Catalytic Asymmetric Dearomatization by Visible‐Light‐Activated [2+2] Photocycloaddition

Abstract

A novel method for the catalytic asymmetric dearomatization by visible-light-activated [2+2] photocycloaddition with benzofurans and one example of a benzothiophene is reported, thereby providing chiral tricyclic structures with up to four stereocenters including quaternary stereocenters. The benzofurans and the benzothiophene are functionalized at the 2-position with a chelating N-acylpyrazole moiety which permits the coordination of a visible-light-activatable chiral-at-rhodium Lewis acid catalyst. Computational molecular modeling revealed the origin of the unusual regioselectivity and identified the heteroatom in the heterocycle to be key for the regiocontrol.

Enantioselective [2+2] Photocycloaddition Reactions of Enones and Olefins with Visible Light Mediated by N,N′‐Dioxide–Metal Complexes

“…Based on the aforementioned efforts along with previousr eports, [4][5][6]26] ap ossible mechanism can be proposed to elucidate the reactionp rocess and chiral induction. As shown in Scheme3b, the substrate 1a coordinates to the chiral terbium-N,N'-dioxide complex to form the new photoactive chiral complex I.T his complex is readily excited by visible light to reach its lowest singlet state (S 1 ,i ntermediate II).…”

“…[2] To date, several successful strategies for asymmetric catalytic [2+ +2] photocycloadditions have been developed separately by the groups of Bach, [3a,b,d, 4] Yoon, [5] and Meggers. [6] Bach and co-workersf irst addressed this reactiont hrough the use of either chiral hydrogen-bondingo rganic sensitizers [3] or chiral Lewis acids [4] as catalysts. The key point of chiral Lewis acid catalysis is the bathochromic shift of the UV absorption of the catalyst-activated substrate, which enables unselective backgroundr eactions to be suppressed throughc areful irradiation with al ight source of appropriate wavelength.…”

“…Recently,M eggers and co-workers described au nique intermolecular [2+ +2] photocycloadditionr eaction catalyzed by ac hiral-at-metal [8] complex in which the chiral catalyst serves as ac hiral Lewis acid and its complex with the substrate could act as the sensitizer. [6] In essence, the development of exogenous chiral ligand-Lewis acid catalysts that enablet he feasible formation of excited-state organic intermediates with longer lifetimes is critical to broadeningt he scope of enantioselectivephotocycloaddition reactions.…”

Visible‐Light‐Promoted Intermolecular Oxidative Dearomatization of β‐Naphthols with N‐Hydroxycarbamates

“…by molecular oxygen regenerates the ground state photocatalyst and produces the superoxide anion. Nucleophilic capture of A by N ‐hydroxycarbamate 2 a delivered the oxygen and alkene‐stabilized radical B , which undergoes single‐electron oxidation by superoxide anion and subsequent deprotonation to give the dearomatized product 3 a .…”

Visible-light-mediated photocatalysis as a new tool for catalytic asymmetric dearomatization (CADA) reactions

“…Very recently, Meggers group [13] demonstrated that visiblelight activated [2 + 2] photocycloadditions can be applied to catalytic asymmetric dearomatization reactions of benzofurans with styrenes (Scheme 2). This novel method provides very high enantioselectivity (98%À99% ee) and forms the tricyclic structures of N-acylpyrazole moieties with up to four stereocenters including a quaternary one.…”