Photocyclization by a triplet-triplet annihilation upconversion pair in water - avoiding UV-light and oxygen removal

We present a formal [2 + 2]-cycloaddition of unsaturated ketones enabled by a green-to-ultraviolet triplettriplet annihilation upconversion (TTA-UC) pair, using commercially available Ru(bpy) (2+)(3) and pyrene as sensitizer and annihilator, respectively. In the developed protocol, visible light irradiation at lambda(max) = 520 nm allows for the reaction to proceed without the need for UV-light and the aqueous medium eliminates the need for oxygen removing protocols. Through this study, the application of the readily available upconversion pair is broadened to include cyclization reactions. We showcase the utility of the system by generating bicyclo[2.1.1]hexanes that are valuable bioisosteres of ortho-substituted benzenes, a promising motif for pharmaceuticals.

Automated summary

We developed a formal [2 + 2]-cycloaddition of unsaturated ketones using a green-to-ultraviolet triplet-triplet annihilation upconversion (TTA-UC) pair, consisting of commercially available Ru(bpy) (2+)(3) as the sensitizer and pyrene as the annihilator. Visible light irradiation at λ(max) = 520 nm was sufficient for the reaction to proceed without the need for UV-light, and the use of an aqueous medium eliminated the requirement for oxygen removing protocols. This study expands the application of the readily available upconversion pair to include cyclization reactions, showcasing its utility in generating valuable bioisosteres for pharmaceuticals, such as bicyclo[2.1.1]hexanes.