Séminaire du Dr Norbert HOFFMANN (CNRS-Université de Reims)

Photochemical reactions play an important role in the context organic synthesis. They provide efficient ways to prepare compounds or compound families which are not or difficultly accessible by more conventional methods of organic synthesis. This is explained by the fact that photochemically excited compounds have a completely different electronic configuration when compared to the corresponding ground state molecules.[1] Also electron transfer and hydrogen transfer steps are facilitated when they occur at the electronically excited state.[2] Often such steps are involved photoredox catalytical reactions which are currently intensively investigated in connection with organic synthesis[3,4]

Our research in this domain deals with intramolecular hydrogen transfer of furanones[5] and imines.[6] We are particularly interested in the stereo- and regioselectivity. Photochemical intra and intermolecular electron and hydrogen transfer leads to unusual products.[7] Physicochemical investigations provide a profound understanding of such processes. Further studies deal with trifluoromethylation[8] and photochemically induced stereo- and regioselective radical addition.

[1] P. Klán, J. Wirz, Photochemistry of Organic Compounds. Wiley, Chichester, 2009. M. Oelgemöller, N. Hoffmann, Studies in organic and physical photochemistry – an interdisciplinary approach. Org. Biomol. Chem. 2016, 14, 7392

[2] N. Hoffmann, Electron and hydrogen transfer in organic photochemical reactions, J. Phys. Org. Chem. 2015, 28, 121. N. Hoffmann, Photochemical Electron and Hydrogen Transfer in Organic Synthesis: The Control of Selectivity, Synthesis 2016, 48, 1782.

[3] C. R. J. Stephenson, T. P. Yoon, D. W. C. MacMillan, Eds., Visible Light Photocatalysis in Organic Chemistry, Wiley VCH, Weinheim, 2018. B. König, Ed. Photocatalysis, 2nd Edition, Walter d Gruyter, Berlin, 2020.

[4] C. Michelin, N. Hoffmann, Photosensitization and Photocatalysis – Perspectives in Organic Synthesis, ACS Catal. 2018, 8, 12046.

[5] R. Jahjah, A. Gassama, V. Bulach, C. Suzuki, M. Abe, N. Hoffmann, A. Martinez, J.-M. Nuzillard, Stereoselective Triplet-Sensitized Radical Reactions of Furanone Derivatives, Chem. Eur. J. 2010, 16, 3341.

[6] C. Lefebvre, C. Michelin, T. Martzel, V. D. Mvondo, V. Bulach, M. Abe, N. Hoffmann, Photochemically induced intramolecular radical cyclization reactions with imines, J. Org. Chem. 2018, 83, 1867.

[7] R. Jahjah, A. Gassama, F. Dumur, S. Marinković, S. Richert, S. Landgraf, A. Lebrun, C. Cadiou, P. Sellès, N. Hoffmann, Photochemical Electron Transfer Mediated Addition of Naphthylamine Derivatives to Electron Deficient Alkenes, J. Org. Chem. 2011, 76, 7104. M. Fréneau, C. Lefebvre, M. A. Gómez Fernández, C. Richard, N. Hoffmann, Photochemical reactivity of phenyl (methyl-tetrazolyl) ketone- Hydrogen atom transfer vs electron transfer, New J. Chem. 2019, 43, 17151.

[8] Q. Lefebvre, N. Hoffmann, M. Rueping, Photoorganocatalyse trifluoromethylation of olefins and (hetero)aromatics in batch and flow, Chem. Commun. 2016 52, 2493