Katalysatorkontrollierte regiodivergente C‐H‐Alkinylierung von Thiophenen**

Silver-Free C-H Activation: Strategic Approaches towards Realizing the Full Potential of C-H Activation in Sustainable Organic Synthesis

The activation of carbon-hydrogen bonds is considered as one of the most attractive techniques in synthetic organic chemistry because it bears the potential to shorten synthetic routes as well as to produce complementary product scopes compared to traditional synthetic strategies. However, many current methods employ silver salts as additives, leading to stoichiometric metal waste and thereby preventing the full potential of C-H activation to be exploited. Therefore, the development of silver-free protocols has recently received increasing attention. Mechanistically, silver can serve various roles in C-H activation and thus, avoiding the use of silver requires different approaches based on the role it serves in a given process. In this Review, we present the comparison of silver-based and silver-free methods. Focusing on the strategic approaches to develop silver-free C-H activation, we provide the reader with the means to develop sustainable methods for C-H activation.

Site-Selective C-H Arylation of Diverse Arenes Ortho to Small Alkyl Groups

Catalytic systems for direct C-H activation of arenes commonly show preference for electronically activated and sterically exposed C-H sites. Here we show that a range of functionally rich and pharmaceutically relevant arene classes can undergo site-selective C-H arylation ortho to small alkyl substituents, preferably endocyclic methylene groups. The C-H activation is experimentally supported as being the selectivity-determining step, while computational studies of the transition state models indicate the relevance of non-covalent interactions between the catalyst and the methylene group of the substrate. Our results suggest that preference for C(sp² )-H activation next to alkyl groups could be a general selectivity mode, distinct from common steric and electronic factors.

Selective Para-C-H Alkynylation of Aniline Derivatives by Pd/S,O-Ligand Catalysis

Herein, we report a nondirected para-selective C-H alkynylation of aniline derivatives by a Pd/S,O-ligand-based catalyst. The reaction proceeds under mild conditions and is compatible with a variety of substituted anilines. The scalability and further derivatizations of the alkynylated products have been also demonstrated.

Mechanism of the Arene-Limited Nondirected C-H Activation of Arenes with Palladium*

Recently palladium catalysts have been discovered that enable the directing-group-free C-H activation of arenes without requiring an excess of the arene substrate, thereby enabling methods for the late-stage modification of complex organic molecules. The key to success has been the use of two complementary ligands, an N-acyl amino acid and an N-heterocycle. Detailed experimental and computational mechanistic studies on the dual-ligand-enabled C-H activation of arenes have led us to identify the catalytically active species and a transition state model that explains the exceptional activity and selectivity of these catalysts. These findings are expected to be highly useful for further method development using this powerful class of catalysts.

Metal-Free Directed C-H Borylation of Pyrroles

Robust strategies to enable the rapid construction of complex organoboronates in selective, practical, low-cost, and environmentally friendly modes remain conspicuously underdeveloped. Here, we develop a general strategy for the site-selective C-H borylation of pyrroles by using only BBr₃ directed by pivaloyl groups, avoiding the use of any metal. The site-selectivity is generally dominated by chelation and electronic effects, thus forming diverse C2-borylated pyrroles against the steric effect. The formed products can readily engage in downstream transformations, enabling a step-economic process to access drugs such as Lipitor. DFT calculations (wB97X-D) demonstrate the preferred positional selectivity of this reaction.