Synthesis and Catalytic Application of Knölker-Type Iron Complexes with a Novel Asymmetric Cyclopentadienone Ligand Design

Abstract

Asymmetric catalysis is an essential tool in modern chemistry, but increasing environmental concerns demand the development of new catalysts based on cheap, abundant, and less toxic iron. As a result, Knölker-type catalysts have emerged as a promising class of iron catalysts for various chemical transformations, notably the hydrogenation of carbonyls and imines, while asymmetric versions are still under exploration to achieve optimal enantio-selectivities. In this work, we report a novel asymmetric design of a Knölker-type catalyst, in which the C2-rotational symmetric cyclopentadienone ligand possesses chiral substituents on the 2- and 5-positions near the active site. Four examples of the highly modular catalyst design were synthesized via standard organic procedures, and their structures were confirmed with NMR, IR, MS, and polarimetry analysis. Density functional theory (DFT) calculations were conducted to elucidate the spatial conformation of the catalysts, and therewith to rationalize the influence of structural alterations. Transfer- and H2-mediated hydrogenations were successfully established, leading to appreciable enantiomeric excesses (ee) values up to 70%. Amongst all reported Knölker-type catalysts, our catalyst design achieves one of the highest ee values for hydrogenation of acetophenone and related compounds.

Recent Catalytic Applications of (Cyclopentadienone)iron Complexes

“…The chiral complexes recently reported by De Wildeman and co‐workers and their applications in ketone AH …”

“…Very recently, De Wildeman and co‐workers reported new chiral CICs possessing a C 2 ‐symmetric cyclopentadienone ligand with stereocenters at positions 2,5 – i.e., closer to the C=O group involved in the catalytic activity (Scheme ) . This kind of design led to a slight improvement in terms of enantioselectivity compared to the Wills' pre‐catalysts, although the ee values remained moderate.…”

Recent Advances in Asymmetric Iron Catalysis

“…Knölker-type iron complexes are known to efficiently catalyze hydrogen transfer reactions [ 86 ]; however, despite their wide application, this class of catalysts suffered from a poor stereocontrol. Recently, the design of a new Knölker-type catalyst L16 ·Fe(0)(CO) 3 , featuring a C2-symmetric cyclopentadienone ligand bearing chiral substituents at the 2 and 5 positions, provided ees up to 70% for the asymmetric hydrogenation of different ketones ( Scheme 31 ) [ 87 ].…”

Bifunctional homogeneous catalysts based on first row transition metals in asymmetric hydrogenation

“…applied in ATH of ketones by De Wildeman et al were applied in AH of a range of ketones bearing alkyl and aryl or heteroaryl substituents (Scheme 36)[68]. The resulting alcohols were obtained with fair to high conversions and low to good enantioselectivities, some being among the highest obtained using chiral Knölker-type catalysts.…”