Elimination ditelluride

Recently, Kondo and coworkers reported on the polymerization of St with diphenyl diselenides (37) as the photoiniferters (Eq. 39) [ 162]. In the photopolymerization of St in the presence of 37a and 37b, the polymer yield and the molecular weight of the polymers increased with reaction time. The chain-end structure of the resulting polymer 38 was characterized. Polymer 38 underwent the reductive elimination of terminal seleno groups by reaction with tri-n-butyltin hydride in the presence of AIBN (Eq. 40). It also afforded the poly(St) with double bonds at both chain ends when it was treated with hydrogen peroxide (Eq. 41). They also reported the polymerization of St with diphenyl ditelluride to afford well-controlled molecular weight and its distribution [163]. 

Diaryl ditellurides are relatively thermolabile compounds and eliminate one tellurium atom by heating at 300°C. ° In the presence of copper metal, however, the extrusion of tellurium is achieved during reflux with toluene or dioxane. " " ... 

The by-product of the elimination, phenyltellurenic acid, is detected, at least partly, as diphenyl ditelluride, and is probably formed by disproportionation and oxidation reactions. 

In every case the product mixture was composed of the expected 2-((aryltelluro) methyl)chromanone as the very major product together with the corresponding elimination product and the diaryl ditelluride. Spectral analysis was facilitated by the possession of authentic samples of the ditellurides and, eventually, of the elimination products. The product ratios, and hence yields, were determined by integration of the NMR spectra. Attempted purification by chromatography on silica gel led only to less pure samples contaminated with increased amounts of ditelluride and elimination products. 

The second method employed for the synthesis of telluraxanthene is similar to that widely used for the preparation of various tricyclic heterocycles containing one or two heteroatoms. It is based on the interaction of 2,2 -dilithiodiphenylmethane with powdered tellurium, by which reaction telluraxanthene 82 has been prepared in 50% yield [81JOM(205)167]. The reaction apparently proceeds through the intermediate bis(telluro-phenolate) 86 which is oxidized to the heterocyclic ditelluride 87. The latter compound eliminates tellurium from its stericaily strained seven-membered ring and converts to telluraxanthene. This mechanistic scheme seems to be corroborated by the observation that the treatment of lithi-oarenes ArLi usually ends in the formation of diarylditellurides Ar2Te2 (74MI1 83MI2). 

The tendency of intermediate heterocyclic ditellurides to eliminate one of the tellurium atoms thus contracting the size of the ring is illustrated by the course of the oxidative cyclization of the dilithio dichalcogenides 78 (84JHC413). Whereas in the case of the diselenolate 78 (M = Se) the expected heterocyclic diselenide 79a is formed, albeit in low yield, the analogous reaction with the ditellurolate 78 (M = Te) results in an extrusion of elemental tellurium affording dibenzotellurophene 79b. 

Based on this unfortunate result, 119 was envisioned a synthesis of the elusive 1,6-epitelluro 119. Thus, the above sequence was repeated but with sodium ditelluride as the reagent the only product formed was that from a simple base-induced elimination, the alkene 118 <2005AJC188>. A very similar study has been published by an Indian... 

The analogous reaction of compound 5 (R = H) with powdered tellurium gives rise to di(2-tellurienyl) telluride in very low yield (11%). The main product of this reaction is, most prohahly, di(2-tellurienyl) ditelluride 6, which transforms to compound 7 by elimination of a tellurium atom. 

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