Because opposite mirror images of pharmaceuticals usually invoke different responses in living systems, the availability of a wide variety of enantiomerically pure compounds is vital to the development of improved drugs. Transition metal complexes promote many organic reactions and represent one of the more promising areas in which to develop new asymmetric synthetic methods. In particular, chiral cyclopentadienyl-metal complexes could mediate a number of useful asymmetric reactions, such as hydrogenation and hydroformylations, among others. However, the chiral cyclopentadienyl ligands reported to date have generally not yielded synthetically useful metal complexes. Proposed herein is the synthesis of new C2 chiral cyclopentadienyl ligands and the derived transition metal complexes. The tetra-alkyl cyclopentadienes described here represent a new class of chiral cyclopentadienyls which may offer several advantages over previously developed ligands, which were only monosubstituted or 1,2-disubstituted. The higher degree of substitution present in these new ligands should result in more significant interactions between the chiral groups and the reactive site on the metal. Furthermore, these ligands are extremely rigid and of well-defined conformation, which should simplify our understanding of their behavior. The C2 symmetry is also advantageous. Only a single stereoisomer will result from coordination of one or two such ligands to a transition metal, since both faces of the free ligand are equivalent. Finally, these materials may be readily prepared from natural sources in very high enantiomeric purity, yielding the desired C2 symmetry from a coupling of two molecules of like chirality. The synthetic approach described herein has precedent in the preparation of achiral cyclopentadienes, but has not been used previously for the construction of chiral ligands. Preliminary results suggest that the proposed route to these materials is indeed viable. The preparation of transition metal complexes containing these ligands will provide new reagents and catalysts for the asymmetric synthesis of a variety of organic compounds.
