Transition-Metal Catalyzed [4+2]-Cycloaddition Reactions: A Sexennial Update

Cycloaddition reactions serve as exceptionally useful strategy to provide atom-economical, efficient and practical access to complex molecules from simple starting materials. The [4+2]-cycloaddition reaction allows for the stereospecific construction of six-membered carbo-/heterocycles. Cycloaddition reactions catalyzed by transition metals are potent methods for the rapid construction of highly functionalized molecular frameworks. This review article summarizes the different reports on transition-metal catalyzed [4+2]-cycloaddition reactions involving various transition metals such as Pd, Cu, Rh, Au, Fe, Ni, Sc, Sn, Hf, Ag, Mn, Dy, Co etc. for the straightforward construction of pharmaceutically active six-membered heterocycles. An emphasis is laid on the mechanistic insights to highlight the unique application of novel processes of transition-metals in these transformations to build intricate polycyclic structures. Cycloaddition reactions are one of the most useful tools for the synthetic organic chemists to construct an array of complex cyclic skeletons among the numerous methodologies for the construction of heterocycles. This review article gathers findings on transition-metal catalyzed [4+2]-cycloaddition reactions for the synthesis of biologically active six-membered heterocycles for the period 2018-2023. Cycloaddition reactions facilitated by transition metals are competent approaches for assembling highly functionalized molecular frameworks quickly. The focus is mainly on the mechanistic insights to demonstrate how these transformations specifically employ transition-metal catalysts such as Palladium, Copper, Rhodium, Gold, Iron, Nickel, Scandium, Tin, Hafnium, Silver, Manganese, Cobalt etc. to produce intricate polycyclic structures present in numerous natural products and bioactive molecules. image