Synthesis of Benzothieno[3,2-b]benzothiophene Propellanes via [2 + 2] Photocycloadditions through Energy Transfer Process

Propellanes are molecules with unique three-dimensional (3D) structures and chemical properties. Owing to their strained and rigid molecular frameworks, propellanes have attracted considerable attention in various fields. Heteroacenes, such as benzothieno[3,2-b]benzothiophene (BTBT), are widely used in materials science. However, there are few reports on the synthesis of heteroacene-derived propellanes. Therefore, the development of general synthetic strategies for constructing heteroacene-derived propellanes and the elucidation of the physical properties of these derivatives are of significant interest. Meanwhile, photocycloaddition via triplet energy transfer (EnT) process with photosensitizers has received considerable attention for the synthesis of 3D molecules from planar substrates. Herein, we report the photochemical synthesis of BTBT propellanes via triplet-sensitized [2 + 2] photocycloadditions. Photochemical reactions of benzothieno[3,2-b]benzothiophene S,S-dioxide (BTBT-DO) with alkenes were performed in the presence of 4-CzIPN under visible light irradiation conditions, affording a variety of BTBT-DO propellanes. Several spectroscopic studies and control experiments support that this reaction proceeds via the EnT process. The BTBT-DO propellanes enable further derivatization, including Pd-catalyzed transformations. This strategy provides an opportunity to access heteroacene-derived propellanes in materials science.