Boratabenzene Zirconium(II) Complexes: An Unusual Annulation with Ethynes

摘要

1-Substituted boratabenzenes can serve as six-electron, rebound, anionic ligands towards a variety of metals. The well-investigated late transition metal complexes, for example l and 2, are closely analogous to the corresponding cyclopenta-dienyl metal compounds. Early transition metal complexes have been more sparingly studied. We recently reported the first zir-conium(IV) boratabenzene complex 3a, which on activation by methylaluminoxane polymerizes ethene with an activity similar to that of [Cp_2ZrCl_2] . Since Cp_2Zr~(II) compounds are particularly useful in organic synthesis, an investigation of the corresponding boratabenzene Zr~(II) complexes is appropriate. We have prepared boratabenzene Zr~(II) complex 4b and find that the PMe_3 ligands aredisplaced under very mild conditions. The reaction of 4b with ethyne involves an unprecedented ring annulation to afford complex 5, which contains the novel ligand 4a-phenyl-8a H-4a-boratanaphthalene.