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

Dearomatization of two-dimensional planar aromatic feedstocks is an attractive strategy for the introduction of three-dimensional vectors into chemical scaffolds to expand chemical space for drug discovery. Here, we demonstrate the dearomative dimerization and skeletal rearrangement of quinolines under polysulfide anion photocatalysis, in which the additive uniquely dictates the reaction courses. In the presence of formate, dearomative dimerization of quinolines is followed by cyclization to form an sp³-rich polyheterocyclic hybrid of a 2,5-methanobenzo[b]azepine and a tetrahydroquinoline in a net-reductive manner. On the other hand, in the presence of triethylamine instead of formate, sequential dimerization and skeletal rearrangement occurs to afford 4-(3-indolylmethyl)quinolines in a redox-neutral manner. These observations enabled the design of a net-reductive skeletal rearrangement of 4-arylquinolines to 3-(arylmethyl)indoles. Detailed mechanistic investigations revealed 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.