Metal Stereogenicity in Asymmetric Transition Metal Catalysis

Abstract

Chiral transition metal catalysts represent a powerful and economic tool for implementing stereocenters in organic synthesis, with the metal center providing a strong chemical activation upon its interaction with substrates or reagents, while the overall chirality of the metal complex achieves the desired stereoselectivity. Often, the overall chiral topology of the metal complex implements a stereogenic metal center, which is then involved in the origin of the asymmetric induction. This review provides a comprehensive survey of reported chiral transition metal catalysts in which the metal formally constitutes a stereocenter. A stereogenic metal center goes along with an overall chiral topology of the metal complex, regardless of whether the ligands are chiral or achiral. Implications for the catalyst design and mechanism of asymmetric induction are discussed for half-sandwich, tetracoordinated, pentacoordinated, and hexacoordinated chiral transition metal complexes containing a stereogenic metal center. The review distinguishes between chiral metal catalysts originating from the coordination to chiral ligands and those which are solely composed of optically inactive ligands (achiral or rapidly interconverting enantiomers) prior to complexation (dubbed "chiral-at-metal" catalysts).

Figure 1

Half-sandwich stereogenic-at-metal complexes for asymmetric catalysis.

Scheme 1. Pioneering Studies on Stereogenic-at-Metal Half-Sandwich Catalysts

Scheme 2. Role of Metal-Centered Chirality in Asymmetric Hydrogenation with Noyori–Ikariya Catalyst

Scheme 3. Stereogenic-at-Metal Half-Sandwich Catalysts for Enantioselective Ring-Closing Nitrene C(sp³)-H Amidation

Figure 2

Tetrahedral stereogenic-at-metal complexes for asymmetric catalysis.

Scheme 4. Stereogenic-at-Mo Metathesis Catalysts Developed by Hoveyda and Schrock

Figure 3

Trigonal bipyramidal and square pyramidal stereogenic-at-metal complexes for asymmetric catalysis. ^§The requirements change if multidentate ligands are involved.

Scheme 5. Pentacoordinated Stereogenic-at-Metal Complexes for Enantioselective Metathesis

Scheme 6. Pentacoordinated Stereogenic-at-Aluminum Salalen Catalyst Applied to Asymmetric Hydrophosphonylation

Figure 4

Topology of octahedral complexes with helical chirality.

Scheme 7. Octahedral Stereogenic-at-Metal Catalysts with One or Two Chiral Bidentate Ligands

Scheme 8. Octahedral Stereogenic-at-Metal Catalysts with Two Bidentate Ligands Formed in Situ

Scheme 9. Heterobimetallic Lanthanide BINOLate Catalysts with Stereogenic Metal Center

Figure 5

Coordination isomers for linear (sequential) tetradentate ligands within an octahedral coordination sphere. cis-α and cis-β feature a stereogenic metal center. The monodentate ligands X are typically labile.

Scheme 10. Octahedral Stereogenic-at-Metal Catalysts with One Chiral Tetradentate Ligand for C–H and C=C Oxygenations

Scheme 11. Other Transformations for Octahedral Stereogenic-at-Metal Catalysts with One Chiral Tetradentate Ligand

Scheme 12. Metallosalen Complexes for Asymmetric Catalysis via cis-β-Coordination Topology

Scheme 13. Scott’s Metallosalen Complex with Biaryl Backbone for Asymmetric Cyclopropanation

Scheme 14. Katsuki’s Titanium Salalen Catalyst for Asymmetric Epoxidation

Scheme 15. Conversion of a Nonstereogenic into a Stereogenic Metal Center for Asymmetric Catalysis

Scheme 16. Yamamoto’s Bis(8-Quinolinolato) Metal Catalysts with 1,1′-Binaphthyl Backbone

Scheme 17. Stereogenic-at-Metal Catalysts with Linear Chiral Pentadentate Ligands

Scheme 18. Stereogenic-at-Metal Catalysts with Branched Chiral Pentadentate Ligands

Scheme 19. Tetrahedral Chiral-at-Zinc Complex for Asymmetric Hetero-Diels–Alder Reaction

Scheme 20. Enantioselective Olefin Metathesis with a Cyclometalated Chiral-at-Ru Complex

Scheme 21. Initial Work by Meggers Using Bis-Cyclometalated Iridium Complexes as Chiral Lewis Acid Catalysts

Scheme 22. Bis-Cyclometalated Benzothiazole Iridium Complexes for Asymmetric Catalysis

Scheme 23. Bis-Cyclometalated Rhodium Complexes for Asymmetric Catalysis

Scheme 24. C₂-Symmetric and Non-C₂-Symmetric Octahedral Chiral-at-Ruthenium for Asymmetric Catalysis

Scheme 25. Chiral-at-Osmium and Chiral-at-Iron Catalysts

Scheme 26. Tetradentate Tripodal Ligand System by Carmona

Scheme 27. Rh(I)-Catalyzed, Asymmetric Hydrogenation of EAC

Scheme 28. Formation of Stereogenic-at-Metal Intermediates XXIIa and XXIIb According to the Halpern–Brown Mechanism