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.

[(E)- and [(Z)-2-[alpha,beta-bis(methoxycarbonyl)vinyl]cyclopentadien-1-ylidene]triphenylphosphorane.

The Ramirez ylide undergoes electrophilic substitution with dialkyl acetylenedicarboxylates, yielding a mixture of the Z and E adducts. The crystal structure analyses of the two adducts formed using dimethylacetylene, viz. dimethyl (E)- and (Z)-1-[2-(triphenylphosphoranylidene)cyclopentadien-1-yl]ethylenedicarboxylate, both C(29)H(25)O(4)P, explain an unusual chemical shift observed for the vinyl H atom of the Z adduct, which had previously precluded a definitive assignment of the isomers. In addition, the structures explain why only one of the isomers reacts further with acetylene esters to produce azulenes with a rare substitution pattern.

A novel azulene synthesis from the Ramirez ylide involving two different modes of its reaction with activated alkynes.

The Ramirez ylide undergoes electrophilic substitution with acetylenedicarboxylates to form Z and E adducts. The latter can react by cycloaddition with another equivalent of the alkyne to provide a new route to novel tetra-substituted azulenes, which show interesting bond localisation and crystal packing effects.