Chromophore Activation by Lewis Acid Coordination

Introduction. Recent interest in Lewis acid catalyzed photochemical reactions has been triggered by the desire to devise enantioselective methods for the construction of typical photochemical reaction products, e.g. cyclobutanes. The existing methods can be crudely classified into four categories based on their mode of action:

(a) The Lewis acid changes the dynamic properties of the excited state(s) by opening a reaction channel that is not accessible in the absence of the Lewis acid (IC = internal conversion, ISC = intersystem crossing).
(b) The Lewis acid alters the energy of the singlet excited state(s), which is typically detected by a bathochromic (or hypsochromic) shift in the UV/Vis spectrum (S0 = electronic ground state, S1 = first excited singlet state).
(c) The Lewis acid modulates the energy of the triplet excited state(s), which is detectable in the phosphorescence spectrum or by quenching studies with a triplet sensitizer the triplet state energy of which is known (T1 = lowest triplet state).
(d) By changing the energy of the excited state, the Lewis acid invariably leads to a change in the redox potential of the excited state making it a stronger oxidant or reductant. If the triplet state is populated, it will be the redox properties of T1 which are altered.


Summary of the Project. The main goal of the project is to explore how Lewis acids modulate the photophysical properties of a given chromophore in an assembly and how this modulation can be exploited for unprecedented selective photochemical reactions. A strategic input is expected from collaborations with theoretical chemistry and spectroscopy by which the excited state properties of the Lewis acid complexes are interrogated. The project aims to discover and to explore salient features of complexes between typical chromophores and Lewis acids. Specifically, carbonyl compounds (aldehydes, ketones, carboxylates), sulfones, and arenes will be studied. Synthetically, the goal is to allow for a selective excitation of a Lewis acid-substrate complex which in turn can be employed to perform catalytic enantioselective, site-selective and type-selective transformations, ideally by visible-light irradiation. It is planned to employ a,b-unsaturated carbonyl compounds (enones) in photochemical reactions which extend beyond [2+2] photocycloaddition chemistry. Relevant reactions – some of which are expected to occur on the singlet hypersurface – include 1,3-migration, E/Z-isomerization followed by subsequent addition, hydrogen abstraction, and photocyclization. In arene photochemistry, the focus will be on reactions that allow for C-C bond formation at the arene core and for a dearomatization of the benzene ring. New binding motifs for Lewis acid coordination are proposed and will be tested in exploratory experiments.
 

Recent Publications

  • Yan, Peng; Stegbauer, Simone; Wu, Qinqin; Kolodzeiski, Elena; Stein, Christopher; Lu, Ping; Bach, Thorsten: Enantioselective Intramolecular ortho Photocycloaddition Reac­tions of 2‐Acetonaphthones. Angew. Chem. Int. Ed. 63, 2024, e202318126 more…
  • Schwinger, Daniel P.; Pickl, Thomas; Bach, Thorsten: Photochemical Isomerization of Cyclohept-1-ene-1-carbaldehyde: Strain-Release Cycloadditions and Ene Reactions. J. Org. Chem., 2023, 12844-12852 more…
  • Proessdorf, Johanna; Jandl, Christian; Pickl, Thomas; Bach, Thorsten: Synthesis of Boronates with a Protoilludane Skeleton. Synthesis 55, 2023, 2311-2318 more…
  • Jeremias, Noah; Peschel, Martin T.; Jaschke, Constantin; de Vivie-Riedle, Regina; Bach, Thorsten: Photochemical Ring Contraction of 5,5-Dialkylcyclopent-2-enones and in situ Trapping by Primary Amines. J. Org. Chem. 88, 2023, 6294-6303 more…
  • Rigotti, Thomas; Bach, Thorsten: Bicyclo[2.1.1]hexanes by Visible Light-Driven Intramolecular Crossed [2+2] Photocycloadditions. Org. Lett. 24, 2022, 8821-8825 more…
  • Schwinger, Daniel P.; Peschel, Martin T.; Rigotti, Thomas; Kabaciński, Piotr; Knoll, Thomas; Thyrhaug, Erling; Cerullo, Giulio; Hauer, Jürgen; de Vivie-Riedle, Regina; Bach, Thorsten: Photoinduced B–Cl Bond Fission in Aldehyde-BCl3 Complexes as a Mechanistic Scenario for C–H Bond Activation. J. Am. Chem. Soc. 144, 2022, 18927-18937 more…
  • Stegbauer, Simone; Jandl, Christian; Bach, Thorsten: Chiral Lewis acid catalysis in a visible light-triggered cycloaddition/rearrangement cascade. Chem. Sci. 13, 2022, 11856-11862 more…
  • Schwinger, Daniel P.; Peschel, Martin T.; Jaschke, Constantin; Jandl, Christian; de Vivie-Riedle, Regina; Bach, Thorsten: Diels–Alder Reaction of Photochemically Generated (E)-Cyclohept-2-enones: Diene Scope, Reaction Pathway, and Synthetic Application. J. Org. Chem. 87, 2022, 4838-4851 more…
  • Rigotti, Thomas; Schwinger, Daniel P.; Graßl, Raphaela; Jandl, Christian; Bach, Thorsten: Enantioselective Crossed Intramolecular [2+2] Photocycloaddition Reactions Mediated by a Chiral Chelating Lewis Acid. Chem. Sci. 13, 2022, 2378-2384 more…

Key Publications

  • Peschel, Martin; Kabacinski, Piotr; Schwinger, Daniel P; Thyrhaug, Erling; Cerullo, Giulio; Bach, Thorsten; Hauer, Jürgen; de Vivie-Riedle, Regina: Activation of 2‐Cyclohexenone by BF3 Coordination: Mechanistic Insights from Theory and Experiment. Angew. Chem. Int. Ed. 60, 2021, 10155-10163 more…
  • Schwinger, Daniel P.; Bach, Thorsten: Chiral 1,3,2-Oxazaborolidine Catalysts for Enantioselective Photochemical Reactions. Acc. Chem. Res. 53, 2020, 1933-1943 more…
  • Leverenz, Malte; Merten, Christian; Dreuw, Andreas; Bach, Thorsten: Lewis Acid Catalyzed Enantioselective Photochemical Rearrangements on the Singlet Potential Energy Surface. J. Am. Chem. Soc. 141, 2019, 20053-20057 more…
  • Stegbauer, Simone; Jeremias, Noah; Jandl, Christian; Bach, Thorsten: Reversal of reaction type selectivity by Lewis acid coordination: the ortho photocycloaddition of 1- and 2-naphthaldehyde. Chem. Sci. 10, 2019, 8566-8570 more…
  • Poplata, Saner; Bauer, Andreas; Storch, Golo; Bach, Thorsten: Intramolecular [2+2] Photocycloaddition of Cyclic Enones: Selectivity Control by Lewis Acids and Mechanistic Implications. Chem. Eur. J. 25, 2019, 8135–8148 more…

Application

If you are interested in joining our team as a PhD student or as a post-doc we would be happy to hear from you. Applications (single .pdf document) should typically include a letter of motivation, CV, an academic transcript of records, and contact information of two references, preferable in English. Prospective PhD students should apply exclusively to apply-crc325@ur.de. Candidates for a post-doctoral position should send their application letters to thorsten.bach@ch.tum.de.