Microwave-assisted Palladium-catalyzed C-H bond Functionalizations Towards the Synthesis of Bio-inspired Heterocycles

Abstract

C-C or C-heteroatom bond formation from direct C-H bond activation of several heteroarenes containing suitable directing groups has now emerged as an efficient and straightforward strategy for the design of complex heterocyclic molecules as well as their late-stage functionalization. The most common problem of several C-H bond activation reactions is high temperature, long reaction time and unwanted side reactions where recent examples of MW assisted C-H bond activation showed the requirements of low temperature and short completion time and thus proved its efficacy in terms of heating effect and conversion rate of conventional heating methods. The schemes discussed in the present review depict the reaction conditions along with a look into the mechanism involved to render a deep understanding of the catalytic role of palladium-catalysis. In some examples, the optimization procedure of the corresponding strategy has been illustrated through tables, i.e., choice of catalyst, solvent screening, loading of the catalyst and percentage yield with different substrates. Each of the described illustrations has been analyzed considering a wide variety of reactants, reaction conditions, and transition metals employed as the catalyst. This review definitely allows to introduce the synthetic chemists in understanding the challenges associated with the previous methods as well as their drawbacks and future opportunities in choosing substrates, catalyst and reaction conditions. This review would be alluring to a wider range of synthetic chemists in academia and industrial R&D sectors working with heterocyclic chemistry. In this short perspective, an outline of recent eloquent examples of a variety of palladium-catalyzed C-H bond activation involving bio-oriented heterocycles achieved in the past ten years is nicely presented and the pros and cons of each strategy are highlighted so that the researchers could get enough scope for further designing and modification of developed protocols.