Sodium Telluride1

Na2Te
(Na2Te)

[12034-41-2]  · Na2Te  · Sodium Telluride  · (MW 173.58) (NaHTe)

[65312-92-7]  · HNaTe  · Sodium Hydrogen Telluride  · (MW 151.60) (Li2Te)

[12136-59-3]  · Li2Te  · Lithium Telluride  · (MW 141.48) (K2Te)

[12142-40-4]  · K2Te  · Potassium Telluride  · (MW 205.80)

(telluride ion and its conjugate acid, TeH-, are reducing agents and powerful nucleophiles that accomplish nucleophilic reductions;2 reduction of nitro groups,3 N-oxides,4 imines,5 azides,6 enones,7 alkenes,7b,8 a,b-epoxy ketones,9 and carbon-halogen10 or carbon-sulfur11 bonds; conversion of optically active glycidyl sulfonate esters to allylic alcohols;2 ester cleavage;12 reductive alkylation of amines;13 Reformatsky reaction;14 synthesis of tellurium compounds1b)

Physical Data: mp 953 °C (dec); d 2.90 g cm-3; pKa (HTe-&b;&ibond; H+ + Te2-) 10.8; E0 (Te2- -> Te + 2e) +0.95 to 1.14 V.

Solubility: sol H2O; insol. THF; purple solutions of sodium polytellurides (Na2Ten) occur in H2O, THF, DMF, and methanol.

Preparative Methods: prepared1b,e in situ (often as a suspension) by reduction of elemental Tellurium with Sodium Hydroxymethanesulfinate (HOCH2SO2Na) in aqueous base, Sodium Borohydride in protic or dipolar aprotic (DMF) solvents,1f Lithium Triethylborohydride in THF, Thiourea Dioxide in aqueous basic THF, Sodium Hydride in dipolar aprotic solvents, Sodium in liquid ammonia or DMF, aqueous Sodium Dithionite, Sodium Naphthalenide in THF, or Hydrazine hydrate in aqueous base or in DMF, or by electrolysis of elemental tellurium. Often, only catalytic amounts of tellurium are required in the reductions of various organic compounds. Buffering solutions of Na2Te with acetic acid provides NaTeH. The latter also is obtained by decreasing the amount of NaBH4 in reduction with that reagent.

Handling, Storage, and Precautions: sodium telluride is a white, crystalline, hygroscopic solid that decomposes to elemental tellurium on exposure to air. Traces of oxygen immediately produce pink to purple solutions of polytelluride anions that are not necessarily harmful for subsequent use. Operation in an inert atmosphere is essential, and degassing of solvents is advised. Avoid strong acids that produce noxious Hydrogen Telluride. The reaction with oxidizing agents may be highly exothermic. All reactions should be carried out in a good fume hood to avoid exposure to possible toxic and malodorous organic tellurium products or byproducts. Decontamination of apparatus or spills is effected by treatment with oxidants such as Chlorox or hydrogen peroxide.

Reductions.

Aromatic mono- and polynitro compounds are reduced to amines by catalytic amounts of Na2Te (eq 1).3a The method is useful when acid sensitive groups are present and is said to give fewer side reactions than does Sodium Sulfide. Use of NaH in DMF to reduce Te gives azobenzenes,3a,b and use of sodium borohydride gives arylhydroxylamines.3c Sodium hydrogen telluride produces azoxybenzene derivatives.3e The latter reagent also reduces amine oxides4 and imines5 to the free amines. Sulfoxides are difficult to reduce and phosphine oxides are inert.4c The in situ formation and reduction of imines by NaTeH is a variation of the reductive alkylation of amines (eq 2).13 Reductions of imines may proceed via iminium salts and are pH sensitive.5b At pH 6, N-methylpyridinium iodide is reduced to N-methylpiperidine, whereas at pH 10-11 a 2:1 mixture of N-methyl-1,2- and -1,4-dihydropyridine is obtained. Azides are selectively reduced to amines by NaTeH,6 thus providing a useful synthesis for the pyrazine (1) (eq 3),6a but the method is not applicable to primary azido ketones. For the selective reduction of the double bond of enones7 and of certain a,b-unsaturated sulfones,11a,15 NaTeH is superior to Na2Te (eq 4).7c In the sulfone case, reduction of the carbon-sulfur bond may occur (eq 5).15 The triple bond of dimethyl acetylenedicarboxylate and of methyl phenylpropiolate is reduced partially.16 Carbon-carbon double bonds conjugated with aromatic systems are reduced by NaTeH (and also PhTeH).7b,8 Addition of HTe- to an isolated, unconjugated double bond followed by reduction with Nickel Boride effects the reduction of alkenes.17a

