Acetylenic tellurides

Like vinylic tellurides, acetylenic tellnrides can be prepared by two general approaches, starting respectively from nncleophilic or electrophilic tellurium species. 

---

A more extensive and detailed study of these reactions (i. e. 32 to 33) was carried out by Dabdoub et al., who found that two equivalents of Cp2Zr(H)Cl are needed for complete consumption of acetylenic tellurides 35 (Scheme 4.24) [51]. Solubilization of Cp2Zr(H)Cl in the reaction medium (THF) is apparently not a sufficient indication of educt consumption when only 1.1 equivalents are employed (e. g., following a proton quench, 58% of the product 37 and 41% of the acetylenic telluride 35 were recovered). Furthermore, care must be taken to avoid Cp2ZrH2, potentially present following the Buchwald route [3] to Cp2Zr(H)Cl, since Csp—Te bond reduction can occur to a significant extent in the presence of this dihydride or of residual LAH. 

As an alternative to hydrozirconation of acetylenic tellurides or selenides, Dabdoub and co-workers have more recently described the first additions of the Schwartz reagent (one equivalent) to acetylenic selenide salts 51 (Scheme 4.30) [52]. Subsequent alkylation at selenium produces 1,1-dimetallo intermediates 52, which are cleanly converted in a one-pot process to stereodefined products 53. It is noteworthy that ketene derivatives 52 are of ( )-geometry, the opposite regiochemistry to that which results from hydrozirconation of acetylenic tellurides (vide supra). This new route also avoids the mixtures of regio-isomers observed when seleno ethers are used as educts. The explanation for the stoichiometric use of Cp2Zr(H)Cl in these reactions, in contrast to the requirement for two equivalents with seleno ethers, may be based on cyclic intermediates 54, in which Li—Cl coordination provides an additional driving force. Curiously, attempted hydrozirconation of the corresponding telluride salt 55 under similar conditions was unsuccessful (Scheme 4.31) (Procedure 12, p. 143). 

The reduction of acetylenic tellurides to the vinylic ones is achieved by treatment with NaBH4 in EtOH. The formal unusual reduction of the carbon triple bond by NaBH4 can be rationalized involving the attack of a hydride ion to the tellurium atom producing a tellurol and an acetylenic anion followed by the addition of the tellurolate anion to the acetylene, DIBAL-H has later been employed as a reducing agent. ... 

Acetylenic tellurides can also be submitted to hydroboration reactions. 

The alkyne hydrozirconation protocol was also applied to acetylenic tellurides furnishing the zirconated vinyl tellnrides in cis fashion and high regioselectivity. Subsequent treatment with tellurenyl halides affords telluro ketene acetals with total retention of configuration. ... 

Acetylenic tellurides (see Section 3.17), on treatment with 3 mol equiv of bromine or iodine, undergo halogenolysis, accompanied by addition of halogen to the acetylenic carbon-carbon bond, giving trihaloalkenes and organyltellurium trihaUdes. ... 

Reaction of acetylenic tellurides with iodine (typical procedurejP To a solution of l-(butyltelluro)heptyne (1.40 g, 5 mmol) in CH2CI2 (10 mL) is added Ij (3.81 g, 15 mmol). The pale yellow colour of the solution turns dark red and a precipitate is formed. The mixture is stirred at room temperature for 30 min and then the solvent is evaporated. The residue is filtered through an Si02 column (eluting with hexane), giving 1,1,2-triiodohep-tene (1.76 g (74%)). 

The Z isomers were achieved by the addition of DIBAL-H to acetylenic tellurides. 

The reactions of acetylenic tellurides are faster than the reaction of the selenium acetylides as assessed in separate comparable experiments. 

Otherwise a variety of acetylene tellurides have been prepared by the reaction of lithium ethynyl tellurolates with alkyl halides. 

Ethylalkynyl reagents of RC CZnEt type were prepared by Te-Zn exchange of Et2Zn (1.5 equiv) with acetylenic telluride, reagents of RC=CZnEt2Li and (RC=C)2Zn type by the treatment of lithium acetylides respectively with Et2Zn or ZnCl2 (0.5 equiv). 

Treatment of acetylenic tellurides (79) with three molecular equivalents of bromine or iodine leads to trihalovinylic derivatives (80). The reactions involve the oxidative halogenolysis of the carbon-tellurium bond followed by addition of the halide to the acetylenic bond. ... 

---