Over the past few decades, transition metal-catalyzed synthetic transformations have gained profound importance as it proves to be a powerful strategy to enable C–C and C–heteroatom bond formation in a highly atom- and step-economical manner.[1] The cross coupling reactions have remained among the most widely used ever since but these requires a preoxidized coupling partner. Similarly, one of the most used palladium catalyzed reactions for organic synthesis of C-C bonds via π-allyl complex is the allylic substitution reaction. This reaction utilizes a broad range of allylic substrates with different leaving groups and provides product with high level of chemo-, regio-, and stereoselectivity. A leaving group is required at the allylic position of the substrate hampers the atom efficiency of this reaction. Notably, the direct allylic C–H bond functionalization reaction has evolved as a promising substitute.[2] It is considerably more atom-and step-economical and provides direct access to allylic functionalized products by using unsaturated hydrocarbons. For many years, Palladium catalysts have found to promote a linear type selective functionalization of terminal alkenes efficiently via π-allyl-palladium intermediate which is further trapped by a soft nucleophile.[3] But using Pd catalysts, branch type selective C–H functionalization of terminal alkenes is very difficult to achieve due to the formation of thermodynamically more stable linear selective products. Similarly, the functionalization of the allylic C–H bond present in the internal alkenes is hard to achieve due to the high thermodynamic stability of the internal olefins. To overcome this, Rh(III) and Ir(III) complexes have recently been emerged as a better alternative. These catalysts are efficient to achieve branch selective C–H bond functionalization of terminal olefins and are also efficient for the allylic C–H bond functionalization of internal olefins. These complexes are used to carry out various allylic oxidation, amination and alkylation reactions.[4] In this seminar, advancements on the direct allylic C–H bond functionalization of different types of olefins with various nucleophiles by using Pd(II), Rh(III) and Ir(III) complexes will be discussed.
References
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