Novel cyclopentadienyl transition metal complexes

Abstract

The preparation of chiral cyclopentadienyl transition metal complexes and their application to enantioselective synthesis was reviewed.

The synthesis of known chiral zirconocene (-)-(n-5-cyclopentadienyl)(n5-{1-[(1S, 2S, 5R)-2-isopropyl-5-methylcyclohexyl]-4,5,6,7-tetrahydroindenyl}) zirconium dichloride was repeated and the yields optimised. Application of this complex to a recently reported co-cyclisation / elimination reaction gave an e.e. of 11%.

A series of complexes were designed and synthesised, based on the structure of (-)-(n-5-cylcopentadienyl)(n5-{1-[(1S, 2S, 5R)-2-isopropyl-5-methylcyclohexyl]-4,5,6,7-'tetrahydroindenyl}) zirconium dichloride. Three novel metallocenes were prepared and X-ray crystal structures obtained. However, these new complexes displayed no significant improvement over the enantioinductive properties of the parent system. A new class of cyclopentadienyl ligand was designed comprising a tethered heteroatom held over one face by a rigid [2,2,2] bicyclic backbone. A synthesis was developed to allow the preparation of a number of ligands with differing heteroatoms and substitution around the cyclopentadienyl ring.

A new ligand with an OCH₃ group held over one face of the cyclopentadienyl ring was prepared. Complexation to CpTiCl₃ gave a mixture from which only one compound could be isolated in 40% yield. NMR and nOe experiments showed that the titanium was bound to the opposite face to the heteroatom linker. Two further complexes bearing ligands of this type were synthesised, however both were difficult to isolate. Nevertheless, comparison of NMR data suggested that in both cases the metal selectivity binds to the least sterically hindered face.

Novel complexes were tested for suitability as propene polymerisation catalysts.

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