Tellurium-carbon bonds reactions with

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. 

The transformation of a carbon-tellurium bond into a carbon-halogen bond can be performed with several types of organotellurium trihalide or diorganotellurium dihalide compounds, in which the organic group is an alkyl, an alkenyl or an aryl substituent. A variety of reaction conditions have been described to realize these transformations, which belong to three main types ... 

Treatment of acetylenic tellurides (79) with three molecular equivalents of bromine or iodine leads to trihalovinylic derivatives (80). The reactions involve the oxidative halogenolysis of the carbon-tellurium bond followed by addition of the halide to the acetylenic bond. ... 

The extreme sensitivity of tellurols to oxygen make the isolation of these tellurium compounds difficult. Therefore, tellurols are almost always used in situ. In the literature, tellurols are sometimes claimed to be the product of the reduction of diaryl ditelluriums in ethanol as the reaction medium. Tellurols are probably present under these conditions in equilibrium with the tellurolates. Whether the tellurols or the tellurolates are the reactive species, for instance, in addition reactions to carbon-carbon multiple bonds, cannot be decided without additional studies. Benzenetellurol, formed in situ by methanolysis of phenyl trimethylsilyl tellurium, was much less reactive towards acetylenes than the tellurium compound obtained by reduction of diphenyl ditellurium with sodium borohydride in ethanol5. 

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. 

Te-NMR spectroscopic investigations ascertained the presence of the isomeric ketonyl tellurium trichlorides in the reaction mixtures. The isomers with tellurium on the less hindered carbon atom are generally the predominant species. Only 2-butanone yielded the 2-oxo-3-butyl tellurium trichloride as the major compound1, in which the tellurium is bonded to the more hindered carbon atom. Morgan2 claimed this compound to be 2-oxo-l-butyl tellurium trichloride. The symmetrical diketonyl tellurium dichlorides with tellurium bonded to the least hindered carbon were the only dichlorides detected1. 

A few reactions were carried out that proved that the telluride anion adds to carbon-carbon triple bonds. The corresponding diorgano telluriums were obtained in low yields. Tellurium, when heated with acetylene, water, and a base, produced divinyl tellurium. Telluride was claimed to be a product of the disproportionation of tellurium3. When this reaction was carried out with aqueous potassium hydroxide and hexamethylphosphoric triamide at 120° in a steel autoclave under 10 atm acetylene, the yield of divinyl tellurium was 30%4. The yield increased to 94% when tin(II) chloride was present in the reaction mixture5 1. 

Arenetellurolates, ethenetellurolates, and alkanetellurolates prepared by reduction of diorgano ditellurium compounds with sodium borohydride in ethanol, THF/ethanol, or DMSO add to acetylenes in regioselective and iran.y-stereoselcctive reactions to produce aryl ethenyl tellurium products either predominantly or exclusively as (Z)-isomers. The yields are almost always higher than 70%. In reactions with acetylenic aldehydes, ketones, carboxylic acids, and esters the arenetellurolate becomes bonded to the carbon atom in a [i-position to the carbonyl group. 

Un symmetrical di vinyl telluriums were synthesized by addition of sodium ethenetellurolate to monosubstituted acetylenes. The presence of sodium hydroxide in the reaction mixture is crucial for the success of these reactions4. Sodium hydroxide converts tellurols, which reduce the carbon-carbon double bonds in vinyl tellurium compounds5, to tellurolates that do not react with double bonds under these conditions. 

Diorgano tellurium dihalides are often the primary products of reactions producing compounds with two tellurium-carbon bonds. Such reactions arc the condensation of tellurium tetrachloride with aromatic compounds (p. 527), the addition of tellurium tetrachloride or organo tellurium trichlorides to carbon-carbon multiple bonds (p. 530, 544), and the alkylation or arylation of organo tellurium trihalides (p. 549). The symmetrical and unsymmetrical diorgano tellurium dihalides are convenient starting materials for the preparation of diorgano tellurium derivatives. 

Tellurium tetrachloride adds across carbon-carbon double bonds of olefins to yield bis[2-chloroalkyl] tellurium dichlorides. The reactions proceed stepwise with 2-chloroalkyl tellurium trichlorides as the primary products that subsequently combine with another molecule of the olefin. The second step may require elevated temperatures. 

Phenyl- 1-trimethylsiloxyethene, the trimethylsilylenol ether of methyl phenyl ketone, added tellurium tetrachloride across the carbon-carbon double bond. Subsequent elimination of chlorotrimethylsilane produced bis[benzoylmethyl] tellurium dichloride. Addition to enols is also the first step in reactions of tellurium tetrachloride with monoketones and 1,3-diketones (p. 534, 535). 

Phenyl tellurium trihalides react with linear olefins and cycloalkenes in methanol or other alcohols. In contrast to reactions in inert organic solvents, in alcoholic media the intermediate adduct between the olefin and the positive phenyldihalotelluro group is attacked by the alcohol and not by the halide ion. Therefore, the product is a 2-alkoxyalkyl phenyl tellurium dihalide. The reactions are regiospecific (tellurium bonding to the less hindered carbon atom) and highly stereospecific (anti-addition). 

A reaction sequence starting with bis[phenyltelluro]mcthane leads to the insertion of a methylene group into the carbon-halogen bond of an alkyl halide. The bis[phenyltelluro] methane is reacted with butyl lithium to yield lithiomethyl phenyl telluride, which combines with an alkyl halide. The resulting alkylmethyl phenyl tellurium is converted to the tellurium dihalide, which in turn decomposes to an alkylmethyl halide when kept under vacuum at 250 6 or heated in DMF in the presence of a sodium halide at 100°7. 

Unsymmetrical diorganyltellurium dihalides are formed upon condensation of aryltellurium trichlorides with activated aromatic compounds and with ketones. The addition of the trichloride across carbon-carbon double bonds in alkenes, as well as the reaction with aryl(trimethyl)silane, hexaphenyldilead, and aryhnercury chlorides leads to the transfer of the aryl group to the tellurium atom. 

The lability of the carbon-tellurium double bond has frequently thwarted attempts to study both telluraldehydes and telluroketones. Tel-lurocarbonyl compounds stabilized by coordination to transition metals have been known since 1980 [80CC635 83AG(E)314 88JOM161]. However, free telluraldehydes were unknown until 1989 when two different synthetic routes were reported. Erker and Hock trapped tellurobenzalde-hyde (92, R = Ph) generated by reaction of ylide (91) with tellurium powder adduct 93 was obtained in low yield [89AG(E)179]. 

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