Telluroxides synthesis

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... 

Catalytic one-pot procedure. Since in the described teUuronium ylide olefmation tellurox-ide is formed as a by-product, and the telluroxide is susceptible to reduction by triphenyl phosphite, a catalytic procedure can be employed, providing a practical one-pot synthesis of a, -unsaturated esters and ketones (method E). By this procedure, a catalytic amount of n-dibutyl telluride reacts with the a-bromoester or a-bromoketone, and the formed tel-luronium salt is converted in situ under phase transfer conditions (solid KjCOj/trace HjO) into the ylide, which reacts in turn with the aldehyde, giving the olefin. Since the reaction is performed in the presence of triphenyl phosphite, the formed dibutyl telluroxide is reduced back to the dibutyl telluride, which is then recycled. 

Diorganyl tellurides are starting materials for the preparation of diorganyltellurium dihahdes, pseudohalides, and carboxylates, of telluroxides, and of teUuronium salts, and are used as ligands for the synthesis of transition metal complexes. 

CT-Telluranes 44 were employed for the synthesis of other derivatives of phenotellurazines with a tricoordinated tellurium atom. The tellurium ylides 46 were obtained in high yields by coupling Te,Te-dibromophenotellurazines with dime-done in the presence of equivalent amounts of EtsN. Alkaline hydrolysis of the dibromides leads to telluroxides 47 in 80-85% yield [82DOK(266) 1164 85KGS757]. 

Tellurium.—Compared with sulphur and selenium little use has been found for tellurium reagents in organic synthesis this year, as is evidenced by the lack of reports. However, the telluroxide (114) is reported as a new mild oxidizing agent. 

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