biaryl

Biaryl refers to a class of organic compounds composed of two aromatic rings joined directly by a single carbon–carbon bond (an aryl–aryl linkage). The simplest example is biphenyl, two phenyl rings connected without an intervening atom. In unsubstituted form, the rings may adopt a near-planar arrangement in some phases, but steric and environmental factors often cause twisting about the aryl–aryl bond, especially in substituted derivatives.

A defining feature of biaryls is the possibility of axial chirality. When bulky substituents reside in the ortho positions, rotation around the aryl–aryl bond can be significantly hindered, giving stable atropisomeric (axially chiral) biaryls. Such compounds are important in asymmetric catalysis and have been developed as ligands and catalysts in various transformations.

Synthesis of biaryls commonly proceeds via carbon–carbon bond-forming cross-couplings. Suzuki–Miyaura coupling (aryl halide with aryl boronate) is widely used, as are Ullmann-type copper couplings and related Chan–Lam reactions. Direct C–H aryl–aryl coupling and nickel-catalyzed variants have expanded the scope to less activated substrates. In many cases, substitution patterns and sterics govern the choice of method.

Biaryls have broad applications in pharmaceuticals, agrochemicals, and materials science. They serve as core motifs in drugs and as building blocks for ligands in asymmetric catalysis. In organic electronics, biaryl frameworks appear in dyes, pigments, and light-emitting materials. Related motifs include binaphthyl and other axially chiral biaryls that exploit restricted rotation for functional properties.