Enantioselective Organo‐Photocatalysis Mediated by Atropisomeric Thiourea Derivatives

Abstract

Can photocatalysis be performed without electron or energy transfer? To address this, organo-photocatalysts that are based on atropisomeric thioureas and display lower excited-state energies than the reactive substrates have been developed. These photocatalysts were found to be efficient in promoting the [2+2] photocycloaddition of 4-alkenyl-substituted coumarins, which led to the corresponding products with high enantioselectivity (77-96 % ee) at low catalyst loading (1-10 mol %). The photocatalytic cycle proceeds by energy sharing via the formation of both static and dynamic complexes (exciplex formation), which is aided by hydrogen bonding.Scheme 1. Enantioselective intramolecular [2+2] photocycloaddition of coumarin 1 promoted by thioureas 3 a-g.

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

“…They designed a chiral thiourea 322 , which can act as hydrogen-bonding template to activate coumarin 319 ( Scheme 104 ). 597 …”

Enhancing the potential of enantioselective organocatalysis with light

“…Other strategies for the enantioselective catalysis of photochemical processes 15,92 have been successively developed. For example, it was demonstrated that a mechanistically similar intramolecular [2+2] photocycloaddition is promoted with high stereoselectivity by chiral thiourea catalysts 93 , traditional ground-state hydrogen-bonding organocatalysts 94 .…”

Photochemical Stereocontrol Using Tandem Photoredox–Chiral Lewis Acid Catalysis

“…The first highly enantioselective photoreactions were not developed until the early 2000s, 13 and only a handful of successful of alternative strategies have emerged since then. 14,15,16 The slow rate of development in this field is particularly surprising considering the rapid rate at which asymmetric catalytic methods were developed for so many other, nonphotochemical reaction types in the last decades of the 20 th century.…”