Synthesis, Characterization and Reactivity of Formal 20 Electron Zirconocene-Pentafulvene Complexes
Résumé
The reaction of low-valent zirconocene reagents formed
either from the Negishi reagent Cp2Zr(n-butyl)2 or from the reduction of
Cp2ZrCl2 with Mg with various pentafulvenes yielded the first
zirconocene−pentafulvene complexes, Cp2ZrFv (Fv = 6,6-di(aryl),
6,6′-adamantylidene, and 6-tert-butylfulvene), which are formal 20-
electron complexes. In addition to NMR spectroscopic analysis and DFT
calculations, three complexes were characterized by X-ray crystallography,
showing a η5:η1 dianionic binding mode of the fulvene to the
metal. The obtained complexes react with carbonyl compounds
(aldehydes and ketones) to unexpectedly afford double-insertion
products, as shown by NMR spectroscopy and X-ray studies. In the
case of aldehydes these diolate complexes were obtained as single
diastereoisomers. The reaction of Cp2ZrFv with internal alkynes did not
result in the formation of insertion products but release of the
coordinated fulvene and reductive dimerization of the alkynes to the corresponding zirconacyclopentadienes.