peak-height and peak-area measurements are linear over a range of approximately 4 orders of magnitude. Of the mineral acids examined in the range up to 2.0 m. nitric acid gives a depressing interference in the range greater than 0.5 m in the zinc method, whereas all of the acids greater than 1.0 m give a slight enhancement of the signal in the sodium borohydride method. The presence of several elements including other hydride-forming elements in 1000-fold ratio to tellurium causes a depressing interference, while enhancing interferences from tungsten and vanadium are observed in the zinc and the sodium borohydride methods, respectively. The present system coupled with the zinc method has been applied to the determination of tellurium in several samples of high-purity copper metal after separation of the analyte from copper by passing an ammoniacal solution of the sample through Chelex-100 resin. The results are in good agreement with those obtained by graphite furnace atomic absorption spectrometry.     

Evidence for anchimeric assistance by the Cp2ZrCl-group in the homolytic cleavage of an OCH3 bond

Anchimeric assistance by a transition metal in the homolysis of a CO bond is discussed for the thermal rearrangement of methyl(zirconocenyidiphenylmethyl)ether $\text{1c}$.     

Determination of antimony in geochemical samples by graphite furnace atomic absorption spectrometry using different matrix modifiers

Determination of antimony by graphite furnace atomic absorption spectrometry using five different matrix modifiers, viz. nitric acid, copper, nickel, molybdenum and palladium, together with L'vov platform was studied. Without matrix modification, antimony was lost in a 1.2-M HCl solution when the thermal pretreatment temperature exceeded 700°C. By using 1.4 M HNO₃ or $\text{1μg}\text{20μl}$ copper solution the thermal pretreatment temperature could be increased up to 900°C. The matrix modification with $\text{2μg}\text{20μl}$ palladium, $\text{4μg}\text{20μl}$ molybdenum, or $\text{20μg}\text{20μl}$ nickel allowed the use of 1300°C as the pretreatment temperature. The best results were obtained with palladium when its concentration exceeded $\text{0.8μg}\text{20μl}$. Chemical interferences were studied with standard solutions spiked with different metal chlorides. The severe interference caused by iron chloride was avoided with ascorbic acid (2.5 %, $\text{w}\text{v}$). The method was tested on some geochemical reference samples.     

Bestimmung von blei und zink in stäuben durch atomabsorption

The contents of lead and zinc in particles of ambient air, in particulate samples of waste gases and in particles of automobile exhaust gases are determined by atomic absorption spectrometry. The samples are dissolved by constantly boiling hydrochloric acid; commonly some nitric acid is added. Unsolvable components are separated by filtration. The concentrations of lead and zinc in the hydrochloric solutions are estimated by use of the acetylene-air-flame.The measuring ranges are between 1 and 10 mg $\text{Pb}\text{l}$, 5 and 50 mg $\text{Pb}\text{l}$ and 50 to 250 mg $\text{Pb}\text{l}$, and between 0,05 and 1,0 mg $\text{Zn}\text{l}$ and 0,5 and 5,0 mg $\text{Zn}\text{l}$. In the hydrochloric solutions of particles sampled in exhaust gases of automobiles the concentrations of lead were determined between 1 and 250 mg $\text{Pb}\text{l}$. The percentage of different particulate samples was found between 0,1 and 1,2 % for Pb and between 0,1 and 74% for Zn.     

Tandem aldol addition / allyl metal addition

Addition of the enol borates $\text{2}$ to aldehydes generated the internally protected aldols $\text{3}$. These were subjected in situ to an allyl metal addition giving the 1,3-diols $\text{7}$ or $\text{13}$. This constitutes a rapid evolution of a carbon skeleton by a highly correlated tandem aldol-addition / allyl metal addition.     

The clemmensen reaction of tigogenin. A reinvestigation

The reaction of tigogenin ($\text{1b}$) with amalgamated zinc yields tetrahydrotigogenin ($\text{2b}$) and the furostan $\text{3}$. While $\text{2b}$ undergoes acid catalyzed elimination to give the rearranged product $\text{7}$, $\text{3}$ spontaneously decomposes to a mixture of $\text{4}$ and $\text{5}$.     

A convenient route to carbocyclic analogs of nucleosides: (±) aristeromycin

Stereocontrolled elaboration of cyclopentadiene-azomethine adducts $\text{1}$⁶ and $\text{2}$⁷ led to a short total synthesis of (±) aristeromycin. In the course of this work a novel zinc reduction ($\text{6}$/$\text{7}$+$\text{9}$) was observed. Also, ozonolysis of the $\text{gem}$-dihalo olefin $\text{10}$ in methanol directly gave the methyl ester $\text{12}$.     

An improved method of synthesis of neplanocin and related cyclopentenyl-containing nucleosides

A facile one-step condensation of the cyclopentenyl α,-tosylate $\text{2c}$ with the alkali metal salts of 6-chloropurine and uracil, afforded the protected carbocyclic nucleosides $\text{5a}$ and $\text{8}$, respectively, which were converted to neplanocin a ($\text{6}$) and cyclopentenyl uracil ($\text{9}$) by established methods.     

Synthesis of “capped porphyrins”

The synthesis of “capped porphyrins” ($\text{10}$), ($\text{18}$), and ($\text{28}$), and their (chloro)iron(III), iron(II), cobalt (II), and zinc(II) complexes is reported. These complexes serve as models for the active site of the oxygen binding haemoproteins. In addition to reversible binding of dioxygen by each of the iron (II) porphyrin complexes, the 1-methyl-imidazole-(“C₃-capped porphyrin”) iron (II) complex ($\text{23}$) reacts reversibly with carbon monoxide, in solution at 25°C.     

