Novel Bicyclization of Enynes and Diynes Promoted by Zirconocene Derivatives and Conversion of Zirconabicycles into Bicyclic Enones via Carbonylation

Article abstract of DOI:10.1021/ja00191a035

Enynes and diynes react with "ZrCp2" (where Cp = η⁵-C5H5) generated by treating Cl2ZrCp2 with Mg and HgCl2 or 2 equiv of an alkyllithium, such as n-BuLi, or a Grignard reagent, such as EtMgBr, and can produce in excellent yields zirconabicycles represented by 2 (M = Zr) and 8, respectively.Their protonolysis can provide the corresponding exocyclic alkenes and conjugated dienes 9, respectively.Iodinolysis of 2 (M = Zr) can give the corresponding diiodides in high yields, while carbonylation of 2 (M = Zr) can produce bicyclic enones 3 (Y = O) in moderate to goodyields.Although the bicyclization reaction fails with terminal alkyne containing substrates, various types of substituents on the alkyne moiety, such as alkyl, alkenyl, aryl, trialkylsilyl, and trialkylstannyl groups, can be accommodated.Investigation of the n-BuLi-Cl2ZrCp2 reaction has revealed that is gives first (n-Bu)2ZrCp2 at -78 deg C, which then decomposes to give Cp2Zr(CH2=CHEt), identified as its PMe3 complex 11.The PMe3-stabilized complex reacts with diphenylacetylene to produce a crystalline compound which has been identified as a zirconacyclopropene, 36a.This demonstrated, for the first time, the feasibility of converting alkynes into zirconacyclopropenes.The reaction of preformed, three-membered zirconacycles with alkynes gives five-membered zirconacycles.The reaction of diphenylacetylene is ca. 150 times as fast as that of (E)-stilbene.These results support a mechanism involving formation of a zirconacyclopropene intermediate followed by its intramolecular carbometalation with the alkene moiety of enynes for the Zr-promoted bicyclization of enynes.