One carbonyl group in 1,2-diketones is reduced to the alcohol,4c and aromatic thioketones are reduced to the alkane.3a Aromatic aldehydes and ketones are reduced to alcohols (34-89% yields) by Na2Te (Te, NaH) in 1-Methyl-2-pyrrolidinone (NMP).17b Other groups that are reduced are as follows: the carbon-carbon bond of certain C-allyl compounds,18 the a-carbon-oxygen bond of a,b-epoxy ketones (eq 6),9 carbon-sulfur bonds of sulfides and sulfones (eqs 5, 7 and 8),11,15b,19 the carbon-nitrogen bond of tertiary nitro compounds,20 gem-diamino compounds,21 and carbamate esters,22 carbon-halogen bonds,10 sulfonyl chlorides (to the sulfinate anion),23 and the sulfur-sulfur bond of disulfides and thiosulfates.24

The reduction of a-halo carbonyl compounds leads to a variant of the Reformatsky reaction (eq 9).14 Wurtz-type couplings have been observed with halides.25 The reduction of optically active glycidyl tosylates and mesylates (obtained via a Sharpless asymmetric epoxidation) by Na2Te to allylic alcohols is accompanied by a transposition of the hydroxy function and the former double bond that enables the synthesis of optically active secondary and tertiary allylic alcohols without the necessity of a resolution (eq 10).2 Epichlorohydrins are transformed to allylic alcohols by treatment with telluride ion.26 Simple epoxides may be reduced to alkenes by a two-step procedure involving nucleophilic opening of the epoxide by NaTeH to a b-hydroxy tellurol and conversion of the alcohol function to a tosylate which, in the presence of pyridine, decomposes to the alkene and elemental tellurium.27

Deprotection of Functional Groups.

Deprotection of various esters to yield carboxylic acids is effected with either Na2Te or NaTeH.12,28 b-Halo alkyl esters are especially reactive (eq 11);12a,b,28c methyl, ethyl, phenacyl,12d,28a and allyl esters12c also are dealkylated readily. Phenols protected as acetate, benzoate,28b carbonate,29 or chloroacetate28c esters or as allyl ethers12c can be regenerated. Amines have been freed from quaternary ammonium salts,27 and diols, from cyclic acetals.30

Heterocyclic Compounds.31

Addition of telluride ion to the triple bonds of 1,3-dialkynes such as 1,4-bis(trimethylsilyl)-1,3-butadiyne is a convenient synthesis of tellurophenes (eq 12);32 six-membered ring systems containing tellurium can likewise be obtained from the appropriate dialkyne.33 A tellurophene synthesis (eq 13) that involves reaction of telluride ion with an acetylenic epichlorohydrin is related to the nucleophilic reduction-transposition reaction exemplified in eq 10.34

Tellurols and Tellurides.35

Reactions of Na2Te and NaTeH with organic halides have found limited use in the synthesis of sodium salts of tellurols. However, a wide variety of symmetrical tellurides are obtained from halides, alkynes, and related compounds with Na2Te (eqs 14 and 15).36a-d Tellurocarboxylates, RCOTe-, are obtained from the acid chloride.36e

Miscellaneous.

Elimination reactions yielding alkenes are observed when the following are treated with NaTeH or Na2Te: 1,2-dihalides,8b,37 1,2-dinitroalkanes,3e b-nitro sulfones,38 and b-bromo azides.6a Conversion of an alcohol to a benzyl ether has been accomplished by addition of NaTeH to an iminium ester derivative of the alcohol.39a,b Lithium telluride is used in the preparation of (Bu3Sn)2Te (from Bu3SnCl) which is an intermediate for (Me2Al)2Te, useful in the synthesis of highly reactive telluroaldehydes and telluroketones.40

Related Reagents.

Hydrogen Selenide; Hydrogen Telluride; Sodium Diselenide; Sodium Hydrogen Sulfide; Sodium Selenide; Tellurium.


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37. (a) Ramasamy, K.; Kalyanasundaram, S. K.; Shanmugam, P. S 1978, 311. (b) Suzuki, H.; Inouye, M. CL 1985, 225.
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Donald C. Dittmer

Syracuse University, NY, USA



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