Organotellurium Compounds, Vol. E12b

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E17c Carbocyclic three- and four-membered ring compounds - - Bd. E17d Carbocyclic three-membered ring compounds -- Bd. E17e Carbocyclic four-membered ring compunds -- Bd. Organo-metall-verbindungen als hilfsmittel in der organische chemie -- Bd. C-Radikale -- Bd. Carbene Carbenoide -- Bd. Carbokationen, Carbokation-Radikale -- Bd. Carbanionen -- Bd. Makromolekulare Stoffe -- Bd. Stereoselective synthesis -- Bd. Synthesis of peptides and peptidomimetics -- Bd. E 22e. E 23a-E23b. Index of experimental procedures -- Bd. Aliphatic compounds I. Carbonyl compounds I -- Bd. Carbonyl compounds II -- Bd.

Substance index : aliphatic compounds II, alkynes, alkenes, alkanes -- Bd. Substance index : cyclic compounds I three- and four-membered monocyclic compounds -- Bd. Substance index : cyclic compounds II, five-membered monocyclic compounds -- Bd. Substance index : cyclic compounds III, six-membered monocyclic compounds without heteroatoms -- Bd. Substance index. Cyclic compounds V.

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Bicyclic compounds I -- Sachregister zu Band E4. Tricyclic Compuonds I -- Bd. Tricyclic compounds II. From the 4th edition and the additional and supplementary volumes to the 4th edition -- Bd.

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E 23m. Substance Index. Polycyclic Compounds I -- Bd. E 23n. Furthermore, these reactions take a considerable time to occur under reflux in the mentioned solvents. An interesting feature of organotellurium IV compounds is the discovery that some of them present important biological properties, 6 , 7 and in a recent publication their unexplored therapeutic potential was pointed out. In this communication we report some reactions previously performed in the above mentioned solvents under reflux or under harsh reaction conditions, simply by keeping the reagents in contact in the absence of solvents for a short period of time under microwave irradiation.

The reaction investigated in more detail was the addition of p -methoxyphenyltellurium trichloride to alkynes, which was already studied using benzene as the solvent. Tellurium tetrachloride is the reagent of choice to prepare a number of organotellurium IV compounds.

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Recently, a more friendly method for the preparation of tellurium tetrachloride was described, consisting in the reaction of elemental tellurium with a six-fold excess of SO 2 Cl 2 under reflux for 72 h, followed by distillation of the excess of SO 2 Cl 2. In the past, the reaction of activated aromatic hydrocarbons with tellurium tetrachloride in carbon tetrachloride under reflux for several hours was the method of choice to prepare aryltellurium trichlorides.

The p -methoxyphenyltellurium trichloride prepared as described above was used to investigate its addition reaction to alkynes under microwave irradiation in the absence of solvents Scheme 3.

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This reaction has been investigated in our 12 , 13 and other 11 laboratories using equimolar amounts of the alkyne and the aryltellurium trichloride in benzene under reflux. In order to compare the traditional method with the one developed in the present work, we repeated some of the previously performed reactions in benzene.

The results are shown in Table 1. In the present method, the reagents were mixed in a pressure resistant tube equipped with a stirring bar and placed in a microwave oven apparatus. The spectral data of 4d and 4e are in agreement with published data. After the time indicated in Table 1 , the tube was opened and the solid was dissolved in chloroform and precipitated with hexane to give analytically pure samples with the yields shown in Table 1.

The extraction, drying and silica gel filtration steps were eliminated. The recrystallization process of the products obtained by this method was much easier as compared with the recrystallization of the products obtained when benzene was used as the reaction solvent.

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  • In all but one case a single product was formed. The structure assignment of the obtained products was made by comparison of their spectral data with those published previously, when a detailed study of the regio- and stereochemistry of the addition products of p -methoxyphenyltellurium trichloride to alkynes was made. These results are in agreement with a previous work. This transformation was important to determine the isomer ratio in the reaction with propargyl alcohol Table 1 , entry 3d , since the chromatographic separation of the tellurium IV compounds failed Scheme 4.

    In conclusion, an easy and ecofriendly method to prepare tellurium IV compounds was developed using microwave irradiation of the reaction mixture in the absence of solvents. Under the conditions employed in this communication, the reactions occur in shorter reaction times and at lower temperatures than in the preceding published methods, avoiding the use of harmful solvents and harsh reaction conditions.

    Analytical thin layer chromatography TLC for monitoring reactions was performed using Merck 0. All reagents are commercial grade and were pretreated before use, when needed all reagents were purchased from Sigma-Aldrich Co. The mixture was refluxed for 72 h until all the tellurium powder was consumed.

    After this time, the excess of SO 2 Cl 2 was removed by distillation under vacuum. A white solid was obtained, which was submitted to high vacuum under a water bath heating to remove any trace of SO 2 Cl 2 and then used without further purification. Yield: After this time all the tellurium powder was consumed and the excess of SO 2 Cl 2 was removed by distillation under vacuum, leaving behind a white solid which was submitted to high vacuum and heating as described above and then used for further reactions without purification.

    Yield: 7. Preparation of p -methoxyphenyltellurium trichloride 2.

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    The procedure described by Cunha et al. The yellow solid obtained was recrystallized from acetic acid. In a glass pressure resistant tube 35 mL equipped with a magnetic stirring bar were placed tellurium tetrachloride prepared as described for 1 1. Yield: 2. Addition of p -methoxyphenyltelluriumtrichloride to alkynes, typical procedure. To a mL round bottomed flask equipped with a reflux condenser and a magnetic stirring bar were added p -methoxyphenyltellurium trichloride prepared as described for 2 15 mmol , the alkyne 15 mmol and benzene 50 mL.

    The mixture was heated under reflux for 8 h and the reaction was monitored by TLC eluting with a mixture of hexane:ethyl acetate The organic phases were dried with MgSO 4 and the solvent was evaporated. The residue was filtered through silica gel eluting with ethyl acetate. After drying, filtering and evaporating the solvent, the residue was dissolved in chloroform and precipitated with hexane for yields see Table 1.