Successive energy-transfer catalytic dearomative reactions of quinolines with bicyclo[1.1.0]butanes for the synthesis of pyridine-fused 3D complicated molecules†

Dearomative photocycloadditions are unique and hard to replace methods for the construction of various polycyclic strained molecules to increase saturation and create three-dimensional (3D) molecular complexity. In this article, we report a facile photochemical strategy for the synthesis of pyridine-fused 3D polycyclic molecules from quinolines and bicyclo[1.1.0]butanes (BCBs) under visible-light conditions. The dearomative reactions proceed via an initial triplet–triplet energy transfer (EnT) enabled [2π + 2σ] cycloaddition to form an adduct, in which the vinylpyridine moiety is still excitable under the same photosensitive conditions. By introducing a suitable alkyl group as an H-donor into a BCB, a 1,6-hydrogen atom transfer (HAT) would occur from the alkyl group to the excited vinylpyridine moiety via a second EnT process, generating a 1,7-diradical; subsequent ring closure produces a seven-membered 2D/3D-fused molecule. The rare 1,6-HAT process was confirmed through dynamic tracking, control experiments, quenching studies and deuterium-labeling experiments. Applying this strategy, we have successfully obtained a series of structurally unique 6-6-5-4-7 ring 3D molecules with wide functional group tolerance and compatibility with various C–H bonds and various quinolines. Meanwhile, it provides a new idea for the construction of polycyclic architectures by utilizing the infrequent 1,6-HAT of an excited olefin to generate 1,7-diradical species.