Zirconium-catalyzed and zirconium-promoted cyclization reactions of non-conjugated dienes with alkylmagnesium halides to give cycloalkylmethylmagnesium derivatives

Article abstract of DOI:10.1016/s0020-1693(98)00064-4

The stoichiometric reaction of certain non-conjugated dienes with n-Bu₂ZrCp₂ provides the corresponding zirconabicycles, such as trans-3-bis(cyclopentadienyl)zirconabicyclo[3.3.0]octane, that can be fully characterized by spectroscopic means. Their treatment with EtMgBr or n-BuMgCl in THF gives the corresponding monocyclic monomagnesium derivatives along with the corresponding alkene-ZrCp₂ derivatives in high yields. In cases where the Grignard reagent is either sterically hindered or lacking β-H, little or no reaction may occur, although some, e.g., s-BuMgCl, react, albeit slowly, to give the expected products in high yields. In cases where either a Grignard reagent in diethyl ether or a dialkylmagnesium (irrespective of solvent) is used, the major product is the corresponding dimagnesio derivative. A couple of intramolecular transmetallation paths are proposed for these cases. The Cp₂ZrCl₂-catalyzed reaction of 1,6-heptadiene with EtMgBr fails to induce the desired bicyclization-ring opening sequence to give the corresponding monocyclic monomagnesium derivatives. On the other hand, the corresponding reaction with n-BuMgBr does proceed as desired to give the monocyclic monomagnesium product which contains a minor amount of the corresponding exo-methylene derivative. Only traces, if any, of the corresponding dimagnesium derivatives reported to be the major products in Et₂O are formed. This procedure has been applied to catalytically convert several other dienes, i.e., (E)-1-phenyl-1,6-heptadiene, 2,4,4-trimethyl-1,6-heptadiene, diallyl(benzyl)amine, 1,7-octadiene and 1,2-diallylbenzene, into the corresponding monocyclized compounds in moderate to excellent combined yields.