Substituted cyclopent adienes and related compounds

Summary

• 1. On the basis of the behavior of methylcyclopentadiene it was confirmed that a characteristic feature of substituted cyclopentadienes is the occurrence of thermodynamic equilibrium between isomers differing in the positions of the intracycle double bonds.

• 2. The equilibrium transformations of substituted cyclopentadienes have a stagewise character, i.e. the proton migrates from the methylene group (the 5-position) and goes predominatly to the adjacent C atom.

• 3. The course of the alkylation of metal derivatives of cyclopentadiene was elucidated for the first time: it was proved that the primary reaction product is the 5-isomer, which in the process of the establishment: of equilibrium is converted into the 1-isomer, and then into an equilibrium mixture of 1- and 2-substituted cyclopentadienes.

• 4. Samples of all three possible methylcyclopentadienes, each almost homogeneous in isomeric composition, were prepared. They were characterized by their individual adducts with maleic anhydride and by some derivatives of these. The mixture of adducts obtained from the equilibrium mixture of cyclopentadienes was separated via their esters.

• 5. The rate of diene condensation with an active dienophile is very much greater than the rates of the isomeric transformations of methylcyclopentadienes.