From Tellurium Ylides

The ylide formed from 10/f-dibenzo[i ,e]tellurin 5,5-dichloride and 1,1-dimethyl-3,5-di-oxocyclohexane reacted with trifluoroacetic acid in hot ethanol to give lOH-dibenzo[b e]tellurin 5,5-bis[trifluoroacetate].  

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The use of ylides derived from tellurium analogues of sulfides 2a and 2b for asymmetric epoxidation has been described [68, 69]. Preliminary experiments using 15 as catalyst gave high enantio- and diastereoselectivities, but poor yield (see Scheme 10.13). 

Ylides from R N2. This reagent is more effective than bis(acetylacetonate)-copper(II) (5, 244) for generation of carbenes from diazo compounds.2 The decomposition proceeds at a lower temperature, even at room temperature. The mild conditions are particularly useful in the preparation of heat-sensitive ylides, such as those of antimony, bismuth, and tellurium. 

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... 

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). 

Ylides of Sulphur, Selenium, Tellurium, and Related Structures 343 (trans cis = 90 10) in 54% yield, but a major by-product was (65), which undoubtedly resulted from proton transfer in the intermediate to re-form a new substituted ylide (66), which added to another molecule of methyl acrylate. A similar proton migration occurred in the reaction of diethylsul-phoniumcyanomethylide with vinyltetramethylenesulphonium bromide, but in this instance a second addition did not occur and the elimination took place directly to afford a sulphonium salt (67). Even though stabilized... 

Ylides of Sulphur, Selenium, Tellurium, and Related Structures 355 pro-S proton from sulphoxides is a matter of continuing disagreement." " There also seems agreement that the gauche relationship of the carbanion and the lone pair on sulphur is the most stable configuration. " " " ... 

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