Zirconocene mediated cyclization and isomerization of 1,3,6-heptatriene

Cyclization of 1,3,6-heptatrienes 1 with Cp2ZrBu2 (Negishi reagent) furnished a mixture of trans- and cis-zirconacyclopentanes that after hydrolysis afforded a mixture of isomeric dimethylcyclopentenes 2 and 3 in ca. 5:1 ratio. An attempt to isomerize the mixture of zirconacyclopentanes by stirring the reaction for 48 h resulted, after hydrolysis, in rather surprising formation of cis-dimethylcyclopentenes 3 with high selectivity. This is the first example of trans to cis isomerization of zirconacyclopentanes. Further reaction of the intermediate zirconacyclopentanes with various electrophiles and a plausible reaction mechanism of the trans to cis isomerization are presented.