Synthesis and Application of Novel Chiral Cp Ligands in Transition Metal Catalysis

Chiral cyclopentadienyl groups and their respective metal complexes have been studied for several decades. However, it is only in the last decade that this field has gained significant momentum, primarily through the introduction of chiral C2-symmetrical 1,2-cyclopentadienes and unsymmetrically substituted chiral 1,2-cyclopentadienes and indenes as precursors for chiral complexes, especially those involving late transition metals. The utilization of chiral C2-symmetrical cyclopentadienyl (Cp) ligands allows for the circumvention of diastereomer formation upon complexation with the desired transition metal center. This is particularly advantageous for unsymmetrically substituted ligands, which often necessitate tedious racemate separation. In all cases, a multi-step synthesis is required, depending on the nature of the chiral backbone. Unsymmetrically substituted Cp rings can bypass the need for a multi-step synthetic approach but generally require the separation of racemates to obtain enantiomerically pure metal complexes. An alternative method involves conjugating achiral Cp-metal complexes to a carrier molecule, which enables the insertion of the conjugate into the active center of an enzyme protein hull. This approach establishes a chiral environment for asymmetric transformations. The resulting metal complexes find extensive applications in C-H functionalization reactions, as well as various cycloaddition reactions.