To Diorgano Ditellurium Compounds

Aryl tellurium trihalides are easily reduced by sodium hydrogen sulfite , sodium or potassium disulfitezinc in refluxing ethanoP, hydrazine hydrate hypophos-phorous acid °, sodium sulfide nonahydrate , sodium borohydride , thiourea diox-ide, 4-methylbenzenesulfonyl hydrazide , or diphenylsilane. The reductions with sulfites are generally carried out in an aqueous medium in the presence of an immiscible organic solvent that extracts the ditellurium. 

Zakharkin, I. V. Pisareva, Izv. Akad. Nauk. SSSR, Ser. Khim. 1987, 877 engl. 803. 

Irgolic Organic Tellurium Compounds with one Te,C Bond 

The reductions with sodium sulfide proceed when the reactants are heated to 100° without solvents. Sensitive organo tellurium trihalides might be decomposed by some of these reducing agents. 8-Chloroalkyl tellurium trichlorides are converted to olefins upon treatment with sodium sulfide, whereas y-chloroalkyl tellurium trichlorides are reduced to ditellurium products. 2-Alkoxyalkyl tellurium trichlorides are reduced without difficulty by sodium disulfite to bis[2-alkoxyalkyl] ditellurium compounds.  

Alkyl and aryl tellurium trihalides with stabilizing electron-donor groups in the molecule that coordinate to the tellurium atom are not reduced by disodium sulfite or hypophosphorous acid to ditellurium compounds but only to tellurium monohalides. 

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Lithium benzenetellurolate8,9 and lithium ethanetellurolate10 were more conveniently obtained from the corresponding diorgano ditellurium compounds and lithium at room temperature with anhydrous diethyl ether or tetrahydrofuran as reaction medium. The reductions are complete when the orange-red color of the ditellurium derivatives has faded to pale yellow. 

The reduction of diorgano ditellurium compounds by sodium borohydride is applicable to both dialkyl, divinyl and diaryl ditellurium compounds (Table 2 p. 166). 

These compounds are liquids or low melting solids with characteristic foul and persistent odors. They can be stored under an inert atmosphere in the refrigerator for long periods without decomposition. At 20° they decompose slowly to diorgano ditellurium derivatives while in air, tellurium is deposited. Protic agents such as water, hydrogen halides, and alcohols cleave the tellurium-group TV element bond. 

Quite a large number of organic tellurium compounds with a C —Te —S or a C-Te —Se unit have been prepared. Most of the compounds are rather stable towards atmospheric agents. The low molecular mass compounds have a tendency to convert to diorgano ditellurium and diselenium derivatives. 

The complexes formed from diorgano ditellurium compounds and copper(I) halides in ethanol/acetonitrile are postulated to have intact Te-Te bonds1,3,4. 

Organo tellurium halides, also named organo tellurenyl halides, are in principle easily accessible through controlled halogenolysis of diorgano ditellurium compounds. However, 2-naphthyl tellurium iodide, prepared in 19592 from bis[2-naphthyl] ditellurium and iodine, remained the only compound of this type until 1971/72, when a series of ortho-carbonyl substituted phenyl tellurium halides were synthesized3-4. Aryl tellurium halides without substituents in the ortho-position to tellurium were isolated and characterized in 1975s. 

Elemental chlorine, bromine, and iodine, sulfuryl chloride and thionyl chloride convert diorgano ditellurium compounds to organo tellurium trihalides (p. 314). The reactions are carried out in inert organic solvents with stoichiometrically required amounts of reagents. Dialkyl ditellurium compounds may lose alkyl halides if the halogenolysis is not performed under mild conditions at low temperatures. When equimolar amounts of halogens and diaryl ditellurium derivatives are combined in appropriate solvents, aryl tellurium halides are formed (p. 239). The formation of aryl tellurium halides is facilitated by stabilizing substituents in the orf/to-position to the tellurium atom or by the presence of thioureas or selenoureas in the reaction mixture. 

