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The metal-catalyzed “cascade C–H activation/annulation” is a highly promising approach to the construction of aromatic and heteroaromatic compounds. In this context, Pd-complexes play a crucial role whereby molecules bearing a directing group can be activated with high catalytic efficiency. These reactions will expand the synthetic potential of Pd-catalyzed C–H activation and subsequent aromatic and aliphatic C–C bond formation. Described herein is an outline of the recent advances in this research subject.

An environment-friendly synthetic protocol was developed to access polyfunctionalized bipyridines from readily available amines, chromone-3-carboxaldehydes, and pyridinylacetonitriles under catalyst-and additive-free conditions using the biorenewable neoteric solvent dihydrolevoglucosenone (Cyrene™). In this strategy, amines served as both a mild-base promoter and a substrate. In addition, water was the only by-product of this reaction. This multi-component protocol provided highly diverse 2,3-, 3,3-, and 3,4-bipyridines in good-to-excellent yields. Operational simplicity, short reaction time, excellent atom economy, and easily obtainable substrates are among the features of this microwave-assisted synthesis. Additionally, the compounds synthesized via this method have demonstrated the ability to detect heavy metals, specifically mercury(II), copper(II), and iron (III) ions.

Base-promoted benzannulation of conjugated N-sulfonylhydrazones and 3-formylchromones for the synthesis of diverse biaryl sulfones is described. The approach facilitates new C−C and C−S bond formation via the cascade diazo formation/Michael addition/ring opening/denitrogenative sulfonylation/intramolecular cycloaddition/ dehydration and introduces diverse functional groups onto biaryl sulfones. The synthesized compounds are converted to aryl sulfones bearing bioactive benzisoxazole and benzofuran frameworks. Moreover, the synthesized biaryl sulfones possess potent turn-on fluorescence sensing and UV absorbance properties.

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The Sc­(OTf)3/BF3·OEt2-catalyzed annulation of 3-formylchromones with functionalized alkenes for the direct construction of 2-hydroxybenzophenones is described. Sc­(OTf)3/BF3·OEt2 acts as a synergistic catalyst, providing rapid synthetic access to diversely and highly functionalized 2-hydroxybenzophenones. This reaction has excellent regio- and chemoselectivities and is suitable for late-stage functionalization. The reaction proceeds via [3 + 3] and [4 + 2] cycloaddition processes, through carbonyl-ene, Diels–Alder, or aldol-type reactions. Furthermore, this protocol tolerates the various functional groups present in natural terpenes and steroids.