Cascade reactions carbon-heteroatom bond formation

Studies performed with stoichiometric palladium complexes have been frequently employed to explore feasibility of the individual steps of the catalytic cycles proposed for the novel catalytic annulation reactions. Herein, evolution of the sophistication of such palladium-catalyzed annulation protocols is demonstrated on selected examples, along with a discussion of the existing evidence for the participation of palladium(IV) complexes. Subsequently, studies performed with stoichiometric isolable or semistable palladium(IV) complexes seeking evidence for the involvement of well-defined palladium(IV) intermediates in analogous cascade carbon-carbon and carbon-heteroatom bond formations are discussed in detail. 

In this review, we have shown the major advances in the growing field of cascade chemistry that have led to regio-, chemo-, and stereoselective formation of several new carbon-carbon and carbon-heteroatom bonds in a stepwise economical fashion by using transition metal-catalyzed reactions. These approaches have already allowed very impressive and rapid construction of unnatural and natural polycyclic compounds of very high molecular complexity. These initial achievements should stimulate the synthetic community to pursue further works notably to develop more efficient and selective strategies that involve new generations of versatile catalysts. Next endeavors will have to focus on green processes as well as asymmetric catalysis. 

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