Stereoselective Synthesis of Dienyl Zirconocene Complexes

Abstract

Several methods are known for the preparation of dienyl zirconocene derivatives such as the hydrozirconation of enynes, the stereospecific reaction of zirconacyclopentadiene derivatives with electrophiles, the reaction of organozirconocene derivatives with carbenoid reagents, and the reaction of vinyl zirconocenes with vinyl halides. All these described methodologies lead to the expected dienyl zirconocenes but the stereoselectivity is always structure dependent. On the other hand, dienyl zirconocenes can also be easily prepared, as unique geometrical isomers, from simple nonconjugated unsaturated enol ethers with (1-butene)ZrCp₂ complexes. This methodology is based on a tandem allylic C–H bond activation–elimination sequence and the mechanism has been mapped out by deuterium labeling experiments. The stereochemical outcome of this process was determined by addition of several electrophiles. When the organometallic derivative was vinylic as well as allylic, an unexpected reversal of the stereochemistry was found during the zirconium to copper transmetalation step.