Reduction of organotellurium trichlorides with sodium borohydride

The frequency of the resonance of 125Te of two organo-ditellurides, R–Te–Te–R (R = 4-CH3C6H4 and 2-(CH3)2NCH2C6H4), in solution undergoes a low-field shift as the concentration of the sample increases. In sharp contrast, the resonance of a sterically hindered ditelluride (R = (C6H5(CH3)2Si)3C) and telluric acid display the opposite effect. While the negative concentration coefficients can be explained by the change in magnetic susceptibility, the positive coefficients are consistent with autoassociation of the molecules through tellurium-centred supramolecular interactions. Although the corresponding equilibrium constants are small, the process is shown to be exothermic. However, the influence of autoassociation is much smaller than the effects of solvent polarity and the conformation of the ditelluride bond.

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125Te NMR provides evidence of autoassociation of organo-ditellurides in solution.

Organic diselenides and ditellurides represent very important classes of compounds, which find valuable applications in organochalcogen chemistry and in organic synthesis. The review describes the main efficient and reliable approaches to organic diselenides and ditellurides and their principal chemical reactions. Examples of syntheses of tri- and polyselenides and -tellurides have been presented. Typical reactions of organic diselenides and ditellurides with nucleophiles, electrophiles, radicals, carbenes and compounds containing the element-element bond have been discussed in the review. Regio- and stereoselective reactions of organic diselenides and ditellurides with unsaturated compounds provide various chalcogenyl and bis(chalcogenyl) derivatives, which are often used in further transformations including stereoselective synthesis of functionalized alkenes. Along with valuable chemical reactions, organic diselenides and ditellurides act as precursors of both nucleophilic and electrophilic selenium and tellurium species, which participate in various useful transformations. The remarkable property of reacting with high regio- and stereoselectivity finds widespread application of selenium and tellurium species in organic synthesis. Besides valuable synthetic applications, organic diselenides and ditellurides exhibit various biological activities including glutathione peroxidase-like action. Aspects of biological activity of these compounds as well as a comparison of selenol–diselenide and thiol–disulfide reversible exchange reactions have been discussed in the review.


Keywords:

diselenides;
ditellurides;
triselenides;
tritellurides;
polyselenides;
polytellurides;
selenium;
tellurium

Organic Diselenides, Ditellurides, Polyselenides and Polytellurides. Synthesis and Reactions

Literature data concerning the synthesis and reactions of diorganyl tellurides, viz., dialkyl, alkyl aryl and diaryl tellurides are systematized and generalized in this review.

Synthesis and Reactions of Diorganyl Tellurides

The chalcogen bonding (ChB) ability of Te is studied in symmetrical diaryl ditellurides ArTeTeAr. Among the two Te σ-holes, the one along the less polarized Te-Te bond was calculated as the more electropositive. This counter-intuitive situation is due to the hyperconjugation contribution from Te lone pair to the σ* of the adjacent Te which coincides with σ-hole along the more polarized Te-Ar bond. ArTeTeAr showed notable structural features in the solid state as a result of intermolecular Te···Te ChB, such as a Te 4 rectangle through dimer aggregation or a triangular Te 3 motif, where one Te interacts with both Te atoms of a neighboring molecule through both its σ-hole and lone pair, in a slightly frustrated geometry. Lewis acidity of ArTeTeAr was also evaluated by NMR with R 3 PO as σ-hole acceptors in different solvents. Thus, 125 Te NMR allowed monitoring Te···O interaction and delivering association constants ( K a ) for 1:1 adducts. The highest value of K a = 90 M -1 was measured for the adduct between ArTeTeAr bearing CF 3 groups and Et 3 PO in cyclohexane. Notably, by using n Bu 3 PO, Te···O interaction was revealed by 19 F- 1 H HOESY showing spatial proximity between CF 3 and CH 3 of n Bu 3 PO.

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Chalcogen Bonding with Diaryl Ditellurides: Evidence from Solid State and Solution Studies.

A kinetic and mechanistic study of the photochemical degradation of a diarylditelluride in toluene solutions containing alcohol

When an aqueous dioxane solution of Na2SO3 or of Na2S2O5 is added to a solution of p-ethoxyphenyltellurium trichloride (RTeCl3) in dioxane the reaction mixture remains homogeneous and the related ditelluride R2Te2 is formed quantitatively. This reaction has been investigated kinetically. The first step in the process is the hydrolysis of the trichloride to give the aryltellurate, RTeO2–. When the water content of the solvent is low this acid-catalysed step becomes rate determining. More usually that step is rapid and the slow step is the reduction of the RTeO2– by HSO3– to give RTeO–. This tellurium(II) intermediate is believed to dimerize to R(HO)TeTe(OH)R before being reduced by another mole of bisulphite to the ditelluride.

A kinetic and mechanistic study of the formation of a diaryl ditelluride in solution by the sulphite reduction of the aryltellurium trichloride

A Kinetic and mechanistic study of the reduction of an aryltellurium trichloride by chromium(II) and vanadium(II) ions

When an aryltellurium trichloride reacts with methanol in a mixed solvent also containing benzene or dioxane the reaction can be followed by monitoring the generation of acid. The first Te–Cl bond cleavage is acid catalysed and is rate determining. It exhibits a small activation enthalpy, but a large and negative activation entropy. The other trihalides have similar activation parameters, The mechanism is believed to involve protonation of a co-ordinated Cl, with dissociative loss of HCl being rate determining. The compounds RTe(O)Cl and RTe(OMe)2Cl have been isolated as solid products but, since the reactions occurring in solution are reversible, the products in dilute solution may be different from these. Kinetic experiments to study the variation of rate with the solvent composition and the nature of the alcohol are also reported, as are results of reactions in aqueous dioxane.

Tahir A. Tahir

Papers

A kinetic and mechanistic study of the photochemical degradation of a diarylditelluride in toluene solutions containing alcohol

A kinetic and mechanistic study of the formation of a diaryl ditelluride in solution by the sulphite reduction of the aryltellurium trichloride

A Kinetic and mechanistic study of the reduction of an aryltellurium trichloride by chromium(II) and vanadium(II) ions

A kinetic and mechanistic study of the solvolysis of aryitellurium trihalides in organic solvents

A Kinetic and Mechanistic Study of the Solvolysis of Aryltellurium Trihalides in Organic Solvents.