Eosin Y-catalyzed cross-dehydrogenative coupling under visible light: metal-free synthetic approaches

Visible-light-driven photocatalysis has emerged as a transformative tool in organic synthesis, enabling diverse bond-forming processes under mild, sustainable conditions. Since 2011, among the range of organic photocatalysts, eosin Y (EY), a low-cost, metal-free organic dye, has gained significant prominence due to its strong visible-light absorption, suitable redox properties, and environmental benignity. Cross-coupling is one of the essential reactions which can enable desired C-C/C-heteroatom (X) (X = N, O, S, P) bond formation. Further cross dehydrogenative coupling (CDC) reactions are known for their unique selectivity as they form desired C-C/C-X bonds directly from C-H bonds without pre-functionalization of the substrates. EY has gained increasing significance in recent CDC reactions owing to its strong visible-light absorption and efficient SET capability, enabling mild oxidative C-H functionalization through metal-free pathways while offering environmentally benign alternatives to transition-metal photocatalysts and improved cross efficiency. Although reviews have broadly covered photoredox catalysis, EY chemistry and CDC, a focused assessment of eosin Y-mediated visible-light-driven CDC reactions remains limited. This review delineates the growing role of EY as a sustainable visible-light photocatalyst in CDC reactions, enabling efficient C-C/C-X bond formation under mild and metal-free conditions with emphasis on substrate scope, reaction conditions, mechanistic pathways, and metal-free versus metal-assisted approaches. Through a critical assessment of reported advances, current limitations, and unresolved challenges, this review highlights future opportunities to develop more selective, efficient, and sustainable cross-dehydrogenative coupling (CDC) methodologies.