A new approach to (±)-apovincamine

(±)-Apovincamine ($\text{2}$) is synthesized by dehydration of the β-hydroxyrster ($\text{7}$) obtained by alkylation of the aldehyde ($\text{6}$) with metal chloroacetate     

Synthesis and 13C nmr characterization of some π-excessive heteropolyaromatic compounds

Several π-excessive heteropolyaromatic compounds, which contain furan and thiophen ring and are possible antifungal agents, have been synthesized in good yields according to two general methods. The first method has been used to prepare compounds possessing thiophens linked by their 2- and 5-positions, such as the ter-aryls $\text{2b}$, $\text{2d}$ and $\text{2a}$. Two precursors of these compounds have been obtained either by the Glaser reaction, or using a novel Pd-mediated reaction. The second method,which consists of the Ni-or Pd-catalyzed heteroarylation of heteroarene halides via cross-coupling with heteroaryl Grignard reagents or zinc halides,has been used to prepare the bi-aryls $\text{1a-e}$, which contain two heteroaromatic units, and the ter-aryl $\text{2c}$. Compound $\text{1e}$ has been also prepared starting from 2-(2-thienyl) furan ($\text{1c}$) by selective lithiation, followed by bromination.The ¹³C NMR signals of $\text{1a-e}$ and $\text{2a-d}$ have been assigned on the basis of the literature data and by relaxation measurements. Relaxation data have been also used to obtain qualitative informations on the conformational equilibria of the bi-aryls $\text{1a}$, $\text{1c}$ and the ter-aryls $\text{2a-d}$.     

Cobalt(II) tetraphenylporphyrin-catalyzed isomerization of electronegative substituted quadricyclanes

Co(II)TPP¹ - catalysed isomerization of a series of electronegative substituted quadricyclanes($\text{1}$) to the corresponding norbornadiene($\text{2}$) was found to proceed via radicophilic attack of the metal to $\text{1}$.     

Synthesis of sesquiterpene antitumor lactones. 7. Studies directed toward the total synthesis of pentalenolactone. Intramolecular ene reaction.

Functionalized cyclopentene $\text{11}$ is cyclized via thermal ene reaction to $\text{12}$. All steps in the synthesis of $\text{12}$ are highly stereoselective.     

On the claisen rearrangement of 1,4-diaryloxy-2-butynes

A proton nmr follow-up provided conclusive evidence for the involvement of two sequential Claisen rearrangements in the thermal rearrangement of 1,4-diaryloxy-2-butynes $\text{1}$to 11a-methylpterocarpans $\text{3}$. A detailed study of the rearrangement of 1,4-diaryloxy-2-butynes $\text{1}$ in polyethyleneglycol-200 (PEG-200), indicated a definite possibility of selective synthesis of either benzofuron (3,2-b )benzofuran $\text{4}$ or benzofuro(2,3-b)benzofuran $\text{5}$ by varying the temperature of the reaction alone     

A kinetic template effect in the metal ion-catalysed formation of arylphosphonium salts

Tertiary phosphines react with $\text{o}$-halobenzaldimines and $\text{o}$-bromodiarylazo compounds, under mild conditions in the presence of a metal ion catalyst, with displacement of the halogen to form the related arylphosphonium salts. The corresponding $\text{o}$-haloaryl compounds do not undergo these reactions.     

(4+2)-cycloaddition of nitroalkenes with ynamines; formation of a 4H-1,2-oxazine 2-oxide derivative

The formation of a thermally unstable (4+2)-cycloadduct, a 4H-1,2-oxazine 2-oxide derivative ($\text{1}$), from the reaction of 1-nitrocyclopentene with 1-phenyl-2-(1-pyrrolidinyl)acetylene has been proven by the structure elucidation of isoxazole derivative $\text{3}$ which results from thermal rearrangement and by the structure determination of the 1,3-dipolar adducts $\text{5}$ of $\text{1}$ with electron-deficient acetylenes.     

Homoallylic substitution reactions of lithium dialkyl cuprates with cyclopropylcarbinyl halides: mechanistic considerations

Highly reactive lithium dialkyl cuprates and l-bromo-l-cyclo-propylalkanes, $\text{4}$, react to give good yields of the homoallylic substitution product, $\text{6}$. Less reactive organocuprates react with $\text{4}$ to give mixtures of $\text{6}$ and the direct substitution product, $\text{7}$. These results are consistent with a copper(I) radical intermediate which undergoes facile rearrangement prior to reductive coupling.     

ESR studies of the alkali metal complexes of verdazyl-labeled benzo-15-crown-5

A verdazyl derivative of benzo-15-crown-5 ($\text{1}$) has been prepared, and the complex formation between the spin labeled crown ether ($\text{1}$) and the alkali metal salts has been studied by ESR spectroscopy.     

Application of the α-alkynone cyclization : synthesis of rac-modhephene

Using the thermal α-alkynone cyclization $\text{8}$ → $\text{9}$ as the key step, modhephene $\text{1}$, a sesquiterpene with a [3.3.3]-propellane skeleton was synthesized.     

The 18-electron electrocyclisation of vinylogous fidecene. an unusual sequence of pericyclic processes

Upon thermal activation the vinylogous fidecene $\text{1}$ is isomerised into the pentacyclic indenoid hydrocarbon $\text{13}$ (benzene, t_{$\text{1}\text{2}$} (150°C) ca. 5 min), the formation of which is explained by an initial conrotatory (“symmetry-forbidden”) 18-electron electrocyclisation followed by two fast hydrogen migrations ([1.9], [1.5]) and 14π-electrocyclisation.