With the growing interest in renewable energy and global warming, it is important to minimize the usage of hazardous chemicals in both academic and industrial research, elimination of waste, and possibly recycle them to obtain better results in greener fashion. The studies under the area of mechanochemistry which cover the grinding chemistry to ball milling, sonication, etc. are certainly of interest to the researchers working on the development of green methodologies. In this review, a collection of examples on recent developments in organic bond formation reactions like carbon–carbon (C–C), carbon–nitrogen (C–N), carbon–oxygen (C–O), carbon–halogen (C–X), etc. is documented. Mechanochemical syntheses of heterocyclic rings, multicomponent reactions and organometallic molecules including their catalytic applications are also highlighted.

Direct catalytic transformation of C–H bonds to new functionalities has provided a powerful strategy to synthesize complex molecular scaffolds in a straightforward way. Unstinting efforts of the synthetic community have helped to overcome the long-standing major challenge of regioselectivity by introducing the directing group concept. However, the full potential of the strategy cannot be realized unless the activated C–H bonds are stereochemically controlled. The enantioselective C–H bond functionalization could provide an imperative tool for a sustainable way of synthesizing chiral complex molecular scaffolds. Despite the intrinsic challenges in achieving stereocontrol, the synthetic community has developed different tools in order to achieve stereoselective C–H bond functionalization. In this review, we discuss the remarkable recent advances in the emerging area of enantioselective C­(sp2)–H bond functionalization to highlight the challenges and opportunities, emphasizing the different techniques developed so far.

Directing group assisted ortho‐C−H activation has been known for the last few decades. In contrast, extending the same approach to achieve activation of the distal meta‐ and para‐C−H bonds in aromatic molecules remained elusive for a long time. The main challenge is the conception of a macrocyclic transition state, which is needed to anchor the metal catalyst close to the target bond. Judicious modification of the chain length, the tether linkage, and the nature of the catalyst‐coordinating donor atom has led to a number of successful studies in the last few years. This Review compiles the significant achievements made in this field of both meta‐ and para‐selectivity using covalently attached directing groups, which are systematically classified on the basis of their mode of covalent attachment to the substrate as well as their chemical nature. This Review aims to create a more heuristic approach for recognizing the suitability of the directing groups for use in future organic transformations.

Palladium(II)‐catalyzed meta‐selective C−H allylation of arenes has been developed utilizing synthetically inert unactivated acyclic internal olefins as allylic surrogates. The strong σ‐donating and π‐accepting ability of pyrimidine‐based directing group facilitates the olefin insertion by overcoming inertness of the typical unactivated internal olefins. Exclusive allyl over styrenyl product selectivity as well as E stereoselectivity were achieved with broad substrate scope, wide functional‐group tolerance, and good to excellent yields. Late‐stage functionalisations of pharmaceuticals were demonstrated. Experimental and computational studies shed light on the mechanism and point to key steric control in the palladacycle, thus determining product selectivities.

Under ball-milling, the organo-oxidant IBX efficiently works at room temperature, in solvent free conditions and in milligram to gram scale.