An important aim of organic synthesis is to synthesise single enantiomers of chiral compounds, and asymmetric transition metal catalysts have an important, and increasing role.
Cyclopentadienyl is by far the most common ligand found in transition metal chemistry, but unlike phosphines, amines etc there are no effective chiral versions. We are investigating new concepts in the design of chiral cyclopentadienyl ligands and their transition metal complexes which should lead to future generations of catalysts.
In one approach we direct metallation to one enantioface of a non-symmetrically substituted cyclopentadiene to provide complexes such as 1 which feature induced planar chirality.[1] Ligands which incorporate a tether to a co-ordinating group are producing exciting results in mid-transition element chemistry (Cr, Ru, Co, Rh), for example formation of 2 in which metal centred asymmetry is also induced.[2]
The selective formation of complexes 1 and 2 rely on a favoured rotomer about the indenyl-chiral centre bond A more powerful design is to fuse a chiral ring to the cyclopentadiene. With this design we have developed a very short and convergent route to novel polycyclic ligands which promise to provide the best solution to date to the unsolved problem of effective chiral cyclopentadienyl ligands.
[1] Asymmetric ethylmagnesiation of alkenes using a novel zirconium catalyst. Bell, L.; Brookings, D. C.; Dawson, G. J.; Whitby, R. J.; Raymond, V. H.; Michael, C. H. Tetrahedron 1998, 54, 14617-14634.
[2] Induction of planar chirality in formation of (eta(5):eta(1)-1- (1-cyclohexyl-2-(diphenylphosphino)ethyl)indenyl)- carbonylrhodium and (eta(5):eta(1)-1-(2-phenyl-2- (diphenylphosphino)ethyl)indenyl)carbonylrhodium. Brookings, D. C.; Harrison, S. A.; Whitby, R. J.; Crombie, B.; Jones, R. V. H. Organometallics 2001, 20, 4574-4583.