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

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. Light excitation of 2‐alkyl‐benzophenones 1 affords transient hydroxy‐o‐quinodinomethanes A. For a long time, because of its high reactivity and fleeting nature, A could not be trapped in a stereoselective catalytic fashion. This microreview describes how the field of enantioselective organocatalysis has provided suitable tools to effectively address this longstanding problem.