Alkynylation of phosphines by alk-l-ynyliodonium salts

In the same way, the propargylphosphonium salt (21) can be rearranged to allenylphos-phonium salt (22)278,279 (reaction 36). 

Nevertheless this kind of isomerization cannot be considered as a general method for the synthesis of all the alk-l-enylphosphoniums, because it often affords thermodynamic mixtures of a,/ - and / ,y-unsaturated phosphonium salts157 280 (equation 37). 

Moreover, these results demonstrate that the stabilizing interaction of a positively charged phosphorus atom with the 7r-electron density of the unsaturation is not the main driving force for the isomerization of the double bond into the a, -position. Therefore, the isomerization ratio generally depends on the actual structure of the alkenyl chain (reactions 38 and 39). 

It depends also, of course, on the reaction conditions282 (reactions 40 and 41). However this kind of isomerization has been used to obtain specifically some dienic phosphonium283 or bisphosphonium193 salts, because, in this case, the stability of the dienic system induced fully selective isomerization (reactions 42 and 43). 

R3PCH=CHCH=CH2 Clin the same way, in the case of the propargylphosphonium salt, two successive isomerizations of the unsaturation from the / ,y to the opposition, combined with a Michael-type addition of nucleophiles on the intermediate allenyl phosphonium, afford various /Mieterosubstituted oc,/ -unsaturated phosphonium salts, which have found interesting applications in organic synthesis (reaction 44). 

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