A Thioxanthone Sensitizer with a Chiral Phosphoric Acid Binding Site: Properties and Applications in Visible Light‐Mediated Cycloadditions

A chiral phosphoric acid with a 2,2’‐binaphthol core was prepared that displays two thioxanthone moieties at the 3,3’‐position as light‐harvesting antennas. Despite its relatively low triplet energy, the phosphoric acid was found to be an efficient catalyst for the enantioselective intermolecular [2+2] photocycloaddition of β‐carboxyl‐substituted cyclic enones (e.r. up to 93:7). Binding of the carboxylic acid to the sensitizer is suggested by NMR studies and by DFT calculations to occur by means of two hydrogen bonds. The binding event not only enables an enantioface differentiation but also modulates the triplet energy of the substrates. Bind me tender: Cyclic enone‐3‐carboxylic acids (X=CH2, O) display a binding motif (in blue) for an assembly with chiral acid 1. The acid acts as photocatalyst by transferring the energy of visible photons to the substrate via its thioxanthone substituents (in yellow) enabling an enantioselective [2+2] photocycloaddition (see scheme).