A Chiral Thioxanthone as an Organocatalyst for Enantioselective [2+2] Photocycloaddition Reactions Induced by Visible Light

Abstract

Thioxanthone 1, which was synthesized in a concise fashion from methyl thiosalicylate, exhibits a significant absorption in the visible light region. It allows for an efficient enantioselective catalysis of intramolecular [2+2] photocycloaddition reactions presumably by triplet energy transfer.

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

“…Simultaneously, work in our laboratories was devoted toward the synthesis of chiral sensitizers, which would incorporate the 1,5,7-trimethyl-3-azabicyclo[3.3.1]nonan-2-one skeleton of templates 107 and ent - 107 . In successive order, the benzophenone 409 , 739 the xanthones 410 and ent - 410 , 740 and the thioxanthone 411 ( 741 ) were synthesized ( Figure 18 ).…”

Enhancing the potential of enantioselective organocatalysis with light

“…In an example of the latter (Fig. 5a) 91 , a visible-light-absorbing thioxanthone moiety was incorporated within the catalyst 42. The lactam functionality of 42 was essential for binding the substrate 40 via a double hydrogen-bond interaction.…”

Cage-confined photocatalysis for wide-scope unusually selective [2 + 2] cycloaddition through visible-light triplet sensitization

“…However, geometric isomerization and racemic background photoreactions facilely induced by UV light make the stereochemical control of cycloaddition a long-standing challenge, especially for acyclic alkenes 8,9 . As a green and clean procedure for chemical process and production that are contemporarily demanded by sustainable development, the visible-light sensitized intra-and intermolecular [2 + 2] cycloaddition of olefins has drawn great attention of chemical society in past decades [10][11][12][13][14][15] , leading to fast progress in [2 + 2] photocycloaddition of nonrigid olefins with excellent regio-and stereoselectivities [16][17][18][19][20] . Nevertheless, owing to steric preference and relative radical stability of excited-state intermediates, hitherto, the stereochemical control is mainly achieved for the thermodynamic favored anti-diastereomers from acyclic olefins, e.g., styrenes, chalcones, and cinnamates 16,[21][22][23][24] .…”