Tellurium stereochemistry

Tellurium, bis(acetophenone)dichloro-photothermography, 6,120 Tellurium, bis(dithiofurancarbamato)-stereochemistry, 1,60 Tellurium, bis(ethyldithiocarbonato)-stereochemistry, 1, 60... 

Tellurium, bis(methyldithiocarbonato)-stereochemistry, 1,60 Tellurium, dichlorodimethyl-lone electron pair, I, 44 Tellurium, pentafluoro-lone electron pair structure, 1,50... 

Tellurium, tetrakis(diethyldithiocarbamato)-stereochemistry, 1, 94 Tellurium, tetrakis(thiourea)-dichloride... 

Tellurium, tris(diethyldithiocarbamato)phenyl-stereochemistry, 1, 82 Tellurium complexes acetylacetone, 2, 370 heterocyclic... 

In this chapter, recent development on the synthesis and stereochemistry of optically active chalcogen compounds over the last 10 years will be described, focusing mainly on chiral selenium and tellurium compounds. 

On the other hand, optically active telluroxides have not been isolated until recently, although it has been surmised that they are key intermediates in asymmetric synthesis.3,4 In 1997, optically active telluroxides 3, stabilized by bulky substituents toward racemization, were isolated for the first time by liquid chromatography on optically active columns.13,14 The stereochemistry was determined by comparing their chiroptical properties with those of chiral selenoxides with known absolute configurations. The stability of the chiral telluroxides toward racemization was found to be lower than that of the corresponding selenoxides, and the racemization mechanism that involved formation of the achiral hydrate by reaction of water was also clarified. Telluroxides 4 and 5, which were thermodynamically stabilized by nitrogen-tellurium interactions, were also optically resolved and their absolute configurations and stability were studied (Scheme 2).12,14... 

Sulfur, Selenium, Tellurium and Polonium Table 1 Compounds of Group VI Elements and their Stereochemistries... 

In this chapter, we will review the use of ylides as enantioselective organocata-lysts. Three main types of asymmetric reaction have been achieved using ylides as catalysts, namely epoxidation, aziridination, and cyclopropanation. Each of these will be dealt with in turn. The use of an ylide to achieve these transformations involves the construction of a C-C bond, a three-membered ring, and two new adjacent stereocenters with control of absolute and relative stereochemistry in one step. These are potentially very efficient transformations in the synthetic chemist s arsenal, but they are also challenging ones to control, as we shall see. Sulfur ylides dominate in these types of transformations because they show the best combination of ylide stability [1] with leaving group ability [2] of the onium ion in the intermediate betaine. In addition, the use of nitrogen, selenium and tellurium ylides as catalysts will also be described. 

The reaction of alkynes with tellurium tetrachloride 16 was recently reinvestigated.68 Aryl alkynes (e.g., 24) react with 16 to give the (Z)-vinylic tellurium trichlorides (e.g., 25) (Equation (5)),47,68,69 whereas alkynes bearing a hydroxyl group at C3 26 give olefins of (ft)-stereochemistry 28 or 29 that reflect the steric demand at C3 (Scheme 7).68,72... 

Bis-vinylic tellurides 68 can be prepared by reacting monosubstituted alkynes with a hydrotelluration system prepared from elemental tellurium and sodium borohydride in a mixture of ethanol/aqueous sodium hydroxide.133 The stereochemistry of the resulting vinylic tellurides is also Z (Equation (7)). 

If unusual vinylic tellurides or vinylic tellurides of defined stereochemistry are required, other vinyl organo-metallics are employed. In recent years, a number of methods of preparing vinylic derivatives of tellurium using organotellurium halides have been developed. 

Representative examples of these reactions are shown in Table 4. For more details, see recent reviews.8 10 Recently, more complex unsaturated systems were subjected to the tellurium-lithium exchange reaction.137,141 In Scheme 98, some examples of such a process are shown. In all cases, the (Z)-stereochemistry of the starting vinylic telluride was preserved.137... 

Higher order alkyl cyanocuprates containing MgBr as the counter-ion 214 couple efficiently with (Z)-vinylic tellurides to give the corresponding tellurium-free olefins with retention of the (Z)-stereochemistry 215.281 By using lower-order cyanocuprates 216, the coupling is efficient even with lithium as the counter-ion (Scheme 114).282... 

An analogous reaction of (Z)-4-octene produced the f/treo-tellurium dibromide, which formed 5-methoxy-3-octene. The stereochemistry of this compound was not determined3. 

In addition to such divalent species, the elements form compounds in formal oxidation states IV and VI with four, five, or six bonds tellurium may give an 8-coordinate ion TeFg". Some examples of compounds of Group 16 elements and their stereochemistries are listed in Table 12-2. They also form cyclic and polycyclic cations (e.g., S2+, Te +) under special conditions. 

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