Dearomative dimerization of quinolines and their skeletal rearrangement to indoles triggered by single-electron transfer

Dearomatization of two-dimensional (2D) planar aromatic feedstocks enables the introduction of three-dimensional (3D) vectors to new molecular scaffolds, vastly expanding the chemical space for drug discovery. Here, we demonstrate that photoinduced single-electron reduction of quinolines under polysulfide anion photocatalysis triggers dearomative dimerization and an unprecedented skeletal rearrangement. In the presence of excess formate as the stoichiometric reductant, a complex polyheterocyclic hybrid of a 2,5-methanobenzo[b]azepine and a tetrahydroisoquinoline is formed in a net-reductive manner, whereas in the presence of a trialkylamine instead of formate, sequential dimerization and skeletal rearrangement occurs to afford 4-(3-indolylmethyl)quinolines in a redox-neutral manner. The remarkable way in which the additive dictates the reaction course showcases the unique tunability of polysulfide anion mediated redox photocatalysis. These observations enabled the design of a net-reductive skeletal rearrangement of 4-arylquinolines to 3-(arylmethyl)indoles. Detailed mechanistic investigations reveal that this umpolung transformation from electron-deficient quinolines to electron-rich indoles is mediated via a 1,2-aryl migration/ring-contraction sequence, as opposed to the more commonly invoked neophyl-like rearrangement.