Catalytic Enantioselective Transformations with Chiral Cyclopentadienyl Ruthenium(II) Complexes

The chemistry of cyclopentadienyl ruthenium(II) complexes plays an important role in ruthenium catalysis because of its high potential for various transformations. However, asymmetric catalysis with chiral cyclopentadienyl ruthenium(II) complexes is still at an infant stage, but rapidly developing because of its expected impact on multiple enantioselective applications.

This thesis discusses the synthesis of a novel class of chiral cyclopentadienyl ligands, which can be readily accessed by a two-step process from alpha, beta-unsaturated aldehydes and cyclopentadiene. The novel C2-symmetric chiral cyclopentadienyl ligands were complexed with ruthenium(II) and successfully applied in asymmetric catalysis.

The cyclopentane-fused CpxRu complex enables the enantioselective synthesis of benzonorcaradienes by coupling of oxa-benzonorbornadienes with internal alkynes in high enantioselectivity (up to 97.5:2.5 er). The reaction mechanism was investigated by computational studies and control experiments.

The chiral cyclopentadienyl ruthenium(II) complexes show promising results for other transformations. An alkylative cycloetherification of allenols and aryl vinyl ketones was investigated using binaphthyl-derived chiral cyclopentadienyl ruthenium(II) complex. After an initial optimization study, allenol and phenyl vinyl ketone gave the desired enantio-enriched cyclic ether with a promising enantioselective ratio of 90:10.

Furthermore, preliminary studies were undertaken for the [2+1] cycloaddition of enynes via ruthenium vinyl carbenes. Model substrate, 1,6-enyne, delivered the desired enantio-enriched bicyclo[3.1.0]hexane in a good yield of 72% (E:Z = 1.6:1), with enantiomeric ratios of 72.5:27.5 and 63.5:36.5 for the E- and Z-isomer, respectively.