1,4-Difunctionalization of 1,3-enynes: new opportunities for the synthesis of allenes <i>via</i> photochemistry and electrochemistry

Allenes, characterized by their unique linear arrangement of orthogonal π-systems and axial chirality, represent an important class of structural motifs widely found in bioactive natural products, pharmaceuticals, and functional materials. Despite their significance, conventional synthetic approaches often suffer from limitations including harsh conditions, narrow scope, and insufficient stereocontrol. In this context, the 1,4-difunctionalization of 1,3-enynes has emerged as a powerful and versatile strategy that enables efficient and selective access to highly functionalized allenes under mild conditions. This review systematically summarizes recent advances in catalytic methodologies, including photoredox/copper, photoredox/nickel, and photoredox/NHC dual catalysis, single photoredox catalysis, electrochemical approaches, and asymmetric catalytic approaches, which leverage radical intermediates to overcome traditional limitations under photochemical and electrochemical conditions. These strategies facilitate the construction of tetrasubstituted allenes with excellent chemo-, regio-, and stereoselectivity, while demonstrating broad substrate versatility and functional group tolerance. Key mechanisms involving radical selective addition, metal-catalyzed cross-coupling, and NHC-catalyzed radical coupling are discussed in detail, highlighting their roles in achieving sustainable and efficient reaction designs. Furthermore, this review outlines future research directions aimed at expanding radical precursor diversity, employing earth-abundant metals, and advancing electrochemical and enantioselective techniques, providing a comprehensive resource for developing novel synthetic routes to allene-based functional molecules.