Diorgano ditellurium compounds and diorgano mercury reagents react also to give diorgano tellurium products. In these reactions both halves of a di tellurium substrate are converted to the desired product. 

The Te —Te bond in diorgano ditellurium compounds is rather weak with a tendency to homolytic cleavage1. Species that act as diradicals may insert into the Te Te bond. Dialkyl or diaryl ditellurium compounds and diazomethane in diethyl ether solution produce bis organotelluro]methanes2-4. 

Diorgano ditellurium compounds eliminate tellurium on heating to form the corresponding diorgano tellurium. These reactions begin above 200° in the absence of a solvent but proceed in good yields only above 300°8. Less stable substrates, e.g., bis[3-methoxyphenyl]... 

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. 

Oxidation of tellurolates generally produces diorgano ditellurium compounds (p. 264,270). However, the oxidation of ethynetellurolates led to the isolation of diethynyl tellurium products. When sodium ethynetellurolates were hydrolyzed with water in the presence of air, the diethynyl tellurium products were obtained in low yields. Oxidation by iodine in DMSO improved the yield to 70%4. The expected ditellurium compounds were postulated as unstable intermediates. 

The ease with which the Te — Te bond in ditellurium compounds is cleaved drastically limits the reactions for the modification of functional groups that are already present in the organic part of the molecules. The conversion of bis[2-carboxyphenyl] ditellurium to bis[chlorocarbonylphenyl] ditellurium by butyl dichloromethyl ether in the presence of anhydrous zinc chloride the acid hydrolysis of the ethylene acetals of bis[4-acetylphenyl] and bis[2-acetylphenyl] ditellurium to the bis[acetylphenyl] ditellurium derivatives, the saponification of bis[co-methoxycarbonylalkyl] ditelluriums to the corresponding bis[m-carboxyalkyl] ditelluriumand the oxidation of bis[2-formylphenyl] ditellurium by silver nitrate to bis[2-carboxyphenyl] ditellurium are the modifications of the organic moieties in diorgano ditellurium compounds that have been carried out successfully. 

Diorgano ditellurium compounds react with transition metal salts and carbonyl complexes to form coordination compounds (Table 5, p. 283). Complexes with the following transition metals have been reported Ti, Cr, Mo, W, Mn, Re, Fe, Ir, Ni, Pd, Pt, Cu, Ag, Cd, Hg, Yb, and U. In many of these complexes, the organyltelluro group bridges the metal atoms in binuclear complexes. The Te —Te bond seems to remain intact upon complexation to mercury halides, rhenium carbonyls, and uranium pentachloride. For details on tellurolatO bridged complexes see p. 212. Complexes with SnCl are also known. Diphenyl ditellurium and bis[4-ethoxyphenyl] ditellurium formed charge-transfer complexes when equimolar amounts of the ditellurium compound and tetracyano-p-quinodime-thane were refluxed in acetonitrile. ... 

Aromatic and aliphatic ditellurium compounds are converted to unsymmetrical diorgano tellurium derivatives in reactions with arenediazonium salts11 -13. 

Tellurolates, which are easily accessible through reduction of diorgano di telluriums, can be alkylated and arylated and added across carbon-carbon triple bonds. In addition, the weakness of the Te —Te bond in diorgano ditelluriums can be exploited to prepare unsymmetrical diorgano telluriums. Many reagents such as organic lithium compounds,... 

A promising method for the synthesis of unsymmctrical diorgano tellurium compounds uses carboxylic acid derivatives and diaryl ditellurium as starting materials. The carboxylic acid chloride, prepared in quantitative yield from the acid and oxalyl chloride is added slowly to a mixture of a diary] ditellurium and the sodium salt of 2-mercaptopyridine N-oxide in toluene at 35°. Under normal laboratory light the aryl tellurium radicals... 

This sequence of reactions transfers the alkyl group of a carboxylic acid to the tellurium atoms of a diaryl ditellurium. Because many carboxylic acids are commercially available at prices much lower than the corresponding alkyl halides, this reaction is a very useful alternative route to unsymmetrical diorgano tellurium compounds. 

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