Tellurium trihalides

N-Bridged tellurium-containing heterocycles are exemplified by derivatives of l,4-tellurazine[2,3-g/i]carbazole 107 and heterocycle 108. One-pot reaction of A-propargylcarbazole with tellurium tetrahalides under conditions of the two-phase tellurohalogenation (Section n,B,l) leads to halogenomethylene derivatives 107 (90KGS126). The reaction proceeds as the spontaneous electrophilic cycliza-tion of initially formed tellurium trihalides 109. 

Tellurium tetrachloride and aryltellurium trichloride, as well as tellurium tetrabro-mide" and aryltellurium tribromides add to acetylenes to produce, respectively, 2-halovinyl tellurium trihalides and dihalides, which can be submitted to further manipulations. 

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. 

Organic tellurium compounds with one Te—C bond containing divalent, tetravalent, or hexavalent tellurium have been prepared. The most widely investigated and best characterized compounds are the diorgano ditellurium derivatives and the organo tellurium trihalides. Pentafluoroethyl tellurium chloride tetrafluoride is the only known example of a hexavalent tellurium compound with one Te-C bond in the molecule. 

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). 

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 tellurium5. 

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. 

The reduction of alkyl tellurium trihalides is a convenient method for the preparation of the corresponding dialkyl ditellurium when the trihalides can be obtained by reactions other than the halogenolysis of the dialkyl ditellurium. Addition of tellurium tetrachloride to olefins and condensation with ketones produces alkyl tellurium trichlorides. Sodium sulfide nonahydrate4 and sodium or potassium disulfite5 8 were used as reducing agents. 

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. 

Tellurinic Acids and Dcrivatcs Organo Tellurium Trihalides... 

Organo tellurium trihalides are the best known and most numerous compounds among the tellurinic acid derivatives. Most of the other derivatives are represented only by a few compounds, some of which have not been adequately characterized. Frequently, different products are claimed for similar reactions with identical starting materials. Much more work needs to be done with the various hydrolysis products of organo tellurium halides before this area of organic tellurium chemistry can be considered to be adequately explored. 

Organo tellurium trifluorides, trichlorides, tribromides, and triiodides are known. Aryl tellurium trihalides are generally more stable than alkyl tellurium trihalides. The sensitivity toward atmospheric agents decreases from the trichlorides to the triiodides. Very few organo tellurium trifluorides have been prepared and little is known about their reactivity. The general reactivity of aryl tellurium trihalides decreases from the chlorides to the iodides. 2-Chloroalkyl tellurium trichlorides, products of the addition of tellurium... 

The compound decomposed on storage with deposition of tellurium1. Tellurium tetrachloride need not be used in substance it can be generated in the reaction mixture from tellurium dioxide and acetyl chloride or chlorotrimethylsilanc. With acetyl bromid tellurium tetrabromide can be formed. When cyclohexene was present in the reaction mixture, 2-halocyclohexyl tellurium trihalides were isolated in approximately 70% yields. Acetic acid, chloroform, or dichloromethane may be used as solvents2. 

When the addition reactions of tellurium tetrachloride or tellurium tetrabromide were carried out in the presence of an alcohol with a cycloalkene as the olefin, trans-2-alkoxycycloalkyl tellurium trihalides were isolated in yields ranging from 30 to 80%4... 

Methyl groups in nitromethane5 and dimethyl sulfite6 are sufficiently activated to condense with tellurium tetrachloride5,6 and tellurium tetrabromide6 to give organo tellurium trihalides. For condensation reactions with carboxylic acid anhydrides see Vol. IX, p. 1152. 

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. 

Table 9 Organo Tellurium Trihalides from Equimolar Amounts of Tellurium T etrahalides and Main Group Organomet all ic Com... 

The halogenolysis of dialkyl ditellurium compounds is a convenient method for the preparation of alkyl tellurium trihalides because dialkyl ditellurium compounds are easily accessible through alkylation of disodium ditelluride (p. 258). Diaryl ditellurium compounds generally obtained via reduction of organo tellurium trichlorides (p. 274), serve as starting materials for the syntheses of aryl tellurium tribromides and triiodides (Vol. IX, p. 1156) that are not obtainable from the hydrocarbons and tellurium telrabromide or tetraiodide. 

Alkyl tellurium halides, RTeX, in general, have no importance as starting materials for the preparation of alkyl tellurium trihalides. The alkyl tellurium halides are synthesized from dialkyl ditellurium compounds and elemental halogens. When alkyl tellurium trihalides are the desired products, there is no reason to prepare the monohalides first. However, when the tellurium monohalide is formed during the introduction of tellurium into an organic molecule, the reaction of this primary product with halogens is the method of choice for the preparation of the tellurium trihalides. This situation is realized for 3-oxo-l-propen-l-yl tellurium halides2,3. 

Oxo-1-propen-1-yl tellurium iodides did not react with iodine. The attempt to prepare mixed (chloride, bromide) tellurium trihalides was unsuccessful a mixture of trichlorides and tribromides was formed. When 1,3-diphenyl-3-oxo-l -propen-l-yl tellurium iodide was reacted with bromine, the bromide and elemental iodine were the only observed products. The 3-oxo-l-propen-l-yl tellurium trihalides are thermally unstable they decompose on mild heating to the tellurium halide and elemental halogen1. 

The Schiff base prepared from 2-formylphenyl butyl tellurium and 1,2-diaminoethane was converted to tellurium trihalides by excess sulfuryl chloride or bromine2. 

The reactivity of the trihalotelluro group makes it difficult to modify the organic moiety of an organo tellurium trihalide. The only known reactions of this type are the exchange of... 

Organo tellurium trihalides form complexes with coordinatively unsaturated metal halides and with ligands having nitrogen, oxygen, or sulfur donor atoms. 

---