Hydroarylation

Hydroarylation is a class of hydrofunctionalization reactions in which an aryl group and a hydrogen atom are added across a multiple bond, typically an alkene or alkyne, to give an aryl-substituted, saturated or partially saturated product. The aryl fragment can be supplied by an aryl–metal species, an aryl halide, an aryl boron reagent, or, in many modern variants, by direct C–H activation of arenes.

In alkenes, hydroarylation generally forms a new C–C bond between the aryl fragment and the double bond, followed by proton transfer. With alkynes, hydroarylation across the triple bond can yield vinyl arenes or, under different conditions, 1,2-diaryl alkanes. Regio- and stereoselectivity are governed by the catalyst, ligand, substituents, and directing groups, leading to control over the position of arylation and the geometry of the product.

Mechanistic approaches include transition-metal-catalyzed hydroarylation via migratory insertion of the alkene or alkyne into a metal–hydride or metal–aryl bond, or via direct C–H activation of arenes followed by coupling to the unsaturated partner. Direct aryl C–H activation enables arylation without preformed aryl partners, contributing to atom-economical routes.

Catalysts commonly used include palladium, nickel, rhodium, cobalt, and iron. Hydrogen sources include H2, hydride donors such as silanes or formates, depending on the system. Many reactions employ directing groups to control site selectivity and to enable less reactive arenes to participate.

Hydroarylation is of interest for the synthesis of complex molecules, including pharmaceuticals and natural products, due to its ability to forge C–C bonds under relatively mild conditions. Limitations include competing side reactions, substrate scope restrictions in some systems, and the need for specialized catalysts and ligands.