Telluration reactions

The ease with which the dipolarophile interacts with vinylacetylenes depends mainly on a spatial factor. The study of the reactions of alkylthiobuten-3-ynones-l and their selenic and telluric analogs with DPNT shows that, in this case, nitrilimine also acts as a nucleophilic agent with a nucleophilic center on the carbon atom of the 1,3-dipole and always adds to the terminal carbon of the enyne system to form l,3-diphenyl-5-/ -2-pyrazolenines. The oxidation of the latter with chloranil leads to alkynylpyrazoles (65ZOR51). 

The synthesis of the tellurium analogues of (24) and (25) requires a different approach, since it is not possible to prepare the necessary amido precursors in significant yields by telluration of [ BuN(H)P( U-N Bu)2PN(H) Bu]. However, the prior hthiation of this P(III)/P(III) system to give (7) followed by reaction with elemental tellurium generates the dianion (24, E = Te) as its dihthium salt (Eq. 5) [37] ... 

The electrodeposition of tellurium and silver has been investigated in dilute aqueous solutions of tellurous acid and Ag " ions (concentrations in the order of 10 to 10 " M) in 0.1 M HCIO4 [164], In particular, cyclic voltammetry experiments were conducted with rotating glassy carbon disk electrodes in baths with various concentration ratios of Ag(I) and Te(IV) precursors, and their outcome was discussed in terms of the voltammetric features. For a Ag(I)/Te(IV) ratio close to 0.8, formation of quasi pure silver telluride, Ag2Te, was reported. The authors, based on their measurements and on account of thermodynamic predictions, assumed that silver is deposited first on the electrode (Ag" + e Ag), and then Te(IV) is reduced on the previous silver deposit with formation of Ag2Te according to the reaction... 

Although selenic and telluric acids contain the central atom in the +6 oxidation state, they are very different. The properties of selenic acid, H2Se04, are very similar to those of H2S04, and many of their salts are similar. The oxyacid that contains Te in the +6 state is HsTeOs which can also be written as Te(OH)s. This acid can be prepared from Te or Te02 by suitable oxidation reactions, and it can be also obtained as a solid hydrate. As expected from the formula, telluric acid is a weak acid, although some salts can be obtained in which one or more protons are replaced. 

Common routes for the synthesis of selenates and tellurates(IV and VI) are the reactions of metal oxides or carbonates with the repsective acids (see Section 4). The disadvantage of this procedure is, that one usually obtains hydrates or, at higher acid concentrations, acidic compounds. Because the oxides E03 and E02 (E=Se, Te) are solids under ambient conditions, solid-state reactions with the respective metal oxides are an alternative route to prepare the anhydrous compounds. 

The tellurate EIg2Te207 is dimorphic. Both forms have been prepared from HgO, Te02, and Te03 by chemical transport reactions with HgCl2 as transport agent.136 In both modifications [Te(VI)Oe] octahedra are connected to infinite... 

The synthetic utility of the described reactions is clearly illustrated by the reductive detel-luration of telluroethers with BujSnH, the entire telluration/detelluration process providing a cyclization under mild conditions for unsaturated alcohols. 

Also, telluric acid can be prepared by oxidation of tellurium or tellurium dioxide with a strong oxidizing agent such as hydrogen peroxide, sodium peroxide, chromic acid, or potassium permanganate in nitric acid. Molecular equations for overall reactions are shown helow ... 

When the reaction is carried out at constant pH and when the ratio of telluric acid to glycerol is large, then... 

Data which allow this equation to be tested must be on solutions which have a high ratio of telluric acid to glycerol. In the table below this is satisfied as the concentration of glycerol in all the solutions is 4.13 X Hrtlf. The pH of the reaction medium is 8.0. 

These data indicate that the complexed telluric add is undergoing no apparent reaction. If any such reaction occurs, its rate constant is very, very small. 

Such effective masking may be traced to two factors (a) the inertness of the telluric acid complexes and (b) the near identity of the demands of oxidant and tellurate on the coordinating properties of the ligand. The decrease in reaction rate here is consistent with a very slow release of ligand from the tellurate complex as established by Edwards and his co-workers (1J). 

Sodium tellurate [10101 -25-8], N TeO (53.7% Te theoretically), is made by oxidizing sodium tellurite solution with hydrogen peroxide. The reaction is exothermic. 

