Telluroxide elimination

The difference in stability between the cychc six- and seven-membered methoxy tel-luroxides is noteworthy. Indeed, while the cyclohexane derivative is stable and isolable, giving ring contraction on treatment with 1 equiv of MCPBA, the cycloheptane derivative is unstable, suffering telluroxide elimination (like cycloheptene formation from cyclohep-tyl phenyl telluride), as will be shown in Section 4.7. 

The telluroxide elimination appeared initially to be of little value compared to the popularity of the analogous selenoxide elimination. However, after the previous results which established the syn stereochemistry of the elimination, more detailed investigations clearly demonstrated the synthetic utility of this methodology. ... 

The addition of EtjN and other amines exhibits a remarkable effect on the telluroxide elimination, improving the alkene yields, suppressing the formation of side products and promoting the elimination even from primary alkylphenyl telluroxides. ... 

General procedure for telluroxide elimination by oxidation of alkyl phenyl tellurides in the presence of base. To a two-necked, round-bottomed flask (25 mL) containing alkyl phenyl telluride (1 mmol), triethylamine (1-2 mmol) and diethyl ether (5 mL) was added solid MCPBA (purity 80%) (2 mmol as pure MCPBA) portionwise at 25°C. The mixture was stirred with a magnetic stirrer for 2 h at the same temperature before being poured into... 

Since the starting tellurides are easily prepared (see Section 3.1.3.2) from the corresponding alkyl bromides and tellurolate ions, and -hydroxyalkyl tellurides by the opening of epoxides with the same reagents, the combined procedures furnish a method for the dehydrobromination of alkyl bromides and for the conversion of epoxides into allylic alcohols. Moreover, combining the telluroxide elimination with the methoxytelluration of olefins (see Sections 3.9.3.2 and 4.4.8.3), allylic and vinylic ethers are easily prepared. 

The telluroxide elimination shows a preference towards the less substituted carbon (as observed by the 2.48-2.50 1 ratio assessed in the elimination of 2-bromooctyl phenyl telluroxide and 2-bromodecyl phenyl telluroxide) that is more marked than that observed for the selenoxide and sulphoxide eliminations. By procedure b (MCPBA oxidation), the ratio is decreased to 1.5-1.7 1. 

The telluroxide elimination proceeds much slowly than selenoxide elimination. The accompanying scheme illustrates these useful synthetic transformations. 

The telluroxide elimination in allylic tellnrides proceedes via a [2,3]-sigmatropic rearrangement affording allylic alcohols after hydrolysis. ... 

The reaction of alkyl phenyl tellurides with excess chloramine-T (A -chloro-A -sodium-p-tolysulphonamide) in refluxing THF leads to olefins, presumably through a tellurosul-phimino intermediate. Owing to the high yields obtained, this method seems to be highly competitive with the telluroxide elimination. 

These compounds are relatively unstable, and treatment with hydrogen peroxide promotes telluroxide elimination to give the corresponding a-methylene ketones. 

These adducts, submitted to MCPBA oxidation, undergo telluroxide elimination, regenerating the original C=C bond. The overall transformation therefore provides a-hydroxy-alkylation of a,jS-unsaturated carbonyl compounds. ... 

The introduction of tellurium into an organic substrate promotes functional groups transformations or presents structural features that can be used for synthetic purposes, if suitable methods to remove tellurium from the resulting structures are available. To date, four main strategies have been explored for this end, namely, the telluroxide elimination, the tellurium/metal exchange, the coupling of tellurides with organometallic species and with alkynes, and the reductive removal via free radicals. 

When the reactions of benzenetellurinyl acetate or trifluoroacetate to olefins were carried out with nitriles as solvents and boron trifluoride diethyl etherate as catalyst, 2-acylamino-1-alkyl phenyl telluroxides were obtained. Telluroxide elimination produces 4,5-dihydrooxazoles2. 

As in the selenium case (Scheme 17) the oxidation of alkyl phenyl telluride with excess MCPBA in the presence of alcohols results in a facile substitution of a PhTe moiety by an alkoxy group. The reaction is assumed to proceed via a similar tellurone-MCPBA adduct intermediate. Oxidation of cycloalkyl telluride (61) was accompanied by ring contraction to produce an acetal (62), < while the bromination-hydrolysis method affords the allylic ether by telluroxide elimination (Scheme 22). ... 

Oxocyclopentyl and 3-oxocyclohexyl phenyl tellurium compounds reacted in tetrahydro-furan with butyraldehyde and benzaldehyde. The products of these aldol reactions were not isolated, but oxidized to the telluroxides that, in turn, were converted to enones by telluroxide elimination s. 

Telluroxide elimination, Alkyl phenyl tellurides on oxidation with CIC6H4CO3H in ether afford alkenes as the major product. In some cases a stable Te(IV) compound is formed, which undergoes elimination on pyrolysis. Examples ... 

Telluroxide elimination,2 Tellurides are converted to alkenes by reaction with ehlorumine-T, presumably via the adduct a (equation I). This elimination proceeds In low yield with t-butyl hydroperoxide. 

Nucleophilic oxirane ring opening with sodium phenyltellurate (Na+ C6H5Te ), followed by telluroxide elimination with base, has been employed as a two-step protocol for isomerizations to allyl alcohols <82TL1177, 83JOM(250)203>. 

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