Phosphoranes cumulated

Addition of an electrophile to the lone pair of oxo-, thioxo- and imino-vinylidenephosphoranes transforms the nucleophilic system into the dipolar ir -ir system of a ketene. The resulting phosphonium salt becomes a true dipolar ketene which, as such, reacts in a known manner (equation 101). Whenever the anion Nu is a stronger nucleophile than the original cumulated ylide, the new alkylidene-phosphorane will be formed, in which compound ElNu has added to the parent vinylidenephosphorane. If the starting phosphacumulene ylide is a stronger nucleophile than Nu , the intermediate salt always reacts with a second molecule of unreacted ylide in a [2 + 2] cycloaddition to give 1,3-cyclobutanedione derivatives. 

A full account has appeared of the alkylation of the ester phosphorane Ph8p= CHC02Et with triethyloxonium borofluoride and of the reactions of the resulting ylide Ph3P=C=C(OEt)2- The cumulated phosphoranes (36) and... 

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