Chiral metallocenes: the synthesis and X-ray crystal structures of TiCl2η5:η5-C5Me4SiMe2C5H3R*) (R* = menthyl or neomenthyl) and related compounds

The syntheses of the chiral ansa-metallocene complexes TiCl2(η5:η5-C5Me4SiMe2C5H3R*) (R* = menthyl (4a) or neomenthyl (4b)) are reported; initially 4a was obtained as a 3:1 mixture of (R:S) diastereoisomers, which differ only in which face of the asymmetrically substituted cyclopentadienyl ring is bonded to the titanium (chiral descriptor shown). The major diastereomer 4aR was crystallized out optically pure from the initial reaction mixture, whereas the 4aS diastereoisomer was isolated after isomerizing a racemic mixture of 4a to a 1:3 mixture of (R:S) diastereoisomers using UV irradiation. The corresponding neomenthyl complex 4b was obtained as a 1.3:1 mixture of diastereoisomers that could not be separated. The optically pure 4aR was converted stereoselectively into the corresponding (R)-TiMe2(η5:η5-C5Me4SiMe2C5H3R*) (R* = menthyl). Syntheses of the related indenyl ligand system C5Me4SiMe2C9H7 (3) is reported but complexation to titanium proved to be problematic although ZrCl2(η5:η5-C5Me4SiMe2C9H6) (6)...