Transition metal complexes: A new era of photosensitizers for Dearomative photocycloaddition/annulation via energy and Electron transfer Photocatalysis.

Dearomative photocycloaddition reactions offer a powerful strategy to construct sp3-rich 3D frameworks adorned with diverse functionalities from readily available 2D feedstocks. This method leverages visible light to drive cascades of energy transfer (EnT) or single electron transfer (SET) mediated by organic or transition metal photosensitizers (e.g., ruthenium (Ru), chromium (Cr), and iridium (Ir)). These reactions represent a compelling alternative to harsh thermal methods, enabling the formation of complex molecules under mild conditions. The inherent potential for dearomatizing polycyclic arenes and heteroarenes makes dearomative cycloaddition a highly sought-after synthetic approach. It stands as a rival to nature-inspired synthesis, allowing for the creation of intricate scaffolds in the lab through the synergistic interplay of visible light and transition metal complexes. Inspired by the prowess of transition metal-mediated dearomative cycloadditions, this comprehensive review delves into the progress made over the past decade in photocatalytic dearomative cycloaddition/annulation reactions. The unique photophysical properties of these metal complexes enable the formation of C C bonds with exceptional efficiency and selectivity. We dissect the mechanistic details of EnT and SET processes, illuminating the critical roles of light absorption, excited state generation, and substrate activation by the metal complexes. Subsequently, we explore the mechanisms, substrate scope, and synthetic transformations of dearomative cycloadditions involving hetero- and carbocyclic arenes (e.g., 2 + 2, 3 + 2, 4 + 2). The potential for discovering therapeutically relevant molecules through this approach will be addressed. We acknowledge the limitations and future directions for advancement. Finally, a miscellaneous section will highlight noteworthy research findings beyond the core focus, further emphasizing the captivating potential of dearomative photocycloadditions. [Display omitted] • Innovative Dearomatization Cycloaddition • Cutting-Edge Visible Light Catalysis • Pioneering Photocatalysis with Transition Metals • Energy /Electron Transfer Catalysis Unleashed • Broad Substrate Scope [ABSTRACT FROM AUTHOR]