Molekularer Sauerstoff als Redox‐Katalysator in intramolekularen Cumarin‐Photocycloadditionen

Abstract

Diese Arbeit wurde von der Universität zu Kçln und dem Fonds der Chemischen Industrie unterstützt. Wir danken Dr. S. Neufeind und Dipl.-Chem. A. M. Heinsch für anregende Diskussionen. Hintergrundinformationen zu diesem Beitrag sind im WWW unter http://dx.doi.org/10.1002/ange.201201222 zu finden.

Enantioselective Organo‐Photocatalysis Mediated by Atropisomeric Thiourea Derivatives

“…Based on this precedence, Bach and co‐workers10c,d have recently developed an enantioselective [2+2] photocycloaddition of 4‐alkenylcoumarin derivatives that is mediated by a chiral Lewis acid. Furthermore, an electron‐transfer‐mediated cycloaddition reaction was recently reported by Griesbeck, Schmalz, and co‐workers 11. Our method provides complementarity to the existing strategies by controlling the excited‐state reactivity through static/dynamic complex formation (exciplex formation) and circumvents the bottleneck of employing catalysts that have higher excited‐state energies than the reactive substrates, thus opening up new avenues for the development of novel organo‐photocatalysts.…”

Recent Visible Light and Metal Free Strategies in [2+2] and [4+2] Photocycloadditions

“…In 2012, Griesbeck and Schmalz introduced the use of α,βunsaturated coumarins as starting materials for intramolecular [2 + 2] photocycloadditions (Scheme 6). [22] During the investigations, they realized that degassed solvents afforded low yields compared to non-degassed ones rationalizing that the presence of oxygen might be involved in the reaction mechanism. Indeed, oxygen proved to play a crucial role in the overall transformation, being the principal quencher of the excited triplet-state coumarin.…”

Enantioselective Organo‐Photocatalysis Mediated by Atropisomeric Thiourea Derivatives

“…[10] Pioneering studies by in the early 1980s established that photoreactions of coumarins can be fine-tuned by the addition of Lewis acids that interact with the carbonyl functional group, which alters the properties of the excited state. [11] Our method provides complementarity to the existing strategies by controlling the excited-state reactivity through static/dynamic complex formation (exciplex formation) and circumvents the bottleneck of employing catalysts that have higher excited-state energies than the reactive substrates, thus opening up new avenues for the development of novel organo-photocatalysts. Furthermore, an electron-transfer-mediated cycloaddition reaction was recently reported by Griesbeck, Schmalz, and co-workers.…”

“…Furthermore, an electron-transfer-mediated cycloaddition reaction was recently reported by Griesbeck, Schmalz, and co-workers. [11] Our method provides complementarity to the existing strategies by controlling the excited-state reactivity through static/dynamic complex formation (exciplex formation) and circumvents the bottleneck of employing catalysts that have higher excited-state energies than the reactive substrates, thus opening up new avenues for the development of novel organo-photocatalysts.…”