Telluroxides

The dimeric tellurium diimide 10.7 undergoes a cycloaddition reaction with BuNCO to generate the Ai,iV -ureatotellurium imide 10.11, which is converted to the corresponding telluroxide 10.12 by reaction with excess BuNCO. " By contrast, BuN=S=N Bu undergoes exchange reactions with isocyanates. 

A series of a-telluranes 44 with X = F, ONO2, OCOMe, and OCOCF3 were prepared by a treatment of the ylides 46 [82DOK(266)1164 85KGS757] or the telluroxides 47 (85KGS757) with protic acids. 

On treatment with an equivalent amount of (Et2N)3P, thiotelluroxide 67a is reduced to give telluroazocine 65 in almost quantitative yield, which can be converted into telluroxide 68 by the usual procedures for a diorganyl telluride to telluroxide transformation. It is worth noting that when treated with Lawesson reagent, the telluroxide 68 forms thiotelluroxide 67a, whereas diaryl telluroxides are reduced by this reagent to diaryl tellurides. 

Sulfoxides (or selenoxides or telluroxides) and sulfones (or selenones or tellurones) may be named by functional class nomenclature [13]. 

On the other hand, optically active telluroxides have not been isolated until recently, although it has been surmised that they are key intermediates in asymmetric synthesis.3,4 In 1997, optically active telluroxides 3, stabilized by bulky substituents toward racemization, were isolated for the first time by liquid chromatography on optically active columns.13,14 The stereochemistry was determined by comparing their chiroptical properties with those of chiral selenoxides with known absolute configurations. The stability of the chiral telluroxides toward racemization was found to be lower than that of the corresponding selenoxides, and the racemization mechanism that involved formation of the achiral hydrate by reaction of water was also clarified. Telluroxides 4 and 5, which were thermodynamically stabilized by nitrogen-tellurium interactions, were also optically resolved and their absolute configurations and stability were studied (Scheme 2).12,14... 

On the other hand, telluronium imides 13 were isolated for the first time in 2002 by optical resolution of their racemic samples on an optically active column by medium-pressure column chromatography.27 The relationship between the absolute configurations and the chiroptical properties was clarified on the basis of their specific rotations and circular dichroism spectra. The racemization mechanism of the optically active telluronium imides, which involved the formation of corresponding telluroxides by hydrolysis of the telluronium imides, was proposed (Scheme 6). 

Oxidation of thiols and related compounds with organoselenium(IV) and organotellurium(rV) compounds 102 Other oxidations with selenoxides and telluroxides 106 Thiolperoxidase and haloperoxidase-like activity of organoselenides and organotellurides 108... 

Selenoxides and telluroxides also function as mild oxidants for the conversion of thiols to disulfides as shown in equations (16) and (17) for the reaction of thiophenol with diphenylselenoxide (47) and diphenyltelluroxide (48). Mechanistically, the oxidation of thiols to disulfides with selenoxides and telluroxides is a multi-step process, which takes advantage of the ease with which tellurium(IV) and selenium(IV) species form trigonal bipryamidal... 

Unlike sulfoxides, which exist primarily as the tetrahedral oxide, both selenoxides and telluroxides add water to give dihydroxy selenanes and telluranes, respectively, which have trigonal bipyramidal structures with two hydroxyl groups as the axial ligands. The hydration step is reversible as shown in Fig. Diphenylselen-... 

The chemistry associated with selenoxides and telluroxides in protic solvents is a dynamic process. If other nucleophiles are present in the solution, then these... 

Once the thiol is introduced to the coordination sphere of the selenoxide or telluroxide, a second slower reaction occurs. This step is associated with reduction of the chalcogen(IV) oxidation state to the chalcogen(II) oxidation state, which was demonstrated with dihydroxy telluranes 52 and 53. In the tellurium(IV) oxidation state of 52 and 53, the 5p orbital of tellurium is involved in the three-center, four-electron bond and cannot interact with the carbon 7r-framework. Long-wavelength absorption maxima for 52 and 53 are found at 510 and 500 nm, respectively in water. Reductive elimination generates a tellurium(II) atom, whose 5p orbital can now... 

Fig. 21 General mechanism for the oxidation of thiols to disulfides with selenoxides or telluroxides.

