1,3,6-HEPTATRIENE

Cyclization and Isomerization of 1,3,6-Heptatriene

Xi Z, et al. Journal of organometallic chemistry, 2002, 663(1-2), 13-20.

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. The mixture of zirconium cyclopentanes was isomerized by stirring the reaction for 48 hours, resulting in the formation of cis-dimethylcyclopentene 3 with high selectivity after hydrolysis.
The overall reaction mechanism: (i) formation of transzirconacyclopentane; (ii) its equilibrium with cis-zirconacyclopentane via bis(olefin)zirconium complex; (iii) cleavage of one of the Zr /C bonds in cis-zirconacyclopentane disrupts the equilibrium. In such a way transzirconacyclopentane is slowly consumed and after the reaction with an electrophile cis-product is isolated.
A typical procedure for the cyclization of 1,3,6- heptatrienes
· To a solution of Cp2ZrBu2, prepared by the reaction of Cp2ZrCl2 (1.0 mmol, 292 mg) with n-BuLi (1.25 ml, 2.0 mmol) at -78 °C in THF (5 ml) was added 3,4- diethyl-1,3,6-heptatriene (1a) (150 mg, 1.0 mmol) at -78 °C and the reaction mixture was warmed to 20 °C within 1 h.
· After that it was quenched with 3 N HCl and extracted with C6H14. Combined extracts were washed with sat. NaHCO3, brine, dried over MgSO4, and concentrated in vacuo. Column chromatography on silica gel (C6H14) afforded 106 mg (70%) of a mixture of 2a/3a (4.7:1).