Photochemical deracemization catalyzed by chiral triplet sensitizers with a lactam binding motif

• Applicant: Professor Dr. Thorsten Bach
• Subject Area: Organic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2019 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 427465615

• Final Report Year: 2022

Final Report Abstract

A deracemization is a process in which a racemic mixture is selectively and ideally quantitatively converted into a specific enantiomer. Due to the fact that enantiomers have identical standard enthalpies of formation a deracemization is impossible in a thermal equilibrium mediated by a chiral catalyst. However, deracemization reactions can be performed photochemically with a chiral triplet sensitizer that displays a hydrogen bonding motif (lactam) and acts as catalyst. Axially chiral allenes and alkenes as well as chiral cyclopropanes and sulfoxides served as substrates and were successfully deracemized. It was shown that not only the preferred binding of one substrate enantiomer to the catalyst but also the the rate of energy transfer within the diastereomeric complexes plays a decisive role for the success of the reaction. A photochemical hydrogen transfer was identified as an additional mode of action for a deracemization. The process is thermally reversible and delivered chiral hydantoins in high enantiomeric excess.

Publications

• Catalytic deracemization of chiral allenes by sensitized excitation with visible light. Nature, 564(7735), 240-243.
  Hölzl-Hobmeier, Alena; Bauer, Andreas; Silva, Alexandre Vieira; Huber, Stefan M.; Bannwarth, Christoph & Bach, Thorsten
  
• Enantioselective Visible‐Light‐Mediated Formation of 3‐Cyclopropylquinolones by Triplet‐Sensitized Deracemization. Angewandte Chemie International Edition, 58(11), 3538-3541.
  Tröster, Andreas; Bauer, Andreas; Jandl, Christian & Bach, Thorsten
  
• Photochemical Deracemization of Chiral Sulfoxides Catalyzed by a Hydrogen-Bonding Xanthone Sensitizer. Synthesis, 51(23), 4417-4416.
  Wimberger, Laura; Kratz, Thilo & Bach, Thorsten
  
• Photochemical Deracemization of Allenes and Subsequent Chirality Transfer. Angewandte Chemie International Edition, 59(31), 12785-12788.
  Plaza, Manuel; Jandl, Christian & Bach, Thorsten
  
• Photochemically Induced Ring Opening of Spirocyclopropyl Oxindoles: Evidence for a Triplet 1,3‐Diradical Intermediate and Deracemization by a Chiral Sensitizer. Angewandte Chemie International Edition, 59(48), 21640-21647.
  Li, Xinyao; Kutta, Roger J.; Jandl, Christian; Bauer, Andreas; Nuernberger, Patrick & Bach, Thorsten
  
• Enantioselective Photochemical Reactions Enabled by Triplet Energy Transfer. Chemical Reviews, 122(2), 1626-1653.
  Großkopf, Johannes; Kratz, Thilo; Rigotti, Thomas & Bach, Thorsten
  
• Photochemical Deracemization at sp3-Hybridized Carbon Centers via a Reversible Hydrogen Atom Transfer. Journal of the American Chemical Society, 143(50), 21241-21245.
  Großkopf, Johannes; Plaza, Manuel; Seitz, Antonia; Breitenlechner, Stefan; Storch, Golo & Bach, Thorsten
  
• Photochemical Deracemization of Primary Allene Amides by Triplet Energy Transfer: A Combined Synthetic and Theoretical Study. Journal of the American Chemical Society, 143(29), 11209-11217.
  Plaza, Manuel; Großkopf, Johannes; Breitenlechner, Stefan; Bannwarth, Christoph & Bach, Thorsten
  
• Photochemical Deracemization of Chiral Alkenes via Triplet Energy Transfer. Journal of the American Chemical Society, 144(23), 10133-10138.
  Kratz, Thilo; Steinbach, Pit; Breitenlechner, Stefan; Storch, Golo; Bannwarth, Christoph & Bach, Thorsten