From Aryl Tellurium Bromides

2-(3-Dimethylaminopropenoyl)-phenyl tellurium bromides cyclized to oxobenzotellurins when heated with hypophosphorous acid in pyridine  

2-Methyl-4-oxo-4fl-l-benzotellurin A solution of 1.98 g (5 mmol) of 2-(3-dimethylamino-2-butenoyl)-phcnyl tellurium bromide and 0.25 g hypophosphorous acid in pyridine is heated under reflux for 20 min, cooled, and hydrolyzed with hydrochloric acid/ice. The resultant mixture is extracted with chloroform, the organic phase is separated, washed with water, dried with anhydrous magnesium sulfate, filtered, and the solvent is evaporated. The residue is chromatographed on a thin layer of silica gel with benzene/ethyl acetate (8/2, v/v) as the mobile phase. The fraction with Rf = 0.7 is isolated and recrystallized from benzene yield 0.61 g (45%) m.p. 100°. 

Phenyl-2-chlorocarbonylethenyl tellurium similarly gave 4-oxo-4//-l-benzotellurin (m.p. 92°) in 56% yields 

7-Methoxy-4-oxo-2-phenyl-4//-l-benzotellurin 8.04 g (20 mmol) of 2-chlorocarbonyl-l-phenylethenyl 3-methoxyphenyl tellurium are dissolved in 80 ml] of dichloromethane under nitrogen, the solution is cooled to — 78°, and 2.94 g (22 mmol) of aluminum chloride are added. The cooling bath is removed, the mixture is 

2-Carboxy-l-organoethen-l-yl tellurium compounds are converted to 4-oxo-4H-1-benzotellurins by treatment with phosphorus pentoxide in methanesulfonic acid  

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Tellurachromanone, and similar heterocycles with thiophene fused to the pyranone ring, are derived from aryl tellurium bromides bearing a 3-./V,Ar-dimethylamino-prop-2-cn-oyl substituent in the ortho-position in refluxing pyridine in the presence of an excess of hypophosphorous acid. These formations can be regarded as the intramolecular addition of the tellurium bromide to the olefin1. 

P) 4-Oxo-4//-l-benzotellurins A. Preparation 1. From Aryl Tellurium Bromides... 

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). 

Tabic 3 Aryl Tellurium Bromides and Iodides from Diaryl Ditcllurium Compounds and... 

Commercially available butyl lithium was frequently used to prepare aryl lithium compounds from aryl bromides. In these cases the aryl butyl tellurium compounds were the products isolated from the reaction mixtures. 

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. 

Aryl tellurium trihalides accept halide ions from tropylium bromide, ammonium halides, phosphonium halides, arsonium halides, sulfonium chlorides, telluronium chlorides, and diphenyl iodonium chloride to form aryltetrahalotellurates(lV)1-5. 

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. 

Diaryl tellurium dihalides in toluene solutions reacted with aryl magnesium bromides in diethyl ether to yield triaryl telluronium salts in yields ranging from 6 to 78% (Vol. IX, p. 1082/3)3-5. Aqueous solutions of the products were generally treated with potassium iodide to convert the telluronium halides to telluronium iodides3,4. 

When treated with potassium iodide, hot, aqueous solutions of triaryl telluronium chlorides precipitate triaryl telluronium iodides1 (Vol. IX, p. 1080). This halogen exchange reaction was used to isolate triaryl telluronium compounds from the mixtures obtained by reacting tellurium tetrachloride with aryl magnesium bromides (Vol. TX, p. 1080). 

Tris[2-thienyl] telluronium iodide5 and 4-bromobutyl tetramethylene telluronium iodide [m.p. 176° (dec)]6 were prepared in this manner. This reaction was employed to isolate telluronium salts as telluronium iodides from reaction mixtures obtained by combining tellurium tetrachloride and aryl magnesium bromides (Vol. IX, p. 1080). 

The phosphonium bromoiodotellurate(II) was also formed from the aryl tellurium iodide and the phosphonium bromide. ... 

Alkyl Aryl Telluriums (Reduction with Thiourea Dioxide) To a solution of 1.0 mmol diaryl ditellurium in 7.5 ml tetrahydrofuran are added 0.108 g (1.0 mmol) thiourea dioxide, 30 mg 2HT (phase transfer reagent), and 2.0 mmol of the alkyl bromide. To this mixture are added 7.5 ml of a 50% aqueous solution of sodium hydroxide. The mixture is stirred for several hours at 20°. The organic phase is separated. The aqueous phase is extraeted three times with 20 ml ethyl acetate. The organic phase and the ethyl aeetate extracts are combined and dried with anhydrous magnesium sulfate. The mixture is filtered and the solvents evaporated from the filtrate. The residue is chromatographed on silica gel with ethyl acetate as mobile phase. 

Aryltelluroformates (533), accessible from the corresponding chloroformates, undergo cleavage of the acyl-tellurium bond on photolysis. In the presence of diphenyldiselenide, the oxyacyl radical (534) is trapped as the corresponding alkyl (phenylseleno)formate (535) in excellent yield. Irradiation of aryl bromides or iodides (536) with potassium tellurocyanate (537) in dimethyl sulfoxide, as both solvent and methyl source, yields aryl methyltellurides (538) in modest yields (9-34%). ... 

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