With Aryl Tellurium Trihalides

The possibility of preparing aryl tellurium halides from equimolar amounts of diaryl ditellurium compounds and aryl tellurium trihalides has hardly been explored. Only phenyl tellurium iodide and 2-biphenylyl tellurium bromide could be obtained by this route. The other aryl tellurium halides (including 3,4-dimethoxyphenyl tellurium chloride) decomposed under the reaction conditions to give diaryl tellurium dihalides and tellurium . 

Irgolic Organic Tellurium Compounds with one Te,C Bond 

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Ammonium aryltetrachlorotellurates were also obtained in yields of 40% or less when diethylamine, trimethylsilyl(diethyl)amine, pyrrolidine, or A-(trimethylsilyl)pyrrolidine were reacted with aryl tellurium trihalides in benzene solution1. The initial products in these reactions are aryl amino tellurium dichlorides, which are then converted to the ammonium tetrachlorotellurates by hydrogen chloride. Hydrogen chloride is formed in the condensation of diethylamine with the aryl tellurium trichlorides, through hydrolysis of chlorotri-methylsilane, or through interaction of the aryl tellurium trichloride with benzene. 

Aryl tellurium bromides and iodides can be obtained by reaction of diaryl ditellurium compounds with an equimolar amount of bromine or iodine, when the solvent is able to dissolve the starting materials but not the aryl tellurium halide. The precipitation of the aryl tellurium halide prevents it from being converted to aryl tellurium trihalides. The aryl tellurium halides thus far isolated and suitable reaction media for their preparation are listed in Table 3 (p. 240). 

Tellurium tetrachloride and tetrabromide react with equimolar quantities of aryl mercury chlorides to produce aryl tellurium trihalides in high yields. This reaction is useful when the trichlorotelluro group is required at a specific position in the aromatic molecule or when tellurium tetrachloride does not condense in an acceptable manner with the aromatic hydrocarbon. The solvent of choice is dioxane, because mercury dichloride precipitates as the dioxane adduct, facilitating the isolation and purification of the aryl tellurium trichlorides. 

Alkyl and aryl tellurium trihalides with stabilizing electron-donor groups in the molecule that coordinate to the tellurium atom are not reduced by disodium sulfite3 or hypophosphorous acid4 to ditellurium compounds but only to tellurium monohalides. 

Aryl tellurium trihalides treated with water are converted to arenetellurinyl halides. The susceptibility to hydrolysis decreases from the trichlorides to the triiodides2,3. With basic aqueous solutions (sodium carbonate, sodium hydroxide) tellurinic acids or tellurinic acid anhydrides are formed2 5. 

Mixed tetrahalotellurates(IV) result from the combination of an aryl tellurium trihalide with an onium salt containing a halide different from that in the tellurium compound. 

Aryl tellurium trihalides were hydrolyzed with aqueous sodium hydroxide solution. The tellurinic acids were precipitated by acidification of the reaction mixture with acetic, hydrochloric1 or sulfuric acid4. 

Tropylium aryltetrahalotellurates can be prepared from tropylium bromide and an aryl tellurium trihalide. A more convenient procedure avoiding the laborious preparation of the tropylium bromide uses cycloheptatricne and triphenylmethyl perchlorate2. The product of this reaction can then be converted to the tetrahalotellurate by treatment with hydrohalic acids. 

The aryl tellurium halides do not have to be isolated but can be generated by reduction of aryl tellurium trihalides and reacted in situ with potassium cyanide4. 

Diaryl ditellurium compounds and aryl tellurium trihalides react in refluxing 1,2-dichloroethane to form diaryl tellurium dihalides and tellurium. The reactions are postulated to proceed via aryl tellurium halides that disproportionate into tellurium and diaryl tellurium dihalides. All reactions of this type thus far carried out on a preparative scale used pairs of reagents with the same aryl group2. 