Against a white surface this colour reaction may be observed even with a 0-00001 per cent, concentration of selenious acid. The reaction is more sensitive with sulphuric acid above 75 per cent, concentration than with dilute acid. The test is not applicable if the sulphuric acid contains iron. Tellurous acid retards the reaction.1... 

When the oxychloride is warmed with potassium perdisulphate, chlorine is evolved if sulphuric acid is present the reaction occurs in the cold. Chlorine is also evolved from selenium oxychloride when warmed with telluric or selenic acid. 

Tellurous acid is rapidly reduced to the element by sulphurous acid in the presence of potassium iodide.3 Owing, however, to the affinity which tellurium has for iodine and the consequent formation of varying proportions of tellurium tetra-iodide, the trustworthiness of the quantitative method based on this reaction is questionable.4... 

Volumetric Estimation.—Tellurium may be determined by oxidation from the tellurous to the telluric condition, using an excess of potassium dichromatc or permanganate and subsequently titrating the excess of oxidising agent with a standard solution of a suitable reducing agent.2 In order to obtain accurate results with the potassium dichromate titration, certain very definite steps in the procedure are essential, and it is necessary to control the course of the reaction, since hydrochloric and telluric acids interact with production of chlorine. 

Tellurium Dioxide as an Acid Anhydride.—When tellurium tetrachloride is treated with water, or when an aqueous solution of a tellurium salt is decomposed by an aqueous alkaline solution, a bulky, colourless precipitate is obtained which is sufficiently soluble in water to impart an acid reaction to the solution and which is more readily soluble than tellurium dioxide in acids or alkalis. A similar precipitate is obtained on acidifying a cold aqueous solution of potassium tellurite with a slight excess of nitric acid. This product is a tellurous acid, possibly H2TeOs, but it is very unstable and spontaneously dehydrates, slowly at the ordinary temperature and rapidly at 40° C. with formation of dioxide.1... 

That the foregoing classes of salts may be derived from different acids is quite feasible, for the second form of telluric acid described, allotelluric acid, obtained by heating the crystalline acid II6Te06 (p. 885), differs from the ortho-acid not only in its greater acidity but in its precipitation reactions.2... 

The difluoroboronate chelate complexes of the derivatives of tellur-achromones 104 (X = O) and tellurathiochromone 104 (X = S), which may be viewed as the corresponding tellurachromylium zwitterions 105, were prepared through the demethylation reaction of the 5-methoxy derivatives 104 under the action of the complex BF3-Et20 (88MI4). 

Using a similar protocol but with phenylacetylene instead of isonitrile, a carbo-telluration product is formed in high yield (Scheme 6.8) [15], The product, containing a vinyltellurium moiety is subjected to a second radical reaction with 2-(ethoxycarbonyl)allyltin, and the corresponding 1,4-diene is formed in good yield. 

Among activated forms of amino acids, mixed anhydrides with inorganic phosphate or phosphate esters require a special discussion because they are universally involved in peptide biosynthesis through the ribosomal and non-ribosomal pathways. These mixed anhydrides have stimulated studies in prebiotic chemistry very early in the history of this field. Amino acyl adenylates 18c have been shown to polymerize in solution [159,160] and in the presence of clays [139]. However, their participation as major activated amino acid species to the prebiotic formation of peptides from amino acids is unlikely for at least two reasons. Firstly, amino acid adenylates that have a significant lifetime in aqueous solution become very unstable as soon as either CO2 or bicarbonate is present at millimolar concentration [137]. Lacey and coworkers [161] were therefore conduced to consider that CO2 was absent in the primitive atmosphere for aminoacyl adenylate to have a sufficient lifetime and then to allow for the emergence of the modern process of amino acid activation and of the translation apparatus. But this proposition is unlikely, as shown by the analysis of geological records in favor of CO2 contents in the atmosphere higher than present levels [128]. It is also in contradiction with most studies of the evolution of the atmosphere of telluric planets [30,32], Secondly, there is no prebiotic pathway available for adenylate formation and ATP proved to be inefficient in this reaction [162]. 

Sodium selenopentathionate 3-hydrate, which was first isolated in 1949,1 is prepared by the reaction used by Norris and Fay2 for the iodometric analysis of selenious acid. Salts of telluropentathionic acid were also first isolated in 1949.3 The reaction utilized in their preparation was also discovered by Norris and Fay.2,4 Tellurous acid cannot be analyzed by a procedure analogous to that which Norris and Fay applied to the determination of selenious acid, because telluropentathionate is not indifferent to iodine but may be titrated directly with it 3... 

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