Tellurium compounds reactions with alkenes

Telluroaldehydes have been generated and trapped for the first time by the reaction of benzylidenetriphenylphosphorane with "activated" tellurium (a method analogous to that previously used to prepare selenoaldehydes) (Scheme 22).71 A wide range of reactive ylides have been converted into the adducts (118) by reaction with borane.72 On heating, (118) rearrange to triphenylphosphine-monoalkylborane adducts (119) which undergo the expected hydroboration reactions with alkenes. A new route to phosphaalkenes (121) is available from the reaction of phosphinomethylenetriphenylphosphoranes (120) with Lewis acids.73 In the case of (120, R2=NPr 2) the compounds (121) can be isolated and in one case an X-ray structure was obtained. However, similar reactions of (120, r2=Bu ) lead to the dimers (122). 

Electrophilic addition reactions of tetravalent tellurium compounds have been reviewed.64 2-Naph-thyltellurium trichloride (ArTeCb) adds to alkenes in an anti stereospecific manner (equation 11), whereas tellurium tetrachloride gives mixtures of 2 1 adducts with both syn and anti addition.72 A one-pot alkene inversion procedure has been developed, based upon TeCU addition to alkenes followed by treatment of the (3-chloroalkyltellurium trichloride adduct with aqueous Na2S (Scheme 37).73 Tellurium compounds such as tellurinyl acetates, ArTe(0)0Ac, prepared in situ through reaction of tellurinic acid anhydrides with acetic acid, can be employed in oxytelluration and aminotelluration procedures (Schemes 38 and 39).74 In the oxytelluration reaction intermediate triacetates of the type RCH2Te(OAc)2Ph are reduced with hydrazine to the corresponding tellurides. 

Reactions of phosphines and phosphites have received some attention but their preparative value is limited. The zwitterion formed from diphenylmethylphosphine and benzyne rearranges to ylide (124) which can be captured by Wittig alkenation, with cyclohexanone, in about 20% yield.159 Some synthetically useful reactions of tellurium and selenium compounds with arynes have been reported. For example, heating diphenyl iodonium carboxylate and bis(p-ethoxyphenyl) ditelluride in dichlorobenzene affords the compound (125).160 The corresponding reactions with diphenyl selenide and diphenyl sulfide... 

Unsymmetrical diorganyltellurium dihalides are formed upon condensation of aryltellurium trichlorides with activated aromatic compounds and with ketones. The addition of the trichloride across carbon-carbon double bonds in alkenes, as well as the reaction with aryl(trimethyl)silane, hexaphenyldilead, and aryhnercury chlorides leads to the transfer of the aryl group to the tellurium atom. 

The additions of tellurium tetrahalides to olefins in the presence of alcohols proceed equally well when the tetrahalides are generated in the reaction mixture from tellurium dioxide and chlorotrimethylsilane in methanoP or concentrated hydrochloric acid/methanol. trans-2-Methoxycyclohexyl tellurium trichloride was obtained in this manner in quantitative yield . In the reactions of terminal alkenes, the TeCl3 group (from TeO and concentrated hydrochloric acid) added according to the Markovnikov rule producing 2-alkoxy-1 -alkyl tellurium trichlorides. The trichlorides were not isolated but were reduced with disodium disulfite to the ditellurium compounds, which, in turn, were converted to the tellurium trichlorides by treatment with sulfuryl chlorides. For data on these tellurium trichlorides see p. 316. The overall yields range from 15 to 70%. 

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