Site-selective C-H functionalization has emerged as an efficient tool in simplifying the synthesis of complex molecules. Most often, directing group (DG)-assisted metallacycle formation serves as an efficient strategy to ensure promising regioselectivity. A wide variety of ortho- and meta-C-H functionalizations stand as examples in this regard. Yet despite this significant progress, DG-assisted selective para-C-H functionalization in arenes has remained unexplored, mainly because it involves the formation of a geometrically constrained metallacyclic transition state. Here we report an easily recyclable, novel Si-containing biphenyl-based template that directs efficient functionalization of the distal p-C-H bond of toluene by forming a D-shaped assembly. This DG allows the required flexibility to support the formation of an oversized pre-transition state. By overcoming electronic and steric bias, para-olefination and acetoxylation were successfully performed while undermining o- and m-C-H activation. The applicability of this D-shaped biphenyl template-based strategy is demonstrated by synthesizing various complex molecules.

Transition metal–catalyzed aryl C−H activation is a powerful synthetic tool as it offers step and atom-economical routes to site-selective functionalization. Compared with proximal ortho-C−H activation, distal (meta- and/or para-) C−H activation remains more challenging due to the inaccessibility of these sites in the formation of energetically favorable organometallic pretransition states. Directing the catalyst toward the distal C−H bonds requires judicious template engineering and catalyst design, as well as prudent choice of ligands. This review aims to summarize the recent elegant discoveries exploiting directing group assistance, transient mediators or traceless directors, noncovalent interactions, and catalyst and/or ligand selection to control distal C−H activation.

The arylation of C–H bonds to generate heteroaryl–aryl (Het–Ar) and arylated quinone (Quin–Ar) compounds has received great attention to achieve sustainable goals in synthetic chemistry. Despite significant advances, arylation of a broad range of Het–Ar and Quin–Ar derivatives remains a challenging task. Herein, a variety of heterocycles are arylated by using arylboronic acids in the presence of catalytic amounts of inexpensive Fe(NO3)3. The C‐arylated quinone compounds can be prepared by reacting arylboronic acids with either quinone or hydroquinone. The present method is operationally simple, scalable, does not require prefunctionalization of the heterocycle or quinone, and can tolerate a wide variety of functional groups in the coupling partners. These qualities are expected to render this method attractive for academic and industrial use.

Strong σ-coordination by a heteroatom containing directing group (DG) is one of the effective strategies for performing site-selective C–H functionalization. Despite tremendous progress in directed ortho-C–H functionalization, selective meta-C–H functionalization using strong σ-coordination remains extremely challenging. Herein, we introduce the 8-nitroquinoline-based DG to ensure the formation of a stable palladacycle which enables selective meta-alkenylation and acetoxylation of arenes. Kinetic experiments, ESI-MS, NMR, and DFT studies provided important information regarding the mechanism of the reaction. The scalability as well as diversification of the products have been examined and are expected to be beneficial in pharmaceutical and material sciences.

An easily removable pyrimidine-based auxiliary has been employed for the remote meta-C-H cyanation of arenes. The scope of this Pd-catalyzed cyanation reaction using copper(I) cyanide as the cyanating agent was demonstrated with benzylsilanes, benzylsulfonates, benzylphophonates, phenethylsulfonates, and phenethyl ether derivatives. The method was utilized for the synthesis of pharmaceutically valuable precursors.