Vinylic tellurides preparation

Treatment of vinylic tellurides prepared by photopromoted carbotelluration of terminal acetylenes (see Section 3.16.3) with aqueous NaOCl, followed by pyrolysis, affords internal acetylenes in good yields. ... 

Bis(trispyrazolylborate)titanium(II) complexes, preparation and reactivity, 4, 266 Bis-vinylic tellurides, preparation, 9, 605 Bis( -xylene)lead(II)bis(tetrachloroaluminate), preparation,... 

Alkyl vinyl tellurides can also be prepared by a sequential reaction of divinylic tel-lurides with lithium and alkyl halides in liquid ammonia. ... 

Preparation of vinyl tellurides. To a mixture of NaH (1 mmol) (80% suspension in oil) and (phenyltelluromethyl)-phosphonate (1 mmol) in THF (5 mL) at reflux under Nj, aldehydes (1 mmol) were added dropwise. Following 3 h at reflux, the solution was cooled to room temperature. Then, saturated aqueous NH4CI was added and the mixture was extracted with diethyl ether. The organic extract was dried (MgS04) and evaporated to give vinyl tellurides, which were purified by column chromatography on silica gel with ethyl acetate/hexane (1 20) or by preparative TLC,... 

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

Transmetallations of vinylic tellurides deserve particular attention. These tellurides (prepared by anti-addition of tellurols to acetylenes, see Section 3.16.1.2) exhibit the Z configuration and therefore generate (Z)-vinyUithiums. These results are in sharp contrast to the earlier tin-lithium exchange performed with vinylstannanes (characterized by the E configuration), giving (ii)-vinyllithiums. ... 

Vinylic organocuprates, prepared by transmetallation of vinylic tellurides with Bu (2-Th)Cu(CN)Li2, are highly reactive towards simple epoxides. ... 

It was later observed" that the above Te-Zn exchange reaction is not a general method to prepare alkenyl Zn reagents, but is restricted to vinylic tellurides bearing Ph, ester, or MejSi groups at the a-position, able to stabilize the formed vinyl zinc. 

TetralkylteUnriiims, prepared in situ by the reaction of tellurium tetrachloride with 4 equiv of alkyllithiums, react with arylacetylenes to afford dialkyl tellurides A, alkylation products B, an alkene C and minor amounts of a vinyl telluride D. ... 

The preparation of a variety of vinylic functionalized C-glycosides involves a photochemical reaction between telluroglycosides with aryl alkynes at 120 °C. Glycosyl radicals were initially formed by photohomolysis of the C-Te bond and readily added to the alkyne a vinyl telluride (e.g., 49) was finally obtained by a further attack of RTe- (Scheme 3.32) [82],... 

Bis-vinylic tellurides 68 can be prepared by reacting monosubstituted alkynes with a hydrotelluration system prepared from elemental tellurium and sodium borohydride in a mixture of ethanol/aqueous sodium hydroxide.133 The stereochemistry of the resulting vinylic tellurides is also Z (Equation (7)). 

To date, a number of vinylic tellurides have been prepared by the methods described above. In the following sections we will comment on the influence of the substrate structure on the yields and regiochemistry of the product. 

Several vinylic tellurides are prepared starting from monosubstituted aryl alkynes.41,42,133-136 The yields are usually high and the reports in the literature mention that the stereochemistry of the product is 100% Z. In the same way, only the formation of 1,2-disubstituted vinylic tellurides is reported (Scheme 36). 

The preparation of the /3-haloenones starting materials sometimes requires acidic reaction conditions. In view of the synthetic potential of the vinylic tellurides (Section 9.13.8.2) associated with the above-commented stereoselectivity, enol phosphates are employed instead as starting materials for the preparation of vinylic tellurides, since enol phosphates can be prepared under very mild basic conditions. 

Vinylic tellurides bearing simple vinyl groups (e.g., CH2=CH, PhCH=CH) can be prepared by reacting the corresponding lithium or Grignard reagents with organotellurium halides.8 ... 

If unusual vinylic tellurides or vinylic tellurides of defined stereochemistry are required, other vinyl organo-metallics are employed. In recent years, a number of methods of preparing vinylic derivatives of tellurium using organotellurium halides have been developed. 

Diorganoditellurides are used as the tellurating agent in a catalytic process involving vinylic boranes 114 to prepare vinylic tellurides 115 (Scheme 68).184... 

Keeping in mind the easy access to vinylic tellurides (see Sections 5.2.3 and 5.2.5), and their fast transformation into widely-used reactive organometallics, we can consider this reaction sequence as a valid alternative to the known methods of generating vinyllithiums and, specially, vinyl cyanocuprates. In addition, the zz/r/r-hydrotel I u ration of alkynes is unique among the known, synthetically useful hydrometallations, since it allows the direct preparation of... 

In this way, simply by controlling the reaction temperature, (Z)- or ( )-enones or enoates could be prepared starting from the same (Z)-vinylic telluride.286... 

Hydrogen telluride, prepared from aluminum telluride and 2 molar hydrochloric acid, added to the double bond in vinyl cyanide and formed bis[2-cyanoethyl tellurium1. 

The same authors also reported a modified sequential reaction, replacing the isocyanide partner with an alkyne (Scheme 61) [172]. Varied functionalized TMS-protected -allylic alcohols could be prepared. In addition, the phenyltellurium group final transfer to vinyl radical 191 afforded vinyl telluride 189, which could be further elaborated. Once again, this reaction proved highly versatile, as basically all kind of carbonyl compounds were suitable (aromatic and aliphatic ketones and aldehydes). Electron-poor alkynes did not give the desired products. 

Internal vinyl tellurides, which are not accessible via hydroteUuration of alkynes, have been prepared from alkynes through a vinyl borane route. 

Vinylic cyanocuprates, prepared from vinylic tellurides, react with bromoalkynes giving conjugated (Z)-enynes and (Z)-enediynes." ... 

Alkenylzinc compounds have been prepared by a Te-Zn exchange reaction on vinylic tellurides under halide-free conditions, with retention of the geometry of the starting tel-luride." ... 

---