Red Light-Blue Light Chromoselective C(sp²)-X Bond Activation by Organic Helicenium-Based Photocatalysis

Chromoselective bond activation has been achieved in organic helicenium (ⁿPr-DMQA⁺)-based photoredox catalysis. Consequently, control over chromoselective C(sp²)-X bond activation in multihalogenated aromatics has been demonstrated. ⁿPr-DMQA⁺ can only initiate the halogen atom transfer (XAT) pathway under red light irradiation to activate low-energy-accessible C(sp²)-I bonds. In contrast, blue light irradiation initiates consecutive photoinduced electron transfer (conPET) to activate more challenging C(sp²)-Br bonds. Comparative reaction outcomes have been demonstrated in the α-arylation of cyclic ketones with red and blue lights. Furthermore, red-light-mediated selective C(sp²)-I bonds have been activated in iodobromoarenes to keep the bromo functional handle untouched. Finally, the strength of the chromoselective catalysis has been highlighted with two-fold functionalization using both photo-to-transition metal and photo-to-photocatalyzed transformations.