Photoinduced Ullmann-Type Cross-Coupling Reactions: Mechanistic Insights and Emerging Challenges

Photoinduced Ullmann-type cross-coupling reactions have become a significant advancement in organic synthesis, providing an efficient means to form C–C and C–heteroatom bonds under milder, light-driven conditions. Utilizing copper catalysis, these reactions present considerable benefits over traditional thermal methods by improving reaction efficiency and promoting more sustainable processes. This review offers an evaluation of recent mechanistic developments, with a focus on the nonchain single-electron transfer (SET) mechanism, which is central to the success of these transformations. The discussion includes an up-to-date overview of both homogeneous and heterogeneous catalytic systems, addressing their practical applications and inherent limitations. In addition, this review identifies key challenges, such as catalyst stability, scalability, and the difficulty of activating less reactive substrates like aryl chlorides. To address these limitations, we propose future research directions aimed at overcoming these obstacles.