Photocatalytic arylation of carbonyl compounds: advancements toward sustainability

In recent decades, there has been a notable increase in interest surrounding the direct functionalization of carbonyl compounds, attributed to their extensive applications across various fields. However, conventional methodologies have predominantly relied on stoichiometric amounts of expensive and often environmentally detrimental metal oxidants, which present significant sustainability challenges in the arylation processes. In contrast to traditional transition metal-catalysed arylations, photochemical methods offer environmentally friendly reaction profiles, greater operational simplicity, and access to novel reactivity modes that are not achievable under thermal conditions. This review highlights recent advancements in visible-light-driven arylation techniques that enable the formation of carbon-carbon (C-C) bonds at either the α- or the β-position, or directly at the carbonyl center of carbonyl compounds. The tutorial review also emphasizes the mechanistic insights, including radical chain propagation, proton-coupled electron transfer (PCET), and energy transfer pathways, that underpin these transformations. By leveraging the potential of photocatalysis, particularly through single-electron transfer (SET) processes and radical-mediated mechanisms, these transformations demonstrate remarkable tolerance for a variety of functional groups and regioselectivity, even within challenging molecular structures by utilizing diverse arylating sources (including aryl diazonium salts, aryl halides, and cyanoarenes). The aim is to provide a concise overview of current methodologies, mechanistic insights, and future directions in photochemical arylation of carbonyl compounds, emphasizing its advancements toward sustainability in modern synthetic organic chemistry.