Transition-Metal-Catalyzed Multicomponent Cycloadditions

Transition-metal-catalyzed multicomponent cycloadditions provide a powerful strategy for constructing complex cyclic frameworks from simple unsaturated building blocks with high atom and step efficiencies. This Review highlights the progress made in transition-metal-catalyzed multicomponent cycloadditions based on the size of the cyclic products formed. First, reactions that construct five-membered rings, including [2 + 2 + 1], [3 + 1 + 1], and [2 + 1 + 1 + 1] cycloadditions, are discussed with an emphasis on key reactive intermediates, such as carbon monoxide, vinylidenes, carbenoids, and nitrenes. Next, six-membered-ring-forming processes, particularly [2 + 2 + 2], [3 + 2 + 1], [4 + 1 + 1], and [2 + 1 + 2 + 1] cycloadditions, are described, highlighting combinations of alkynes, alkenes, and heteroatom-containing components. The Review then covers strategies for constructing seven-membered rings via [3 + 2 + 2] and [4 + 2 + 1] cycloadditions. Finally, emerging methods that enable the formation of eight- and nine-membered rings through higher-order multicomponent cycloadditions are described. This Review aims to provide a comprehensive overview of the conceptual framework, mechanistic principles, and synthetic possibilities of transition-metal-catalyzed multicomponent cycloadditions by organizing them according to ring size and cycloaddition components.