Symmetrical dialkyl tellurides

In addition to the selective synthesis of selenides and diselenides, symmetrical dialkyl tellurides [5a, 47-49, 50a, 58] and ditellurides [48a, 51a, c, 59] can be prepared with high selectivity by alkylation of Na2Te and Na2Te2, respectively, with alkyl halides (Scheme 15.20). 

Tetraalkylammonium borohydrides react with elemental selenium and tellurium to form the symmetrical dialkyl selenides and tellurides [44]. Diphenyl selenide yields the alkyl phenyl selenides. 

Since symmetrical dialkyl and diaryl tellurides are the most employed in organic synthesis, and often exhibit structural or biological interest, their preparation will be examined in detail, focusing on the methods and procedures that are considered as the most popular. 

Alkali tellurides 13 and 14, or ditellurides 15, are potent nucleophiles and react rapidly with alkyl halides. Table 1 presents some representative examples of such reactions leading to dialkyl tellurides 3 and dialkyl ditellurides 4. Alkali organotellurolates 2 are also potent nucleophiles and are used to prepare symmetrical and unsymmetrical tellurides (Table 2). 

Unsymmetrical dialkyl tellurium derivatives were prepared by mixing an aqueous disodium telluride solution with equimolar amounts of two different alkyl halides. All three possible dialkyl tellurium products are formed. The unsymmetrical dialkyl tellurium is the predominant species. It can be separated from the symmetrical compounds by chromatography1. This one-pot procedure takes less time to complete than the alternative route employing alkyl tellurolates (p. 387) and was used to prepare unsymmetrical dialkyl telluriums containing radioactive tellurium. Sequential addition of two alkyl halides produced only symmetrical dialkyl telluriums. 

Good yields of secondary amines are achieved using both the methods in the reactions of aromatic and aliphatic aldehydes as well as of dialkyl ketones and cycloalkanones with ahphatic and alicychc amines (and ammonia). Anilines give low yields, but when 2 equiv is used in the sodium hydrogen telluride method, the yields are improved, hi the reaction of ammonia with aldehydes, symmetrical secondary amines are obtained, whereas glu-taraldehyde and amines lead to N-substituted piperidines. 

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