Addition of aryl tellurium trihalides to cyclohexene produces trails-] -aryldihalotelluro-2-halocyclohexanes when the reactions are carried out in chloroform, and trans-1-aryldihalotelluro-2-methoxycyclohexanes when methanol is used as the reaction medium. Treatment of the 2-chlorocyclohexyl 4-methoxyphenyl tellurium dichloride with terf.-butyl hydroperoxide in acetic acid yielded tram-l,2-dichlorocyclohexane and aryl chloride. Bis[4-methoxyphenyl] tellurium dichloride did not yield any aryl chloride under these conditions1. Pyrolysis of tran.s-2-methoxycydohexyl phenyl tellurium dibromidc afforded Crum- l-bromo-2-methoxycyclohexane1. 

Generally, the reduction of aryl tellurium trihalides by a variety of reducing agents leads to diaryl ditellurium compounds. However, the reduction of aryl tellurium trihalides with stabilizing groups in the or//to-position to the tellurium atom, by sodium hydrogen sulfite, disodium disulfite, or hydrazine hydrate " to aryl tellurium halides shows that the reduction can be stopped at the tellurium halide stage. 

Aryl and alkyl tellurium halides react with chlorine or bromine in an organic solvent to give aryl tellurium trihalides (p. 317). 

When solutions of aniline, Wmethylanilines, or amino(methyl)benzenes are mixed with solutions of tellurium tetrachloride, complexes are formed. These complexes react in refluxing methanol to yield aryl tellurium trihalides The trihalotelluro group is claimed to occupy the position para to the amino substituent. ... 

Aryl butyl tellurium compounds reacted with excess sulfuryl chloride, bromine, or iodine in refluxing chloroform with cleavage of the tellurium-butyl bond and formation of the aryl tellurium trihalide" ... 

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. 

Halogen exchange reactions may be used to prepare tetrahalotellurates(IV) that are not accessible through reactions of aryl tellurium trihalides and onium halides. Treatment of tetrachloro- or bromotrichlorotellurates with dilute hydrobromic acid produces the tetrabromotellurates. The conversion of bromotellurates to tetrachlorotellurates requires concentrated hydrochloric acid. Chlorine and bromine are easily replaced by iodine when the tellurates are reacted with excess potassium iodide in dilute hydrochloric acid. The iodotellurates cannot be converted to the bromo- or chlorotellurates ... 

Aryltellurium trichlorides 5 (R = aryl, X = Cl) are usually yellow, very stable crystalline solids, with a slight odor of hydrochloric acid, which probably arises from the reaction of 5 with the air moisture. Contact of 5 with metallic spatulas, with moist solvents, or prolonged exposure to light must be avoided. These compounds, however, can be handled in the air with no risk of decomposition. Aryltellurium tribromides (5, R = aryl, X = Br) are yellow crystalline solids, and the triiodides (5, R = aryl, X = 1) are dark red solids. The aryltellurium tribromides and triiodides were less explored for synthetic purposes, in contrast to the aryltellurium trichlorides, which were frequently used in several synthetic transformations. The aliphatic tellurium trihalides are less stable than the aromatic ones and were much less studied and used for preparative purposes. 

A 4 1 molar ratio of thiosemicarbazone/aryl tellurium trichloride is necessary for the success of these reactions. No product was obtained with a 2 1 molar ratio of the reactants. The l,3-diphenyl-3-oxo-l-propen-1-yl tellurium trihalides were quantitatively reduced to the corresponding tellurium halides by hypophosphorous acid3. 

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. 

The reactivity of the tellurium trihalides decreases from the chlorides to the iodides. The aryl tellurium triiodides do not form diaryl tellurium diiodides and do not react with acetone6. 

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. 

Aryl tellurium trichlorides and N-, S-, or 0-donor ligands form 1 1 complexes when the components are combined in an organic solvent. Several complexes with a 1 2 stoichiometry and one complex with a 1 4 stoichiometry were also prepared. The few complexes of alkyl tellurium trihalides that are reported in the literature all have 1 1 stoichiometry The following organic compounds served as ligands. ... 

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