Carboxyl-Assisted meta-Selective C-H Functionalizations of Benzylsulfonamides

Pd-catalysed C-H functionalisation of free carboxylic acids

Pd-catalysed C-H functionalisation of free carboxylic acids has drawn significant attention over the last few years due to the predominance of carboxylic acid moieties in pharmaceuticals and agrochemicals. But their coordinating ability was overlooked and masked by exogenous directing groups for a long time. Even other crucial roles of carboxylic acids as additives and steric inducers that directly influence the mode of a reaction have been widely neglected. This review aims to embrace all of the diverse aspects of carboxylic acids except additive and steric effects by concisely and systematically describing their versatile role in Pd-catalysed proximal and distal C-H activation reactions that could be implemented in the pharmaceutical and agrochemical industries. In addition, the mechanistic perspectives along with several recent strategies developed in the last few years discussed here will serve as educational resources for future research.

Empirical Guidelines for the Development of Remote Directing Templates through Quantitative and Experimental Analyses

The ability to differentiate and selectively activate remote C-H bonds represents a perennial challenge in the field of C-H activation. Since its first report in 2012, a now-established "directing template" (DT) approach remains demonstrably effective for the functionalization of remote C-H bonds. As selectivity is hypothesized to be principally determined by the optimal positioning of the reactive catalyst to a target C-H bond, a DT's spatial factors are particularly important toward achieving high selectivity, though a systematic study on its requisite factors remain unelucidated. Through an in-depth analysis of 119 structurally unique published remote DTs, this report summarizes the key factors that are central toward achieving high selectivity at defined aryl positions, which are experimentally corroborated through the development of new aliphatic meta and para-selective DTs for electronically unbiased arenes. These empirical rules, which summarize key distance and geometric factors, are expected to be useful tools for the future development of site-selective arene C-H activation as well as other reactions that rely on covalent/noncovalent DT-mediated remote regioselection.

Emergence of Pyrimidine-Based meta-Directing Group: Journey from Weak to Strong Coordination in Diversifying meta-C-H Functionalization

C-H activation has emerged as a powerful transformative synthetic tool to construct complex molecular frameworks, which are ubiquitous in natural products, medicines, dyes, polymers, and many more. However, reactivity and selectivity, arising from the inertness of C-H bonds and their overabundance in organic molecules, are the two major fundamental challenges in developing various carbon-carbon (C-C) and carbon-heteroatom (C-X) bond formation reactions via C-H activation technique. Functional groups with coordinating capacity to the transition metal catalysts, profoundly known as directing groups (DGs), have shown great promise in exerting selective C-H activation, often called site-selective or regioselective transformation of a target molecule. Advent of directing group (DG)-assisted strategies not only has resolved the selectivity issues but also offers a unique solution to the rapid synthesis of complex molecules in a convenient and predictable manner. Our laboratory, in this regard, is fascinated by the prospect of DG-assisted distal C-H functionalization of arenes, in which the target C-H bond is remotely located from the existing directing group. Notably, in opposition to proximal ortho-C-H activation, which proceeded via an energetically favorable five- to seven-membered metallacycle, distal C-H activation remained a formidable challenge as it required formation of a large macrocyclic metallacycle. Therefore, designing a suitable directing template that would maintain the required distance and geometric relationship between the target C-H bond and the appended directing auxiliary in order to ensure the prolific delivery of the metal catalyst to the closest proximity of targeted distal C-H bond was the key to success. In this regard, the Yu group devised an elegant "U-shaped" template for the first time to execute distal meta-C-H activation recruiting a cyano-based directing group. Our initial effort to diversify the scope of meta-C-H functionalization using a cyano-based template led us to realize that the "cyano-based DGs" are intrinsically limited with weak coordinating ability, competitive binding mode (end-on vs side-on), and incompatibility with acidic and basic reaction conditions. In search of a robust directing auxiliary, we were intrigued by the possibility of using the strongly coordinating ability of pyrimidine and quinoline-based DGs.In this Account, we describe our journey from the weakly coordinating cyano-based DG to the strongly coordinating pyrimidine-based DG to achieve diverse meta-C-H functionalization of electronically and sterically unbiased arenes. While some of the functionalizations were achieved by finding suitable reaction conditions, others were led by mechanistic understanding. Notably, initial development in this realm was constrained with short linkers, in which the DG was attached to the arene of interest through 2-4 atoms. In later studies, we demonstrated that the selective meta-C-H activation can be attained even though the DG is 10-atoms away from the targeted arene. More importantly, a transient DG was successfully utilized to deliver meta-C-H olefination of arenes via in situ imine formation, which provided a step-economic route to meta-C-H activation.We hope that this Account will stimulate further template design and will provide a guiding platform for the future development of distal meta-C-H functionalization.

Enantioselective Cross-Coupling of Electron-Deficient Alkenes via Ir-Catalyzed Vinylic sp2 C-H Alkylation

A chiral Ir-catalyzed reaction of α-aryl-α,β-unsaturated amides with β-substituted acrylates proceeded to give formal conjugate adducts in high yield and ee (up to 99% yield and up to 95% ee). This is the first example of the enantioselective cross-coupling of two different electron-deficient alkenes via vinylic sp² C-H activation, and polyfunctionalized chiral compounds were obtained.