On November 11, 2024, Professor Thorsten Bach from the Technical University of Munich delivered an insightful presentation at the 168th Science Lecture hosted by the College of Science at SUSTech. His lecture, titled “Catalytic Photochemical Deracemization Reactions”, showcased groundbreaking advancements in photochemical stereochemistry. The event was attended by Bin TAN, Associate Dean of the College of Science, and Xin-Yuan LIU, Head of the Department of Chemistry, who presented Prof. Bach with an honorary certificate in recognition of his achievements. 

Prof. Bach’s lecture centered on photochemical deracemization, a process that involves the temporary removal of stereogenic elements followed by their reintroduction via short-lived intermediates. His research explores deracemization of racemic substrates exhibiting axial or point chirality through energy transfer or reversible hydrogen atom transfer pathways.

During his talk, Prof. Bach demonstrated how the binding between chiral photocatalysts and racemic substrates enables deracemization, facilitating the conversion of one enantiomer (A) into its mirror image (ent-A). Using the example of allene deracemization, he detailed the mechanism of triplet energy transfer. An excited photocatalyst, such as thioxanthone, undergoes intersystem crossing to reach a triplet state, subsequently transferring energy to the matching enantiomer of the allene, e.g., A. Upon returning to the singlet state, two enantiomers, A and ent-A, of the starting allene are statistically formed. The matched enantiomer A then participates in another cycle of energy transfer and subsequent processes. Over multiple cycles, the matched enantiomer A is gradually depleted, leading to the enrichment of the mismatched enantiomer ent-A. Prof. Bach emphasized the critical role of substrate-catalyst proximity during interaction, noting that energy transfer efficiency is highly dependent on this distance.

He further elaborated on a series of energy transfer-based deracemization reactions, supported by spectroscopic evidence of intermediates. Additionally, he introduced an alternative deracemization pathway involving reversible hydrogen atom transfer and provided numerous examples. To conclude, Prof. Bach highlighted the use of diketopiperazine substrates in stereochemical editing, underscoring its value in expanding the scope of deracemization reactions.

Following the lecture, professors and students engaged in an in-depth discussion with Prof. Bach, addressing key aspects of his work. Topics included the potential for alternative binding pathways and the feasibility of targeting C–H bonds with higher bond dissociation energies. The exchanges led to a deeper understanding of the challenges and future directions in this field.

Q1: The reaction is limited to the amide or lactam group to have the interaction with the photocatalyst. Will you develop another binding pathway?

A1: There are many groups, like the amide group, which we will try in the future.

Q2: Is it possible to use another C–H bond with higher bond dissociation energy to do deracemization? Is the carbonyl group necessary?

A2: No, we think the carbonyl group is not necessary. We still have a chance to solve another C–H bond, but now we cannot achieve good results.