Organocatalytic Strategies to Stereoselectively Trap Photochemically Generated Hydroxy‐o‐quinodimethanes

Abstract

Light excitation of ortho‐alkyl aromatic ketones and aldehydes gives access to hydroxy‐o‐quinodimethanes. These reactive electron‐rich intermediates are sufficiently long‐lived to productively engage in chemical processes, mainly acting as dienes in [4+2]‐cycloadditions with electron‐poor alkenes. Since the early discovery of this photoenolization mechanism in 1961, a variety of transformations has been developed, providing a photochemical alternative to classical Diels–Alder chemistry. However, enantioselective catalytic versions of the photenolization/Diels–Alder sequence have remained elusive until recently. This review describes how the field of enantioselective organocatalysis has provided suitable tools to stereoselectively trap photochemically generated hydroxy‐o‐quinodimethanes. Recent studies have also demonstrated that the chemistry is not limited to cycloaddition‐type manifolds, but can be expanded in order to develop intermolecular enantioselective addition processes.

Unlocking the Synthetic Potential of Light-Excited Aryl Ketones: Applications in Direct Photochemistry and Photoredox Catalysis

“…25,26 More recently, it has been disclosed that the use of other types of electrophiles, such as ,-unsaturated carbonyl compounds, 27 activated imines, 28 -fluoroketones 29 or CO 2 , 30 lead to the exclusive formation of functionalized benzophenones 9 (Scheme 12, right pathway). 31 Overall, the photoenolization process enables straightforward access to a versatile intermediate 2′ that can be exploited in alternative synthetic scenarios, including natural product synthesis, 25a-d macromolecule linkages 32 and polymer chemistry. 33 Our next efforts were focused on the implementation of 2-methylbenzophenone-driven reactions under a MFP setup.…”

Visible Light-Induced Ligation via o-Quinodimethane Thioethers

“…33 A large number of synthetically valuable transformations including light-induced reactions based on o-QDMs have been reported, 34 including various organocatalytic strategies to for enantioselective trapping of o-QDMs. 35 Especially the generation of hetero aza-o-xylylenes (heteroatom analogue o-QDMs) was repeatedly used to enable elegant syntheses of complex structural motifs. [36][37][38][39] To the best of our knowledge, there is only one report that indicates the formation of o-QDMs using visible light: Furukawa…”

“…Underpinned by time-dependent density functional theory (TD-DFT) calculations, this investigation allowed for an in-depth understanding of the mechanism and photophysical properties of the reaction . A large number of synthetically valuable transformations including light-induced reactions based on o -QDMs have been reported, including various organocatalytic strategies for enantioselective trapping of o -QDMs . Especially the generation of hetero aza- o -xylylenes (heteroatom analogue o -QDMs) was repeatedly used to enable elegant syntheses of complex structural motifs. − To the best of our knowledge, there is only one report that indicates the formation of o -QDMs using visible light: Furukawa and co-workers reported in 1993 that 8,13-dihydrobenzo­[ g ]­naphtho­[1,8- bc ]­[1,5]­diselenonine forms o -QDMs under exposure of ambient laboratory light .…”

Asymmetric Photocatalysis Enabled by Chiral Organocatalysts

“…The substrate scope, advantages and limitations, and proposed mechanism of each reaction are discussed. The Review is focused on visible‐light‐induced asymmetric photocatalytic reactions enabled by chiral organocatalysts; therefore, unless they are closely related, asymmetric reactions that take place under UV‐light irradiation are not included [59–66] …”