Over the past few decades, transition metal-catalyzed synthetic transformation has 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 pre functionalized substrates. Compared to C(sp) H and C(sp2) H bonds, C(sp3) H bonds are more ubiquitous but also more challenging to be activated owing to their weak coordination ability with transition metals. Among various kinds of C(sp3) H bonds, allylic C(sp3) H bonds bearing alkene units have gained particular attention because it can be utilized as a starting material for the synthesis of a wide range of organic chemical building blocks apart from the allylic functionalization. This alkene also significantly facilitates the activation of C H bond leading to milder, more efficient and controllable selective C H transformation.[2] The direct allylic C H functionalization is an efficient method to avoid the functional group manipulations classically needed to obtain pre oxidized substrates. Therefore, a special interest in the direct functionalization of allylic C H bonds has arisen in the past few years. Various metals are able to promote such activation and palladium is one of the most attractive ones to accomplish this task in terms of diverse reactivity. 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] Tremendous progress has been made towards the development of increasingly elaborate nucleophiles and catalysts to facilitate the aforementioned reaction. A series of novel reactions based on new catalytic systems have been developed to produce useful molecules and complex natural products. The control of branch/linear selectivity and enantioselectivity has also been realized in the latest reports.[4] Meanwhile, the Pd(II)-catalysed asymmetric allylic C H functionalization has also been successfully demonstrated.[5] In this seminar, advancements on the direct allylic C–H bond functionalization of different types of olefins with various nucleophiles by using Pd complexes will be discussed. Latest reports on Pd catalyzed branch selective functionalization as well as asymmetric allylic C-H functionalization will also be discussed.
References
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