A Re-Evaluation of the Electrophilic Substitution Reactions of the Ramirez Ylide

Cyclopentadienylidenetriphenylphosphorane (the Ramirez ylide), unexpectedly and contrary to a number
of earlier reports, has been shown to be like pyrrole in undergoing electrophilic substitution on the
cyclopentadienide ring at either the 3- or the 2-position, depending on the electrophile. Formylation
under Vilsmeier conditions and addition of tetracyanoethylene occurs at the 3-position, while activated
acetylenes and the nitrosyl electrophile substitute at the 2 position. The 3-formylated product was reduced
to the 3-methyl derivative and it also reacted under Knoevenagel conditions to give a number of novel
condensation products. The results of single-crystal X-ray crystallographic analyses are given for four of
the compounds studied, and a careful 2D NMR analysis of all of the compounds was performed in order
to develop a reliable method for the unambiguous assignment of the regiochemistry of adduct formation.