Tellurium-carbon bond cleavage

Tellurium-lithium exchange in cyclic systems, like the telluranaphthalene 120, has been demonstrated by Sashida. The possibility of a two-fold tellurium-carbon bond cleavage introduces a route to doubly hthiated compounds. When compound 120 is treated with an excess of n-butyllithium in THF, the lithiated lithium tellurate 121 is formed, which is interconverting to the doubly hthiated compound 122 (Scheme 44, verified by deuteriolysis). 

Brominations are performed by dropping solutions containing the stoichiometrically required amount of bromine to the stirred solutions of the diorgano tellurium compounds. To prevent tellurium-carbon bond cleavage, which may occur with dialkyl and alkyl aryl tellurium compounds, the reactions should be performed at or below 20°. The likelihood of Te —C bond cleavage decreases from chlorine to iodine. 

Tellurium-carbon bond cleavage was also observed when bis[trifluoromethyl] tellurium dihalides9 and dibenzyl tellurium dihalides10 were exposed to elemental halogens. 

The cleavage of the tellurium-carbon bond in alkyl, aryl, and alkenyl tellurium trihalides has been investigated as a pathway to organic halides. 

The lO-E-4 chalcogen(IV) species diphenylselenium(IV) dibromide (1, Fig. 1) and diphenyltellurium(IV) dibromide (2, Fig. 1) oxidize thiophenol to diphenyl disulfide in nearly quantitative yield as shown in equations (13) and (14). Tellurium(IV) dihalides 6-11 also oxidize thiophenol to diphenyl disulfide and benzene selenol to diphenyl diselenide. Similarly, the 12-Te-5 molecule dioxatellurapentalene 45 (Fig. 19) is a mild oxidant for ethylmercaptan, thiophenol, and benzene selenol giving diethyl disulfide, diphenyl disulfide, and diphenyl diselenide in essentially quantitative yield. As shown in equation (15), 1,1,5,5,9,9-hexachloro-1,5,9-tritelluracyclododecane oxidizes six molecules of thiophenol to diphenyl disulfide and 1,5,9-tritelluracyclododecane in 90% yield. In contrast, 12-Te-5 pertellurane 44 and 12-Se-5 perselenane 46 do not oxidize thiophenol to diphenyl disulfide. Instead, these molecules undergo a nucleophilic addition of thiophenol followed by cleavage of the tellurium-carbon or selenium-carbon bond. ... 

Cleavage of a carbon to sulfur bond by a lithium reagent is a well known entry to organolithium reagents. This process has been used to synthesize a-nitrogen carbanions, and hence SMA derivatives.67 165 This study has been extended to tellurium and selenium starting materials as well.166... 

The precipitation of black tellurium frequently encountered in reactions carried out with tellurols or tellurolates indicates that the carbon-tellurium bond is prone to cleavage under rather mild conditions. Alkanetellurols and -tellurolates appear to be less stable than the aromatic compounds. Exposure of tellurolates to air and water causes at least partial decomposition of arenetellurolates7 and ethynetellurolates8 to elemental tellurium. 

Dimethyl ditellurium and tetramethyldistibane formed, within a few minutes at 20° in an exothermic reaction dime thyIstibane methyl tellurium in quantitative yield2. The reaction with tetraethyldistibane is best carried out at — 40° to prevent cleavage of carbon-antimony bonds. 

---