Orthoarylation

Orthoarylation refers to the installation of an aryl substituent at the ortho position of an aromatic ring. In practice, it is achieved by methods that direct arylation to the ortho C–H bond relative to a coordinating group on the substrate, most commonly through directed C–H activation or cross-coupling strategies.

Most widely used approaches employ transition-metal catalysts, especially palladium. A typical catalytic cycle involves coordination of a directing group (for example, a carbonyl-containing amide, pyridine, or related moiety) to the metal, enabling selective activation of the ortho C–H bond to form a cyclometalated intermediate. An aryl source—such as an aryl iodide or bromide, a diazonium salt, or an organoboron reagent—is then coupled to the activated aryl ring and the product is released by reductive elimination, with the catalyst regenerated. Oxidants or reoxidants are often required to maintain the catalytic cycle.

Substrates bearing suitable directing groups can be simple arenes or complex, naturally occurring products and pharmaceuticals. The choice of directing group, metal, ligands, and arylating reagent determines scope and selectivity. In addition to Pd-based systems, other metals such as nickel or copper, as well as photoredox-enabled protocols, have been reported to achieve ortho-arylation.

Applications include synthesis of biaryl motifs, late-stage functionalization of complex molecules, and preparation of pharmaceutical and material building blocks. Key challenges include the need for a directing group, control of mono- versus diarylation, and compatibility with sensitive functionalities.