Tellurium Reagents

R2 = allylic, PhCH2, CH2C=CH (alkyne and allene product) TL 36 6495 (1995) 

RLi=R U (R = alkyl, vinylic, aiyl, allylic, alkynyl), UCR2CN, LiCH(OR)CN, UCH2C02R, UCH2COR, UCH(SR)2 

E+ = RX, CO2, RCHO, Me3SiCl, RO2CCI, PhSCl, H00NM62, QCONMej, B(0R)3 JACS 101 769 (1979) 

Davies and C. L. Goodfellow, Advances in Metal-Organic Chemistry, Ed. L. S. Liebeskind, JA1 Press, Greenwich, CT (1991), Vol 2 (review) 

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Preparation of the More Important Inorganic Tellurium Reagents... 

PREPARATION OF THE MORE IMPORTANT INORGANIC TELLURIUM REAGENTS... 

In several cases the use of tellurium reagents is clearly advantageous compared to the usual previously described methods because of the mildness of the experimental conditions and the great selectivity. 

The tellurium reagents are more effective than their selenium analogues. The lithium salts seem to be the most reactive. 

Several methods achieving the debromination of v/c-dibromides by means of tellurium reagents are well established. These methods are particularly advantageous compared to the conventional ones in terms of the mildness of the experimental conditions, good yields, lack of important side reactions and inermess of several functionalities to the employed reagents. A relevant characteristic of these reactions is the high E2-type stereospecificity demonstrated by the formation of olefins with Z and E geometry from threo-and eryf/iro-dibromides, respectively. 

The removal of functional groups from the a-carbon of carbonyl compounds is an important transformation in organic synthesis. Anionic tellurium reagents offer additional useful methods to attain this. 

Some investigations directed to the dehalogenation of polyhalogenated organic derivatives by means of tellurium reagents have been reported. 

Compared to the analogous supported selenoxide, the tellurium reagent requires shorter reaction times and milder experimental conditions. 

Reaction of oxidizing tellurium reagents with the C=C bond... 

The first reported radical reaction promoted by tellurium reagent was probably the conversion of allylic halides into the coupled 1,5-dienes by treatment with telluride anions. The reaction, which gives the best results when employing the reagent prepared in situ from elemental tellurium and lithium triethylborohydride, proceeds through the intermediacy of the thermally unstable bis-allylic telluride followed by extrusion of tellurium and coupling of the formed allylic radicals. 

N. Petragnani u. J. V. Comasseto, Synthesis 1991, 793-817 . .Tellurium Reagents in Organic Synthesis Recent Advances" (zu Aminen fuhrende Reduktionen. S. 800 ff.). 

Review.1 This review covers preferred methods of preparation of the more important organotellurium reagents. Most of these are aromatic derivatives of tellurium, which are more stable and less volatile than aliphatic ones. Even though synthetic use of tellurium reagents is relatively new, the review covers 119 references, including several for the year 1985. 

Similarly, iminium salts 3590 93 and enamines 3691 94 (Scheme 19) are reduced to the corresponding amines 37 by means of tellurium reagents. 

The reduction of nitro compounds 38 by tellurium reagents depends on the structure of the substrate and on the solvent used. For example, phenyl tellurol90 and sodium telluride95 reduce nitroarenes to anilines 39 in protic solvents. When an aprotic solvent is used, sodium telluride converts the nitroarenes 38 into the respective azo compounds 40 (Scheme 20).25... 

The reaction of nitroalkanes and dinitroalkanes with sodium hydrogen telluride gives nitrosoalkane dimers and olefins, respectively.96 The reduction of other nitrogenated species such as hydroxylamines, azides, nitroso, azo, and azoxy compounds can also be performed by using tellurium reagents.6,11,12... 

Oxidizing tellurium reagents react with olefins affording the products shown in Scheme 75. 

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