Biaryls

Biaryls are organic compounds in which two aryl rings are connected by a single carbon–carbon bond. The simplest member is biphenyl, consisting of two phenyl rings linked directly. The biaryl motif is widespread in chemistry and materials, appearing in natural products, pharmaceuticals, ligands, and polymers.

Rotation about the aryl–aryl bond varies with substitution. In many unsubstituted or lightly substituted biaryls, the rings rotate freely at ambient temperature. However, bulky ortho substituents or rigid linkages can hinder rotation sufficiently to create atropisomerism, producing stable, enantiomeric forms defined by axial chirality rather than by stereocenters.

Biaryls are typically formed through cross-coupling methods and related transformations. Common routes include Suzuki–Miyaura coupling, Ullmann-type couplings, and other aryl–aryl bond-forming reactions such as direct oxidative coupling or C–H activation processes. Both symmetric biaryls (identical rings) and nonsymmetric biaryls (different rings) can be prepared, enabling a wide range of substituent patterns and electronic properties.

Applications of biaryls span catalysis, materials, and biology. They serve as key scaffolds in ligands for asymmetric catalysis, notably via chiral biaryl backbones such as binaphthyl derivatives. Examples include BINAP and BINOL-related ligands. Biaryl units also appear in dyes, liquid crystals, and pharmaceutical agents, with substituents tuning planarity, conjugation, and metabolic stability to achieve desired physical and chemical properties.