Tellurium structural rearrangement

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 359 was proposed that sulphene formation was involved in each reaction, with the triphenylphosphine adding to the sulphene and displacing SO2 to afford the phosphonium ylide, which was protonated by the conjugate acid of the amine. Sulphenes have been reported to react with tropone to afford an adduct which rearranged on heating to an o-hydroxystyrene (lOQ. ... 

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