By telluroxide elimination

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 starting telluroxides (see Section 3.1) are prepared by hydrolysis of the corresponding tellurium dibromides (method a) or by direct oxidation of the tellurides (MCPBA is the more appropriate oxidizing agenH) (method b). By the first method the telluroxides are obtained as hydrates and normally higher elimination yields are achieved. 

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 

Moreover the 2-pyridyltelluro moiety was shown to be a better leaving group than the usual phenyltelluro moiety. The following important features should be noted. 

---

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. 

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>. 

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. 

Ley and Barton s observation that di-4-methoxyphenyltelluride could be used catalytically was the first entry into the use of in situ generated selenoxides or telluroxides as catalysts. As shown in Fig. 8, a variety of different nucleophiles can be introduced via the selenoxide or telluroxide followed by reductive elimination to generate oxidized product and reduced selenide or telluride. If the nucleophile is relatively inert to oxidation by hydrogen peroxide, then the reduced selenide or telluride can be reoxidized by hydrogen peroxide and the overall oxidation of the nucleophile becomes catalytic in the selenide or telluride. In the case of thiols, disulfides are the final product and the selenides or tellurides exhibit thiolperox-idase-like activity 60-62 64 82 83 If halide salts (chloride, bromide, iodide) are the nucleophiles, then positive halogen sources are the oxidized products and the selenides and tellurides exhibit haloperoxidase-like activity.84-88 The phenoxypro-pyltelluride 59 has been used as a catalyst for the iodination and bromination of a variety of organic substrates as shown in Fig. 24.87... 

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. 

Alkyl phenyl telluroxides and cycloalkyl phenyl telluroxides, with the exception of cyclohexyl derivatives, are unstable compounds, suffering the elimination reaction at room temperature. In contrast, n-alkyl phenyl telluroxides and cyclohexyl phenyl telluroxide are stable compounds and undergo elimination only by prolonged heating in toluene or THF, or by pyrolysis at 200-240°C. 

---