Design of a Photocatalytic [2+2] Cycloaddition Reaction Using Redox-Tag Strategy.

The design of photocatalytic processes is important for a sustainable society. Key to these photocatalytic reactions is electron transfer. This article is focused on titanium dioxide photocatalyzed organic synthesis and the design of a new [2+2] cycloaddition reaction based on the electron transfer process. Electron transfer - not only between the substrate and the photocatalyst but also inter- and intramolecularly - is crucial for the reaction design. Radical cations were generated by the photocatalyst and trapped by alkenes. The resultant cyclobutyl radical cations were immediately reduced by the aryl rings via intramolecular electron transfer to obtain cyclobutane rings. The outcome of the reaction was controlled by substitution of the aryl ring and the linker connecting the aryl ring to the enol ether. The carefully designed substrates were highly effective for photocatalytic cycloaddition.