ISBN 9783843953542

978-3-8439-5354-2, Reihe Organische Chemie

Thilo Kratz
Enantioselective Photocatalysis: Alkene Deracemization and [2π+2σ] Photocycloadditions Enabled by Triplet Energy Transfer

190 Seiten, Dissertation Technische Universität München (2023), Softcover, A5

Zusammenfassung / Abstract

Photochemical deracemization allows the conversion of a racemic mixture into a single enantiomer without the loss of material. This can be achieved by isomerization of the respective molecules. The aim of this dissertation was the design of alkene substrates suitable for enantioselective E/Z-isomerization enabled by triplet energy transfer. A concise synthesis of tetrasubstituted alkenes was developed and axially chiral alkenes were obtained in moderate to good yields. The developed method was applicable for various cyclic ketones. Subsequent photochemical deracemization employing a chiral thioxanthone sensitizer produced the axially chiral alkenes with high enantioselectivities and good yields. Dilution experiments monitored by 1H NMR revealed different strength of association of the substrate enantiomers to the catalyst whereupon a mechanism for the triplet sensitized alkene deracemization was proposed. Additionally, the well established [2π+2π] photocycloaddition was extended by photocycloaddition of highly ring strained bicyclo[1.1.0]butanes allowing the synthesis of complex, three dimensional molecular structures. The [2π+2σ] photocycloaddition of bicyclo[1.1.0]butanes with quinolones and other heterocycles was examined with the goal to develop an enantioselective reaction. Various 3- and 4-substituted quinolones underwent sensitized [2π+2σ] photocycloaddition with a selected bicyclo[1.1.0]butane in good yields. First results of an enantioslective [2π+2σ] photocycloaddition with bicyclo[1.1.0]butanes were already promising with enantioselectivities up to 73%.