Transition-Metal-Catalyzed C-H Activation-Enabled [3+2] Annulations for the Synthesis of Functionalized Indanones

Transition-metal-catalyzed C–H activation-enabled [3+2] annulation has emerged as an efficient and atom-economical strategy for the synthesis of indanone frameworks, which are important structural motifs in natural products, pharmaceuticals, agrochemicals, and functional materials. Compared with conventional cyclization methods, these annulation approaches provide rapid access to structurally diverse indanones with high regioselectivity and minimal prefunctionalization requirements. Recent advances involving palladium, rhodium, cobalt, nickel, and ruthenium catalysts in combination with alkenes and strained ring systems have significantly expanded the synthetic toolbox for indanone construction. This review summarizes recent developments in transition-metal-catalyzed [3+2] annulation reactions of simple arenes with alkenes, categorized according to reaction mechanisms and types of metal catalysts employed. Particular emphasis is placed on substrate scope, mechanistic insights, current limitations, and future opportunities for the development of sustainable and selective annulation methodologies.