Gram-Scale Photocatalysis with a Stable and Recyclable Chromium(III) Photocatalyst in Acetonitrile and in Water

The use of stable photosensitizers made on a multigram-scale from cheap and available starting materials offers an extraordinary opportunity to translate small-scale proof-of-concept photoreactions to widespread applications in non-specialized laboratories, thereby promoting the implementation of light-driven syntheses. Herein, we report an easy, multigram-scale preparation of the exceptionally photo- and redoxstable chromium(III) complex [Cr(tpe)2]3+ (tpe = 1,1,1-tris(pyrid-2-yl)ethane) at room temperature. This photocatalyst was successfully employed in acetonitrile and in the “green” solvent water as demonstrated in visible-light induced Schenck-Ene reactions, Newman-Kwart rearrangements, radical cation Diels-Alder cycloadditions, oxidative decarboxylation and trifluoromethylation reactions with excellent substrate conversions and high turnover numbers. Gram-scale photocatalytic reactions were conducted using a simple round-bottom flask as reaction vessel and irradiation with energy-efficient blue LEDs or even with sunlight. The stable photocatalyst was successfully recycled in up to 90–92 % yields after the photoreactions.