Several other oxidation reactions of selenoxides and telluroxides are summarized in Fig. 23. Ley, Barton, and co-workers discovered that di-4-methoxyphenyltellur-oxide (54) could be used catalytically as an oxidant in the presence of 1,2-dibromotetrachloroethane. After reduction of the telluroxide to the telluride, the di-4-methoxyphenyltelluride (24) debrominated the 1,2-dibromotetrachloroethane to give the tellurium(IV) dibromide, which was hydrolyzed in situ to give the telluroxide 54. This process was used to oxidize phosphines to phosphine oxides and... 

Fig. 23 Other oxidation reactions using delenoxides and telluroxides as mild oxidants.

The catalytic cycle for the thiolperoxidase and haloperoxidase-like activity of diorganoselenides and tellurides is summarized in Fig. 25. Stopped-flow spectroscopy has been used to elicit mechanistic details of the cycle. " " Following oxidation to the selenoxide or telluroxide, the catalytic cycle for thiolperoxidase-like activity is shown in Fig. 21. The details of the haloperoxidase-like cycle are not as well defined. Using dihydroxytellurane 52 as a substrate, the addition of 0.5 M sodium iodide in pH 6.8 buffer gave a fast reaction with a second-... 

The final products of oxidation of diarylselenides and tellurides (and sulfides as well) in the presence of nucleophiles are the corresponding chalcogen (IV) compounds. In the presence of water, the selenoxide or telluroxide (or the corresponding dihydroxy selenane or tellurane) is the final product. This still leaves several possible pathways, leveraged from early mechanistic studies done using electrochemical techniques on diaryl sulfides and outlined by Engman (Fig. 32). In these pathways, the initial radical cation can react with a nucleophile present in solution, or the dication resulting from further oxidation or disproportionation can do so. 

Diorganyl tellurides are easily oxidized to the corresponding telluroxides. Dialkyl tel-Inrides are especially sensitive to oxidation, and their exposnre to air, either neat or in soln-tion, canses the formation of dialkyl telluroxides. Diaryl tellnrides are much more stable and are oxidized only by chemical methods. 

Diaryl telluroxides have recently attained outstanding importance as mild and selective oxidizing reagents (see Section 4.4.1). 

Two methods are well established and generally employed to prepare diaryl telluroxides. 

Di-p-anisyl telluroxide (typical procedure) Di-p-anisyltellurium dichloride (8.00 g, 23 mmol) is stirred at 95°C in 5% aqueous NaOH (100 mL) for 1 h. After cooling at 0°C the white solid is separated by filtration, washed with cold H2O (3X10 mL) and dried nnder vacnnm over P2O5 giving the telluroxide (5.60 g (81%) m.p. 187-189°C). 

Bis(p-methoxyphenyl) telluroxides as a mild and selective oxidizing reagent... 

Oxidation with bis(p-methoxyphenyl) telluroxide (general procedure All the reactions are performed at room temperature under Nj in CHCI3 or CHjClj. Approximately 10 mL of solvent is used for every 100 mg of substrate. For thiocarbonyl derivatives, 1.1 equiv of the reagent is used, for thiols 0.55 equiv. The mixtures are concentrated by evaporation and submitted to thin layer or column chromatography to isolate the products. An2Te is always recovered in a yield of 64-96%, while sulphur or selenium is always recovered in near quantitative yields from reaction with thio- or selenocarbonyl derivatives. 

The following compounds are unaffected by bis(p-methoxyphenyl) telluroxide dithi-olanes, enamines, aldehydes, ketones, alcohols, pyrroles, indoles, amino acids, aromatic amines, monohydroxyarenes, esters, hindered thiocarbonates, isonitriles, oximes, arylhy-drazones, sulphides, and selenides. ... 

A useful modification of the bis(p-methoxyphenyl) telluroxide reagent is its immobilization on a polymeric resin. This polymer-supported reagent, prepared from p-methoxyphenyl tellurocyanate and polylp-Uthiostyrene), exhibits several advantages over the monomeric reagent, such as easier product work-up and multiple recycle from the spent reagent. 

Bis(p-methoxyphenyl) tellurium diacetate or ditosylate is generated by the electrolysis of the parent telluride in the presence of BU4N+ acetate or tosylate. Like the telluroxide, they accomplish the conversion of thioamides into nitriles and 1,2,4-thiadiazoles. Taking into consideration of these facts, the above conversion can be performed in an electrochemical... 

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