Mechanistic insights into excited-state palladium catalysis for C-S bond formations and dehydrogenative sulfonylation of amines

Catalytic Asymmetric Cascade Dearomatization of Indoles via a Photoinduced Pd-Catalyzed 1,2-Bisfunctionalization of Butadienes

Photoinduced Pd-catalyzed bisfunctionalization of butadienes with a readily available organic halide and a nucleophile represents an emerging and attractive method to assemble versatile alkenes bearing various functional groups at the allylic position. However, enantiocontrol and/or diastereocontrol in the C-C or C-X bond-formation step have not been solved due to the open-shell process. Herein, we present a cascade asymmetric dearomatization reaction of indoles via photoexcited Pd-catalyzed 1,2-biscarbonfunctionalization of 1,3-butadienes, wherein asymmetric control on both the nucleophile and electrophile part is achieved for the first time in photoinduced bisfunctionalization of butadienes. This method delivers structurally novel chiral spiroindolenines bearing two contiguous stereogenic centers with high diastereomeric ratios (up to >20 : 1 dr) and good to excellent enantiomeric ratios (up to 97 : 3 er). Experimental and computational studies of the mechanism have confirmed a radical pathway involving excited-state palladium catalysis. The alignment and non-covalent interactions between the substrate and the catalyst were found to be essential for stereocontrol.

Photoexcitation of Distinct Divalent Palladium Complexes in Cross-Coupling Amination Under Air

The development of metal complexes that function as both photocatalyst and cross-coupling catalyst remains a challenging research topic. So far, progress has been shown in palladium(0) excited-state transition metal catalysis for the construction of carbon-carbon bonds where the oxidative addition of alkyl/aryl halides to zero-valent palladium (Pd0 ) is achievable at room temperature. In contrast, the analogous process with divalent palladium (PdII ) is uphill and endothermic. For the first time, we report that divalent palladium can act as a light-absorbing species that undergoes double excitation to realize carbon-nitrogen (C-N) cross-couplings under air. Differently substituted aryl halides can be applied in the mild, and selective cross-coupling amination using palladium acetate as both photocatalyst and cross-coupling catalyst at room temperature. Density functional theory studies supported by mechanistic investigations provide insight into the reaction mechanism.